Identification of E6AP-independent degradation targets of HPV E6

The high-risk Human Papillomavirus (HPV) E6 oncoprotein is known to contribute to human malignancy by targeting several of its cellular substrates through the ubiquitin-mediated degradation pathway. Previous studies have revealed that E6 interacts with the E6AP ubiquitin-protein ligase and directs its ubiquitylation activity toward several specific cellular proteins, one of the most important of which is p53. However, the role of E6AP in the degradation of many other E6 substrates is still ambiguous because loss of E6AP also induces a loss of E6 expression. To examine this further, we used CRISPR-edited E6AP knockout cells to perform E6 degradation assays in the presence of a catalytically inactive mutant form of E6AP, thus ensuring the stabilization of E6 but with the ligase itself being functionally inactive. Using this system, we found that E6 can mediate the degradation of several PDZ domain-containing proteins independently of E6AP ubiquitin ligase activity. This study thus opens up ways to investigate other possible components of the cellular ubiquitin proteasome pathway that E6 might utilize to target these substrates.