Vaccinia virus entry into cells is dependent on a virion surface protein encoded by the A28L gene

A Pseudovirus Nanoparticle Displaying the Vaccinia Virus L1 Protein Elicited High Neutralizing Antibody Titers and Provided Complete Protection to Mice against Mortality Caused by a Vaccinia Virus Challenge

The recent worldwide incidence of mpox infection and concerns about future emerging variants of mpox viruses highlight the need for the development of a new generation of mpox vaccines. To achieve this goal, we utilized our norovirus S nanoparticle vaccine platform to produce and evaluate two pseudovirus nanoparticles (PVNPs), S-L1 and S-J1. These PVNPs displayed the L1 neutralizing antigen target of the vaccinia virus and a yet-untested J1 antigen of the mpox virus, respectively, with the aim of creating an effective nanoparticle-based mpox vaccine. Each self-assembled PVNP consists of an inner shell resembling the interior layer of the norovirus capsid and multiple L1 or J1 antigens on the surface. The PVNPs improved the antibody responses toward the displayed L1 or J1 antigens in mice, resulting in significantly greater L1/J1-specific IgG and IgA titers than those elicited by the corresponding free L1 or J1 antigens. After immunization with the S-L1 PVNPs, the mouse sera exhibited high neutralizing antibody titers against the vaccinia virus, and the S-L1 PVNPs provided mice with 100% protection against mortality caused by vaccinia virus challenge. In contrast, the S-J1 PVNPs induced low neutralizing antibody titers and conferred mice weak protective immunity. These data confirm that the L1 protein is an excellent vaccine target and that the readily available S-L1 PVNPs are a promising mpox vaccine candidate worthy of further development.

Structural and functional analyses of viral H2 protein of the vaccinia virus entry fusion complex

IMPORTANCE

Vaccinia virus infection requires virus-cell membrane fusion to complete entry during endocytosis; however, it contains a large viral fusion protein complex of 11 viral proteins that share no structure or sequence homology to all the known viral fusion proteins, including type I, II, and III fusion proteins. It is thus very challenging to investigate how the vaccinia fusion complex works to trigger membrane fusion with host cells. In this study, we crystallized the ectodomain of vaccinia H2 protein, one component of the viral fusion complex. Furthermore, we performed a series of mutational, biochemical, and molecular analyses and identified two surface loops containing 170LGYSG174 and 125RRGTGDAW132 as the A28-binding region. We also showed that residues in the N-terminal helical region (amino acids 51-90) are also important for H2 function.

Structural and functional analysis of vaccinia viral fusion complex component protein A28 through NMR and molecular dynamic simulations

Host cell entry of vaccinia virus (a poxvirus) proceeds through multiple steps that involve many viral proteins to mediate cell infection. Upon binding to cells, vaccinia virus membrane fuses with host membranes via a viral entry fusion protein complex comprising 11 proteins: A16, A21, A28, F9, G3, G9, H2, J5, L1, L5 and O3. Despite vaccinia virus having two infectious forms, mature and enveloped, that have different membrane layers, both forms require an identical viral entry fusion complex for membrane fusion. Components of the poxvirus entry fusion complex that have been structurally assessed to date share no known homology with all other type I, II and III viral fusion proteins, and the large number of fusion protein components renders it a unique system to investigate poxvirus-mediated membrane fusion. Here, we determined the NMR structure of a truncated version of vaccinia A28 protein. We also expressed a soluble H2 protein and showed that A28 interacts with H2 protein at a 1:1 ratio in vitro. Furthermore, we performed extensive in vitro alanine mutagenesis to identify A28 protein residues that are critical for H2 binding, entry fusion complex formation, and virus-mediated membrane fusion. Finally, we used molecular dynamic simulations to model full-length A28-H2 subcomplex in membranes. In summary, we characterized vaccinia virus A28 protein and determined residues important in its interaction with H2 protein and membrane components. We also provide a structural model of the A28-H2 protein interaction to illustrate how it forms a 1:1 subcomplex on a modeled membrane.

Functional epitopes and neutralizing antibodies of vaccinia virus

Smallpox is an infectious disease caused by the variola virus, and it has a high mortality rate. Historically it has broken out in many countries and it was a great threat to human health. Smallpox was declared eradicated in 1980, and Many countries stopped nation-wide smallpox vaccinations at that time. In recent years the potential threat of bioterrorism using smallpox has led to resumed research on the treatment and prevention of smallpox. Effective ways of preventing and treating smallpox infection have been reported, including vaccination, chemical drugs, neutralizing antibodies, and clinical symptomatic therapies. Antibody treatments include anti-sera, murine monoclonal antibodies, and engineered humanized or human antibodies. Engineered antibodies are homologous, safe, and effective. The development of humanized and genetically engineered antibodies against variola virus via molecular biology and bioinformatics is therefore a potentially fruitful prospect with respect to field application. Natural smallpox virus is inaccessible, therefore most research about prevention and/or treatment of smallpox were done using vaccinia virus, which is much safer and highly homologous to smallpox. Herein we summarize vaccinia virus epitope information reported to date, and discuss neutralizing antibodies with potential value for field application.

Human mpox: Biology, epidemiology, therapeutic options, and development of small molecule inhibitors

Although monkeypox (mpox) has been endemic in Western and Central Africa for 50 years, it has not received sufficient prophylactic and therapeutical attention to avoid evolving into an epidemic. From January 2022 to January 2023, more than 84,000 of mpox cases were reported from 110 countries worldwide. Case numbers appear to be rising every day, making mpox an increasing global public health threat for the foreseeable future. In this perspective, we review the known biology and epidemiology of mpox virus, together with the latest therapeutic options available for mpox treatment. Further, small molecule inhibitors against mpox virus and the future directions in this field are discussed as well.

A succession of two viral lattices drives vaccinia virus assembly

During its cytoplasmic replication, vaccinia virus assembles non-infectious spherical immature virions (IV) coated by a viral D13 lattice. Subsequently, IV mature into infectious brick-shaped intracellular mature virions (IMV) that lack D13. Here, we performed cryo-electron tomography (cryo-ET) of frozen-hydrated vaccinia-infected cells to structurally characterise the maturation process in situ. During IMV formation, a new viral core forms inside IV with a wall consisting of trimeric pillars arranged in a new pseudohexagonal lattice. This lattice appears as a palisade in cross-section. As maturation occurs, which involves a 50% reduction in particle volume, the viral membrane becomes corrugated as it adapts to the newly formed viral core in a process that does not appear to require membrane removal. Our study suggests that the length of this core is determined by the D13 lattice and that the consecutive D13 and palisade lattices control virion shape and dimensions during vaccinia assembly and maturation.

