|The beta subunit of the Sec61 complex facilitates cotranslational protein transport and interacts with the signal peptidase during translocation. |
Kalies, K U, et al.
J. Cell Biol., 141: 887-94 (1998)
The Sec61 complex is the central component of the protein translocation apparatus of the ER membrane. We have addressed the role of the beta subunit (Sec61beta) during cotranslational protein translocation. With a reconstituted system, we show that a Sec61 complex lacking Sec61beta 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 Sec61beta kinetically facilitates cotranslational translocation, but is not essential for it. Using chemical cross-linking we show that Sec61beta 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 Sec61beta and SPC25 and between Sec61beta and Sec61alpha 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 activity.
|Protein translocation into proteoliposomes reconstituted from purified components of the endoplasmic reticulum membrane. |
Görlich, D and Rapoport, T A
Cell, 75: 615-30 (1993)
We have reproduced the process of protein transport across and of protein integration into the mammalian endoplasmic reticulum membrane by the use of proteoliposomes reconstituted from pure phospholipids and purified membrane proteins. The transport of some proteins requires only two membrane protein complexes: the signal recognition particle receptor, needed for targeting of a nascent chain to the membrane, and a novel complex, the Sec61p complex, that consists of Sec61p and two smaller polypeptides. The translocation of other proteins also needs the presence of the translocating chain-association membrane (TRAM) protein. The integration of two membrane proteins of different topologies into the membrane does not require additional components. These results indicate a surprising simplicity of the basic translocation machinery. They suggest that the Sec61p complex binds the ribosome during translocation and forms the postulated protein-conducting channel.