Nuclear import of p53 in relation to MDM2-mediated degradation and ubiquitination / 中华肿瘤杂志

ABSTRACT

<p><b>OBJECTIVE</b>To study the function of p53 nuclear import in murine double minute 2 (MDM2)-mediated ubiquitination and degradation.</p><p><b>METHODS</b>Plasmid containing mutant p53-GFP was constructed by site-directed mutagenesis by which 5 amino scid residues in the nuclear localization signal (NLS) were replaced by alanine to produce mutant p53KRKKK-GFP. After being fused with pEGF-Nuc (NLS containing SV40) to produce p53KRKKK-NLS-GFP, it was transfected into U20S cells. Localization, degradation and ubiquitination of p53 and MDM2 proteins were assessed by fluorescent staining, Western blot and ubiquitination analysis in MDM2 or MDM2-NLS co-transfected U20S cells.</p><p><b>RESULTS</b>p53KRKKK-GFP was located in cytoplasm, and was not degraded by either MDM2 or MDM2-NLS mutation, but could be ubiquitinated; p53KRKKK-NLS-GFP could be brought back to nucleus by SV-40 NLS, so could be both degraded and ubiquitinated by either MDM2 or MDM2-NLS; Wild type p53 and mutant NLS could be ubiquitinated by either wild type MDM2 or mutant NLS. Ubiquitination happened to be even more efficient in cytoplasm when p53KRKKK and MDM2-NLS co-localization, but not degraded.</p><p><b>CONCLUSION</b>Nuclear import is required for p53 degradation mediated by MDM2, but not for ubiquitination. p53 can be efficiently ubiquitinated in cytoplasm.</p>