Nuclear import strategies of high risk HPV16 L1 major capsid protein

Pseudotyped Virus for Papillomavirus

Papillomavirus is difficult to culture in vitro, which limits its related research. The development of pseudotyped virus technology provides a valuable research tool for virus infectivity research, vaccine evaluation, infection inhibitor evaluation, and so on. Depending on the application fields, different measures have been developed to generate various kinds of pseudotyped papillomavirus. L1-based and L2-based HPV vaccines should be evaluated using different pseudotyped virus system. Pseudotyped papillomavirus animal models need high-titer pseudotyped virus and unique handling procedure to generate robust results. This paper reviewed the development, optimization, standardization, and application of various pseudotyped papillomavirus methods.

How SARS-CoV-2 and Other Viruses Build an Invasion Route to Hijack the Host Nucleocytoplasmic Trafficking System

The host nucleocytoplasmic trafficking system is often hijacked by viruses to accomplish their replication and to suppress the host immune response. Viruses encode many factors that interact with the host nuclear transport receptors (NTRs) and the nucleoporins of the nuclear pore complex (NPC) to access the host nucleus. In this review, we discuss the viral factors and the host factors involved in the nuclear import and export of viral components. As nucleocytoplasmic shuttling is vital for the replication of many viruses, we also review several drugs that target the host nuclear transport machinery and discuss their feasibility for use in antiviral treatment.

Capsid assembly is regulated by amino acid residues asparagine 47 and 48 in the VP2 protein of porcine parvovirus

Porcine parvovirus (PPV) is a major cause of reproductive failure in swine and has caused substantial losses throughout the world. Viral protein 2 (VP2) of PPV is a major structural protein that can self-assemble into virus-like particles (VLP) with hemagglutination (HA) activity. In order to identify the essential residues involved in the mechanism of capsid assembly and to further understand the function of HA, we analyzed a series of deletion mutants and site-directed mutations within the N-terminal of VP2 using the Escherichia coli system. Our results showed that deletion of the first 47 amino acids from the N-terminal of the VP2 protein did not affect capsid assembly, and further truncation to residue 48 Asparagine (Asn, N) caused detrimental effects. Site-directed mutagenesis experiments demonstrated that residue 47Asn reduced the assembly efficiency of PPV VLP, while residue 48Asn destroyed the stability, hemagglutination, and self-assembly characteristics of the PPV VP2 protein. Results from native PAGE inferred that macromolecular polymers were critical intermediates of the VP2 protein during the capsid assembly process. Site-directed mutation at 48Asn did not affect the ability of monomers to form into oligomers, but destroyed the ability of oligomers to assemble into macromolecular particles, influencing both capsid assembly and HA activity. Our findings provide valuable information on the mechanisms of PPV capsid assembly and the possibility of chimeric VLP vaccine development by replacing the first 47 amino acids at the N-terminal of the VP2 protein.

DNA damage response is hijacked by human papillomaviruses to complete their life cycle

The DNA damage response (DDR) is activated when DNA is altered by intrinsic or extrinsic agents. This pathway is a complex signaling network and plays important roles in genome stability, tumor transformation, and cell cycle regulation. Human papillomaviruses (HPVs) are the main etiological agents of cervical cancer. Cervical cancer ranks as the fourth most common cancer among women and the second most frequent cause of cancer-related death worldwide. Over 200 types of HPVs have been identified and about one third of these infect the genital tract. The HPV life cycle is associated with epithelial differentiation. Recent studies have shown that HPVs deregulate the DDR to achieve a productive life cycle. In this review, I summarize current findings about how HPVs mediate the ataxia-telangiectasia mutated kinase (ATM) and the ATM-and RAD3-related kinase (ATR) DDRs, and focus on the roles that ATM and ATR signalings play in HPV viral replication. In addition, I demonstrate that the signal transducer and activator of transcription-5 (STAT)-5, an important immune regulator, can promote ATM and ATR activations through different mechanisms. These findings may provide novel opportunities for development of new therapeutic targets for HPV-related cancers.

