MdmX Inhibits ARF Mediated Mdm2 Sumoylation

Abstract

Mdm2, by virtue of an intrinsic E3 ubiquitin ligase activity, is capable ofautoubiquitination and the ubiquitination of the p53 tumor suppressor protein.Additionally, Hdm2 has been reported to undergo a p14ARF-dependentsumoylationwith concurrent Hdm2 stabilization. In this present work, we reportthat MdmX can undergo ARF-mediated sumoylation similar to that reported forMdm2. When coexpressed, MdmX overexpression results in a dose-dependentinhibition of Mdm2 sumoylation and a concurrent increase in Mdm2ubiquitination. This switch from Mdm2 sumoylation to Mdm2 ubiquitination mayexplain the destablization of Mdm2 previously observed in cells overexpressingboth ARF and MdmX. Given that MdmX can heterodimerize with Mdm2 andseparately associate with ARF we employed a series of MdmX mutants toexamine how MdmX blocks Mdm2 sumoylation. A MdmX miniprotein capable ofbinding to ARF, but not p53 or Mdm2 was able to competitively inhibit Mdm2sumoylation and reverse ARF mediated activation of p53 transactivation. Takentogether, these results demonstrate that MdmX can affect post-translationalmodification and stability of Mdm2 and p53 activity through interaction with ARF.