Purification and biophysical characterization of a minimal functional domain and of an N-terminal Zn2+-binding fragment from the human papillomavirus type 16 E6 protein

Structure, Genome, Infection Cycle and Clinical Manifestations Associated with Human Papillomavirus

A small, non-enveloped, obligatory parasite, Human papillomavirus (HPV) is known to be the cause of a range of malignancies. These entail benign infections like genital warts as well as malignant, life-threatening conditions such as cervical cancer. Since a very high mortality rate is associated with HPV caused cancers (cervical cancer is a 2nd leading cause of death caused due to cancer among women globally), there is an escalating need to understand and search for ways to combat such medical conditions. Under the same light, the given article provides an insight into the world of this versatile pathogen. Distinct aspects related to HPV have been discussed here. Emphasis has been laid upon the composition, function and assembly of capsid proteins (structural studies) and various genetic elements and their gene products (genomic studies). The essence of the mechanism behind the development of persistent infection and modes responsible for the transmission of the infectious particles has been briefly covered. Finally, the review outlines various infections and diseases caused by HPV with a major focus on their clinical and histological manifestations.

Structural Insights in Multifunctional Papillomavirus Oncoproteins

Since their discovery in the mid-eighties, the main papillomavirus oncoproteins E6 and E7 have been recalcitrant to high-resolution structure analysis. However, in the last decade a wealth of three-dimensional information has been gained on both proteins whether free or complexed to host target proteins. Here, we first summarize the diverse activities of these small multifunctional oncoproteins. Next, we review the available structural data and the new insights they provide about the evolution of E6 and E7, their multiple interactions and their functional variability across human papillomavirus (HPV) species.

Molecular characterization, tissue tropism, and genetic variability of the novel Mupapillomavirus type HPV204 and phylogenetically related types HPV1 and HPV63

HPV204 is the only newly identified Mupapillomavirus (Mu-PV) type in more than a decade. To comprehensively characterize HPV204, we performed a detailed molecular analysis of the viral genome and evaluated its clinical relevance in comparison to the other Mu-PVs, HPV1 and HPV63. The 7,227-bp long genome of HPV204 exhibits typical genomic organization of Mu-PVs with eight open reading frames (ORFs) (E6, E7, E1, E2, E8, E4, L2, and L1). We developed three type-specific quantitative real-time PCRs and used them to test a representative collection (n = 1,006) of various HPV-associated benign and malignant neoplasms, as well as samples of clinically normal cutaneous, mucosal, and mucocutaneous origins. HPV204, HPV1, and HPV63 were detected in 1.1%, 2.7%, and 1.9% of samples tested, respectively, and were present in skin and mucosa, suggesting dual tissue tropism of all Mu-PVs. To evaluate the etiological role of Mu-PVs in the development of HPV-associated neoplasms, Mu-PV viral loads per single cell were estimated. HPV1 and HPV63 were present in high viral copy numbers in 3/43 and 1/43 cutaneous warts, respectively, and were identified as the most likely causative agents of these warts. HPV204 viral load was extremely low in a single HPV204-positive cutaneous wart (7.4 × 10⁻⁷ viral copies/cell). Hence, etiological association between HPV204 and the development of cutaneous warts could not be established. To the best of our knowledge, this is the first study to evaluate the genetic variability of Mu-PVs by sequencing complete LCR genomic regions of HPV204, HPV1, and HPV63. We detected several nucleotide substitutions and deletions within the LCR genomic regions of Mu-PVs and identified two genetic variants of HPV204 and HPV63 and five genetic variants of HPV1.

Mutagenic Potential ofBos taurus Papillomavirus Type 1 E6 Recombinant Protein: First Description

