Calcium sensor which most probably regulates fusion of vesicles with membranes. Binds calcium and phospholipids. May be involved in calcium dependent neurotransmitter release through the interaction with UNC13A. May be involved in calcium-dependent spontaneous release of neurotransmitter in absence of action potentials in neuronal cells. Regulates Ca(2+)-dependent secretory lysosome exocytosis in mast cells. Interacts (via N-terminus) with UNC13A. Interacts with cytoplasmic dynein light chain DYNLT1 (By similarity). Interacts with UNC13D (By similarity). Brain and mast cells. C2 domain 1 is involved in binding calcium and phospholipids.