Positively charged termini of the L2 minor capsid protein are necessary for papillomavirus infection

Cryo EM Analysis Reveals Inherent Flexibility of Authentic Murine Papillomavirus Capsids

Human papillomavirus (HPV) is a significant health burden and leading cause of virus-induced cancers. However, studies have been hampered due to restricted tropism that makes production and purification of high titer virus problematic. This issue has been overcome by developing alternative HPV production methods such as virus-like particles (VLPs), which are devoid of a native viral genome. Structural studies have been limited in resolution due to the heterogeneity, fragility, and stability of the VLP capsids. The mouse papillomavirus (MmuPV1) presented here has provided the opportunity to study a native papillomavirus in the context of a common laboratory animal. Using cryo EM to solve the structure of MmuPV1, we achieved 3.3 Å resolution with a local symmetry refinement method that defined smaller, symmetry related subparticles. The resulting high-resolution structure allowed us to build the MmuPV1 asymmetric unit for the first time and identify putative L2 density. We also used our program ISECC to quantify capsid flexibility, which revealed that capsomers move as rigid bodies connected by flexible linkers. The MmuPV1 flexibility was comparable to that of a HPV VLP previously characterized. The resulting MmuPV1 structure is a promising step forward in the study of papillomavirus and will provide a framework for continuing biochemical, genetic, and biophysical research for papillomaviruses.

Novel Production of Bovine Papillomavirus Pseudovirions in Tobacco Plants

Vaccine efficacy requires the production of neutralising antibodies which offer protection against the native virus. The current gold standard for determining the presence of neutralising antibodies is the pseudovirion-based neutralisation assay (PBNA). PBNAs utilise pseudovirions (PsVs), structures which mimic native virus capsids, but contain non-viral nucleic material. PsVs are currently produced in expensive cell culture systems, which limits their production, yet plant expression systems may offer cheaper, safer alternatives. Our aim was to determine whether plants could be used for the production of functional PsVs of bovine papillomavirus 1 (BPV1), an important causative agent of economically damaging bovine papillomas in cattle and equine sarcoids in horses and wild equids. BPV1 capsid proteins, L1 and L2, and a self-replicating reporter plasmid were transiently expressed in Nicotiana benthamiana to produce virus-like particles (VLPs) and PsVs. Strategies to enhance particle yields were investigated and optimised protocols were established. The PsVs' ability to infect mammalian cells and express their encapsidated reporter genes in vitro was confirmed, and their functionality as reagents in PBNAs was demonstrated through their neutralisation by several different antibodies. This is the first report of BPV PsVs expressed in plants and demonstrates the potential for the development of therapeutic veterinary vaccines in planta.

Refolding and in vitro characterization of human papillomavirus 16 minor capsid protein L2

The minor capsid protein L2 of papillomaviruses exhibits multiple functions during viral entry including membrane interaction. Information on the protein is scarce, because of its high tendency of aggregation. We determined suitable conditions to produce a functional human papillomavirus (HPV) 16 L2 protein and thereby provide the opportunity for extensive in vitro analysis with respect to structural and biochemical information on L2 proteins and mechanistic details in viral entry. We produced the L2 protein of high-risk HPV 16 in Escherichia coli as inclusion bodies and purified the protein under denaturing conditions. A successive buffer screen resulted in suitable conditions for the biophysical characterization of 16L2. Analytical ultracentrifugation of the refolded protein showed a homogenous monomeric species. Furthermore, refolded 16L2 shows secondary structure elements. The N-terminal region including the proposed transmembrane region of 16L2 shows alpha-helical characteristics. However, overall 16L2 appears largely unstructured. Refolded 16L2 is capable of binding to DNA indicating that the putative DNA-binding regions are accessible in refolded 16L2. Further the refolded protein interacts with liposomal membranes presumably via the proposed transmembrane region at neutral pH without structural changes. This indicates that 16L2 can initially interact with membranes via pre-existing structural features.

DNA plasmid vaccine carrying Chlamydia trachomatis (Ct) major outer membrane and human papillomavirus 16L2 proteins for anti-Ct infection

Chlamydia trachomatis (Ct) is one of the most frequently encountered sexual infection all over the world, yielding tremendous reproductive problems (e.g. infertility and ectopic pregnancy) in the women. This work described the design of a plasmid vaccine that protect mice from Ct infection, and reduce productive tract damage by generating effective antibody and cytotoxic T cell immunity. The vaccine, s was composed of MOMP multi-epitope and HPV16L2 genes carried in pcDNA plasmid (i.e. pcDNA3.1/MOMP/HPV16L). In transfection, the vaccine expressed the chimeric genes (i.e. MOMP and HPV16L2), as demonstrated via western blot, RT-PCR and fluorescence imaging. In vitro, the vaccine transfected COS-7 cells and expressed the proteins corresponding to the genes carried in the vaccine. Through intramuscular immunization in BALB/c mice, the vaccine induced higher levels of anti-Ct IgG titer, anti-HPV16L2 IgG titer in serum and IgA titer in local mucosal secretions, compared to plasmid vaccines that carry only Ct MOMP multi-epitope or HPV16L2 chimeric component only. In mice intravaginally challenged with Ct, the vaccines pcDNA3.1/MOMP/HPV16L2 generated a higher level of genital protection compared to other vaccine formulations. Additionally, histochemical staining indicated that pcDNA3.1/MOMP/HPV16L2 eliminated mouse genital tract tissue pathologies induced by Ct infection. This work demonstrated that pcDNA/MOMP/HPV16L2 vaccine can protect against Ct infection by regulating antibody production, cytotoxic T cell killing functions and reducing pathological damage in mice genital tract. This work can potentially offer us a new vaccine platform against Ct infection.

Cell receptor screening for human papillomavirus invasion by using a polyoxometalate-peptide assembly as a probe

The present study constructed a competitive recognition system using cell receptor screening for human papillomavirus (HPV) invasion by using the hybrid-assembly of polyoxometalates (POMs) and cationic peptides as a platform. The fine tuning both of the surface charge of POMs and peptide sequence were precisely performed to develop a luminescence switch of POMs, leading to the establishment of a ternary system to identify which types of glycosaminoglycans (GAGs) are potential cell receptors for HPV infection. In addition, the method was successfully applied to construct a hybrid-assembly with the recombined HPV 16 L1 pentamers from Escherichia coli and perform GAGs screening, which validated the system's potential for practical applications. In particular, the intrinsic mechanism for each competitive partner in the system was explained well by using isothermal titration calorimetry (ITC) and time-resolved fluorescence spectra. The present method will be helpful to extend the protocol to other systems by using peptides and POMs with similar properties, and ultimately, we hope it will promote the development of anti-viral agents.

A central region in the minor capsid protein of papillomaviruses facilitates viral genome tethering and membrane penetration for mitotic nuclear entry

Incoming papillomaviruses (PVs) depend on mitotic nuclear envelope breakdown to gain initial access to the nucleus for viral transcription and replication. In our previous work, we hypothesized that the minor capsid protein L2 of PVs tethers the incoming vDNA to mitotic chromosomes to direct them into the nascent nuclei. To re-evaluate how dynamic L2 recruitment to cellular chromosomes occurs specifically during prometaphase, we developed a quantitative, microscopy-based assay for measuring the degree of chromosome recruitment of L2-EGFP. Analyzing various HPV16 L2 truncation-mutants revealed a central chromosome-binding region (CBR) of 147 amino acids that confers binding to mitotic chromosomes. Specific mutations of conserved motifs (IVAL286AAAA, RR302/5AA, and RTR313EEE) within the CBR interfered with chromosomal binding. Moreover, assembly-competent HPV16 containing the chromosome-binding deficient L2(RTR313EEE) or L2(IVAL286AAAA) were inhibited for infection despite their ability to be transported to intracellular compartments. Since vDNA and L2 were not associated with mitotic chromosomes either, the infectivity was likely impaired by a defect in tethering of the vDNA to mitotic chromosomes. However, L2 mutations that abrogated chromatin association also compromised translocation of L2 across membranes of intracellular organelles. Thus, chromatin recruitment of L2 may in itself be a requirement for successful penetration of the limiting membrane thereby linking both processes mechanistically. Furthermore, we demonstrate that the association of L2 with mitotic chromosomes is conserved among the alpha, beta, gamma, and iota genera of Papillomaviridae. However, different binding patterns point to a certain variance amongst the different genera. Overall, our data suggest a common strategy among various PVs, in which a central region of L2 mediates tethering of vDNA to mitotic chromosomes during cell division thereby coordinating membrane translocation and delivery to daughter nuclei.

