Plays a regulatory role in calcium-dependent exocytosis and neurotransmitter release. Inhibits membrane fusion between transport vesicles and the plasma membrane. May modulate the assembly of trans-SNARE complexes between transport vesicles and the plasma membrane. Inhibits translocation of GLUT4 from intracellular vesicles to the plasma membrane. Competes with STXBP1 for STX1 binding (By similarity). Interacts with STX1A and STX1B via its v-SNARE homology domain. Part of a complex that contains STX1, STXBP5, SNAP25 and SYT1. Part of a complex that contains STXBP5, STX4A and SNAP23 (By similarity). Cytoplasmic, and associated with vesicular membranes and the plasma membrane. Detected at synapses and on synaptic vesicles (By similarity). Belongs to the WD repeat L(2)GL family.