Transport and Golgi organization protein 1 homolog (Mia3)

Plays a role in the transport of cargos that are too big 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 freight loading of COL7A1 in endoplasmic reticulum exit sites by binding to COPII coat subunits Sec23/24 and directing SH3-bound COL7A1 into a growing carrier. Doesn't play a part in global protein secretion and is apparently specific to COL7A1 freight loading.