The CCP20 domain of SVEP1 contacts the D1-D2 headpiece domains of TIE1.

(A) Comparative PAE plots of full-length SVEP1 and the TIE1 ectodomain displays a primary binding site for TIE1 (domains D1-D2) by AF3-multimer modeling, that localizes to the CCP19-21 stretch, notably encompassing CCP20.. The PAE plot for full-length SVEP1 and the TIE2 ectodomain does not show an equivalent prediction for the same CCP19-21 region. Labelled SVEP1 domains: PTX (Pentraxin), SEA (sperm protein, enterokinase and agrin) module, VWA (von Willebrand factor type A), statistics flSVEP1-ectoTIE1 pLDDT = 66.1; pTM = 0.34; ipTM = 0.48; statistics flSVEP1-ectoTIE2 pLDDT = 0.68, ipTM = 0.28, pTM = 0.34 (B) A discrete AF2.3 model of TIE1 bound to CCP19-21 shows that the Ig-like D1-D2 headpiece domains of TIE1 (linked by a Cys-rich D3 to an ‘open’ or flared-out D4 Ig module) are in contact with CCP20. By contrast, the AF2.3 model of TIE2’s N-terminal domains is more loosely perched between the CCP20 and CCP21 domains with a qualitatively lower affinity due to reduced sidechain contacts. (C) Quantitative analysis of TIE1-SVEP1 interaction using SPR assays. In the absence of other proteins, a 70kD version of SVEP1 (CCP15-CCP24, amino acids: 2261-2890; depicted in diagram fashion) containing the CCP20 domain binds TIE1 (KD = 62.5 nM), but does not bind TIE2. (D) Negative staining shows binding of human C-term SVEP1-Strep II (reaching from CCP15 to the C-Term) and TIE1, labelled with 10nm Au. PAE: Predicted aligned error; AF3: AlphaFold 3; CCP: complement control protein (sushi repeats).

ANG1/2 strengthens the binding capacity of SVEP1 and TIE1.

(A) AF2.3-modeling of the SVEP1 CCP19-21 domain and TIE1 together with ANG1/2. The affinity of TIE1 for SVEP1 is considerably heightened by the co-binding of ANG1 or ANG2 C-terminal domains to the CCP20 module. (B) The Interaction between the complete C-terminal SVEP1 (spanning from the EGF1 domain to C-terminus) and the ectodomain of TIE1 was analyzed by ELISA binding assays. SVEP1 proteins were immobilized on 96 well plastic plates and overlayed with serial dilutions of TIE1 fused to an ALFA-tag, in the absence or presence of equal amounts of ANG2 as indicated. Bound TIE1 proteins were detected by HRP conjugated anti-ALFA-tag nanobody. ANG2 binding was detected using an HRP conjugated anti-Biotin antibody against the biotinylated protein. KD values are lower in the presence of ANG1 (KD=82,4) or ANG2 (KD=55,6), reflecting a higher affinity than SVEP1 and TIE1 in the absence of angiopoietins (KD=189,0). (C) Co-immunoprecipitation of human TIE1 co-transfected with either human ANG1 or ANG2 in 293T HEK cells. Human SVEP1-Strep II (a 150 kDa version reaching from CCP15 to the C-Term) was immunoprecipitated, and associated TIE1 and ANG1/2 were detected via Western blot analysis. (D) Quantification of the TIE1 levels with SVEP1 levels as a reference confirmed a significant increase in binding affinity of SVEP1 and TIE1 together with ANG2 (**p.adj = 0.008; Mann–Whitney U test; Values are presented as means ± SD; individual data points for each experiment). TL, total lysate.

Simultaneous stimulation of LECs with SVEP1 and ANG2 leads to phosphorylation of TIE1 and AKT as well as nuclear exclusion of FOXO1 in vitro.

(A) Western blot analysis of TIE1 phosphorylation at Y1007 residue in SVEP1 and/or ANG2 stimulated LECs after immunoprecipitation of TIE1. TIE1 is phosphorylated after simultaneous stimulation with SVEP1 and ANG2. The antibody anti–phospho-TIE2 (pY992, AF2720 R&D Systems) detects both, TIE1 phosphorylation (at Y1007 residue) and TIE2 phosphorylation (at Y992 residue), but can be distinguished by size (Brouillard et al. 2024). (B) Western blot analysis of p-AKT and AKT in the cell lysate of SVEP1 and ANG2 stimulated LECs. Knock-down of either TIE1 or TIE2 leads to reduced induction of phosphorylation of AKT by SVEP1-ANG2 (C) FOXO1 (red) and DAPI (blue) staining of LECs stimulated with only SVEP1 or ANG2, or in combination. FOXO1 staining is restricted to the nucleus of unstimulated (control) LECs, whereas in cells stimulated with SVEP1 and ANG2 together FOXO1 is in the cytoplasm. (D) Quantification of the pTIE1/total TIE1 ratios shown in A. (N = 5; *p adj = 0,045; Mann–Whitney U test; Values are presented as means ± SD) (E) Quantification of the pAKT/total AKT ratios shown in B. Values are presented as means ± SD, N = 3/6; Mann–Whitney U test; Control vs. ANG2_SVEP1 p.adj = 0.028 *; SVEP1 vs. ANG2 p.adj = 0.022 *; ANG2 vs. ANG2_SVEP1 p.adj = 0.028 *; ANG2_SVEP1 vs. aANG2AB_SVEP1 p.adj = 0.091. (F) The ratio of nuclear FOXO1/cytoplasmic FOXO1 decreases after stimulation with either ANG2 or SVEP1. Stimulation of LECs with simultaneous SVEP1 and ANG2 leads to a further reduction of the nuclear FOXO1/cytoplasmic FOXO1 ratio. Each datapoint represents the mean value per image of the ratio of nuclear to cytoplasmic FOXO1 per cell (Mann–Whitney U test; Values are presented as means ± SD; N=4).

