Protein carboxyl methyltransferase from cow eye lens

Protein carboxyl methyltransferase activity (S-adenosyl-L-methionine: protein carboxyl-0-methyltransferase; E.C. 2.1.1.24) has been detected in crude soluble extracts of cow eye lens. The activity incorporates methyl groups from S-adenosyl-L-methionine into endogenous lens proteins in vitro, and several of these species co-migrate electrophoretically with lens crystallins. A 2600-fold purification of the enzyme free of endogenous substrates was achieved by gel filtration and affinity chromatography. The lens methyltransferase has a native molecular weight of approximately 27,000, and catalyzes the substoichiometric incorporation of highly alkali-labile methyl ester groups into a broad range of protein substrates. The lens enzyme appears to be similar to that found in human erythrocytes, which specifically recognizes and modifies D-aspartic acid residues in aged proteins in a postulated degradative or racemization-repair pathway (McFadden, P.N., and Clarke, S. (1982) Proc. Natl. Acad. Sci. U.S.A. 79, 2460-2464).