Involved in vesicular protein trafficking. Mainly functions in the early secretory pathway but also in post-Golgi membranes. 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 p24delta5 at endoplasmic reticulum export sites for endoplasmic reticulum exit and coupled transport to the Golgi apparatus. 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 p24delta5. Cycles between the endoplasmic reticulum and Golgi via COPI and COPII dependent pathways. The cytoplasmic C-terminal domain contains an Arg-Val motif, which is involved in the anterograde ER-to-Golgi transport of the protein. Belongs to the EMP24/GP25L family.