The human discs large protein 1 interacts with and maintains connexin 43 at the plasma membrane in keratinocytes.

Gap junction channels, composed of connexins, allow direct cell-to-cell communication. Connexin 43 (Cx43; also known as GJA1) is widely expressed in tissues, including the epidermis. In a previous study of human papillomavirus-positive cervical epithelial tumour cells, we identified Cx43 as a binding partner of the human homologue of Drosophila Discs large (Dlg1; also known as SAP97). Dlg1 is a member of the membrane associated-guanylate kinase (MAGUK) scaffolding protein family, which is known to control cell shape and polarity. Here, we show that Cx43 also interacts with Dlg1 in uninfected keratinocytes in vitro and in keratinocytes, dermal cells and adipocytes in normal human epidermis in vivo. Depletion of Dlg1 in keratinocytes did not alter Cx43 transcription but was associated with a reduction in Cx43 protein levels. Reduced Dlg1 levels in keratinocytes resulted in a reduction in Cx43 at the plasma membrane with a concomitant reduction in gap junctional intercellular communication and relocation of Cx43 to the Golgi compartment. Our data suggest a key role for Dlg1 in maintaining Cx43 at the plasma membrane in keratinocytes.

HPV-18 E6 enhances the interaction between EMILIN2 and SNX27 to promote WNT signaling.

Oncogenic HPV E6 proteins have a PDZ-binding motif (PBM) which plays important roles in both the viral life cycle and tumor development. The PBM confers interaction with a large number of different PDZ domain-containing substrates, one of which is Sorting Nexin 27. This protein is part of the retromer complex and plays an important role in endocytic sorting pathways. It has been shown that at least two SNX27 interacting partners, GLUT1 and TANC2, are aberrantly trafficked due to the E6 PBM-dependent interaction with SNX27. To investigate further which other components of the endocytic trafficking pathway might be affected by the SNX27-HPV E6 interaction, we analyzed the SNX27 proteome interaction profile in a previously described HeLa cell line expressing GFP-SNX27, both in the presence and absence of the HPV-18 E6 oncoprotein. In this study, we identify a novel interacting partner of SNX27, secreted glycoprotein EMILIN2, whose release is blocked by HPV18 E6 in a PBM-dependent manner. Mechanistically, E6 can block EMILIN2 interaction with the WNT1 ligand, thereby enhancing WNT1 signaling and promoting cell proliferation. IMPORTANCE: This study demonstrates that HPV E6 blocks EMILIN2 inhibition of WNT1 signaling, thereby enhancing cell proliferation in HPV-positive tumor cells. This involves a novel mechanism whereby the E6 PBM actually contributes toward enhancing the interaction between SNX27 and EMILIN2, suggesting that the mode of recognition of SNX27 by E6 and EMILIN2 is different. This is the first example of the E6 PBM altering a PDZ domain-containing protein to enhance potential substrate recognition.

Virology under the Microscope-a Call for Rational Discourse.

Viruses have brought humanity many challenges: respiratory infection, cancer, neurological impairment and immunosuppression to name a few. Virology research over the last 60+ years has responded to reduce this disease burden with vaccines and antivirals. Despite this long history, the COVID-19 pandemic has brought unprecedented attention to the field of virology. Some of this attention is focused on concern about the safe conduct of research with human pathogens. A small but vocal group of individuals has seized upon these concerns - conflating legitimate questions about safely conducting virus-related research with uncertainties over the origins of SARS-CoV-2. The result has fueled public confusion and, in many instances, ill-informed condemnation of virology. With this article, we seek to promote a return to rational discourse. We explain the use of gain-of-function approaches in science, discuss the possible origins of SARS-CoV-2 and outline current regulatory structures that provide oversight for virological research in the United States. By offering our expertise, we - a broad group of working virologists - seek to aid policy makers in navigating these controversial issues. Balanced, evidence-based discourse is essential to addressing public concern while maintaining and expanding much-needed research in virology.

Loss of the E6AP Ubiquitin Ligase Induces p53-Dependent Phosphorylation of Human Papillomavirus 18 E6 in Cells Derived from Cervical Cancer.

