The Role of the Ubiquitin Proteasome Pathway in Papillomavirus Pathogenicity

Tomaić, Vjekoslav (2009). The Role of the Ubiquitin Proteasome Pathway in Papillomavirus Pathogenicity. PhD thesis The Open University.



To establish an infection in the host cell, Papillomaviruses (PVs) encode two major oncoproteins, E6 and E7, which interact with numerous cellular proteins and interfere with many cellular pathways. We use HPV E6 and RhPV-1 E7 oncoproteins as tools to investigate some of the most important characteristics of the high-risk HPV E6 proteins: their association with the ubiquitin pathway and ability to direct proteasomal degradation of PDZ domain-containing proteins. We show that E6 protein levels are dependent on the E6AP ubiquitin ligase: in its absence, E6 is degraded in a proteasome dependent manner. A proteomic analysis for HPV-18 E6 interacting partners showed that HPV-18 E6 interacts with EDD, another HECT domain ubiquitin ligase. EDD does not direct the degradation of p53 or PDZ domain-containing substrates, but appears to regulate E6AP levels, with consequent effects on E6 protein levels and its p53 targeting. These studies demonstrate a complex interplay between E6, EDD and E6AP for regulating E6’s degradation of its substrate proteins. Rhesus papillomavirus 1 (RhPV-1) is a high-risk mucosal papillomavirus, but its E6 protein has no PDZ-binding motif. However, we show a remarkable evolutionary conservation, with the PDZ-binding motif present on the RhPV-1 E7 protein instead. Furthermore, this directs the binding of RhPV-1 E7 to Par3, a PDZ domain-containing protein controlling the polarity regulation pathway also controlled by hDlg and hScrib, the PDZ domain-containing targets of HPV-18 and HPV-16 E6. RhPV-1 E7 degrades Par3 analogously to HPV E6’s degradation of its PDZ substrates, and also appears to interact with EDD. These studies demonstrate that PDZ domain-containing cell polarity regulators and critical components of the ubiquitin proteasome pathway are common targets of evolutionarily diverse oncogenic mucosal papillomaviruses. This suggests that these pathways represent essential steps in the viral life cycles and in these viruses’ ability to induce malignancy.

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