Plays a regulatory role in calcium-dependent exocytosis and neurotransmitter release (By similarity). 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. Competes with STXBP1 for STX1 binding. Inhibits translocation of GLUT4 from intracellular vesicles to the plasma membrane. Part of a complex that contains STX1, STXBP5, SNAP25 and SYT1 (By similarity). Interacts with STX1A and STX4A via its v-SNARE homology domain. Part of a complex that contains STXBP5, STX4A and SNAP23. Cytoplasmic, and associated with vesicular membranes and the plasma membrane. Detected in heart, spleen, lung, skeletal muscle, liver and kidney (at protein level). Detected in brain, particularly in the olfactory bulb and in hippocampus. Detected in the tenia tecta and in the piriform layer of the brain cortex. Belongs to the WD repeat L(2)GL family.