INHIBITION OF P53 DNA-BINDING BY HUMAN PAPILLOMAVIRUS E6 PROTEINS

Transformation by the human papillomavirus (HPV) early gene products, E6 and E7, involves their interaction with cellular proteins p53 and Rb. Using glutathione S-transferase (GST) fusion proteins, we found that HPV E6 bound human p53 and that the relative efficiency of binding varied such that the GST-HPV type 16 E6 (16E6) protein bound p53 with highest affinity, followed by GST-31E6, GST-18E6, and GST-11E6. The GST-E6 fusion proteins were sufficient for binding p53 purified from a baculovirus expression system as well as in vitro translation sources, while no association was observed with GST-18E7 or a GST-16E6 mutant bearing a five-amino-acid deletion in E6. When the site-specific DNA binding activity of p53 was examined in the presence of GST-E6 proteins, an inhibition of DNA binding was observed. The degree of inhibition correlated with the relative affinity of different E6 proteins for p53; thus, GST-16E6 was the most potent inhibitor of p53 DNA binding activity, and GST-11E6 was the least effective. Prevention of p53 DNA binding is likely to play a role in the abrogation of the transcriptional activity of p53 by HPV E6 and provides a further mechanism for E6 disruption of p53 growth suppressor function in addition to its role in directing specific degradation of p53 through the ubiquitin-mediated pathway. The variation in inhibition of DNA binding seen with the various E6 proteins may thus contribute to the differences in oncogenic potential seen among the HPV types.