title: The Vesicle Priming Factor CAPS Functions as a Homodimer via C2 Domain Interactions to Promote Regulated Vesicle Exocytosis.
abstract: Neurotransmitters and peptide hormones are secreted by regulated vesicle exocytosis. CAPS (also known as CADPS) is a 145-kDa cytosolic and peripheral membrane protein required for vesicle docking and priming steps that precede Ca2+-triggered vesicle exocytosis. CAPS binds phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) and SNARE proteins and is proposed to promote SNARE protein complex assembly for vesicle docking and priming. We characterized purified soluble CAPS as mainly monomer in equilibrium with small amounts of dimer. However, the active form of CAPS bound to PC12 cell membranes or to liposomes containing PI(4,5)P2and Q-SNARE proteins was mainly dimer. CAPS dimer formation required its C2 domain based on mutation or deletion studies. Moreover, C2 domain mutations or deletions resulted in a loss of CAPS function in regulated vesicle exocytosis, indicating that dimerization is essential for CAPS function. Comparison of the CAPS C2 domain to a structurally defined Munc13-1 C2A domain dimer revealed conserved residues involved in CAPS dimerization. We conclude that CAPS functions as a C2 domain-mediated dimer in regulated vesicle exocytosis. The unique tandem C2-PH domain of CAPS may serve as a PI(4,5)P2-triggered switch for dimerization. CAPS dimerization may be coupled to oligomeric SNARE complex assembly for vesicle docking and priming.
authors.affiliation: From the Department of Biochemistry, Integrated Program in Biochemistry, University of Wisconsin, Madison, Wisconsin 53706, and.
authors.forename: Matt
authors.initials: M
authors.lastname: Petrie
authors.affiliation: From the Department of Biochemistry, Program of Molecular and Cellular Pharmacology, and.
authors.forename: Joseph
authors.initials: J
authors.lastname: Esquibel
authors.affiliation: From the Department of Biochemistry, Integrated Program in Biochemistry, University of Wisconsin, Madison, Wisconsin 53706, and.
authors.forename: Greg
authors.identifier: http://orcid.org/0000-0003-4363-441X
authors.initials: G
authors.lastname: Kabachinski
authors.affiliation: From the Department of Biochemistry, Integrated Program in Biochemistry, University of Wisconsin, Madison, Wisconsin 53706, and.
authors.forename: Stephanie
authors.initials: S
authors.lastname: Maciuba
authors.affiliation: the Department of Pharmacology, University of Cambridge, Cambridge CB2 1PD, United Kingdom.
authors.forename: Hirohide
authors.initials: H
authors.lastname: Takahashi
authors.affiliation: the Department of Pharmacology, University of Cambridge, Cambridge CB2 1PD, United Kingdom.
authors.forename: J Michael
authors.identifier: http://orcid.org/0000-0003-0499-7795
authors.initials: JM
authors.lastname: Edwardson
authors.affiliation: From the Department of Biochemistry, Integrated Program in Biochemistry, University of Wisconsin, Madison, Wisconsin 53706, and Program of Molecular and Cellular Pharmacology, and tfmartin@wisc.edu.
authors.forename: Thomas F J
authors.identifier: http://orcid.org/0000-0002-5204-0202
authors.initials: TF
authors.lastname: Martin
mesh_terms: D000818:Animals; D002135:Calcium-Binding Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D005089:Exocytosis / Q000502:physiology*; D009419:Nerve Tissue Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D016716:PC12 Cells; D019269:Phosphatidylinositol 4,5-Diphosphate / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D000072417:Protein Domains; D055503:Protein Multimerization / Q000502:physiology*; D050682:Q-SNARE Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D051381:Rats; D022142:Secretory Vesicles / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*
chemical_list: C485812:Cadps protein, rat; D002135:Calcium-Binding Proteins; D009419:Nerve Tissue Proteins; D019269:Phosphatidylinositol 4,5-Diphosphate; D050682:Q-SNARE Proteins; C103912:Unc13a protein, rat
keywords: SNARE proteins; dimerization; exocytosis; inositol phospholipid; vesicles