Papillomaviruses can cause carcinogenic malignancies such as cervical cancer. Like most DNA viruses, papillomaviruses must deliver their genome to the cell nucleus during initial infection, where it is expressed and replicated. However, papillomaviruses make use of unconventional mechanisms for genome delivery. They reside on the cell surface for protracted, hour-long times, before they are taken up by a novel endocytic mechanism. Moreover, they are delivered to the trans-Golgi-network by non-canonical endosomal trafficking prior to nuclear delivery. For entry into the nucleus, papillomaviruses access the nuclear space after nuclear envelope breakdown during mitosis unlike most other intranuclear viruses. The detailed mechanism how the viral genome is directed to nascent nuclei during mitosis remains elusive. Our previous work suggested that the minor capsid protein L2 may tether the incoming viral genome to mitotic chromosomes to direct it to the nascent nuclei. This work identifies a conserved central region in L2 protein to be necessary and sufficient for tethering. Moreover, it demonstrates that this mechanism is conserved across different papillomavirus genera. Importantly, this report also provides evidence that the processes of nuclear import by tethering and membrane penetration are mechanistically linked.

Cryoelectron Microscopy Maps of Human Papillomavirus 16 Reveal L2 Densities and Heparin Binding Site

Human papillomavirus (HPV) is a significant health burden and leading cause of virus-induced cancers. The current commercial vaccines are genotype specific and provide little therapeutic benefit to patients with existing HPV infections. Host entry mechanisms represent an excellent target for alternative therapeutics, but HPV receptor use, the details of cell attachment, and host entry are inadequately understood. Here we present near-atomic resolution structures of the HPV16 capsid and HPV16 in complex with heparin, both determined from cryoelectron micrographs collected with direct electron detection technology. The structures clarify details of capsid architecture for the first time, including variation in L1 major capsid protein conformation and putative location of L2 minor protein. Heparin binds specifically around the capsid icosahedral vertices and may recapitulate the earliest stage of infection, providing a framework for continuing biochemical, genetic, and biophysical studies.

Furin Cleavage of L2 during Papillomavirus Infection: Minimal Dependence on Cyclophilins

Despite an abundance of evidence supporting an important role for the cleavage of minor capsid protein L2 by cellular furin, direct cleavage of capsid-associated L2 during human papillomavirus 16 (HPV16) infection remains poorly characterized. The conserved cleavage site, close to the L2 N terminus, confounds observation and quantification of the small cleavage product by SDS-PAGE. To overcome this difficulty, we increased the size shift by fusing a compact protein domain, the Propionibacterium shermanii transcarboxylase domain (PSTCD), to the N terminus of L2. The infectious PSTCD-L2 virus displayed an appreciable L2 size shift during infection of HaCaT keratinocytes. Cleavage under standard cell culture conditions rarely exceeded 35% of total L2. Cleavage levels were enhanced by the addition of exogenous furin, and the absolute levels of infection correlated to the level of L2 cleavage. Cleavage occurred on both the HaCaT cell surface and extracellular matrix (ECM). Contrary to current models, experiments on the involvement of cyclophilins revealed little, if any, role for these cellular enzymes in the modulation of furin cleavage. HPV16 L2 contains two consensus cleavage sites, Arg5 (2RHKR5) and Arg12 (9RTKR12). Mutant PSTCD-L2 viruses demonstrated that although furin can cleave either site, cleavage must occur at Arg12, as cleavage at Arg5 alone is insufficient for successful infection. Mutation of the conserved cysteine residues revealed that the Cys22-Cys28 disulfide bridge is not required for cleavage. The PSTCD-L2 virus or similar N-terminal fusions will be valuable tools to study additional cellular and viral determinants of furin cleavage.

IMPORTANCE

Furin cleavage of minor capsid protein L2 during papillomavirus infection has been difficult to directly visualize and quantify, confounding efforts to study this important step of HPV infection. Fusion of a small protein domain to the N terminus greatly facilitates direct visualization of the cleavage product, revealing important characteristics of this critical process. Contrary to the current model, we found that cleavage is largely independent of cyclophilins, suggesting that cyclophilins act either in parallel to or downstream of furin to trigger exposure of a conserved N-terminal L2 epitope (RG-1) during infection. Based on this finding, we strongly caution against using L2 RG-1 epitope exposure as a convenient but indirect proxy of furin cleavage.

Human Papillomavirus Biology, Pathogenesis, and Potential for Drug Discovery: A Literature Review for HIV Nurse Clinical Scientists

Persistent oncogenic human papillomavirus (HPV) infection increases the probability that precancerous anal high-grade squamous intraepithelial lesions will progress to invasive anal cancer. Anal neoplasia associated with HPV disproportionately affects HIV-infected individuals, especially men who have sex with men. Prevention is limited to HPV vaccine recommendations, highlighting the need for new treatments. The purpose of this review is to provide HIV information to nurse clinical scientists about HPV-related cancer to highlight the connection between: (a) HPV biology and pathogenesis and (b) the development of drugs and novel therapeutic methods using high-throughput screening. PubMed and CINAHL were used to search the literature to determine HPV-related epidemiology, biology, and use of high-throughput screening for drug discovery. Several events in the HPV life cycle have the potential to be developed into biologic targets for drug discovery using the high-throughput screening technique, which has been successfully used to identify compounds to inhibit HPV infections.

Direct binding of retromer to human papillomavirus type 16 minor capsid protein L2 mediates endosome exit during viral infection

Trafficking of human papillomaviruses to the Golgi apparatus during virus entry requires retromer, an endosomal coat protein complex that mediates the vesicular transport of cellular transmembrane proteins from the endosome to the Golgi apparatus or the plasma membrane. Here we show that the HPV16 L2 minor capsid protein is a retromer cargo, even though L2 is not a transmembrane protein. We show that direct binding of retromer to a conserved sequence in the carboxy-terminus of L2 is required for exit of L2 from the early endosome and delivery to the trans-Golgi network during virus entry. This binding site is different from known retromer binding motifs and can be replaced by a sorting signal from a cellular retromer cargo. Thus, HPV16 is an unconventional particulate retromer cargo, and retromer binding initiates retrograde transport of viral components from the endosome to the trans-Golgi network during virus entry. We propose that the carboxy-terminal segment of L2 protein protrudes through the endosomal membrane and is accessed by retromer in the cytoplasm.

Human papillomavirus species-specific interaction with the basement membrane-resident non-heparan sulfate receptor

Using a cell culture model where virus is bound to the extracellular matrix (ECM) prior to cell surface binding, we determined that human papillomavirus type 16 (HPV16) utilizes ECM resident laminin (LN) 332 as an attachment receptor for infectious entry. In presence of LN332, soluble heparin can function as ligand activator rather than competitive inhibitor of HPV16 infection. We also show that the ability to use LN332 binding as a productive attachment step for infectious entry is not conserved amongst HPV types. In the alpha genus, species 9 members (HPV16) attach to ECM via LN332, while members of species 7 (HPV18) are completely inhibited by heparin pre-incubation due to an inability to use LN332. Since HPV species 7 and 9 are preferentially associated with adenocarcinoma and squamous cell carcinoma of the cervix, respectively, our data provide first evidence that pre-entry events may contribute to the anatomical-site preference of HPV species.

