Isoform 1 acts as a RAB27A effector protein and plays a role in cytotoxic granule exocytosis in lymphocytes. It is required for cytotoxic granule docking at the immunologic synapse. Isoform 4 binds phosphatidylserine (PS) and phosphatidylinositol-4,5-bisphosphate (PIP2) and promotes the recruitment of glucagon-containing granules to the cell membrane in pancreatic alpha cells. Binding to PS is inhibited by Ca2+ while binding to PIP2 is Ca2+ insensitive.