Together with STX4 and VAMP2, may play a role in insulin-dependent movement of GLUT4 and in docking/fusion of intracellular GLUT4-containing vesicles with the cell surface in adipocytes. Interacts with DOC2B; the interaction is direct, occurs at the cell membrane, excludes interaction with STX4 and regulates glucose-stimulated insulin secretion (By similarity). Interacts with STX4. In platelets, predominantly cytosolic. Low amounts membrane-associated. Megakaryocytes and platelets. Phosphorylated by PKC in platelets in response to thrombin stimulation; phosphorylation inhibits binding to STX4. Belongs to the STXBP/unc-18/SEC1 family.