The human papillomavirus replication cycle, and its links to cancer progression: a comprehensive review

HPVs (human papillomaviruses) infect epithelial cells and their replication cycle is intimately linked to epithelial differentiation. There are over 200 different HPV genotypes identified to date and each displays a strict tissue specificity for infection. HPV infection can result in a range of benign lesions, for example verrucas on the feet, common warts on the hands, or genital warts. HPV infects dividing basal epithelial cells where its dsDNA episomal genome enters the nuclei. Upon basal cell division, an infected daughter cell begins the process of keratinocyte differentiation that triggers a tightly orchestrated pattern of viral gene expression to accomplish a productive infection. A subset of mucosal-infective HPVs, the so-called ‘high risk’ (HR) HPVs, cause cervical disease, categorized as low or high grade. Most individuals will experience transient HR-HPV infection during their lifetime but these infections will not progress to clinically significant cervical disease or cancer because the immune system eventually recognizes and clears the virus. Cancer progression is due to persistent infection with an HR-HPV. HR-HPV infection is the cause of >99.7% cervical cancers in women, and a subset of oropharyngeal cancers, predominantly in men. HPV16 (HR-HPV genotype 16) is the most prevalent worldwide and the major cause of HPV-associated cancers. At the molecular level, cancer progression is due to increased expression of the viral oncoproteins E6 and E7, which activate the cell cycle, inhibit apoptosis, and allow accumulation of DNA damage. This review aims to describe the productive life cycle of HPV and discuss the roles of the viral proteins in HPV replication. Routes to viral persistence and cancer progression are also discussed.

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.

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.

Regulation of the life cycle of HPVs by differentiation and the DNA damage response

HPVs are the causative agents of cervical and other anogenital cancers. HPVs infect stratified epithelia and link their productive life cycles to cellular differentiation. Low levels of viral genomes are stably maintained in undifferentiated cells and productive replication or amplification is restricted to differentiated suprabasal cells. Amplification is dependent on the activation of the ATM DNA damage factors that are recruited to viral replication centers and inhibition of this pathway blocks productive replication. The STAT-5 protein appears to play a critical role in mediating activation of the ATM pathway in HPV-positive cells. While HPVs need to activate the DNA damage pathway for replication, cervical cancers contain many genomic alterations suggesting that this pathway is circumvented during progression to malignancy.

Transportin acts to regulate mitotic assembly events by target binding rather than Ran sequestration

Transportin-specific molecular tools are used to show that the mitotic cell contains importin β and transportin “global positioning system” pathways that are mechanistically parallel. Transportin works to control where the spindle, nuclear membrane, and nuclear pores are formed by directly affecting assembly factor function.

The nuclear import receptors importin β and transportin play a different role in mitosis: both act phenotypically as spatial regulators to ensure that mitotic spindle, nuclear membrane, and nuclear pore assembly occur exclusively around chromatin. Importin β is known to act by repressing assembly factors in regions distant from chromatin, whereas RanGTP produced on chromatin frees factors from importin β for localized assembly. The mechanism of transportin regulation was unknown. Diametrically opposed models for transportin action are as follows: 1) indirect action by RanGTP sequestration, thus down-regulating release of assembly factors from importin β, and 2) direct action by transportin binding and inhibiting assembly factors. Experiments in Xenopus assembly extracts with M9M, a superaffinity nuclear localization sequence that displaces cargoes bound by transportin, or TLB, a mutant transportin that can bind cargo and RanGTP simultaneously, support direct inhibition. Consistently, simple addition of M9M to mitotic cytosol induces microtubule aster assembly. ELYS and the nucleoporin 107–160 complex, components of mitotic kinetochores and nuclear pores, are blocked from binding to kinetochores in vitro by transportin, a block reversible by M9M. In vivo, 30% of M9M-transfected cells have spindle/cytokinesis defects. We conclude that the cell contains importin β and transportin “global positioning system”or “GPS” pathways that are mechanistically parallel.