Bovine papillomavirus (BPV) is considered a useful model to study HPV oncogenic process. BPV interacts with the host chromatin, resulting in DNA damage, which is attributed to E5, E6, and E7 viral oncoproteins activity. However, the oncogenic mechanisms of BPV E6 oncoprotein per se remain unknown. This study aimed to evaluate the mutagenic potential of Bos taurus papillomavirus type 1 (BPV-1) E6 recombinant oncoprotein by the cytokinesis-block micronucleus assay (CBMNA) and comet assay (CA). Peripheral blood samples of five calves were collected. Samples were subjected to molecular diagnosis, which did not reveal presence of BPV sequences. Samples were treated with 1 μg/mL of BPV-1 E6 oncoprotein and 50 μg/mL of cyclophosphamide (positive control). Negative controls were not submitted to any treatment. The samples were submitted to the CBMNA and CA. The results showed that BPV E6 oncoprotein induces clastogenesis per se, which is indicative of genomic instability. These results allowed better understanding the mechanism of cancer promotion associated with the BPV E6 oncoprotein and revealed that this oncoprotein can induce carcinogenesis per se. E6 recombinant oncoprotein has been suggested as a possible vaccine candidate. Results pointed out that BPV E6 recombinant oncoprotein modifications are required to use it as vaccine.

Mass spectrometric determination of disulfide bonds in the biologically active recombinant HBx protein of hepatitis B virus

Many proteins rely on disulfide bonds formed between pairs of cysteines for the stability of their folded state and to keep regulatory control over their functions. The hepatitis B virus-encoded HBx oncoprotein is known to perform an overwhelming array of functions in the cell and has been implicated in the development of hepatocellular carcinoma. However, its structure has not been elucidated. HBx carries nine conserved cysteine residues that have proven to be crucial for its various functions. However, the status of disulfide bonds between the cysteine residues reported in previous studies remains discrepant because of the use of refolded recombinant HBx that may contain non-native disulfides. Now we have determined the disulfide linkages in soluble and biologically active recombinant maltose binding protein-HBx fusion protein using matrix-assisted laser desorption ionization time-of-flight mass spectrometry. We report four disulfide linkages in HBx protein, viz., between Cys(7) and Cys(69), Cys(61) and Cys(115), Cys(78) and Cys(137), and Cys(17) and Cys(143), based on the differential mobility of corresponding disulfide-linked peptide ions under reducing and nonreducing conditions. Cys(148) was observed to be free. Site-directed mutagenesis of Cys(143) and Cys(148) with serine and functional analyses of these mutants affirmed the importance of these residues in the ability of HBx to potentiate Cdk2/cyclin E kinase activity and transcriptionally activate promoter reporter gene activity. Thus, this study identifies native disulfide linkages in the structure of a biologically active viral oncoprotein.

Role of proteomics in translational research in cervical cancer

Cervical cancer is one of the leading causes of cancer morbidity and mortality in women worldwide. More than 98% of cases are related to a human papillomavirus (HPV) infection. Infection with specific subtypes of HPV has been strongly implicated in cervical carcinogenesis. The identification and functional verification of host proteins associated with HPV E6 and E7 oncoproteins may provide useful information for understanding cervical carcinogenesis and the development of cervical cancer-specific markers. In addition, proteomic profiling of altered proteins by anticancer drugs on cervical cancer cells may contribute to providing the fundamental resources for investigation of disease-specific target proteins, elucidation of the novel mechanisms of action and development of new drugs. The advent of proteomics has provided the hope of discovering novel biological markers for use in the screening, early diagnosis and prediction of response to therapy. This review describes the studies where profiles of protein expression in cervical cancer have been generated.

Papillomavirus E6 oncoproteins

Papillomaviruses induce benign and malignant epithelial tumors, and the viral E6 oncoprotein is essential for full transformation. E6 contributes to transformation by associating with cellular proteins, docking on specific acidic LXXLL peptide motifs found on these proteins. This review examines insights from recent studies of human and animal E6 proteins that determine the three-dimensional structure of E6 when bound to acidic LXXLL peptides. The structure of E6 is related to recent advances in the purification and identification of E6 associated protein complexes. These E6 protein-complexes, together with other proteins that bind to E6, alter a broad array of biological outcomes including modulation of cell survival, cellular transcription, host cell differentiation, growth factor dependence, DNA damage responses, and cell cycle progression.

Human papillomavirus type 16 E6 protein inhibits DNA fragmentation via interaction with DNA fragmentation factor 40

The E6 oncoprotein of human papillomavirus (HPV) is critical in cervical cancer development. Using the yeast two-hybrid assay, we showed that HPV-16 E6 (16E6) interacts with one of the DNA fragmentation factors (DFFs), DFF40, which mediates DNA degradation during apoptosis. Furthermore, 16E6 interacts with DFF40 through its zinc finger motif 2 and a bridge section linking the two zinc finger motifs. DNA fragmentation assays disclosed that 16E6 binding to DFF40 leads to blockage of DNA cleavage. Our data collectively suggest that suppression of DNA fragmentation through 16E6-DFF40 interaction is a central event promoting tumorigenesis.

