Participates in the regulation of synaptic vesicle docking and fusion through interaction with GTP-binding proteins (PubMed:21689256). Essential for neurotransmission and binds syntaxin, a component of the synaptic vesicle fusion machinery probably in a 1:1 ratio. Can interact with syntaxins 1, 2, and 3 but not syntaxin 4. May play a role in determining the specificity of intracellular fusion reactions. Interacts with SYTL4 (PubMed:12058058). Interacts with STX1A (PubMed:10746715). Interacts with alpha-synuclein/SNCA; this interaction controls SNCA self-replicating aggregation (By similarity). Interacts with RAB3A; this interaction promotes RAB3A dissociation from the vesicle membrane (PubMed:21689256). Interacts with CABP5 (By similarity). Brain and spinal cord. Highly enriched in axons. Faint levels are detectable at embryonic day 14, with levels rising at later embryonic ages and peaking at postnatal day 7. Belongs to the STXBP/unc-18/SEC1 family.