Prophylactic papillomavirus vaccines

Human papillomaviruses (HPV) are the causative agents of cervical cancer, the third most common cancer in women. The development of prophylactic HPV vaccines Gardasil® and Cervarix® targeting the major oncogenic HPV types is now the frontline of cervical cancer prevention. Both vaccines have been proven to be highly effective and safe although there are still open questions about their target population, cross-protection, and long-term efficacy. The main limitation for a worldwide implementation of Gardasil® and Cervarix® is their high cost. To develop more affordable vaccines research groups are concentrated in new formulations with different antigens including capsomeres, the minor capsid protein L2 and DNA. In this article we describe the vaccines' impact on HPV-associated disease, the main open questions about the marketed vaccines, and current efforts for the development of second-generation vaccines.

Vesicular trafficking of incoming human papillomavirus 16 to the Golgi apparatus and endoplasmic reticulum requires γ-secretase activity

The route taken by papillomaviruses from the cell surface to the nucleus during infection is incompletely understood. Here, we developed a novel human papillomavirus 16 (HPV16) pseudovirus in which the carboxy terminus of the minor capsid protein L2 is exposed on the exterior of the intact capsid prior to cell binding. With this pseudovirus, we used the proximity ligation assay immune detection technique to demonstrate that during entry HPV16 L2 traffics into and out of the early endosome prior to Golgi localization, and we demonstrated that L2 enters the endoplasmic reticulum during entry. The cellular membrane-associated protease, γ-secretase, is required for infection by HPV16 pseudovirus and authentic HPV16. We also showed that inhibition of γ-secretase does not interfere substantively with virus internalization, initiation of capsid disassembly, entry into the early endosome, or exit from this compartment, but γ-secretase is required for localization of L2 and viral DNA to the Golgi apparatus and the endoplasmic reticulum. These results show that incoming HPV16 traffics sequentially from the cell surface to the endosome and then to the Golgi apparatus and the endoplasmic reticulum prior to nuclear entry.

The human papillomaviruses are small nonenveloped DNA viruses responsible for approximately 5% of all human cancer deaths, but little is known about the process by which these viruses transit from the cell surface to the nucleus. Here we show that incoming HPV16, the most common high-risk HPV, traffics though a series of vesicular compartments during infectious entry, including the endosome, Golgi apparatus, and endoplasmic reticulum. Furthermore, we show that γ-secretase, a cellular membrane-associated protease, is required for entry of the L2 minor capsid protein and viral DNA into the Golgi apparatus and endoplasmic reticulum. These studies reveal a new pathway of cell entry by DNA viruses and suggest that components of this pathway are candidate antiviral targets.

Coat as a dagger: the use of capsid proteins to perforate membranes during non-enveloped DNA viruses trafficking

To get access to the replication site, small non-enveloped DNA viruses have to cross the cell membrane using a limited number of capsid proteins, which also protect the viral genome in the extracellular environment. Most of DNA viruses have to reach the nucleus to replicate. The capsid proteins involved in transmembrane penetration are exposed or released during endosomal trafficking of the virus. Subsequently, the conserved domains of capsid proteins interact with cellular membranes and ensure their efficient permeabilization. This review summarizes our current knowledge concerning the role of capsid proteins of small non-enveloped DNA viruses in intracellular membrane perturbation in the early stages of infection.

Impact of inhibitors and L2 antibodies upon the infectivity of diverse alpha and beta human papillomavirus types

The licensed human papillomavirus (HPV) vaccines elicit type-restricted immunity but do not target cutaneous HPV types of the beta genus that are associated with non-melanoma skin cancer in immune-compromised patients, and it is unclear if these diverse types share a common mechanism of infection. Residues 11-88 of minor capsid protein L2 contain cross-protective epitopes, and vaccination with concatamers of this region derived from as many as eight alpha HPV (L2 α11-88x8) is being developed as an alternative prophylactic vaccine with potentially broader efficacy. There is also interest in developing broadly protective topical microbicides, such as carrageenan or heparin that block HPV receptor interactions, or small molecule inhibitors of infection. Here we have examined several inhibitors of HPV infection and antisera to L2 α11-88x8 for their breadth of activity against infection by 34 HPV types from within both the alpha and beta families using pseudovirions (PsV) carrying a luciferase reporter as surrogates for native virus. We observed that both heparin and carrageenan prevented infection by mucosatropic HPV types, but surprisingly PsV of several epidermotropic alpha4 and beta HPV types exhibited increased infectivity especially at low inhibitor concentrations. Furin and γ-secretase inhibitors and L2 α11-88x8 antiserum blocked infection by all HPV PsV types tested. These findings suggest that the distinct tropism of mucosal and cutaneous HPV may reflect distinct cell surface receptor interactions, but a common uptake mechanism dependent upon furin and γ-secretase proteolytic activities. Carrageenan, which is being tested as a vaginal microbicide, broadly inhibited infection by the high-risk mucosatropic HPV PsV, but not most skin tropic alpha and beta HPV. Vaccination with an L2 multimer derived exclusively from alpha papillomavirus sequences induced antibodies that broadly neutralized PsV of all 34 HPVs from within both the alpha and beta families, suggesting each displays conserved L2 neutralizing epitopes.

Multiple heparan sulfate binding site engagements are required for the infectious entry of human papillomavirus type 16

Human papillomavirus (HPV) entry is accompanied by multiple receptor-induced conformational changes (CCs) affecting both the major and minor capsid proteins, L1 and L2. Interaction of heparan sulfate (HS) with L1 is essential for successful HPV16 entry. Recently, cocrystallization of HPV16 with heparin revealed four distinct binding sites. Here we characterize mutant HPV16 to delineate the role of engagement with HS binding sites during infectious internalization. Site 1 (Lys278, Lys361), which mediates primary binding, is sufficient to trigger an L2 CC, exposing the amino terminus. Site 2 (Lys54, Lys356) and site 3 (Asn57, Lys59, Lys442, Lys443) are engaged following primary attachment and are required for infectious entry. Site 2 mutant particles are efficiently internalized but fail to undergo an L1 CC on the cell surface and subsequent uncoating in the endocytic compartment. After initial attachment to the cell, site 3 mutants undergo L1 and L2 CCs and then accumulate on the extracellular matrix (ECM). We conclude that the induction of CCs following site 1 and site 2 interactions results in reduced affinity for the primary HS binding site(s) on the cell surface, which allows engagement with site 3. Taken together, our findings suggest that HS binding site engagement induces CCs that prepare the virus for downstream events, such as the exposure of secondary binding sites, CCs, transfer to the uptake receptor, and uncoating.

Virus-like particles for the prevention of human papillomavirus-associated malignancies

As compared with peptide- or protein-based vaccines, naked DNA vectors and even traditional attenuated or inactivated virus vaccines, virus-like particles (VLPs) are an attractive vaccine platform, as they offer a combination of safety, ease of production and both high-density B-cell epitope display and intracellular presentation of T-cell epitopes that induce potent humoral and cellular immune responses, respectively. Indeed, HPV vaccines based on VLP production by recombinant expression of major capsid antigen L1 in yeast (Gardasil(®), Merck & Co., NJ, USA) or insect cells (Cervarix(®), GlaxoSmithKline, London, UK) have been licensed for the prevention of cervical and anogenital infection and disease associated with the genotypes targeted by each vaccine. However, these HPV vaccines have not been demonstrated as effective to treat existing infections, and efforts to develop a therapeutic HPV vaccine continue. Furthermore, current HPV L1-VLP vaccines provide type-restricted protection, requiring highly multivalent formulations to broaden coverage to the dozen or more oncogenic HPV genotypes. This raises the complexity and cost of vaccine production. The lack of access to screening and high disease burden in developing countries has spurred efforts to develop second-generation HPV vaccines that are more affordable, induce wider protective coverage and offer therapeutic coverage against HPV-associated malignancies. Given the previous success with L1-VLP-based vaccines against HPV, VLPs have been also adopted as platforms for many second-generation HPV and non-HPV vaccine candidates with both prophylactic and therapeutic intent. In this article, the authors examine the progress and challenges of these efforts, with a focus on how they inform VLP vaccine design.

