Featured Article - May 2009
Short description: Pirh2 promotes the ubiquitylation of the transcriptionally active form of p53 using an unusual two-site interaction strategy.Nat. Struc. Mol. Biol. 15, 1334-1342 (2008)
p53 has a vital role in preventing cancer and suppressing tumors by instructing cells to commit suicide by apoptosis or by preventing cell division until DNA damage within the cell is repaired. p53 protein levels in the cell are tightly regulated by ubiquitin E3 ligases, and, because of their key role in the p53 pathway, these enzymes are of interest as therapeutic targets.
E3 ligases negatively regulate p53 activity by promoting its ubiquitylation and its consequent degradation by the 26S proteasome. The best characterized p53 E3 ligase is Mdm2, but several others have been identified recently, including Pirh2, COP1, TOPORS and ARF-BP1. In common with many E3 ligases, Pirh2 contains a RING-H2 domain, but its structural and exact role are unclear.
Using NMR spectroscopy, Sheng et al. 1 from PSI NESG determined the three-dimensional structure of the three individual domains of Pirh2: an N-terminal domain (NTD), a RING domain and a C-terminal domain (CTD). Pirh2 binds nine zinc ions: six are located in the NTD, two in the RING domain and one in the CTD. Pirh2 contains a novel zinc-coordinating motif, conserved in eukaryotes, that consists of a three-turn b-spiral held together by three zinc ions.
The authors started mapping the interaction between Pirh2 and p53 and uncovered a surprise. Pirh2 interacts with p53 at two sites: a primary binding site in the CTD of Pirh2 (residues 249–256) that targets the p53 tetramerization domain; and a secondary site in the NTD (residues 120–137) that interacts with the p53 DNA-binding domain. By contrast, Mdm2 targets p53's transactivation domain, which is distinct from the tetramerization and DNA-binding domains.
Further investigations revealed that Pirh2 binds only to tetrameric p53, and not to dimeric and monomeric forms. This is significant because p53 is only transcriptionally active as a tetramer. In vivo experiments by Sheng et al. show that Pirh2 efficiently targets and degrades only tetrameric, transcriptionally active p53 in the cell.
The authors propose a new working model for Pirh2-mediated p53 ubiquitylation. They suggest that the C-terminal interaction with the tetramerization domain provides most of the binding energy, and that the NTD interacts transiently with one or more of the p53 DNA-binding domains. This would position p53 ready for interaction with a ubiquitin-charged E2 enzyme, promoting ubiquitylation and downregulating p53 protein levels within cells after they have recovered from DNA damage or other stress.
E3 ligases are the main regulators of p53 protein levels and are key targets for researchers aiming to enhance growth inhibition and apoptosis in cancer cells. The finding that Pirh2 binds to different sites from Mdm2 offers an alternative strategy for p53 regulation.
Y. Sheng et al. Molecular basis of Pirh2-mediated p53 ubiquitylation. Nature Struct.
Mol. Biol. 15, 1334-1342 (2008). doi:10.1038/nsmb.1521