ALTERATION OF CELL CYCLE-DEPENDENT HISTONE PHOSPHORYLATIONS BY OKADAIC ACID - INDUCTION OF MITOSIS-SPECIFIC H3 PHOSPHORYLATION AND CHROMATIN CONDENSATION IN MAMMALIAN INTERPHASE CELLS

Effects of okadaic acid (OA), a protein phosphatase inhibitor, on chro matin structure and phosphorylation of histones were examined using He La and N18 cells. The chromatin condensation in HeLa cells was mild an d resemble prometaphase nuclei, while the condensation in N18 cells wa s extensive and chromatin became a compact body, H2A in HeLa cells was extensively and consistently phosphorylated at the same site througho ut the cell cycle, and H3 was demonstrated to be phosphorylated at the mitosis-specific site Ser(10). In contrast, H1 phosphorylation was ra pidly decreased in most sites within 3 h. The reduction of H1 phosphor ylation was accompanied by a quantitative change in the set of H1 phos phopeptides. During the early phase of the OA treatment, H1 phosphoryl ation was transiently elevated in tandem, whereas H3 phosphorylation r eached a maximum somewhat later, The results suggest that mitosis-spec ific events (cdc2/H1 kinase activation, H1 super-phosphorylation, mito sis-specific H3 phosphorylation and chromatin condensation) induced by OA are sequentially associated, The changes appear to reflect a molec ular mechanism similar to that operating in normal mitosis.