L1 interaction domains of papillomavirus l2 necessary for viral genome encapsidation

Critical Residues Involved in the Coassembly of L1 and L2 Capsid Proteins of Human Papillomavirus 16

Human papillomaviruses (HPV) are small DNA viruses associated with cervical cancer, warts, and other epithelial tumors. Structural studies have shown that the HPV capsid consists of 360 copies of the major capsid protein, L1, arranged as 72 pentamers in a T=7 icosahedral lattice, coassembling with substoichiometric amounts of the minor capsid protein, L2. However, the residues involved in the coassembly of L1 and L2 remain undefined due to the lack of structure information. Here, we investigated the solvent accessibility surfaces (SASs) of the central cavity residues of the HPV16 L1 pentamer in the crystal structure because those internal exposed residues might mediate the association with L2. Twenty residues in L1 protein were selected to be analyzed, with four residues in the lumen of the L1 pentamer identified as important: F256, R315, Q317, and T340. Mutations to these four residues reduced the PsV (pseudovirus) infection capacity in 293FT cells, and mutations to R315, Q317, and T340 substantially perturb L2 from coassembling into L1 capsid. Compared with wild-type (WT) PsVs, these mutant PsVs also have a reduced ability to become internalized into host cells. Finally, we identified a stretch of negatively charged residues on L2 (amino acids [aa] 337 to 340 [EEIE]), mutations to which completely abrogate L2 assembly into L1 capsid and subsequently impair the endocytosis and infectivity of HPV16 PsVs. These findings shed light on the elusive coassembly between HPV L1 and L2. IMPORTANCE Over 200 types of HPV have been isolated, with several high-risk types correlated with the occurrence of cervical cancer. The HPV major capsid protein, L1, assembles into a T=7 icosahedral viral shell, and associates with the minor capsid protein, L2, which plays a critical role in the HPV life cycle. Despite the important role of the L2 protein, its structure and coassembly with L1 remain elusive. In this study, we analyzed the amino acid residues at the proposed interface between L1 and L2. Certain mutations at these sites decreased the amount of L2 protein assembled into the capsid, which, in turn, led to a decrease in viral infectivity. Knowledge about these residues and the coassembly of L1 and L2 could help to expand our understanding of HPV biology and aid in the development of countermeasures against a wide range of HPV types by targeting the L2 protein.

Head and Neck Cancer Susceptibility and Metabolism in Fanconi Anemia

Fanconi anemia (FA) is a rare inherited, generally autosomal recessive syndrome, but it displays X-linked or dominant negative inheritance for certain genes. FA is characterized by a deficiency in DNA damage repair that results in bone marrow failure, and in an increased risk for various epithelial tumors, most commonly squamous cell carcinomas of the head and neck (HNSCC) and of the esophagus, anogenital tract and skin. Individuals with FA exhibit increased human papilloma virus (HPV) prevalence. Furthermore, a subset of anogenital squamous cell carcinomas (SCCs) in FA harbor HPV sequences and FA-deficient laboratory models reveal molecular crosstalk between HPV and FA proteins. However, a definitive role for HPV in HNSCC development in the FA patient population is unproven. Cellular metabolism plays an integral role in tissue homeostasis, and metabolic deregulation is a known hallmark of cancer progression that supports uncontrolled proliferation, tumor development and metastatic dissemination. The metabolic consequences of FA deficiency in keratinocytes and associated impact on the development of SCC in the FA population is poorly understood. Herein, we review the current literature on the metabolic consequences of FA deficiency and potential effects of resulting metabolic reprogramming on FA cancer phenotypes.

Human Papillomaviruses Target the DNA Damage Repair and Innate Immune Response Pathways to Allow for Persistent Infection

Persistent infection with high-risk human papillomaviruses (HPVs) is the major risk factor associated with development of anogenital and oropharyngeal cancers. Initial infection by HPVs occurs into basal epithelial cells where viral genomes are established as nuclear episomes and persist until cleared by the immune response. Productive replication or amplification occurs upon differentiation and is dependent upon activation of the ataxia-telangiectasia mutated (ATM), ataxia telangiectasia and RAD3-related (ATR) DNA damage repair (DDR) pathways. In addition to activating DDR pathways, HPVs must escape innate immune surveillance mechanisms by antagonizing sensors, adaptors, interferons and antiviral gene expression. Both DDR and innate immune pathways are key host mechanisms that crosstalk with each other to maintain homeostasis of cells persistently infected with HPVs. Interestingly, it is still not fully understood why some HPV infections get cleared while others do not. Targeting of these two processes with antiviral therapies may provide opportunities for treatment of cancers caused by high-risk HPVs.

High resolution cryo EM analysis of HPV16 identifies minor structural protein L2 and describes capsid flexibility

Human papillomavirus (HPV) is a significant health burden and leading cause of virus-induced cancers. HPV is epitheliotropic and its replication is tightly associated with terminal keratinocyte differentiation making production and purification of high titer virus preparations for research problematic, therefore alternative HPV production methods have been developed for virological and structural studies. In this study we use HPV16 quasivirus, composed of HPV16 L1/L2 capsid proteins with a packaged cottontail rabbit papillomavirus genome. We have achieved the first high resolution, 3.1 Å, structure of HPV16 by using a local subvolume refinement approach. The high resolution enabled us to build L1 unambiguously and identify L2 protein strands. The L2 density is incorporated adjacent to conserved L1 residues on the interior of the capsid. Further interpretation with our own software for Icosahedral Subvolume Extraction and Correlated Classification revealed flexibility, on the whole-particle level through diameter analysis and local movement with inter-capsomer analysis. Inter-capsomer expansion or contraction, governed by the connecting arms, showed no bias in the magnitude or direction of capsomer movement. We propose that papillomavirus capsids are dynamic and capsomers move as rigid bodies connected by flexible linkers. The resulting virus structure will provide a framework for continuing biochemical, genetic and biophysical research for papillomaviruses. Furthermore, our approach has allowed insight into the resolution barrier that has previously been a limitation in papillomavirus structural studies.

