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. This protein is required for collagen VII (COL7A1) secretion by loading COL7A1 into transport carriers. It may participate in cargo loading of COL7A1 at endoplasmic reticulum exit sites by binding to COPII coat subunits Sec23/24 and guiding SH3-bound COL7A1 into a growing carrier. Does not play a role in global protein secretion and is apparently specific to COL7A1 cargo loading. However, it may participate in secretion of other proteins in cells that do not secrete COL7A1. It is also specifically required for the secretion of lipoproteins by participating in their export from the endoplasmic reticulum (PubMed:27138255, PubMed:19269366). Required for correct assembly of COPII coat components at endoplasmic reticulum exit sites (ERES) and for the localization of SEC16A and membrane-bound ER-resident complexes consisting of MIA2 and PREB/SEC12 to ERES (PubMed:28442536). Interacts with MIA2 (PubMed:21525241). Interacts (via SH3 domain) with COL7A1 (PubMed:19269366). Interacts with the COPII coat subunits SEC23A, SEC23B and maybe SEC24C (PubMed:19269366, PubMed:27551091, PubMed:28442536). May interact with APOB and MIA2 (PubMed:27138255). Isoform 1 and isoform 4 interact with SEC16A (PubMed:28442536). Localizes at endoplasmic reticulum exit sites (ERES), also known as transitional endoplasmic reticulum (tER) (PubMed:32101163). SEC16A is required for its proper localization to ERES. After loading of COL7A1 into transport carriers, it is not incorporated into COPII carriers and remains in the endoplasmic reticulum membrane. Broadly expressed, except in bone marrow and peripheral blood mononuclear cells. Down-regulated in melanoma tissue. 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. Although 2 transmembrane domains are predicted, PubMed:19269366 showed that it only contains one transmembrane domain. The other predicted transmembrane region is probably a hairpin-type region embedded into the membrane, which does not cross the membrane. It is unclear which of the 2 predicted transmembrane regions is the transmembrane or the hairpin-type region. The disease may be caused by variants affecting the gene represented in this entry. Belongs to the MIA/OTOR family. Tango1 subfamily. Truncated N-terminus. Truncated N-terminus.