Phosphorylation of the tumor suppressor protein p53 by mitogen-activated protein kinases.

The p53 tumor suppressor protein is tightly regulated in the cell and is phosphorylated at multiple sites by several different protein kinases. We have investigated the phosphorylation of p53 by mitogen-activated protein (MAP) kinase, a protein kinase that plays a central role in mediating many mitogenic and differentiation signals. Recombinant wild-type mouse p53 was phosphorylated in vitro by activated recombinant p42-MAP kinase but not by inactive MAP kinase or by the activating protein, MAP kinase kinase. Phosphorylation of p53 by MAP kinase occurred at two N-terminal sites, threonine residues 73 and 83. Tryptic phosphopeptides of recombinant p53 phosphorylated in vitro by MAP kinase comigrated on two-dimensional maps with p53 from SV3T3 cells labeled in vivo with [32P]orthophosphate, suggesting that MAP kinase targets a site in p53 that is phosphorylated in the cell. Following serum stimulation of quiescent C57MG cells, two p53 kinases, which were resolved by chromatography on Mono Q, were stimulated 15-20-fold within 5 min. Each of these kinase activities co-eluted with myelin basic protein kinase activity and could be inactivated following treatment with protein phosphatase 2A, a serine/threonine phosphatase, or leukocyte antigen receptor, a protein tyrosine phosphatase, suggesting that these activities were members of the MAP kinase family. The two kinase activities from the lysates targeted the same phosphorylation sites on p53 as the purified recombinant MAP kinase. These protein kinase activities were also stimulated following exposure of the cells to ultraviolet radiation, but with slightly delayed kinetics. Phorbol ester treatment of SV3T3 cells led to increased phosphorylation of the peptide containing the residues targeted by MAP kinase. The data suggest that p53 may be phosphorylated by MAP kinase physiologically and that this interaction may be involved in the cell's response to UV exposure, growth factor stimulation, or transformation by oncogenes.