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 (By similarity). Interacts with STX17. Interacts with STX5A. Interacts with the COG complex via COG4 (By similarity). Belongs to the STXBP/unc-18/SEC1 family.