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J. Biol. Chem., Vol. 282, Issue 42, 30618-30628, October 19, 2007

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The Transmembrane Domain Is Sufficient for Sbh1p Function, Its Association with the Sec61 Complex, and Interaction with Rtn1p^*

Dejiang Feng¹², Xueqiang Zhao¹, Christina Soromani^¶3, Jaana Toikkanen, Karin Römisch^¶4, Shruthi S. Vembar^||, Jeffrey L. Brodsky^||5, Sirkka Keränen, and Jussi Jäntti⁶

From the VTT Biotechnology, FI-02044 VTT, Espoo, Finland, the Institute of Biotechnology, P. O. Box 56, 00014 University of Helsinki, Helsinki, Finland, the ^¶University of Cambridge, Cambridge Institute for Medical Research and Department of Clinical Biochemistry, Hills Road, Cambridge CB2 2XY, United Kingdom, and the ^||Department of Biological Sciences, University of Pittsburgh, Pittsburgh, Pennsylvania 15260

The Sec61 protein translocation complex in the endoplasmic reticulum (ER) membrane is composed of three subunits. The -subunit, called Sec61p in yeast, is a multispanning membrane protein that forms the protein conducting channel. The functions of the smaller, carboxyl-terminally tail-anchored subunit Sbh1p, its close homologue Sbh2p, and the subunit Sss1p are not well understood. Here we show that co-translational protein translocation into the ER is reduced in sbh1 sbh2 cells, whereas there is a limited reduction of post-translational tranlocation and no effect on export of a mutant form of -factor precursor for ER-associated degradation in the cytosol. The translocation defect and the temperature-sensitive growth phenotype of sbh1 sbh2 cells were rescued by expression of the transmembrane domain of Sbh1p alone, and the Sbh1p transmembrane domain was sufficient for coimmunoprecipitation with Sec61p and Sss1p. Furthermore, we show that Sbh1p co-precipitates with the ER transmembrane protein Rtn1p. Sbh1p-Rtn1p complexes do not appear to contain Sss1p and Sec61p. Our results define the transmembrane domain as the minimal functional domain of the Sec61 homologue Sbh1p in ER translocation, identify a novel interaction partner for Shb1p, and imply that Sbh1p has additional functions that are not directly linked to protein translocation in association with the Sec61 complex.

Received for publication, March 2, 2007 , and in revised form, August 14, 2007.

^* This work was supported in part by Academy of Finland Grants 52096 and 204373 (to S. K.), Academy Research Fellow Grant 211171 (to J. J.), and the Magnus Ehrnrooth Foundation (to J. J.), and was part of the research program "VTT Industrial Biotechnology" supported by the Academy of Finland, Finnish Center of Excellence Program, 2000–2005, Project 64330. 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.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Tables S1 and S2 and Figs. S1 and S2.

¹ Both authors contributed equally to this work.

² Present address: Children's Hospital Oakland Research Institute, 5700 Martin Luther King Jr. Way, Oakland, CA 94609.

³ Supported by a Medical Research Council studentship.

⁴ Supported by Senior Fellowship 042216 from The Wellcome Trust.

⁵ Supported by Grant GM75061 from the National Institutes of Health.

⁶ To whom correspondence should be addressed: Institute of Biotechnology, P.O. Box 56, 00014 University of Helsinki, Helsinki, Finland. Tel.: 358-9-19159722; Fax: 358-9-19159570; E-mail: jussi.jantti{at}helsinki.fi.

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