The b Subunit of the Sec61 Complex Facilitates Cotranslational Protein Transport and Interacts with the Signal Peptidase during Translocation

The Sec61 complex is the central component of the protein translocation apparatus of the ER membrane. We have addressed the role of the b subunit (Sec61 b ) during cotranslational protein translocation. With a reconstituted system, we show that a Sec61 complex lacking Sec61 b is essentially inactive when elongation and membrane targeting of a nascent chain occur at the same time. The translocation process is perturbed at a step where the nascent chain would be inserted into the translocation channel. However, if sufficient time is given for the interaction of the nascent polypeptide with the mutant Sec61 complex, translocation is almost normal. Thus Sec61 b kinetically facilitates cotranslational translocation, but is not essential for it. Using chemical cross-linking we show that Sec61 b not only interacts with subunits of the Sec61 complex but also with the 25-kD subunit of the signal peptidase complex (SPC25), thus demonstrating for the first time a tight interaction between the SPC and the Sec61 complex. Interestingly, the cross-links between Sec61 b and SPC25 and between Sec61 b and Sec61 a depend on the presence of membrane-bound ribosomes, suggesting that these interactions are induced when translocation is initiated. We propose that the SPC is transiently recruited to the translocation site, thus enhancing its ac-