Nuclear import of cutaneous beta genus HPV8 E7 oncoprotein is mediated by hydrophobic interactions between its zinc-binding domain and FG nucleoporins

We have previously discovered and characterized the nuclear import pathways for the E7 oncoproteins of mucosal alpha genus HPVs, type 16 and 11. Here we investigated the nuclear import of cutaneous beta genus HPV8 E7 protein using confocal microscopy after transfections of HeLa cells with EGFP-8E7 and mutant plasmids and nuclear import assays in digitonin-permeabilized HeLa cells. We determined that HPV8 E7 contains a nuclear localization signal (NLS) within its zinc-binding domain that mediates its nuclear import. Furthermore, we discovered that a mostly hydrophobic patch 65LRLFV69 within the zinc-binding domain is essential for the nuclear import and localization of HPV8 E7 via hydrophobic interactions with the FG nucleoporins Nup62 and Nup153. Substitution of the hydrophobic residues within the 65LRLFV69 patch to alanines, and not R66A mutation, disrupt the interactions between the 8E7 zinc-binding domain and Nup62 and Nup153 and consequently inhibit nuclear import of HPV8 E7.

Nuclear import of high risk HPV16 E7 oncoprotein is mediated by its zinc-binding domain via hydrophobic interactions with Nup62

We previously discovered that nuclear import of high risk HPV16 E7 is mediated by a cNLS located within the zinc-binding domain via a pathway that is independent of karyopherins/importins (Angeline et al., 2003; Knapp et al., 2009). In this study we continued our characterization of the cNLS and nuclear import pathway of HPV16 E7. We find that an intact zinc-binding domain is essential for the cNLS function in mediating nuclear import of HPV16 E7. Mutagenesis of cysteine residues to alanine in each of the two CysXXCys motifs involved in zinc-binding changes the nuclear localization of the EGFP-16E7 and 2xEGFP-16E7 mutants. We further discover that a patch of hydrophobic residues, 65LRLCV69, within the zinc-binding domain of HPV16 E7 mediates its nuclear import via hydrophobic interactions with the FG domain of the central channel nucleoporin Nup62.

Characterization of the transport signals that mediate the nucleocytoplasmic traffic of low risk HPV11 E7

We previously discovered that nuclear import of low risk HPV11 E7 is mediated by its zinc-binding domain via a pathway that is independent of karyopherins/importins (Piccioli et al., 2010. Virology 407, 100-109). In this study we mapped and characterized a leucine-rich nuclear export signal (NES), 76IRQLQDLLL84, within the zinc-binding domain that mediates the nuclear export of HPV11 E7 in a CRM1-dependent manner. We also identified a mostly hydrophobic patch 65VRLVV69 within the zinc-binding domain that mediates nuclear import of HPV11 E7 via hydrophobic interactions with the FG-repeats domain of Nup62. Substitutions of hydrophobic residues to alanine within the 65VRLVV69 sequence disrupt the nuclear localization of 11E7, whereas the R66A mutation has no effect. Overall the data support a model of nuclear entry of HPV11 E7 protein via hydrophobic interactions with FG nucleoporins at the nuclear pore complex.

The papillomavirus major capsid protein L1

The elegant icosahedral surface of the papillomavirus virion is formed by a single protein called L1. Recombinant L1 proteins can spontaneously self-assemble into a highly immunogenic structure that closely mimics the natural surface of native papillomavirus virions. This has served as the basis for two highly successful vaccines against cancer-causing human papillomaviruses (HPVs). During the viral life cycle, the capsid must undergo a variety of conformational changes, allowing key functions including the encapsidation of the ~8 kb viral genomic DNA, maturation into a more stable state to survive transit between hosts, mediating attachment to new host cells, and finally releasing the viral DNA into the newly infected host cell. This brief review focuses on conserved sequence and structural features that underlie the functions of this remarkable protein.

The nuclear localization of low risk HPV11 E7 protein mediated by its zinc binding domain is independent of nuclear import receptors

We investigated the nuclear import of low risk HPV11 E7 protein using 1) transfection assays in HeLa cells with EGFP fusion plasmids containing 11E7 and its domains and 2) nuclear import assays in digitonin-permeabilized HeLa cells with GST fusion proteins containing 11E7 and its domains. The EGFP-11E7 and EGFP-11cE7(39-98) localized mostly to the nucleus. The GST-11E7 and GST-11cE7(39-98) were imported into the nuclei in the presence of either Ran-GDP or RanG19V-GTP mutant and in the absence of nuclear import receptors. This suggests that 11E7 enters the nucleus via a Ran-dependent pathway, independent of nuclear import receptors, mediated by a nuclear localization signal located in its C-terminal domain (cNLS). This cNLS contains the zinc binding domain consisting of two copies of Cys-X-X-Cys motif. Mutagenesis of Cys residues in these motifs changed the localization of the EGFP-11cE7/-11E7 mutants to cytoplasmic, suggesting that the zinc binding domain is essential for nuclear localization of 11E7.

