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J. Biol. Chem., Vol. 283, Issue 15, 9509-9512, April 11, 2008

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GCUNC45 Is the First Hsp90 Co-chaperone to Show /β Isoform Specificity^*

From the Department of Biochemistry and Molecular Biology, Mayo Clinic College of Medicine, Rochester, Minnesota 55905

Hsp90 is an essential molecular chaperone required for the normal functioning of many key regulatory proteins in eukaryotic cells. Vertebrates have two closely related isoforms of cytosolic Hsp90 (Hsp90 and Hsp90β). However, specific functions for each isoform are largely unknown, and no Hsp90 co-chaperone has been reported to distinguish between the two isoforms. In this study, we show that the Hsp90 co-chaperone GCUNC45 bound preferentially to the β isoform of Hsp90 in vitro. GCUNC45 efficiently blocked the progression of progesterone receptor chaperoning in an in vitro functional system when Hsp90β was used, but did so with much less efficacy when Hsp90 was used. Knockdown experiments in HeLa cells showed that GCUNC45 is required for the normal cellular distribution of Hsp90β, but not Hsp90. This is the first example of a co-chaperone with isoform selectivity, and this approach may open novel avenues to understanding the functional differences between Hsp90 isoforms.

Received for publication, January 24, 2008 , and in revised form, February 15, 2008.

^* This work was supported by National Institutes of Health Grant RO1DK4624. 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: Mayo Clinic College of Medicine, 200 First St. SW, Rochester, MN 55905. Tel.: 507-284-1928; Fax: 507-284-2053; E-mail: chadli.ahmed{at}mayo.edu.

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