ENHANCED KINASE-ACTIVITY IN SV40-TRANSFORMED CELLS MAY BE COMPENSATEDBY ENHANCED PHOSPHATASE-ACTIVITY IN REVERTANTS AS REFLECTED BY PHOSPHORYLATION OF P53

Transformation of rat cells with SV40 large T antigen results in activ ation of protein kinases and hyperphosphorylation of the tumor suppres sor protein p53. In searching for cellular targets involved in SV40-me diated transformation, flat revertants of SV40-transformed rat cells h ad been isolated carrying a presumptive defect in a cellular gene (Bau er et al: J Virol 61: 1821, 1987). In this investigation, we asked whe ther the phosphorylation state of p53 might be affected in the reverta nts. Two-dimensional phosphopeptide analyses revealed indeed a charact eristic reduction of phosphorylation of p53 compared to the parental t ransformed cells. However, when we employed the phosphatase inhibitor okadaic acid in vivo hyperphosphorylation of p53 resumed indicating th at the kinases involved in phosphorylation of p53 were fully active bu t counterbalanced by enhanced phosphatase activity. Indeed, the phosph ate turnover of p53 in vivo and phosphatase activity towards p53 in vi tro was higher in the revertants than in the parental transformants. T hese findings demonstrate that the transformation state of a cell corr elates with the phosphorylation state of p53 which in turn can be regu lated in different ways, enhanced kinase activity in transformed cells may be counteracted by enhanced phosphatase activity in revertant cel ls.