Cancer-causing human papillomavirus (HPV) E6 oncoproteins contain a well-characterized phosphoacceptor site within the PDZ (PSD-95/Dlg/ZO-1) binding motif (PBM) at the C terminus of the protein. Previous studies have shown that the threonine or serine residue in the E6 PBM is subject to phosphorylation by several stress-responsive cellular kinases upon the induction of DNA damage in cervical cancer-derived cells. However, there is little information about the regulation of E6 phosphorylation in the absence of DNA damage and whether there may be other pathways by which E6 is phosphorylated. In this study, we demonstrate that loss of E6AP results in a dramatic increase in the levels of phosphorylated E6 (pE6) despite the expected overall reduction in total E6 protein levels. Furthermore, phosphorylation of E6 requires transcriptionally active p53 and occurs in a manner that is dependent upon DNA-dependent protein kinase (DNA PK). These results identify a novel feedback loop, where loss of E6AP results in upregulation of p53, leading to increased levels of E6 phosphorylation, which in turn correlates with increased association with 14-3-3 and inhibition of p53 transcriptional activity. IMPORTANCE This study demonstrates that the knockdown of E6AP from cervical cancer-derived cells leads to an increase in phosphorylation of the E6 oncoprotein. We show that this phosphorylation of E6 requires p53 transcriptional activity and the enzyme DNA PK. This study therefore defines a feedback loop whereby activation of p53 can induce phosphorylation of E6 and which in turn can inhibit p53 transcriptional activity independently of E6's ability to target p53 for degradation.

Regulation of HPV E7 Stability by E6-Associated Protein (E6AP).

High-risk human papillomaviruses (HPVs) are responsible for most human cervical cancers, and uncontrolled expression of the two key viral oncoproteins, E6 and E7, stimulates the induction of carcinogenesis. Previous studies have shown that both E6 and E7 are closely associated with different components of the ubiquitin proteasome pathway, including several ubiquitin ligases. Most often these are utilized to target cellular substrates for proteasome-mediated degradation, but in the case of E6, the E6AP ubiquitin ligase plays a critical role in controlling E6 stability. We now show that knockdown of E6AP in HPV-positive cervical cancer-derived cells causes a marked decrease in E7 protein levels. This is due to a decrease in the E7 half-life and occurs in a proteasome-dependent manner. In an attempt to define the underlying mechanism, we show that E7 can also associate with E6AP, albeit in a manner different from that of E6. In addition, we show that E6AP-dependent stabilization of E7 also leads to an increase in the degradation of E7's cellular target substrates. Interestingly, ectopic overexpression of E6 oncoprotein results in lower levels of E7 protein through sequestration of E6AP. We also show that increased E7 stability in the presence of E6AP increases the proliferation of the cervical cancer-derived cell lines. These results demonstrate a surprising interplay between E6 and E7, in a manner which is mediated by the E6AP ubiquitin ligase. IMPORTANCE This is the first demonstration that E6AP can directly help stabilize the HPV E7 oncoprotein, in a manner similar to that observed with HPV E6. This redefines how E6 and E7 can cooperate and potentially modulate each other's activity and further highlights the essential role played by E6AP in the viral life cycle and malignancy.

HPV-18E6 Inhibits Interactions between TANC2 and SNX27 in a PBM-Dependent Manner and Promotes Increased Cell Proliferation.

