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 (PubMed:27138255, PubMed:21525241, PubMed:25202031, PubMed:27170179). Plays a role in the secretion of lipoproteins, pre-chylomicrons and pre-VLDLs, by participating in their export from the endoplasmic reticulum (PubMed:27138255). Thereby, may play a role in cholesterol and triglyceride homeostasis (By similarity). Required for collagen VII (COL7A1) secretion by loading COL7A1 into transport carriers and recruiting PREB/SEC12 at the endoplasmic reticulum exit sites (PubMed:21525241, PubMed:25202031, PubMed:27170179). Interacts with MIA3 (PubMed:21525241, PubMed:25202031, PubMed:27138255, PubMed:27170179). Interacts with the COPII coat subunits SEC23A, SEC23B and maybe SEC24C (PubMed:21525241, PubMed:27551091). Interacts with PREB; recruits PREB to endoplasmic reticulum exit sites (PubMed:25202031, PubMed:27170179). Interacts with APOB (PubMed:27138255). Localizes to endoplasmic reticulum exit sites (ERES), also known as transitional endoplasmic reticulum (tER). Highly expressed in liver and weakly in testis. Expression was higher in patients with severe fibrosis or inflammation and chronic hepatitis (PubMed:12586826). Isoform 1 is specifically expressed in lung, testis, small intestine, colon, pancreas, kidney, liver and prostate (PubMed:27138255). Isoform 8 is expressed only in testis (at the protein level). Isoform 8 (at protein level) and isoform 9 are expressed in cutaneous T-cell lymphoma (CTCL) cell lines, colorectal carcinomas, breast carcinomas and melanoma. Isoform 9, but not isoform 5A, is expressed in head and neck squamous cell carcinoma (PubMed:12839582). 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:21525241, PubMed:27551091). The coiled coil domains mediate interaction with MIA3 (PubMed:21525241). The first coiled coil domain mediates interaction with PREB (PubMed:25202031). Autoantibodies against MIA2 are present in several cancer types, including benign meningioma and cutaneous T-cell lymphoma (CTCL). Readthrough transcript producing a functional fusion protein MIA2-CTAGE5 with similarity to MIA3. Belongs to the MIA/OTOR family. Extended N-terminus. Probable cloning artifact. Truncated N-terminus.