Insights into the Organization of the Poxvirus Multicomponent Entry-Fusion Complex from Proximity Analyses in Living Infected Cells

Poxviruses are exceptional in having a complex entry-fusion complex (EFC) that is comprised of 11 conserved proteins embedded in the membrane of mature virions. However, the detailed architecture is unknown and only a few bimolecular protein interactions have been demonstrated by coimmunoprecipitation from detergent-treated lysates and by cross-linking. Here, we adapted the tripartite split green fluorescent protein (GFP) complementation system in order to analyze EFC protein contacts within living cells. This system employs a detector fragment called GFP1-9 comprised of nine GFP β-strands. To achieve fluorescence, two additional 20-amino-acid fragments called GFP10 and GFP11 attached to interacting proteins are needed, providing the basis for identification of the latter. We constructed a novel recombinant vaccinia virus (VACV-GFP1-9) expressing GFP1-9 under a viral early/late promoter and plasmids with VACV late promoters regulating each of the EFC proteins with GFP10 or GFP11 attached to their ectodomains. GFP fluorescence was detected by confocal microscopy at sites of virion assembly in cells infected with VACV-GFP1-9 and cotransfected with plasmids expressing one EFC-GFP10 and one EFC-GFP11 interacting protein. Flow cytometry provided a quantitative way to determine the interaction of each EFC-GFP10 protein with every other EFC-GFP11 protein in the context of a normal infection in which all viral proteins are synthesized and assembled. Previous EFC protein interactions were confirmed, and new ones were discovered and corroborated by additional methods. Most remarkable was the finding that the small, hydrophobic O3 protein interacted with each of the other EFC proteins. IMPORTANCE Poxviruses are enveloped viruses with a DNA-containing core that enters cells following fusion of viral and host membranes. This essential step is a target for vaccines and therapeutics. The entry-fusion complex (EFC) of poxviruses is unusually complex and comprised of 11 conserved viral proteins. Determination of the structure of the EFC is a prerequisite for understanding the fusion mechanism. Here, we used a tripartite split green fluorescent protein assay to determine the proximity of individual EFC proteins in living cells. A network connecting components of the EFC was derived.

Investigating Viruses During the Transformation of Molecular Biology: Part II

My scientific career started at an extraordinary time, shortly after the discoveries of the helical structure of DNA, the central dogma of DNA to RNA to protein, and the genetic code. Part I of this series emphasizes my education and early studies highlighted by the isolation and characterization of numerous vaccinia virus enzymes, determination of the cap structure of messenger RNA, and development of poxviruses as gene expression vectors for use as recombinant vaccines. Here I describe a shift in my research focus to combine molecular biology and genetics for a comprehensive understanding of poxvirus biology. The dominant paradigm during the early years was to select a function, isolate the responsible proteins, and locate the corresponding gene, whereas later the common paradigm was to select a gene, make a mutation, and determine the altered function. Motivations, behind-the-scenes insights, importance of new technologies, and the vital roles of trainees and coworkers are emphasized.

Mutations Near the N Terminus of Vaccinia Virus G9 Protein Overcome Restrictions on Cell Entry and Syncytium Formation Imposed by the A56/K2 Fusion Regulatory Complex

The entry/fusion complex (EFC) consists of 11 conserved proteins embedded in the membrane envelope of mature poxvirus particles. Poxviruses also encode proteins that localize in cell membranes and negatively regulate superinfection and syncytium formation. The vaccinia virus (VACV) A56/K2 fusion regulatory complex associates with the G9/A16 EFC subcomplex, but functional support for the importance of this interaction was lacking. Here, we describe serially passaging VACV in nonpermissive cells expressing A56/K2 as an unbiased approach to isolate and analyze escape mutants. Viruses forming large plaques in A56/K2 cells increased in successive rounds of infection, indicating the occurrence and enrichment of adaptive mutations. Sequencing of genomes of passaged and cloned viruses revealed mutations near the N terminus of the G9 open reading frame but none in A16 or other genes. The most frequent mutation was His to Tyr at amino acid 44; additional escape mutants had a His-to-Arg mutation at amino acid 44 or a duplication of amino acids 26 to 39. An adaptive Tyr-to-Cys substitution at amino acid 42 was discovered using error-prone PCR to generate additional mutations. Myristoylation of G9 was unaffected by the near-N-terminal mutations. The roles of the G9 mutations in enhancing plaque size were validated by homologous recombination. The mutants exhibited enhanced entry and spread in A56/K2 cells and induced syncytia at neutral pH in HeLa cells despite the expression of A56/K2. The data suggest that the mutations perturb the interaction of G9 with A56/K2, although some association was still detected in detergent-treated infected cell lysates.IMPORTANCE The entry of enveloped viruses is achieved by the fusion of viral and cellular membranes, a critical step in infection that determines host range and provides targets for vaccines and therapeutics. Poxviruses encode an exceptionally large number of proteins comprising the entry/fusion complex (EFC), which enables infection of diverse cells. Vaccinia virus (VACV), the prototype member of the poxvirus family, also encodes the fusion regulatory proteins A56 and K2, which are displayed on the plasma membrane and may be beneficial by preventing reinfection and cell-cell fusion. Previous studies showed that A56/K2 interacts with the G9/A16 EFC subcomplex in detergent-treated cell extracts. Functional evidence for the importance of this interaction was obtained by serially passaging wild-type VACV in cells that are nonpermissive because of A56/K2 expression. VACV mutants with amino acid substitutions or duplications near the N terminus of G9 were enriched because of their ability to overcome the block to entry imposed by A56/K2.

