Plays a role in vesicle maturation during exocytosis as a target of the diacylglycerol second messenger pathway. Involved in neurotransmitter release by acting in synaptic vesicle priming prior to vesicle fusion and participates in the activity-dependent refilling of readily releasable vesicle pool (RRP). Essential for synaptic vesicle maturation in most excitatory/glutamatergic but not inhibitory/GABA-mediated synapses. Facilitates neuronal dense core vesicles fusion as well as controls the location and efficiency of their synaptic release (By similarity). Also involved in secretory granule priming in insulin secretion. Plays a role in dendrite formation by melanocytes (By similarity). Interacts with the N-termini of STX1A and/or STX1B1 and DOC2A (PubMed:8999968, PubMed:9195900, PubMed:9736751). Interacts with BSN (PubMed:12163476, PubMed:14734538). Interacts with RIMS1 which recruits UNC13A to the active zone (PubMed:11343654, PubMed:16704978). Forms homodimers via its first C2 domain. Also interacts via this domain with the zinc finger domain of RIMS2 (PubMed:16052212, PubMed:16732694). Part of a complex consisting of ERC2, RIMS1 and UNC13A (PubMed:12163476). Also part of a complex consisting of UNC13A, RIMS2 and RAB3A. Interacts with FBXO45 (via SRY domain); leading to the degradation of UNC13A by the proteasome (By similarity). Translocated to the plasma membrane in response to phorbol ester binding. Expressed in brain, with highest levels in the olfactory bulb, striatum, cerebral cortex, hippocampus and cerebellum. Also expressed in pancreatic islet cells. First detected at birth, after which expression level is steadily increasing until it reaches a plateau at P15. The C2 domains are not involved in calcium-dependent phospholipid binding. The C-terminal region containing both MHD domains and the third C2 domain is required for synaptic vesicle priming activity. Belongs to the unc-13 family.