1. Biochemistry and Chemical Biology

A secretory kinase complex regulates extracellular protein phosphorylation

  1. Jixin Cui
  2. Junyu Xiao
  3. Vincent S Tagliabracci
  4. Jianzhong Wen
  5. Meghdad Rahdar
  6. Jack E Dixon  Is a corresponding author
  1. University of California, San Diego, United States
  2. Peking University, China
  3. Merck & Co, United States
  4. ISIS Pharmaceuticals, United States
https://doi.org/10.7554/eLife.06120
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  1. Jixin Cui
  2. Junyu Xiao
  3. Vincent S Tagliabracci
  4. Jianzhong Wen
  5. Meghdad Rahdar
  6. Jack E Dixon
(2015)
A secretory kinase complex regulates extracellular protein phosphorylation
eLife 4:e06120.
https://doi.org/10.7554/eLife.06120
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Abstract

Although numerous extracellular phosphoproteins have been identified, the protein kinases within the secretory pathway have only recently been discovered, and their regulation is virtually unexplored. Fam20C is the physiological Golgi casein kinase, which phosphorylates many secreted proteins and is critical for proper biomineralization. Fam20A, a Fam20C paralog, is essential for enamel formation, but the biochemical function of Fam20A is unknown. Here we show that Fam20A potentiates Fam20C kinase activity and promotes the phosphorylation of enamel matrix proteins in vitro and in cells. Mechanistically, Fam20A is a pseudokinase that forms a functional complex with Fam20C, and this complex enhances extracellular protein phosphorylation within the secretory pathway. Our findings shed light on the molecular mechanism by which Fam20C and Fam20A collaborate to control enamel formation, and provide the first insight into the regulation of secretory pathway phosphorylation.

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Author details

  1. Jixin Cui

    Department of Pharmacology, University of California, San Diego, San Diego, United States
    Competing interests
    The authors declare that no competing interests exist.

  2. Junyu Xiao

    State Key Laboratory of Protein and Plant Gene Research, School of Life Sciences, Peking University, Beijing, China
    Competing interests
    The authors declare that no competing interests exist.

  3. Vincent S Tagliabracci

    Department of Pharmacology, University of California, San Diego, San Diego, United States
    Competing interests
    The authors declare that no competing interests exist.

  4. Jianzhong Wen

    Discovery Bioanalytics, Merck & Co, Rahway, United States
    Competing interests
    The authors declare that no competing interests exist.

  5. Meghdad Rahdar

    ISIS Pharmaceuticals, Carlsbad, United States
    Competing interests
    The authors declare that no competing interests exist.

  6. Jack E Dixon

    Department of Pharmacology, University of California, San Diego, San Diego, United States
    For correspondence
    jedixon@ucsd.edu
    Competing interests
    The authors declare that no competing interests exist.

Ethics

Animal experimentation: Procedures involving mice were reviewed and approved by the Institutional Animal Care and Use Committee (IACUC) at the UC San Diego (Protocol #S03039).

Reviewing Editor

  1. Tony Hunter, Salk Institute, United States

Publication history

  1. Received: December 16, 2014
  2. Accepted: March 18, 2015
  3. Accepted Manuscript published: March 19, 2015 (version 1)
  4. Version of Record published: May 6, 2015 (version 2)

Copyright

© 2015, Cui et al.

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

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  1. Jixin Cui
  2. Junyu Xiao
  3. Vincent S Tagliabracci
  4. Jianzhong Wen
  5. Meghdad Rahdar
  6. Jack E Dixon
(2015)
A secretory kinase complex regulates extracellular protein phosphorylation
eLife 4:e06120.
https://doi.org/10.7554/eLife.06120

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Further reading

    1. Biochemistry and Chemical Biology
    2. Structural Biology and Molecular Biophysics

    Structure of Fam20A reveals a pseudokinase featuring a unique disulfide pattern and inverted ATP-binding

    Jixin Cui et al.
    Research Advance Updated

    Mutations in FAM20A cause tooth enamel defects known as Amelogenesis Imperfecta (AI) and renal calcification. We previously showed that Fam20A is a secretory pathway pseudokinase and allosterically activates the physiological casein kinase Fam20C to phosphorylate secreted proteins important for biomineralization (Cui et al., 2015). Here we report the nucleotide-free and ATP-bound structures of Fam20A. Fam20A exhibits a distinct disulfide bond pattern mediated by a unique insertion region. Loss of this insertion due to abnormal mRNA splicing interferes with the structure and function of Fam20A, resulting in AI. Fam20A binds ATP in the absence of divalent cations, and strikingly, ATP is bound in an inverted orientation compared to other kinases. Fam20A forms a dimer in the crystal, and residues in the dimer interface are critical for Fam20C activation. Together, these results provide structural insights into the function of Fam20A and shed light on the mechanism by which Fam20A mutations cause disease.

    1. Biochemistry and Chemical Biology

    Protein Kinases: What is the point of pseudokinases?

    Saravanan Raju, Andrey S Shaw
    Insight

    The interaction between an active kinase and an ‘inactive’ pseudokinase provides clues about how these enzymes were regulated in the past, and how this regulation has evolved.

    1. Biochemistry and Chemical Biology

    A network of cytosolic (co)chaperones promotes the biogenesis of mitochondrial signal-anchored outer membrane proteins

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    Research Article Updated

    Signal-anchored (SA) proteins are anchored into the mitochondrial outer membrane (OM) via a single transmembrane segment at their N-terminus while the bulk of the proteins is facing the cytosol. These proteins are encoded by nuclear DNA, translated on cytosolic ribosomes, and are then targeted to the organelle and inserted into its OM by import factors. Recently, research on the insertion mechanisms of these proteins into the mitochondrial OM have gained a lot of attention. In contrast, the early cytosolic steps of their biogenesis are unresolved. Using various proteins from this category and a broad set of in vivo, in organello, and in vitro assays, we reconstituted the early steps of their biogenesis. We identified a subset of molecular (co)chaperones that interact with newly synthesized SA proteins, namely, Hsp70 and Hsp90 chaperones and co-chaperones from the Hsp40 family like Ydj1 and Sis1. These interactions were mediated by the hydrophobic transmembrane segments of the SA proteins. We further demonstrate that interfering with these interactions inhibits the biogenesis of SA proteins to a various extent. Finally, we could demonstrate direct interaction of peptides corresponding to the transmembrane segments of SA proteins with the (co)chaperones and reconstitute in vitro the transfer of such peptides from the Hsp70 chaperone to the mitochondrial Tom70 receptor. Collectively, this study unravels an array of cytosolic chaperones and mitochondrial import factors that facilitates the targeting and membrane integration of mitochondrial SA proteins.