The Human Papillomavirus (HPV) E6* Proteins from High-risk, Mucosal HPVs Can Direct Degradation of Cellular Proteins in the Absence of Full-length E6 Protein

The E6 oncoproteins from high-risk mucosotrophic human papillomaviruses (HPVs) target a range of cellular proteins for proteasome-mediated degradation. Apart from the tumor suppressor p53 and proapoptotic Bcl-2 family member Bak, many targets contain class 1 PDZ domains and are involved in cell junction stability and signaling. The targeting mechanism is considered to function by the E6 protein acting as an adaptor molecule linking a cellular ubiquitin ligase to the target protein. In each case, whether the target is the p53 tumor suppressor or a member of the group of PDZ domain-containing targets, this mechanism relies on a direct interaction between E6 and its cellular target. This study focuses on the impact of the HPV type 18 (HPV-18) E6*I protein on the stability of Akt, Dlg, MAGI-1, MAGI-2, and Scribble. We show that HPV-18 E6* expression can downregulate the expression levels of Akt, Dlg, and Scribble in the absence of full-length HPV-18 E6 protein. The reduction in Dlg levels by E6* is independent of transcription and does not require a direct interaction between the two proteins although the proteasome pathway is involved. Further, we provide evidence that activation of certain signal transduction pathways has a profound effect on the targeting of Dlg by E6* and suggest that high-risk HPV E6 oncoproteins can target certain substrates both directly and indirectly through the E6* proteins and may cooperate in their degradation.