Functions in intracellular vesicle trafficking through interaction with SNAREs (soluble N-ethylmaleimide-sensitive factor- attachment protein receptors) which are essential proteins for membrane fusion. By promoting the assembly of functional STX11/SNAP23/VAMP8 SNARE complexes at the plasma membrane, regulates cytotoxic granule exocytosis in natural killer (NK) cells. {ECO:0000269|PubMed:19804848, ECO:0000269|PubMed:19884660, ECO:0000269|PubMed:26771955, ECO:0000269|PubMed:28265073}. This is the function of STXBP2 (syntaxin binding protein 2, Ensembl gene identifier ENSG00000076944).