The C₂ and PH domains of CAPS constitute an effective PI(4,5)P2-binding unit essential for Ca²⁺-regulated exocytosis

Ca²⁺-dependent activator proteins for secretion (CAPSs) are required for Ca²⁺-regulated exocytosis in neurons and neuroendocrine cells. CAPSs contain a pleckstrin homology (PH) domain that binds PI(4,5)P2-membrane. There is also a C₂ domain residing adjacent to the PH domain, but its function remains unclear. In this study, we solved the crystal structure of the CAPS-1 C₂PH module. The structure showed that the C₂ and PH tandem packs against one another mainly via hydrophobic residues. With this interaction, the C₂PH module exhibited enhanced binding to PI(4,5)P2-membrane compared with the isolated PH domain. In addition, we identified a new PI(4,5)P2-binding site on the C₂ domain. Disruption of either the tight interaction between the C₂ and PH domains or the PI(4,5)P2-binding sites on both domains significantly impairs CAPS-1 function in Ca²⁺-regulated exocytosis at the Caenorhabditis elegans neuromuscular junction (NMJ). These results suggest that the C₂ and PH domains constitute an effective unit to promote Ca²⁺-regulated exocytosis.