L2, the minor capsid protein of papillomavirus

The capsid protein L2 plays major roles in both papillomavirus assembly and the infectious process. While L1 forms the majority of the capsid and can self-assemble into empty virus-like particles (VLPs), L2 is a minor capsid component and lacks the capacity to form VLPs. However, L2 co-assembles with L1 into VLPs, enhancing their assembly. L2 also facilitates encapsidation of the ∼8 kbp circular and nucleosome-bound viral genome during assembly of the non-enveloped T=7d virions in the nucleus of terminally differentiated epithelial cells, although, like L1, L2 is not detectably expressed in infected basal cells. With respect to infection, L2 is not required for particles to bind to and enter cells. However L2 must be cleaved by furin for endosome escape. L2 then travels with the viral genome to the nucleus, wherein it accumulates at ND-10 domains. Here, we provide an overview of the biology of L2.

The high risk HPV16 L2 minor capsid protein has multiple transport signals that mediate its nucleocytoplasmic traffic

In this study we examined the transport signals contributing to HPV16 L2 nucleocytoplasmic traffic using confocal microscopy analysis of enhanced green fluorescent protein-L2 (EGFP-L2) fusions expressed in HeLa cells. We confirmed that both nuclear localization signals (NLSs), the nNLS (1MRHKRSAKRTKR12) and cNLS (456RKRRKR461), previously characterized in vitro (Darshan et al., 2004), function independently in vivo. We discovered that a middle region rich in arginine residues (296SRRTGIRYSRIGNKQTLRTRS316) functions as a nuclear retention sequence (NRS), as mutagenesis of critical arginine residues within this NRS reduced the fraction of L2 in the nucleus despite the presence of both NLSs. Significantly, the infectivity of HPV16 pseudoviruses containing either RR297AA or RR297EE within the L2 NRS was strongly reduced both in HaCaT cells and in a murine challenge model. Experiments using Ratjadone A nuclear export inhibitor and mutation-localization analysis lead to the discovery of a leucine-rich nuclear export signal ((462)LPYFFSDVSL) mediating 16L2 nuclear export. These data indicate that HPV16 L2 nucleocytoplasmic traffic is dependent on multiple functional transport signals.

Interaction of synthetic HPV-16 capsid peptides with heparin: thermodynamic parameters and binding mechanism

Capsid proteins binding cell surface proteoglycans is a key early event in human papillomavirus (HPV) infection. The positively charged sequences at the C-terminus of the L1 protein and the N- and C-termini of the L2 protein of HPV-16 can efficiently bind to heparin receptors, which were characterized in the present study by quantitative isothermal titration calorimetry experiments primarily, fluorescence spectroscopy, and static right-angle light scattering. The binding constant, K, was at an order of magnitude of 10(7) M(-1) for the two peptides at the N- and C-termini of HPV-16 L2 and segment b at the C-terminus of HPV-16 L1, while that for other L1 analogues were of a smaller order, illustrating that the heparin binding is a typical sequence-specific and -dependent phenomenon. These results suggest that, in addition to L1, the L2 protein may participate in cell surface attachment during HPV infection. Furthermore, the calorimetry results demonstrated that hydrophobic interactions and hydrogen bonding are involved in peptide binding to heparin in addition to the essential electrostatic interactions. Meanwhile, circular dichroism spectroscopy revealed that binding to heparin does not induce obvious secondary structural changes in the peptides.

Heparan Sulfate Proteoglycans in Infection

To cause infections, microbial pathogens elaborate a multitude of factors that interact with host components. Using these host–pathogen interactions to their advantage, pathogens attach, invade, disseminate, and evade host defense mechanisms to promote their survival in the hostile host environment. Many viruses, bacteria, and parasites express adhesins that bind to cell surface heparan sulfate proteoglycans (HSPGs) to facilitate their initial attachment and subsequent cellular entry. Some pathogens also secrete virulence factors that modify HSPG expression. HSPGs are ubiquitously expressed on the cell surface of adherent cells and in the extracellular matrix. HSPGs are composed of one or several heparan sulfate (HS) glycosaminoglycan chains attached covalently to specific core proteins. For most intracellular pathogens, cell surface HSPGs serve as a scaffold that facilitates the interaction of microbes with secondary receptors that mediate host cell entry. Consistent with this mechanism, addition of HS or its pharmaceutical functional mimic, heparin, inhibits microbial attachment and entry into cultured host cells, and HS-binding pathogens can no longer attach or enter cultured host cells whose HS expression has been reduced by enzymatic treatment or chemical mutagenesis. In pathogens where the specific HS adhesin has been identified, mutant strains lacking HS adhesins are viable and show normal growth rates, suggesting that the capacity to interact with HSPGs is strictly a virulence activity. The goal of this chapter is to provide a mechanistic overview of our current understanding of how certain microbial pathogens subvert HSPGs to promote their infection, using specific HSPG–pathogen interactions as representative examples.

Papillomavirus infection requires gamma secretase

The mechanism by which papillomaviruses breach cellular membranes to deliver their genomic cargo to the nucleus is poorly understood. Here, we show that infection by a broad range of papillomavirus types requires the intramembrane protease γ secretase. The γ-secretase inhibitor (S,S)-2-[2-(3,5-difluorophenyl)-acetylamino]-N-(1-methyl-2-oxo-5-phenyl-2,3-dihydro-1H-benzo[e][1,4]diazepin-3-yl)-propionamide (compound XXI) inhibits infection in vitro by all types of papillomavirus pseudovirions tested, with a 50% inhibitory concentration (IC(50)) of 130 to 1,000 pM, regardless of reporter construct and without impacting cellular viability. Conversely, XXI does not inhibit in vitro infection by adenovirus or pseudovirions derived from the BK or Merkel cell polyomaviruses. Vaginal application of XXI prevents infection of the mouse genital tract by human papillomavirus type 16 (HPV16) pseudovirions. Nicastrin and presenilin-1 are essential components of the γ-secretase complex, and mouse embryo fibroblasts deficient in any one of these components were not infected by HPV16, whereas wild-type and β-secretase (BACE1)-deficient cells were susceptible. Neither the uptake of HPV16 into Lamp-1-positive perinuclear vesicles nor the disassembly of capsid to reveal both internal L1 and L2 epitopes and bromodeoxyuridine (BrdU)-labeled encapsidated DNA is dependent upon γ-secretase activity. However, blockade of γ-secretase activity by XXI prevents the BrdU-labeled DNA encapsidated by HPV16 from reaching the ND10 subnuclear domains. Since prior studies indicate that L2 is critical for endosomal escape and targeting of the viral DNA to ND10 and that γ secretase is located in endosomal membranes, our findings suggest that either L2 or an intracellular receptor are cleaved by γ secretase as papillomavirus escapes the endosome.

Study of infectious virus production from HPV18/16 capsid chimeras

Using the HPV18 genome as the backbone, we exchanged the HPV18 L2 or L1 genes with those of HPV16. The intertypical exchange of HPV18 L1 with the HPV16 L1 produced genomes that efficiently replicated and produced infectious virus. Genomes containing an intertypical exchange of HPV18 L2 for the HPV16 L2 failed to produce infectious virus in multiple independently derived cell lines. Using chimeric constructs of individual capsid proteins, we identified a type-specific domain at the N-terminus of the HPV18L1 capsid protein, which interferes with its ability to cooperate with the HPV16 L2 protein to form infectious viral particles. Deletion of this domain allows for the cooperation of the HPV18 L1 protein and HPV16 L2 protein and production of infectious progeny. In addition, cooperation of this N-terminal HPV18 L1 deletion mutant protein with the wild-type HPV18 L2 protein efficiently replicates infectious virus but changes occur in the viral structure.

Proteoglycans in host-pathogen interactions: molecular mechanisms and therapeutic implications

Many microbial pathogens subvert proteoglycans for their adhesion to host tissues, invasion of host cells, infection of neighbouring cells, dissemination into the systemic circulation, and evasion of host defence mechanisms. Where studied, specific virulence factors mediate these proteoglycan-pathogen interactions, which are thus thought to affect the onset, progression and outcome of infection. Proteoglycans are composites of glycosaminoglycan (GAG) chains attached covalently to specific core proteins. Proteoglycans are expressed ubiquitously on the cell surface, in intracellular compartments, and in the extracellular matrix. GAGs mediate the majority of ligand-binding activities of proteoglycans, and many microbial pathogens elaborate cell-surface and secreted factors that interact with GAGs. Some pathogens also modulate the expression and function of proteoglycans through known virulence factors. Several GAG-binding pathogens can no longer attach to and invade host cells whose GAG expression has been reduced by mutagenesis or enzymatic treatment. Furthermore, GAG antagonists have been shown to inhibit microbial attachment and host cell entry in vitro and reduce virulence in vivo. Together, these observations underscore the biological significance of proteoglycan-pathogen interactions in infectious diseases.