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.

[HPV: Carcinogenic implications and preventive measures]

Human oncogenic papillomaviruses (HPV) have an increasingly prominent role in the genesis of many cancers. The oncogenic mechanisms associated with HPV are now better known and make it possible to explain the etiopathogenesis of the association. HPV status is now sought for certain cancers and conditions both prognosis and management of patients. Preventive antiviral vaccination has become a real public health issue and aims to effectively reduce the prevalence of cervical, anal and oropharynx cancer, HPV-associated. However, vaccination against HPV still lags behind. The purpose of this review is to redefine the involvement of HPV in several cancers as well as current therapeutic challenges of HPV-related cancers, notably in term of prevention.

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.

Naturally occurring capsid protein variants L1 of human papillomavirus genotype 16 in Morocco

HPV L1 protein is a corner stone in HPV structure, it's involved in the formation of the viral capsid; widely used as a systematic material and considered as the main component in vaccines development and production. The present study aims to characterize genetic variation of L1 gene of HPV 16 specimens and to evaluate in silico the impact of major variants on the epitope change affecting its conformational structure. A fragment of L1 gene from 35 HPV 16 confirmed specimens were amplified by PCR and sequenced. Overall, five amino acids residues changes were reported: T390P in 16 specimens, M425I and M431I in 2 cases, insertion of Serine at 460 and aspartic acid deletion at position 477 in all analyzed cases. The 3D generated model showed that T389P amino acid substitution is located in the H-I loop; the two substitutions M424I and M430I are both located in the H2 helice. The Serine insertion and aspartic acid deletion are located in the H4 helice and B-C loop, respectively. Superimposition of sequences' structures showed that they share a very similar conformation highlighting that the reported amino acids variations don't affect the structure of the L1 protein. However T389P, located in the H-I loop identified as an immunogenetic region of L1 capsid, was reported in 51.4% of cases could interact with vaccines induced monoclonal antibodies suggesting a potential impact on the efficacy of available anti-HPV vaccines.

Human Papillomavirus Major Capsid Protein L1 Remains Associated with the Incoming Viral Genome throughout the Entry Process

During infectious entry, acidification within the endosome triggers uncoating of the human papillomavirus (HPV) capsid, whereupon host cyclophilins facilitate the release of most of the major capsid protein, L1, from the minor capsid protein L2 and the viral genome. The L2/DNA complex traffics to the trans-Golgi network (TGN). After the onset of mitosis, HPV-harboring transport vesicles bud from the TGN, followed by association with mitotic chromosomes. During this time, the HPV genome remains in a vesicular compartment until the nucleus has completely reformed. Recent data suggest that while most of L1 protein dissociates and is degraded in the endosome, some L1 protein remains associated with the viral genome. The L1 protein has DNA binding activity, and the L2 protein has multiple domains capable of interacting with L1 capsomeres. In this study, we report that some L1 protein traffics with L2 and viral genome to the nucleus. The accompanying L1 protein is mostly full length and retains conformation-dependent epitopes, which are recognized by neutralizing antibodies. Since more than one L1 molecule contributes to these epitopes and requires assembly into capsomeres, we propose that L1 protein is present in the form of pentamers. Furthermore, we provide evidence that the L1 protein interacts directly with viral DNA within the capsid. Based on our findings, we propose that the L1 protein, likely arranged as capsomeres, stabilizes the viral genome within the subviral complex during intracellular trafficking.IMPORTANCE After internalization, the nonenveloped human papillomavirus virion uncoats in the endosome, whereupon conformational changes result in a dissociation of a subset of the major capsid protein L1 from the minor capsid protein L2, which remains in complex with the viral DNA. Recent data suggest that some L1 protein may accompany the viral genome beyond the endosomal compartment. We demonstrate that conformationally intact L1 protein, likely still arranged as capsomeres, remains associated with the incoming viral genome throughout mitosis and transiently resides in the nucleus until after the viral DNA is released from the transport vesicle.

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.

Papillomavirus assembly: An overview and perspectives

Papillomavirus life cycle is tightly coupled to epithelial cell differentiation, which has hindered the investigation of many aspects of papillomavirus biology, including virion assembly. The development of in vitro production methods of papillomavirus pseudoviruses, and the production of "native" virus in raft cultures have facilitated the study of some aspects of the assembly process. In this paper we review the current knowledge of papillomavirus assembly, directions for future research, and the implications of these studies on new therapeutic interventions.

Cruising the cellular highways: How human papillomavirus travels from the surface to the nucleus

The non-enveloped human papillomaviruses (HPVs) specifically target epithelial cells of the skin and mucosa. Successful infection requires a lesion in the stratified tissue for access to the basal cells. Herein, we discuss our recent progress in understanding binding, internalization, uncoating, and intracellular trafficking of HPV particles. Our focus will be on HPV type 16, which is the most common HPV type associated with various anogenital and oropharyngeal carcinomas. The study of HPV entry has revealed a number of novel cellular pathways utilized during infection. These include but are not restricted to the following: a previously uncharacterized form of endocytosis, membrane penetration by a capsid protein, the use of retromer complexes for trafficking to the trans-Golgi network, the requirement for nuclear envelope breakdown and microtubule-mediated transport during mitosis for nuclear entry, the existence of membrane-bound intranuclear vesicles harboring HPV genome, and the requirement of PML protein for efficient transcription of incoming viral genome. The continued study of these pathways may reveal new roles in basic biological cellular processes.