Cancer-causing HPV E6 oncoproteins contain a PDZ-binding motif at the extreme carboxy terminus, which plays an important role in the viral life cycle and in the development of malignancy. Through this motif, HPV E6 targets a large number of cellular substrates, many of which are involved in processes related to the regulation of cell polarity. Recent studies also demonstrated E6's PDZ binding motif (PBM)-dependent association with SNX27, with a potential role in the perturbation of endocytic transport. Here, we have performed a proteomic analysis to identify SNX27-interacting partners whose binding to SNX27 is specifically perturbed in an E6-dependent manner. Extracts of HeLa cells that express GFP-tagged SNX27, transfected with control siRNA or siRNA targeting E6AP, were subject to GFP immunoprecipitation followed by mass spectroscopy, which identified TANC2 as an interacting partner of SNX27. Furthermore, we demonstrate that HPV E6 inhibits association between SNX27 and TANC2 in a PBM-dependent manner, resulting in an increase in TANC2 protein levels. In the absence of E6, SNX27 directs TANC2 toward lysosomal degradation. TANC2, in the presence of HPV-18E6, enhances cell proliferation in a PBM-dependent manner, indicating that HPV E6 targets the SNX27-mediated transport of TANC2 to promote cellular proliferation. IMPORTANCE While a great deal is known about the role of the E6 PDZ binding motif (PBM) in modulating the cellular proteins involved in regulating cell polarity, much less is known about the consequences of E6's interactions with SNX27 and the endocytic sorting machinery. We reasoned that a potential consequence of such interactions could be to affect the fate of multiple SNX27 endosomal partners, such as transmembrane proteins or soluble accessory proteins. Using a proteomic approach in HPV-18-positive cervical tumor-derived cells, we demonstrate that TANC2 is an interacting partner of SNX27, whose interaction is blocked by E6 in a PBM-dependent manner. This study therefore begins to shed new light on how E6 can regulate the endocytic transport of multiple SNX27-binding proteins, thereby expanding our understanding of the functions of the E6 PBM.

Repression of Memo1, a Novel Target of Human Papillomavirus Type 16 E7, Increases Cell Proliferation in Cervical Cancer Cells.

Human papillomavirus (HPV)-induced carcinogenesis is associated with unregulated expression of the oncoproteins E6 and E7. HPV E7 is a viral protein that lacks enzymatic activity; however, it can target several cellular proteins to induce cell transformation and promote uncontrolled proliferation. Although several E7 targets have been described, there are still gaps in the understanding of how this oncoprotein drives cells toward malignancy. Here, using a small HPV type 16 (HPV16) E7 peptide in a proteomic approach, we report Memo1 as a new E7 binding partner, interacting through the aspartic and glutamic acid residues (E80 and D81) in the C-terminal region of HPV16 E7. Furthermore, we demonstrate that HPV16 E7 targets Memo1 for proteasomal degradation through a Cullin2-dependent mechanism. In addition, we show that overexpression of Memo1 decreases cell transformation and proliferation and that reduction of Memo1 levels correlate with activation of Akt and an increase in invasion of HPV-positive cervical cancer cell lines. Our results show a novel HPV E7 interacting partner and describe novel functions of Memo1 in the context of HPV-induced malignancy. IMPORTANCE Although numerous targets have been reported to interact with the HPV E7 oncoprotein, the mechanisms involved in HPV-induced carcinogenesis and the maintenance of cell transformation are still lacking. Here, through pulldown assays using a peptide encompassing the C-terminal region of HPV16 E7, we report Memo1 as a novel E7 interactor. High levels of Memo1 correlated with reduced cell proliferation and, concordantly, knockdown of Memo1 resulted in Akt activation in HPV-positive cell lines. These results highlight new mechanisms used by HPV oncoproteins to modulate proliferation pathways in cervical cancer cells and increase our understanding of the link between Memo1 protein and cancer.

MAML1-induced HPV E6 oncoprotein stability is required for cellular proliferation and migration of cervical tumor-derived cells.

