Control of p53 in the DNA Damage Response
The activity of the tumour suppressor protein p53 is critically controlled by proteolysis. When cells are exposed to a variety of stress stimuli including hypoxia, DNA damage or the action of certain oncogenes this degradative pathway is inhibited and p53 protein levels rise inducing cell cycle arrest and apoptosis. Two cellular proteins, Mdm2 and ARF have been discovered to play a critical role in regulating the specific stability of p53. Mdm2, binds to the N terminus of p53 recognising a specific peptide motif and targets p53 to the proteosome. We have established in vivo and in vitro assays for the ubquitination of p53 by Mdm2. The Mdm2 protein acts as a specific E3 ubiquitin ligase and the ARF protein binds to Mdm2 and inhibits its ligase activity. The recognition of p53 as a substrate for ubiquitination requires several discrete properties including the N terminal Mdm2 recognition motif, the oligomerisation domain and C terminal lysine residues. It also requires that the Mdm2 protein and p53 enjoy the correct sub cellular localisation. We are searching for other substrates of this pathway. We have recently localised the ARF Mdm2 interaction using pepscan libraries of ARF. In addition to its affects on the subcellular localisation of Mdm2 ARF and ARF derived peptides act directly as inhibitors of the E3 activity of Mdm2. Recently we have also discovered that p53 is modified by the small ubiquitin like protein SUMO and this modification may inhibit the degradation of p53. The ability to induce the p53 response with non genotoxic agents combined with the recognition that p53 mutant human tumours lack the Mdm2 dependant degradation pathway opens up many exciting new approaches to drug discovery in the p53 pathway.
David Lane, Carol Bindon, Dimitris Xirodimas, Christine Blattner, Carol Midgley, Mark Saville, Alison Sparks, Sonia Lain, 1Joana Desterro, 1Manuel Rodriguez and 1Ron Hay
University of Dundee,