ATP-dependent inositide phosphorylation required for Ca2+-activated secretion

REGULATED fusion of secretory granules with the plasma membrane in secretory cells requires ATP, Ca2+ and cytosolic1-3 as well as membrane4 proteins. ATP-dependent steps in Ca2+-activated secretion from PC 12 cells require three cytosolic PEP proteins (priming in exocytosis proteins, PEP1-3)5,6, the identity of which will provide insights into the required ATP-using reactions. PEP3 was recently identified as phosphatidylinositol transfer protein (PtdlnsTP)6, and here we report that PEP1 consists of the type I phosphatidylinositol-4-phosphate 5-kinase (PtdlnsPSK). The roles of PEP3/PtdInsTP and PEP1/ PtdlnsPSK in sequential phosphoinositide recruitment and phosphorylation explains their synergistic activity in ATP-dependent priming. Moreover, inhibition of Ca2+-activated secretion by PtdIns(4,5)P ₂-specific antibodies and phospholipase C implies that 5-phosphorylated inositides play a novel, necessary role in the regulated secretory pathway. The results indicate that lipid kinase-mediated phosphorylation is an important basis for ATP use in the exocytotic pathway.