MdmX inhibits ARF mediated Mdm2 sumoylation

Mdm2, by virtue of an intrinsic E3 ubiquitin ligase activity, is capable of autoubiquitination and the ubiquitination of the p53 tumor suppressor protein. Additionally, Hdm2 has been reported to undergo a p14ARF-dependent sumoylation with concurrent Hdm2 stabilization. In this present work, we report that MdmX can undergo ARF-mediated sumoylation similar to that reported for Mdm2. When coexpressed, MdmX overexpression results in a dose-dependent inhibition of Mdm2 sumoylation and a concurrent increase in Mdm2 ubiquitination. This switch from Mdm2 sumoylation to Mdm2 ubiquitination may explain the destablization of Mdm2 previously observed in cells overexpressing both ARF and MdmX. Given that MdmX can heterodimerize with Mdm2 and separately associate with ARF we employed a series of MdmX mutants to examine how MdmX blocks Mdm2 sumoylation. A MdmX miniprotein capable of binding to ARF, but not p53 or Mdm2 was able to competitively inhibit Mdm2 sumoylation and reverse ARF mediated activation of p53 transactivation. Taken together, these results demonstrate that MdmX can affect post-translational modification and stability of Mdm2 and p53 activity through interaction with ARF.