Experimental Evolution To Isolate Vaccinia Virus Adaptive G9 Mutants That Overcome Membrane Fusion Inhibition via the Vaccinia Virus A56/K2 Protein Complex

For cell entry, vaccinia virus requires fusion with the host membrane via a viral fusion complex of 11 proteins, but the mechanism remains unclear. It was shown previously that the viral proteins A56 and K2 are expressed on infected cells to prevent superinfection by extracellular vaccinia virus through binding to two components of the viral fusion complex (G9 and A16), thereby inhibiting membrane fusion. To investigate how the A56/K2 complex inhibits membrane fusion, we performed experimental evolutionary analyses by repeatedly passaging vaccinia virus in HeLa cells overexpressing the A56 and K2 proteins to isolate adaptive mutant viruses. Genome sequencing of adaptive mutants revealed that they had accumulated a unique G9R open reading frame (ORF) mutation, resulting in a single His44Tyr amino acid change. We engineered a recombinant vaccinia virus to express the G9^H44Y mutant protein, and it readily infected HeLa-A56/K2 cells. Moreover, similar to the ΔA56 virus, the G9^H44Y mutant virus on HeLa cells had a cell fusion phenotype, indicating that G9^H44Y-mediated membrane fusion was less prone to inhibition by A56/K2. Coimmunoprecipitation experiments demonstrated that the G9^H44Y protein bound to A56/K2 at neutral pH, suggesting that the H44Y mutation did not eliminate the binding of G9 to A56/K2. Interestingly, upon acid treatment to inactivate A56/K2-mediated fusion inhibition, the G9^H44Y mutant virus induced robust cell-cell fusion at pH 6, unlike the pH 4.7 required for control and revertant vaccinia viruses. Thus, A56/K2 fusion suppression mainly targets the G9 protein. Moreover, the G9^H44Y mutant protein escapes A56/K2-mediated membrane fusion inhibition most likely because it mimics an acid-induced intermediate conformation more prone to membrane fusion.IMPORTANCE It remains unclear how the multiprotein entry fusion complex of vaccinia virus mediates membrane fusion. Moreover, vaccinia virus contains fusion suppressor proteins to prevent the aberrant activation of this multiprotein complex. Here, we used experimental evolution to identify adaptive mutant viruses that overcome membrane fusion inhibition mediated by the A56/K2 protein complex. We show that the H44Y mutation of the G9 protein is sufficient to overcome A56/K2-mediated membrane fusion inhibition. Treatment of virus-infected cells at different pHs indicated that the H44Y mutation lowers the threshold of fusion inhibition by A56/K2. Our study provides evidence that A56/K2 inhibits the viral fusion complex via the latter's G9 subcomponent. Although the G9^H44Y mutant protein still binds to A56/K2 at neutral pH, it is less dependent on low pH for fusion activation, implying that it may adopt a subtle conformational change that mimics a structural intermediate induced by low pH.

Nanoscale polarization of the entry fusion complex of vaccinia virus drives efficient fusion

To achieve efficient binding and subsequent fusion, most enveloped viruses encode between one and five proteins1. For many viruses, the clustering of fusion proteins-and their distribution on virus particles-is crucial for fusion activity2,3. Poxviruses, the most complex mammalian viruses, dedicate 15 proteins to binding and membrane fusion4. However, the spatial organization of these proteins and how this influences fusion activity is unknown. Here, we show that the membrane of vaccinia virus is organized into distinct functional domains that are critical for the efficiency of membrane fusion. Using super-resolution microscopy and single-particle analysis, we found that the fusion machinery of vaccinia virus resides exclusively in clusters at virion tips. Repression of individual components of the fusion complex disrupts fusion-machinery polarization, consistent with the reported loss of fusion activity5. Furthermore, we show that displacement of functional fusion complexes from virion tips disrupts the formation of fusion pores and infection kinetics. Our results demonstrate how the protein architecture of poxviruses directly contributes to the efficiency of membrane fusion, and suggest that nanoscale organization may be an intrinsic property of these viruses to assure successful infection.

A comparative review of viral entry and attachment during large and giant dsDNA virus infections

Viruses enter host cells via several mechanisms, including endocytosis, macropinocytosis, and phagocytosis. They can also fuse at the plasma membrane and can spread within the host via cell-to-cell fusion or syncytia. The mechanism used by a given viral strain depends on its external topology and proteome and the type of cell being entered. This comparative review discusses the cellular attachment receptors and entry pathways of dsDNA viruses belonging to the families Adenoviridae, Baculoviridae, Herpesviridae and nucleocytoplasmic large DNA viruses (NCLDVs) belonging to the families Ascoviridae, Asfarviridae, Iridoviridae, Phycodnaviridae, and Poxviridae, and giant viruses belonging to the families Mimiviridae and Marseilleviridae as well as the proposed families Pandoraviridae and Pithoviridae. Although these viruses have several common features (e.g., topology, replication and protein sequence similarities) they utilize different entry pathways to infect wide-range of hosts, including humans, other mammals, invertebrates, fish, protozoa and algae. Similarities and differences between the entry methods used by these virus families are highlighted, with particular emphasis on viral topology and proteins that mediate viral attachment and entry. Cell types that are frequently used to study viral entry are also reviewed, along with other factors that affect virus-host cell interactions.

Membrane fusion during poxvirus entry

Poxviruses comprise a large family of enveloped DNA viruses that infect vertebrates and invertebrates. Poxviruses, unlike most DNA viruses, replicate in the cytoplasm and encode enzymes and other proteins that enable entry, gene expression, genome replication, virion assembly and resistance to host defenses. Entry of vaccinia virus, the prototype member of the family, can occur at the plasma membrane or following endocytosis. Whereas many viruses encode one or two proteins for attachment and membrane fusion, vaccinia virus encodes four proteins for attachment and eleven more for membrane fusion and core entry. The entry-fusion proteins are conserved in all poxviruses and form a complex, known as the Entry Fusion Complex (EFC), which is embedded in the membrane of the mature virion. An additional membrane that encloses the mature virion and is discarded prior to entry is present on an extracellular form of the virus. The EFC is held together by multiple interactions that depend on nine of the eleven proteins. The entry process can be divided into attachment, hemifusion and core entry. All eleven EFC proteins are required for core entry and at least eight for hemifusion. To mediate fusion the virus particle is activated by low pH, which removes one or more fusion repressors that interact with EFC components. Additional EFC-interacting fusion repressors insert into cell membranes and prevent secondary infection. The absence of detailed structural information, except for two attachment proteins and one EFC protein, is delaying efforts to determine the fusion mechanism.

