Analysis of Dynamic Changes in Post- translational Modifications of Human Histones during Cell Cycle by Mass Spectrometry*□S

The N-terminal tails of the four core histones are subject to several types of covalent post-translational modifica-tions 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 ar-rested in phase G1, S, and G2/M. For all core histones, the modifications in the G1 and S phases were largely identi-cal but drastically different during mitosis. Modification changes between S and G2/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 phospho-rylation on H2A was constant during the cell cycle. A los