While a small proportion of high-risk (HR) alpha (α) human papillomaviruses (HPVs) is associated with numerous human malignancies, of which cervical cancer is the most prevalent, beta (ß) HPVs predominantly act as co-factors in skin carcinogenesis. A characteristic feature of both α- and ß-E6 oncoproteins is the presence of the LXXLL binding motif, which α-E6s utilize to form a complex with E6AP and which enables ß-E6s to interact with MAML1. Here we show that multiple α-E6 oncoproteins bind to MAML1 via the LXXLL binding motif and that this results in increased protein stability. Moreover, ß-E6 oncoprotein stability is also dependent on the interaction with MAML1. Additionally, in the absence of MAML1, endogenous HPV-8 E6 and HPV-18 E6 are rapidly degraded at the proteasome. Ablation of both E6AP and MAML1 leads to an even more profound downregulation of α-E6 protein expression, whereas this is not observed with ß-E6. This highly suggests that there is one cellular pool for most of ß-E6 that interacts solely with MAML1, whereas there are two cellular pools of HR α-E6, one forming a complex with MAML1 and the other interacting with E6AP. Furthermore, MAML1 induces HPV-8 E6 shuttling from the nucleus to the cytosolic fraction, while MAML1 interaction with HR E6 induces a drastic nuclear and membrane upregulation of E6. Interestingly, the HR α-E6/MAML1 complex does not affect targeting of some of the known HR E6 cellular substrates such as p53 and DLG1. However, MAML1 and E6AP joint co-expression with HR α-E6 leads to a significant increase in cellular proliferation, whereas silencing MAML1 decreases wound closure in HeLa cells. These results demonstrate that HR α-E6 interaction with MAML1 results in a stable form of E6, which likely modulates MAML1's normal cellular activities, one consequence of which being an increased proliferative capacity of HPV-transformed cancer cells. Thus, this study shows a novel function of the α-E6 oncoprotein and how it's activity might affect HPV-induced pathogenesis.

Human Papillomavirus 16 L2 Recruits both Retromer and Retriever Complexes during Retrograde Trafficking of the Viral Genome to the Cell Nucleus.

Previous studies have identified an interaction between the human papillomavirus (HPV) L2 minor capsid protein and sorting nexins 17 and 27 (SNX17 and SNX27) during virus infection. Further studies show the involvement of both retromer and retriever complexes in this process since knockdown of proteins from either complex impairs infection. In this study, we show that HPV L2 and 5-ethynyl-2'-deoxyuridine (EdU)-labeled pseudovirions colocalize with both retromer and retriever, with components of each complex being bound by L2 during infection. We also show that both sorting nexins may interact with either of the recycling complexes but that the interaction between SNX17 and HPV16 L2 is not responsible for retriever recruitment during infection, instead being required for retromer recruitment. Furthermore, we show that retriever recruitment most likely involves a direct interaction between L2 and the C16orf62 subunit of the retriever, in a manner similar to that of its interaction with the VPS35 subunit of retromer.IMPORTANCE Previous studies identified sorting nexins 17 and 27, as well as the retromer complex, as playing a role in HPV infection. This study shows that the newly identified retriever complex also plays an important role and begins to shed light on how both sorting nexins contribute to retromer and retriever recruitment during the infection process.

Human Papillomavirus infection requires the CCT Chaperonin Complex.

Human papillomavirus (HPV) infection is a multi-step process that implies complex interactions of the viral particles with cellular proteins. The HPV capsid includes the two structural proteins L1 and L2, that play crucial roles on infectious viral entry. L2 is particularly relevant for the intracellular trafficking of the viral DNA towards the nucleus. Here, using proteomic studies we identified CCT proteins as novel interaction partners of HPV-16 L2. The CCT multimeric complex is an essential chaperonin which interacts with a large number of protein targets. We analysed the binding of different components of the CCT complex to L2. We confirmed the interaction of this structural viral protein with the CCT subunit 3 (CCT3) and we found that this interaction requires the N-terminal region of L2. Defects in HPV-16 pseudoviral particle (PsVs) infection were revealed by siRNA-mediated knockdown of some CCT subunits. While a substantial drop in the viral infection was associated with the ablation of CCT component 2, even more pronounced effects on infectivity were observed upon depletion of CCT component 3. Using confocal immunofluorescence assays, CCT3 co-localised with HPV PsVs at early times after infection, with L2 being required for this to occur. Further analysis showed the colocalization of several other subunits of CCT with the PsVs. Moreover, we observed a defect in capsid uncoating and a change in PsVs intracellular normal processing when ablating CCT3. Taken together, these studies demonstrate the importance of CCT chaperonin during HPV infectious entry.ImportanceSeveral of the mechanisms that function during the infection of target cells by HPV particles have been previously described. However, many aspects of this process remain unknown. In particular, the role of cellular proteins functioning as molecular chaperones during HPV infections has been only partially investigated. To the best of our knowledge, we describe here for the first time, a requirement of the CCT chaperonin for HPV infection. The role of this cellular complex seems to be determined by the binding of its component 3 to the viral structural protein L2. However, CCT's effect on HPV infection most probably comprises the whole chaperonin complex. Altogether, these studies define an important role for the CCT chaperonin in the processing and intracellular trafficking of HPV particles and in subsequent viral infectious entry.