Intracellular Transport of Vaccinia Virus in HeLa Cells Requires WASH-VPEF/FAM21-Retromer Complexes and Recycling Molecules Rab11 and Rab22

Vaccinia virus, the prototype of the Orthopoxvirus genus in the family Poxviridae, infects a wide range of cell lines and animals. Vaccinia mature virus particles of the WR strain reportedly enter HeLa cells through fluid-phase endocytosis. However, the intracellular trafficking process of the vaccinia mature virus between cellular uptake and membrane fusion remains unknown. We used live imaging of single virus particles with a combination of various cellular vesicle markers, to track fluorescent vaccinia mature virus particle movement in cells. Furthermore, we performed functional interference assays to perturb distinct vesicle trafficking processes in order to delineate the specific route undertaken by vaccinia mature virus prior to membrane fusion and virus core uncoating in cells. Our results showed that vaccinia virus traffics to early endosomes, where recycling endosome markers Rab11 and Rab22 are recruited to participate in subsequent virus trafficking prior to virus core uncoating in the cytoplasm. Furthermore, we identified WASH-VPEF/FAM21-retromer complexes that mediate endosome fission and sorting of virus-containing vesicles prior to virus core uncoating in the cytoplasm.

IMPORTANCE

Vaccinia mature virions of the WR strain enter HeLa cells through fluid phase endocytosis. We previously demonstrated that virus-containing vesicles are internalized into phosphatidylinositol 3-phosphate positive macropinosomes, which are then fused with Rab5-positive early endosomes. However, the subsequent process of sorting the virion-containing vesicles prior to membrane fusion remains unclear. We dissected the intracellular trafficking pathway of vaccinia mature virions in cells up to virus core uncoating in cytoplasm. We show that vaccinia mature virions first travel to early endosomes. Subsequent trafficking events require the important endosome-tethered protein VPEF/FAM21, which recruits WASH and retromer protein complexes to the endosome. There, the complex executes endosomal membrane fission and cargo sorting to the Rab11-positive and Rab22-positive recycling pathway, resulting in membrane fusion and virus core uncoating in the cytoplasm.

From crescent to mature virion: vaccinia virus assembly and maturation

Vaccinia virus (VACV) has achieved unprecedented success as a live viral vaccine for smallpox which mitigated eradication of the disease. Vaccinia virus has a complex virion morphology and recent advances have been made to answer some of the key outstanding questions, in particular, the origin and biogenesis of the virion membrane, the transformation from immature virion (IV) to mature virus (MV), and the role of several novel genes, which were previously uncharacterized, but have now been shown to be essential for VACV virion formation. This new knowledge will undoubtedly contribute to the rational design of safe, immunogenic vaccine candidates, or effective antivirals in the future. This review endeavors to provide an update on our current knowledge of the VACV maturation processes with a specific focus on the initiation of VACV replication through to the formation of mature virions.

Initial characterization of vaccinia virus B4 suggests a role in virus spread

Currently, little is known about the ankyrin/F-box protein B4. Here, we report that B4R-null viruses exhibited reduced plaque size in tissue culture, and decreased ability to spread, as assessed by multiple-step growth analysis. Electron microscopy indicated that B4R-null viruses still formed mature and extracellular virions; however, there was a slight decrease of virions released into the media following deletion of B4R. Deletion of B4R did not affect the ability of the virus to rearrange actin; however, VACV811, a large vaccinia virus deletion mutant missing 55 open reading frames, had decreased ability to produce actin tails. Using ectromelia virus, a natural mouse pathogen, we demonstrated that virus devoid of EVM154, the B4R homolog, showed decreased spread to organs and was attenuated during infection. This initial characterization suggests that B4 may play a role in virus spread, and that other unidentified mediators of actin tail formation may exist in vaccinia virus.

Role of the vaccinia virus O3 protein in cell entry can be fulfilled by its Sequence flexible transmembrane domain

The vaccinia virus O3 protein, a component of the entry-fusion complex, is encoded by all chordopoxviruses. We constructed truncation mutants and demonstrated that the transmembrane domain, which comprises two-thirds of this 35 amino acid protein, is necessary and sufficient for interaction with the entry-fusion complex and function in cell entry. Nevertheless, neither single amino acid substitutions nor alanine scanning mutagenesis revealed essential amino acids within the transmembrane domain. Moreover, replication-competent mutant viruses were generated by randomization of 10 amino acids of the transmembrane domain. Of eight unique viruses, two contained only two amino acids in common with wild type and the remainder contained one or none within the randomized sequence. Although these mutant viruses formed normal size plaques, the entry-fusion complex did not co-purify with the mutant O3 proteins suggesting a less stable interaction. Thus, despite low specific sequence requirements, the transmembrane domain is sufficient for function in entry.

Enveloped virus labeling via both intrinsic biosynthesis and metabolic incorporation of phospholipids in host cells

An alternative method for labeling fully replicative enveloped viruses was developed, in which both the biosynthesis and metabolic incorporation of phospholipids in host cells were simultaneously utilized to introduce an azide group to the envelope of the vaccinia virus by taking advantage of the host-derived lipid membrane formation mechanism. Such an azide group could be subsequently used to fluorescently label the envelope of the virus via a bioorthogonal reaction. Furthermore, simultaneous dual-labeling of the virus through the virus replication was realized skillfully by coupling this envelope labeling strategy with "replication-intercalation labeling" of viral nucleic acid. For the first time, it is by natural propagation of the virus in its host cells in the presence of fluorophores that simultaneous dual-labeling of living viruses can be mildly realized with high efficiency in facile and mild conditions.