Production of Human papillomavirus pseudovirions in plants and their use in pseudovirion-based neutralisation assays in mammalian cells

Human papillomaviruses (HPV) cause cervical cancer and have recently also been implicated in mouth, laryngeal and anogenital cancers. There are three commercially available prophylactic vaccines that show good efficacy; however, efforts to develop second-generation vaccines that are more affordable, stable and elicit a wider spectrum of cross-neutralising immunity are still ongoing. Testing antisera elicited by current and candidate HPV vaccines for neutralizing antibodies is done using a HPV pseudovirion (PsV)-based neutralisation assay (PBNA). PsVs are produced by transfection of mammalian cell cultures with plasmids expressing L1 and L2 capsid proteins, and a reporter gene plasmid, a highly expensive process. We investigated making HPV-16 PsVs in plants, in order to develop a cheaper alternative. The secreted embryonic alkaline phosphatase (SEAP) reporter gene and promoter were cloned into a geminivirus-derived plant expression vector, in order to produce circular dsDNA replicons. This was co-introduced into Nicotiana benthamiana plants with vectors expressing L1 and L2 via agroinfiltration, and presumptive PsVs were purified. The PsVs contained DNA, and could be successfully used for PBNA with anti-HPV antibodies. This is the first demonstration of the production of mammalian pseudovirions in plants, and the first demonstration of the potential of plants to make DNA vaccines.

A Cell-Free Assembly System for Generating Infectious Human Papillomavirus 16 Capsids Implicates a Size Discrimination Mechanism for Preferential Viral Genome Packaging

We have established a cell-free in vitro system to study human papillomavirus type 16 (HPV16) assembly, a poorly understood process. L1/L2 capsomers, obtained from the disassembly of virus-like particles (VLPs), were incubated with nuclear extracts to provide access to the range of cellular proteins that would be available during assembly within the host cell. Incorporation of a reporter plasmid "pseudogenome" was dependent on the presence of both nuclear extract and ATP. Unexpectedly, L1/L2 VLPs that were not disassembled prior to incubation with a reassembly mixture containing nuclear extract also encapsidated a reporter plasmid. As with HPV pseudoviruses (PsV) generated intracellularly, infection by cell-free particles assembled in vitro required the presence of L2 and was susceptible to the same biochemical inhibitors, implying the cell-free assembled particles use the infectious pathway previously described for HPV16 produced in cell culture. Using biochemical and electron microscopy analyses, we observed that, in the presence of nuclear extract, intact VLPs partially disassemble, providing a mechanistic explanation to how the exogenous plasmid was packaged by these particles. Further, we provide evidence that capsids containing an <8-kb pseudogenome are resistant to the disassembly/reassembly reaction. Our results suggest a novel size discrimination mechanism for papillomavirus genome packaging in which particles undergo iterative rounds of disassembly/reassembly, seemingly sampling DNA until a suitably sized DNA is encountered, resulting in the formation of a stable virion structure.

IMPORTANCE

Little is known about papillomavirus assembly biology due to the difficulties in propagating virus in vitro. The cell-free assembly method established in this paper reveals a new mechanism for viral genome packaging and will provide a tractable system for further dissecting papillomavirus assembly. The knowledge gained will increase our understanding of virus-host interactions, help to identify new targets for antiviral therapy, and allow for the development of new gene delivery systems based on in vitro-generated papillomavirus vectors.

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.

Multivalent human papillomavirus l1 DNA vaccination utilizing electroporation

Objectives

Naked DNA vaccines can be manufactured simply and are stable at ambient temperature, but require improved delivery technologies to boost immunogenicity. Here we explore in vivo electroporation for multivalent codon-optimized human papillomavirus (HPV) L1 and L2 DNA vaccination.

Methods

Balb/c mice were vaccinated three times at two week intervals with a fusion protein comprising L2 residues ∼11−88 of 8 different HPV types (11−88×8) or its DNA expression vector, DNA constructs expressing L1 only or L1+L2 of a single HPV type, or as a mixture of several high-risk HPV types and administered utilizing electroporation, i.m. injection or gene gun. Serum was collected two weeks and 3 months after the last vaccination. Sera from immunized mice were tested for in-vitro neutralization titer, and protective efficacy upon passive transfer to naive mice and vaginal HPV challenge. Heterotypic interactions between L1 proteins of HPV6, HPV16 and HPV18 in 293TT cells were tested by co-precipitation using type-specific monoclonal antibodies.

Results

Electroporation with L2 multimer DNA did not elicit detectable antibody titer, whereas DNA expressing L1 or L1+L2 induced L1-specific, type-restricted neutralizing antibodies, with titers approaching those induced by Gardasil. Co-expression of L2 neither augmented L1-specific responses nor induced L2-specific antibodies. Delivery of HPV L1 DNA via in vivo electroporation produces a stronger antibody response compared to i.m. injection or i.d. ballistic delivery via gene gun. Reduced neutralizing antibody titers were observed for certain types when vaccinating with a mixture of L1 (or L1+L2) vectors of multiple HPV types, likely resulting from heterotypic L1 interactions observed in co-immunoprecipitation studies. High titers were restored by vaccinating with individual constructs at different sites, or partially recovered by co-expression of L2, such that durable protective antibody titers were achieved for each type.

