Inhibits translocation of GLUT4 from intracellular vesicles to the plasma membrane (By similarity). 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. Competes with STXBP1 for STX1 binding. Part of a complex that contains STXBP5, STX4A and SNAP23 (By similarity). Interacts with STX1A and STX4A via its v-SNARE homology domain. Part of a complex that contains STX1, STXBP5, SNAP25 and SYT1. Cytoplasmic, and associated with vesicular membranes and the plasma membrane. Detected at synapses and on synaptic vesicles. Isoform 1 is detected in heart, brain, lung, liver, skeletal muscle, kidney and testis. Isoform 2 is detected in brain and in testis. Isoform 3 is detected in testis. Phosphorylation by PKA reduces interaction with STX1A and enhances synaptic neurotransmitter release. Belongs to the WD repeat L(2)GL family.