Cancer-Causing Human Papillomavirus E6 Proteins Display Major Differences in the Phospho-Regulation of Their PDZ Interactions

Abstract

Previous studies have shown that the cancer-causing high-risk human papillomavirus (HPV) E6 oncoproteins have PDZ binding potential, an activity which is important for their ability to support the viral life cycle and to cooperate in the induction of malignancy. However, PDZ interactions are not constitutive, and they can be negatively regulated by phosphorylation within the E6 PDZ binding motif (PBM). In this study, we have investigated the differential regulation of the HPV E6 PBMs from diverse highrisk HPV types. We show that, depending on the HPV type, PDZ binding activity can be regulated by phosphorylation with protein kinase A (PKA) or AKT, which, in turn, inhibits PDZ recognition. Such regulation is highly conserved between E6 proteins derived from HPV-16, HPV-18, and HPV-58 while being somewhat weaker or absent from other types such as HPV-31, HPV-33, and HPV-51. In the case of HPV31, PKA phosphorylation occurs within the core of the E6 protein and has no effect on PDZ interactions, and this demonstrates a surprising degree of heterogeneity among the different high-risk HPV E6 oncoproteins in how they are regulated by different cellular signaling pathways. IMPORTANCEThis study demonstrated that the cancer-causing HPV E6 oncoproteins are all subject to posttranslational modification of their extreme C-terminal PDZ binding motifs through phosphorylation. However, the identities of the kinase are not the same for all HPV types. This demonstrates a very important divergence between these HPVs, and it suggests that changes in cell signaling pathways have different consequences for different high-risk virus infections and their associated malignancies.H uman papillomaviruses (HPVs) are the causative agents of cervical cancer, which remains a leading cause of death in women throughout the world. Over 120 different HPV types have been identified, 12 of which are defined as cancer causing (1, 2). Of these, HPV-16 and HPV-18 are the most important, accounting for approximately 70% of cervical cancers. The remaining cancers are caused by other high-risk (HR) HPV types, which include 2). HPVinduced carcinogenesis arises from the combined activity of the two major viral oncoproteins E6 and E7, which, by deregulating multiple cellular pathways, including cell cycle control and apoptosis, ultimately induce cell immortalization and, eventually, malignancy (3, 4).A unique characteristic of the HR HPV E6 oncoproteins is the presence of a class I PDZ (PSD-95/Dlg/ZO-1) binding motif (PBM) at the extreme carboxy terminus, which is absent in the low-risk (LR) non-cancer-causing HPV E6 proteins (5, 6). This region of E6 allows it to interact with a number of cellular PDZ domain-containing proteins, many of which are involved in the regulation of cell junctional integrity and cell signaling pathways (reviewed in reference 7). The first such targets to be identified were the cell polarity regulators Discs Large (hDlg1) (5,7,8) and Scribble (hScrib) (9), which were shown to be degraded by HPV-16 and HPV-18 E6 in a protea...

The CXCL12/CXCR4 Signaling Pathway: A New Susceptibility Factor in Human Papillomavirus Pathogenesis

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HPV16 E6 promotes cervical cancer cell migration and invasion by downregulation of NHERF1

“…As minor variations in the PBM can have a dramatic effect on PDZ targets interacted with the HPV E6 oncoproteins . Thus, subtle variations in the PBM region between HPV16 E6 (with a c‐terminal sequence of SSRTRRETQL) and HPV18 E6 (ERLQRRRETQV) result in a significant change in their binding profile . Both HPV16 E6 and HPV18 E6 are able to target DLG1 and SCRIB, but HPV16 E6 preferentially binds SCRIB over DLG1 and vice versa for HPV18 E6 .…”

The human papillomavirus E7 oncoprotein as a regulator of transcription

“…Similar to other viral oncoproteins such as HPV E6 and human adenovirus E1A and E1B, HPV E7 is a phosphoprotein, and its phosphorylation is thought to be important for its function and regulation (Barbosa, Edmonds et al 1990; Helt and Galloway 2003; Ching, Dobner et al 2012; Boon, Tomaic et al 2015). The primary site of phosphorylation is a pair of serines at positions 31 and 32 in CR2 which are phosphorylated by casein kinase II (CKII)(Firzlaff, Galloway et al 1989; Barbosa, Edmonds et al 1990; Chien, Parker et al 2000).…”