Methylation of calmodulin at carboxylic acid residues in erythrocytes. A non-regulatory covalent modification?
The physiological role of protein carboxy-group methylation reactions in human erythrocytes was studied with calmodulin as an endogenous methyl-group acceptor. The steady-state degree of calmodulin carboxy-group methylation is substoichiometric both in intact cells and in a lysed-cell system (about 0.0003 mol of methyl groups/mol of polypeptide). Purified erythrocyte calmodulin is a substrate for a partially purified erythrocyte carboxy-group methyltransferase and can be methylated to the extent of about 0.0007-0.001 mol of methyl groups/mol of polypeptide. This erythrocyte protein methyltransferase displays an apparent specificity for atypical racemized and/or isomerized D-aspartate and L-isoaspartate residues [McFadden & Clarke (1982) Proc. Natl. Acad. Sci. U.S.A. 79, 2460-2464; Murray & Clarke (1984) J. Biol. Chem. 259, 10722-10732]. Exposure of calmodulin to elevated temperatures before methylation results in racemization of aspartate and/or asparagine residues, and may result in isoaspartate formation as well. The methylatability of these samples also increases as a function of time of heating, independent of the pH (over the range pH 5-9) or Ca2+ concentration; the most significant increase occurs during the initial 60 min, when calmodulin retains a fraction of its biological activity. These results are consistent with the hypothesis that methylation of calmodulin may occur at these uncommon aspartate residues, but are not consistent with a regulatory role for the methylation reaction.
D001216:Asparagine / Q000378:metabolism; D001224:Aspartic Acid / Q000378:metabolism; D002147:Calmodulin / Q000097:blood / Q000378:metabolism*; D002264:Carboxylic Acids / Q000378:metabolism; D004591:Electrophoresis, Polyacrylamide Gel; D004912:Erythrocytes / Q000378:metabolism*; D006358:Hot Temperature; D006801:Humans; D006868:Hydrolysis; D008745:Methylation; D012259:Ribonuclease, Pancreatic / Q000097:blood
D002147:Calmodulin; D002264:Carboxylic Acids; D001224:Aspartic Acid; D001216:Asparagine; D012259:Ribonuclease, Pancreatic