EVIDENCE THAT THE ENDOGENOUS HISTONE H1 PHOSPHATASE IN HELA MITOTIC CHROMOSOMES IS PROTEIN PHOSPHATASE-1, NOT PROTEIN PHOSPHATASE 2A

Histone H1 is highly phosphorylated in mitotic HeLa cells, but is quic kly dephosphorylated in vivo at the end of mitosis and in vitro follow ing cell lysis. We show here that okadaic acid and microcystin-LR bloc k the in vitro dephosphorylation of H1 and that they do so directly by inhibiting the histone H1 phosphatase rather than by some indirect me chanism. The concentrations of microcystin and okadaic acid required f or inhibition strongly suggest that the histone H1 phosphatase is eith er PP1 or an unknown protein phosphatase with okadaic acid-sensitivity similar to PP1, The histone H1 phosphatase is predominantly located i n chromosomes with at most one copy for every 86 nucleosomes. This ten ds to support its identification as PP1, since localization in mitotic chromosomes is a characteristic of PP1 but not of the other known oka daic acid-sensitive protein phosphatases. We also show that treatment of metaphase-arrested HeLa cells with staurosporine and olomoucine, in hibitors of p34(cdc2) other protein kinases, rapidly induces reassembl y of interphase nuclei and dephosphorylation of histone H1 without chr omosome segregation. This result indicates that protein kinase activit y must remain elevated to maintain a mitotic block. Using this as a mo del system for the M- to G(1)-phase transition, we present evidence fr om inhibitor studies suggesting that the in vivo histone H1 phosphatas e may be either PP1 or another phosphatase with similar okadaic acid-s ensitivity, but not PP2A.