Solution structure analysis of the HPV16 E6 oncoprotein reveals a self-association mechanism required for E6-mediated degradation of p53

The viral oncoprotein E6 is an essential factor for cervical cancers induced by "high-risk" mucosal HPV. Among other oncogenic activities, E6 recruits the ubiquitin ligase E6AP to promote the ubiquitination and subsequent proteasomal degradation of p53. E6 is prone to self-association, which long precluded its structural analysis. Here we found that E6 specifically dimerizes through its N-terminal domain and that disruption of the dimer interface strongly increases E6 solubility. This allowed us to raise structural data covering the entire HPV16 E6 protein, including the high-resolution NMR structures of the two zinc-binding domains of E6 and a robust data-driven model structure of the N-terminal domain homodimer. Interestingly, homodimer interface mutations that disrupt E6 self-association also inactivate E6-mediated p53 degradation. These data suggest that E6 needs to self-associate via its N-terminal domain to promote the polyubiquitination of p53 by E6AP.

Detecting genomic regions associated with a disease using variability functions and Adjusted Rand Index

BACKGROUND

The identification of functional regions contained in a given multiple sequence alignment constitutes one of the major challenges of comparative genomics. Several studies have focused on the identification of conserved regions and motifs. However, most of existing methods ignore the relationship between the functional genomic regions and the external evidence associated with the considered group of species (e.g., carcinogenicity of Human Papilloma Virus). In the past, we have proposed a method that takes into account the prior knowledge on an external evidence (e.g., carcinogenicity or invasivity of the considered organisms) and identifies genomic regions related to a specific disease.

RESULTS AND CONCLUSION

We present a new algorithm for detecting genomic regions that may be associated with a disease. Two new variability functions and a bipartition optimization procedure are described. We validate and weigh our results using the Adjusted Rand Index (ARI), and thus assess to what extent the selected regions are related to carcinogenicity, invasivity, or any other species classification, given as input. The predictive power of different hit region detection functions was assessed on synthetic and real data. Our simulation results suggest that there is no a single function that provides the best results in all practical situations (e.g., monophyletic or polyphyletic evolution, and positive or negative selection), and that at least three different functions might be useful. The proposed hit region identification functions that do not benefit from the prior knowledge (i.e., carcinogenicity or invasivity of the involved organisms) can provide equivalent results than the existing functions that take advantage of such a prior knowledge. Using the new algorithm, we examined the Neisseria meningitidis FrpB gene product for invasivity and immunologic activity, and human papilloma virus (HPV) E6 oncoprotein for carcinogenicity, and confirmed some well-known molecular features, including surface exposed loops for N. meningitidis and PDZ domain for HPV.

Strategies for bacterial expression of protein-peptide complexes: application to solubilization of papillomavirus E6

E6 is a small oncoprotein involved in tumorigenesis induced by papillomaviruses (PVs). E6 often recognizes its cellular targets by binding to short motifs presenting the consensus LXXLL. E6 proteins have long resisted structural analysis. We found that bovine papillomavirus type 1 (BPV1) E6 binds the N-terminal LXXLL motif of the cellular protein paxillin with significantly higher affinity as compared to other E6/peptide interactions. Although recombinant BPV1 E6 was poorly soluble in the free state, provision of the paxillin LXXLL peptide during BPV1 E6 biosynthesis greatly enhanced the protein's solubility. Expression of BPV1 E6/LXXLL peptide complexes was carried out in bacteria in the form of triple fusion constructs comprising, from N- to C-terminus, the soluble carrier protein maltose binding protein (MBP), the LXXLL motif and the E6 protein. A TEV protease cleavage site was placed either between MBP and LXXLL motif or between LXXLL motif and E6. These constructs allowed us to produce highly concentrated samples of BPV1 E6, either covalently fused to the C-terminus of the LXXLL motif (intra-molecular complex) or non-covalently bound to it (inter-molecular complex). Heteronuclear NMR measurements were performed and showed that the E6 protein was folded with similar conformations in both covalent and non-covalent complexes. These data open the way to novel structural and functional studies of the BPV1 E6 in complex with its preferential target motif.

