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 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 (By similarity). Interacts with cytoplasmic dynein light chain DYNLT1. 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. May interact with UNC13A; the interaction mediates targeting to the plasma membrane. Translocates to the plasma membrane in a calcium-dependent manner. Expressed in brain; highly enriched in neurons. C2 domain 1 is involved in binding calcium and phospholipids. C2 domain 2 may also play a role in the calcium-dependent targeting to membranes (By similarity).