Structural and biochemical characterization of the vaccinia virus envelope protein D8 and its recognition by the antibody LA5

Smallpox vaccine is considered a gold standard of vaccines, as it is the only one that has led to the complete eradication of an infectious disease from the human population. B cell responses are critical for the protective immunity induced by the vaccine, yet their targeted epitopes recognized in humans remain poorly described. Here we describe the biochemical and structural characterization of one of the immunodominant vaccinia virus (VACV) antigens, D8, and its binding to the monoclonal antibody LA5, which is capable of neutralizing VACV in the presence of complement. The full-length D8 ectodomain was found to form a tetramer. We determined the crystal structure of the LA5 Fab-monomeric D8 complex at a resolution of 2.1 Å, as well as the unliganded structures of D8 and LA5-Fab at resolutions of 1.42 Å and 1.6 Å, respectively. D8 features a carbonic anhydrase (CAH) fold that has evolved to bind to the glycosaminoglycan (GAG) chondroitin sulfate (CS) on host cells. The central positively charged crevice of D8 was predicted to be the CS binding site by automated docking experiments. Furthermore, sequence alignment of various poxvirus D8 orthologs revealed that this crevice is structurally conserved. The D8 epitope is formed by 23 discontinuous residues that are spread across 80% of the D8 protein sequence. Interestingly, LA5 binds with a high-affinity lock-and-key mechanism above this crevice with an unusually large antibody-antigen interface, burying 2,434 Å(2) of protein surface.

Poxvirus cell entry: how many proteins does it take?

For many viruses, one or two proteins enable cell binding, membrane fusion and entry. The large number of proteins employed by poxviruses is unprecedented and may be related to their ability to infect a wide range of cells. There are two main infectious forms of vaccinia virus, the prototype poxvirus: the mature virion (MV), which has a single membrane, and the extracellular enveloped virion (EV), which has an additional outer membrane that is disrupted prior to fusion. Four viral proteins associated with the MV membrane facilitate attachment by binding to glycosaminoglycans or laminin on the cell surface, whereas EV attachment proteins have not yet been identified. Entry can occur at the plasma membrane or in acidified endosomes following macropinocytosis and involves actin dynamics and cell signaling. Regardless of the pathway or whether the MV or EV mediates infection, fusion is dependent on 11 to 12 non-glycosylated, transmembrane proteins ranging in size from 4- to 43-kDa that are associated in a complex. These proteins are conserved in poxviruses making it likely that a common entry mechanism exists. Biochemical studies support a two-step process in which lipid mixing of viral and cellular membranes is followed by pore expansion and core penetration.

The myristate moiety and amino terminus of vaccinia virus l1 constitute a bipartite functional region needed for entry

Vaccinia virus (VACV) L1 is a myristoylated envelope protein which is required for cell entry and the fusion of infected cells. L1 associates with members of the entry-fusion complex (EFC), but its specific role in entry has not been delineated. We recently demonstrated (Foo CH, et al., Virology 385:368-382, 2009) that soluble L1 binds to cells and blocks entry, suggesting that L1 serves as the receptor-binding protein for entry. Our goal is to identify the structural domains of L1 which are essential for its functions in VACV entry. We hypothesized that the myristate and the conserved residues at the N terminus of L1 are critical for entry. To test our hypothesis, we generated mutants in the N terminus of L1 and used a complementation assay to evaluate their ability to rescue infectivity. We also assessed the myristoylation efficiency of the mutants and their ability to interact with the EFC. We found that the N terminus of L1 constitutes a region that is critical for the infectivity of VACV and for myristoylation. At the same time, the nonmyristoylated mutants were incorporated into mature virions, suggesting that the myristate is not required for the association of L1 with the viral membrane. Although some of the mutants exhibited altered structural conformations, two mutants with impaired infectivity were similar in conformation to wild-type L1. Importantly, these two mutants, with changes at A4 and A5, undergo myristoylation. Overall, our results imply dual differential roles for myristate and the amino acids at the N terminus of L1. We propose a myristoyl switch model to describe how L1 functions.

Vaccinia mature virus fusion regulator A26 protein binds to A16 and G9 proteins of the viral entry fusion complex and dissociates from mature virions at low pH

Vaccinia mature virus enters cells through either endocytosis or plasma membrane fusion, depending on virus strain and cell type. Our previous results showed that vaccinia virus mature virions containing viral A26 protein enter HeLa cells preferentially through endocytosis, whereas mature virions lacking A26 protein enter through plasma membrane fusion, leading us to propose that A26 acts as an acid-sensitive fusion suppressor for mature virus (S. J. Chang, Y. X. Chang, R. Izmailyan R, Y. L. Tang, and W. Chang, J. Virol. 84:8422-8432, 2010). In the present study, we investigated the fusion suppression mechanism of A26 protein. We found that A26 protein was coimmunoprecipitated with multiple components of the viral entry-fusion complex (EFC) in infected HeLa cells. Transient expression of viral EFC components in HeLa cells revealed that vaccinia virus A26 protein interacted directly with A16 and G9 but not with G3, L5 and H2 proteins of the EFC components. Consistently, a glutathione S-transferase (GST)-A26 fusion protein, but not GST, pulled down A16 and G9 proteins individually in vitro. Together, our results supported the idea that A26 protein binds to A16 and G9 protein at neutral pH contributing to suppression of vaccinia virus-triggered membrane fusion from without. Since vaccinia virus extracellular envelope proteins A56/K2 were recently shown to bind to the A16/G9 subcomplex to suppress virus-induced fusion from within, our results also highlight an evolutionary convergence in which vaccinia viral fusion suppressor proteins regulate membrane fusion by targeting the A16 and G9 components of the viral EFC complex. Finally, we provide evidence that acid (pH 4.7) treatment induced A26 protein and A26-A27 protein complexes of 70 kDa and 90 kDa to dissociate from mature virions, suggesting that the structure of A26 protein is acid sensitive.

Antiviral activity of the EB peptide against zoonotic poxviruses

Background

The EB peptide is a 20-mer that was previously shown to have broad spectrum in vitro activity against several unrelated viruses, including highly pathogenic avian influenza, herpes simplex virus type I, and vaccinia, the prototypic orthopoxvirus. To expand on this work, we evaluated EB for in vitro activity against the zoonotic orthopoxviruses cowpox and monkeypox and for in vivo activity in mice against vaccinia and cowpox.

