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Reciprocal action of Casein Kinase Iε on core planar polarity proteins regulates clustering and asymmetric localisation

  1. Helen Strutt  Is a corresponding author
  2. Jessica Gamage
  3. David Strutt  Is a corresponding author
  1. University of Sheffield, United Kingdom
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Cite this article as: eLife 2019;8:e45107 doi: 10.7554/eLife.45107

Abstract

The conserved core planar polarity pathway is essential for coordinating polarised cell behaviours and the formation of polarised structures such as cilia and hairs. Core planar polarity proteins localise asymmetrically to opposite cell ends and form intercellular complexes that link the polarity of neighbouring cells. This asymmetric segregation is regulated by phosphorylation through poorly understood mechanisms. We show that loss of phosphorylation of the core protein Strabismus in the Drosophila pupal wing increases its stability and promotes its clustering at intercellular junctions, and that Prickle negatively regulates Strabismus phosphorylation. Additionally, loss of phosphorylation of Dishevelled – which normally localises to opposite cell edges to Strabismus – reduces its stability at junctions. Moreover, both phosphorylation events are independently mediated by Casein Kinase Iε. We conclude that Casein Kinase Iε phosphorylation acts as a switch, promoting Strabismus mobility and Dishevelled immobility, thus enhancing sorting of these proteins to opposite cell edges.

https://doi.org/10.7554/eLife.45107.001

Introduction

Phosphorylation is a widespread means of controlling protein activity, regulating protein-protein interactions, protein stability and conformation (Hunter, 2007). The activity of most signalling pathways is regulated by phosphorylation of pathway components. This includes the ‘core’ planar polarity pathway (Singh and Mlodzik, 2012; Butler and Wallingford, 2017): however, compared to other signalling pathways, the molecular mechanisms are poorly understood.

The core planar polarity proteins (hereafter, the ‘core proteins’) regulate the production of polarised structures or polarised cell behaviours in the plane of a tissue. This includes polarised production of cilia and of stereocilia bundles in the inner ear, and the coordinated polarisation of tissue movements necessary for convergence and extension of the body axis (Devenport, 2016; Davey and Moens, 2017; Butler and Wallingford, 2017). In Drosophila, the core pathway controls the production of polarised hairs and bristles on many adult tissues, for example the trichomes that emerge from the distal edge of each cell in the adult wing.

The core pathway specifies polarised structures via the asymmetric localisation of pathway components. In the Drosophila pupal wing, the seven-pass transmembrane protein Frizzled (Fz), and the cytoplasmic proteins Dishevelled (Dsh) and Diego (Dgo) localise to distal cell ends, where the trichome will emerge. The four-pass transmembrane protein Strabismus (Stbm, also known as Van Gogh [Vang]) and Prickle (Pk) localise to proximal cell ends, and the atypical cadherin Flamingo (Fmi, also known as Starry Night [Stan]) localises to both proximal and distal cell ends (Figure 1A). Fmi mediates homophilic adhesion that is important for coupling polarity between cells (reviewed in Goodrich and Strutt, 2011; Devenport, 2016; Butler and Wallingford, 2017).

Planar polarity and the cloud model of core protein localisation.

(A) Core polarity proteins localise to proximal or distal edges of pupal wing cells (left), where they form intercellular complexes (right). (B) Live image of a 28 hr APF pupal wing expressing Stbm-EGFP. Asymmetrically localised core proteins cluster into membrane subdomains (puncta, yellow circle). The cyan circle indicates a non-puncta domain on the proximal-distal cell edges. (C) Diagram illustrating the possible organisation of the core polarity proteins. In non-puncta junctional regions, complexes associate at low density, in both orientations, and have relatively high mobility (left). Feedback interactions between the core proteins leads to complex sorting and complexes align in the same orientation. This promotes higher order multimerisation (red connectors) and reduced mobility (middle and right). (D) Diagram illustrating the position of the conserved phosphorylation site clusters (orange boxes) in the Stbm protein. The positions of the four transmembrane domains (black boxes, TM1-4) are also shown.

https://doi.org/10.7554/eLife.45107.002

The overall direction of polarisation is determined by tissue-specific global cues (Aw and Devenport, 2017). Polarity is then thought to be refined and amplified by feedback interactions between the core proteins. Mathematical modelling has suggested that feedback may involve destabilisation of complexes of opposite orientation and/or stabilisation of complexes in the same orientation. This can lead to sorting of complexes such that they all align in the same direction (Amonlirdviman et al., 2005; Le Garrec et al., 2006; Burak and Shraiman, 2009; Schamberg et al., 2010).

With regard to possible stabilising mechanisms, core protein asymmetry is associated with clustering of proteins into punctate membrane subdomains (Figure 1B, Strutt et al., 2011; Cho et al., 2015) and reduced core protein turnover (Strutt et al., 2011; Butler and Wallingford, 2015; Chien et al., 2015; Strutt et al., 2016). Based on a detailed study of core protein organisation in puncta, we recently proposed that core proteins form a non-stoichiometric ‘cloud’ around a Fmi-Fz nucleus (Strutt et al., 2016). Feedback interactions lead to sorting of complexes, and multiple protein-protein interactions are thought to promote a phase transition into higher order ‘signalosome-like’ structures, where arrays of complexes of the same orientation are stabilised (Figure 1C, Strutt et al., 2016). Interestingly, Stbm stoichiometry was found to be much higher than that of the other core proteins (Strutt et al., 2016). The reasons for this are unclear, but could relate to a role for Stbm in promoting higher order structures. Furthermore, Pk may stabilise Stbm by promoting complex clustering (Tree et al., 2002; Bastock et al., 2003; Cho et al., 2015).

Mechanisms of destabilisation may include competitive binding between core proteins (Tree et al., 2002; Carreira-Barbosa et al., 2003; Jenny et al., 2005; Amonlirdviman et al., 2005). More specifically, Pk (a ‘proximal’ complex component) is known to destabilise Fz and/or Dsh (‘distal’ components) in the same cell (Warrington et al., 2017). In addition, Pk has been suggested to destabilise complexes containing Stbm and Fmi (Cho et al., 2015). However, knowledge of additional molecular mechanisms by which core proteins might become destabilised or clustered together is very poor, and post-translational modifications such as phosphorylation are likely to be a key element.

Indeed, core protein phosphorylation is essential for feedback amplification of asymmetry. In particular, reduced activity of Casein Kinase Iε (CKIε, also known as Discs Overgrown [Dco] or Doubletime [Dbt] in flies) causes planar polarity defects and a reduction in core protein asymmetry (Strutt et al., 2006; Klein et al., 2006; Kelly et al., 2016; Yang et al., 2017). Interestingly, CKIε has been implicated in phosphorylation of both Stbm and Dsh. CKIε was first found to bind to and phosphorylate the vertebrate Dsh homologue (Dvl) in canonical Wnt signalling (Peters et al., 1999; McKay et al., 2001). In planar polarity in flies, Dsh phosphorylation correlates with its recruitment to cellular junctions by Fz (Axelrod, 2001; Shimada et al., 2001), where it is incorporated into stable complexes (Strutt et al., 2016), and decreased Dsh phosphorylation is seen in dco mutants (Strutt et al., 2006).

The exact phosphorylation sites for CKIε in Dsh/Dvl are not well defined, but a mutation of a serine/threonine-rich region upstream of the PDZ domain affects Dvl recruitment to membranes in Xenopus (Ossipova et al., 2005). Moreover, mutation of one of these residues (S236 in fly Dsh) blocks phosphorylation of Dsh by Dco in vitro (Klein et al., 2006). However, a transgene in which these residues were mutated largely rescued the planar polarity defects of dsh mutants in the adult fly wing (Strutt et al., 2006; but see also Penton et al., 2002).

More recently, CKIε has been implicated in phosphorylating Stbm and its vertebrate homologue Vangl2 (Gao et al., 2011; Kelly et al., 2016; Yang et al., 2017). In particular, Wnt gradients were proposed to lead to a gradient of Vangl2 phosphorylation and asymmetry in the vertebrate limb (Gao et al., 2011). CKIε promotes Stbm/Vangl2 phosphorylation in cell culture (Gao et al., 2011; Kelly et al., 2016; Yang et al., 2017). Two clusters of conserved serine and threonine residues were identified as CKIε phosphorylation sites. Mutation of some or all of these residues leads to a loss of Stbm/Vangl2 phosphorylation in cell culture, and defects in planar polarisation (Gao et al., 2011; Ossipova et al., 2015; Kelly et al., 2016; Yang et al., 2017).

The fact that CKIε has been implicated in phosphorylating both Stbm/Vangl2 and Dsh/Dvl in cell culture leads to the question of whether both proteins are bona fide targets in vivo. For instance, both Fz and Dsh/Dvl have been proposed to promote Stbm/Vangl2 phosphorylation by CKIε (Kelly et al., 2016; Yang et al., 2017). Thus, it is possible that only Stbm/Vangl2 are direct targets of CKIε and that Stbm/Vangl2 phosphorylation has a secondary effect on Fz-Dsh/Dvl behaviour. Moreover, mechanistic insight into how these phosphorylation events affect core protein sorting and asymmetry is lacking.

Here, we demonstrate that CKIε has independent and reciprocal actions on Dsh and Stbm during planar polarity signalling in Drosophila. We use phosphorylation site mutations in Stbm to show that lack of Stbm phosphorylation leads to its clustering in ‘mixed’ puncta that contain complexes in both orientations. CKIε-dependent phosphorylation increases Stbm turnover at junctions, and thus promotes complex sorting, while phosphorylation of Dsh decreases its turnover. Pk negatively regulates Stbm phosphorylation and increases Stbm stability. These results support a direct role for Dco in phosphorylating both Stbm and Dsh in vivo in planar polarity signalling.

Results

Stbm phosphorylation sites are essential for core protein asymmetry

Previous work identified two conserved clusters of serine and threonine residues within vertebrate Vangl2, which are phosphorylated in tissue culture (Figure 1D, Gao et al., 2011; Yang et al., 2017). P[acman]-stbm rescue constructs (Strutt et al., 2016) were generated, in which all serine/threonine residues in clusters I and II were mutated to alanine (phosphomutant ‘S[All]A’) or glutamic acid (phosphomimetic ‘S[All]E’). These residues are in regions of Stbm predicted to be unstructured (data not shown), so this was not expected to alter the secondary structure of Stbm. Neither the phosphomutant nor the phosphomimetic form of Stbm rescued the trichome orientation of stbm null mutants, while wild-type P[acman]-stbm in the same genomic site gave complete rescue (Figure 2A–D and Figure 2—figure supplement 1D). The failure of the phosphomimetic version to rescue may be because glutamic acid does not completely substitute for phosphorylated serine and threonine residues within Stbm. Alternatively, Stbm may need to cycle between phosphorylated and unphosphorylated forms in order to function in planar polarity, and the mutated proteins are unable to perform this cycling.

Figure 2 with 2 supplements see all
Disrupted trichome orientation in Stbm phosphomutants and phosphomimetics.

(A–D) Adult wings from stbm6 mutant flies (A), P[acman]-stbm stbm6 flies (B), P[acman]-stbmS[All]A stbm6 flies (C), or P[acman]-stbmS[All]E stbm6 flies (D). Scale bar 100 µm. (E,F) 28 hr APF pupal wings, carrying twin clones of P[acman]-stbm stbm6, marked by β-gal immunolabelling (red), next to P[acman]-stbmS[All]A stbm6 (E) or P[acman]-stbmS[All]E stbm6 (F). Wings immunolabelled for Stbm in green. (E',F') Stbm immunolabelling overlaid with polarity nematics (red lines), where the length of line indicates mean cell polarity and the orientation indicates direction of polarity. Yellow boxes indicate zoomed regions shown in E'' and F'' (wild-type regions) or E''' and F''' (mutant regions). Scale bar 5 µm. (G,H) Quantitation of mean polarity and variation in polarity angle, of 28 hr APF pupal wings immunolabelled for Stbm in twin clones of P[acman]-stbm stbm6 and P[acman]-stbmS[All]A stbm6 (G, n = 11) or P[acman]-stbmS[All]E stbm6 (H, n = 13). Values from the same wing are linked by black bars, mean and 95% confidence intervals are listed. Paired t-tests were used to compare values in the same wings, ***p≤0.001. (I) Quantitation of mean intensity of Stbm immunolabelling at junctions of 28 hr APF pupal wings, shown as a ratio of signal in P[acman]-stbmS[All]A stbm6 (n = 12) or P[acman]-stbmS[All]E stbm6 (n = 12) compared to P[acman]-stbm stbm6 (wt) in each wing. Error bars are 95% confidence intervals. One-sample t-tests were used to determine if the ratio differed from 1.0, ***p≤0.001. (J) Western blot probed with Stbm antibody, of extracts from 28 hr APF pupal wings from wild-type, P[acman]-stbmS[All]A stbm6 or P[acman]-stbmS[All]E stbm6 flies. The asterisk indicates a non-specific band. Actin was used as a loading control. (K) Quantitation of Stbm levels from western blots, from wild-type (n = 3), P[acman]-stbmS[All]A stbm6 (n = 4) or P[acman]-stbmS[All]E stbm6 (n = 4) pupal wings. Error bars are 95% confidence intervals. Levels were compared to wild-type by ANOVA with Dunnett’s multiple comparisons test, no significant differences were seen. (L,M) Quantitation of puncta number (L) and mean puncta amount (M), from 28 hr APF pupal wings immunolabelled for Stbm, shown as a ratio of signal in P[acman]-stbmS[All]A stbm6 (n = 6) or P[acman]-stbmS[All]E stbm6 (n = 8) compared to P[acman]-stbm stbm6 (wt) in each wing. Puncta were detected using Fmi immunolabelling. Error bars are 95% confidence intervals. One-sample t-tests were used to determine if the ratio differed from 1.0, **≤0.01, ***p≤0.001.

https://doi.org/10.7554/eLife.45107.003

Core protein localisation in phosphomutant and phosphomimetic pupal wings was then examined. Twin clones were made, in which tissue expressing wild-type Stbm was juxtaposed to tissue expressing mutant forms of Stbm, both in the absence of endogenous stbm gene activity. In keeping with the strong trichome orientation defects, a strong decrease in core protein asymmetry was seen in pupal wings expressing either phosphomutant or phosphomimetic forms of Stbm (Figure 2E–H, Figure 2—figure supplement 1H and I, Figure 2—source data 1). There was also a slight increase in overall levels of phosphomutant or phosphomimetic Stbm at cellular junctions, compared to wild-type Stbm (Figure 2I, Figure 2—source data 1). We conclude that the phosphorylation sites in Stbm are necessary for its correct asymmetric localisation and to orient trichomes in the adult wing.