Clinical validation of full HR-HPV genotyping HPV Selfy assay according to the international guidelines for HPV test requirements for cervical cancer screening on clinician-collected and self-collected samples.

BACKGROUND: According to international guidelines, Human Papillomavirus (HPV) DNA tests represent a valid alternative to Pap Test for primary cervical cancer screening, provided that they guarantee balanced clinical sensitivity and specificity for cervical intraepithelial neoplasia grade 2 or more (CIN2+) lesions. The study aimed to assess whether HPV Selfy (Ulisse BioMed - Trieste, Italy), a full-genotyping HPV DNA test that detects and differentiates 14 high-risk HPV (HR-HPV) types, meets the criteria for primary cervical cancer screening described in the international guidelines, on clinician-collected as well as on self-collected samples. METHODS: For each participant woman, consecutively referring to Azienda Sanitaria Universitaria Giuliano Isontina (Trieste, Italy) and CRO-National Cancer Institute (Aviano, Italy) for the cervical cancer screening program, the following samples were tested: (a) a clinician-collected cervical specimen, analyzed with the reference test (Hybrid Capture®2 test, HC2) and HPV Selfy; and (b) a self-collected vaginal sample, analyzed with HPV Selfy. Enrolled women were also asked to fulfill a questionnaire about self-sampling acceptability. As required by guidelines, a non-inferiority test was conducted to compare the clinical performance of the test under evaluation with its reference test. RESULTS: HPV Selfy clinical sensitivity and specificity resulted non-inferior to those of HC2. By analysis of a total of 889 cervical liquid-based cytology samples from a screening population, of which 98 were from women with CIN2+, HPV Selfy showed relative sensitivity and specificity for CIN2+ of 0.98 and 1.00 respectively (non-inferiority score test: P = 0.01747 and P = 0.00414, respectively); the test reached adequate intra- and inter-laboratory reproducibility. Moreover, we demonstrated that the performance of HPV Selfy on self-collected vaginal samples was non-inferior to the performance obtained on clinician-collected cervical specimen (0.92 relative sensitivity and 0.97 relative specificity). Finally, through HPV Selfy genotyping, we were able to describe HPV types prevalence in the study population. CONCLUSIONS: HPV Selfy fulfills all the requirements of the international Meijer's guidelines and has been clinically validated for primary cervical cancer screening purposes. Moreover, HPV Selfy has also been validated for self-sampling according to VALHUDES guidelines. Therefore, at date, HPV Selfy is the only full-genotyping test validated both for screening purposes and for self-sampling. Trial registration ASUGI Trieste n. 16008/2018; CRO Aviano n.17149/2018.