Mechanisms of cell entry by human papillomaviruses: an overview

As the primary etiological agents of cervical cancer, human papillomaviruses (HPVs) must deliver their genetic material into the nucleus of the target cell. The viral capsid has evolved to fulfil various roles that are critical to establish viral infection. The particle interacts with the cell surface via interaction of the major capsid protein, L1, with heparan sulfate proteoglycans. Moreover, accumulating evidence suggests the involvement of a secondary receptor and a possible role for the minor capsid protein, L2, in cell surface interactions.

The entry of HPV in vitro is initiated by binding to a cell surface receptor in contrast to the in vivo situation where the basement membrane has recently been identified as the primary site of virus binding. Binding of HPV triggers conformational changes, which affect both capsid proteins L1 and L2, and such changes are a prerequisite for interaction with the elusive uptake receptor. Most HPV types that have been examined, appear to enter the cell via a clathrin-dependent endocytic mechanism, although many data are inconclusive and inconsistent. Furthermore, the productive entry of HPV is a process that occurs slowly and asynchronously and it is characterised by an unusually extended residence on the cell surface.

Despite the significant advances and the emergence of a general picture of the infectious HPV entry pathway, many details remain to be clarified. The impressive technological progress in HPV virion analysis achieved over the past decade, in addition to the improvements in general methodologies for studying viral infections, provide reasons to be optimistic about further advancement of this field.

This mini review is intended to provide a concise overview of the literature in HPV virion/host cell interactions and the consequences for endocytosis.

Role of L2 cysteines in papillomavirus infection and neutralization

Vaccination of mice with minor capsid protein L2 or passive transfer with the L2-specific neutralizing monoclonal antibody RG-1 protects against human papillomavirus type 16 (HPV16) challenge. Here we explored the nature of the RG-1 epitope and its contribution to viral infectivity. RG-1 bound equivalently HPV16 L2 residues 17-36 with or without an intact C22-C28 disulphide bridge. HPV16 L2 mutations K20A, C22A, C22S, C28A, C28S, or P29A prevented RG-1 binding, whereas Y19A, K23A or Q24A had no impact. Mutation of either C22 or C28 to alanine or serine compromises HPV16 pseudoviral infectivity both in vitro and in the murine vaginal tract, but does not impact pseudovirion assembly. Despite their lack of infectivity, HPV16 pseudovirions containing C22S or C28S mutant L2 bind to cell surfaces, are taken up, and expose the 17-36 region on the virion surface as for wild type HPV16 pseudovirions suggesting normal furin cleavage of L2. Mutation of the second cysteine residue in Bovine papillomavirus type 1 (BPV1) L2 to serine (C25S) dramatically reduced the infectivity of BPV1 pseudovirions. Surprisingly, in contrast to the double mutation in HPV16 L2, the BPV1 L2 C19S, C25S double mutation reduced BPV1 pseudovirion infectivity of 293TT cells by only half.

Developing vaccines against minor capsid antigen L2 to prevent papillomavirus infection

A subset of human papillomavirus (HPV) genotypes is responsible for approximately 5% of all cancer deaths globally, and uterine cervical carcinoma accounts for the majority of these cases. The impact of HPV is greatest for women who do not have access to effective secondary preventive measures, and consequently over 80% of cervical cancer deaths worldwide occur in developing nations. The understanding that persistent infection by this 'oncogenic' subset of HPV genotypes is necessary for the development of cervical carcinoma has driven the development of preventive vaccines. Two preventive vaccines comprising recombinant HPV L1 virus-like particles (VLPs) have been licensed. However, the current cost of these vaccines precludes sustained global delivery, and they target only two of the approximately 15 known oncogenic HPV types, although approximately 70% of cervical cancer cases are attributed to these two types and there is evidence for some degree of cross-protection against other closely related types. A possible approach to broader immunity at lower cost is to consider vaccination against L2. L2 vaccines can be produced inexpensively and they also have the promise of conferring much broader cross-type protective immunity than that observed with L1 VLP immunization. However, L2 vaccine development lags behind L1 VLP vaccines and several technical hurdles remain.

Genital transmission of HPV in a mouse model is potentiated by nonoxynol-9 and inhibited by carrageenan

Genital human papillomavirus (HPV) infection is the most common sexually transmitted infection, and virtually all cases of cervical cancer are attributable to infection by a subset of HPVs (reviewed in ref. 1). Despite the high incidence of HPV infection and the recent development of a prophylactic vaccine that confers protection against some HPV types, many features of HPV infection are poorly understood. It remains worthwhile to consider other interventions against genital HPVs, particularly those that target infections not prevented by the current vaccine. However, productive papillomavirus infection is species- and tissue-restricted, and traditional models use animal papillomaviruses that infect the skin or oral mucosa. Here we report the development of a mouse model of cervicovaginal infection with HPV16 that recapitulates the establishment phase of papillomavirus infection. Transduction of a reporter gene by an HPV16 pseudovirus was characterized by histology and quantified by whole-organ, multispectral imaging. Disruption of the integrity of the stratified or columnar genital epithelium was required for infection, which occurred after deposition of the virus on the basement membrane underlying basal keratinocytes. A widely used vaginal spermicide, nonoxynol-9 (N-9), greatly increased susceptibility to infection. In contrast, carrageenan, a polysaccharide present in some vaginal lubricants, prevented infection even in the presence of N-9, suggesting that carrageenan might serve as an effective topical HPV microbicide.

Identification of a dynein interacting domain in the papillomavirus minor capsid protein l2

Papillomaviruses enter cells via endocytosis (H. C. Selinka et al., Virology 299:279-287, 2002). After egress from endosomes, the minor capsid protein L2 accompanies the viral DNA to the nucleus and subsequently to the subnuclear promyelocytic leukemia protein bodies (P. M. Day et al., Proc. Natl. Acad. Sci. USA 101:14252-14257, 2004), suggesting that this protein may be involved in the intracytoplasmic transport of the viral genome. We now demonstrate that the L2 protein is able to interact with the microtubule network via the motor protein dynein. L2 protein was found attached to microtubules after uncoating of incoming human papillomavirus pseudovirions. Based on immunofluorescence and coimmunoprecipitation analyses, the L2 region interacting with dynein is mapped to the C-terminal 40 amino acids. Mutations within this region abrogating the L2/dynein interaction strongly reduce the infectivity of pseudoviruses, indicating that this interaction mediates the minus-end-directed transport of the viral genome along microtubules towards the nucleus.

Molecular biology of human papillomavirus infection and cervical cancer

HPVs (human papillomaviruses) infect epithelial cells and cause a variety of lesions ranging from common warts/verrucas to cervical neoplasia and cancer. Over 100 different HPV types have been identified so far, with a subset of these being classified as high risk. High-risk HPV DNA is found in almost all cervical cancers (>99.7%), with HPV16 being the most prevalent type in both low-grade disease and cervical neoplasia. Productive infection by high-risk HPV types is manifest as cervical flat warts or condyloma that shed infectious virions from their surface. Viral genomes are maintained as episomes in the basal layer, with viral gene expression being tightly controlled as the infected cells move towards the epithelial surface. The pattern of viral gene expression in low-grade cervical lesions resembles that seen in productive warts caused by other HPV types. High-grade neoplasia represents an abortive infection in which viral gene expression becomes deregulated, and the normal life cycle of the virus cannot be completed. Most cervical cancers arise within the cervical transformation zone at the squamous/columnar junction, and it has been suggested that this is a site where productive infection may be inefficiently supported. The high-risk E6 and E7 proteins drive cell proliferation through their association with PDZ domain proteins and Rb (retinoblastoma), and contribute to neoplastic progression, whereas E6-mediated p53 degradation prevents the normal repair of chance mutations in the cellular genome. Cancers usually arise in individuals who fail to resolve their infection and who retain oncogene expression for years or decades. In most individuals, immune regression eventually leads to clearance of the virus, or to its maintenance in a latent or asymptomatic state in the basal cells.

