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J. Biol. Chem., Vol. 279, Issue 44, 46135-46142, October 29, 2004

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Hsp90 Enhances Degradation of Oxidized Calmodulin by the 20 S Proteasome^*

Jennifer E. Whittier, Yijia Xiong, Martin C. Rechsteiner, and Thomas C. Squier¶

From the Cell Biology and Biochemistry Group, Biological Sciences Division, Pacific Northwest National Laboratory, Richland, Washington 99352 and the Department of Biochemistry, University of Utah School of Medicine, Salt Lake City, Utah 84132

The 20 S proteasome has been suggested to play a critical role in mediating the degradation of abnormal proteins under conditions of oxidative stress and has been found in tight association with the molecular chaperone Hsp90. To elucidate the role of Hsp90 in promoting the degradation of oxidized calmodulin (CaM_ox), we have purified red blood cell 20 S proteasomes free of Hsp90 and assessed their ability to degrade CaM_ox in the absence or presence of Hsp90. Purified 20 S proteasome does not degrade CaM_ox unless Hsp90 is added. CaM_ox degradation is sensitive to both proteasome and Hsp90-specific inhibitors and is further enhanced in the presence of 2 mM ATP. Irrespective of the presence of Hsp90, we find that unoxidized CaM is not significantly degraded. Direct binding measurements demonstrate that Hsp90 selectively associates with CaM_ox; essentially no binding is observed between Hsp90 and unoxidized CaM. These results indicate that Hsp90 in association with the 20 S proteasome can selectively associate with oxidized and partially unfolded CaM to promote degradation by the proteasome.

Received for publication, June 1, 2004 , and in revised form, August 16, 2004.

^* This work was supported by a grant from the National Institutes of Health (AG17996). 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.

^¶ To whom correspondence should be addressed: Pacific Northwest National Laboratory, P. O. Box 999, Mail Stop P7-53; Richland, WA 99352. Tel.: 509-376-2218; Fax: 509-372-1632; E-mail: thomas.squier{at}pnl.gov.

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