Protein carboxymethyltransferase and its physiological significance

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Protein carboxylmethyltransferase is a cytosolic enzyme that methylates side-chain carboxyl groups in proteinaceous substrates, with S-adenosyl L-methionine as the methyl donor. However, natural substrates as well as the physiological function of this enzyme have been unknown. Therefore, in this study we intended to examine its endogenous substrates in brain and found the following observations.Several polypeptides were methylated when brain slices were incubated with L-methionine or when cytosolic fraction were incubated with S-adenosyl L-methionine. Two methyl-accepting proteins in the cytoplasm were identified as tubulin and high molecular weight microtubule-associated proteins. The muthyl-moiety transferred to these proteins was labile at alkaline pH. The high molecular weight microtubule-associated proteins showed higher methyl-accepting activity than tubulin or ovalbumin, which was used as a standard substrate: about 20 mmol of high molecular weight microtubule-associated protein … More s, 2 mmol of tubulin and 10 mmol of ovalbumin were methylated per mol of each protein.Several other methyl-accepting proteins were found in brain cells, although these proteins have not yet been identified. In particular, the cytosolic fraction contains a 30 KD protein as a major methyl-accepting protein in addition to microtubule proteins. This protein may have some significant role in cellular activities, because no 30 KD protein methylated in vitro with S-adenosyl L-methionine was detected in the brains of aged rats.It is well-known that microtubules plays a great role in cell-mitosis. It is also known that in cancer cells cell-division occurrs frequently and actively, whereas brain cells do not divide. Therefore we studied the methyl-accepting activity of microtubules of ascites-hepatoma cells and found that the microtubules was also a good substrate for this enzyme. There seems no relation between cell division and carboxylmethylation.Further studies are required to determine the physiological significance of this reaction. Less