Nuclear import strategies of high-risk HPV18 L2 minor capsid protein

We have investigated the nuclear import strategies of high-risk HPV18 L2 minor capsid protein. HPV18 L2 interacts with Kap alpha2 adapter, and Kap beta2 and Kap beta3 nuclear import receptors. Moreover, binding of RanGTP to either Kap beta2 or Kap beta3 inhibits their interaction with L2, suggesting that these Kap beta/L2 complexes are import competent. Mapping studies show that HPV18 L2 contains two NLSs: in the N-terminus (nNLS) and in the C-terminus (cNLS), both of which can independently mediate nuclear import. Both nNLS and cNLS form a complex with Kap alpha2beta1 heterodimer and mediate nuclear import via a classical pathway. The nNLS is also essential for the interaction of HPV18 L2 with Kap beta2 and Kap beta3. Interestingly, both nNLS and cNLS interact with the viral DNA and this DNA binding occurs without nucleotide sequence specificity. Together, the data suggest that HPV18 L2 can interact via its NLSs with several Kaps and the viral DNA and may enter the nucleus via multiple import pathways mediated by Kap alpha2beta1 heterodimers, Kap beta2 and Kap beta3.

Factors influencing subcellular localization of the human papillomavirus L2 minor structural protein

Two structural proteins form the capsids of papillomaviruses. The major structural protein L1 is the structural determinant of the capsids and is present in 360 copies arranged in 72 pentamers. The minor structural protein L2 is estimated to be present in twelve copies per capsid. Possible roles for L2 in interaction with cell surface receptors and in virion uptake have been suggested. As previously reported, L2 localizes in subnuclear domains identified as nuclear domain 10 (ND10). As it was demonstrated that L2 is able to recruit viral and cellular proteins to ND10, a possible role for L2 as a mediator in viral assembly has been proposed. In this study, we determined factors influencing the localization of L2 at ND10. Under conditions of moderate L2 expression level and in the absence of heterologous viral components, we observed that, in contrast to previous reports, L2 is mainly distributed homogeneously throughout the nucleus. L2, however, is recruited to ND10 at a higher expression level or in the presence of viral components derived from vaccinia virus or from Semliki Forest virus. We observed that translocation of L2 to ND10 is not a concentration-dependent accumulation but rather seems to be triggered by yet unidentified cellular factors. In contrast to HPV 11 and 16 L2, the HPV 18 L2 protein seems to require L1 for efficient nuclear accumulation.

A membrane-destabilizing peptide in capsid protein L2 is required for egress of papillomavirus genomes from endosomes

Papillomaviruses are internalized via clathrin-dependent endocytosis. However, the mechanism by which viral genomes pass endosomal membranes has not been elucidated. In this report we show that the minor capsid protein L2 is required for egress of viral genomes from endosomes but not for initial uptake and uncoating and that a 23-amino-acid peptide at the C terminus of L2 is necessary for this function. Pseudogenomes encapsidated by L1 and L2 lacking this peptide accumulated in vesicular compartments similar to that observed with L1-only viral particles, and these mutant pseudoviruses were noninfectious. This L2 peptide displayed strong membrane-disrupting activity, induced cytolysis of bacteria and eukaryotic cells in a pH-dependent manner, and permeabilized cells after exogenous addition. Fusions between green fluorescent protein and the L2 peptide integrated into cellular membranes like the wild type but not like C-terminal mutants of L2. Our data indicate that the L2 C terminus facilitates escape of viral genomes from the endocytic compartment and that this feature is conserved among papillomaviruses. Furthermore, the characteristic of this peptide differs from the classical virus-encoded membrane-penetrating peptides.

Cleavage of the papillomavirus minor capsid protein, L2, at a furin consensus site is necessary for infection

Papillomaviruses (PV) comprise a large family of nonenveloped DNA viruses that include the oncogenic PV types that are the causative agents of human cervical cancer. As is true of many animal DNA viruses, PV are taken into the cell by endocytosis and must escape from the endosomal compartment to the cytoplasm to initiate infection. Here we show that this step depends on the site-specific enzymatic cleavage of the PV minor virion protein L2 at a consensus furin recognition site. Cleavage by furin, a cell-encoded proprotein convertase, is known to be required for endosome escape by many bacterial toxins. However, to our knowledge, furin has not been previously implicated in the viral entry process. This step is potentially a target for PV inhibition.

Cross-neutralization of cutaneous and mucosal Papillomavirus types with anti-sera to the amino terminus of L2

Vaccination with papillomavirus L2 has been shown to induce neutralizing antibodies that protect against homologous type infection and cross-neutralize a limited number of genital HPVs. Surprisingly, we found that antibodies to bovine papillomavirus (BPV1) L2 amino acids 1-88 induced similar titers of neutralizing antibodies against Human papillomavirus (HPV)16 and 18 and BPV1 pseudoviruses and also neutralized HPV11 native virions. These antibodies also neutralized each of the other pseudovirus types tested, HPV31, HPV6 and Cottontail rabbit papillomavirus (CRPV) pseudoviruses, albeit with lower titers. HPV16, HPV18, HPV31, HPV6 and CRPV L2 anti-sera also displayed some cross-neutralization, but the titers were lower and did not encompass all pseudoviruses tested. This study demonstrates the presence of broadly cross-neutralizing epitopes at the N-terminus of L2 that are shared by cutaneous and mucosal types and by types that infect divergent species. BPV1 L2 was exceptionally effective at inducing cross-neutralizing antibodies to these shared epitopes.

The papillomavirus life cycle

Papillomaviruses infect epithelial cells, and depend on epithelial differentiation for completion of their life cycle. The expression of viral gene products is closely regulated as the infected basal cell migrates towards the epithelial surface. Expression of E6 and E7 in the lower epithelial layers drives cells into S-phase, which creates an environment that is conducive for viral genome replication and cell proliferation. Genome amplification, which is necessary for the production of infectious virions, is prevented until the levels of viral replication proteins rise, and depends on the co-expression of several viral proteins. Virus capsid proteins are expressed in cells that also express E4 as the infected cell enters the upper epithelial layers. The timing of these events varies depending on the infecting papillomavirus, and in the case of the high-risk human papillomaviruses (HPVs), on the severity of neoplasia. Viruses that are evolutionarily related, such as HPV1 and canine oral papillomavirus (COPV), generally organize their productive cycle in a similar way, despite infecting different hosts and epithelial sites. In some instances, such as following HPV16 infection of the cervix or cottontail rabbit papillomavirus (CRPV) infection of domestic rabbits, papillomaviruses can undergo abortive infections in which the productive cycle of the virus is not completed. As with other DNA tumour viruses, such abortive infections can predispose to cancer.

Interaction of tSNARE syntaxin 18 with the papillomavirus minor capsid protein mediates infection

The papillomavirus capsid mediates binding to the cell surface and passage of the virion to the perinuclear region during infection. To better understand how the virus traffics across the cell, we sought to identify cellular proteins that bind to the minor capsid protein L2. We have identified syntaxin 18 as a protein that interacts with bovine papillomavirus type 1 (BPV1) L2. Syntaxin 18 is a target membrane-associated soluble N-ethylmaleimide-sensitive factor-attachment protein receptor (tSNARE) that resides in the endoplasmic reticulum (ER). The ectopic expression of FLAG-tagged syntaxin 18, which disrupts ER trafficking, blocked BPV1 pseudovirion infection. Furthermore, the expression of FLAG-syntaxin 18 prevented the passage of BPV1 pseudovirions to the perinuclear region that is consistent with the ER. Genetic studies identified a highly conserved L2 domain, DKILK, comprising residues 40 to 44 that mediated BPV1 trafficking through the ER during infection via an interaction with the tSNARE syntaxin 18. Mutations within the DKILK motif of L2 that did not significantly impact virion morphogenesis or binding at the cell surface prevented the L2 interaction with syntaxin 18 and disrupted BPV1 infection.

