Oncogenic human papillomavirus E6 proteins target the discs large tumour suppressor for proteasome-mediated degradation

Previous studies have shown that the oncogenic HPV E6 proteins form a complex with the human homologue of the Drosophila tumour suppressor protein, discs large (Dlg), This is mediated by the carboxy terminus of the E6 proteins and involves recognition of at least one PDZ domain of Dig, This region of E6 is not conserved amongst E6 proteins from the low risk papillomavirus types and, hence, binding of HPV E6 proteins to Dig correlates with the oncogenic potential of these viruses. We have performed studies to investigate the consequences of the interaction between E6 and Dig, Mutational analysis of both the HPV18 E6 and Dig proteins has further defined the regions of E6 and Dig necessary for complex formation. Strikingly, co-expression of wild type HPV18 E6 with Dig in vitro or in vivo results in a dramatic decrease in the amount of Dig protein, whereas mutants of E6 which fail to complex with Dig have minimal effect on Dig protein levels. The oncogenic HPV16 E6 also decreased the Dig levels, but this was not observed with the low risk HPV11 E6 protein. Moreover, a region within the first 544 amino acids of Dig containing the three PDZ domains confers susceptibility to E6 mediated degradation. Finally, treatment of cells with a proteasome inhibitor overrides the capacity of E6 to degrade Dig, These results demonstrate that Dig is targeted by high risk HPV E6 proteins for proteasome mediated degradation.