Carboxylmethylation of calmodulin inhibits calmodulin-dependent phosphorylation in rat brain membranes and cytosol

Protein carboxylmethyltransferase, using S-adenosylmethionine (AdoMet) as a methyl donor, forms baselabile carboxylmethylesters on acidic amino acids of various protein substrates. Calmodulin (CaM), an acidic protein, is an excellent substrate for protein carboxylmethyltransferase. Using purified rat brain protein carboxylmethyltransferase, we have investigated the effects of carboxylmethylation of CaM on its subsequent ability to stimulate Ca⁺²-CaM-dependent phosphorylations in rat brain cytosol and membrane preparations. Incubation of CaM, purified protein carboxylmethyltransferase, and AdoMet resulted in the time- and temperature-dependent formation of carboxylmethylesters, with CaM showing greater methylation than other protein substrates. Acidic gel electrophoresis of CaM after incubation with protein carboxylmethyltransferase and AdoMet revealed a single radioactive band that co-migrated with native CaM. The functional consequences of carboxylmethylation were investigated by preincubating CaM with protein carboxylmethyltransferase, followed by determination of Ca⁺²-CaM-stimulated phosphorylation in rat brain membranes depleted of CaM. Carboxylmethylated CaM was less able to stimulate Ca⁺²-CaM-dependent phosphorylation in membrane proteins. This inhibition of Ca⁺²-CaM-dependent phosphorylation was reversed by including S-adenosylhomocysteine, an inhibitor of protein carboxylmethyltransferase, in the preincubation with protein carboxylmethyltransferase and AdoMet. Similar results were obtained by preincubating rat brain cytosol with purified protein carboxylmethyltransferase and AdoMet. In this preparation, Ca²⁺-stimulated phosphorylations were inhibited, with maximal inhibition noted at 100 μM AdoMet. These results suggest that carboxylmethylation of CaM may provide an important biochemical mechanism for regulation of Ca²⁺-CaM-dependent reactions in nervous tissue.