Findings

In yield reduction assays, EB had an EC₅₀ of 26.7 μM against cowpox and 4.4 μM against monkeypox. The EC₅₀ for plaque reduction was 26.3 μM against cowpox and 48.6 μM against monkeypox. A scrambled peptide had no inhibitory activity against either virus. EB inhibited cowpox in vitro by disrupting virus entry, as evidenced by a reduction of the release of virus cores into the cytoplasm. Monkeypox was also inhibited in vitro by EB, but at the attachment stage of infection. EB showed protective activity in mice infected intranasally with vaccinia when co-administered with the virus, but had no effect when administered prophylactically one day prior to infection or therapeutically one day post-infection. EB had no in vivo activity against cowpox in mice.

Conclusions

While EB did demonstrate some in vivo efficacy against vaccinia in mice, the limited conditions under which it was effective against vaccinia and lack of activity against cowpox suggest EB may be more useful for studying orthopoxvirus entry and attachment in vitro than as a therapeutic against orthopoxviruses in vivo.

Transcriptional repression and RNA silencing act synergistically to demonstrate the function of the eleventh component of the vaccinia virus entry-fusion complex

Poxviruses have an elaborate system for infecting cells comprising several proteins for attachment and a larger number dedicated to membrane fusion and entry. Thus far, 11 proteins have been identified as components of the vaccinia virus (VACV) entry-fusion complex (EFC), and 10 of these proteins have been shown to be required for entry. J5, the remaining functionally uncharacterized component of the complex, is conserved in all poxviruses, has a predicted C-terminal transmembrane domain, and is an N-terminally truncated paralog of two other EFC proteins. To determine the role of J5, we constructed a mutant that inducibly regulates J5 transcription. Although the virus yield was reduced only about 80% without inducer, the inability to isolate a J5 deletion mutant suggested an essential function. To enhance stringency, we employed RNA silencing alone and together with transcriptional repression of the inducible mutant. The yield of infectious virus was reduced 4- to 5-fold by repression, 2-fold by silencing, and 60-fold by the combination of the two. Virus particles made under the latter conditions appeared to contain a full complement of proteins excluding J5 but had very low infectivity. Further studies indicated that after binding to cells, J5-deficient virions had a defect in core entry and an inability to induce syncytium formation. In addition, we confirmed that J5 is associated with the EFC by affinity purification. These data indicate that J5 is a functional component of the EFC and highlights the advantage of combining transcriptional repression and RNA silencing for stringent reduction of gene expression.

Sequence-divergent chordopoxvirus homologs of the o3 protein maintain functional interactions with components of the vaccinia virus entry-fusion complex

Composed of 35 amino acids, O3 is the smallest characterized protein encoded by vaccinia virus (VACV) and is an integral component of the entry-fusion complex (EFC). O3 is conserved with 100% identity in all orthopoxviruses except for monkeypox viruses, whose O3 homologs have 2 to 3 amino acid substitutions. Since O3 is part of the EFC, high conservation could suggest an immutable requirement for interaction with multiple proteins. Chordopoxviruses of other genera also encode small proteins with a characteristic predicted N-terminal α-helical hydrophobic domain followed by basic amino acids and proline in the same relative genome location as that of VACV O3. However, the statistical significance of their similarity to VACV O3 is low due to the large contribution of the transmembrane domain, their small size, and their sequence diversity. Nevertheless, trans-complementation experiments demonstrated the ability of a representative O3-like protein from each chordopoxvirus genus to rescue the infectivity of a VACV mutant that was unable to express endogenous O3. Moreover, recombinant viruses expressing O3 homologs in place of O3 replicated and formed plaques as well or nearly as well as wild-type VACV. The O3 homologs expressed by the recombinant VACVs were incorporated into the membranes of mature virions and, with one exception, remained stably associated with the detergent-extracted and affinity-purified EFC. The ability of the sequence-divergent O3 homologs to coordinate function with VACV entry proteins suggests the conservation of structural motifs. Analysis of chimeras formed by swapping domains of O3 with those of other proteins indicated that the N-terminal transmembrane segment was responsible for EFC interactions and for the complementation of infectivity.

Role of sulfatide in vaccinia virus infection

BACKGROUND INFORMATION

Vaccinia virus (VACV) was used as a surrogate of variola virus (genus Orthopoxvirus), the causative agent of smallpox, to study orthopoxvirus infection. VACV infects cells via attachment and fusion of the viral membrane with the host cell membrane. Glycosphingolipids, expressed in multiple organs, are major components of lipid rafts and have been associated with the infectious route of several pathogens.

RESULTS

We demonstrate that the VACV-WR (VACV Western-Reserve strain) displays no binding to Cer (ceramide) or to Gal-Cer (galactosylceramide), but binds to a natural sulfated derivative of these molecules: the Sulf (sulfatide) 3' sulfogalactosylceramide. The interaction between Sulf and VACV-WR resulted in a time-dependent inhibition of virus infection. Virus cell attachment was the crucial step inhibited by Sulf. Electron microscopy showed that SUVs (small unilamellar vesicles) enriched in Sulf bound to VACV particles. Both the A27 and L5 viral membrane proteins were shown to interact with Sulf, indicating that they could be the major viral ligands for Sulf. Soluble Sulf was successful in preventing mortality, but not morbidity, in a lethal mouse model infection with VACV-WR.

CONCLUSIONS

Together the results suggest that Sulf could play a role as an alternate receptor for VACV-WR and probably other Orthopoxviruses.

Vaccinia extracellular virions enter cells by macropinocytosis and acid-activated membrane rupture

Double membrane-bound vaccinia extracellular virions actively promote their own macropinocytosis. This, followed by acid-induced rupture of the outer membrane in endocytic vesicles, exposes the inner membrane for fusion with the endocytic membrane and release into the cytosol.

Vaccinia virus (VACV), the model poxvirus, produces two types of infectious particles: mature virions (MVs) and extracellular virions (EVs). EV particles possess two membranes and therefore require an unusual cellular entry mechanism. By a combination of fluorescence and electron microscopy as well as flow cytometry, we investigated the cellular processes that EVs required to infect HeLa cells. We found that EV particles were endocytosed, and that internalization and infection depended on actin rearrangements, activity of Na⁺/H⁺ exchangers, and signalling events typical for the macropinocytic mechanism of endocytosis. To promote their internalization, EVs were capable of actively triggering macropinocytosis. EV infection also required vacuolar acidification, and acid exposure in endocytic vacuoles was needed to disrupt the outer EV membrane. Once exposed, the underlying MV-like particle presumably fused its single membrane with the limiting vacuolar membrane. Release of the viral core into the host cell cytosol allowed for productive infection.

