ABSTRACT
Protein methylation, including that of histones, has been reviewed comprehensively by W. K. Paik and J. Kim. Among the various postsynthetic modifications that have been reported to occur in nuclear proteins, N-methylation occupies a special position since, in contrast to acetylation and phosphorylation, it is—at least in histones—not subject to turnover. For the assessment of in vitro methylation of histones or nonhistone chromosomal (NHC) proteins, most authors have used methyl-labeled S-adenosylmethionine as the precursor. Increased methylation of NHC proteins and histones was observed in transplantable hepatomas, and in rat liver cells in which proliferation was induced by subtotal hepatectomy or by treatment with triiodothyronine. Radiomethylation of NHC proteins was achieved by incubating the glands in the presence of (methyl-3H)-methionine and an inhibitor of protein synthesis. The tissue was subsequently sectioned and the sections treated with nuclease, 0.2 N HCl and organic solvents to remove nucleic acids, histones, and lipids.