SVEP1, TIE1 and ANG1/2 can form 2:2:2 complexes mediated by SVEP1.

(A) Larger hexameric complexes of SVEP1-TIE1 D1-D4-ANG1/2 in a 2:2:2 stoichiometry could be confidently modeled with AF3. They show a symmetric assembly linked by predicted TIE1 dimerization sites at FN3-FN2’ as well as the D3 and D4 domains. Similar to the 1:1:1 complex of SVEP1-TIE1-ANG1/2, ANG1/2 makes contact with the D1 and D2 domains of TIE1 by inclusion of the SVEP1 chain (CCP19-21). (B) Schematic model of SVEP1 enabling the multimerization of TIE1 receptors on the cell surface by cross-lacing of TIE1 dimers and N-terminal coiled-coil dimers (or greater assemblies) of ANG1/2. (C) Schematic model of TIE1 clustering via only ANG1/2 mediated multimerization.

Statistics and (A) PDBePISA analysis of a SVEP1-TIE1ectodomain model vs a SVEP1-TIE2 ectodomain model depicting interface surface area, Gibbs free energy, salt bridges and hydrogen bond contacts. (B) and (C) statistics table, bonds and PAE plot of TIE1 (B) or TIE2 (C). PAE plot: A =SVEP1 CCP19-21, B=TIE1 or TIE2.

SVEP1 TIE1 interfaces.

(A) Salt bridges E2568-K159 and E2701-K152 between SVEP1 CCP20 (green) and D1/D2 of TIE1 ectodomain (cyan). Electrostatic bonds are highlighted in yellow.. (B) Hydrogen bond network of SVEP1 CCP20 (green) and D1/D2 of TIE1 ectodomain (cyan). H-bonds are highlighted in white. (C) F2566 of SVEP1 (green) fits in the hydrophobic pocket of TIE1 (cyan) partially formed by P202 and L203. (D and E) Surface electrostatic potential representation of SVEP1 (green) bound to TIE1 (cyan) detailing that the mainly acidic interaction surface of SVEP1 (D) is opposed by a basic interaction surface of TIE1 (E). (F and G) Hydrophobicity surface representations of SVEP1 (green) bound to TIE1 (cyan). The hydrophobic side chains of F2566 and F2583 fit into a hydrophobic cavity of TIE1.

(A) Different versions of human SVEP1-StrepTac (spanning from CCP15 to the C-Term) were immunoprecipitated and associated TIE1 was detected via Western blot analysis. TIE1 bound to CCP19-21 as well as CCP20-21, but not the CCP19-21 mutant containing the E2568A-G2569A mutant or CCP19-20. (B) Binding of SVEP1 (wt or E2568A-G2569A) to TIE1 (wt or P202L L203F) was analyzed by ELISA. Both mutations, either in TIE1 or in SVEP1 inhibit binding between TIE1 and SVEP1.

(A) AlphaFold 2.3 multimer output scores and (B) PAE plots of SVEP1-TIE1-ANG1/2 complexes. A is SVEP1 CCP19-21, B is TIE1 ecotdomain, C is the ANG1/2 fibrinogen like domain. (C) Bonding residues of each interface (D) and (E) PDBePISA analysis of interfaces of SVEP1-TIE1-ANG1/2 top ranked models. The conformation of the TIE1-ANG1/2 heteromers in the SVEP1-bound pose resembles the solved structures of TIE2 bound to either ANG1 or ANG2. However, the weaker binding of TIE2 to CCP19-21 of SVEP1 fails to nucleate the formation of the TIE2 ternary complex with SVEP1.

(A,B,C) Surface electrostatic potential of the interaction surfaces present in the SVEP1-TIE1-ANG1 complex demonstrating complementary charge distributions. (D,E,F) Surface hydrophobicity of the interaction surfaces present in the SVEP1-TIE1-ANG1 complex. (G,H,I) Surface electrostatic potential of the interaction surfaces present in the SVEP1-TIE1-ANG2 complex. (J,K,L) Surface hydrophobicity of the interaction surfaces present in the SVEP1-TIE1-ANG2 complex. All electrostatic potentials and hydrophobicity surfaces were generated and visualized with UCSF Chimera 1.19.

ANG1 and ANG2 bind in close proximity to the site where TIE1 and SVEP1 interact.

Negative staining of human SVEP1 binding to TIE1 with and without the presence of ANG1 or ANG2. (A) hSVEP 150 kDa. (B) hSVEP-hTIE-10 nm Au. (C) hSVEP-ANG1-5nm Au, (D) hSVEP-ANG2-5nm Au. (E) hSVEP-hTIE-10 nm Au-ANG1-5 nm Au. (F) hSVEP-hTIE-10 nm Au-ANG2-5 nm Au. (G) hSVEP-hTIE-10 nm Au-ANG1/2-5 nm Au. Parts of panels A and B (control, SVEP1-TIE1-10 nm Au) have also been shown in main figure 1. Scale bar: 25 nm.

(A) AF3 output metrics (B) PAE plots demonstrating confident predictions of the complexes shown in Figure 4. (C) and (D) The side (C) and top (D) view of the model hexamer shows the side-by-side, antiparallel packing of TIE1 D3 domains that produces the domain-swapping of D4 Ig domains. E) Top view of the Fn2 – Fn3 domains.