Positively charged synthetic peptides from structural proteins of papillomaviruses abrogate human papillomavirus infectivity

Human papillomavirus (HPV) virus-like particles (VLP) and synthetic peptides corresponding to positively-charged sequences of the major and minor capsid proteins were tested for their efficacy in inhibiting the infectivity of HPV 31 pseudovirions by blocking virus entry into cells. A greater than 80% reduction of transfection was observed with one HPV-31 peptide at a concentration of 10 microg/ml. Moreover, the blocking was not type-specific since similar reduction in transfection was observed with peptides from other HPV types at a concentration of 60 microg/ml. This concentration was non-toxic for the cells. These findings indicate that some of the positively-charged sequences of the L1 and L2 HPV capsid proteins of papillomavirus are compounds that might be locally active against sexually transmitted papillomavirus. The findings provide further evidence that cellular glycosamino-glycans (GAGs) are functional receptors for HPVs.

The positively charged termini of L2 minor capsid protein required for bovine papillomavirus infection function separately in nuclear import and DNA binding

During the papillomavirus (PV) life cycle, the L2 minor capsid protein enters the nucleus twice: in the initial phase after entry of virions into cells and in the productive phase to mediate encapsidation of the newly replicated viral genome. Therefore, we investigated the interactions of the L2 protein of bovine PV type 1 (BPV1) with the nuclear import machinery and the viral DNA. We found that BPV1 L2 bound to the karyopherin alpha2 (Kap alpha2) adapter and formed a complex with Kap alpha2beta1 heterodimers. Previous data have shown that the positively charged termini of BPV1 L2 are required for BPV1 infection after the binding of the virions to the cell surface. We determined that these BPV1 L2 termini function as nuclear localization signals (NLSs). Both the N-terminal NLS (nNLS) and the C-terminal NLS (cNLS) interacted with Kap alpha2, formed a complex with Kap alpha2beta1 heterodimers, and mediated nuclear import via a Kap alpha2beta1 pathway. Interestingly, the cNLS was also the major DNA binding site of BPV1 L2. Consistent with the promiscuous DNA encapsidation by BPV1 pseudovirions, this DNA binding occurred without nucleotide sequence specificity. Moreover, an L2 mutant encoding a scrambled version of the cNLS, which supports production of virions, rescued the DNA binding but not the Kap alpha2 interaction. These data support a model in which BPV1 L2 functions as an adapter between the viral DNA via the cNLS and the Kaps via the nNLS and facilitates nuclear import of the DNA during infection.

The l2 minor capsid protein of human papillomavirus type 16 interacts with a network of nuclear import receptors

The L2 minor capsid proteins enter the nucleus twice during viral infection: in the initial phase after virion disassembly and in the productive phase when, together with the L1 major capsid proteins, they assemble the replicated viral DNA into virions. In this study we investigated the interactions between the L2 protein of high-risk human papillomavirus type 16 (HPV16) and nuclear import receptors. We discovered that HPV16 L2 interacts directly with both Kapbeta(2) and Kapbeta(3). Moreover, binding of Ran-GTP to either Kapbeta(2) or Kapbeta(3) inhibits its interaction with L2, suggesting that the Kapbeta/L2 complex is import competent. In addition, we found that L2 forms a complex with the Kapalpha(2)beta(1) heterodimer via interaction with the Kapalpha(2) adapter. In agreement with the binding data, nuclear import of L2 in digitonin-permeabilized cells could be mediated by either Kapalpha(2)beta(1) heterodimers, Kapbeta(2), or Kapbeta(3). Mapping studies revealed that HPV16 L2 contains two nuclear localization signals (NLSs), in the N terminus (nNLS) and C terminus (cNLS), that could mediate its nuclear import. Together the data suggest that HPV16 L2 interacts via its NLSs with a network of karyopherins and can enter the nucleus via several import pathways mediated by Kapalpha(2)beta(1) heterodimers, Kapbeta(2), and Kapbeta(3).

Establishment of papillomavirus infection is enhanced by promyelocytic leukemia protein (PML) expression

Previous studies have suggested that most papillomaviruses enter the host cell via clathrin-dependent receptor-mediated endocytosis but have not addressed later steps in viral entry. To examine these events, we followed the localization of L2 and packaged DNA after entry of infectious virions or L1/L2 pseudovirions. Confocal microscopic analyses of HeLa cells showed a time-dependent uncoating of capsids in cytoplasmic vesicles and the accumulation of both L2 and viral DNA at distinct nuclear domains identified as nuclear domain 10 (ND10). Both L2 and the pseudogenome had a punctate distribution and localized to ND10 in promyelocytic leukemia protein (PML)-expressing cells, whereas L2 had a diffuse nuclear distribution in PML-/- cells. The number of pseudovirus-infected cells was an order of magnitude higher in the PML+ cells compared with the PML-/- cells, and viral genome transcription after infection with authentic bovine papillomavirus virions was similarly elevated in PML+ cells. The results identify a role for PML in the enhancement of viral infectivity in the early part of the life cycle. We propose a model in which L2 chaperones the viral genome to ND10 to efficiently initiate viral transcription.

Vaccination to prevent and treat cervical cancer

Human papillomaviruses (HPVs) are the primary etiologic agents of cervical cancer. Thus, cervical cancer and other HPV-associated malignancies might be prevented or treated by HPV vaccines. Transmission of papillomavirus may be prevented by the generation of antibodies to capsid proteins L1 and L2 that neutralize viral infection. However, because the capsid proteins are not expressed at detectable levels by infected basal keratinocytes or in HPV-transformed cells, therapeutic vaccines generally target nonstructural early viral antigens. Two HPV oncogenic proteins, E6 and E7, are critical to the induction and maintenance of cellular transformation and are coexpressed in the majority of HPV-containing carcinomas. Thus, therapeutic vaccines targeting E6 and E7 may provide the best option for controlling HPV-associated malignancies. Various candidate therapeutic HPV vaccines are currently being tested whereby E6 and/or E7 are administered in live vectors, as peptides or protein, in nucleic acid form, as components of chimeric virus-like particles, or in cell-based vaccines. Encouraging results from experimental vaccination systems in animal models have led to several prophylactic and therapeutic vaccine clinical trials. If these preventive and therapeutic HPV vaccines prove successful in patients, as they have in animal models, then oncogenic HPV infection and its associated malignancies may be controllable by vaccination.

Dissection of human papillomavirus type 33 L2 domains involved in nuclear domains (ND) 10 homing and reorganization

We have recently shown that the minor capsid protein L2 of human papillomavirus type 33 (HPV33) recruits the transcriptional repressor Daxx into nuclear domains (ND) 10 and causes the loss of the transcriptional activator Sp100 from these subnuclear structures. In order to dissect L2 domains involved in nuclear translocation, ND10 homing, loss of Sp100, and recruitment of Daxx, a detailed deletion mutagenesis of L2 was performed. Using immunofluorescence and green fluorescent protein fusions, we have identified two nuclear localization signals (NLS) in the central and C-terminal part of L2, respectively, homologous to previously identified NLS in HPV6B L2 (Sun et al., 1995). We mapped the ND10 localization domain to within a 30 amino acid peptide in the C-terminal half of L2. L2-induced attraction of Daxx into ND10, coimmunoprecipitation of L2 and Daxx, as well as induction of the loss of Sp100 from ND10 require an intact ND10 localization domain. This domain contains conserved PXXP motives characteristic of some protein/protein interacting domains. Our data also suggest that the Daxx/L2 interaction may be the driving force for L2 accumulation in ND10.

