ARF6 regulates a plasma membrane pool of phosphatidylinositol(4,5)bisphosphate required for regulated exocytosis

A DP-ribosylation factor (ARF) 6 regulates endosomal plasma membrane trafficking in many cell types, but is also suggested to play a role in Ca2+-dependent dense-core vesicle (DCV) exocytosis in neuroendocrine cells. In the present work, expression of the constitutively active GTPase-defective ARF6(Q67L) mutant in PC12 cells was found to inhibit Ca2+-dependent DCV exocytosis. The inhibition of exocytosis was accompanied by accumulation of ARF(Q67L), phosphatidylinositol 4,5-bisphosphate (PIP2), and the phosphatidylinositol 4-phosphate 5-kinase type I (PIP5KI) on endosomal membranes with their corresponding depletion from the plasma membrane. That the depletion of PIP2 and PIP5K from the plasma membrane caused the inhibition of DCV exocytosis was demonstrated directly in permeable cell reconstitution studies in which overexpression or addition of PIP5KIgamma restored Ca2+-dependent exocytosis. The restoration of exocytosis in ARF6(Q67L)-expressing permeable cells unexpectedly exhibited a Ca2+ dependence, which was attributed to the dephosphorylation and activation of PIP5K. Increased Ca2+ and dephosphorylation stimulated the association of PIP5KIgamma with ARF6. The results reveal a mechanism by which Ca2+ influx promotes increased ARF6-dependent synthesis Of PIP2. We conclude that ARF6 plays a role in Ca2+-dependent DCV exocytosis by regulating the activity of PIP5K for the synthesis of an essential plasma membrane pool of PIP2.