Identification of p53 Sequence Elements That Are Required for MDM2-Mediated Nuclear Export

It has been demonstrated that MDM2 can differentially regulate subcellular distribution of p53 and its close structural homologue p73. In contrast to MDM2-mediated p53 nuclear export, p73 accumulates in the nucleus as aggregates that colocalize with MDM2. Distinct distribution patterns of p53 and p73 suggest the existence of unique structural elements in the two homologues that determine their MDM2-mediated relocalization in the cell. Using a series of p53/p73 chimeric proteins, we demonstrate that three regions of p53 are involved in the regulation of MDM2-mediated nuclear export. The DNA binding domain (DBD) is involved in the maintenance of a proper conformation that is required for functional activity of the nuclear export sequence (NES) of p53. The extreme C terminus of p53 harbors several lysine residues whose ubiquitination by MDM2 appears to be the initial event in p53 nuclear export, as evidenced by the impaired nucleocytoplasmic shuttling of p53 mutants bearing simultaneous substitutions of lysines 370, 372, 373, 381, 382, and 386 to arginines (6KR) or alanines (6KA). Finally, the region between the DBD and the oligomerization domain of p53, specifically lysine 305, also plays a critical role in fully revealing p53NES. We conclude that MDM2-mediated nuclear export of p53 depends on a series of ubiquitination-induced conformational changes in the p53 molecule that lead to the activation of p53NES. In addition, we demonstrate that the p53NES may be activated without necessarily disrupting the p53 tetramer.