MDM2-ARF complex regulates p53 sumoylation.

The p53 tumor suppressor is regulated by MDM2-mediated ubiquitination and degradation. Ubiquitination of p53 is regulated by ARF, which binds to MDM2 and inhibits its E3 ligase function. P53 is also subjected to modification by conjugation of SUMO-1. We found that a p53 mutant deficient for MDM2 binding (p53(14Q19S)) is poorly sumoylated in vivo compared to wild-type p53. Overexpression of MDM2 increases the level of p53 sumoylation, which is further stimulated by expression of ARF. Stimulation of p53 sumoylation requires a highly conserved region (102-116) encoded by exon 2 of ARF and correlates with the ability of ARF to target p53 to the nucleolus. An MDM2 deletion mutant (MDM2(Delta222-437)) with activated cryptic nucleolar localization signal also targets p53 to the nucleolus and efficiently promotes p53 sumoylation in the absence of ARF. Direct targeting of p53 to the nucleolus enhances its sumoylation in an MDM2- and ARF-dependent fashion. These results show that p53 sumoylation is regulated by MDM2- and ARF-mediated nucleolar targeting.