Transformation of rat cells with SV40 large T antigen results in activation of protein kinases and hyperphosphorylation of the tumor suppressor protein p53. In searching for cellular targets involved in SV40-mediated transformation, flat revertants of SV40-transformed rat cells had been isolated carrying a presumptive defect in a cellular gene (Bauer et al: J Virol 61: 1821, 1987). In this investigation, we asked whether the phosphorylation state of p53 might be affected in the revertants. Two-dimensional phosphopeptide analyses revealed indeed a characteristic reduction of phosphorylation of p53 compared to the parental transformed cells. However, when we employed the phosphatase inhibitor okadaic acid in vivo hyperphosphorylation of p53 resumed indicating that the kinases involved in phosphorylation of p53 were fully active but counterbalanced by enhanced phosphatase activity. Indeed, the phosphate turnover of p53 in vivo and phosphatase activity towards p53 in vitro was higher in the revertants than in the parental transformants. These findings demonstrate that the transformation state of a cell correlates with the phosphorylation state of p53 which in turn can be regulated in different ways, enhanced kinase activity in transformed cells may be counteracted by enhanced phosphatase activity in revertant cells.

Related Articles