Human immunodeficiency virus type 1 (HIV-1) induces the cytoplasmic retention of heterogeneous nuclear ribonucleoprotein A1 by disrupting nuclear import: implications for HIV-1 gene expression

Human immunodeficiency virus type 1 (HIV-1) co-opts host proteins and cellular machineries to its advantage at every step of the replication cycle. Here we show that HIV-1 enhances heterogeneous nuclear ribonucleoprotein (hnRNP) A1 expression and promotes the relocalization of hnRNP A1 to the cytoplasm. The latter was dependent on the nuclear export of the unspliced viral genomic RNA (vRNA) and to alterations in the abundance and localization of the FG-repeat nuclear pore glycoprotein p62. hnRNP A1 and vRNA remain colocalized in the cytoplasm supporting a post-nuclear function during the late stages of HIV-1 replication. Consistently, we show that hnRNP A1 acts as an internal ribosomal entry site trans-acting factor up-regulating internal ribosome entry site-mediated translation initiation of the HIV-1 vRNA. The up-regulation and cytoplasmic retention of hnRNP A1 by HIV-1 would ensure abundant expression of viral structural proteins in cells infected with HIV-1.

Identification of the nuclear localization and export signals of high risk HPV16 E7 oncoprotein

The E7 oncoprotein of high risk human papillomavirus type 16 (HPV16) binds and inactivates the retinoblastoma (RB) family of proteins. Our previous studies suggested that HPV16 E7 enters the nucleus via a novel Ran-dependent pathway independent of the nuclear import receptors (Angeline, M., Merle, E., and Moroianu, J. (2003). The E7 oncoprotein of high-risk human papillomavirus type 16 enters the nucleus via a nonclassical Ran-dependent pathway. Virology 317(1), 13-23.). Here, analysis of the localization of specific E7 mutants revealed that the nuclear localization of E7 is independent of its interaction with pRB or of its phosphorylation by CKII. Fluorescence microscopy analysis of enhanced green fluorescent protein (EGFP) and 2xEGFP fusions with E7 and E7 domains in HeLa cells revealed that E7 contains a novel nuclear localization signal (NLS) in the N-terminal domain (aa 1-37). Interestingly, treatment of transfected HeLa cells with two specific nuclear export inhibitors, Leptomycin B and ratjadone, changed the localization of 2xEGFP-E7(38-98) from cytoplasmic to mostly nuclear. These data suggest the presence of a leucine-rich nuclear export signal (NES) and a second NLS in the C-terminal domain of E7 (aa 38-98). Mutagenesis of critical amino acids in the putative NES sequence ((76)IRTLEDLLM(84)) changed the localization of 2xEGFP-E7(38-98) from cytoplasmic to mostly nuclear suggesting that this is a functional NES. The presence of both NLSs and an NES suggests that HPV16 E7 shuttles between the cytoplasm and nucleus which is consistent with E7 having functions in both of these cell compartments.

Possible role for cellular karyopherins in regulating polyomavirus and papillomavirus capsid assembly

Polyomavirus and papillomavirus (papovavirus) capsids are composed of 72 capsomeres of their major capsid proteins, VP1 and L1, respectively. After translation in the cytoplasm, L1 and VP1 pentamerize into capsomeres and are then imported into the nucleus using the cellular alpha and beta karyopherins. Virion assembly only occurs in the nucleus, and cellular mechanisms exist to prevent premature capsid assembly in the cytosol. We have identified the karyopherin family of nuclear import factors as possible "chaperones" in preventing the cytoplasmic assembly of papovavirus capsomeres. Recombinant murine polyomavirus (mPy) VP1 and human papillomavirus type 11 (HPV11) L1 capsomeres bound the karyopherin heterodimer alpha2beta1 in vitro in a nuclear localization signal (NLS)-dependent manner. Because the amino acid sequence comprising the NLS of VP1 and L1 overlaps the previously identified DNA binding domain, we examined the relationship between karyopherin and DNA binding of both mPy VP1 and HPV11 L1. Capsomeres of L1, but not VP1, bound by karyopherin alpha2beta1 or beta1 alone were unable to bind DNA. VP1 and L1 capsomeres could bind both karyopherin alpha2 and DNA simultaneously. Both VP1 and L1 capsomeres bound by karyopherin alpha2beta1 were unable to assemble into capsids, as shown by in vitro assembly reactions. These results support a role for karyopherins as chaperones in the in vivo regulation of viral capsid assembly.