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

BACKGROUND: Oncogenic Human Papillomaviruses (HPVs) base their transforming potential on the action of both E6 and E7 viral oncoproteins, which perform cooperative or antagonistic actions and thus interfere with a variety of relevant cellular targets. Among them, the expression of some PDZ-containing polarity proteins, as DLG1 and hScrib, is altered during the HPV life cycle and the consequent malignant transformation. Together with the well-established interference of E6 with PDZ proteins, we have recently shown that E7 viral oncoprotein is also responsible for the changes in abundance and localization of DLG1 observed in HPV-associated lesions. Given that the mechanisms involved remained only partially understood, we here thoroughly analyse the contribution of a crucial E7 post-translational modification: its CKII-dependent phosphorylation. Moreover, we extended our studies to hScrib, in order to investigate possible conserved regulatory events among diverse PDZ targets of HPV. METHODS: We have acutely analysed the expression of DLG1 and hScrib in restrictive conditions for E7 phosphorylation by CKII in epithelial culture cells by western blot and confocal fluorescence microscopy. We made use of genome-edited HPV-positive cells, specific inhibitors of CKII activity and transient expression of the viral oncoproteins, including a mutant version of E7. RESULTS: We here demonstrate that the functional phosphorylation of E7 oncoprotein by the CKII cellular kinase, a key regulatory event for its activities, is also crucial to counteract the E6-mediated degradation of the PDZ-polarity protein DLG1 and to promote its subcellular redistribution. Moreover, we show that the CKII-dependent phosphorylation of E7 is able to control the expression of another PDZ target of HPV: hScrib. Remarkably, we found this is a shared feature among different oncogenic HPV types, suggesting a common path towards viral pathogenesis. CONCLUSIONS: The present study sheds light into the mechanisms behind the misexpression of PDZ-polarity proteins during HPV infections. Our findings stress the relevance of the CKII-mediated regulation of E7 activities, providing novel insights into the joint action of HPV oncoproteins and further indicating a conserved and most likely crucial mechanism during the viral life cycle and the associated transformation.

Human Papillomavirus 58 E7 T20I/G63S Variant Isolated from an East Asian Population Possesses High Oncogenicity.

Human papillomavirus (HPV) type 58 is the third most commonly detected HPV type in cervical cancer among Eastern Asians. Our previous international epidemiological studies revealed that HPV58 carrying an E7 natural variant, T20I/G63S (designated V1), was associated with a higher risk of cervical cancer. We recently showed that V1 possesses a greater ability to immortalize and transform primary cells, as well as degrading pRB more effectively, than the prototype and other common variants. In this study, we performed a series of phenotypic and molecular assays using physiologically relevant in vitro and in vivo models to compare the oncogenicity of V1 with that of the prototype and other common natural variants. Through activation of the AKT and K-Ras/extracellular signal-regulated kinase (ERK) signaling pathways, V1 consistently showed greater oncogenicity than the prototype and other variants, as demonstrated by increased cell proliferation, migration, and invasion, as well as induction of larger tumors in athymic nude mice. This study complements our previous epidemiological and molecular observations pinpointing the higher oncogenicity of V1 than that of the prototype and all other common variants. Since V1 is more commonly found in eastern Asia, our report provides insight into the design of HPV screening assays and selection of components for HPV vaccines in this region.IMPORTANCE Epidemiological studies have revealed that a wild-type variant of HPV58 carrying an E7 variation, T20I/G63S (V1), is associated with a higher risk of cervical cancer. We previously reported that this increased oncogenicity could be the result of the virus's greater ability to degrade pRB, thereby leading to an increased ability to grow in an anchorage-independent manner. In addition to this, this report further showed that this HPV variant induced activation of the AKT and K-Ras/ERK signaling pathways, thereby explaining its genuine oncogenicity in promoting cell proliferation, migration, invasion, and formation of tumors, all to a greater extent than the prototype HPV58 and other common variants.

The HPV-18 E7 CKII phospho acceptor site is required for maintaining the transformed phenotype of cervical tumour-derived cells.

The Human Papillomavirus E7 oncoprotein plays an essential role in the development and maintenance of malignancy, which it achieves through targeting a number of critical cell control pathways. An important element in the ability of E7 to contribute towards cell transformation is the presence of a Casein Kinase II phospho-acceptor site within the CR2 domain of the protein. Phosphorylation is believed to enhance E7 interaction with a number of different cellular target proteins, and thereby increase the ability of E7 to enhance cell proliferation and induce malignancy. However, there is little information on how important this site in E7 is, once the tumour cells have become fully transformed. In this study, we have performed genome editing of the HPV-18 E7 CKII recognition site in C4-1 cervical tumour-derived cells. We first show that mutation of HPV18 E7 S32/S34 to A32/A34 abolishes CKII phosphorylation of E7, and subsequently we have isolated C4-1 clones containing these mutations in E7. The cells continue to proliferate, but are somewhat more slow-growing than wild type cells, reach lower saturation densities, and are also more susceptible to low nutrient conditions. These cells are severely defective in matrigel invasion assays, partly due to downregulation of matrix metalloproteases (MMPs). Mechanistically, we find that phosphorylation of E7 plays a direct role in the ability of E7 to activate AKT signaling, which in turn is required for optimal levels of MMP secretion. These results demonstrate that the E7 CKII phospho-acceptor site thus continues to play an important role for E7's activity in cells derived from cervical cancers, and suggests that blocking this activity of E7 could be expected to have therapeutic potential.

