Alteration of Cell Cycle-dependent Histone Phosphorylations by Okadaic Acid

Abstract

Effects of okadaic acid (OA), a protein phosphatase inhibitor, on chromatin structure and phosphorylation of histones were examined using HeLa and N18 cells. The chromatin condensation in HeLa cells was mild and resemble prometaphase nuclei, while the condensation in N18 cells was extensive and chromatin became a com- pact body. H2A in HeLa cells was extensively and con- sistently phosphorylated at the same site throughout the cell cycle, and H3 was demonstrated to be phospho- rylated at the mitosis-specific site Ser10. In contrast, H1 phosphorylation was rapidly decreased in most sites within 3 h. The reduction of H1 phosphorylation was accompanied by a quantitative change in the set of H1 phosphopeptides. During the early phase of the OA treatment, H1 phosphorylation was transiently elevated in tandem, whereas H3 phosphorylation reached a max- imum somewhat later. The results suggest that mitosis- specific events (cdc2/H1 kinase activation, H1 super- phosphorylation, mitosis-specific H3 phosphorylation and chromatin condensation) induced by OA are se- quentially associated. The changes appear to reflect a molecular mechanism similar to that operating in nor- mal mitosis.