Participates in the regulation of synaptic vesicle docking and fusion through interaction with GTP-binding proteins (By similarity). 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 (By similarity). Interacts with SYTL4 (By similarity). Interacts with STX1A (By similarity). 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 (By similarity). Interacts with CABP5 (By similarity). Expressed in the inner and outer plexiform layers of the retina (at protein level). Belongs to the STXBP/unc-18/SEC1 family.