E6 proteins from diverse papillomaviruses self-associate both in vitro and in vivo

Papillomavirus E6 oncoproteins bind and often provoke the degradation of many cellular proteins important for the control of cell proliferation and/or cell death. Structural studies on E6 proteins have long been hindered by the difficulties of obtaining highly concentrated samples of recombinant E6. Here, we show that recombinant E6 proteins from eight human papillomavirus strains and one bovine papillomavirus strain exist as oligomeric and multimeric species. These species were characterized using a variety of biochemical and biophysical techniques, including analytical gel filtration, activity assays, surface plasmon resonance, electron microscopy and Fourier transform infrared spectroscopy. The characterization of E6 oligomers is facilitated by the fusion to the maltose binding protein, which slows the formation of higher-order multimeric species. The proportion of each oligomeric form varies depending on the viral strain considered. Oligomers appear to consist of folded units, which, in the case of high-risk mucosal human papillomavirus E6, retain binding to the ubiquitin ligase E6-associated protein and the capacity to degrade the proapoptotic protein p53. In addition to the small-size oligomers, E6 proteins spontaneously assemble into large organized multimeric structures, a process that is accompanied by a significant increase in the beta-sheet secondary structure content. Finally, co-localisation experiments using E6 equipped with different tags further demonstrate the occurrence of E6 self-association in eukaryotic cells. The ensemble of these data suggests that self-association is a general property of E6 proteins that occurs both in vitro and in vivo and might therefore be functionally relevant.

The EVER proteins as a natural barrier against papillomaviruses: a new insight into the pathogenesis of human papillomavirus infections

Infections by human papillomaviruses (HPVs) are the most frequently occurring sexually transmitted diseases. The crucial role of genital oncogenic HPV in cervical carcinoma development is now well established. In contrast, the role of cutaneous HPV in skin cancer development remains a matter of debate. Cutaneous beta-HPV strains show an amazing ubiquity. The fact that a few oncogenic genotypes cause cancers in patients suffering from epidermodysplasia verruciformis is in sharp contrast to the unapparent course of infection in the general population. Our recent investigations revealed that a natural barrier exists in humans, which protects them against infection with these papillomaviruses. A central role in the function of this HPV-specific barrier is played by a complex of the zinc-transporting proteins EVER1, EVER2, and ZnT-1, which maintain cellular zinc homeostasis. Apparently, the deregulation of the cellular zinc balance emerges as an important step in the life cycles not only of cutaneous but also of genital HPVs, although the latter viruses have developed a mechanism by which they can break the barrier and impose a zinc imbalance. Herein, we present a previously unpublished list of the cellular partners of EVER proteins, which points to future directions concerning investigations of the mechanisms of action of the EVER/ZnT-1 complex. We also present a general overview of the pathogenesis of HPV infections, taking into account the latest discoveries regarding the role of cellular zinc homeostasis in the HPV life cycle. We propose a potential model for the mechanism of function of the anti-HPV barrier.

Determinants of stability for the E6 protein of papillomavirus type 16

E6 is an oncoprotein produced by human papillomavirus (HPV). The E6 protein from high-risk HPV type 16 contains two zinc-binding domains with two C-x-x-C motifs each. E6 exerts its transforming functions through formation of a complex with E6AP, which binds p53 and stimulates its degradation. There have been few biophysical and structural studies due to difficulty in preparation of soluble protein; here we describe the preparation of soluble E6 constructs including the two separated zinc-binding domains of E6. These proteins are used to examine the extent to which the two domains cooperate to mediate E6 function, how zinc influences the behavior of E6 protein, and which domains mediate aggregation. We demonstrate, using p53 degradation, E6AP binding, and hDlg (human homolog of the Drosophila discs large tumor suppressor protein) PDZ (postsynaptic density/disc large/zonula occludens) protein binding assays, that these soluble proteins are active, and, using NMR, circular dichroism, and fluorescence spectroscopies, we show that they are folded and stable. We show that the separated N-terminal and C-terminal domains interact, but nonproductively, for E6 function. The two domains bind zinc differently with higher affinity associated with the C-terminal domain. Analyses using surface plasmon resonance and circular dichroism and fluorescence spectroscopies show that aggregation is mediated more through the N-terminal domain than through the C-terminal domain. These studies allow a model in which the C-terminal zinc-binding domain of E6 recruits a target protein such as hDlg and the N-terminal domain is mostly responsible for recruiting a ubiquitin ligase to mediate target protein degradation.

