Chromatin-associated protein phosphatase 1 regulates aurora-B and histone H3 phosphorylation

Proper chromosome condensation requires the phosphorylation of histone and nonhistone chromatin proteins. We have used an in vitro chromosome assembly system based on Xenopus egg cytoplasmic extracts to study mitotic histone H3 phosphorylation, We identified a histone H3 Ser(10) kinase activity asso ciated with isolated mitotic chromosomes. The histone H3 kinase was not aff ected by inhibitors of cyclin-dependent kinases, DNA-dependent protein kina se, p90(rsk), or cAMP-dependent protein kinase, The activity could be selec tively eluted from mitotic chromosomes and immunoprecipitated by specific a nti-X aurora-B/AIRK2 antibodies. This activity was regulated by phosphoryla tion. Treatment of X aurora-B immunoprecipitates with recombinant protein p hosphatase 1 (PP1) inhibited kinase activity. The presence of PP1 on chroma tin suggested that PP1 might directly regulate the X aurora-B associated ki nase activity. Indeed, incubation of isolated interphase chromatin with the PP1-specific inhibitor I2 and ATP generated an H3 kinase activity that was also specifically immunoprecipitated by anti-X aurora-B antibodies. Noneth eless, we found that stimulation of histone H3 phosphorylation in interphas e cytosol does not drive chromosome condensation or targeting of 13 S conde nsin to chromatin, In summary, the chromosome-associated mitotic histone H3 Ser(10) kinase is associated with X aurora-B and is inhibited directly in interphase chromatin by PP1.