Evaluation of the association with cervical cancer of polymorphisms in syndecan-1, a heparan sulfate proteoglycan involved with viral cell entry

Infection with 1 of approximately 15 oncogenic human papillomaviruses is known to be linked to the development of all histologic forms of cervical cancer. We evaluated whether polymorphisms in syndecan-1 (SDC-1), a gene whose protein product is believed to be involved in human papillomavirus entry into epithelial cells, were associated with histologic subtypes of cervical cancer. A total of 293 in situ/invasive adenocarcinoma cases, 260 in situ/invasive squamous cell carcinoma cases, and 478 controls from two studies conducted in the Eastern United States and Seattle area were evaluated. DNA from peripheral blood was used for testing. We sequenced 5 exons and 60 nucleotides upstream of the start codon for SDC-1 in a random subset of 50 cases and 50 controls from the Eastern U.S. Study and identified two polymorphisms (E84E, rs2230924 and Pro-27 C --> T, rs11544860). PCR-based testing was done to evaluate risk associated with these two polymorphisms. Polymorphisms of SDC-1 were not associated with risk of squamous cell carcinomas of the cervix. Similarly, there was no evidence for an association between SDC-1 exon 3 polymorphisms and risk of cervical adenocarcinomas. A marginally significant increase in risk of cervical adenocarcinoma was associated with the presence of the Pro-27 polymorphism (pooled odds ratios, 1.6; 95% confidence intervals, 0.99-2.6), an effect that was restricted to the Eastern U.S. Study. Our results indicate a lack of association between SDC-1 polymorphisms and risk of squamous cell carcinomas of the cervix. An association between SDC-1 Pro-27 polymorphism and cervical adenocarcinoma cannot be ruled out.

Nuclear import of bovine papillomavirus type 1 E1 protein is mediated by multiple alpha importins and is negatively regulated by phosphorylation near a nuclear localization signal

Papillomavirus DNA replication occurs in the nucleus of infected cells and requires the viral E1 protein, which enters the nuclei of host epithelial cells and carries out enzymatic functions required for the initiation of viral DNA replication. In this study, we investigated the pathway and regulation of the nuclear import of the E1 protein from bovine papillomavirus type 1 (BPV1). Using an in vitro binding assay, we determined that the E1 protein interacted with importins alpha3, alpha4, and alpha5 via its nuclear localization signal (NLS) sequence. In agreement with this result, purified E1 protein was effectively imported into the nucleus of digitonin-permeabilized HeLa cells after incubation with importin alpha3, alpha4, or alpha5 and other necessary import factors. We also observed that in vitro binding of E1 protein to all three alpha importins was significantly decreased by the introduction of pseudophosphorylation mutations in the NLS region. Consistent with the binding defect, pseudophosphorylated E1 protein failed to enter the nucleus of digitonin-permeabilized HeLa cells in vitro. Likewise, the pseudophosphorylation mutant showed aberrant intracellular localization in vivo and accumulated primarily on the nuclear envelope in transfected HeLa cells, while the corresponding alanine replacement mutant displayed the same cellular location pattern as wild-type E1 protein. Collectively, our data demonstrate that BPV1 E1 protein can be transported into the nucleus by more than one importin alpha and suggest that E1 phosphorylation by host cell kinases plays a regulatory role in modulating E1 nucleocytoplasmic localization. This phosphoregulation of nuclear E1 protein uptake may contribute to the coordination of viral replication with keratinocyte proliferation and differentiation.

