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 neu-rons and neuroendocrine cells. CAPSs contain a pleckstrin homology (PH) domain that binds PI(4,5)P2-mem-brane. 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 sug-gest that the C2 and PH domains constitute an effective unit to promote Ca2+-regulated exocytosis.