Mdm2 mutant defective in binding p300 promotes ubiquitination but not degradation of p53

Turnover of the p53 tumor suppressor protein is mediated by Mdm2 through th e ubiquitin proteolysis pathway. p300, a co-activator for p53, also partici pates in this process by complexing with Mdm2. We now report that the mutan t Mdm2, defective in p53 binding, does not promote p53 ubiquitination and d egradation in vivo or inhibit p53 transcriptional activation. By contrast, the mutant Mdm2, defective in p300 binding, still retains its activity to p romote p53 ubiquitination and to inhibit p53 transcriptional activation but fails in promoting p53 degradation. We also show that both wild-type Mdm2 and the mutant Mdm2, defective in p300 binding, can promote the ubiquitinat ion of cancer-derived p53 mutants, but only wild-type Mdm2 can cause their degradation. Furthermore, adenoviral oncoprotein, 12S.E.1A, but not its del etion mutant that lacks p300 binding, was shown to decrease in vivo ubiquit ination of mutant p53. Taken together, these results provide genetic eviden ce that p300 plays a pivotal role in the regulation of Mdm2-mediated p53 tu rnover by integrating the cellular ubiquitination and proteolytic processes .