Mechanism of large protein secretion regulated by small GTPase and complex localized at the ER

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Collagen synthesized in the ER is too big to fit into conventional transport COPII carriers. How collagen exports from the ER is still unclear. We have previously identified cTAGE5/TANGO1 complex as a cargo receptor for collagen VII at ER exit sites. Here we show that cTAGE5 interacts with Sec12, a guanine-nucleotide exchange factor of Sar1 at ER exit sites. Upon cTAGE5 depletion, Sec12 delocalizes from the ER exit sites. These results imply that collagen VII secretion from the ER is regulated not only by cTAGE5/TANGO1 complex but with the activity of Sar1 small GTPase.