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© The Rockefeller University Press, 0021-9525/1998//887 $5.00
The Journal of Cell Biology, Volume 141, Number 4, , 1998 887-894


Articles

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



Kai-Uwe Kalies*, Tom A. Rapoport{ddagger}, and Enno Hartmann*

* Abteilung Biochemie II, Zentrum Biochemie und Molekulare Zellbiologie, Georg-August-Universität, 37073 Göttingen, Germany; and {ddagger} Harvard Medical School, Department of Cell Biology, Boston, Massachusetts 02115

The Sec61 complex is the central component of the protein translocation apparatus of the ER membrane. We have addressed the role of the β subunit (Sec61β) during cotranslational protein translocation. With a reconstituted system, we show that a Sec61 complex lacking Sec61β 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 Sec61β kinetically facilitates cotranslational translocation, but is not essential for it.

Using chemical cross-linking we show that Sec61β 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 Sec61β and SPC25 and between Sec61β and Sec61{alpha} 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.


Abbreviations used in this paper: BMH, bis-maleimidohexane; PK-RM, puromycin and high salt–treated rough microsomes; RM, rough microsomes; SPC, signal peptidase complex; SPC25, the 25-kD subunit of the signal peptidase complex; SRP, signal recognition particle; TRAM, translocating chain–associating membrane.

The work was supported by a grant from the Deutsche Forschungsgemeinschaft.

Address all correspondence to Dr. E. Hartmann, Abteilung Biochemie II, Zentrum Biochemie und Molekulare Zellbiologie, Georg-August-Universität, Gossler Strasse 12 d, 37073 Göttingen, Germany. Tel.: (49) (551) 395973. Fax: (49) (30) 94063363.



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