Involved in vesicular protein trafficking. Mainly functions in the early secretory pathway. Thought to act as cargo receptor at the lumenal side for incorporation of secretory cargo molecules into transport vesicles and to be involved in vesicle coat formation at the cytoplasmic side (By similarity). Interacts with p24beta2 at endoplasmic reticulum export sites for endoplasmic reticulum exit and coupled transport to the Golgi apparatus. Once in the Golgi, interacts very efficiently with the COPI machinery for retrograde transport back to the endoplasmic reticulum. Probably oligomerizes with other members of the EMP24/GP25L family (By similarity). Associates with the COPI vesicle coat (coatomer) (By similarity). Associates with the COPII vesicle coat (coatomer) (By similarity). Interacts with p24beta2. Cycles between the endoplasmic reticulum and Golgi via COPI and COPII dependent pathways. Mainly located in endoplasmic reticulum. The cytoplasmic C-terminal domain contains a functional dilysine-retrieval motif, which is involved in the retrograde Golgi-to-ER transport of the protein. Belongs to the EMP24/GP25L family.