Acquisition of a phospho-acceptor site enhances HPV E6 PDZ-binding motif functional promiscuity.

All cancer-causing human papillomavirus (HPV) E6 oncoproteins have a C-terminal PDZ-binding motif (PBM), which correlates with oncogenic potential. Nonetheless, several HPVs with little or no oncogenic potential also have an E6 PBM, with minor sequence differences affecting PDZ protein selectivity. Furthermore, certain HPV types have a phospho-acceptor site embedded within the PBM. We therefore compared HPV-18, HPV-66 and HPV-40 E6 proteins to examine the possible link between the ability to target multiple PDZ proteins and the acquisition of a phospho-acceptor site. The mutation of essential residues in HPV-18E6 reduces its phosphorylation, and fewer PDZ substrates are bound. In contrast, the generation of consensus phospho-acceptor sites in HPV-66 and HPV-40 E6 PBMs increases the PDZ proteins recognized. Thus, although phosphorylation of the E6 PBM and PDZ protein recognition are mutually exclusive, they are closely linked, with the acquisition of a phospho-acceptor site also contributing to an expansion in the number of PDZ proteins bound.

Human papillomavirus oncoproteins and post-translational modifications: generating multifunctional hubs for overriding cellular homeostasis.

Human papillomaviruses (HPVs) are major human carcinogens, causing around 5% of all human cancers, with cervical cancer being the most important. These tumors are all driven by the two HPV oncoproteins E6 and E7. Whilst their mechanisms of action are becoming increasingly clear through their abilities to target essential cellular tumor suppressor and growth control pathways, the roles that post-translational modifications (PTMs) of E6 and E7 play in the regulation of these activities remain unclear. Here, we discuss the direct consequences of some of the most common PTMs of E6 and E7, and how this impacts upon the multi-functionality of these viral proteins, and thereby contribute to the viral life cycle and to the induction of malignancy. Furthermore, it is becoming increasingly clear that these modifications, may, in some cases, offer novel routes for therapeutic intervention in HPV-induced disease.

Phosphorylation of Human Papillomavirus Type 16 L2 Contributes to Efficient Virus Infectious Entry.

The human papillomavirus (HPV) capsid comprises two viral proteins, L1 and L2, with the L2 component being essential to ensure efficient endocytic transport of incoming viral genomes. Several studies have previously reported that L1 and L2 are posttranslationally modified, but it is uncertain whether these modifications affect HPV infectious entry. Using a proteomic screen, we identified a highly conserved phospho-acceptor site on the HPV-16 and bovine papillomavirus 1 (BPV-1) L2 proteins. The phospho-modification of L2 and its presence in HPV pseudovirions (PsVs) were confirmed using anti-phospho-L2-specific antibodies. Mutation of the phospho-acceptor sites of both HPV-16 and BPV-1 L2 resulted in the production of infectious virus particles, with no differences in efficiencies of packaging the reporter DNA. However, these mutated PsVs showed marked defects in infectious entry. Further analysis revealed a defect in uncoating, characterized by a delay in the exposure of a conformational epitope on L1 that indicates capsid uncoating. This uncoating defect was accompanied by a delay in the proteolysis of both L1 and L2 in mutated HPV-16 PsVs. Taken together, these studies indicate that phosphorylation of L2 during virus assembly plays an important role in optimal uncoating of virions during infection, suggesting that phosphorylation of the viral capsid proteins contributes to infectious entry.IMPORTANCE The papillomavirus L2 capsid protein plays an essential role in infectious entry, where it directs the successful trafficking of incoming viral genomes to the nucleus. However, nothing is known about how potential posttranslational modifications may affect different aspects of capsid assembly or infectious entry. In this study, we report the first phospho-specific modification of the BPV-1 and HPV-16 L2 capsid proteins. The phospho-acceptor site is very highly conserved across multiple papillomavirus types, indicating a highly conserved function within the L2 protein and the viral capsid. We show that this modification plays an essential role in infectious entry, where it modulates susceptibility of the incoming virus to capsid disassembly. These studies therefore define a completely new means of regulating the papillomavirus L2 proteins, a regulation that optimizes endocytic processing and subsequent completion of the infectious entry pathway.

