involved in the organization of the cytomatrix at the nerve terminals active zone (CAZ) which regulates neurotransmitter release. Essential in regulated neurotransmitter release from a subset of brain glutamatergic synapses. Involved in the formation of the retinal photoreceptor ribbon synapses. Interacts with CAST1, RIMS1 and UNC13A. Part of a complex consisting of ERC2, RIMS1 and BSN. Localized to the active zone of presynaptic density. Mice lacking functional BSN showed a reduced excitability attributed to inactivation of a fraction of brain glutamatergic synapses. At these synapses, vesicles are clustered and docked in normal numbers, but are unable to fuse. In retina, mutants lacking functional BSN showed normal retinal anatomy, but synapses lacked anchoring of the photoreceptor ribbon to the presynaptic active zone resulting in impaired photoreceptor synaptic transmission. Two alternatively spliced isoforms have been reported. Note: This description may include information from UniProtKB.