Critical role for Daxx in regulating Mdm2

The tumour suppressor p53 induces apoptosis or cell-cycle arrest in response to genotoxic and other stresses(1,2). In unstressed cells, the anti-proliferative effects of p53 are restrained by mouse double minute 2 ( Mdm2), a ubiquitin ligase ( E3) that promotes p53 ubiquitination and degradation(3). Mdm2 also mediates its own degradation through auto-ubiquitination. It is unclear how the cis- and trans-E3 activities of Mdm2, which have opposing effects on cell fate, are differentially regulated. Here, we show that death domain-associated protein ( Daxx) 4 is required for Mdm2 stability. Downregulation of Daxx decreases Mdm2 levels, whereas overexpression of Daxx strongly stabilizes Mdm2. Daxx simultaneously binds to Mdm2 and the deubiquitinase Hausp, and it mediates the stabilizing effect of Hausp on Mdm2. In addition, Daxx enhances the intrinsic E3 activity of Mdm2 towards p53. On DNA damage, Daxx dissociates from Mdm2, which correlates with Mdm2 self-degradation. These findings reveal that Daxx modulates the function of Mdm2 at multiple levels and suggest that the disruption of the Mdm2-Daxx interaction may be important for p53 activation in response to DNA damage.