Studies of Histone Methylation during the HeLa S-3 Cell Cycle

Abstract

Abstract The rate and specificity of histone methylation were examined throughout the HeLa S-3 cell cycle in cultures synchronized by selective detachment in mitosis or by double thymidine block. (a) Of all the cellular fractions tested, only the 0.25 n H2SO4-soluble nuclear fraction contained proteins which appeared to be methylated at high rates. Within this fraction histone polypeptides f2a1 and f3 were methylated at the highest rates, f2b and f2a2 were methylated at lower rates and f1 polypeptides were not methylated at all. (b) Lysine residues in histone fractions f2a, f2b, and f3 were methylated throughout the cell cycle with maximum rates of methylation occurring in G2 phase and mitosis. Only histone f3 polypeptides contained significant amounts of methylated arginine and histidine residues. These residues were methylated at maximum rates during the S phase of the cell cycle. (c) Mitotic cells contained a higher percentage of methylated histone molecules than cells at other stages of the cell cycle and it was calculated that if there was only one methylation site in each of the molecules, from 80 to 100% of all f2a1, f2b, and f3 polypeptides were methylated. (d) The ratios of e, e-N-dimethyllysine to e-N-monomethyllysine in the histone fractions of the heteroploid malignant HeLa S-3 cell were about half the ratios of these amino acids reported found in the corresponding histones of normal cells. (e) The turnover rate of histone methyl groups was about 1 to 2% per hour depending upon the fraction.