Papillomavirus E6 proteins

The papillomaviruses are small DNA viruses that encode approximately eight genes, and require the host cell DNA replication machinery for their viral DNA replication. Thus papillomaviruses have evolved strategies to induce host cell DNA synthesis balanced with strategies to protect the cell from unscheduled replication. While the papillomavirus E1 and E2 genes are directly involved in viral replication by binding to and unwinding the origin of replication, the E6 and E7 proteins have auxillary functions that promote proliferation. As a consequence of disrupting the normal checkpoints that regulate cell cycle entry and progression, the E6 and E7 proteins play a key role in the oncogenic properties of human papillomaviruses with a high risk of causing anogenital cancers (HR HPVs). As a consequence, E6 and E7 of HR HPVs are invariably expressed in cervical cancers. This article will focus on the E6 protein and its numerous activities including inactivating p53, blocking apoptosis, activating telomerase, disrupting cell adhesion, polarity and epithelial differentiation, altering transcription and reducing immune recognition.

A single-codon mutation converts HPV16 E6 oncoprotein into a potential tumor suppressor, which induces p53-dependent senescence of HPV-positive HeLa cervical cancer cells

High-risk mucosal human papillomaviruses (HPV), mainly HPV16 and HPV18, are implicated in cervical carcinogenesis. HPV16 E6 oncoprotein binds and often targets for degradation numerous cell proteins, including the tumor suppressor p53 and several PDZ domain proteins. Here, we show that a single-point mutation, F47R, is sufficient to convert the HPV16 E6 oncoprotein into a suppressor of HPV-positive HeLa cervical cancer cells proliferation. The E6 F47R mutant is defective for polyubiquitination and subsequent degradation of p53. When expressed in HPV-positive cervical cancer cells, E6 F47R acts as a dominant negative mutant by counteracting the p53 degradation activity of endogenous E6 and restoring high p53 protein levels. Moreover, the prolonged expression of E6 F47R leads to suppression of HeLa cells proliferation through the induction of premature senescence. This phenotype is independent on the PDZ-binding activity of E6. F47R-senescent HeLa cells exhibit a sustained expression of p53, hMDM2 and p21(CIP) proteins and a reduced expression of endogenous HPV18 E6 protein. Finally, small interfering RNAs directed against p53 counteract the effect of E6 F47R expression, indicating that E6 F47R-induced cellular senescence is strongly dependent on p53 signaling pathway.

Novel interaction between HPV E6 and BARD1 (BRCA1-associated ring domain 1) and its biologic roles

Human papillomaviruses (HPVs), which are associated with the majority of cervical cancers, encode a transforming protein, E6, which interacts with the p53 tumor suppressor protein. There is a wide effort focused on searching for the target of the involvement of p53-independent HPV 16 E6-interacting proteins. We identified Breast Cancer 1 Gene (BRCA1)-associated ring domain protein 1 (BARD1) as a binding partner of E6 and investigated its biological function in cervical cancer cells. In vivo co-immunoprecipitation assay was performed to determine whether E6-BARD1 interaction occurred. We then used a degradation assay to determine whether E6-mediated inactivation of BARD1 transactivation function was associated with BARD1 degradation. A mutation assay revealed the site of interaction of E6 with BARD1. The effect of BARD1 on p53 transcriptional activity was tested using BARD1 knockdown and overexpression systems. BARD1 was not degraded by E6, and, instead, formed a physical complex with E6. Moreover, the mutations of the metal motif zinc-finger region decreased the ability of E6 to interact with BARD1. Transient transfection of BARD1 increased the p53-mediated activation of p21(WAF1) promoter despite the presence of E6. Additionally, the existence of BARD1 inactivated the expression of E6 in cervical cancer cells. These findings suggest that BARD1 may regulate the transcriptional activities of p53 as tumor suppressors.

