By interacting with the collagen cargo receptor component cTAGE5, Sec12 concentrates at ER exit sites and generates the high levels of GTP-bound Sar1 necessary for export of collagen to the Golgi.

Two functionally irreplaceable and molecularly separable modules in cTAGE5 are both required for collagen VII export from the ER. The concentration of Sec12 induced by cTAGE5 serves for efficient production of activated Sar1 around ER exit sites, and the GTPase cycle of Sar1 seems to be required for collagen VII export from the ER.