May participate in the regulation of synaptic vesicle docking and fusion, possibly through interaction with GTP-binding proteins. 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). Binds SYTL4 (By similarity). Interacts with STX1A (By similarity). Interacts with alpha-synuclein/SNCA; this interaction controls SNCA self-replicating aggregation (By similarity). Interacts with CABP5 (By similarity). Belongs to the STXBP/unc-18/SEC1 family.