Drosophila S2 cells are non-permissive for vaccinia virus DNA replication following entry via low pH-dependent endocytosis and early transcription

Vaccinia virus (VACV), a member of the chordopox subfamily of the Poxviridae, abortively infects insect cells. We have investigated VACV infection of Drosophila S2 cells, which are useful for protein expression and genome-wide RNAi screening. Biochemical and electron microscopic analyses indicated that VACV entry into Drosophila S2 cells depended on the VACV multiprotein entry-fusion complex but appeared to occur exclusively by a low pH-dependent endocytic mechanism, in contrast to both neutral and low pH entry pathways used in mammalian cells. Deep RNA sequencing revealed that the entire VACV early transcriptome, comprising 118 open reading frames, was robustly expressed but neither intermediate nor late mRNAs were made. Nor was viral late protein synthesis or inhibition of host protein synthesis detected by pulse-labeling with radioactive amino acids. Some reduction in viral early proteins was noted by Western blotting. Nevertheless, synthesis of the multitude of early proteins needed for intermediate gene expression was demonstrated by transfection of a plasmid containing a reporter gene regulated by an intermediate promoter. In addition, expression of a reporter gene with a late promoter was achieved by cotransfection of intermediate genes encoding the late transcription factors. The requirement for transfection of DNA templates for intermediate and late gene expression indicated a defect in viral genome replication in VACV-infected S2 cells, which was confirmed by direct analysis. Furthermore, VACV-infected S2 cells did not support the replication of a transfected plasmid, which occurs in mammalian cells and is dependent on all known viral replication proteins, indicating a primary restriction of DNA synthesis.

Third-generation smallpox vaccines: challenges in the absence of clinical smallpox

Smallpox, a disease caused by variola virus, is estimated to have killed hundreds of millions to billions of people before it was certified as eradicated in 1980. However, there has been renewed interest in smallpox vaccine development due in part to zoonotic poxvirus infections and the possibility of a re-emergence of smallpox, as well as the fact that first-generation smallpox vaccines are associated with relatively rare, but severe, adverse reactions in some vaccinees. An understanding of the immune mechanisms of vaccine protection and the use of suitable animal models for vaccine efficacy assessment are paramount to the development of safer and effective smallpox vaccines. This article focuses on studies aimed at understanding the immune responses elicited by vaccinia virus and the various animal models that can be used to evaluate smallpox vaccine efficacy. Harnessing this information is necessary to assess the effectiveness and potential usefulness of new-generation smallpox vaccines.

The neutralizing antibody response to the vaccinia virus A28 protein is specifically enhanced by its association with the H2 protein

The vaccinia virus (VACV) entry-fusion complex (EFC) is composed of at least nine membrane proteins. Immunization of mice with individual EFC genes induced corresponding protein-binding antibody but failed to protect against VACV intranasal challenge and only DNA encoding A28 elicited low neutralizing antibody. Because the A28 and H2 proteins interact, we determined the effect of immunizing with both genes simultaneously. This procedure greatly enhanced the amount of antibody that bound intact virions, neutralized infectivity, and provided partial protection against respiratory challenge. Neither injection of A28 and H2 plasmids at different sites or mixing A28 and H2 sera enhanced neutralizing antibody. The neutralizing antibody could be completely removed by binding to the A28 protein alone and the epitope was located in the C-terminal segment. These data suggest that the interaction of H2 with A28 stabilizes the immunogenic form of A28, mimicking an exposed region of the entry-fusion complex on infectious virions.

Lipid membranes in poxvirus replication

Poxviruses replicate in the cytoplasm, where they acquire multiple lipoprotein membranes. Although a proposal that the initial membrane arises de novo has not been substantiated, there is no accepted explanation for its formation from cellular membranes. A subsequent membrane-wrapping step involving modified trans-Golgi or endosomal cisternae results in a particle with three membranes. These wrapped virions traverse the cytoplasm on microtubules; the outermost membrane is lost during exocytosis, the middle one is lost just prior to cell entry, and the remaining membrane fuses with the cell to allow the virus core to enter the cytoplasm and initiate a new infection.

Appraising the apoptotic mimicry model and the role of phospholipids for poxvirus entry

Entry of vaccinia virus (VACV) into cells occurs by fusion with the plasma membrane and via a low pH-dependent endosomal pathway, presumably involving unidentified cellular receptors. In addition to approximately 25 viral proteins, the membrane of VACV mature virions contains several phospholipids including phosphatidylserine (PS). A recent model posits that PS flags virions as apoptotic debris to activate a common cellular uptake pathway to gain cell entry, perhaps through an interaction with a PS-specific cell surface receptor. To evaluate the apoptotic mimicry model, we reconstituted the membrane of detergent-extracted virions with several different phospholipids. Although the ability of the L-stereoisomer of PS to reconstitute infectivity was confirmed, the nonbiologically relevant D-stereoisomer of PS, and phosphatidylglycerol, which are not normally present in the virion membrane, functioned as well. Regardless of which phospholipid reconstituted infectivity, virus entry was inhibited by a neutralizing monoclonal antibody to a virion surface protein and by the drugs blebbistatin and bafilomycin A1, suggesting that in each case virus uptake was specific and occurred by a similar mechanism involving macropinocytosis and a low-pH endocytic pathway. Lipid-reconstituted and nonreconstituted, membrane-extracted virions were equally capable of binding to cells. However, the physical association of phospholipids with virus particles during membrane reconstitution correlated directly with rescue of particle infectivity and cell entry capability. Our results support a role for PS in poxvirus entry, but demonstrate that other phospholipids, not known to signal uptake of apoptotic debris, can function similarly.

