Novel effect of the high risk-HPV E7 CKII phospho-acceptor site on polarity protein expression

Identification and characterisation of novel potential phospho-acceptor sites in HPV-16 E7

Several studies have described functional regulation of high-risk human papillomaviruses (HPVs), E6 and E7 oncoproteins via posttranslational modifications (PTMs). However, how these PTMs modulate the activity of E6 and E7, particularly in their targeting of cellular proteins, is not completely understood. In this study, we show that HPV16 E7 can be phosphorylated by casein kinase I (CKI) and glycogen synthase kinase 3 (GSK3). This principal phosphorylation occurs at threonine residues 5 and 7 with a more minor role for residues 19-20 in the N-terminal region of 16 E7. Intriguingly, whilst mutational analyses suggest that residues 5 and 7 may be dispensable for the transformation of primary baby rat kidney cells by E7, intact residues 19 and 20 are required. Furthermore, negative charges at these residues (TT19-20DD) enhance the pRb-E7 interaction and cells display increased proliferation and invasion capacities. Using a proteomic approach with a phosphorylated peptide spanning the TT19-20 region of HPV16 E7, we have identified a panel of new, phospho-specific E7 interacting partners. These results shed new light on the complexity of N-terminal phosphorylation of E7 and how this can contribute towards expanding the repertoire of E7 targeted pathways.

Viral manipulation of cell polarity signalling

Cell polarity refers to the asymmetric distribution of biomacromolecules that enable the correct orientation of a cell in a particular direction. It is thus an essential component for appropriate tissue development and function. Viral infections can lead to dysregulation of polarity. This is associated with a poor prognosis due to viral interference with core cell polarity regulatory scaffolding proteins that often feature PDZ (PSD-95, DLG, and ZO-1) domains including Scrib, Dlg, Pals1, PatJ, Par3 and Par6. PDZ domains are also promiscuous, binding to several different partners through their C-terminal region which contain PDZ-binding motifs (PBM). Numerous viruses encode viral effector proteins that target cell polarity regulators for their benefit and include papillomaviruses, flaviviruses and coronaviruses. A better understanding of the mechanisms of action utilised by viral effector proteins to subvert host cell polarity sigalling will provide avenues for future therapeutic intervention, while at the same time enhance our understanding of cell polarity regulation and its role tissue homeostasis.