Dependence on Mdm2 for Mdm4 inhibition of p53 activity.

Both Mdm2 and Mdm4 inhibit p53 activity by masking of its transcriptional activation domain. In addition, Mdm2 functions as an E3 ubiquitin ligase, targeting p53 for degradation. The Mdm4 amino terminus binds wild type and mutant p53 while its RING domain, which lacks E3 ligase activity, is required for heterodimerization with Mdm2. To determine how these domains of Mdm4 regulate p53, we generated mouse models with either a deletion of the Mdm4 RING domain (Mdm4) or all of Mdm4 (Mdm4) on a hypomorphic (p53) background. Mdm4 mice exhibited elevated p53 levels and activity, albeit to a lesser extent than mice with complete Mdm4 loss, indicating that the amino terminus of Mdm4 contributes to p53 inhibition. Moreover, in the absence of Mdm2, neither the deletion of the Mdm4 RING domain nor the complete loss of Mdm4 further increased p53 protein levels on a mutant p53 background, indicating that Mdm4 modulates Mdm2 in its regulation of p53 stability. Collectively, our findings suggest that Mdm4 contributes to p53 inhibition by modulating Mdm2 activity via both its amino terminus and RING domains.