Quiescent non-permissive cells re-enter the cell cycle upon infection with the DNA tumor virus SV40. Before the expression of virus specific proteins and other cellular reactions there is an induced expression of the growth suppressor protein p53. p53 is known to be a substrate for protein kinase CK2 and in addition it is tightly associated with CK2 and both proteins are implicated in cell cycle regulation. No phosphorylation of p53 was observed in vivo until late in G(1)- or early S-phase. Immunopurified p53 from the early G(1)-phase of the cell cycle was not phosphorylated by an associated protein kinase activity. Furthermore, protein kinase CK2 could not phosphorylate p53 from the early G(1)-phase of the cell cycle and also immunopurified p53 from late G(1)- and S-phase which were dephosphorylated by alkaline and acid phosphatases. p53 from cells in S-phase is an efficient substrate. Moreover, in the presence of okadaic acid, a potent inhibitor of protein phosphatase PP2a, phosphorylation of p53 is detectable early in G(1)-phase of the cell cycle.

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