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Ca2+-dependent activator protein (also called CAPS) Pleckstrin homology (PH) domain CADPS/CAPS consists of two members, CAPS1 which regulates catecholamine release from neuroendocrine cells and CAPS2 which is involved in the release of two neurotrophins, brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) from cerebellar granule cells. CADPS plays an important role in vesicle exocytosis in neurons and endocrine cells where it functions to prime the exocytic machinery for Ca2+-triggered fusion. Priming involves the assembly of trans SNARE complexes. The initial interaction of vesicles with target membranes is mediated by diverse stage-specific tethering factors or multi-subunit tethering complexes. CADPS and Munc13 proteins are proposed to be the functional homologs of the stage-specific tethering factors that prime membrane fusion. Interestingly, regions in the C-terminal half of CADPS are similar to the C-terminal region of Munc13-1 that was reported to bind syntaxin-1. CADPS has independent interactions with each of the SNARE proteins (Q-SNARE and R-SNARE) required for vesicle fusion. CADPS interacts with Q-SNARE proteins syntaxin-1 (H3 SNARE) and SNAP-25 (SN1) and might promote Q-SNARE heterodimer formation. Through its N-terminal R-SNARE VAMP-2 interactions, CADPS bound to heterodimeric Q-SNARE complexes could be involved in catalyzing the zippering of VAMP-2 into recipient complexes. It also contains a central PH domain that binds to phosphoinositide 4,5 bisphosphate containing liposomes. Membrane association may also be mediated by binding to phosphatidlyserine via general electrostatic interactions. PH domains have diverse functions, but in general are involved in targeting proteins to the appropriate cellular location or in the interaction with a binding partner. They share little sequence conservation, but all have a common fold, which is electrostatically polarized. Less than 10% of PH domains bind phosphoinositide phosphates (PIPs) with high affinity and specificity. PH domains are distinguished from other PIP-binding domains by their specific high-affinity binding to PIPs with two vicinal phosphate groups: PtdIns(3,4)P2, PtdIns(4,5)P2 or PtdIns(3,4,5)P3 which results in targeting some PH domain proteins to the plasma membrane. A few display strong specificity in lipid binding. Any specificity is usually determined by loop regions or insertions in the N-terminus of the domain, which are not conserved across all PH domains. PH domains are found in cellular signaling proteins such as serine/threonine kinase, tyrosine kinases, regulators of G-proteins, endocytotic GTPases, adaptors, as well as cytoskeletal associated molecules and in lipid associated enzymes. |
Jiyao W et al.(2023). "The conserved domain database in 2023.",