SPECIFIC BINDING OF PHOSPHATIDYLINOSITOL 4,5-BISPHOSPHATE TO CALCIUM-DEPENDENT ACTIVATOR PROTEIN FOR SECRETION (CAPS), A POTENTIAL PHOSPHOINOSITIDE EFFECTOR PROTEIN FOR REGULATED EXOCYTOSIS

The calcium-dependent activator protein for secretion (CAPS) is a nove l neural/endocrine-specific cytosolic and peripheral membrane protein required for the Ca2+-regulated exocytosis of secretory vesicles, CAPS acts at a stage in exocytosis that follows ATP-dependent priming, whi ch involves the essential synthesis of phosphatidylinositol 4,5-bispho sphate (PtdIns(4,5)P-2). In the present studies, CAPS is shown to bind liposomes that contain acidic phospholipids and binding was markedly enhanced by inclusion of PtdIns(4,5)P-2 but not other phosphoinositide s in the absence of Ca2+. PtdIns(4,5)P-2, but not other phosphoinositi des including PtdIns(3,4)P-2 and PtdIns(3,4,5)P-3, altered the suscept ibility of CAPS to proteolysis by trypsin and proteinase K, suggesting that phosphoinositide binding promoted a conformational change, Photo affinity labeling studies with a photoactivatable benzoylcinnimidyl ac yl chain derivative of PtdIns(4,5)P-2 confirmed the phosphoinositide-b inding properties of CAPS and suggested a hydrophobic aspect of the in teraction, CAPS, as one of very few characterized proteins with a bind ing specificity for 4-,5-phosphorylated inositides over 3-phosphorylat ed inositides, may function in regulated exocytosis as an effector of PtdIns(4,5)P-2.