PDZ Domain-Containing Protein NHERF-2 Is a Novel Target of Human Papillomavirus 16 (HPV-16) and HPV-18.

Cancer-causing human papillomavirus (HPV) E6 oncoproteins have a class I PDZ-binding motif (PBM) on their C termini, which play critical roles that are related to the HPV life cycle and HPV-induced malignancies. E6 oncoproteins use these PBMs to interact with, to target for proteasome-mediated degradation, a plethora of cellular substrates that contain PDZ domains and that are involved in the regulation of various cellular pathways. In this study, we show that both HPV-16 and HPV-18 E6 oncoproteins can interact with Na+/H+ exchange regulatory factor 2 (NHERF-2), a PDZ domain-containing protein, which among other cellular functions also behaves as a tumor suppressor regulating endothelial proliferation. The interaction between the E6 oncoproteins and NHERF-2 is PBM dependent and results in proteasome-mediated degradation of NHERF-2. We further confirmed this effect in cells derived from HPV-16- and HPV-18-positive cervical tumors, where we show that NHERF-2 protein turnover is increased in the presence of E6. Finally, our data indicate that E6-mediated NHERF-2 degradation results in p27 downregulation and cyclin D1 upregulation, leading to accelerated cellular proliferation. To our knowledge, this is the first report to demonstrate that E6 oncoproteins can stimulate cell proliferation by indirectly regulating p27 through targeting a PDZ domain-containing protein.IMPORTANCE This study links HPV-16 and HPV-18 E6 oncoproteins to the modulation of cellular proliferation. The PDZ domain-containing protein NHERF-2 is a tumor suppressor that has been shown to regulate endothelial proliferation; here, we demonstrate that NHERF-2 is targeted by HPV E6 for proteasome-mediated degradation. Interestingly, this indirectly affects p27, cyclin D1, and CDK4 protein levels and, consequently, affects cell proliferation. Hence, this study provides information that will improve our understanding of the molecular basis for HPV E6 function, and it also highlights the importance of the PDZ domain-containing protein NHERF-2 and its tumor-suppressive role in regulating cell proliferation.

Oncogenic comparison of human papillomavirus type 58 E7 variants.

Human papillomavirus 58 (HPV58) ranks the second or third in East Asian cervical cancers. Current studies on HPV58 are scarce and focus on the prototype. Previously, we identified the three most common circulating HPV58 E7 strains contained amino acid alterations: G41R/G63D (51%), T20I/G63S (22%) and T74A/D76E (14%) respectively. Among them, the T20I/G63S variant (V1) had a stronger epidemiological association with cervical cancer. We therefore suggested that V1 possessed stronger oncogenicity than the other two variants. Here, we performed phenotypic assays to characterize and compare their oncogenicities with HPV58 E7 prototype. Our results showed that overexpression of V1 conferred a higher colony-forming ability to primary murine epithelial cells than prototype (P < 0.05) and other variants, implicating its higher immortalising potential. Further experiments showed that both V1 and prototype enhanced the anchorage-independent growth of NIH/3T3 cells (P < 0.001), implicating their stronger transforming power than the two other variants. Moreover, they possessed an increased ability to degrade pRb (P < 0.001), which is a major effector pathway of E7-driven oncogenesis. Our work represents the first study to compare the oncogenicities of HPV58 E7 prototype and variants. These findings deepened our understanding of HPV58 and might inform clinical screening and follow-up strategy.

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.