Enzymatic carboxyl methylation of calcium-binding proteins.

The enzymes protein-carboxyl methylase and protein methylesterase reversibly modify the charge and structure of proteins by adding and removing methyl groups on free carboxyl groups of proteins. Since this posttranslational system has been implicated in biological processes that required calcium, the carboxyl methylation of calmodulin was investigated. Calmodulin was an excellent substrate for both protein-carboxyl methylase and protein methylesterase. Carboxyl methylation of calmodulin resulted in inactivation, since methylated calmodulin was less capable of activating cyclic nucleotide phosphodiesterase. To determine whether the carboxyl methylation of calmodulin was simply a test tube reaction or a biochemical reaction normally occurring in intact cells, two different cell lines were labeled with [methyl-3H]methionine. Calmodulin was isolated by affinity chromatography and was found to be carboxyl methylated. Finally, calcineurin was also an excellent substrate for the methylase, suggesting that other calcium-binding proteins may be affected by protein methylation-demethylation.