Western blotting of pupal wing extracts confirmed that endogenous Stbm is phosphorylated in vivo, with the majority of protein existing in a phosphorylated state (Figure 2—figure supplement 2A). As expected, the phosphomutant form showed increased mobility on SDS-PAGE, while the phosphomimetic form had a similar mobility to that of endogenous Stbm (Figure 2J). However, as expected, the mobility of phosphomimetic Stbm was not sensitive to phosphatase treatment (Figure 2—figure supplement 2A). Overall cellular protein levels were similar to wild-type (Figure 2K, Figure 2—source data 1).

It has been suggested that residue 5 in cluster II, and residues 120 and 122 in cluster I are ‘founder sites’, such that phosphorylation on these leads to a cascade of phosphorylation on neighbouring residues (Gao et al., 2011; Ossipova et al., 2015; Kelly et al., 2016; Yang et al., 2017). However, P[acman]-stbm constructs simultaneously mutant for all three founder sites fully rescued trichome polarity in adult wings, and core protein asymmetry in pupal wings was normal (Figure 2—figure supplement 1, Figure 2—source data 1). This contrasts with the work of Kelly et al. (2016), who reported planar polarity defects in flies after mutation of serines 120 and 122 to alanine. The difference in our results could be due to abnormal or uneven expression of the tub-StbmS2A rescue construct used in Kelly et al. (2016). Furthermore, mutation of all phosphorylation sites in only cluster I or cluster II revealed that the phosphorylation sites within cluster I are sufficient for correct core protein asymmetry, and are responsible for most of the retardation in mobility on SDS-PAGE (Figure 2—figure supplement 2B–H, Figure 2—source data 1).

Uncoupling of puncta formation and asymmetry in Stbm phosphomutants

Interestingly, previous data has suggested a correlation between core protein asymmetric localisation to opposite cell ends, and the formation of large junctional puncta (Strutt et al., 2011; Cho et al., 2015). In keeping with this idea, fewer puncta were observed in Stbm phosphomimetic flies, and non-puncta material also increased (Figure 2F). However, in the Stbm phosphomutant, Stbm still appeared to cluster into puncta, despite the loss of asymmetry (Figure 2E).

To quantitate puncta size, we thresholded images using the same threshold value in wild-type and mutant regions of the same wings. Fmi co-immunolabelling was used to select puncta, as overall junctional levels of Fmi do not change in either the phosphomutant or the phosphomimetic tissue (see below). This revealed that a similar number of puncta were seen in the phosphomutant as in wild-type tissue, while many fewer puncta were seen in the phosphomimetic (Figure 2L, Figure 2—source data 1). The mean amount of Stbm in puncta was however slightly reduced in both phosphomutant and phosphomimetic tissue (Figure 2M, Figure 2—source data 1). As the phosphomutant forms a similar number of puncta to wild-type, this suggests that the coupling between puncta formation and core protein asymmetry is lost in Stbm phosphomutants.

Other core proteins co-localised with both phosphomutant and phosphomimetic Stbm (Figure 3A–D). Overall levels of Fmi and Dsh in junctions were similar to wild-type, while Fz levels were slightly decreased in both cases, and Pk levels were lower in phosphomutant and higher in phosphomimetic wings (Figure 3—figure supplement 1A–J, Figure 3—source data 1).

Figure 3 with 1 supplement see all
Stbm phosphomutants form ‘mixed’ puncta.

(A–D) 28 hr APF pupal wings expressing P[acman]-stbmS[All]A stbm6 (A,C) or P[acman]-stbmS[All]E stbm6 (B,D). Blue immunolabelled tissue in (D) is wild-type. Wings immunolabelled for Stbm in green and Fmi (A,B) or Fz (C,D) in red. Arrowheads point to examples of puncta in which Stbm and Fmi or Fz co-localise. Scale bar 5 µm. (E,F) Schematic of twin clone experiment. Groups of cells express either EGFP-Dgo (green) or mApple-Dgo (red), and puncta are examined at the boundaries between them. (E) In wild-type wings, Dgo localises to distal cell ends, so puncta on distal clone boundaries contain either only EGFP-Dgo or only mApple-Dgo. (F) In phosphomutant wings, Dgo is not asymmetrically localised. If individual puncta are polarised (left), puncta on clones boundaries will contain either EGFP-Dgo or mApple-Dgo, regardless of whether the boundary is proximal or distal. If individual puncta are not polarised (right), puncta will contain both EGFP-Dgo and mApple-Dgo. (G,H) 28 hr APF pupal wings carrying twin clones of P[acman]-EGFP-dgo dgo380 next to P[acman]-mApple-dgo dgo380, in a wild-type (G) or a P[acman]-stbmS[All]A stbm6 mutant background (H). EGFP fluorescence is in green and mApple fluorescence is in red. White arrowheads indicate specific puncta on clone boundaries. Puncta are labelled with either green or red Dgo in wild-type, but in stbmS[All]A tissue, puncta on clone boundaries appear yellow, as they contain both green and red Dgo. (I) Schematic of ‘mixed’ punctum, containing core protein complexes in both orientations (compare to Figure 1C, right).

https://doi.org/10.7554/eLife.45107.007

Stbm phosphomutant puncta contain complexes in both orientations

Core protein complexes within junctional puncta are highly polarised compared to other junctional regions (Strutt et al., 2011; Cho et al., 2015; Strutt et al., 2016). This is consistent with puncta containing arrays of core protein complexes, all aligned in the same direction (Figure 1C, right). As Stbm phosphomutant puncta are no longer associated with overall asymmetry we asked whether individual puncta are still polarised, or whether phosphomutant puncta have a different organisation. To test this, we made adjacent twin clones of EGFP-tagged Dgo next to mApple-tagged Dgo. In both wild-type and phosphomutant backgrounds, EGFP-Dgo and mApple-Dgo co-localise with other core proteins in puncta, as expected (Figure 3—figure supplement 1K and L). In a wild-type background, Dgo localised predominantly to distal cell ends, as previously reported (Das et al., 2004), so puncta on clone boundaries contained either EGFP-Dgo (green) or mApple-Dgo (red) (Figure 3E,G). We could envisage two scenarios for Stbm phosphomutant puncta on clone boundaries: if individual puncta were polarised, we would expect to see puncta containing either EGFP-Dgo or mApple-Dgo, but Dgo in puncta could localise to any cell edge (Figure 3F, left). Alternatively if individual puncta were not polarised, co-localisation of EGFP-Dgo and mApple-Dgo would be seen (Figure 3F, right). Such co-localisation of EGFP-Dgo and mApple-Dgo was indeed observed (Figure 3H). This indicates that individual puncta (at least at this optical resolution) contain complexes in both orientations (Figure 3I), and that inhibition of Stbm phosphorylation disrupts sorting of complexes.

Phosphorylation of Stbm regulates its turnover at junctions

In a wild-type situation, the alignment of core proteins in the same orientation within puncta correlates with low protein turnover, as measured by Fluorescence Recovery After Photobleaching (FRAP) assays (Strutt et al., 2011). As the Stbm phosphomutant forms abnormal puncta with complexes in both orientations, we investigated the turnover of phosphomutant Stbm. A ‘hub-and-spoke’ FRAP methodology was used, in which the junctions in the equivalent of half a cell are bleached (Warrington et al., 2017, Figure 4A). This avoids excessive bleaching of total protein in any single cell, while allowing junctions of all orientations to be sampled, regardless of whether they are enriched for core proteins or contain puncta.

Figure 4 with 2 supplements see all
Stbm phosphomutants have increased stability at junctions.

(A) Schematic of hub-and-spoke FRAP. The orange dotted line shows the ‘hub-and-spoke’ region that is bleached, which corresponds to three cell vertices and the equivalent of three cell junctions. The stable fraction, as determined by FRAP, is multiplied by the total initial intensity within the bleached region to give a stable amount of protein at junctions for each wing. (B) Stable and unstable amounts of EGFP-tagged protein in 28 hr APF pupal wings after hub-and-spoke FRAP. Flies were P[acman]-stbm-EGFP stbm6/+ (n = 11), P[acman]-stbmS[All]A-EGFP stbm6/P[acman]-stbmS[All]A stbm6 (n = 9) and P[acman]-stbmS[All]E-EGFP stbm6/P[acman]-stbmS[All]E stbm6 (n = 11). Data are normalised to the total intensity in Stbm-EGFP. Error bars are 95% confidence intervals, and ANOVA with Dunnett’s multiple comparisons test was used to compare stable amounts (asterisks in dark orange columns), unstable amounts (asterisks in light orange columns) or total amounts (blue asterisks above the columns) to the Stbm-EGFP control, p≤0.05*, p≤0.001***. (C) Summary diagram showing the effect of phosphorylation and dephosphorylation on Stbm turnover. (D) Schematic of puncta FRAP and boundary FRAP. In puncta FRAP, an elliptical region surrounding a punctum is bleached (Figure 4E–G), whilst in boundary FRAP an entire junction on a clone boundary is bleached (Figure 6G). Note that as puncta of different genotypes are different sizes, stable fractions in puncta FRAP cannot be translated into stable amounts. (E–G) Stable fraction of EGFP-tagged protein in puncta in 28 hr APF pupal wings. (E) P[acman]-stbm-EGFP stbm6 (n = 6), P[acman]-stbmS[All]A-EGFP stbm6 (n = 7), P[acman]-stbmS[All]E-EGFP stbm6 (n = 9). (F) fmi-EGFP/+ (n = 9), P[acman]-stbmS[All]A stbm6/P[acman]/stbmS[All]A stbm6 fmi-EGFP (n = 10), P[acman]-stbmS[All]E stbm6/P[acman]/stbmS[All]E stbm6 fmi-EGFP (n = 10). (G) fz-EGFP/+ (n = 9), P[acman]-stbmS[All]A stbm6; fz-EGFP/+ (n = 8), P[acman]-stbmS[All]E stbm6; fz-EGFP/+ (n = 7). The fluorescence recovery was fitted to an exponential curve for each genotype, and the graph shows the stable fraction (1-Ymax) and the 95% confidence intervals. Stable fractions were compared to control (wild-type Stbm or Stbm-EGFP) using an extra sum of squares F test, p≤0.05*, p≤0.001***.

https://doi.org/10.7554/eLife.45107.010

Hub-and-spoke FRAP showed that the stable amount of phosphomutant Stbm-EGFP at junctions was increased compared to wild-type Stbm-EGFP, while the unstable amount was unchanged (Figure 4B, Figure 4—figure supplement 1, Figure 4—source data 1). This suggests that Stbm phosphorylation promotes turnover of Stbm. Consistent with this, the stable amount of phosphomimetic Stbm-EGFP did not change compared to wild-type, but the unstable amount increased (Figure 4B, Figure 4—figure supplement 1, Figure 4—source data 1). This suggests that phosphomimetic Stbm accumulates at junctions even when not stably incorporated into complexes.

Taken together with our previous data that there is less phosphomimetic Stbm incorporated into puncta (Figure 2L,M), this supports a model in which phosphorylation promotes Stbm turnover, while lack of phosphorylation promotes Stbm clustering in complexes (Figure 4C). As both protein turnover and clustering are thought to be required for sorting of core proteins to opposite cell ends, these data are consistent with the idea that Stbm normally cycles between phosphorylated and unphosphorylated states during complex sorting and establishment of asymmetry.

We also measured the turnover of Stbm within puncta (Figure 4D), as these are normally sites of high core protein asymmetry and stability, but contain complexes of mixed orientation in Stbm phosphomutants. In keeping with the hub-and-spoke FRAP, an increase in the stable fraction of phosphomutant Stbm-EGFP (i.e. decreased turnover) was seen in puncta, while the stable fraction was decreased in phosphomimetic Stbm-EGFP puncta (Figure 4E, Figure 4—source data 1, note that as puncta size varies between genotypes we were not able to translate stable fractions into stable amounts).

As core protein complexes are thought to be sorted via feedback interactions, we considered how the altered stability of Stbm phosphomutants and phosphomimetics affected the localisation and stability of other core proteins. Interestingly, the stable fraction of Fz-EGFP and Fmi-EGFP within puncta was decreased in both a Stbm phosphomutant and a Stbm phosphomimetic background (Figure 4F and G, Figure 4—source data 1). We interpret this to mean that inhibiting Stbm phosphorylation promotes excess clustering and stability of Stbm within complexes; but the presence of oppositely oriented complexes may promote negative feedback interactions, leading to destabilisation of other complex components. In phosphomutant Stbm wings, this competition between complex stabilisation and destabilisation results in a net increase in Stbm stability, but a net decrease in Fmi and Fz stability. In the phosphomimetic, the result is a net decrease in stability of all three core proteins. In both cases negative feedback between unsorted complexes may prevent puncta growing to the same size as wild-type puncta (see Figure 2M).

A role for Stbm in promoting clustering of complexes of the same orientation may be a mechanism for feedback amplification of asymmetry. Interestingly, wings expressing one copy of either phosphomutant or phosphomimetic Stbm and one copy of wild-type Stbm failed to polarise, suggesting that both mutant forms act as dominant negatives (Figure 4—figure supplement 2, Figure 4—source data 1, see also Yang et al., 2017). Large puncta were observed in the phosphomutant heterozygotes (Figure 4—figure supplement 2B), consistent with a model in which excess clustering of phosphomutant Stbm leads to recruitment of wild-type Stbm into abnormal, mixed puncta.

In vivo regulation of Stbm by Dco kinase

Experiments in cultured cells have suggested that at least some of the phosphorylation of Stbm or Vangl2 could be mediated by the kinase CKIε (Dco in flies, Gao et al., 2011; Kelly et al., 2016; Yang et al., 2017). However, this has proven difficult to verify in vivo. Stbm migration on SDS-PAGE was not altered in dco hypomorphs (Figure 5—figure supplement 1A, see also Kelly et al., 2016). However, we did see a subtle increase in Stbm migration after expression of dominant-negative Dco in pupal wings (Figure 5—figure supplement 1B).

