Tertiary structural rearrangements upon oxidation of Methionine145 in calmodulin promotes targeted proteasomal degradation

The selectivity underlying the recognition of oxidized calmodulin (CaM) by the 20S proteasome in complex with Hsp90 was identified using mass spectrometry. We find that degradation of oxidized CaM (CaMox) occurs in a multistep process, which involves an initial cleavage that releases a large N-terminal fragment (A1-F92) as well as multiple smaller carboxyl-terminus peptides ranging from 17 to 26 amino acids in length. These latter small peptides are enriched in methionine sulfoxides (MetO), suggesting a preferential degradation around MetO within the carboxyl-terminal domain. To confirm the specificity of CaMox degradation and to identify the structural signals underlying the preferential recognition and degradation by the proteasome/Hsp90, we have investigated how the oxidation of individual methionines affect the degradation of CaM using mutants in which all but selected methionines in CaM were substituted with leucines. Substitution of all methionines with leucines except Met144 and Met145 has no detectable effect on the structure of CaM, permitting a determination of how site-specific substitutions and the oxidation of Met144 and Met145 affects the recognition and degradation of CaM by the proteasome/Hsp90. Comparable rates of degradation are observed upon the selective oxidation of Met144 and Met145 in CaM-L7 relative to that observed upon oxidation of all nine methionines in wild-type CaM. Substitution of leucines for either Met144 or Met145 promotes a limited recognition and degradation by the proteasome that correlates with decreases in the helical content of CaM. The specific oxidation of Met144 has little effect on rates of proteolytic degradation by the proteasome/Hsp90 or the structure of CaM. In contrast, the specific oxidation of Met145 results in both large increases in the rate of degradation by the proteasome/Hsp90 and significant circular dichroic spectral shape changes that are indicative of changes in tertiary rather than secondary structure. Thus, tertiary structural changes resulting from the site-specific oxidation of a single methionine (i.e., Met145) promote the degradation of CaM by the proteasome/Hsp90, suggesting a mechanism to regulate cellular metabolism through the targeted modulation of CaM abundance in response to oxidative stress.

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 (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.