Analysis of dynamic changes in post-translational modifications of human histones during cell cycle by mass spectrometry

The N-terminal tails of the four core histones are subject to several types of covalent post-translational modifications that have specific roles in regulating chromatin structure and function. Here we present an extensive analysis of the core histone modifications occurring through the cell cycle. Our MS experiments characterized the modification patterns of histones from HeLa cells arrested in phase G1, S, and G(2)/M. For all core histones, the modifications in the G1 and S phases were largely identical but drastically different during mitosis. Modification changes between S and G(2)/M phases were quantified using the SILAC (stable isotope labeling by amino acids in cell culture) approach. Most striking was the mitotic phosphorylation on histone H3 and H4, whereas phosphorylation on H2A was constant during the cell cycle. A loss of acetylation was observed on all histones in G2/M-arrested cells. The pattern of cycle-dependent methylation was more complex: during G(2)/M, H3 Lys(27) and Lys(36) were decreased, whereas H4 Lys(20) was increased. Our results show that mitosis was the period of the cell cycle during which many modifications exhibit dynamic changes.