To confirm a role for Dco in regulating Stbm phosphorylation, we examined the turnover of Stbm by FRAP when either dominant-negative or wild-type Dco were overexpressed. Overexpression of dominant-negative Dco led to an increase in the stable amount of Stbm-EGFP at junctions, phenocopying the results from Stbm phosphomutants (Figure 5A, Figure 5—figure supplement 2A–C, Figure 5—source data 1). Expression of dominant-negative Dco had no further effect on the stable amount of phosphomutant Stbm (Figure 5A, Figure 5—figure supplement 2A–C, Figure 5—source data 1). Conversely, overexpression of wild-type Dco, which would be expected to promote excess phosphorylation, caused an increase in the unstable amount of Stbm, similar to the Stbm phosphomimetic (Figure 5B, Figure 5—figure supplement 2D–F, Figure 5—source data 1).

Figure 5 with 2 supplements see all
Regulation of Stbm phosphorylation and turnover by Dco.

(A–C) Stable and unstable amounts of EGFP-tagged protein, in 28 hr APF pupal wings after hub-and-spoke FRAP. Flies were (A) P[acman]-stbm-EGFP stbm6/+ (n = 11), P[acman]-stbm-EGFP stbm6/en-GAL4; UAS-dcoK38R/+ (n = 11), P[acman]-stbmS[All]A-EGFP stbm6/P[acman]-stbmS[All]A stbm6 (n = 9) and P[acman]-stbmS[All]A-EGFP stbm6/P[acman]-stbmS[All]A stbm6 en-GAL4; UAS-dcoK38R/+ (n = 15); (B) P[acman]-stbm-EGFP stbm6/+ (n = 11), UAS-Dco/+; P[acman]-stbm-EGFP-stbm6/en-GAL4 (n = 11) and P[acman]-stbmS[All]E-EGFP stbm6/P[acman]-stbmS[All]E stbm6 (n = 11); (C) dsh1; P[acman]-stbm-EGFP stbm6/+ (n = 12) and dsh1; P[acman]-stbm-EGFP stbm6/en-GAL4; UAS-dcoK38R/+ (n = 10). Wings were imaged in the posterior compartment. Data are normalised to the total intensity in Stbm-EGFP and error bars are 95% confidence intervals. Stable amounts (asterisks in dark shaded columns), unstable amounts (asterisks in light shaded columns) or total amounts (blue asterisks above the columns) were compared between genotypes, using ANOVA with Holm-Sidak’s multiple comparisons test to compare pairs of samples with and without DcoK38R (A), ANOVA with Dunnett’s multiple comparisons test to compare with the Stbm-EGFP control (B) or an unpaired t-test (C). p≤0.05*, p≤0.01**, p≤0.001***.

https://doi.org/10.7554/eLife.45107.014

As Dco is known to act on Dsh as well as Stbm, we then asked whether the effect of Dco on Stbm turnover was independent of any effect on Dsh. Overall levels of Stbm are decreased in a dsh1 mutant background (a planar polarity-specific mutation), leading to a decrease in the absolute amount of stable Stbm (Figure 6B, Figure 6—figure supplement 1C–E, Figure 6—source data 1). However, expression of dominant-negative Dco in a dsh mutant still caused an increase in the stable amount of Stbm (Figure 5C, Figure 5—figure supplement 2G–I, Figure 5—source data 1). Therefore, we conclude that Dco affects Stbm turnover independently of Dsh and supports a model in which Dco regulates Stbm turnover by direct phosphorylation of Stbm.

Figure 6 with 4 supplements see all
Pk reduces Stbm phosphorylation and promotes Stbm stability.

(A,C,E) Western blots probed with Stbm antibody, of pupal wing extracts. (A) Wings from wild-type, fzP21, pkpk-sple13, dsh1 or P[acman]-stbmS[All]A stbm6 flies at 28 hr APF, two biological replicates are shown for each genotype. (C) Wings from wild-type, Actin-GAL4, tub-GAL80ts, UAS-pk or Actin-GAL4, tub-GAL80ts, UAS-dgo flies, raised at 29°C for 25 hr APF. (E) Wings from wild-type, fzP21, dsh1, pkpk-sple13, pkpksple13; fzP21 or dsh1; pkpk-sple13 flies at 28 hr APF. The asterisk indicates a non-specific band. Actin (A,E) or α-Tubulin (C) was used as a loading control. (B,D) Stable and unstable amounts of Stbm-EGFP in pupal wings after hub-and-spoke FRAP. Flies were (B) P[acman]-stbm-EGFP stbm6/+ in a wild-type (n = 11), fzP21 (n = 9), pkpk-sple13 (n = 10) or dsh1 (n = 12) background, at 28 hr APF; (D) P[acman]-stbm-EGFP stbm6/+ in a wild-type (n = 9) or Actin-GAL4, tub-GAL80ts, UAS-pk (n = 8) background, flies raised at 29°C for 25 hr APF. Data are normalised to the total intensity in Stbm-EGFP. Error bars are 95% confidence intervals, and stable amounts (asterisks in dark shaded columns), unstable amounts (asterisks in light shaded columns) or total amounts (blue asterisks above the columns) were compared to the control (Stbm-EGFP) using ANOVA with Dunnett’s multiple comparisons test (B) or an unpaired t-test (D), p≤0.05*, p≤0.001**, p≤0.001***. (F,G) Diagram to illustrate boundary FRAP experiment (F), and stable and unstable amounts of Stbm-EGFP on illustrated clone boundaries in 28 hr APF pupal wings (G). Flies were P[acman]-stbm-EGFP stbm6 next to stbm6 (column 1, n = 10), P[acman]-stbm-EGFP stbm6 next to pkpk-sple13 stbm6 (column 2, n = 10), P[acman]-stbm-EGFP pkpk-sple13 stbm6 next to stbm6 (column 3, n = 12) and P[acman]-stbm-EGFP pkpk-sple13 stbm6 next to stbm-EGFP pkpk-sple13 stbm6 (column 4, n = 10). Error bars are 95% confidence intervals, and ANOVA with Tukey’s multiple comparisons test was used to compare stable amounts (asterisks in dark shaded columns), unstable amounts (no significant differences were seen) or total amounts (blue asterisks above the columns) between all genotypes. Comparisons to column one are shown, p≤0.001**, p≤0.001***. (H–J) 28 hr APF pupal wings from flies carrying pkpk-sple13 clones in a wild-type background (H), in a P[acman]-stbmS[All]A stbm6 background (I) or in a P[acman]-stbmS[All]E stbm6 background (J). Clones immunolabelled for Stbm (green) and marked by loss of β-gal immunolabelling (H) or loss of Pk immunolabelling (I,J) in red. Scale bar 5 µm. (K) Quantitation of mean puncta amount in 28 hr APF pupal wings, shown as a ratio of puncta amount in pkpk-sple13 mutant tissue compared to wild-type tissue in the same wing. Puncta amount is quantitated from wings immunostained for Stbm in a wild-type background (n = 6), a P[acman]-stbmS[All]A stbm6 background (n = 10) or a P[acman]-stbmS[All]E stbm6 background (n = 8). Error bars are 95% confidence intervals. One-sample t-tests were used to determine if the ratio differed from 1.0, **≤0.01, ***p≤0.001.

https://doi.org/10.7554/eLife.45107.018

Stbm phosphorylation and turnover are negatively regulated by Pk

If Stbm phosphorylation normally controls Stbm turnover and this is important for core protein asymmetry, we considered the possibility that Stbm phosphorylation might be regulated by other core proteins. Interestingly, we saw a decrease in Stbm migration on SDS-PAGE in a pk mutant background (Figure 6A). This decrease in migration was lost after phosphatase treatment (Figure 6—figure supplement 2A), suggesting that Pk negatively regulates Stbm phosphorylation. Consistent with this, loss of Pk did not alter migration of either the Stbm phosphomutant or the phosphomimetic on SDS-PAGE (Figure 6—figure supplement 2B).

Overexpression of Pk causes excess clustering of core proteins into large junctional puncta (Tree et al., 2002; Bastock et al., 2003), which contain complexes in both orientations (Figure 6—figure supplement 1A B), as seen for Stbm phosphomutant puncta. Interestingly, this also led to increased migration of Stbm on SDS-PAGE, suggesting decreased phosphorylation (Figure 6C). Thus, Pk overexpression mimics Stbm phosphomutant phenotypes, while loss of Pk has the opposite effect.

Previous studies have suggested that Fz or Dsh might promote Stbm phosphorylation (Kelly et al., 2016; Yang et al., 2017). This would support a model in which Fz or Dsh recruit a kinase, thus phosphorylating and destabilising Stbm in complexes in the opposite orientation. However, we did not see any change in the migration of endogenous Stbm on SDS-PAGE in fz or dsh mutants, with Stbm still migrating more slowly than phosphomutant Stbm (Figure 6A). This differs from the work of Kelly et al. (2016), who observed increased mobility of FLAG-tagged Stbm in fz mutants. We do not know why our results differ, but it is possible that tagging Stbm at the C-terminus with FLAG affects its function. Further support for our data comes from the observation that the hyperphosphorylation seen in pk mutants is retained in pk; fz or dsh; pk mutants, suggesting that neither Fz nor Dsh is needed for this hyperphosphorylation (Figure 6E). However, loss of Fz or Dsh did lead to a decrease in the stable amounts of Stbm at junctions (Figure 6B, Figure 6—figure supplement 1C–E, Figure 6—source data 1). This could be because Fz and Dsh normally promote Stbm recruitment across cell junctions, which would be consistent with the previously reported stabilisation of Fz across cell junctions by Stbm and Pk (Warrington et al., 2017).

Regulation of Stbm turnover and clustering by Pk

Hyperphosphorylation of Stbm in a pk mutant was accompanied by increased Stbm turnover (Figure 6B, Figure 6—figure supplement 1C–E, Figure 6—source data 1), suggesting that Pk may stabilise Stbm by inhibiting its phosphorylation. In contrast, Pk overexpression leads to decreased Stbm turnover (Figure 6D, Figure 6—figure supplement 1F–H, Figure 6—source data 1).

How might Pk regulate Stbm phosphorylation and turnover? One possibility is that the role of Pk is simply to promote complex sorting, which has been shown to occur by Pk destabilising Fz within the same cell, acting via Dsh (Warrington et al., 2017). It is possible that when complexes sort out into arrays of the same orientation, clustering of Stbm leads to reduced accessibility to the kinase and thus reduced phosphorylation. We do not favour this ‘indirect’ model, as hyperphosphorylation of Stbm is not seen in all situations where complexes are thought to be unsorted, for example in fz or dsh mutants (Figure 6A).

An alternative model is that Pk directly regulates Stbm phosphorylation, perhaps by regulating its clustering (see Discussion). To investigate this, FRAP experiments were carried out on clone boundaries, in which Pk activity was present only in the same cell as Stbm, or only in neighbouring cells (Figure 6F). In an otherwise wild-type background, Stbm-EGFP strongly accumulates on boundaries next to stbm mutant cells, where it is highly stable (Figure 6G, column 1, Figure 6—figure supplement 3A and E–G, Figure 6—source data 1). Stbm-EGFP still accumulates on such boundaries in a pk mutant background, but its stability is significantly decreased (Figure 6G, column 4, Figure 6—figure supplement 3D–G, Figure 6—source data 1). If Pk is present only in Stbm-EGFP expressing cells, the phenotype resembles that of the ‘wild-type’ situation, and Stbm is highly stable (Figure 6G, column 2, Figure 6—figure supplement 3B and E–G, Figure 6—source data 1). In contrast, if Pk is absent only in Stbm-EGFP expressing cells, Stbm-EGFP is unstable (Figure 6G, column 3, Figure 6—figure supplement 3C and E–G, Figure 6—source data 1). This indicates that Pk acts in the same cell to stabilise Stbm and supports a direct role for Pk in regulating Stbm phosphorylation and turnover.

If a major role of Pk was to regulate Stbm phosphorylation, and this phosphorylation regulates Stbm clustering into puncta, we would also expect Stbm phosphomutant clustering to be largely insensitive to loss of Pk. In wild-type wings, loss of Pk causes a reduction in puncta size (Figure 6H and K, Figure 6—source data 1, Strutt et al., 2011). As expected, loss of Pk had less effect on either Stbm phosphomutant or Stbm phosphomimetic puncta (Figure 6I–K, Figure 6—source data 1). This again supports the conclusion that Pk directly regulates Stbm clustering by modulating the Dco-dependent phosphorylation of Stbm. In contrast, loss of Dsh caused a reduction in phosphomutant and phosphomimetic puncta size, as is also seen in the presence of wild-type Stbm (Figure 6—figure supplement 4A and B, Strutt et al., 2011). This is consistent with a model in which Fz and Dsh regulate Stbm clustering indirectly by promoting intercellular complex formation.

Regulation of Dsh junctional localisation by Dco kinase in vivo is independent of Stbm

Having established that Dco regulates Stbm phosphorylation, and this controls the turnover and clustering of Stbm, we next questioned whether these mechanisms are sufficient to explain all the effects of Dco on the core proteins. In particular, does Dco also directly regulate Dsh in any way, or does Dco only regulate Stbm, which then leads to secondary effects on Dsh phosphorylation?

A number of lines of evidence suggest a direct effect of Dco on Dsh. Firstly, we analysed the effect of expressing dominant-negative Dco on EGFP-Dsh turnover. We saw a reduction in overall junctional levels of EGFP-Dsh, leading to a proportionate decrease in the stable amount of Dsh (Figure 7A, Figure 7—figure supplement 1A–C, Figure 7—source data 1). Junctional levels of EGFP-Dsh were also decreased in the absence of Stbm (compare Figure 7A and B, Figure 7—figure supplement 1E–G, Figure 7—source data 1), but were further decreased when dominant-negative Dco was expressed (Figure 7B, Figure 7—figure supplement 1E–G, Figure 7—source data 1). This argues for a role for Dco in regulating Dsh levels at cell junctions independently of Stbm. Furthermore, neither Dsh levels nor Dsh phosphorylation were altered in Stbm phosphomutants (Figure 7C, Figure 3—figure supplement 1E,F,I and J, Figure 3—source data 1), whereas a decrease in Dsh phosphorylation was seen in dco hypomorphs (Figure 7—figure supplement 2A and B, Figure 7—source data 1, Strutt et al., 2006).

Figure 7 with 3 supplements see all
Phosphorylation of Dsh by Dco promotes core protein asymmetry.

