An intact HDM2 RING-finger domain is required for nuclear exclusion of p53

The p53 tumour-suppressor protein is negatively regulated by HDM2. Recent r eports indicate that the leucine-rich nuclear-export sequence (NES) of HDM2 enables it to shuttle to the cytoplasm, and that this activity is required for degradation of p53. However, it is unclear whether HDM2 is involved in nuclear export of p53, partly because p53 has itself been shown to contain a functional NES within its tetramerization domain. Here we show that co-e xpression of HDM2 with green fluorescent protein (GFP)-tagged p53 causes re distribution of p53 from the nucleus to the cytoplasm of the cell. This act ivity is dependent on binding of p53 to HDM2, and requires an intact p53 NE S, but is independent of the HDM2 NES. A mutant of the HDM2 RING-finger dom ain that is unable to ubiquitinate p53 does not cause relocalization of p53 , indicating that ubiquitin ligation or other activities of this region of HDM2 may be necessary for its regulation of p53 localization.