It is shown here that one can induce prematurely condensed chromosomes (PCCs) in G1-phase human (HeLa) and mouse (FT210) cells by treating them with the protein phosphatase inhibitor calyculin A. However, histone H1 is not phosphorylated in these G1-PCCs. It has previously been proposed that histone H1 phosphorylation is responsible for mitotic chromosome condensation, but our results suggest that this is not the case. They indicate instead that phosphorylation of histone H1 is not required for chromosome condensation. It is known that the Cdk1 protein kinase, which triggers mitosis and also phosphorylates histone H1, cannot be activated in G1-phase because mitotic cyclins are not present. Since calyculin A induces PCCs in G1-phase in the absence of active Cdk1, our results suggest that inactivation of protein phosphatases may be just as important for the onset of chromosome condensation and other mitotic events as the activation of protein kinases.
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