The control of p53 ubiquitination by MDM2 provides a model system to define how an E3-ligase functions on a conformationally flexible substrate. The mechanism of MDM2-mediated ubiquitination of p53 has been analyzed by deconstructing, in vitro, the MDM2-dependent ubiquitination reaction. Surprisingly, ligands binding to the hydrophobic cleft of MDM2 do not inhibit its E3-ligase function. However, peptides from within the DNA binding domain of p53 that bind the acid domain of MDM2 inhibit ubiquitination of p53, localizing a motif that harbors a key ubiquitination signal. The binding of ligands to the N-terminal hydrophobic cleft of MDM2 reactivates, in vitro and in vivo, MDM2-catalyzed ubiquitination of p53F19A, a mutant p53 normally refractory to MDM2-catalyzed ubiquitination. We propose a model in which the interaction between the p53-BOX-I domain and the N terminus of MDM2 promotes conformational changes in MDM2 that stabilize acid-domain interactions with a ubiquitination signal in the DNA binding domain of the p53 tetramer.

Although the N-terminal BOX-I domain of the tumor suppressor protein p53 contains the primary docking site for MDM2, previous studies demonstrated that RNA stabilizes the MDM2⅐p53 complex using a p53 mutant lacking the BOX-I motif. In vitro assays measuring the specific activity of MDM2 in the ligand-free and RNAbound state identified a novel MDM2 interaction site in the core domain of p53. As defined using phage-peptide display, the RNA⅐MDM2 isoform exhibited a notable switch in peptide binding specificity, with enhanced affinity for novel peptide sequences in either p53 or small nuclear ribonucleoprotein-U (snRNP-U) and substantially reduced affinity for the primary p53 binding site in the BOX-I domain. The consensus binding site for the RNA⅐MDM2 complex within p53 is SGXLLGESXF, which links the S9 -S10 ␤-sheets flanking the BOX-IV and BOX-V motifs in the core domain and which is a site of reversible conformational flexibility in p53. Mutation of conserved amino acids in the linker at Ser 261 and Leu 264 , which bridges the S9 -S10 ␤-sheets, stimulated p53 activity from reporter templates and increased MDM2-dependent ubiquitination of p53. Furthermore, mutation of the conserved Phe 270 within the S10 ␤-sheet resulted in a mutant p53, which binds more stably to RNA⅐MDM2 complexes in vitro and which is strikingly hyper-ubiquitinated in vivo. Introducing an Ala 19 mutation into the p53 F270A protein abolished both RNA⅐MDM2 complex binding and hyper-ubiquitination in vivo, thus indicating that p53 F270A protein hyper-ubiquitination depends upon MDM2 binding to its primary site in the BOX-I domain. Together, these data identify a novel MDM2 binding interface within the S9 -S10 ␤-sheet region of p53 that plays a regulatory role in modulating the rate of MDM2-dependent ubiquitination of p53 in cells.

We have discovered that a small peptide is sufficient to mimic p21(WAF1) function and inhibit the activity of a critical G1 cyclin-Cdk complex, preventing pRb phosphorylation and producing a G1 cell-cycle arrest in tissue culture cell systems. This makes cyclin D1-Cdk4 a realistic and exciting target for the design of novel synthetic compounds that can act as anti-proliferative agents in human cells.

These results demonstrate that a p16-derived peptide can mediate three of the known functions of p16: firstly, it interacts with cdk4 and cdk6; secondly, it inhibits pRb phosphorylation in vitro and in vivo; and thirdly, it blocks entry into S phase. The fact that one small synthetic peptide can enter the cells directly from the tissue culture medium to inhibit pRb phosphorylation and block cell-cycle progression makes this an attractive approach for future peptidometic drug design. Our results suggest a novel and exciting means by which the function of the p16 suppressor gene can be restored in human tumours.

The p53-inducible gene product p21 WAF1/CIP1 plays a critical role in regulating the rate of tumor incidence, and identifying mechanisms of its post-translational regulation will define key pathways that link growth control to p21-dependent tumor suppression. A eukaryotic cell model system has been developed to determine whether protein kinase signaling pathways that phosphorylate human p21 exist in vivo and whether such pathways regulate the binding of p21 to one of its key target proteins, proliferating cell nuclear antigen (PCNA). Although human p21 expressed in Sf9 cells is able to form a complex with human PCNA, the inclusion of cell-permeable phosphatase inhibitors renders p21 protein inactive for PCNA binding. The treatment of this inactive isoform of p21 with alkaline phosphatase restores its binding to PCNA, suggesting that p21 expressed in Sf9 cells is subject to reversible phosphorylation at a key regulatory site(s). A biochemical approach was subsequently used to map the phosphorylation sites within p21, whose modification in vitro can inhibit p21-PCNA complex formation, to the C-terminal domain at residues Thr 145 or Ser 146 . A phospho-specific antibody was developed that only bound to full-length p21 protein after phosphorylation in vitro at Ser 146 , and this reagent was further used to demonstrate that the inactive isoform of p21 recovered from Sf9 cells treated with phosphatase inhibitors had been phosphorylated in vivo at Ser 146 . These data identify the first phosphorylation site within the C-terminal regulatory domain of p21 whose modification in vivo modulates p21-PCNA interactions and define a eukaryotic cell model that can be used to study post-translational signaling pathways that regulate p21.