Calcium sensor which positively regulates SNARE-dependent fusion of vesicles with membranes. Binds phospholipids in a calcium-dependent manner and may act at the priming stage of fusion by modifying membrane curvature to stimulate fusion. Involved in calcium-triggered exocytosis in chromaffin cells and calcium-dependent spontaneous release of neurotransmitter in absence of action potentials in neuronal cells. Involved both in glucose-stimulated insulin secretion in pancreatic cells and insulin-dependent GLUT4 transport to the plasma membrane in adipocytes. Interacts with cytoplasmic dynein light chain DYNLT1. May interact with UNC13A; the interaction mediates targeting to the plasma membrane (By similarity). Probably interacts with the SNARE (soluble N-ethylmaleimide-sensitive factor attached protein receptor) complex composed of SNAP25, STX1A and VAMP2; the interaction is calcium-dependent and competitive with SYT1. Interacts with STX4; the interaction is calcium-dependent, increased by insulin and glucose, and mediates vesicle fusion with plasma membrane in pancreatic cells and adipocytes. Interacts with STXBP3; the interaction is direct, occurs at the cell membrane and regulates glucose-stimulated insulin secretion. Translocates to the plasma membrane in a calcium-dependent manner. Widely expressed. Expressed in pancreatic islet cells (at protein level). C2 domain 1 is involved in binding calcium and phospholipids. According to PubMed:19033398, the C2 domain 2 may also play a role in the calcium-dependent targeting to membranes. Mice are viable and fertile without gross abnormalities.