Plays a role in the transport of cargos that are too large to fit into COPII-coated vesicles and require specific mechanisms to be incorporated into membrane-bound carriers and exported from the endoplasmic reticulum. Plays a role in the secretion of lipoproteins, pre-chylomicrons and pre-VLDLs, by participating in their export from the endoplasmic reticulum (By similarity). Thereby, may play a role in cholesterol and triglyceride homeostasis (PubMed:21807889). Required for collagen VII (COL7A1) secretion by loading COL7A1 into transport carriers and recruiting PREB/SEC12 at the endoplasmic reticulum exit sites (By similarity). Interacts with MIA3 (By similarity). Interacts with the COPII coat subunits SEC23A, SEC23B and maybe SEC24C (By similarity) (PubMed:21807889). Interacts with PREB; recruits PREB to endoplasmic reticulum exit sites. Interacts with APOB (By similarity). Localizes to endoplasmic reticulum exit sites (ERES), also known as transitional endoplasmic reticulum (tER). Isoform 1 is expressed in liver (at protein level) (PubMed:12586826, PubMed:21807889). Isoform 2 is highly expressed in liver and weakly in testis (PubMed:12586826). Expression is seen at 12.5 dpc and 14.5 dpc embryonic stages. The proline-rich domain (PRD) contains repeated PPP motifs. A single PPP motif is necessary and sufficient to mediate interaction with the COPII coat subunits SEC23A and SEC23B (PubMed:21807889). The coiled coil domains mediate interaction with MIA3. The first coiled coil domain mediates interaction with PREB (By similarity). Readthrough transcript producing a functional fusion protein MIA2-CTAGE5 with similarity to MIA3. Belongs to the MIA/OTOR family.