Protein Carboxyl Methylation-Demethylation May be Involved in Gene Activation

In 1965 Axelrod and Daly (1) reported the presence of a methanol-forming enzyme in bovine pituitary gland. Subsequent reports by Liss et al. (2–4) and Kim and Paik (5,6) described a protein-carboxyl-O-methyltransferase (EC 2.1.1.24) which catalyzed the transfer of the methyl group from S-adenosylmethionine to the carboxyl side chain of glutamate and/or aspartate in various proteins (Scheme 1). This reaction results in the formation of protein-methyl esters which were first thought to undergo spontaneous hydrolysis to form methanol under physiological conditions. However, Gagnon et al. (7) found that various tissues from the rat contained a protein methyl esterase which would readily hydrolyze protein-methyl esters to produce methanol. Fetters et al. (8) came to similar conclusions. The protein methyl esters which resulted from the methylation of ACTH via a highly purified protein carboxyl methyl transferase from thymus were stable for several hours at pH 6.5 at 37°C. However, the resultant protein methyl esters were hydrolyzed rapidly upon the further addition of nucleoplasm or cytoplasm (Fig. 1). The resultant [3H]methanol was oxidized via an alcohol dehydrogenase or oxidase. Although alcohol dehydrogenase has been found to be primarily a liver enzyme, we found that alcohol was oxidized by extracts prepared from several different tissues, including thymus (8,9).