Characterization of the nuclear localization signal of high risk HPV16 E2 protein

The E2 protein of high risk human papillomavirus type 16 (HPV16) contains an amino-terminal (N) domain, a hinge (H) region and a carboxyl-terminal (C) DNA-binding domain. Using enhanced green fluorescent protein (EGFP) fusions with full length E2 and E2 domains in transfection assays in HeLa cells, we found that the C domain is responsible for the nuclear localization of E2 in vivo, whereas the N and H domains do not contain additional nuclear localization signals (NLSs). Deletion analysis of EGFP-E2 and EGFP-cE2 determined that the C domain contains an alpha helix cNLS that overlaps with the DNA-binding region. Mutational analysis revealed that the arginine and lysine residues in this cNLS are essential for nuclear localization of HPV16 E2. Interestingly, these basic amino acid residues are well conserved among the E2 proteins of BPV-1 and some high risk HPV types but not in the low risk HPV types, suggesting that there are differences between the NLSs and corresponding nuclear import pathways between these E2 proteins.

The l2 minor capsid protein of low-risk human papillomavirus type 11 interacts with host nuclear import receptors and viral DNA

Analysis of the interactions of low-risk human papillomavirus type 11 (HPV11) L2 with karyopherin beta (Kap beta) nuclear import receptors revealed that L2 interacted with Kap beta 1, Kap beta 2, and Kap beta 3 and formed a complex with the Kap alpha 2 beta 1 heterodimer. HPV11 L2 contains two nuclear localization signals (NLSs)-in the N terminus and the C terminus-that could mediate its nuclear import via a classical pathway. Each NLS was functional in vivo, and deletion of both of them abolished L2 nuclear localization. Both NLSs interacted with the viral DNA. Thus, HPV11 L2 can interact with several karyopherins and the viral DNA and may enter the nucleus via multiple pathways.

Human papillomaviruses: basic mechanisms of pathogenesis and oncogenicity

Human papillomaviruses (HPVs) are small double-stranded DNA viruses that infect the cutaneous and mucosal epithelium. Infection by specific HPV types has been linked to the development of cervical carcinoma. HPV infects epithelial cells that undergo terminal differentiation and so encode multiple mechanisms to override the normal regulation of differentiation to produce progeny virions. Two viral proteins, E6 and E7, alter cell cycle control and are the main arbitrators of HPV-induced oncogenesis. Recent data suggest that E6 and E7 also play a major role in the inhibition of the host cell innate immune response to HPV. The E1 and E2 proteins, in combination with various cellular factors, mediate viral replication. In addition, E2 has been implicated in both viral and cellular transcriptional control. Despite decades of research, the function of other viral proteins still remains unclear. While prophylactic vaccines to block genital HPV infection will soon be available, the widespread nature of HPV infection requires greater understanding of both the HPV life cycle as well as the mechanisms underlying HPV-induced carcinogenesis.

Nuclear Transport of Trimeric Assembly Intermediates Exerts a Morphogenetic Control on the Icosahedral Parvovirus Capsid

The connection between nuclear transport and morphogenesis of a large macromolecular entity has been investigated using the karyophylic capsid of the parvovirus minute virus of mice (MVM) as a model. The VP1 (82 kDa) and VP2 (63 kDa) proteins forming the T = 1 icosahedral MVM capsid at the respective 1:5 molar ratio of synthesis, could be covalently cross-linked with dimethyl suberimidate into two types of oligomeric assemblies, which were present at stoichiometric amounts in infected cell extracts and purified viral particles. The larger species contained VP1 and corresponded in size (200 kDa) to a heterotrimer of one VP1 and two VP2 subunits. The smaller species contained VP2 only and corresponded in size (180 kDa) to a homotrimer. The introduction of bulky residues or the truncation of side-chains involved in multiple interactions at the interfaces between trimers of VPs in the MVM capsid, produced the accumulation of trimeric intermediates that were competent in nuclear translocation but not in capsid assembly. These results indicate that MVM maturation proceeds by cytoplasmic oligomerization of the capsid subunits into two types of trimers, which are the assembly intermediates competent to translocate across the nuclear membrane. Consistent with this conclusion, mutations at basic residues that inactivate a previously identified beta-stranded nuclear localization motif, which notably are not involved in inter or intra-subunit contacts, led to cytoplasmic retention of the two types of trimers, with no evidence for other assembly intermediates. Although a fraction of the VP1-containing trimers were translocated into the nucleus driven by the conventional nuclear transport signal of VP1 N terminus, their further assembly in the absence of the VP2-only trimers yielded large molecular mass amorphous aggregates. Therefore, the nuclear transport stoichiometry of assembly intermediates may exert a morphogenetic quality control on macromolecular complexes like the MVM capsid.

