FURTHER CHARACTERIZATION OF THE F1-HISTONE PHOSPHOKINASE OF METAPHASE-ARRESTED ANIMAL CELLS

Exponentially growing Chinese hamster cells are found to contain two major phosphokinase activities with specificity for the phosphorylation of F1 (lysine-rich) histone. These two activities, designated KI and KII, were extracted with 0.35 M NaCl and fractionated in 0.2 M NaCl by Sephadex G-200 gel filtration. KI, which is similar to the ubiquitous cyclic 3',5'-adenosine monophosphate (cAMP)-dependent phosphokinase, differs from KII by several criteria. KII is mol wt 90,000, cAMP independent, rapidly turned over in vivo, low K_m for ATP, and phosphorylates F1 histone at several unique sites. Comparative examination of metaphase-arrested (M) and counterpart interphase (I) cells for these two activities reveals that KII is responsible for the overall high activity in M-arrested cells. Pulse labeling of cells with ³²P during traverse of the G₂-M phase of the cell cycle reveals an in vivo tryptic-phosphopeptide pattern in whole unfractionated F1 which is unique to M cells. Seven major phosphopeptides derived by in vitro phosphorylation of F1 with the KII enzyme correspond to these M cell-specific phosphorylation sites observed in vivo. It is suggested that KII activity predominates during the G₂-M transition and that F1 is its natural in vivo substrate.