The E6 protein of the cutaneous human papillomavirus type 8 can stimulate the viral early and late promoters by distinct mechanisms

The expression of the proteins encoded by human papillomaviruses (HPVs) is tightly linked to the differentiation program of the infected keratinocytes. The late promoter, expressing the structural proteins, becomes activated in the differentiated keratinocytes, while the early promoter is also active in the basal layers. We have shown previously that the viral transcriptional regulator E2 and the cellular coactivator p300 cooperate in activation of gene expression of HPV8, which infects the skin and is associated with epidermodysplasia verruciformis. Here we demonstrate that this activation is further stimulated after overexpression of the E6 oncoprotein of HPV8 (8E6). RNase protection experiments revealed that 8E6 efficiently cooperates with 8E2 and p300 in activation of the late promoter. In addition, the early promoter, which did not respond to 8E2 and/or p300, was stimulated more than fourfold by 8E6. Our data suggest that both promoters are activated via distinct mechanisms, since the activation of the early promoter was achieved by the N-terminal moiety of 8E6; in contrast, its C-terminal half was sufficient for late promoter activation. This was markedly reduced by the deletion of amino acids 132 to 136 of 8E6, which also abolished the binding to p300, indicating that a direct interaction between 8E6 and p300 is involved. Moreover, a 45-amino-acid segment within the C/H3 region of p300 is required for 8E6 to stimulate the coactivator function of p300. Our results demonstrate for the first time that an E6 oncoprotein of HPV directly contributes to the regulation of HPV gene expression.

Formation of well-defined soluble aggregates upon fusion to MBP is a generic property of E6 proteins from various human papillomavirus species

Protein aggregation is a main barrier hindering structural and functional studies of a number of interesting biological targets. The E6 oncoprotein of Human Papillomavirus strain 16 (E6(16)) is difficult to express under a native soluble form in bacteria. Produced as an unfused sequence, it forms inclusion bodies. Fused to the C-terminus of MBP, it is mainly produced in the form of soluble high molecular weight aggregates. Here, we produced as MBP-fusions seven E6 proteins from other HPV strains (5, 11, 18, 33, 45, 52, and 58) belonging to four different species, and we compared their aggregation state to that of MBP-E6(16). Using a fast mutagenesis method, we changed most non-conserved cysteines to the isosteric residue serine to minimize disulfide bridge-mediated aggregation during purification. Static and dynamic light scattering measurements, ultracentrifugation and electron microscopy demonstrated the presence in all MBP-E6 preparations of soluble high-molecular weight aggregates with a well-defined spherical shape. These aggregated particles are relatively monodisperse but their amount and their size vary depending on the conditions of expression and the strain considered. For all strains, minimal aggregate formation occurs when the expression is performed at 15 degrees C. Such observations suggest that the assembly of MBP-E6 aggregates takes place in vivo during protein biosynthesis, rather than occurring during purification. Finally, we show that all MBP-E6 preparations contain two zinc ions per protein monomer, suggesting that E6 domains within the high molecular weight aggregates possess a native-like fold, which enables correct coordination to the metal center.

Molecular dissection of the mycobacterial stringent response protein Rel

Latency in Mycobacterium tuberculosis poses a barrier in its complete eradication. Overexpression of certain genes is one of the factors that help these bacilli survive inside the host during latency. Among these genes, rel, which leads to the expression of Rel protein, plays an important role by synthesizing the signaling molecule ppGpp using GDP and ATP as substrates, thereby changing bacterial physiology. In Gram-negative bacteria, the protein is thought to be activated in vivo in the presence of ribosome by sensing uncharged tRNA. In the present report, we show that Rel protein from Mycobacterium smegmatis, which is highly homologous to M. tuberculosis Rel, is functional even in the absence of ribosome and uncharged tRNA. From the experiments presented here, it appears that the activity of Rel(Msm) is regulated by the domains present at the C terminus, as the deletion of these domains results in higher synthesis activity, with little change in hydrolysis of ppGpp. However, in the presence of tRNA, though the synthesis activity of the full-length protein increases to a certain extent, the hydrolysis activity undergoes drastic reduction. Full-length Rel undergoes multimerization involving interchain disulfide bonds. The synthesis of pppGpp by the full-length protein is enhanced in the reduced environment in vitro, whereas the hydrolysis activity does not change significantly. Mutations of cysteines to serines result in monomerization with a simultaneous increase in the synthesis activity. Finally, it has been possible to identify the unique cysteine, of six present in Rel, required for tRNA-mediated synthesis of ppGpp.

