Ring Finger And WD Repeat Containing Domain Protein 3 (RFWD3)

In unstressed cells, the tumor suppressor p53 is maintained at low levels by ubiquitin-mediated proteolysis mainly through Mdm2. An E3 ubiquitin ligase RFWD3 forms a complex with Mdm2 and p53 to synergistically ubiquitinate p53 and is required to stabilize p53 in the late response to DNA damage. This process is regulated by the DNA damage checkpoint, because RFWD3 is phosphorylated by ATM/ATR kinases and the phosphorylation mutant fails to stimulate p53 ubiquitination. In vitro experiments suggest that RFWD3 is a p53 E3 ubiquitin ligase and that RFWD3–Mdm2 complex restricts the polyubiquitination of p53 by Mdm2. RFWD3 as a positive regulator of p53 stability when the G₁ cell cycle checkpoint is activated and provides an explanation for how p53 is protected from degradation in the presence of high levels of Mdm2.