Allosteric activation of latent p53 tetramers.

ABSTRACT

BACKGROUND: The DNA-binding activity of p53 is essential to its function as a tumour suppressor. Point mutations that abolish this activity have been found to occur frequently in the p53 genes of human cancer cells. Wild-type p53 protein assembles into oligomers with latent DNA-binding activity that can be activated in vitro by phosphorylation of a carboxy-terminal regulatory region, catalyzed by protein kinase C or casein kinase II. We have investigated the mechanism underlying this post-translational regulation of p53. Specifically, we have asked the following questions. First, whether the carboxy-terminal regulatory site contributes to p53's ability to form tetramers. Second, whether the latent DNA-binding activity of p53 can be activated in vivo. And third, whether the activation of p53 is reversible. RESULTS: Biophysical molecular-sizing analysis shows that both latent and activated forms of p53 are tetramers. Using a novel method, we have further established that p53 remains tetrameric when bound to DNA. We have also found that p53 can indeed be activated in vivo p53 prepared from cells can be separated into activated and latent forms. Finally, we generated a monoclonal antibody specific for the casein kinase II target site in the carboxy-terminal regulatory region of p53, and used it to demonstrate the allosteric inhibition of in vitro and in vivo activated forms of p53. CONCLUSIONS: p53 protein assembles naturally as a tetramer that can be converted between latent and activated forms by a concerted, allosteric transition. The highly purified, reconstituted system that we have developed, in which the DNA-binding activity of p53 can be reversibly regulated, should facilitate the discovery of agents that can modulate the DNA-binding activity of p53--particularly those that can activate mutant p53 proteins and that may have potential in the design of anti-cancer drugs.