Activation of human CDC2 protein as a histone H1 kinase is associated with complex formation with the p62 subunit

p34 kinase, the product of the CDC2 gene, is a cell-cycle regulated protein kinase that is most active during mitosis. In HeLa cells, p34 kinase has previously been shown to exist in both a low- and a high-molecular-mass form, the latter of which is only found in cells in the G₂/M phase of the cell cycle and contains a 62-kDa subunit. Here we show that although each form of the kinase phosphorylates casein in vitro, only the high-molecular-mass form uses histone H1 as substrate. The high-molecular-mass form of p34 kinase from nocodazole-treated HeLa cells was purified 6700-fold. The apparent molecular mass of the mitotic CDC2-encoded protein kinase complex was 220 kDa. The purified enzyme phosphorylated not only its endogenous 62-kDa subunit but also phosphorylated histone H1 with a K(m) of 3 μM and used ATP 40 times more efficiently than GTP (K(m) 54 μM and 2 mM, respectively). The enzyme activity was unaffected by cAMP, calcium/calmodulin, or by the heat-stable inhibitor of cAMP-dependent protein kinase. These characteristics are typical of growth-associated histone H1 kinase from different organisms. These results suggest that CDC2 protein may be activated as a M-phase-specific protein kinase in part by its association with the p62 subunit.