Discussion

Multivalent vaccination via in vivo electroporation requires spatial separation of individual type L1 DNA vaccines.

The transcription factors TBX2 and TBX3 interact with human papillomavirus 16 (HPV16) L2 and repress the long control region of HPVs

The minor capsid protein L2 of human papillomaviruses (HPVs) has multiple functions during the viral life cycle. Although L2 is required for effective invasion and morphogenesis, only a few cellular interaction partners are known so far. Using yeast two-hybrid screening, we identified the transcription factor TBX2 as a novel interaction partner of HPV type 16 (HPV16) L2. Coimmunoprecipitations and immunofluorescence analyses confirmed the L2-TBX2 interaction and revealed that L2 also interacts with TBX3, another member of the T-box family. Transcription of the early genes during HPV infection is under the control of an upstream enhancer and early promoter region, the long control region (LCR). In promoter-reporter gene assays, we observed that TBX2 and TBX3 repress transcription from the LCR and that this effect is enhanced by L2. Repression of the HPV LCR by TBX2/3 seems to be a conserved mechanism, as it was also observed with the LCRs of different HPV types. Finally, interaction of TBX2 with the LCR was detected by chromatin immunoprecipitation, and we found a strong colocalization of L2 and TBX2 in HPV16-positive cervical intraepithelial neoplasia (CIN) I-II tissue sections. These results suggest that TBX2/3 might play a role in the regulation of HPV gene expression during the viral life cycle.

Principles of polyoma- and papillomavirus uncoating

Virus particles are vehicles for transmission of the viral genetic information between infected and uninfected cells and organisms. They have evolved to self-assemble, to serve as a protective shell for the viral genome during transfer, and to disassemble when entering a target cell. Disassembly during entry is a complex, multi-step process typically termed uncoating. Uncoating is triggered by multiple host-cell interactions. During cell entry, these interactions occur sequentially in different cellular compartments that the viruses pass through on their way to the site of replication. Here, we highlight the general principles of uncoating for two structurally related virus families, the polyoma- and papillomaviruses. Recent research indicates the use of different compartments and cellular interactions for uncoating despite their structural similarity.

Cyclophilins facilitate dissociation of the human papillomavirus type 16 capsid protein L1 from the L2/DNA complex following virus entry

Human papillomaviruses (HPV) are composed of the major and minor capsid proteins, L1 and L2, that encapsidate a chromatinized, circular double-stranded DNA genome. At the outset of infection, the interaction of HPV type 16 (HPV16) (pseudo)virions with heparan sulfate proteoglycans triggers a conformational change in L2 that is facilitated by the host cell chaperone cyclophilin B (CyPB). This conformational change results in exposure of the L2 N terminus, which is required for infectious internalization. Following internalization, L2 facilitates egress of the viral genome from acidified endosomes, and the L2/DNA complex accumulates at PML nuclear bodies. We recently described a mutant virus that bypasses the requirement for cell surface CyPB but remains sensitive to cyclosporine for infection, indicating an additional role for CyP following endocytic uptake of virions. We now report that the L1 protein dissociates from the L2/DNA complex following infectious internalization. Inhibition and small interfering RNA (siRNA)-mediated knockdown of CyPs blocked dissociation of L1 from the L2/DNA complex. In vitro, purified CyPs facilitated the dissociation of L1 pentamers from recombinant HPV11 L1/L2 complexes in a pH-dependent manner. Furthermore, CyPs released L1 capsomeres from partially disassembled HPV16 pseudovirions at slightly acidic pH. Taken together, these data suggest that CyPs mediate the dissociation of HPV L1 and L2 capsid proteins following acidification of endocytic vesicles.

Human papillomavirus L2 facilitates viral escape from late endosomes via sorting nexin 17

The human papillomavirus (HPV) L2 capsid protein plays an essential role during the early stages of viral infection, but the molecular mechanisms underlying its mode of action remain obscure. Using a proteomic approach, we have identified the adaptor protein, sorting nexin 17 (SNX17) as a strong interacting partner of HPV L2. This interaction occurs through a highly conserved SNX17 consensus binding motif, which is present in the majority of HPV L2 proteins analysed. Using mutants of L2 defective for SNX17 interaction, or siRNA ablation of SNX17 expression, we demonstrate that the interaction between L2 and SNX17 is essential for viral infection. Furthermore, loss of the L2-SNX17 interaction results in enhanced turnover of the L2 protein and decreased stability of the viral capsids, and concomitantly, there is a dramatic decrease in the efficiency with which viral genomes transit to the nucleus. Indeed, using a range of endosomal and lysosomal markers, we show that capsids defective in their capacity to bind SNX17 transit much more rapidly to the lysosomal compartment. These results demonstrate that the L2-SNX17 interaction is essential for viral infection and facilitates the escape of the L2-DNA complex from the late endosomal/lysosomal compartments.

Inhibition of nuclear entry of HPV16 pseudovirus-packaged DNA by an anti-HPV16 L2 neutralizing antibody

Rabbit anti-HPV16 L2 serum (anti-P56/75) neutralizes multiple oncogenic human papillomaviruses (HPVs). We inoculated HeLa cells with HPV16 pseudovirus (16PV) and with anti-P56/75-bound 16PV (16PV-Ab). Both 16PV and 16PV-Ab attached equally well to the cell surface. However, the cell-attached L1 protein of 16PV became trypsin-resistant after incubation at 37°C, whereas approximately 20% of the cell-attached 16PV-Ab L1 remained trypsin-sensitive. Confocal microscopy of HeLa cells inoculated with 16PV revealed packaged DNA in the nucleus at 22h after inoculation; however, nuclear DNA was not detected in cells inoculated with 16PV-Ab. Electron microscopy of HeLa cells inoculated with 16PV showed particles located in multivesicular bodies, lamellar bodies, and the cytosol after 4h; no cytosolic particles were detected after inoculation with 16PV-Ab. These data suggest that anti-P56/75 inhibits HPV infection partly by blocking viral entry and primarily by blocking the transport of the viral genome to the nucleus.