(A,B) Stable and unstable amounts of EGFP-Dsh in 28 hr APF pupal wings after hub-and-spoke FRAP. (A) Flies were dshV26/+; P[acman]-EGFP-dsh/+ (n = 14), and dshV26/+; P[acman]-EGFP-dsh/en-GAL4; UAS-dcoK38R/+ (n = 10). (B) Flies were dshV26/+; P[acman]-EGFP-dsh stbm6/stbm6 (n = 12), and dshV26/+; P[acman]-EGFP-dsh stbm6/stbm6 en-GAL4; UAS-dcoK38R/+ (n = 7). Wings were imaged in the posterior compartment. Data are normalised to the total intensity in EGFP-Dsh. Error bars are 95% confidence intervals, and unpaired t-tests were used to compare stable amounts (asterisks in dark shaded columns), unstable amounts (asterisks in light shaded columns) or total amounts (blue asterisks above the columns) between samples with and without DcoK38R, p≤0.05*, p≤0.001***. (C) Western blot probed with Dsh antibody, of extracts from 28 hr APF pupal wings from wild-type, P[acman]-stbmS[All]A stbm6 or P[acman]-stbmS[All]E stbm6 flies. Actin was used as a loading control. (D–G) Adult wings expressing en-GAL4, UAS-dco in a wild-type background (D) or in flies heterozygous for fzP21 (E), stbm6 (F) or dshV26 (G). Regions of the wing with abnormal trichome polarity are outlined in yellow. Scale bar 100 µm. (H) Quantitation of trichome swirling in UAS-dco/+; en-GAL4/+ (n = 12), UAS-dco/+; en-GAL4/+; fzP21/+ (n = 12), UAS-dco/+; en-GAL4/stbm6 (n = 12) and UAS-dco/dshV26; en-GAL4/+ (n = 12). Graph shows the wing area next to the posterior cross vein in which trichome polarity was abnormal. Data are normalised to the area of the swirl in the UAS-dco control. Error bars are 95% confidence intervals, and ANOVA with Dunnett’s multiple comparisons test was used to compare to the UAS-dco control, p≤0.001***. (I,J) 28 hr APF pupal wings from dshV26; dsh-GFP/+ (I) or dshV26; dshST8-GFP/+ (J). Wings immunolabelled for GFP (green) or Fmi (red). Scale bar 5 µm. (K) Quantitation of mean polarity and variation in polarity angle, of 28 hr APF pupal wings immunolabelled for Fmi in dshV26; dsh-GFP/+ (n = 9) or dshV26; dshST8-GFP/+ (n = 9) flies. Error bars are 95% confidence intervals, and samples were compared using an unpaired t-test, ***p≤0.001.

https://doi.org/10.7554/eLife.45107.024

We then examined whether regulation of Dsh by Dco is important in establishing asymmetry. As discussed above, expression of dominant negative Dco caused a decrease in total levels of Dsh at junctions (Figure 7A, Figure 7—figure supplement 1C, Figure 7—source data 1). However, a decrease in Dsh levels alone is not sufficient to affect asymmetry: halving Dsh dosage halves Dsh levels at junctions (Figure 7—figure supplement 1C, Figure 7—source data 1), with little effect on the stable fraction (Figure 7—figure supplement 1D, Figure 7—source data 1) or asymmetry (Strutt et al., 2016).

However, two lines of evidence suggest that Dco-mediated phosphorylation of Dsh is functionally important for asymmetry. Firstly, overexpression of Dco caused strong trichome swirling in the adult wing (Figure 7D, Cong et al., 2004; Klein et al., 2006; Strutt et al., 2006), and this was accompanied by excess Dsh phosphorylation (Figure 7—figure supplement 2C and D, Figure 7—source data 1). Notably, these trichome polarity defects were suppressed by halving dsh gene dosage, but not stbm or fz gene dosage (Figure 7E–H, Figure 7—source data 1). This genetic interaction supports a direct role for Dco in regulating Dsh phosphorylation and core protein asymmetry.

Secondly, we analysed dsh mutant flies carrying a dsh genomic rescue construct in which eight putative Dco phosphorylation sites were mutated to alanine (dshST8-GFP, Figure 7—figure supplement 3A, Strutt et al., 2006). These flies exhibit only mild defects in trichome polarity (Strutt et al., 2006), but core protein asymmetry was not previously examined. As expected, core protein asymmetry was normal in dsh mutant flies carrying a wild-type Dsh rescue construct (Figure 7I and K, Figure 7—source data 1). However, core protein asymmetry was much reduced in dshST8-GFP flies (Figure 7J and K, Figure 7—source data 1), despite levels at junctions being similar to levels of wild-type Dsh (Figure 7—figure supplement 3B, Figure 7—source data 1). Interestingly, DshST8-GFP has a small but significantly increased rate of turnover at cell junctions, compared to wild-type Dsh-GFP (Figure 7—figure supplement 3C, Figure 7—source data 1).

Taken together, these results support a model in which Dco-mediated phosphorylation of Dsh regulates its recruitment into junctional complexes and that this is essential for core protein asymmetry.

Discussion

In this paper, we describe a dual role for CKIε/Dco kinase in regulating planar polarity in the fly pupal wing. In the first case, Dco promotes phosphorylation of Stbm. Stbm phosphorylation acts as a switch, changing Stbm from a stable immobile form that can enter junctional complexes, to an unstable mobile form that can redistribute within cells (Figure 4C). Inhibiting Stbm phosphorylation causes an increase in Stbm stability at junctions that prevents sorting of complexes: thus complexes are ‘locked’ in an unsorted state. In contrast, hyperphosphorylation of Stbm destabilises Stbm, allowing it to leave junctions, hence permitting complex sorting. A second role for Dco is to mediate Dsh phosphorylation, which increases Dsh localisation at junctions. Significantly, the effects of Dco on Dsh are independent of Stbm and vice versa.

In our ‘cloud model’ (Figure 1C, Strutt et al., 2016), we envisage that multiple binding interactions drive a phase transition from a loosely packed, disordered association of core proteins in non-puncta, towards a highly cross-linked array of complexes within puncta that are all aligned in the same orientation. Stbm is well-placed to be a key component driving such a clustering mechanism, as not only can it multimerise with itself (Jenny et al., 2003), but it also has a high stoichiometry within junctions (Figure 1C, Strutt et al., 2016). Also consistent with a role for Stbm in complex clustering is the observation that Stbm phosphorylation site mutants act as dominant negatives, recruiting wild-type Stbm into non-polarised puncta. Phosphorylation may inhibit a clustering mechanism, due to an increase in negative charge (reviewed in Wu, 2013; Bergeron-Sandoval et al., 2016; Boeynaems et al., 2018).

Interestingly, excess clustering of unphosphorylated Stbm in unsorted complexes is also expected to lead to destabilising feedback interactions with the other core components. When Stbm is unphosphorylated, the increase in Stbm stability is sufficient for Stbm to ‘win’ over Fmi and Fz. Thus, there is an overall increase in Stbm stability in phosphomutant Stbm puncta, that is accompanied by decreased stability of Fmi and Fz (Figure 4E–G).

Pk both promotes Stbm stability and reduces its phosphorylation. A role for Pk in increasing Stbm stability is not surprising, as overexpression of Pk is known to cause excess clustering of the core proteins (Bastock et al., 2003; Tree et al., 2002). We can envisage a number of mechanisms by which Pk could affect Stbm phosphorylation. Warrington et al. (2017) provided evidence that Pk has two roles: firstly, it acts via Dsh to destabilise Fz in the same cell (Figure 8A); secondly, it acts via Stbm to stabilise Fz in adjacent cells (Figure 8B). In the first case, Pk would promote sorting of complexes, and one possibility is that Stbm is inaccessible to the kinase in sorted complexes, and thus Pk is indirectly reducing Stbm phosphorylation by promoting sorting. Arguing against this, loss of fz or dsh also abolishes core protein asymmetry, but no hyperphosphorylation is seen. Our boundary FRAP experiments instead support Pk acting directly in the same cell to stabilise Stbm. We therefore propose a mechanism whereby direct binding of Pk to Stbm protects Stbm from phosphorylation.

Model for how Pk and phosphorylation of Stbm regulate complex sorting and clustering.

(A) Pk regulates complex sorting by destabilising Fz, in a Dsh-dependent manner (Warrington et al., 2017). (B) Pk also acts via Stbm to stabilise Fz (red arrow, Warrington et al., 2017). Our new data suggest that Pk promotes Stbm stability by protecting Stbm from phosphorylation by Dco. Furthermore, phosphorylation of Dsh (yellow stars) by Dco promotes Dsh stability. (C) When Stbm is hyperphosphorylated (yellow stars) it is more mobile and promotes complex sorting.

https://doi.org/10.7554/eLife.45107.029

Interestingly, Stbm has a significantly higher stoichiometry within junctions than Pk (Strutt et al., 2016). One possibility is that Stbm forms multimers, and that association of Pk with these multimers causes a conformational change that reduces accessibility to kinase-binding sites. Alternatively, Pk might recruit a phosphatase (albeit no candidates for such a phosphatase are known). The reduced negative charge might then allow Stbm to form higher order structures, which promotes clustering of the entire core protein complex into puncta (Figure 6—figure supplement 4C).

Puncta formation in both wild-type and phosphomutants is also dependent on Dsh. Dsh is another a good candidate for promoting clustering as it too can multimerise (Schwarz-Romond et al., 2007; Gammons et al., 2016), and thus puncta formation may be dependent on clustering on both sides of the complex. Moreover, direct interactions between Stbm and Dsh (Park and Moon, 2002; Bastock et al., 2003) may promote clustering of unsorted complexes in the absence of phosphorylation.

Feedback models for core protein asymmetry suggest that particular components of the core pathway signal to other components to either stabilise or destabilise them (Amonlirdviman et al., 2005; Burak and Shraiman, 2009; Le Garrec et al., 2006; Schamberg et al., 2010). An attractive model would be that Fz or Dsh recruits a kinase which phosphorylates Stbm and destabilises complexes of the opposite orientation (Kelly et al., 2016; Yang et al., 2017). Consistent with this, a proportion of Dco localises to apicolateral junctions in pupal wings (Strutt et al., 2006). However, we do not see any change in Stbm phosphorylation in fz or dsh mutants, nor are Fz and Dsh required for the hyperphosphorylation of Stbm seen in pk mutants. Therefore, we conclude that Stbm phosphorylation is more likely to be constitutive. Such constitutive phosphorylation would be sufficient to keep Stbm mobile and allow complex sorting (Figure 8C); and Pk would then counterbalance this and promote complex stability (Figure 8B). The balance between Stbm phosphorylation/complex mobility and Pk binding (leading to reduced Stbm phosphorylation) would resolve over time towards a more stable state as complexes segregate to opposite cell edges.

We note that in normal development, Stbm downregulates Pk levels (Strutt et al., 2013b). This suggests Pk levels are finely tuned, in order to prevent unrestrained clustering (as seen when Pk is overexpressed).

We also provide evidence that Dco regulates Dsh phosphorylation and junctional levels independently of Stbm. Our findings are consistent with previous observations that Dsh phosphorylation correlates with its recruitment by Fz into junctional complexes (Axelrod, 2001; Shimada et al., 2001). The mechanism by which Dsh phosphorylation acts in planar polarity remains to be elucidated, but our data show that dco overexpression phenotypes are suppressed by reduced dsh gene dosage, and that Dsh phosphomutants have reduced core protein asymmetry in pupal wings. Furthermore, a small but significant decrease in Dsh stability at junctions is observed in Dsh phosphomutants. Overall, our data are consistent with a model in which phosphorylation of Dsh promotes its stable association at junctions (Figure 8B).

In summary, we propose that Dco regulates the asymmetric localisation of the core proteins by reciprocal actions on Stbm and Dsh. Dco regulates Stbm phosphorylation and turnover and causes it to leave junctions, while phosphorylation of Dsh by Dco promotes its junctional association.

