Endogenous substrates for protein carboxyl methyltransferase in cytosolic fractions of bovine brain.

A method of polyacrylamide gel electrophoresis utilizing the discontinuous pH-stacking gel format, the cationic detergent cetylpyridinium chloride, and an acidic buffer system has been applied to detection of specific substrates for protein carboxyl methyltransferase (PCM, EC 2.1.1.24) in cytosol fractions of bovine cerebral cortex. This electrophoresis system produces a high-resolution separation of proteins while preventing spontaneous hydrolysis of protein carboxyl methyl esters. Separation occurs largely on the basis of molecular weight. By running polyacrylamide gels at 4 degrees C or 25 degrees C, it was possible to demonstrate that any specific methyl-accepting protein is modified to form a labile methyl ester rather than the more stable N-derivative. Using this system, we have found that partially purified fractions of PCM contain a variety of endogenous methyl-accepting proteins. The apparent specificity of these substrates varies widely; some apparently abundant proteins show little or no methylation, while other apparently less abundant proteins exhibit a relatively high degree of methylation. One protein, with an apparent Mr of 46,000, exhibited an exceptional degree of methylation. Two distinct classes of protein carboxyl methyl esters could be distinguished by their differing susceptibility to nonenzymatic hydrolysis. The possible relevance of our findings to the recent suggestion that PCM specifically methylates abnormal D-aspartyl residues in age-racemized proteins is considered.