Overexpression of Mdm2 and MdmX Fusion Proteins Alters p53 Mediated Transactivation, Ubiquitination, and Degradation

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Abstract
Mdm2 and MdmX function as cellular regulators of the p53 tumor suppressor protein. Mdm2, a p53 inducible protein, negatively regulates p53 by inhibiting p53 transcriptional activity and promoting ubiquitin mediated proteasome degradation. The Mdm2 ring finger domain has been shown to possess E3 ligase activity and to be a necessary domain for targeting p53 degradation. MdmX, a p53 binding protein sharing a high degree of structural homology with Mdm2, has emerged as another negative regulator of the p53 tumor suppressor. MdmX has also been shown to block p53 transactivation but unlike Mdm2 cannot induce p53 degradation. Since MdmX also possesses a ring finger domain that allows MdmX to associate with Mdm2, this study focused on elucidating how the ring and zinc fingers of these two proteins affected p53 function. We have generated a series of fusion proteins between Mdm2 and MdmX by swapping the ring finger domains with or without the zinc finger domains and examined how these fusions regulated p53 induced transactivation, ubiquitination, and degradation. All fusions inhibited the transcriptional activity of p53. In the absence of Mdm2, none of the fusion proteins could trigger p53 ubiquitination or degradation. However, in a cell line with endogenous Hdm2, Mdm2:X fusions containing the ring finger domain with or without the zinc finger domain demonstrated p53 ubiquitination presumably through stabilization of Hdm2. Additionally, an Mdm2:XZFRF fusion also degraded p53 when endogenous Hdm2 was present. Results from immunofluorescence studies suggest that p53 is colocalized to the cytoplasm when coexpressed with a Mdm2:X fusion (Mdm2:XZFRF) and that this fusion is capable of stabilizing endogenous Hdm2. Since none of the fusions triggered p53 ubiquitination in cells lacking Mdm2, these results indicate that the E3 ligase domain within the ring finger of Mdm2 when part of MdmX and the MdmX ring finger fused to Mdm2 were not sufficient to trigger p53 ubiquitination, in vivo.