SV40 Large T Antigen Induces a Protein Kinase Responsible for Phosphorylation of the Cellular Protein p53

Abstract

The overall phosphorylation state of p53 from normal versus transformed cells was compared by labeling cells with [³⁵S]methionine or [³²P]phosphate and isolating p53 from cell extracts by immunoprecipitation and subsequent SDS-polyacrylamide gel electrophoresis. In transformed cells both T antigen and p53 are heavily phosphorylated (Fig. 1, lane a). Phosphorylation of p53 occurs mainly at serine and to a minor extent at threonine residues (not shown). In contrast, p53 from normal cells is hardly phosphorylated (not shown). The rate of synthesis of p53 appears to be similar, whereas the metabolic stability differs considerably in both cell lines, the half-lives being less than 30 min in normal cells and more than 5 h in transformed cells. In the transformed cells. p53 seems to be converted to a second form with a slightly higher electrophoretic mobility (Fig. 1, lane c). This conversion is complete in about 5 h with a half-time of about 2 h (data not shown).