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.

Papillomavirus pseudovirions packaged with the L2 gene induce cross-neutralizing antibodies

BACKGROUND

Current vaccines against HPVs are constituted of L1 protein self-assembled into virus-like particles (VLPs) and they have been shown to protect against natural HPV16 and HPV18 infections and associated lesions. In addition, limited cross-protection has been observed against closely related types. Immunization with L2 protein in animal models has been shown to provide cross-protection against distant papillomavirus types, suggesting that the L2 protein contains cross-neutralizing epitopes. However, vaccination with L2 protein or L2 peptides does not induce high titers of anti-L2 antibodies. In order to develop a vaccine with the potential to protect against other high-risk HPV types, we have produced HPV58 pseudovirions encoding the HPV31 L2 protein and compared their capacity to induce cross-neutralizing antibodies with that of HPV L1 and HPV L1/L2 VLPs.

METHODS

The titers of neutralizing antibodies against HPV16, HPV18, HPV31 and HPV58 induced in Balb/c mice were compared after immunization with L2-containing vaccines.

RESULTS

Low titers of cross-neutralizing antibodies were detected in mice when immunized with L1/L2 VLPs, and the highest levels of cross-neutralizing antibodies were observed in mice immunized with HPV 58 L1/L2 pseudovirions encoding the HPV 31 L2 protein.

CONCLUSIONS

The results obtained indicate that high levels of cross-neutralizing antibodies are only observed after immunization with pseudovirions encoding the L2 protein. HPV pseudovirions thus represent a possible new strategy for the generation of a broad-spectrum vaccine to protect against high-risk HPVs and associated neoplasia.

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.

Viral entry mechanisms: human papillomavirus and a long journey from extracellular matrix to the nucleus

Papillomaviruses are epitheliotropic non-enveloped double-stranded DNA viruses, whose replication is strictly dependent on the terminally differentiating tissue of the epidermis. They induce self-limiting benign tumors of skin and mucosa, which may progress to malignancy (e.g. cervical carcinoma). Prior to entry into basal cells, virions attach to heparan sulfate moieties of the basement membrane. This triggers conformational changes, which affect both capsid proteins, L1 and L2, and such changes are a prerequisite for interaction with the elusive uptake receptor. These processes are very slow, resulting in an uptake half-time of up to 14 h. This minireview summarizes recent advances in our understanding of cell surface events, internalization and the subsequent intracellular trafficking of papillomaviruses.

Two highly conserved cysteine residues in HPV16 L2 form an intramolecular disulfide bond and are critical for infectivity in human keratinocytes

BACKGROUND

Minor capsid protein L2 performs an indispensable but uncharacterized role in human papillomavirus infections. A neutralizing B cell epitope has recently been mapped to the N-terminus of HPV16 L2, residues 17-36, and exposure of this region of L2 has been implicated in translocation of incoming virions from the endo/lysosomal compartment to the cellular cytoplasm. Here we examine the redox state of Cys22 and Cys28 two highly conserved cysteines located within this epitope. We also investigate the infectivity of virions containing L2 single and double cysteine point mutants.

METHODOLOGY AND PRINCIPAL FINDINGS

Denaturing/non-reducing gel analysis and thiol labeling experiments of wild type and cysteine mutant HPV16 virion particles strongly support the existence of a buried intramolecular C22-C28 disulfide bond. The disulfide was confirmed by tandem mass spectrometry of L2 protein from non-reduced virions. Single C22S and C28S and the double C22/28S mutants were non-infectious but had no apparent defects in cell binding, endocytosis, or trafficking to lysosomes by 8 h post infection. During infection with L2 mutant particles, there was a marked decrease in L2 levels compared to wild type L2-containing virions, suggesting a failure of mutant L2/genome complexes to exit the endo/lysosomal compartment.

CONCLUSIONS AND SIGNIFICANCE

L2 residues C22 and C28 are bound as an intramolecular disulfide bond in HPV16 virions and are necessary for infectivity. Previous work has suggested that the furin-dependent exposure of the 17-36 epitope and subsequent interaction of this region with an unknown receptor is necessary for egress from the endo/lysosomal compartment and infection. Identification of the C22-C28 disulfide suggests that reduction of this disufide bond may be necessary for exposure of 17-36 and HPV16 infection.

Structure, attachment and entry of polyoma- and papillomaviruses

Polyoma- (PY) and Papillomavirus (PV) virions have remarkable structural equivalence although no discernable sequence similarities among the capsid proteins can be detected. Their similarities include the overall surface organization, the presence of 72 capsomeres composed of five molecules of the major capsid proteins, VP1 and L1, respectively, the structure of the core segment of capsomeres with classical antiparallel "jelly roll" beta strands as the major feature, and the linkage of neighboring capsomeres by invading C-terminal arms. Differences include the size of surface exposed loops that contain the dominant neutralizing epitopes, the details of the intercapsomeric interactions, and the presence of 2 or 1 minor capsid proteins, respectively. These differences may affect the dramatic differences observed in receptor binding and internalization pathways utilized by these viruses, but as detailed later even structural differences cannot completely explain receptor and pathway usage. In recent years, technical advances aiding the study of entry processes have allowed the identification of novel endocytic compartments and an appreciation of the links between endocytic pathways that were previously thought to be completely separable. This review is intended to highlight recent advances in our understanding of virus receptor interactions and their consequences for endocytosis and intracellular trafficking.