Structural and functional analysis of E6 oncoprotein: insights in the molecular pathways of human papillomavirus-mediated pathogenesis

Oncoprotein E6 is essential for oncogenesis induced by human papillomaviruses (HPVs). The solution structure of HPV16-E6 C-terminal domain reveals a zinc binding fold. A model of full-length E6 is proposed and analyzed in the context of HPV evolution. E6 appears as a chameleon protein combining a conserved structural scaffold with highly variable surfaces participating in generic or specialized HPV functions. We investigated surface residues involved in two specialized activities of high-risk genital HPV E6: p53 tumor suppressor degradation and nucleic acid binding. Screening of E6 surface mutants identified an in vivo p53 degradation-defective mutant that fails to recruit p53 to ubiquitin ligase E6AP and restores high p53 levels in cervical carcinoma cells by competing with endogeneous E6. We also mapped the nucleic acid binding surface of E6, the positive potential of which correlates with genital oncogenicity. E6 structure-function analysis provides new clues for understanding and counteracting the complex pathways of HPV-mediated pathogenesis.

The role of HPV E6 and E7 oncoproteins in HPV-associated cervical carcinogenesis

Cervical cancer is one of the leading world causes of cancer morbidity and mortality in woman, with more than 98% related to a human papillomavirus (HPV) infection origin. Infection with specific subtypes of HPV has been strongly implicated in cervical carcinogenesis. The identification and functional verification of host proteins associated with HPV E6 and E7 oncoproteins may provide useful information in understanding cervical carcinogenesis and the development of cervical cancer-specific markers. The advent of functional genomics and proteomics has provided hope of discovering novel biological markers for use in the screening, early diagnosis, prognostication and prediction of response to therapy. Herein, we review the studies where the profiles of host proteins associated with HPV E6 and E7 oncoproteins in cervical cancer were generated.

Kinetic Analysis of the Interactions of Human Papillomavirus E6 Oncoproteins with the Ubiquitin Ligase E6AP Using Surface Plasmon Resonance

Cervical cancers evolve from lesions generated by genital human papillomaviruses (HPV). "Low-risk" genital HPVs cause benign proliferations whereas "high-risk" types have the potential to progress into cancer. High-risk HPV E6 oncoproteins interact with the ubiquitin ligase E6AP and target several cellular proteins, including p53 and proteins of the MAGI family, towards ubiquitin-mediated degradation. E6AP, like other E6 binding proteins such as E6BP, IRF-3 and paxillin, interacts with E6 via a consensus leucine-charged motif. Here we have investigated the kinetics of the interactions of a 15-mer peptide containing the LxxvarphiLsh motif of E6AP with E6. For this we have developed a Biacore assay based on antibody-capture on the sensor surface of GST- and/or MBP-E6AP peptide constructs followed by E6 protein injection. Our experiments show that E6 oncoproteins from four major high-risk (16, 18, 33 and 58) HPV types bind to E6AP with equilibrium dissociation constants in the low micromolar range. The kinetic dissociation parameters of these interactions are remarkably similar. On the other hand, low-risk HPV 11 E6 does not interact with E6AP even at relatively high concentrations. We also show that the two zinc-binding domains of E6 are required for E6AP recognition. Finally, we have analysed the binding properties of site-directed mutants of the E6AP-derived peptide. We demonstrate the importance for binding of conserved aliphatic side-chains and the moderate role of the global negative charge of the peptide. This work provides the first quantitative data on an HPV E6-mediated interaction, which support the current models of E6AP-mediated degradation.

1H and 15N resonance assignment, secondary structure and dynamic behaviour of the C-terminal domain of human papillomavirus oncoprotein E6

