PNNL

Abstract

Mass spectrometry was used to identify possible linkages between the oxidation of specific methionines in calmodulin (CaM) and its degradation by the proteasome. Degradation of oxidized CaM (CaMox) occurs in a multistep process, which involves an initial cleavage that releases a large fragment (Q41-D131) and a carboxyl-terminus peptide (G132-K148) enriched in methionine sulfoxide [Met(O)] prior to rebinding and complete digestion. Rates of CaM degradation correlate with the oxidation of Met145, which is preferentially oxidized by hydrogen peroxide prior to the oxidation of Met144. The specificity of CaMox degradation was confirmed using CaM mutants in which the majority of methionines were replaced by leucines using site-directed mutagenesis, permitting the site-specific oxidation of Met144 or Met145. Oxidation of both Met144 and Met145 results in the selective degradation of CaMox by the proteasome with rates comparable to that observed for fully oxidized CaM. Oxidation of Met144 has little effect on proteolytic degradation; rather, oxidation of Met145 targets CaMox for proteasomal degradation. The selective oxidation of either Met144 or Met145 has little effect on the secondary structure of CaM; rather, oxidation-induced tertiary structural changes mediate the recognition and degradation of CaMox by the proteasome. Thus, oxidation of Met145 can function as a sensor that has the potential to regulate cellular metabolism through the targeted modulation of CaM abundance in response to oxidative stress.