Multiple sites of methyl esterification of calmodulin in intact human erythrocytes

Aspartyl and asparaginyl residues are susceptible to spontaneous chemical degradation reactions that result in the formation of isomerized and racemized aspartyl residues. At least a subset of these abnormal residues are recognized by a widely distributed protein D-aspartyl/L-isoaspartyl methyltransferase (EC 2.1.1.77) that can participate in their conversion to normal L-aspartyl residues. We have used this methyltransferase as a probe to identify modified aspartyl and asparaginyl residues in peptides and proteins. In purified calmodulin from bovine brain, major sites of methylation were found to originate from the Asp-2 residue near the amino terminus and the Asp-78 residue in the alpha-helix that connects the two globular calcium-binding domains. When purified calmodulin was incubated at physiological temperature and pH in the absence of calcium, additional methylation sites were found in three of the four calcium-binding sites. In this work we have analyzed the methyl esterification of human calmodulin catalyzed by this enzyme in intact erythrocytes. On the basis of results from peptide mapping studies, Asp-2, Asp-78/80, and residues in calcium-binding domains III and IV appear to be methylated. Methylation of sites in the calcium-binding regions appears to reflect the low concentration of free calcium in human erythrocytes. We also found that calmodulin isolated from erythrocytes and methylated in vitro contains major methylation sites at Asp-2 and Asp-78/80 but not in the calcium-binding sites. Comparison of the number of available methylation sites of calmodulin in intact cells and in material aged in vitro supports the hypothesis that repair processes can occur in erythrocytes.