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J. Biol. Chem., Vol. 280, Issue 50, 41262-41269, December 16, 2005

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The Yeast Hsp110 Sse1 Functionally Interacts with the Hsp70 Chaperones Ssa and Ssb^*

Lance Shaner, Harald Wegele, Johannes Buchner¶, and Kevin A. Morano1

From the Department of Microbiology and Molecular Genetics, University of Texas Medical School, Houston, Texas 77030, Pharma Development, Roche Diagnostics GmbH, Nonnenwald 2, 82377, Penzberg, Germany, and the ^¶Institut fuer Organische Chemie & Biochemie, Technische Universitaet Muenchen, Lichtenbergstrasse 4, 85747 Garching, Germany

There is growing evidence that members of the extended Hsp70 family of molecular chaperones, including the Hsp110 and Grp170 subgroups, collaborate in vivo to carry out essential cellular processes. However, relatively little is known regarding the interactions and cellular functions of Sse1, the yeast Hsp110 homolog. Through co-immunoprecipitation analysis, we found that Sse1 forms heterodimeric complexes with the abundant cytosolic Hsp70s Ssa and Ssb in vivo. Furthermore, these complexes can be efficiently reconstituted in vitro using purified proteins. Binding of Ssa or Ssb to Sse1 was mutually exclusive. The ATPase domain of Sse1 was found to be critical for interaction as inactivating point mutations severely reduced interaction with Ssa and Ssb. Sse1 stimulated Ssa1 ATPase activity synergistically with the co-chaperone Ydj1, and stimulation required complex formation. Ssa1 is required for post-translational translocation of the yeast mating pheromone -factor into the endoplasmic reticulum. Like ssa mutants, we demonstrate that sse1 cells accumulate prepro--factor, but not the co-translationally imported protein Kar2, indicating that interaction between Sse1 and Ssa is functionally significant in vivo. These data suggest that the Hsp110 chaperone operates in concert with Hsp70 in yeast and that this collaboration is required for cellular Hsp70 functions.

Received for publication, April 1, 2005 , and in revised form, August 9, 2005.

^* This work was supported by Research Scholar Grant MBC-103134 from the American Cancer Society (to K. A. M.) and grants from the Deutsche Forschungsgemeinschaft (to J. B.). 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 To whom correspondence should be addressed: Dept. of Microbiology and Molecular Genetics, University of Texas Medical School, 6431 Fannin St., Houston, TX 77030. Tel.: 713-500-5890; Fax: 713-500-5499; E-mail: kevin.a.morano{at}uth.tmc.edu.

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