Regulation of Mdm2-directed degradation by the C terminus of p53.

The stability of the p53 tumor suppressor protein is regulated by interaction with Mdm2, the product of a p53-inducible gene. Mdm2-targeted degradation of p53 depends on the interaction between the two proteins and is mediated by the proteasome. We show here that in addition to the N-terminal Mdm2 binding domain, the C terminus of p53 participates in the ability of p53 to be degraded by Mdm2. In contrast, alterations in the central DNA binding domain of p53, which change the conformation of the p53 protein, do not abrogate the sensitivity of the protein to Mdm2-mediated degradation. The importance of the C-terminal oligomerization domain to Mdm2-targeted degradation of p53 is likely to reflect the importance of oligomerization of the full-length p53 protein for interaction with Mdm2, as previously shown in vitro. Interestingly, the extreme C-terminal region of p53, outside the oligomerization domain, was also shown to be necessary for efficient degradation, and deletion of this region stabilized the protein without abrogating its ability to bind to Mdm2. Mdm2-resistant p53 mutants were not further stabilized following DNA damage, supporting a role for Mdm2 as the principal regulator of p53 stability in cells. The extreme C terminus of the p53 protein has previously been shown to contain several regulatory elements, raising the possibility that either allosteric regulation of p53 by this domain or interaction between this region and a third protein plays a role in determining the sensitivity of p53 to Mdm2-directed degradation.