Evolutionary and structural analyses of alpha-papillomavirus capsid proteins yields novel insights into L2 structure and interaction with L1

Background

PVs (PV) are small, non-enveloped, double-stranded DNA viruses that have been identified as the primary etiological agent for cervical cancer and their potential for malignant transformation in mucosal tissue has a large impact on public health. The PV family Papillomaviridae is organized into multiple genus based on sequential parsimony, host range, tissue tropism, and histology. We focused this analysis on the late gene products, major (L1) and minor (L2) capsid proteins from the family Papillomaviridae genus Alpha-papillomavirus. Alpha-PVs preferentially infect oral and anogenital mucosa of humans and primates with varied risk of oncogenic transformation. Development of evolutionary associations between PVs will likely provide novel information to assist in clarifying the currently elusive relationship between PV and its microenvironment (i.e., the single infected cell) and macro environment (i.e., the skin tissue). We attempt to identify the regions of the major capsid proteins as well as minor capsid proteins of alpha-papillomavirus that have been evolutionarily conserved, and define regions that are under constant selective pressure with respect to the entire family of viruses.

Results

This analysis shows the loops of L1 are in fact the most variable regions among the alpha-PVs. We also identify regions of L2, involved in interaction with L1, as evolutionarily conserved among the members of alpha- PVs. Finally, a predicted three-dimensional model was generated to further elucidate probable aspects of the L1 and L2 interaction.

Arrangement of L2 within the papillomavirus capsid

Papillomaviruses are a family of nonenveloped DNA tumor viruses. Some sexually transmitted human papillomavirus (HPV) types, including HPV type 16 (HPV16), cause cancer of the uterine cervix. Papillomaviruses encode two capsid proteins, L1 and L2. The major capsid protein, L1, can assemble spontaneously into a 72-pentamer icosahedral structure that closely resembles native virions. Although the minor capsid protein, L2, is not required for capsid formation, it is thought to participate in encapsidation of the viral genome and plays a number of essential roles in the viral infectious entry pathway. The abundance of L2 and its arrangement within the virion remain unclear. To address these questions, we developed methods for serial propagation of infectious HPV16 capsids (pseudoviruses) in cultured human cell lines. Biochemical analysis of capsid preparations produced using various methods showed that up to 72 molecules of L2 can be incorporated per capsid. Cryoelectron microscopy and image reconstruction analysis of purified capsids revealed an icosahedrally ordered L2-specific density beneath the axial lumen of each L1 capsomer. The relatively close proximity of these L2 density buttons to one another raised the possibility of homotypic L2 interactions within assembled virions. The concept that the N and C termini of neighboring L2 molecules can be closely apposed within the capsid was supported using bimolecular fluorescence complementation or "split GFP" technology. This structural information should facilitate investigation of L2 function during the assembly and entry phases of the papillomavirus life cycle.

Bovine papillomavirus type 1 infection is mediated by SNARE syntaxin 18

Events that lead to viral infections include the binding of the virus to the target cells, internalization of the virus into the cells, and the ability of the viral genome to be expressed. These steps are mediated by cellular and viral proteins and are temporally regulated. The papillomavirus capsid consists of two virally encoded capsid proteins, L1 and L2. Much is known about the role of the major capsid protein L1 compared to what is known of the role of the L2 protein. We identified the interaction of the L2 protein with SNARE protein syntaxin 18, which mediates the trafficking of vesicles and their cargo between the endoplasmic reticulum, the cis-Golgi compartment, and possibly the plasma membrane. Mutations of L2 residues 41 to 44 prevented the interaction of L2 protein with syntaxin 18 in cotransfection experiments and resulted in noninfectious pseudovirions. In this paper, we describe that syntaxin 18 colocalizes with infectious bovine papillomavirus type 1 (BPV1) pseudovirions during infection but does not colocalize with the noninfectious BPV1 pseudovirions made with an L2 mutant at residues 41 to 44. We show that an antibody against BPV1 L2 residues 36 to 49 (alpha L2 36-49) binds to in vitro-generated BPV1 pseudoviral capsids and does not coimmunoprecipitate syntaxin 18- and BPV1 L2-transfected proteins. alpha L2 36-49 was able to partially or completely neutralize infection of BPV1 pseudovirions and genuine virions. These results support the dependence of syntaxin 18 during BPV1 infection and the ability to interfere with infection by targeting the L2-syntaxin 18 interaction and further define the infectious route of BPV1 mediated by the L2 protein.

Minor capsid proteins of simian virus 40 are dispensable for nucleocapsid assembly and cell entry but are required for nuclear entry of the viral genome

We investigated the roles of simian virus 40 capsid proteins in the viral life cycle by analyzing point mutants in Vp1 and Vp2/3, as well as a deletion mutant lacking the Vp2/3 coding sequence. The Vp1 mutants (V243E and L245E) and the Vp2/3 mutants (F157E-I158E and P164R-G165E-G166R) were previously shown to be defective in Vp1-Vp2/3 interaction and to be noninfectious or poorly infectious, respectively. Here, we show that all these point mutants form stable particles following DNA transfection into cells. The Vp2/3-mutant particles contained very low levels of Vp2/3, whereas the Vp1 mutant particles contained no detectable Vp2/3. As expected, the deletion mutant also formed particles that were noninfectious. We further characterized the two Vp1 point mutants and the deletion mutant. All three mutant particles comprised Vp1 and histone-associated viral DNA, and all were able to enter cells. However, the mutant complexes failed to associate with host importins (owing to the loss of the Vp2/3 nuclear localization signal), and the mutant viral DNAs prematurely dissociated from the Vp1s, suggesting that the nucleocapsids did not enter the nucleus. Consistently, all three mutant particles failed to express large T antigen. Together, our results demonstrate unequivocally that Vp2/3 is dispensable for the formation of nucleocapsids. Further, the nucleocapsids' ability to enter cells implies that Vp1 contains the major determinants for cell attachment and entry. We propose that the major role of Vp2/3 in infectivity is to mediate the nuclear entry of viral DNA.

