Turnover of the p53 tumor suppressor protein is mediated by Mdm2 through the ubiquitin proteolysis pathway. p300, a co-activator for p53, also participates in this process by complexing with Mdm2. We now report that the mutant Mdm2, defective in p53 binding, does not promote p53 ubiquitination and degradation in vivo or inhibit p53 transcriptional activation. By contrast, the mutant Mdm2, defective in p300 binding, still retains its activity to promote 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 ubiquitination of cancer-derived p53 mutants, but only wild-type Mdm2 can cause their degradation. Furthermore, adenoviral oncoprotein, 12S.E.1A, but not its deletion mutant that lacks p300 binding, was shown to decrease in vivo ubiquitination of mutant p53. Taken together, these results provide genetic evidence that p300 plays a pivotal role in the regulation of Mdm2-mediated p53 turnover by integrating the cellular ubiquitination and proteolytic processes.
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