Syntaxin-binding protein 3, also known as Platelet Sec1 protein, Protein unc-18 homolog 3, Protein unc-18 homolog C, Unc-18C, Unc18-3 and STXBP3, is a cytoplasm protein which belongs to the STXBP/unc-18/SEC1 family. STXBP3 is expressed in cells that exhibit granule exocytosis, such as neutrophils, mast cells, platelets and endothelial cells. STXBP3, 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. STXBP3 participates in the consolidation and secretion of secondary and tertiary granules. STXBP3 contains one SEC1 domain. Phosphorylation at Ser129 may stimulate granule release. Human STXBP3 shares 90% aa identity with mouse STXBP3. STXBP3 interacts with DOC2B; the interaction is direct, occurs at the cell membrane, excludes interaction with STX4 and regulates glucose-stimulated insulin secretion. Interacts with STX4.