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Two purified isozymes of protein carboxyl methyltransferase from bovine brain catalyze the substoichiometric transfer of methyl groups in vitro from S-adenosyl-L-[methyl-3H]methionine to several erythrocyte membrane proteins, which include bands 2.1, 3, and 4.1, as well as several integral membrane polypeptides. D-Aspartic acid beta-[3H]methyl ester has been isolated from proteolytic digests of these methylated proteins, suggesting that protein D-aspartyl residues can serve as methyl-acceptor sites for the two brain enzymes. This formation of D-aspartic acid beta-[3H]methyl ester is competitively inhibited by the peptide L-Val-L-Tyr-L-Pro-L-isoAsp-Gly-L-Ala, which contains an L-aspartyl residue in an unusual beta-peptide linkage. Since this peptide is a stoichiometric substrate for the brain methyltransferases, it appears that one enzymatic activity can catalyze methyl ester formation at both D-aspartyl and L-isoaspartyl sites. In these respects, the activity of both brain isozymes closely resembles those previously described for the erythrocyte enzyme. The results are discussed in terms of a model in which derivatized aspartyl residues in proteins, arising by either racemization or isomerization, are recognized by the methyltransferase; the enzyme may function in either the metabolism or correction of the altered structures. The presence of a similar enzyme in both translationally active (brain) and inactive (erythrocyte) tissues suggests that the reactions are of general importance to cellular integrity.

proceedings of the national academy of sciences of the united states of america

Mammalian brain and erythrocyte carboxyl methyltransferases are similar enzymes that recognize both D-aspartyl and L-isoaspartyl residues in structurally altered protein substrates.