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J. Biol. Chem., Vol. 281, Issue 12, 7899-7906, March 24, 2006

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The Brl Domain in Sec63p Is Required for Assembly of Functional Endoplasmic Reticulum Translocons^*

From the Faculty of Life Sciences, Michael Smith Building, University of Manchester, Oxford Road, Manchester M13 9PT, United Kingdom

Protein translocation into the endoplasmic reticulum occurs at pore-forming structures known as translocons. In yeast, two different targeting pathways converge at a translocation pore formed by the Sec61 complex. The signal recognition particle-dependent pathway targets nascent precursors co-translationally, whereas the Sec62p-dependent pathway targets polypeptides post-translationally. In addition to the Sec61 complex, both pathways also require Sec63p, an integral membrane protein of the Hsp40 family, and Kar2p, a soluble Hsp70 located in the ER lumen. Using a series of mutant alleles, we demonstrate that a conserved Brl (Brr2-like) domain in the COOH-terminal cytosolic region of Sec63p is essential for function both in vivo and in vitro. We further demonstrate that this domain is required for assembly of two oligomeric complexes of 350 and 380 kDa, respectively. The larger of these corresponds to the heptameric "SEC complex" required for post-translational translocation. However, the 350-kDa complex represents a newly defined hexameric SEC' complex comprising Sec61p, Sss1p, Sbh1p, Sec63p, Sec71p, and Sec72p. Our data indicate that the SEC' complex is required for co-translational protein translocation across the yeast ER membrane.

Received for publication, October 20, 2005 , and in revised form, December 13, 2005.

^* The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ Supported by studentships from the Biotechnology and Biological Sciences Research Council.

² Supported by a grant from the Wellcome Trust.

³ To whom correspondence should be addressed. Tel.: 44-161-275-5104; Fax: 44-161-275-5082; E-mail: colin.stirling{at}manchester.ac.uk.

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