Plays a role in vesicle trafficking and exocytosis inhibition. In pancreatic beta-cells, inhibits insulin secretion probably by interacting with and regulating STX1A and STX4, key t-SNARE proteins involved in the fusion of insulin granules to the plasma membrane. Also plays a role in neurotransmitter release by inhibiting basal acetylcholine release from axon terminals and by preventing synaptic fatigue upon repetitive stimulation (By similarity). Promotes as well axonal outgrowth (PubMed:25504045). . This is the function of ENSG00000145087 (STXBP5L, syntaxin binding protein 5L).