Molecular basis of Pirh2-mediated p53 ubiquitylation

Pirh2 is one of several ubiquitin ligases known to modify and negatively regulate p53. Solution studies reveal the structures of the three Pirh2 domains and indicate that the C-terminal domain of Pirh2 interacts with the p53 tetramerization domain. Additional data suggest that Pirh2 preferentially modifies the tetrameric, transcriptionally active form of p53 for proteasome-mediated degradation. Pirh2 (p53-induced RING-H2 domain protein; also known as Rchy1) is an E3 ubiquitin ligase involved in a negative-feedback loop with p53. Using NMR spectroscopy, we show that Pirh2 is a unique cysteine-rich protein comprising three modular domains. The protein binds nine zinc ions using a variety of zinc coordination schemes, including a RING domain and a left-handed β-spiral in which three zinc ions align three consecutive small β-sheets in an interleaved fashion. We show that Pirh2-p53 interaction is dependent on the C-terminal zinc binding module of Pirh2, which binds to the tetramerization domain of p53. As a result, Pirh2 preferentially ubiquitylates the tetrameric form of p53 in vitro and in vivo , suggesting that Pirh2 regulates protein turnover of the transcriptionally active form of p53.