Appears to be involved in vesicular protein trafficking, mainly in the early secretory pathway. In COPI vesicle-mediated retrograde transport involved in the coatomer recruitment to membranes of the early secretory pathway. Increases coatomer-dependent activity of ARFGAP2. Thought to play a crucial role in the specific retention of p24 complexes in cis-Golgi membranes; specifically contributes to the coupled localization of TMED2 and TMED10 in the cis-Golgi network. May be involved in organization of intracellular membranes, such as of the ER-Golgi intermediate compartment and the Golgi apparatus. Involved in ER localization of PTPN2 (By similarity). Monomer and homodimer in endoplasmic reticulum. Predominantly monomeric and to lesser extent homodimeric in endoplasmic reticulum-Golgi intermediate compartment and cis-Golgi network. Probably oligomerizes with other members of the EMP24/GP25L family such as TMED2, TMED7 and TMED10. Interacts with TMED5. Interacts (via C-terminus) with COPG1; the interaction involves dimeric TMED9. Interacts with PTPN2 and SPAST. Interacts with STX17; the interaction is direct (By similarity). Cycles between compartments of the early secretatory pathway. N-linked glycosylated containing high mannose. Belongs to the EMP24/GP25L family.