Predicting the Nuclear Localization Signals of 107 Types of HPV L1 Proteins by Bioinformatic Analysis

In this study, 107 types of human papillomavirus (HPV) L1 protein sequences were obtained from available databases, and the nuclear localization signals (NLSs) of these HPV L1 proteins were analyzed and predicted by bioinformatic analysis. Out of the 107 types, the NLSs of 39 types were predicted by PredictNLS software (35 types of bipartite NLSs and 4 types of monopartite NLSs). The NLSs of the remaining HPV types were predicted according to the characteristics and the homology of the already predicted NLSs as well as the general rule of NLSs. According to the result, the NLSs of 107 types of HPV L1 proteins were classified into 15 categories. The different types of HPV L1 proteins in the same NLS category could share the similar or the same nucleocytoplasmic transport pathway. They might be used as the same target to prevent and treat different types of HPV infection. The results also showed that bioinformatic technology could be used to analyze and predict NLSs of proteins.

Dissection of a novel nuclear localization signal in open reading frame 29 of varicella-zoster virus

Open reading frame 29 (ORF29) of varicella-zoster virus (VZV) encodes a 120-kDa single-stranded DNA binding protein (ORF29p) that is not packaged in the virion and is expressed during latency. During lytic infection, ORF29p is localized primarily to infected cell nuclei. In contrast, ORF29p is found exclusively in the cytoplasm in neurons of the dorsal root ganglia obtained at autopsy from seropositive latently infected patients. ORF29p accumulates in the nuclei of neurons in dorsal root ganglia obtained at autopsy from patients with active zoster. The localization of this protein is, therefore, tightly correlated with the proposed VZV lytic/latent switch. In this report, we have investigated the nuclear import mechanism of ORF29p. We identified a novel nuclear targeting domain bounded by amino acids 9 to 154 of ORF29p that functions independent of other VZV-encoded factors. In vitro import assays in digitonin-permeabilized HeLa cells reveal that ORF29p is transported into the nucleus by a Ran-, karyopherin alpha- and beta-dependent mechanism. These data are further supported by the demonstration that a glutathione S-transferase-karyopherin alpha fusion interacts with ORF29p, but not with a protein containing a point mutation in its nuclear localization signal (NLS). Therefore, the region of ORF29p responsible for its nuclear targeting is also involved in the association with karyopherin alpha. As a result of this interaction, this noncanonical NLS appears to hijack the classical cellular nuclear import machinery. Elucidation of the mechanisms governing ORF29p nuclear targeting could shed light on the VZV reactivation process.

Nuclear import in viral infections

The separation of transcription in the nucleus and translation in the cytoplasm requires nucleo-cytoplasmic exchange of proteins and RNAs. Viruses have evolved strategies to capitalize on the nucleo-cytoplasmic trafficking machinery of the cell. Here, we first discuss the principal mechanisms of receptor-mediated nuclear import of proteinaceous cargo through the nuclear pore complex, the gate keeper of the cell nucleus. We then focus on viral strategies leading to nuclear import of genomes and subgenomic particles. Nucleo-cytoplasmic transport is directly important for those viruses that are replicating in the nucleus, such as DNA tumor viruses and RNA viruses, including parvoviruses, the DNA retroviruses hepadnaviruses, RNA-retrotransposons and retroviruses, adenoviruses, herpesviruses, papovaviruses, and particular negative-sense RNA viruses, such as the orthomyxovirus influenza virus. The viral strategies of nuclear import turn out to be surprisingly diverse. Their investigation continues to give insight into how nucleic acids pass in and out of the nucleus.

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).

