HPV-18 E6*I protein modulates the E6-directed degradation of p53 by binding to full-length HPV-18 E6.

We have previously demonstrated that ectopic expression of the HPV-18 E6*I protein has an antiproliferative effect in cells derived from HPV-containing cervical tumours. This effect correlated with the ability of E6*I to inhibit the E6-mediated degradation of p53 both in vitro and in vivo and with an increase in p53 transcriptional trans-activation. The observation that the E6*I protein can interact with both full-length HPV-18 E6 and E6-AP proteins in vitro indicated the mechanism by which this activity was mediated. In this study we describe a mutational strategy to attempt to differentiate between the E6-AP and full-length HPV-18 E6 interactions, with respect to the biological function of E6*I. We identify regions of the E6*I protein essential for its interaction with full-length E6 and important for its interaction with E6-AP. We show that a mutant of E6*I which is unable to bind to full-length HPV-18 E6 protein is unable to inhibit the E6-directed degradation of p53 and is also unable to inhibit the proliferation of a cervical tumour-derived cell line. Finally, we show that inhibition of transformed cell growth by E6*I protein correlates with its ability to induce apoptosis in a p53-dependent manner. These results raise the intriguing possibility of using E6*I as a basis for therapeutic intervention in HPV-associated tumours.