Identification of amino acid residues within simian virus 40 capsid proteins Vp1, Vp2, and Vp3 that are required for their interaction and for viral infection

Interaction of simian virus 40 (SV40) major capsid protein Vp1 with the minor capsid proteins Vp2 and Vp3 is an integral aspect of the SV40 architecture. Two Vp3 sequence elements mediate Vp1 pentamer binding in vitro, Vp3 residues 155 to 190, or D1, and Vp3 residues 222 to 234, or D2. Of the two, D1 but not D2 was necessary and sufficient to direct the interaction with Vp1 in vivo. Rational mutagenesis of Vp3 residues (Phe157, Ile158, Pro164, Gly165, Gly166, Leu177, and Leu181) or Vp1 residues (Val243 and Leu245), based on a structural model of the SV40 Vp1 pentamer complexed with Vp3 D1, was carried out to disrupt the interaction between Vp1 and Vp3 and to study the consequences of these mutations for viral viability. Altering these residues to bulky, charged residues blocked the interaction in vitro. When these alterations were introduced into the viral genome, they reduced viral viability. Mutants with alterations in Vp1 Val243, Leu245, or both to glutamate were nearly nonviable, whereas those with Vp3 alterations reduced, but did not eliminate, viability. Our results defined the residues of Vp1 and the minor capsid proteins that are essential for both the interaction of the capsid proteins and viral viability in permissive cells.

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.

Human papillomavirus 16 minor capsid protein L2 helps capsomeres assemble independently of intercapsomeric disulfide bonding

The human papillomavirus (HPV) capsomeres (pentamers of major capsid protein L1), which constitute along with L2 the virion capsid, can assemble themselves alone into the L1-capsid particles in vivo and in vitro, depending on intercapsomeric disulfide bonds. To study a possible role of L2 in capsid assembly, we examined the interaction between HPV16 L2 and capsomeres under the conditions that inhibit the formation of disulfide bonds in vitro and in vivo. The purified L2 bound to free capsomeres prepared by disassembling L1-capsids but not to the L1-capsids in vitro. And the L2 was found to help capsomeres assemble into smaller capsid-like particles independently of intercapsomeric disulfide bonding. Similar particles were obtained from the Sf9 cells co-infected with baculoviruses expressing L2 and an L1 mutant that lacks a C-terminal cysteine (C428S) and can form capsomeres but no capsids when expressed alone. These findings suggest that L2, which is known to bind both viral DNA and L1, may contribute to the formation of the virion by linking viral DNA and capsomeres and by helping capsomeres assemble before the virion capsid structure is completed by dintercapsomeric disulfide bonding.

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.

The minor capsid protein L2 contributes to two steps in the human papillomavirus type 31 life cycle

Prior studies, which have relied upon the use of pseudovirions generated in heterologous cell types, have led to sometimes conflicting conclusions regarding the role of the minor capsid protein of papillomaviruses, L2, in the viral life cycle. In this study we carry out analyses with true virus particles assembled in the natural host cell to assess L2's role in the viral infectious life cycle. For these studies we used the organotypic (raft) culture system to recapitulate the full viral life cycle of the high-risk human papillomavirus HPV31, which was either wild type or mutant for L2. After transfection, the L2 mutant HPV31 genome was able to establish itself as a nuclear plasmid in proliferating populations of poorly differentiated (basal-like) human keratinocytes and to amplify its genome to high copy number, support late viral gene expression, and cause formation of virus particles in human keratinocytes that had been induced to undergo terminal differentiation. These results indicate that aspects of both the nonproductive and productive phases of the viral life cycle occur normally in the absence of functional L2. However, upon the analysis of the virus particles generated, we found an approximate 10-fold reduction in the amount of viral DNA encapsidated into L2-deficient virions. Furthermore, there was an over-100-fold reduction in the infectivity of L2-deficient virus. Because the latter deficiency cannot be accounted for solely by the 10-fold decrease in encapsidation, we conclude that L2 contributes to at least two steps in the production of infectious virus.

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).

Nuclear localization but not PML protein is required for incorporation of the papillomavirus minor capsid protein L2 into virus-like particles

Recent reports suggest that nuclear domain(s) 10 (ND10) is the site of papillomavirus morphogenesis. The viral genome replicates in or close to ND10. In addition, the minor capsid protein, L2, accumulates in these subnuclear structures and recruits the major capsid protein, L1. We have now used cell lines deficient for promyelocytic leukemia (PML) protein, the main structural component of ND10, to study the role of this nuclear protein for L2 incorporation into virus-like particles (VLPs). L2 expressed in PML protein knockout (PML(-/-)) cells accumulated in nuclear dots, which resemble L2 aggregates forming at ND10 in PML protein-containing cells. These L2 assemblies also attracted L1 and the transcriptional repressor Daxx, suggesting that they are functional in the absence of PML protein. In addition, L2-containing VLPs assembled in PML(-/-) cells. In order to analyze whether incorporation of L2 into VLPs requires any specific subcellular localization, an L1 mutant defective for nuclear transport and L2 mutants deficient in nuclear translocation and/or ND10 localization were constructed. Using this approach, we identified two independent L2 domains interacting with L1. Mutant L2 proteins not accumulating in ND10 were incorporated into VLPs. Mutant L1 protein, which assembled into VLPs in the cytoplasm, did not incorporate L2 defective for nuclear translocation. The same mutant L2 protein, which passively diffuses into the nucleus, is incorporated into wild-type L1-VLPs in the nucleus. Our data demonstrate that the incorporation of L2 into VLPs requires nuclear but not ND10 localization.

