Hsp90 enhances degradation of oxidized calmodulin by the 20 S proteasome

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 (CaMox), we have purified red blood cell 20 S proteasomes free of Hsp90 and assessed their ability to degrade CaMox in the absence or presence of Hsp90. Purified 20 S proteasome does not degrade CaMox unless Hsp90 is added. CaMox 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 CaMox; 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.