Materials and methods

Key resources table
Reagent type
(species)
or resource
DesignationSource or
reference
IdentifiersAdditional
information
Genetic reagent (Drosophila melanogaster)stbm6Wolff and Rubin, 1998, PMID 9463361BDSC:6918; FLYB:FBal0062423; RRID:BDSC_6918FlyBase symbol: Vangstbm-6
Genetic reagent (Drosophila melanogaster)pkpk-sple13Gubb et al., 1999, PMID:10485852BDSC:41790; FLYB:FBal0060943; RRID:BDSC_41790FlyBase symbol:
pkpk-sple-13
Genetic reagent (Drosophila melanogaster)dshV26Perrimon and Mahowald, 1987, PMID:3803719BDSC:6331; FLYB:FBal0003140; RRID:BDSC_6331FlyBase symbol: dsh3
Genetic reagent (Drosophila melanogaster)dsh1Bloomington Drosophila Stock CenterBDSC:5298; FLYB FBal0003138; RRID:BDSC_5298
Genetic reagent (Drosophila melanogaster)dgo380Feiguin et al., 2001, PMID:11703927BDSC:41786; FLYB:FBal0141190; RRID:BDSC_41786
Genetic reagent (Drosophila melanogaster)fmi-EGFPStrutt et al., 2016, PMID:27926869Knock-in of EGFP to the C-terminus of fmi in the endogenous locus
Genetic reagent (Drosophila melanogaster)fz-EGFPStrutt et al., 2016, PMID:27926869Knock-in of EGFP to the C-terminus of fz in the endogenous locus
Genetic reagent (Drosophila melanogaster)P[acman]-EGFP-dsh attP40 (2L) 25C6Strutt et al., 2016, PMID:27926869P[acman] transgene with EGFP recombineered at the N-terminus of dsh
Genetic reagent (Drosophila melanogaster)P[acman]-EGFP-dgo attP40 (2L) 25C6Strutt et al., 2016, PMID:27926869P[acman] transgene with EGFP recombineered at the N-terminus of dgo
Genetic reagent (Drosophila melanogaster)P[acman]-stbm attP40 (2L) 25C6Strutt et al., 2016, PMID:27926869P[acman] transgene
Genetic reagent (Drosophila melanogaster)P[acman]-stbm-EGFP attP40 (2L) 25C6Strutt et al., 2016, PMID:27926869P[acman] transgene with EGFP recombineered at the C-terminus of stbm
Genetic reagent (Drosophila melanogaster)P[acman]-stbm-EGFP VK1 (2R) 59D3This paperP[acman] transgene with EGFP recombineered at the C-terminus of stbm
Genetic reagent (Drosophila melanogaster)P[acman]-EGFP-stbm attP40 (2L) 25C6This paperP[acman] transgene with EGFP recombineered at the N-terminus of stbm
Genetic reagent (Drosophila melanogaster)P[acman]-mApple-stbm attP40 (2L) 25C6This paperP[acman] transgene with mApple recombineered at the N-terminus of stbm
Genetic reagent (Drosophila melanogaster)P[acman]-mApple-dgo attP40 (2L) 25C6This paperP[acman] transgene with mApple recombineered at the N-terminus of dgo
Genetic reagent (Drosophila melanogaster)P[acman]-EGFP-stbmS[5,120,122]A attP40 (2L) 25C6This paperP[acman] transgene with EGFP at the N-terminus of stbm, and with phosphorylation site mutations S[5,120,122]A
Genetic reagent (Drosophila melanogaster)P[acman]-EGFP-stbmS[5,120,122]E attP40 (2L) 25C6This paperP[acman] transgene with EGFP at the N-terminus of stbm, and with phosphorylation site mutations S[5,120,122]E
Genetic reagent (Drosophila melanogaster)P[acman]-EGFP-stbmS[All]A attP40 (2L) 25C6This paperP[acman] transgene with EGFP at the N-terminus of stbm, and with phosphorylation site mutations S[5-17, 113-122]A
Genetic reagent (Drosophila melanogaster)P[acman]-stbmS[All]A attP40 (2L) 25C6This paperP[acman] transgene with phosphorylation site mutations S[5-17, 113-122]A
Genetic reagent (Drosophila melanogaster)P[acman]-stbmS[All]A VK31 (3L) 62E1This paperP[acman] transgene with phosphorylation site mutations S[5-17, 113-122]A
Genetic reagent (Drosophila melanogaster)P[acman]-stbmS[All]E attP40 (2L) 25C6This paperP[acman] transgene with phosphorylation site mutations S[5-17, 113-122]E
Genetic reagent (Drosophila melanogaster)P[acman]-stbmS[5-17]A attP40 (2L) 25C6This paperP[acman] transgene with phosphorylation site mutations S[5-17]A
Genetic reagent (Drosophila melanogaster)P[acman]-stbmS[113-122]A attP40 (2L) 25C6This paperP[acman] transgene with phosphorylation site mutations S[113-122]A
Genetic reagent (Drosophila melanogaster)P[acman]-stbmS[5-17]E attP40 (2L) 25C6This paperP[acman] transgene with phosphorylation site mutations S[5-17]E
Genetic reagent (Drosophila melanogaster)P[acman]-stbmS[113-122]E attP40 (2L) 25C6This paperP[acman] transgene with phosphorylation site mutations S[113-122]E
Genetic reagent (Drosophila melanogaster)P[acman]-stbmS[All]A-EGFP attP40 (2L) 25C6This paperP[acman] transgene with EGFP at the C-terminus of stbm, and with phosphorylation site mutations S[5-17, 113-122]A
Genetic reagent (Drosophila melanogaster)P[acman]-stbmS[All]E-EGFP VK31 (3L) 62E1This paperP[acman] transgene with EGFP at the C-terminus of stbm, and with phosphorylation site mutations S[5-17, 113-122]E
Genetic reagent (Drosophila melanogaster)attB-UAS-pk VK20 (3R) 99F8This paperpk gene under control of the UAS promoter
Genetic reagent (Drosophila melanogaster)attB-UAS-dgo VK20 (3R) 99F8This paperdgo gene under control of the UAS promoter
Genetic reagent (Drosophila melanogaster)CaSpeR-dsh-GFPAxelrod, 2001, PMID:11358862dsh genomic rescue construct, with GFP at the C-terminus
Genetic reagent (Drosophila melanogaster)CaSpeR-dshST8-GFPStrutt et al., 2006, PMID:16824921dsh genomic rescue construct with GFP at the C-terminus, and with phosphorylation site mutations S[236-247]A
Genetic reagent (Drosophila melanogaster)UAS-dcoK38RStrutt et al., 2006, PMID:16824921Dominant negative dco, under control of the UAS promoter
Genetic reagent (Drosophila melanogaster)UAS-dcoSekine et al., 2008, PMID:18258753dco gene under control of the UAS promoter
Genetic reagent (Drosophila melanogaster)ptc-GAL4Bloomington
Drosophila Stock Center
BDSC:2017; FLYB:FBti0002124; RRID:BDSC_2017FlyBase symbol: P{GawB}ptc559.1
Genetic reagent (Drosophila melanogaster)en-GAL4Bloomington
Drosophila Stock Center
BDSC:30564; FLYB:FBti0003572; RRID:BDSC_30564FlyBase symbol: P{en2.4-GAL4}e16E
Genetic reagent (Drosophila melanogaster)Actin-GAL425Bloomington Drosophila Stock CenterBDSC:3953; FLYB:FBti0012293; RRID:BDSC_3953FlyBase symbol: P{AyGAL4}25
Genetic reagent (Drosophila melanogaster)tub-GAL80ts20Bloomington Drosophila Stock CenterBDSC:7019; FLYB:FBti0027796; RRID:BDSC_7019FlyBase symbol: P{tubP-GAL80ts}20
Genetic reagent (Drosophila melanogaster)Ubx-FLPBloomington Drosophila Stock CenterBDSC:42718; FLYB:FBti0150334: RRID:BDSC_42718FlyBase symbol: P{Ubx-FLP}1
Genetic reagent (Drosophila melanogaster)hs-FLPG5 attP2Bloomington Drosophila Stock CenterBDSC:55816; FLYB:FBti0160507: RRID:BDSC_55816FlyBase symbol: P{hs-FLPG5}attP2
Antibodyrabbit polyclonal anti-StbmWarrington et al., 2013, PMID:23364328RRID:AB_25700771/1000 (immunolabelling)
Antibodyrat polyclonal anti-StbmStrutt and Strutt, 2008, PMID:18804371RRID:AB_25697161/1000 (immunolabelling)
Antibodyaffinity purified rabbit polyclonal anti-FzBastock and Strutt, 2007, PMID:17652348RRID:AB_28014211/300 (immunolabelling)
Antibodyaffinity purified rat polyclonal anti-PkStrutt et al., 2013a, PMID:23487316RRID:AB_25697201/25 (immunolabelling)
Antibodyrat polyclonal anti-DshStrutt et al., 2006, PMID:16824921RRID:AB_28014191/1000 (immunolabelling)
Antibodymouse monoclonal anti-Fmi #74Usui et al., 1999, PMID:10490098RRID:AB_26195833 µg/ml (immunolabelling)
Antibodyaffinity purified rabbit polyclonal anti-GFPAbcamAbcam:ab6556; RRID:AB_3055641/4000 (immunolabelling)
Antibodymouse monoclonal anti-ß-gal 40-1aDSHBRRID:AB_23145091/200 (immunolabelling)
Antibodyrabbit polyclonal anti-ß-galMP Biomedicals/
Cappel
MP Biochemicals:0855976 (Cappel:55976); RRID:AB_23349341/4000 (immunolabelling)
Antibodyrabbit polyclonal anti-StbmRawls and Wolff, 2003 PMID:126424921/20000 (western blotting)
Antibodyaffinity purified rabbit polyclonal anti-DshStrutt et al., 2006, PMID:16824921RRID:AB_28014201/200 (western blotting)
Antibodymouse monoclonal anti-Actin AC40Sigma-AldrichSigma:A4700; RRID:AB_4767301/5000 (western blotting)
Antibodymouse monoclonal anti-Tubulin DM1ASigma-AldrichSigma:T9026; RRID:AB_4775931/10000 (western blotting)
Software, algorithmProgResC14 version 1.7.3Jenoptik
Software, algorithmNIS Elements AR version 4.60Nikon
Software, algorithmImage Lab version 4.1BioRad Laboratories
Software,
algorithm
ImageJ version 2.0.0-r65/1.51 shttps://fiji.sc
Software, algorithmPacking AnalyzerAigouy et al., 2010, PMID:20813263
Software, algorithmMATLAB_R2014bMathworks
Software, algorithmMembrane intensity and Polarity measurement scripts (MATLAB)Strutt et al., 2016, PMID:27926869
Software, algorithmPuncta measurement script (MATLAB)Strutt et al., 2019, PMID: 30661800
Software, algorithmGraphPad Prism version 7.0 cGraphPad Software, Inc.

Drosophila genetics

Drosophila melanogaster flies were grown on standard cornmeal/agar/molasses media at 18°C or 25°C, unless otherwise described.

Fly strains are described in FlyBase. fzP21, stbm6, pkpk-sple13, dshV26 are null alleles, and dsh1 gives a strong planar polarity phenotype, but functions normally in Wingless signalling (Axelrod et al., 1998; Boutros et al., 1998).

P[acman] constructs (BACPAC resources) were recombineered using standard methods. N-terminal fusions of P[acman]-EGFP-stbm, P[acman]-mApple-stbm and P[acman]-mApple-dgo used plasmid PL452-N-EGFP (Addgene) as a source vector, or a modified version PL452-N-mApple where mApple replaced EGFP. Gene-specific primers were used to amplify EGFP/mApple and the selection cassette, and the resulting fragment was then recombineered into the relevant P[acman] construct, in frame with the open reading frame. The kanamycin cassette was then excised, leaving a single LoxP site between the EGFP/mApple tag and the open-reading frame. Phosphomutants were generated using recombineering with positive-negative selection into P[acman]-stbm (Strutt et al., 2016), P[acman]-stbm-EGFP (Strutt et al., 2016) or P[acman]-EGFP-stbm. These were exact mutations, leaving no foreign sequence. The open-reading frame of dgo was cloned into attB-pUAST using standard methods.

P[acman] constructs were integrated into the genome via ΦC31-mediated recombination into the attP40 landing site on 2L, the VK1 site on 2R or the VK31 site on 3L. P[acman]-stbm lines were recombined or double balanced with stbm6 and P[acman]-dgo lines were recombined with dgo380, or with stbm6 dgo380. attB-UAS-pk (Warrington et al., 2017) and attB-UAS-dgo were integrated into the VK20 landing site on 3L. Transgenics were made by Genetivision.

fmi-EGFP and fz-EGFP knock-ins and P[acman]-stbm, P[acman]-stbm-EGFP, P[acman]-EGFP-dsh and P[acman]-EGFP-dgo (all in attP40) are described in Strutt et al. (2016). Other P element transgene insertions were CaSpeR-dsh-GFP (Axelrod, 2001), CaSpeR-dshST8-GFP (Strutt et al., 2006), UAS-dcoK38R (Strutt et al., 2006) and UAS-dco (Sekine et al., 2008).

Flies were raised at 25°C and dissected or imaged 28 hr after puparium formation (APF), unless otherwise indicated. Flies raised at 29°C were imaged after 25 hr. To avoid dosage compensation effects, females of dshV26/+; P[acman]-EGFP-dsh/+ were used. Mitotic clones were induced using the FLP/FRT system and either Ubx-FLP or hs-FLP. For expression of UAS-dcoK38R and UAS-dco, flies were crossed to en-GAL4 at 25°C, or Actin-GAL4, tub-GAL80ts at 29°C. Expression of attB-UAS-pk and attB-UAS-dgo used ptc-GAL4 at 25°C or Actin-GAL4, tub-GAL80ts at 29°C.

Full genotypes for each figure are shown in Table 1.

