Regulation of p53 by isoforms, stoichiometry, and ubiquitination

Half of the all the cancer cases contain mutations in the p53 gene. The majority of these mutations are missense mutations that occur in the central DNA binding domain. These missense mutations abolish the DNA binding ability of p53 and cause p53 inactivation. Besides, several studies also showed that the DNA-binding defective mutants exert a dominant-negative effect on wild-type p53. Since p53 functions as a tetramer, it is still not known that how many of these mutants are required to inactivate a tetramer. In my study, I showed that the two common DNA-binding defective mutants [p53(R249S) and p53(R273H)] could not efficiently impair the transcriptional activity of p53. At least three mutants were required to inactivate a tetramer. On the other hand, the transactivation-defective mutant p53N? was a powerful inhibitor of p53. Only one p53N? was sufficient to inactivate a tetramer.;As p53 plays an extremely critical role in the prevention of tumorigenesis, it is important to know how it is regulated. It is well known that p53 is regulated by MDM2 in an ubiquitin-mediated proteolytic pathway. However, the precise sites of ubiquitination are still not well defined. I found that apart from the previously described COOH-terminal region of p53, the DNA-binding domain contained additional ubiquitin acceptor sites. I showed that mutation of the COOH-terminal lysine residues or deletion of the entire COOH-terminal region did not entirely abolish the ubiquitination of p53. Nevertheless, removal of the DNA-binding domain decreased ubiquitination and increased p53 stability.;Recently, a novel p53 isoform Deltap53 (Delta257-322) has been discovered. This isoform contains an incomplete DNA binding domain and lacks the nuclear localization signal (NLS). It was reported that Deltap53 is transcriptionally active towards p21CIP1/WAF1 and 14-3-3sigma, but not MDM2 and Bax. However, I found that in contrast to wild-type p53, Deltap53 did not transactivate p21CIP1/WAF1 and MDM2. Furthermore, I also explored whether like other DNA-binding defective mutants of p53, Deltap53 has the potential to interact with p53 and act in a dominant-negative manner. I found that Deltap53 was ineffective in inhibiting the activity of p53. In addition, due to the deletion of nuclear localization signal, Deltap53 was not imported into the nucleus. Similar to p53, Deltap53 could be ubiquitinated by MDM2 but with a shorter half-life. Forcing Deltap53 into the nucleus could restore the protein stability but not the transactivation function. Finally, I showed that targeting Deltap53 into the nucleus could enhance the dominant-negative activity of Deltap53.