E6 is a viral oncoprotein implicated in cervical cancers, produced by human papillomaviruses (HPVs). E6 contains two putative zinc-binding domains of about 75 residues each. The difficulty in producing recombinant E6 has long hindered the obtention of structural data. Recently, we described the expression and purification of E6-C 4C/4S, a stable, folded mutant of the C-terminal domain of HPV16 E6. Here, we have produced 15N-labelled samples of E6-C 4C/4S for structural studies by NMR. We have assigned most 1H and 15N resonances and identified the elements of secondary structure of the domain. The domain displays an original alpha/beta topology with roughly equal proportions of alpha-helix and beta-sheet. The PDZ-binding region of E6, located at the extreme C-terminus of the domain, is in a random conformation. Mass spectrometry demonstrated the presence of one zinc ion per protein molecule. Kinetics of replacement of zinc by cadmium followed by 1H,15N-HSQC experiments revealed specific frequency changes for the zinc-binding cysteines and their immediate neighbours. NMR spectra were affected by severe line-broadening effects which seriously hindered the assignment work. Investigation of these effects by 15N relaxation experiments showed that they are due to heterogeneous dynamic behaviour with mus-ms time scale motions occurring in localised regions of the monomeric domain.

Genomic and Proteomic Expression Patterns in HPV-16 E6 Gene Transfected Stable Human Carcinoma Cell Lines

cDNA microarray and proteomics studies were performed to analyze the genomic and proteomic expression patterns in HPV-16 E6 gene transfected stable human carcinoma cell lines. Among 1024 known genes and ESTs tested by cDNA microarray, we found 50 upregulated and 35 downregulated genes in RC10.1 HPV-16 E6 transfected human colon adenocarcinoma cells compared to RKO cells, and 27 upregulated and 43 downregulated genes in A549E6 HPV-16 E6 transfected human lung adenocarcinoma cells compared to A549 cells. Employing two dimensional gel electrophoresis and MALDI-TOF-MS, the global pattern of protein expressions in RC10.1 human colon adenocarcinoma and A549E6 human lung adenocarcinoma cell lines stably expressing the HPV 16-E6 gene were compared with those of RKO and A549 cell lines to generate a differential protein expression catalog. We found 13 upregulated and 13 downregulated proteins in RC10.1 (E6-expressing RKO) cells compared to RKO cells and 12 upregulated and 14 downregulated proteins in A549E6 (E6-expressing A549) cells compared to A549 cells. The identified genes and proteins were classified into several groups according to the subcellular function. Expressing pattern of three genes and proteins (CDK5, Bak, and I-TRAF) were matched in both analyses of cDNA microarray and proteomics. These powerful approaches using cDNA microarray and proteomics could provide in-depth information on the impact of HPV-16 E6-related genes and proteins. Differential gene and protein expression patterns by transfection of HPV-16 E6 will provide the nucleus of valuable resource for investigation of the biochemical basis of cervical carcinogenesis. Further understanding of this data base may provide valuable resources for developing novel diagnostic markers and therapeutic targets of cervical cancer.

Protein expression and refolding--a practical guide to getting the most out of inclusion bodies

The release of sequence data, particularly from a number of medically and biotechnologically important genomes, is increasing in an exponential fashion. In light of this, elucidating the structure and function of proteins, particularly in a "high throughput" manner, is an important quest. The production of recombinant proteins however is not always straightforward, with a number of proteins falling prey to low expression problems, a high susceptibility to proteolysis and the often despised production of inclusion bodies. Whilst expression as inclusion bodies can often be advantageous, their solubilization and renaturation is often a time consuming and empirical process. In this review, we aim to outline some of the more common approaches that have been applied to a variety of proteins and address issues associated with their handling.

Protein mutagenesis with monodispersity-based quality probing: selective inactivation of p53 degradation and DNA-binding properties of HPV E6 oncoprotein

Interpretation of protein mutagenesis experiments requires the ability to distinguish functionally relevant mutations from mutations affecting the structure. When a protein is expressed soluble in bacteria, properly folded mutants are expected to remain soluble whereas misfolded mutants should form insoluble aggregates. However, this rule may fail for proteins fused to highly soluble carrier proteins. In a previous study, we analysed the biophysical status of HPV oncoprotein E6 fused to the C-terminus of maltose-binding protein (MBP) and found that misfolded E6 moieties fused to MBP formed soluble aggregates of high molecular weight. By contrast, preparations of properly folded E6 fused to MBP were monodisperse. Here, we have used this finding to evaluate the quality of 19 MBP-fused E6 site-directed mutants by using a light scattering assay performed in a fluorimeter. This assay guided us to rule out structurally defective mutants and to obtain functionally relevant E6 mutants selectively altered for two molecular activities: degradation of tumour suppressor p53 and DNA recognition.