Plays a role in SNARE-pin assembly and Golgi-to-ER retrograde transport via its interaction with COG4. Involved in vesicular transport between the endoplasmic reticulum and the Golgi. Interacts with STX17 (Probable). Interacts with the COG complex via COG4. Interacts with STX5A. Highly expressed in testis. Detected at lower levels in brain, astrocytes, heart and small intestine. Up-regulated in astrocytes upon reoxygenation after hypoxia. Belongs to the STXBP/unc-18/SEC1 family.