Table 1
List of full genotypes used in each figure.
https://doi.org/10.7554/eLife.45107.030
Figure
Figure 2Aw; stbm6
Figure 2Bw; P[acman]-stbm [attP40] FRT40 stbm6
Figure 2Cw; P[acman]-stbmS[All]A [attP40] FRT40 stbm6
Figure 2Dw; P[acman]-stbmS[All]E [attP40] FRT40 stbm6
Figure 2E, G, I, L, My w Ubx-FLP; P[acman]-stbm [attP40] arm-lacZ FRT40 stbm6 / P[acman]-stbmS[All]A [attP40] FRT40 stbm6
Figure 2F, H, I, L, My w Ubx-FLP; P[acman]-stbm [attP40] arm-lacZ FRT40 stbm/ P[acman]-stbmS[All]E [attP40] FRT40 stbm6
Figure 2J, Kw
w; P[acman]-stbmS[All]A [attP40] FRT40 stbm6
w; P[acman]-stbmS[All]E [attP40] FRT40 stbm6
Figure 3A, Cy w Ubx-FLP; P[acman]-stbm [attP40] arm-lacZ FRT40 stbm/ P[acman]-stbmS[All]A [attP40] FRT40 stbm6
Figure 3B, Dy w Ubx-FLP; P[acman]-stbm [attP40] arm-lacZ FRT40 stbm/ P[acman]-stbmS[All]E [attP40] FRT40 stbm6
Figure 3Gw hs-FLP; P[acman]-EGFP-dgo [attP40] FRT40 dgo380 / P[acman]-mApple-dgo [attP40] FRT40 dgo380
Figure 3Hw; P[acman]-EGFP-dgo [attP40] FRT40 stbm6 dgo380 / P[acman]-mApple-dgo [attP40] FRT40 stbm6 dgo380; P[acman-stbmS[All]A [VK31] / P[acman]-stbmS[All]A [VK31] hs-FLPG5 [attP2]
Figure 4Bw; P[acman]-stbm-EGFP [attP40] FRT40 stbm/ +
w; P[acman]-stbmS[All]A [attP40] FRT40 stbm/ P[acman]-stbmS[All]A-EGFP [attP40] FRT40 stbm6
w; P[acman]-stbmS[All]E [attP40] FRT40 stbm/ stbm6; P[acman]-stbmS[All]E-EGFP [VK31] / +
Figure 4Ew; P[acman]-stbm-EGFP [attP40] FRT40 stbm6
w; P[acman]-stbmS[All]A-EGFP [attP40] FRT40 stbm6
w; stbm6; P[acman]-stbmS[All]E-EGFP [VK31]
Figure 4Fw; fmi-EGFP/+
w; P[acman]-stbmS[All]A [attP40] FRT42 stbm6fmi-EGFP/P[acman]-stbmS[All]A [attP40] FRT40 stbm6
w; P[acman]-stbmS[All]E [attP40] FRT42 stbm6 fmi-EGFP/P[acman]-stbmS[All]E [attP40] FRT40 stbm6
Figure 4Gw; fz-EGFP/+
w; P[acman]-stbmS[All]A [attP40] FRT40 stbm6/stbm6; P[acman]-stbmS[All]A [VK31] fz-EGFP / +
w; P[acman]-stbmS[All]E [attP40] FRT40 stbm6; fz-EGFP / +
Figure 5Aw; P[acman]-stbm-EGFP [attP40] FRT40 stbm/ +
w; P[acman]-stbm-EGFP [attP40] FRT40 stbm/ en-GAL4; UAS-dcoK38R / +
w; P[acman]-stbmS[All]A [attP40] FRT40 stbm/ P[acman]-stbmS[All]A-EGFP [attP40] FRT40 stbm6
w; P[acman]-stbmS[Al]A-EGFP [attP40] FRT40 stbm/ P[acman]-stbmS[All]A [attP40] en-GAL4, stbm6; UAS-dcoK38R / +
Figure 5Bw; P[acman]-stbm-EGFP [attP40] FRT40 stbm/ +
w UAS-dco/w; P[acman]-stbm-EGFP [attP40] FRT40 stbm6/en-GAL4
w; P[acman]-stbmS[All]E [attP40] FRT40 stbm/ stbm6; P[acman]-stbmS[All]E-EGFP [VK31] / +
Figure 5Cw dsh1; P[acman]-stbm-EGFP [attP40] FRT40 stbm/ +
w dsh1; P[acman]-stbm-EGFP [attP40] FRT40 stbm6/ en-GAL4; UAS-dcoK38R / +
Figure 6Aw
w; fzP21
w; pkpk-sple13
w dsh1
w; P[acman]-stbmS[All]A [attP40] FRT40 stbm6
Figure 6Bw; P[acman]-stbm-EGFP [attP40] FRT40 stbm/ +w; P[acman]-stbm-EGFP [attP40] FRT40 stbm/ +; fzP21
w; P[acman]-stbm-EGFP [attP40] FRT42 pkpk-sple13 stbm/ FRT42 pkpk-sple13
w dsh1; P[acman]-stbm-EGFP [attP40] FRT40 stbm/ +
Figure 6Cw
w; Actin-GAL4, tub-GAL80ts / +; UAS-pk [VK20] / +w; Actin-GAL4, tub-GAL80ts / +; UAS-dgo [VK20] / +
Figure 6Dw; P[acman]-stbm-EGFP [attP40] FRT40 stbm/ +w; P[acman]-stbm-EGFP [attP40] FRT40 stbm/Actin-GAL4, tub-GAL80ts; UAS-pk [VK20] / +
Figure 6Ew; stbm6
w
w; fzP21
w dsh1
w; pkpk-sple13
w; pkpk-sple13; fzP21
w dsh1; pkpk-sple13
Figure 6Gy w Ubx-FLP; FRT42 stbm6 P[acman]-stbm-EGFP [VK1] / FRT42 stbm6, Ubi-mRFP-nls
y w Ubx-FLP; FRT42 stbm6 P[acman]-stbm-EGFP [VK1] / FRT42 pkpk-sple13 stbm6, Ubi-mRFP-nls
y w Ubx-FLP; FRT42 pkpk-sple13 stbm6P[acman]-stbm-EGFP [VK1] / FRT42 stbm6, Ubi-mRFP-nls
y w Ubx-FLP; FRT42 pkpk-sple13 stbm6P[acman]-stbm-EGFP [VK1] / FRT42 pkpk-sple13 stbm6, Ubi-mRFP-nls
Figure 6H, Ky w Ubx-FLP; FRT42 arm-lacZ / FRT42 pkpk-sple13
Figure 6I, Ky w Ubx-FLP; P[acman]-stbmS[All]A [attP40] FRT42 stbm/ P[acman]-stbmS[All]A [attP40] FRT42 pkpk-sple13 stbm6
Figure 6J, Ky w Ubx-FLP; P[acman]-stbmS[All]E [attP40] FRT42 stbm/ P[acman]-stbmS[All]E [attP40] FRT42 pkpk-sple13 stbm6
Figure 7Ay w dshV26 FRT18 / w; P[acman]-EGFP-dsh [attP40]/+
y w dshV26 FRT18 / w; P[acman]-EGFP-dsh [attP40] / en-GAL4; UAS-dcoK38R / +
Figure 7By w dshV26 FRT18 / w; P[acman]-EGFP-dsh [attP40] stbm/ stbm6
y w dshV26 FRT18 / w; P[acman]-EGFP-dsh [attP40] stbm/ en-GAL4, stbm6; UAS-dcoK38R / +
Figure 7Cw
w; P[acman]-stbmS[All]A [attP40] FRT40 stbm6
w; P[acman]-stbmS[All]E [attP40] FRT40 stbm6
Figure 7D, Hw UAS-dco/w; en-GAL4/+
Figure 7E, Hw UAS-dco/w; en-GAL4 / +; fzP21 / +
Figure 7F, Hw UAS-dco / w; en-GAL4 / stbm6
Figure 7G, Hw UAS-dco / w dshV26; en-GAL4 / +
Figure 7I, Hy w dshV26 FRT18; pCaSpeR-dsh-GFP / +
Figure 7J, Ky w dshV26 FRT18; pCaSpeR-dshST8-GFP / +
Figure 2—figure supplement 1Aw; P[acman]-EGFP-stbm [attP40] FRT40 stbm6
Figure 2—figure supplement 1Bw; P[acman]-EGFP-stbmS[5,120,122]A [attP40] FRT40 stbm6
Figure 2—figure supplement 1Cw; P[acman]-EGFP-stbmS[5,120,122]E [attP40] FRT40 stbm6
Figure 2—figure supplement 1Dw; P[acman]-EGFP-stbmS[All]A [attP40] FRT40 stbm6
Figure 2—figure supplement 1E, Iy w Ubx-FLP; P[acman]-EGFP-stbm [attP40] arm-lacZ FRT40 stbm/ P[acman]-stbm [attP40] FRT40 stbm6
Figure 2—figure supplement 1F, Iy w Ubx-FLP; P[acman]-EGFP-stbmS[5,120,122]A [attP40] FRT40 stbm/ P[acman]-EGFP-stbm [attP40] arm-lacZ FRT40 stbm6
Figure 2—figure supplement 1G, Iy w Ubx-FLP; P[acman]-EGFP-stbmS[5,120,122]E [attP40] FRT40 stbm/ P[acman]-EGFP-stbm [attP40] arm-lacZ FRT40 stbm6
Figure 2—figure supplement 1H, Iy w Ubx-FLP; P[acman]-EGFP-stbmS[All]A [attP40] FRT40 stbm/ P[acman]-EGFP-stbm [attP40] arm-lacZ FRT40 stbm6
Figure 2—figure supplement 2Aw
w; P[acman]-stbmS[All]A [attP40] FRT40 stbm6
w; P[acman]-stbmS[All]E [attP40] FRT40 stbm6
Figure 2—figure supplement 2Bw
w; P[acman]-stbmS[5-17]A [attP40] FRT40 stbm6
w; P[acman]-stbmS[113-122]A [attP40] FRT40 stbm6
Figure 2—figure supplement 2C, Gy w Ubx-FLP; P[acman]-stbmS[5-17]A [attP40] FRT40 stbm/ P[acman]-stbm [attP40] arm-lacZ FRT40 stbm6
Figure 2—figure supplement 2D, Hy w Ubx-FLP; P[acman]-stbmS[5-17]E [attP40] FRT40 stbm/ P[acman]-stbm [attP40] arm-lacZ FRT40 stbm6
Figure 2—figure supplement 2E, Gy w Ubx-FLP; P[acman]-stbmS[113-122]A [attP40] FRT40 stbm/ P[acman]-stbm [attP40] arm-lacZ FRT40 stbm6
Figure 2—figure supplement 2F, Hy w Ubx-FLP; P[acman]-stbmS[113-122]E [attP40] FRT40 stbm/ P[acman]-stbm [attP40] arm-lacZ FRT40 stbm6
Figure 3–figure supplement 1A, C, E, G, Iy w Ubx-FLP; P[acman]-stbm [attP40] arm-lacZ FRT40 stbm6] / P[acman]-stbmS[All]A [attP40] FRT40 stbm6
Figure 3–figure supplement 1B, D, F, H, Jy w Ubx-FLP; P[acman]-stbm [attP40] arm-lacZ FRT40 stbm/ P[acman]-stbmS[All]E [attP40] FRT40 stbm6
Figure 3—figure supplement 1Kw; P[acman]-EGFP-dgo [attP40] FRT40 dgo380
Figure 3—figure supplement 1Lw; P[acman]-EGFP-dgo [attP40] FRT40 stbm6dgo380; P[acman-stbmS[All]A [VK31]
Figure 4—figure supplement 1A, C, Dw; P[acman]-stbm-EGFP [attP40] FRT40 stbm/ +
Figure 4—figure supplement 1A, C, Ew; P[acman]-stbmS[All]A [attP40] FRT40 stbm/ P[acman]-stbmS[All]A-EGFP [attP40] FRT40 stbm6
Figure 4—figure supplement 1A, C, Fw; P[acman]-stbmS[All]E [attP40] FRT40 stbm
stbm6; P[acman]-stbmS[All]E-EGFP [VK31] / +
Figure 4—figure supplement 2A-Dw
Figure 4—figure supplement 2B, Dw; P[acman]-stbmS[All]A [attP40] FRT40 stbm/ +
Figure 4—figure supplement 2C, Dw; P[acman]-stbmS[All]E [attP40] FRT40 stbm/ +
Figure 5—figure supplement 1Aw
w; FRT82 dco/ FRT82 dco5B2.6
Figure 5—figure supplement 1Bw
w; Actin-GAL4, tub-GAL80ts / +; UAS-dcoK38R / +
Figure 5—figure supplement 2A-Cw; P[acman]-stbm-EGFP [attP40] FRT40 stbm6/+w; P[acman]-stbm-EGFP [attP40] FRT40 stbm/ en-GAL4; UAS-dcoK38R / +
w; P[acman]-stbmS[All]A [attP40] FRT40 stbm/ P[acman]-stbmS[All]A-EGFP [attP40] FRT40 stbm6
w; P[acman]-stbmS[Al]A-EGFP [attP40] FRT40 stbm/ P[acman]-stbmS[All]A [attP40] en-GAL4, stbm6; UAS-dcoK38R / +
Figure 5—figure supplement 2D-Fw; P[acman]-stbm-EGFP [attP40] FRT40 stbm/ +
w UAS-dco / w; P[acman]-stbm-EGFP [attP40] FRT40 stbm/ en-GAL4
w; P[acman]-stbmS[All]E [attP40] FRT40 stbm/ stbm6; P[acman]-stbmS[All]E-EGFP [VK31] / +
Figure 5—figure supplement 2G-Iw dsh1; P[acman]-stbm-EGFP [attP40] FRT40 stbm/ +
w dsh1; P[acman]-stbm-EGFP [attP40] FRT40 stbm/ en-GAL4; UAS-dcoK38R / +
Figure 6—figure supplement 2Aw
w; pkpk-sple13
Figure 6—figure supplement 2Bw; P[acman]-stbmS[All]E [attP40] FRT40 stbm6
w; P[acman]-stbmS[All]E [attP40] FRT42 pkpk-sple13 stbm6
w; P[acman]-stbmS[All]A [attP40] FRT40 stbm6
w; P[acman]-stbmS[All]A [attP40] FRT42 pkpk-sple13 stbm6
Figure 6—figure supplement 1Ay w Ubx-FLP; P[acman]-EGFP-stbm [attP40] FRT40 stbm/ P[acman]-mApple-stbm [attP40] FRT40 stbm6
Figure 6—figure supplement 1By w Ubx-FLP; P[acman[-EGFP-Stbm [attP40] FRT40 ptc-GAL4 stbm/ P[acman]-mApple-Stbm [attP40] FRT40 stbm6; UAS-Pk [VK20] / +
Figure 6—figure supplement 1C-Ew; P[acman]-stbm-EGFP [attP40] FRT40 stbm6/+
w; P[acman]-stbm-EGFP [attP40] FRT40 stbm6/+; fzP21
w; P[acman]-stbm-EGFP [attP40] FRT42 pkpk-sple13 stbm/ FRT42 pkpk-sple13
w dsh1; P[acman]-stbm-EGFP [attP40] FRT40 stbm/ +
Figure 6—figure supplement 1F-Hw; P[acman]-stbm-EGFP [attP40] FRT40 stbm/ +w; P[acman]-stbm-EGFP [attP40] FRT40 stbm/Actin-GAL4, tub-GAL80ts; UAS-pk [VK20] / +
Figure 6—figure supplement 3A, E-Gy w Ubx-FLP; FRT42 stbm6 P[acman]-stbm-EGFP [VK1] / FRT42 stbm6, Ubi-mRFP-nls
Figure 6—figure supplement 3B, E-Gy w Ubx-FLP; FRT42 stbm6 P[acman]-stbm-EGFP [VK1] / FRT42 pkpk-sple13 stbm6, Ubi-mRFP-nls
Figure 6—figure supplement 3C, E-Gy w Ubx-FLP; FRT42 pkpk-sple13 stbm6P[acman]-stbm-EGFP [VK1] / FRT42 stbm6, Ubi-mRFP-nls
Figure 6—figure supplement 3D, E-Gy w Ubx-FLP; FRT42 pkpk-sple13 stbm6P[acman]-stbm-EGFP [VK1]/ FRT42 pkpk-sple13 stbm6, Ubi-mRFP-nls
Figure 6—figure supplement 4Aw dshV26 FRT19A/y w Ubx-FLP FRT19A; P(acman)-StbmS(All)A [attP40] FRT40 stbm[6]
Figure 6—figure supplement 4Bw dshV26 FRT19A / y w Ubx-FLP FRT19A; P(acman)-StbmS(All)E [attP40] FRT40 stbm[6]
Figure 7—figure supplement 1A-Dy w dshV26 FRT18 / w; P[acman]-EGFP-dsh [attP40] / +
y w dshV26 FRT18 / w; P[acman]-EGFP-dsh [attP40] / en-GAL4; UAS-dcoK38R / +
y w dshV26 FRT18; P[acman]-EGFP-dsh [attP40] / +
Figure 7—figure supplement 1E-Gy w dshV26 FRT18 / w; P[acman]-EGFP-dsh [attP40] stbm/ stbm6
y w dshV26 FRT18 / w; P[acman]-EGFP-dsh [attP40] stbm/ en-GAL4, stbm6; UAS-dcoK38R / +
Figure 7—figure supplement 2A, Bw
w; FRT82 dco/ FRT82 dco5B2.6
Figure 7—figure supplement 2C, Dw
UAS-dco / w; en-GAL4 / +
Figure 7—figure supplement 3B, Cy w dshV26 FRT18; pCaSpeR-dsh-GFP / +
y w dshV26 FRT18; pCaSpeR-dshST8-GFP / +

Adult wing preparations

Adult wings were dehydrated in isopropanol and mounted in GMM (50% methyl salicylate, 50% Canada Balsam), and incubated overnight on a 60°C hot plate to clear. Wings were photographed at 20x magnification. To quantify trichome swirling, ImageJ was used to draw around a region near the posterior cross vein in which trichomes deviated significantly away from their normal orientation. Data were compared using ANOVA with Dunnett’s multiple comparisons test.

