The distribution and turnover of labeled methyl groups in histone fractions of cultured mammalian cells

Random cultures of mammalian cells were labeled with [ 14C-Me]-methionine. Cells were harvested either directly after labeling or after varying periods of further growth in unlabeled medium. Harvested cells were fractionated into cytosol, nucleoplasm, histone, and residual protein fractions. The distribution of radiomethyl activity among the amino acids of each fraction was determined by hydrolysis and ion-exchange chromatography, while the distribution of volatile radiomethyl components was determined by hydrolysis and gas chromatography. The specific activities of methylated lysine residues were found to be considerably higher in histones than in any other fraction, while the nucleoplasm and residual proteins contained methyl arginine at higher specific activities than did the histones. Among the histone fractions, only F2b was found to contain methylated arginine. Among the fractions, only histones were found to contain methanol as their main volatile component after brief alkaline hydrolysis. A comparison of the half-lives of histone fractions and their methyl lysine and methyl arginine components indicated that histone methylation was irreversible.