Phosphatidylinositol 4,5-bisphosphate regulates SNARE-dependent membrane fusion

Phosphatidylinositol 4,5-bisphosphate (PI 4,5-P-2) on the plasma membrane is essential for vesicle exocytosis but its role in membrane fusion has not been determined. Here, we quantify the concentration of PI 4,5-P-2 as similar to 6 mol% in the cytoplasmic lea. et of plasma membrane microdomains at sites of docked vesicles. At this concentration of PI 4,5-P-2 soluble NSF attachment protein receptor (SNARE)-dependent liposome fusion is inhibited. Inhibition by PI 4,5-P-2 likely results from its intrinsic positive curvature-promoting properties that inhibit formation of high negative curvature membrane fusion intermediates. Mutation of juxtamembrane basic residues in the plasma membrane SNARE syntaxin-1 increase inhibition by PI 4,5-P-2, suggesting that syntaxin sequesters PI 4,5-P-2 to alleviate inhibition. To de. ne an essential rather than inhibitory role for PI 4,5-P-2, we test a PI 4,5-P-2-binding priming factor required for vesicle exocytosis. Ca2+-dependent activator protein for secretion promotes increased rates of SNARE-dependent fusion that are PI 4,5-P-2 dependent. These results indicate that PI 4,5-P-2 regulates fusion both as a fusion restraint that syntaxin-1 alleviates and as an essential cofactor that recruits protein priming factors to facilitate SNARE-dependent fusion.