Immunolabelling

Pupal wings were dissected at 28 hr after puparium formation (APF) at 25°C. Briefly, pupae were removed from their pupal case and fixed for 25–60 min in 4% paraformaldehyde in PBS, depending on antibody combinations. Wings were then dissected and the outer cuticle removed, and were blocked for 1 hr in PBS containing 0.2% Triton X100 (PTX) and 10% normal goat serum. Primary and secondary antibodies were incubated overnight at 4°C in PTX with 10% normal goat serum, and all washes were in PTX. After immunolabelling, wings were post-fixed in 4% paraformaldehyde in PBS for 30 min. Wings were mounted in 25 µl of PBS containing 10% glycerol and 2.5% DABCO, pH7.5. Wings expressing mApple-tagged proteins were mounted in 12.5 µl Vectashield, as this preserved the fluorescence for longer.

Primary antibodies for immunolabelling were rabbit anti-Stbm (Warrington et al., 2013), rat anti-Stbm (Strutt and Strutt, 2008), affinity purified rabbit anti-Fz (Bastock and Strutt, 2007), affinity purified rat anti-Pk (Strutt et al., 2013a), rat anti-Dsh (Strutt et al., 2006), mouse monoclonal anti-Fmi (DSHB, Usui et al., 1999), rabbit anti-GFP (Abcam cat#6556), mouse monoclonal anti-ß-gal 40-1a (DSHB) and rabbit anti-ß-gal (Cappel).

Western blotting

For pupal wing westerns, 28 hr APF pupal wings were dissected directly into sample buffer. One pupal wing equivalent was used per lane. For phosphatase treatments, 6 hr APF prepupal wing extracts were made in lysis buffer (50 mM Tris-HCl pH7.5, 150 mM NaCl, 0.5% Triton X-100, 1 x protease inhibitors [Roche]). Lysates were treated with 400 units lambda phosphatase (NEB) for 1 hr at 30°C, before addition of sample buffer.

Western blots were probed with rabbit anti-Stbm (Rawls and Wolff, 2003), affinity purified rabbit anti-Dsh (Strutt et al., 2006), mouse monoclonal anti-Actin AC-40 (DSHB) and mouse monoclonal anti-α-Tubulin DM1-A (Sigma). SuperSignal West Dura Extended Duration Substrate (Thermo Scientific) was used for detection and a BioRad ChemiDoc XRS + was used for imaging. To quantitate total protein levels, intensities from three or four biological replicates were quantified using ImageJ. Data were compared using ANOVA with Tukey’s multiple comparisons test.

For comparing levels of phosphorylated and unphosphorylated Dsh, bands on western blots migrated too close together to quantitate absolute band intensities. ImageJ was used to generate a band profile for each lane, and the maximum values of the phosphorylated and unphosphorylated bands were measured. Data is expressed as a ratio of this maximum intensity, and ratios from four biological replicate samples were compared using unpaired t-tests.

Imaging of fixed samples

Pupal wings were imaged on a Nikon A1R GaAsP confocal microscope using a 60x NA1.4 apochromatic lens. Wings without clones were imaged posterior to vein 4; wings containing clones were imaged wherever they appeared in the wing. 9 Z-slices separated by 150 nm were imaged at a pixel size of 70–80 nm, and the three brightest slices around apicolateral junctions were selected and averaged for each channel in ImageJ.

Membrane masks and polarity nematics were generated in Packing Analyzer (Aigouy et al., 2010). MATLAB scripts were used to calculate mean membrane intensity (Strutt et al., 2016). Polarity magnitude (maximum asymmetry ratio on a cell-by-cell basis) and variation in polarity angle were also calculated using MATLAB scripts (Strutt et al., 2016).

To compare puncta between wild-type and mutant tissue, a MATLAB script was used to select puncta using the same threshold value in wild-type and mutant regions of the same wings (Strutt et al., 2019). Puncta number per unit area was calculated, as well as mean puncta amount (puncta area multiplied by mean puncta intensity).

Values for control and mutant regions of the same wings (for clones) were expressed as a ratio and compared using one sample t-tests; or were compared between images using unpaired t-tests or ANOVA for more than two genotypes. For all experiments n = number of wings.

Live imaging

For live imaging, a small piece of cuticle was removed from over the pupal wings of 28 hr APF pupae, and the exposed wing was mounted in a drop of Halocarbon 700 oil in a glass-bottomed dish. For FRAP analysis, images were 256 × 256 pixels, with a pixel size of 100 nm, and a pinhole of 1.2 AU. ‘Hub-and-spoke’ ROIs of 3–4 µm2 were selected, that covered a vertex and three half-cell edges. Alternatively elliptical ROIs were selected to bleach puncta (ROIs 1–1.5 µm2) or entire junctions between two cells on a clone boundary (ROIs 2–2.5 µm2). Three pre-bleach images were taken at two frames/sec, and ROIs were then bleached using a 488 nm Argon laser at 80% with eight passes (1 s total time), which resulted in 60–75% bleaching. Immediately following bleaching, five images were taken at 5 s intervals, followed by 10 images at 10 s intervals and 26 images at 15 s intervals. Laser power was adjusted to maintain constant power between different imaging sessions. If only EGFP was being imaged, a long pass GFP filter was used. If mRFP was present in the stock, EGFP was detected using a 525–550 band pass filter.

FRAP processing

For data analysis, ImageJ was used to manually reselect up to six bleached regions in each image for each time point. The laser off background was subtracted, and the values were corrected for acquisition bleaching and normalised against the average of the prebleach values. Data were then plotted on an xy graph using Prism (v7 Graphpad), bleached regions within the same wing were averaged and a one-phase exponential curve was fitted for each wing. Multiple wings were then combined and an exponential association curve was fitted. An extra-sum-of-squares F test was used to compare curve plateaux (y[max]), and stable fractions were calculated as 1-y[max].

For hub-and-spoke and boundary FRAP experiments, the stable amount of protein was calculated by measuring the intensity of the ROIs from the three pre-bleach images, and averaging per wing. The intensity was then corrected for distance from the coverslip as previously described (Strutt et al., 2016), and this value was then multiplied by the stable fraction (1-y[max]) for each wing. The stable amounts were then averaged across wings.

Overall junctional intensities, and stable and unstable amounts were compared between genotypes using unpaired t-tests, or one-way ANOVA for more than two genotypes. Post-hoc tests were used to compare individual samples: Dunnett’s multiple comparison test was used to compare the control to the rest of the genotypes in the experiment; Tukey-Kramer’s multiple comparison test to compare all genotypes within an experiment; and Holm-Šídák’s multiple comparison test was used to compare genotypes pair-wise.

Each experiment was performed on multiple wings from different pupae, which represent biological replicates (n = number of wings). For each wing, 4 ROIs were selected for FRAP analysis, and these were treated as technical replicates and were averaged per wing to produce a y[max] and a stable amount per wing.

Based on the mean intensity and standard deviation of a control set of wings, we calculated that a sample size of 6 wings per genotype would allow detection of differences of 20% in the means, in a pair-wise comparison, with a power of 0.8 and α 0.05 (using G*Power). As standard deviations were larger for some genotypes, we aimed for 10 wings per genotype. Data was excluded if the ROI recovery curve failed the ‘replicates test for lack of fit’ in GraphPad Prism, or if the wing moved out of focus during the course of imaging.

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Decision letter

  1. Jeremy Nathans
    Reviewing Editor; Johns Hopkins University School of Medicine, United States
  2. Utpal Banerjee
    Senior Editor; University of California, Los Angeles, United States

In the interests of transparency, eLife includes the editorial decision letter and accompanying author responses. A lightly edited version of the letter sent to the authors after peer review is shown, indicating the most substantive concerns; minor comments are not usually included.

Thank you for submitting your article "Reciprocal action of Casein Kinase Iε on core planar polarity proteins regulates clustering and asymmetric localisation" for consideration by eLife. Your article has been reviewed by three peer reviewers, and the evaluation has been overseen by a Reviewing Editor and Utpal Banerjee as the Senior Editor. The reviewers have opted to remain anonymous.

As you will see, all of the reviewers were impressed with the importance and novelty of your work. I am including the three reviews at the end of this letter, as there are a variety of specific and useful suggestions in them. We appreciate that the reviewers' comments cover a broad range of suggestions for improving the manuscript. We look forward to receiving your revised manuscript.

Reviewer #1:

The manuscript examines the role the Casein kinase Dco plays in the regulation of core PCP proteins, especially Stbm and Dsh. The study confirms recent findings the Dco affects Stbm phosphorylation, and extends these by examining the effects of phosphorylation site mutants on Drosophila wing PCP, and on Stbm polarization, stability at junctions, puncta size, etc. Thus they go well beyond the study of Kelly et al., 2016 especially in vivo, and in detail some of their results disagree. Intriguingly, phosphomutant and phosphomimetic Stbm both have abnormal and even dominant negative effects on PCP and are associated in decreased and increased junctional stability, respectively. The authors do a nice job incorporating this into their model.

Previous work had suggested that Stbm phosphorylation could be regulated by Dsh and Fz. However, the authors find little evidence of this in vivo from fz and dsh mutants. Instead, they find a strong effect from Pk. While they do not have a firm understanding of how this works, they provide some persuasive arguments that it is likely fairly direct, rather than through simple assembly of the PCP complex or through Fz or Dsh.

Finally, they go on to show that Dco also has an independent effect on Dsh levels and phosphorylation.

While this paper lacks the zing of recent studies on polarity-inducing feedback mechanisms, it is certainly a solid study that extends previous work on the role of PCP component phosphorylation. I think it deserves publication.

Reviewer #2:

In this study, Strutt et al. investigate the role of phosphorylation in the establishment of planar cell polarity (PCP) in Drosophila. Using immunofluorescence, live imaging, FRAP, biochemistry, and genetic mosaic approaches, they provide evidence that Stbm is less stable and more mobile when phosphorylated, and conversely, more stable when unphosphorylated. In contrast to Stbm, Dsh was found to be more stable when phosphorylated. They also provide evidence that Pk negatively regulates Stbm phosphorylation. Their findings suggest that Casein Kinase Ie mediates Stbm phosphorylation, confirming other reports, and extend this finding by providing genetic evidence that Casein Kinase Ie phosphorylates both Stbm and Dsh independently. They propose a model for how modifying differential protein stability could be means to regulate sorting of PCP proteins.

Mechanisms that regulate the establishment and maintenance of PCP is of high interest to the fields of polarity and developmental biology. Although Stbm/Vang and Dsh/Dvl have been known to be phosphorylated for some time, and that their phosphorylation is important for PCP, it was not known how phosphorylation regulates PCP protein dynamics and function. By providing further support that Casein Kinase Ie-mediates phosphorylation of Stbm and Dsh and by demonstrating that phosphorylation alters PCP protein stability and sorting, this study provides important and substantial contributions to the field. A strength of the study is the use of tissue extracts from transgenic and mutant lines to measure phosphorylation states (as opposed to transfected cells), but the reliance on very subtle band shifts as the sole means to detect phosphorylation is a weakness. The manuscript text is generally well written, however the data and figures could have been presented more clearly. While statistics used were appropriate and thorough, the quantification is often confusing and needs to be made more transparent. Though the findings are significant and impactful with some mechanistic detail, there are concerns that the work does not go far enough to extend current knowledge and has limited accessibility in its current state for the broad readership of eLife.

Major comments:

- Inclusion of a table of Drosophila genotypes used with direct reference of how they are labeled/referred to in figures their phenotypes would be very helpful.

- No biochemical evidence is shown to support that direct binding of Pk to Stbm protects Stbm from phosphorylation (as discussed in the Discussion, fourth paragraph).

- Figure 1 lays the foundation for all subsequent figures and could be presented more clearly for a broader audience. For example, the schematic in A makes sense to a PCP specialist but needs more context for someone less familiar with the field. Zoomed images for E-F will help better visualize trichome orientation, and a reorganization of E-M to better pair panels of the same genotypes would help. Additionally, an overlay of polarity vectors for I-J would be helpful as Stbm asymmetry is subtle and not apparent to the untrained eye.

- The authors state that "only small puncta were observed in Stbm phosphomimetic flies, and non-puncta material also increased (Figure 1J). However, in the Stbm phosphomutant, Stbm still clustered into large puncta, despite the loss of asymmetry (Figure 1I)." Quantification of the puncta sizes in Figure 1I-J and Figure 2A-D are lacking and necessary as many of the future conclusions rely on these observations.

- Figure 2G-H is a critical piece of data to support the authors model that -P promotes sorting of opposite PCP complexes, and should be presented more convincingly. Panel H shows Diego localized to both sides of cell junctions in Stbm phosphomutants, suggesting PCP complexes of opposite orientations are mixed. According to the model, puncta in wild type flies represent sorted PCP complexes, but is this observed during intermediate stages of polarization? Diego is clearly segregated in the fully polarized control in panel G, but at earlier stages, do they see puncta along a single junction that are sorted in opposite orientations? This control would strengthen the argument that it is the sorting of opposite complexes within puncta that is defective in their mutants. According to the model, stbmS(AII)E complexes/puncta should be sorted and thus, that data should be included stbmS(AII)E phosphomimetic for G-H panels.

- Figure 3. Stable to unstable amounts should be displayed separately to better see the changes, it is hard to compare the relative amounts as currently displayed (this holds true for all FRAP data).

- Figure 5. The western blot bands in A demonstrating a shift in Stbm are not obvious. This is main evidence that Pk influences Stbm phosphorylation, but it's not wholly convincing. In general, detecting changes in -P solely by band shifts is a general weakness of the overall approach since these differences are very subtle, hard to quantify, and open to other interpretations. Although this is probably beyond scope of this paper, generating a phospho-specific Ab or performing mass spec analyses of -P sites would go a long way towards strengthening the results and lending support to the overall model.

- The model in Figure 7 would benefit from inclusion of Dco. In addition, do the authors think that Pk destabilizes Fz via inhibiting Dco-dependent -P of Dsh?