Carboxy-fluorescein diacetate, succinimidyl ester labeled papillomavirus virus-like particles fluoresce after internalization and interact with heparan sulfate for binding and entry

Human papillomaviruses (HPVs) infect epithelial cells and are associated with genital carcinoma. Most epithelial cell lines express cell-surface glycosaminoglycans (GAGs) usually found attached to the protein core of proteoglycans. Our aim was to study how GAGs influenced HPV entry. Using a human keratinocyte cell line (HaCaT), preincubation of HPV virus-like particles (VLPs) with GAGs showed a dose-dependent inhibition of binding. The IC(50) (50% inhibition) was only 0.5 microg/ml for heparin, 1 microg/ml for dextran sulfate, and 5-10 microg/ml for heparan sulfate from mucosal origin. Mutated chinese hamster ovary (CHO) cell lines lacking heparan sulfate or all GAGs were unable to bind HPV VLPs. Here we also report a method to study internalization by using VLPs labeled with carboxy-fluorescein diacetate, succinimidyl ester, a fluorochrome that is only activated after cell entry. Pretreatment of labeled HPV VLPs with heparin inhibited uptake, suggesting a primary interaction between HPV and cell-surface heparan sulfate.

Interaction of L2 with beta-actin directs intracellular transport of papillomavirus and infection

Viruses that replicate in the nucleus, including the primary causative agent of cervical cancer, human papillomavirus type 16 (HPV16), must first cross the cytoplasm. We compared the uptake of HPV16 virus-like particles (VLPs) either with or without the minor capsid protein L2. Whereas VLPs containing only the major capsid protein L1 were diffusely distributed within the cytoplasm even 6 h post-infection, VLPs comprising both L1 and L2 exhibited a radial distribution in the cytoplasm and accumulated in the perinuclear region of BPHE-1 cells within 2 h. L2 of HPV16 or bovine papillomavirus was shown to bind to a 43-kDa cellular protein that was subsequently identified as beta-actin by matrix-assisted laser desorption ionization time-of-flight analysis. A conserved domain comprising residues 25-45 of HPV16 L2 was sufficient for interaction with beta-actin. HPV16 L2 residues 25-45 fused to green fluorescent protein, but not green fluorescent protein alone, colocalized with actin and caused cell retraction and disruption of the microfilament network. Finally, wild-type L2, but not L2 with residues 25-45 deleted, facilitated HPV16 pseudovirion infection. Thus, binding of beta-actin by L2 residues 25-45 facilitates transport of HPV16 across the cytoplasm during infection, and blockade of this novel interaction may be useful for prophylaxis.

Cell surface-binding motifs of L2 that facilitate papillomavirus infection

Human papillomavirus type 16 (HPV16) is the primary etiologic agent of cervical carcinoma, whereas bovine papillomavirus type 1 (BPV1) causes benign fibropapillomas. However, the capsid proteins, L1 and L2, of these divergent papillomaviruses exhibit functional conservation. A peptide comprising residues 1 to 88 of BPV1 L2 binds to a variety of cell lines, but not to the monocyte-derived cell line D32, and blocks BPV1 infection of mouse C127 cells. Residues 13 to 31 of HPV16 L2 and BPV1 L2 residues 1 to 88 compete for binding to the cell surface, and their binding, unlike that of HPV16 L1/L2 virus-like particles, is unaffected by heparinase or trypsin pretreatment of HeLa cells. A fusion of HPV16 L2 peptide 13-31 and GFP binds (K(d), approximately 1 nM) to approximately 45,000 receptors per HeLa cell. Furthermore, mutation of L2 residues 18 and 19 or 21 and 22 significantly reduces both the ability of the HPV16 L2 13-31-GFP fusion protein to bind to SiHa cells and the infectivity of HPV16 pseudovirions. Antibody to BPV1 L2 peptides comprising residues 115 to 135 binds to intact BPV1 virions, but fails to neutralize at a 1:10 dilution. However, deletion of residues 91 to 129 from L2 abolishes the infectivity of BPV1, but not their binding to the cell surface. In summary, L2 residues 91 to 129 contain epitopes displayed on the virion surface and are required for infection, but not virion binding to the cell surface. Upon the binding of papillomavirus to the cell surface, residues 13 to 31 of L2 interact with a widely expressed, trypsin- and heparinase-resistant cell surface molecule and facilitate infection.

Positively charged sequences of human papillomavirus type 16 capsid proteins are sufficient to mediate gene transfer into target cells via the heparan sulfate receptor

Using synthetic peptides we have shown that positively charged sequences present at the C terminus of the L1 protein and the N and C termini of the L2 protein of human papillomavirus type 16 (HPV-16) bind to both DNA and heparan sulfate receptors. Moreover, these short amino acid sequences are sufficient to mediate gene transfer in COS-7 cells. The L1 proteins of other HPVs were shown to contain one or two DNA- and heparin-binding sequences that have the capacity to transfer genes. These DNA-binding sequences also recognized the enhancing packaging sequence of bovine papillomavirus type 1. The results suggest that the L2 protein could participate in DNA packaging during maturation of virions.

Interaction of human papillomavirus type 16 L2 with cellular proteins: identification of novel nuclear body-associated proteins

Two structural proteins form the Papillomavirus (PV) capsids. While the functions of the major structural protein L1 are well established, the exact functions for the minor structural protein L2 are much less well defined, except for some information on a role in viral entry and maturation of infectious virions. To gain more insight in the function of L2 we used the yeast two hybrid system with the Human Papillomavirus (HPV) 11 L2 and HPV16 L2 as bait proteins to isolate putative cellular interaction partners. We identified four proteins interacting with L2 proteins of at least two different HPV types and this interaction was confirmed in vitro by pull-down assays. Further evidence for this interaction was obtained by in vivo localization studies. Two of the proteins, the previously described PATZ and a novel protein, designated PLINP, were localized in discrete nuclear domains and colocalized with L2. The third protein, designated PMSP, is a newly identified cytoplasmic protein which was recruited to nuclear dots when coexpressed with L2. The fourth protein interacting with HPV16, 11 and 1 L2, the tubular-nephritis antigen related protein (TIN-Ag-RP), shows a cytoplasmic as well as a membrane bound subcellular distribution. Taken together, our data indicate that L2 of HPVs with different phenotypes interacts with several cellular host proteins, recruits one of them to the nucleus, and is complexed with at least three cellular proteins in specific nuclear domains. These findings suggest an HPV type-independent modulatory function of L2 on host-cell functions that involves discrete nuclear domains and alteration of the subcellular distribution of cellular proteins. The interacting cellular proteins identified may play a role in the viral life cycle and establishment of viral persistence.

DNA binding of L1 is required for human papillomavirus morphogenesis in vivo

The role of putative DNA-binding domains of human papillomavirus (HPV) capsid proteins for DNA encapsidation in vivo is still unknown. We have now analyzed mutants of the major capsid protein L1 of HPV type 33, which are defective for DNA binding, for their ability to encapsidate DNA using an in vivo packaging approach. Since the DNA-binding domain and the nuclear localization signal (NLS) of L1 overlap, both a carboxy-terminal deletion mutant (L1-1/470) and a substitution mutant (L1-1/477M9) were analyzed. L1-1/477M9 has the classical NLS replaced by a noncanonical NLS taken from the human hnRNP protein A1. The mutant proteins were defective for DNA binding in contrast to wild-type (wt) L1 proteins of several HPV types. L1-1/470 localized to the cytoplasm when expressed in human cell lines, whereas L1-1/477M9 was found mainly in the nucleus. When coexpressed with wt L2, mutant L1 proteins colocalized with L2 in nuclear domains 10 (ND10) similar to wt L1 and assembled into virus-like particles incorporating L2. Wt L1 and L2 but not mutant L1 proteins and wt L2 efficiently encapsidated DNA and transferred it into cells, suggesting that physical interaction of L1 with DNA is essential for DNA inclusion into VLPs. We also compared the infectivity of histone-free DNA-containing pseudovirions generated in vitro with the infectivity of pseudovirions generated in vivo, which most likely harbor DNA in the form of chromatin. Pseudovirions generated in vivo were, on a per genome basis, four to five times more infectious than pseudovirions generated in vitro, even though no structural or biochemical difference was obvious. This indicates that chromatin may enhance the infectivity of pseudovirions.