The E7 oncoprotein of high-risk human papillomavirus type 16 enters the nucleus via a nonclassical Ran-dependent pathway

E7, the major transforming protein of high-risk human papillomavirus (HPV), type 16, binds and inactivates the retinoblastoma protein (pRb), and the Rb-related proteins p107 and p130. HPV16 E7 is a nuclear protein lacking a classical basic nuclear localization signal. In this study we investigated the nuclear import of HPV16 E7 oncoprotein in digitonin-permeabilized HeLa cells. HPV16 E7 nuclear import was independent of pRb, as an E7(DeltaDLYC) variant defective in pRb binding was imported into the nuclei of digitonin-permeabilized cells as efficiently as wild-type E7 in the presence of exogenous cytosol. Interestingly, we discovered that HPV16 E7 is imported into the nuclei via a novel pathway different from those mediated by Kap alpha2beta1 heterodimers, Kap beta1, or Kap beta2. Nuclear accumulation of E7 required Ran and was not inhibited by the RanG19V-GTP variant, an inhibitor of Kap beta mediated import pathways. Together the data suggest that HPV16 E7 translocates through the nuclear pores via a nonclassical Ran-dependent pathway, independent of the main cytosolic Kap beta import receptors.

Human papillomavirus types 16, 31, and 58 use different endocytosis pathways to enter cells

The early steps of the intracellular trafficking of human papillomavirus type 16 (HPV-16), -31, and -58 pseudovirions were studied by investigating the effects of drugs acting at defined points of endocytosis pathways on virus-like particle-mediated pseudoinfection by overexpression of a dominant-negative form of the Eps15 protein to inhibit clathrin-mediated endocytosis and by electron microscopy. The results obtained suggested the involvement of clathrin-mediated endocytosis in HPV-16 and HPV-58 entry and caveola-mediated endocytosis in HPV-31 entry.

The L1 major capsid protein of human papillomavirus type 11 interacts with Kap beta2 and Kap beta3 nuclear import receptors

We have previously shown that the L1 major capsid protein of low-risk HPV11 binds to the Kap alpha2 adapter and enters the nucleus via a Kap alpha2beta1-mediated pathway. In this study, we discovered that HPV11 L1 capsomeres bind to Kap beta2 import receptor, known to mediate nuclear import of hnRNP A1 via interaction with its nuclear localization signal termed M9. Significantly, binding of HPV11 L1 capsomeres to Kap beta2 inhibited the nuclear import of Kap beta2, and its specific M9-containing cargo. Interestingly, HPV11 L1 capsomeres also interacted with Kap beta3 import receptor and inhibited Kap beta3 nuclear import. Moreover, the L1 capsomeres of high-risk HPV-16 shared these activities. These data suggest that HPV L1 major capsid proteins interact with Kap beta2 and Kap beta3, and they may inhibit the Kap beta2- and Kap beta3-mediated nuclear import pathways during the productive phase of the viral life cycle when the virions are assembled and released.

Nuclear entry of high-risk human papillomavirus type 16 E6 oncoprotein occurs via several pathways

The E6 oncoprotein of high-risk human papillomavirus type 16 (HPV16) interacts with several nuclear transcription factors and coactivators in addition to cytoplasmic proteins, suggesting that nuclear import of HPV16 E6 plays a role in the cellular transformation process. In this study we have investigated the nuclear import pathways of HPV16 E6 in digitonin-permeabilized HeLa cells. We found that HPV16 E6 interacted with the karyopherin (Kap) alpha2 adapter and could enter the nucleus via a classical Kap alpha2beta1-mediated pathway. Interestingly, HPV16 E6 also interacted, via its basic nuclear localization signal (NLS) located at the C terminus, with both Kap beta1 and Kap beta2 import receptors. Binding of RanGTP to these Kap betas inhibited their interaction with HPV16 E6 NLS. In agreement with these binding data, nuclear import of the HPV16 E6 oncoprotein in digitonin-permeabilized HeLa cells could be mediated by either Kap beta1 or Kap beta2. Nuclear import via these pathways required RanGDP and was independent of GTP hydrolysis by Ran. Significantly, an E6(R124G) mutant had reduced nuclear import activity, and the E6 deletion mutant lacking (121)KKQR(124) was not imported into the nucleus. The data reveal that the HPV16 E6 oncoprotein interacts via its C-terminal NLS with several karyopherins and exploits these interactions to enter the nucleus of host cells via multiple pathways.