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The p53 gene product has been implicated in both human and animal tumorigenesis. p53 complexes with the transforming proteins encoded by several different DNA tumour viruses. We demonstrate that human p53 is phosphorylated by the mammalian p34cdc2 kinase in vitro and coprecipitates with p34cdc2 in vivo. Our observations suggest that phosphorylation of p53 by p34cdc2 kinase may regulate the known activities of p53 in the initiation steps of DNA replication in mammalian cells.

Several lines of evidence suggest that the cellular phosphoprotein p53, overproduced in a variety of neoplastic cell types (reviewed in Crawford 1983; Jenkins and Sturzbecher 1988; Oren 1985; Rotter and Wolf 1985), is an oncogene product, directly involved in the process of malignant transformation. p53 cDNA expression constructs immortalise both adult (Jenkins et al. 1984, 1985) and embryo (Jenkins et al. 1985; Rovinski and Benchimol 1988) cells and cooperate with an activated ras oncogene in malignant transformation (Eliyahu et al. 1984; Jenkins et al. 1984; Parada et al. 1984; Rovinski and Benchimol 1988; Finlay et al. 1988). The potential of wild-type p53 to score in transformation assays is dependent upon powerful heterologous promoter/enhancers driving p53 expression (Jenkins et al. 1984, 1985). Immortalisation and ras complementation are distinct activities of p53 and can be separated by deletion mutagenesis (Jenkins et al. 1985). Generation of specific variant p53 species can give rise to protein products with increased stability and enhanced transforming/biological activity (Jenkins et al. 1985), indicating that the p53 gene can be mutationally activated. Near identical arrangements of the endogenous p53 gene coding sequences have been identified in vivo and are associated with malignant transformation by Friend leukaemia virus (Rovinski et al. 1987). Subsequently, further in vitro-generated p53 mutants with enhanced activity in cotransformation assays have been identified (Finlay et al. 1988).

Immunopurified mouse p53 proteins were used to gain experimental access to the mechanisms underlying nonprimate p53 directed suppression of SV40 origin directed DNA replication in vivo. In replication competent HeLa cell extracts containing exogenous T antigen, mouse p53 blocks T antigen dependent DNA synthesis as in vivo. However, in transcription competent HeLa extracts, mouse p53 has no effect either on overall transcription or on the ability of immunopurified T antigen to downregulate SV40 early transcription. We show that although mouse p53 has no significant effect on T antigen encoded activities such as ATPase and DNA binding, helicase activity is somewhat reduced suggesting that the in vivo suppression by mouse p53 of SV40 replication may be due, at least in part, to direct modulation of T antigen function.