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J. Biol. Chem., Vol. 280, Issue 50, 41252-41261, December 16, 2005

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Hsp110 Cooperates with Different Cytosolic HSP70 Systems in a Pathway for de Novo Folding^*

Alice Yen-Wen Yam1, Véronique Albanèse12, Hen-Tzu Jill Lin, and Judith Frydman3

From the Department of Biological Sciences and BioX Program, Stanford University, Stanford, California 94305-5020 and University of California at San Francisco Mass Spectrometry Facility, University of California, San Francisco, California 94143

Molecular chaperones such as Hsp70 use ATP binding and hydrolysis to prevent aggregation and ensure the efficient folding of newly translated and stress-denatured polypeptides. Eukaryotic cells contain several cytosolic Hsp70 subfamilies. In yeast, these include the Hsp70s SSB and SSA as well as the Hsp110-like Sse1/2p. The cellular functions and interplay between these different Hsp70 systems remain ill-defined. Here we show that the different cytosolic Hsp70 systems functionally interact with Hsp110 to form a chaperone network that interacts with newly translated polypeptides during their biogenesis. Both SSB and SSA Hsp70s form stable complexes with the Hsp110 Sse1p. Pulse-chase analysis indicates that these Hsp70/Hsp110 teams, SSB/SSE and SSA/SSE, transiently associate with newly synthesized polypeptides with different kinetics. SSB Hsp70s bind cotranslationally to a large fraction of nascent chains, suggesting an early role in the stabilization of nascent chains. SSA Hsp70s bind mostly post-translationally to a more restricted subset of newly translated polypeptides, suggesting a downstream function in the folding pathway. Notably, loss of SSB dramatically enhances the cotranslational association of SSA with nascent chains, suggesting SSA can partially fulfill an SSB-like function. On the other hand, the absence of SSE1 enhances polypeptide binding to both SSB and SSA and impairs cell growth. It, thus, appears that Hsp110 is an important regulator of Hsp70-substrate interactions. Based on our data, we propose that Hsp110 cooperates with the SSB and SSA Hsp70 subfamilies, which act sequentially during de novo folding.

Received for publication, April 1, 2005 , and in revised form, August 8, 2005.

^* This work was supported by National Institutes of Health Grant GM56433. Mass spectrometry analysis at the University of California at San Francisco Mass Spectrometry Facility was supported by National Institutes of Health Grant RR01614. 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 These authors contributed equally to this work.

² Recipient of postdoctoral fellowships from the Fondation pour la Recherche Médicale and the Association pour la Recherche contre le Cancer, France.

³ To whom correspondence should be addressed. Tel.: 650-725-7833; Fax: 650-724-4927; E-mail: jfrydman{at}stanford.edu.

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