Cotranslational Stabilization of Sec62/63 within the ER Sec61 Translocon Is Controlled by Distinct Substrate-Driven Translocation Events

The ER Sec61 translocon is a large macromolecular machine responsible for partitioning secretory and membrane polypeptides into the lumen, cytosol, and lipid bilayer. Because the Sec61 protein-conducting channel has been isolated in multiple membrane-derived complexes, we determined how the nascent polypeptide modulates translocon component associations during defined cotranslational translocation events. The model substrate preprolactin (pPL) was isolated principally with Sec61 alpha beta gamma upon membrane targeting, whereas higher-order complexes containing OST, TRAP, and TRAM were stabilized following substrate translocation. Blocking pPL translocation by passenger domain folding favored stabilization of an alternate complex that contained Sec61, Sec62, and Sec63. Moreover, Sec62/63 stabilization within the translocon occurred for native endogenous substrates, such as the prion protein, and correlated with a delay in translocation initiation. These data show that cotranslational translocon contacts are ultimately controlled by the engaged nascent chain and the resultant substrate-driven translocation events.