Interactions between papillomavirus L1 and L2 capsid proteins

The human papillomavirus (HPV) capsid consists of 360 copies of the major capsid protein, L1, arranged as 72 pentamers on a T=7 icosahedral lattice, with substoichiometric amounts of the minor capsid protein, L2. In order to understand the arrangement of L2 within the HPV virion, we have defined and biochemically characterized a domain of L2 that interacts with L1 pentamers. We utilized an in vivo binding assay involving the coexpression of recombinant HPV type 11 (HPV11) L1 and HPV11 glutathione S-transferase (GST) L2 fusion proteins in Escherichia coli. In this system, L1 forms pentamers, GST=L2 associates with these pentamers, and L1+L2 complexes are subsequently isolated by using the GST tag on L2. The stoichiometry of L1:L2 in purified L1+L2 complexes was 5:1, indicating that a single molecule of L2 interacts with an L1 pentamer. Coexpression of HPV11 L1 with deletion mutants of HPV11 L2 defined an L1-binding domain contained within amino acids 396 to 439 near the carboxy terminus of L2. L2 proteins from eight different human and animal papillomavirus serotypes were tested for their ability to interact with HPV11 L1. This analysis targeted a hydrophobic region within the L1-binding domain of L2 as critical for L1 binding. Introduction of negative charges into this hydrophobic region by site-directed mutagenesis disrupted L1 binding. L1-L2 interactions were not significantly disrupted by treatment with high salt concentrations (2 M NaCl), weak detergents, and urea concentrations of up to 2 M, further indicating that L1 binding by this domain is mediated by strong hydrophobic interactions. L1+L2 protein complexes were able to form virus-like particles in vitro at pH 5.2 and also at pH 6.8, a pH that is nonpermissive for assembly of L1 protein alone. Thus, L1/L2 interactions are primarily hydrophobic, encompass a relatively short stretch of amino acids, and have significant effects upon in vitro assembly.

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.

The ND10 component promyelocytic leukemia protein relocates to human papillomavirus type 1 E4 intranuclear inclusion bodies in cultured keratinocytes and in warts

Human papillomavirus type 1 (HPV1) E4 protein is associated with cytoplasmic and nuclear inclusions in productively infected keratinocytes. Here we have used transient expression of HPV1 E4 (also known as E1E4) protein in keratinocytes to reproduce formation of E4 inclusions. Immunofluorescence analysis showed that progressive formation of inclusions correlated with diminished colocalization between E4 and keratin intermediate filaments (IFs). Our results support a model in which the HPV1 E4-keratin IF association is transient, occurring only at an early stage of inclusion formation. We also demonstrate that E4 induces relocation of the promyelocytic leukemia protein (PML) from multiple intranuclear speckles (ND10 bodies) to the periphery of nuclear E4 inclusions and that this activity is specific to full-length E4 protein. Analysis of HPV1-induced warts demonstrated that nuclear PML-E4 inclusions were present in productively infected keratinocytes, indicating that reorganization of PML occurs during the virus's replication cycle. It has been suggested that ND10 bodies are the sites for papillomavirus genome replication and virion assembly. Our finding that E4 induces reorganization of ND10 bodies in vitro and in vivo is further strong evidence that these domains play an important role in the papillomavirus life cycle. This study indicates that HPV1 is analogous to other DNA viruses that disrupt or reorganize ND10 domains, possibly to increase efficiency of virus infection. We hypothesize that HPV1 E4-induced reorganization of PML is necessary for efficient replication of the virus during the virus-producing phase.

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.

Assembly and translocation of papillomavirus capsid proteins

The major and minor capsid proteins of polyomavirus are preassembled in the cytoplasm and translocated to the nucleus only as a VP1-VP2/VP3 complex. In this study, we describe independent nuclear translocation of the L1 major protein and the L2 minor capsid protein of human papillomavirus type 33 by several approaches. First, we observed that expression and nuclear translocation of L2 in natural lesions precede expression of L1. Second, using a cell culture system for coexpression, we found that accumulation of L2 in nuclear domain 10 (ND10) subnuclear structures precedes L1 by several hours. In contrast, complexes of L2 and mutants of L1 forced to assemble in the cytoplasm are translocated directly to ND10, like L2 expressed alone. Interestingly, accumulation of wild-type L1 is observed only after L2-induced release of the ND10-associated protein Sp100. Third, nuclear translocation of L2 but not of L1 was blocked by the proteasome inhibitor MG132. Our data suggest that L1 and L2 interaction occurs after L2-induced reorganization of ND10 subnuclear domains.

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

Coexpression of bovine papillomavirus L1 with L2 mutants lacking either eight N-terminal or nine C-terminal amino acids that encode positively charged domains resulted in wild-type levels of viral genome encapsidation. Despite wild-type binding to the cell surface, the resulting virions were noninfectious. An L2 mutant encoding a scrambled version of the nine C-terminal residues restored infectivity, in contrast to an L2 mutant encoding a scrambled version of the N-terminal residues.