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J. Biol. Chem., Vol. 281, Issue 44, 33739-33748, November 3, 2006

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Sgt1p Is a Unique Co-chaperone That Acts as a Client Adaptor to Link Hsp90 to Skp1p^*

From the Section of Molecular and Cellular Biology, University of California, Davis, California 95616

Sgt1p is a conserved, essential protein required for kinetochore assembly in both yeast and animal cells. Sgt1p has homology to both TPR and p23 domains, sequences often found in proteins that interact with and regulate the molecular chaperone, Hsp90. The presence of these domains and the recent findings that Sgt1p interacts with Hsp90 has led to the speculation that Sgt1p and Hsp90 form a co-chaperone complex. To test this possibility, we have used purified recombinant proteins to characterize the in vitro interactions between yeast Sgt1p and Hsp82p (an Hsp90 homologue in yeast). We show that Sgt1p interacts directly with Hsp82p via its p23 homology region in a nucleotide-dependent manner. However, Sgt1p binding does not alter the enzymatic activity of Hsp82p, suggesting that it is distinct from other co-chaperones. We find that Sgt1p can form a ternary chaperone complex with Hsp82p and Sti1p, a well characterized Hsp90 co-chaperone. Sgt1p interacts with its binding partner Skp1p through its TPR domains and links Skp1p to the core Hsp82p-Sti1p co-chaperone complex. The multidomain nature of Sgt1p and its ability to bridge the interaction between Skp1p and Hsp82p argue that Sgt1p acts as a "client adaptor" recruiting specific clients to Hsp82p co-chaperone complexes.

Received for publication, April 21, 2006 , and in revised form, August 30, 2006.

^* This work was funded by an American Cancer Society Grant RSG-02-035-01 (to K. B. K.) and by Grant P20 MD000262 from the Research Infrastructure in Minority Institutions Program, NCMHD, National Institutes of Health. 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 Figs. S1 and S2.

¹ To whom correspondence should be addressed: University of California, One Shields Ave., Briggs 149, Davis, California 95616. Tel.: 530-754-5044; Fax: 530-752-3085; E-mail: kbkaplan{at}ucdavis.edu.

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