Reviewer #3:

The manuscript by Strutt and colleagues addresses the mechanisms by which protein complexes become asymmetrically distributed on cell junctions, a process important for establishing planar cell polarity. The authors identified an important role of CK1 in phosphorylating core proteins and thereby regulating their clustering/membrane association. This work gives interesting new insights into the role that phosphorylation plays in establishing the asymmetry of core proteins at cell junctions. The manuscript is well written and the data are convincing for the most part. I have a few concerns, which I expect could be addressed within a reasonable time by the authors:

Figure 1J, L The authors claim that in Stbm phosphomimetic flies only small puncta are observed, whereas in Stbm phosphomutant flies, Stbm clusters in large puncta. The puncta are not obvious from these images. Higher magnification images should be shown and puncta size needs to be quantified. Moreover, the authors should mention whether stbm[6] mutants still express Stbm protein, i.e. do these stainings show Stbm[6] and phosphomutant Stbm or only the phosphomutant Stbm?

Figure 2G, H I can see that in the Stbm phosphomutant flies Diego forms "non-polarized" puncta, however, I do not see evidence that these puncta co-localize with Stbm, as the authors claim.

Figure 3A. The authors should briefly explain the concept of the 'hub-and-spoke' FRAP methods, so that readers do not have to look up the reference in order to understand the experiment. Moreover, images of the FRAP experiment should be shown (e.g. in the supplement) to allow readers to evaluate the data.

Figure 3 FRAP analysis allows to estimate the mobile fraction of a protein, and the authors use this method to show that phosphorylation of Stbm increases its mobile fraction. It is however, less clear how the authors infer from the FRAP analysis that phosphorylation suppresses clustering in complexes. While complex formation might be one mechanism to reduce the mobile fraction of a protein, it is not the only one. The authors should elaborate on this issue.

https://doi.org/10.7554/eLife.45107.033

Author response

Reviewer #2:

[…] A strength of the study is the use of tissue extracts from transgenic and mutant lines to measure phosphorylation states (as opposed to transfected cells), but the reliance on very subtle band shifts as the sole means to detect phosphorylation is a weakness. The manuscript text is generally well written, however the data and figures could have been presented more clearly. While statistics used were appropriate and thorough, the quantification is often confusing and needs to be made more transparent. Though the findings are significant and impactful with some mechanistic detail, there are concerns that the work does not go far enough to extend current knowledge and has limited accessibility in its current state for the broad readership of eLife.

Major comments:

- Inclusion of a table of Drosophila genotypes used with direct reference of how they are labeled/referred to in figures their phenotypes would be very helpful.

We have added a table of all genotypes associated with each figure.

- No biochemical evidence is shown to support that direct binding of Pk to Stbm protects Stbm from phosphorylation (as discussed in the Discussion, fourth paragraph).

We have discussed other possible models for how Pk might regulate Stbm phosphorylation in the manuscript, but we found evidence arguing against the other models that we considered. We agree however that we don't have direct evidence for the idea that binding of Pk to Stbm protects Stbm from phosphorylation; this is why we put the associated model figure in the supplementary data rather than in the main manuscript. However we think it is useful to suggest hypotheses about the mechanism, as this is then a basis for future studies. In the future we will be looking for direct evidence for this model, but such experiments are beyond the scope of the current manuscript.

- Figure 1 lays the foundation for all subsequent figures and could be presented more clearly for a broader audience. For example, the schematic in A makes sense to a PCP specialist but needs more context for someone less familiar with the field. Zoomed images for E-F will help better visualize trichome orientation, and a reorganization of E-M to better pair panels of the same genotypes would help. Additionally, an overlay of polarity vectors for I-J would be helpful as Stbm asymmetry is subtle and not apparent to the untrained eye.

These are all good points, and we have added extra introductory panels to the first figure. The adult wings in (now in Figure 2 and Figure 2—figure supplement 1) have been replaced with higher magnification images. We have also overlaid polarity vectors on the images in Figure 2 and Figure 2 supplements 1 and 2. Finally, we have also included higher magnification images of puncta in these genotypes, in response to reviewer 3.

- The authors state that "only small puncta were observed in Stbm phosphomimetic flies, and non-puncta material also increased (Figure 1J). However, in the Stbm phosphomutant, Stbm still clustered into large puncta, despite the loss of asymmetry (Figure 1I)." Quantification of the puncta sizes in Figure 1I-J and Figure 2A-D are lacking and necessary as many of the future conclusions rely on these observations.

We have now included measurements of puncta number and size in Figure 2. We previously attempted to threshold images using Stbm immunolabelling, using the same threshold for wild-type and mutant regions of the same wing. However as overall junctional levels of Stbm increase in both the phosphomutant and the phosphomimetic, this was problematic and resulted in apparent increased values in both cases. We have now got round this issue by using Fmi immunolabelling to threshold the images: as Fmi co-localises with Stbm and junctional levels of Fmi do not change in the phosphomutants, this successfully identifies puncta without "spreading" into neighbouring non-puncta regions. The major finding is that puncta number is similar in wild-type and phosphomutant; suggesting a similar degree of "punctateness"; while the phosphomimetic is less punctate. The amount of Stbm in puncta (puncta area multiplied by puncta intensity) is however slightly lower in both the phosphomutant and phosphomimetic. We think the smaller puncta size in phosphomutant and phosphomimetic may be due to the fact that negative feedback limits the growth of individual puncta (as seen in our FRAP experiments using Fmi-EGFP and Fz-EGFP).

We have modified the manuscript to take into account these quantitations; in particular we have stated that the phosphomimetic forms fewer puncta rather than that it forms only small puncta.

However, the main conclusion still stands: that the Stbm phosphomimetic clusters into puncta more readily than the phosphomimetic and thus the coupling between puncta formation and asymmetry is lost.

- Figure 2G-H is a critical piece of data to support the authors model that -P promotes sorting of opposite PCP complexes, and should be presented more convincingly. Panel H shows Diego localized to both sides of cell junctions in Stbm phosphomutants, suggesting PCP complexes of opposite orientations are mixed. According to the model, puncta in wild type flies represent sorted PCP complexes, but is this observed during intermediate stages of polarization? Diego is clearly segregated in the fully polarized control in panel G, but at earlier stages, do they see puncta along a single junction that are sorted in opposite orientations? This control would strengthen the argument that it is the sorting of opposite complexes within puncta that is defective in their mutants. According to the model, stbmS(AII)E complexes/puncta should be sorted and thus, that data should be included stbmS(AII)E phosphomimetic for G-H panels.

We have not looked at Dgo localisation at earlier developmental stages, but we speculate that the sorting would be less good than at 28 hr APF. We expect that sorting within puncta would be reduced; and that most puncta would contain complexes in both orientations rather than seeing puncta along a single junction that are sorted in opposite orientations.

We also don't predict that individual Stbm phosphomimetic complexes/puncta should be sorted. Our data suggest that Stbm phosphomimetics behave like pk mutants, in which Stbm is hyperphosphorylated. In our model hyperphosphorylation promotes Stbm mobility and thus the puncta that do form are unstable. This means that complexes are continually forming and unforming and that complexes of the same orientation fail to accumulate. It would be interesting to test this directly, but we are unable to do the twin clone experiment with our current reagents, as P[acman]-stbmS[All]E, P[acman]-mApple-dgo and P[acman]-EGFP-dgo are in the same attP site.

- Figure 3. Stable to unstable amounts should be displayed separately to better see the changes, it is hard to compare the relative amounts as currently displayed (this holds true for all FRAP data).

We agree that it is not easy to compare the unstable amounts in the current graphs. However, if we plot stable and unstable amounts separately, we will "lose" the other important data regarding total intensity. We have however added extra supplementary figures for all FRAP data, showing the total intensity, stable amount and unstable amounts as separate plots.

- Figure 5. The western blot bands in A demonstrating a shift in Stbm are not obvious. This is main evidence that Pk influences Stbm phosphorylation, but it's not wholly convincing. In general, detecting changes in -P solely by band shifts is a general weakness of the overall approach since these differences are very subtle, hard to quantify, and open to other interpretations. Although this is probably beyond scope of this paper, generating a phospho-specific Ab or performing mass spec analyses of -P sites would go a long way towards strengthening the results and lending support to the overall model.

We agree that the band shifts are subtle, and that is why we have shown duplicates of the critical samples. We have now also included an extra supplementary figure panel (Figure 6—figure supplement 1A) in response to reviewer 1, to show that the decreased migration of Stbm in pk mutants is sensitive to phosphatase.

We also agree that phospho-specific antibodies would be nice, but also that this is beyond the scope of this paper. Furthermore, we note that other researchers have made phospho-specific antibodies (e.g. for Dsh, Yanfeng et al., 2011) and it is difficult to get enough sensitivity on westerns in an in vivo context.

- The model in Figure 7 would benefit from inclusion of Dco. In addition, do the authors think that Pk destabilizes Fz via inhibiting Dco-dependent -P of Dsh?

We have added Dco to the model diagram (now in Figure 8).

In our previous work, we proposed two mechanisms by which Pk could destabilise Fz. In the first, Dsh would normally protect Fz from endocytosis (e.g. by promoting multimerisation); and that Pk binding to Dsh could block this protective function. In the second mechanism, we proposed that binding of Pk to Fz/Dsh could lead to recruitment of an enzyme that mediates a change in post-translational modification of Dsh or Fz. In either model, we could envisage either a direct role for Dco (e.g. Pk locally inhibits Dco function or promotes a phosphatase that opposes Dco activity) or indirect roles (e.g. multimerised Dsh is a better target for Dco or is protected from a phosphatase, and Pk opposes multimerisation thus indirectly leading to a change in the Dco-mediated phosphorylation of Dsh). However, we think we might be speculating beyond what the data can really support if we included such ideas in the manuscript.

Reviewer #3:

[…] I have a few concerns, which I expect could be addressed within a reasonable time by the authors:

Figure 1J, L The authors claim that in Stbm phosphomimetic flies only small puncta are observed, whereas in Stbm phosphomutant flies, Stbm clusters in large puncta. The puncta are not obvious from these images. Higher magnification images should be shown and puncta size needs to be quantified. Moreover, the authors should mention whether stbm[6] mutants still express Stbm protein, i.e. do these stainings show Stbm[6] and phosphomutant Stbm or only the phosphomutant Stbm?

We have now shown higher magnification images of the puncta in Figure 2 and Figure 2—figure supplements 1 and 2. We have also provided quantitations of puncta number and size (see response to reviewer 2).

We have also clarified in the manuscript that the clones of the phosphorylation site mutants are generated in the absence of endogenous Stbm.

Figure 2G, H, I can see that in the Stbm phosphomutant flies Diego forms "non-polarized" puncta, however, I do not see evidence that these puncta co-localize with Stbm, as the authors claim.

We have now shown in Figure 3—figure supplement 1 that EGFP-Dgo co-localises in puncta with the other core proteins, in both a wild-type and a Stbm phosphomutant background.

Figure 3A. The authors should briefly explain the concept of the 'hub-and-spoke' FRAP methods, so that readers do not have to look up the reference in order to understand the experiment. Moreover, images of the FRAP experiment should be shown (e.g. in the supplement) to allow readers to evaluate the data.

We have added more explanation to the text to better explain the methodology. We have also added some example FRAP images (pre-bleach, immediately post-bleach and after 500 sec recovery) for the first experiment, and the entire FRAP datasets are in the source data.

Figure 3 FRAP analysis allows to estimate the mobile fraction of a protein, and the authors use this method to show that phosphorylation of Stbm increases its mobile fraction. It is however, less clear how the authors infer from the FRAP analysis that phosphorylation suppresses clustering in complexes. While complex formation might be one mechanism to reduce the mobile fraction of a protein, it is not the only one. The authors should elaborate on this issue.

We agree that in general a reduction in the mobile fraction of a protein can be caused by mechanisms other than complex formation; for instance other explanations could be reduced vesicular trafficking or tethering to another structure such as the cytoskeleton. However, our model is based not only on the FRAP data but on the fact that we see changes in cluster formation (puncta) in our images (now quantitated in the revised manuscript). Taken together, we think our current working model is the most plausible. We would prefer not to add further speculation to the Discussion, which is already quite long.

https://doi.org/10.7554/eLife.45107.034

Article and author information

Author details

  1. Helen Strutt

    Department of Biomedical Science, University of Sheffield, Sheffield, United Kingdom
    Contribution
    Formal analysis, Investigation, Methodology, Writing—original draft, Writing—review and editing
    For correspondence
    h.strutt@sheffield.ac.uk
    Competing interests
    No competing interests declared
    ORCID icon "This ORCID iD identifies the author of this article:" 0000-0003-4365-2271
  2. Jessica Gamage

    Department of Biomedical Science, University of Sheffield, Sheffield, United Kingdom
    Contribution
    Investigation, Writing—review and editing
    Competing interests
    No competing interests declared
  3. David Strutt

    Department of Biomedical Science, University of Sheffield, Sheffield, United Kingdom
    Contribution
    Conceptualization, Supervision, Funding acquisition, Methodology, Writing—review and editing
    For correspondence
    d.strutt@sheffield.ac.uk
    Competing interests
    No competing interests declared
    ORCID icon "This ORCID iD identifies the author of this article:" 0000-0001-8185-4515

Funding

Wellcome Trust (100986/Z/13/Z)

  • Helen Strutt
  • David Strutt

Medical Research Council (G0900203-1/1)

  • Jessica Gamage

Wellcome Trust (210630/Z/18/Z)

  • Helen Strutt
  • David Strutt

Medical Research Council (G1000405-1/1)

  • Jessica Gamage

The funders had no role in study design, data collection and interpretation, or the decision to submit the work for publication.

Acknowledgements

We thank the Bloomington Drosophila Stock Center for fly stocks, Tanya Wolff and the Developmental Studies Hybridoma Bank for antibodies, and Genetivision for generating transgenics. Natalia Bulgakova, Katie Fisher, Simon Fellgett and Melissa Gammons are thanked for comments on the manuscript, and the fly room staff for excellent technical support. Imaging was performed in the Wolfson Light Microscopy Facility. The work was funded by Wellcome Senior Fellowship awards to DS [100986/Z/13/Z and 210630/Z/18/Z] and an MRC studentship (grant numbers G0900203-1/1 and G1000405-1/1).

Senior Editor

  1. Utpal Banerjee, University of California, Los Angeles, United States

Reviewing Editor

  1. Jeremy Nathans, Johns Hopkins University School of Medicine, United States

Publication history

  1. Received: January 11, 2019
  2. Accepted: May 14, 2019
  3. Accepted Manuscript published: May 15, 2019 (version 1)
  4. Version of Record published: May 30, 2019 (version 2)

Copyright

© 2019, Strutt et al.

This article is distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use and redistribution provided that the original author and source are credited.

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