The Vesicle Priming Factor CAPS Functions as a Homodimer via C2 Domain Interactions to Promote Regulated Vesicle Exocytosis.


Type
Article
Change log
Authors
Petrie, Matt 
Esquibel, Joseph 
Kabachinski, Greg 
Maciuba, Stephanie 
Takahashi, Hirohide 
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)P2 and 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.

Description
Keywords
SNARE proteins, dimerization, exocytosis, inositol phospholipid, vesicles, Animals, Calcium-Binding Proteins, Exocytosis, Nerve Tissue Proteins, PC12 Cells, Phosphatidylinositol 4,5-Diphosphate, Protein Domains, Protein Multimerization, Q-SNARE Proteins, Rats, Secretory Vesicles
Journal Title
J Biol Chem
Conference Name
Journal ISSN
0021-9258
1083-351X
Volume Title
291
Publisher
Elsevier BV