Assembly and disassembly of the capsid-like external scaffold of immature virions during vaccinia virus morphogenesis

Infectious poxvirus particles are unusual in that they are brick shaped and lack symmetry. Nevertheless, an external honeycomb lattice comprised of a capsid-like protein dictates the spherical shape and size of immature poxvirus particles. In the case of vaccinia virus, trimers of 63-kDa D13 polypeptides form the building blocks of the lattice. In the present study, we addressed two questions: how D13, which has no transmembrane domain, associates with the immature virion (IV) membrane to form the lattice structure and how this scaffold is removed during the subsequent stage of morphogenesis. Interaction of D13 with the A17 membrane protein was demonstrated by immunoaffinity purification and Western blot analysis. In addition, the results of immunogold electron microscopy indicated a close association of A17 and D13 in crescents, as well as in vesicular structures when crescent formation was prevented. Further studies indicated that binding of A17 to D13 was abrogated by truncation of the N-terminal segment of A17. The N-terminal region of A17 was also required for the formation of crescent and IV structures. Disassembly of the D13 scaffold correlated with the processing of A17 by the I7 protease. When I7 expression was repressed, D13 was retained on aberrant virus particles. Furthermore, the morphogenesis of IVs to mature virions was blocked by mutation of the N-terminal but not the C-terminal cleavage site on A17. Taken together, these data indicate that A17 and D13 interactions regulate the assembly and disassembly of the IV scaffold.

Vaccinia virus strain differences in cell attachment and entry

Vaccinia virus (VACV) strain WR can enter cells by a low pH endosomal pathway or direct fusion with the plasma membrane at neutral pH. Here, we compared attachment and entry of five VACV strains in six cell lines and discovered two major patterns. Only WR exhibited pH 5-enhanced rate of entry following neutral pH adsorption to cells, which correlated with sensitivity to bafilomycin A1, an inhibitor of endosomal acidification. Entry of IHD-J, Copenhagen and Elstree strains were neither accelerated by pH 5 treatment nor prevented by bafilomycin A1. Entry of the Wyeth strain, although not augmented by pH 5, was inhibited by bafilomycin A1. WR and Wyeth were both relatively resistant to the negative effects of heparin on entry, whereas the other strains were extremely sensitive due to inhibition of cell binding. The relative sensitivities of individual vaccinia virus strains to heparin correlated inversely with their abilities to bind to and enter glycosaminoglycan-deficient sog9 cells but not other cell lines tested. These results suggested that that IHD-J, Copenhagen and Elstree have a more limited ability than WR and Wyeth to use the low pH endosomal pathway and are more dependent on binding to glycosaminoglycans for cell attachment.

Inhibition of Vaccinia virus entry by a broad spectrum antiviral peptide

Concerns about the possible use of Variola virus, the causative agent of smallpox, as a weapon for bioterrorism have led to renewed efforts to identify new antivirals against orthopoxviruses. We identified a peptide, EB, which inhibited infection by Vaccinia virus with an EC(50) of 15 microM. A control peptide, EBX, identical in composition to EB but differing in sequence, was inactive (EC50>200 microM), indicating sequence specificity. The inhibition was reversed upon removal of the peptide, and EB treatment had no effect on the physical integrity of virus particles as determined by electron microscopy. Viral adsorption was unaffected by the presence of EB, and the addition of EB post-entry had no effect on viral titers or on early gene expression. The addition of EB post-adsorption resulted in the inhibition of beta-galactosidase expression from an early viral promoter with an EC(50) of 45 microM. A significant reduction in virus entry was detected in the presence of the peptide when the number of viral cores released into the cytoplasm was quantified. Electron microscopy indicated that 88% of the virions remained on the surface of cells in the presence of EB, compared to 37% in the control (p<0.001). EB also blocked fusion-from-within, suggesting that virus infection is inhibited at the fusion step. Analysis of EB derivatives suggested that peptide length may be important for the activity of EB. The EB peptide is, to our knowledge, the first known small molecule inhibitor of Vaccinia virus entry.

Using fluorescent proteins to study poxvirus morphogenesis

Fluorescent protein (FP) fusions not only allow for the convenient visualization of a protein of -interest's subcellular localization but also permit the real-time monitoring of their subcellular trafficking. The subcellular fluorescent pattern of FP-fusions can also serve as a visual marker for various subcellular processes using either live or static microscopy. We have employed FP-fusions for the study of poxvirus morphogenesis. Fusion of FP with either a viral core protein or an extracellular virion-specific protein can serve as a visual read-out for normal poxvirus morphogenesis at the subcellular level. Recombinant viruses expressing a FP-fusion, in conjunction with the deletion of a gene involved in either morphogenesis or egress, usually display an aberrant FP pattern. Functional domains in the missing protein are then mapped by complementation in-trans followed by fluorescent microscopy for analysis of the FP pattern. The methods presented here describe how to infect and transfect cells for trans-complementation for the purpose of functional domain mapping. The imaging and analysis of these cells is described.

Engineering the vaccinia virus L1 protein for increased neutralizing antibody response after DNA immunization

Background

The licensed smallpox vaccine, comprised of infectious vaccinia virus, has associated adverse effects, particularly for immunocompromised individuals. Therefore, safer DNA and protein vaccines are being investigated. The L1 protein, a component of the mature virion membrane that is conserved in all sequenced poxviruses, is required for vaccinia virus entry into host cells and is a target for neutralizing antibody. When expressed by vaccinia virus, the unglycosylated, myristoylated L1 protein attaches to the viral membrane via a C-terminal transmembrane anchor without traversing the secretory pathway. The purpose of the present study was to investigate modifications of the gene expressing the L1 protein that would increase immunogenicity in mice when delivered by a gene gun.

Results

The L1 gene was codon modified for optimal expression in mammalian cells and potential N-glycosylation sites removed. Addition of a signal sequence to the N-terminus of L1 increased cell surface expression as shown by confocal microscopy and flow cytometry of transfected cells. Removal of the transmembrane domain led to secretion of L1 into the medium. Induction of binding and neutralizing antibodies in mice was enhanced by gene gun delivery of L1 containing the signal sequence with or without the transmembrane domain. Each L1 construct partially protected mice against weight loss caused by intranasal administration of vaccinia virus.

Conclusion

Modifications of the vaccinia virus L1 gene including codon optimization and addition of a signal sequence with or without deletion of the transmembrane domain can enhance the neutralizing antibody response of a DNA vaccine.

Vaccinia virus L1 binds to cell surfaces and blocks virus entry independently of glycosaminoglycans

L1 and A28 are vaccinia virus (VACV) envelope proteins which are essential for cellular entry. However, their specific roles during entry are unknown. We tested whether one or both of these proteins might serve as receptor binding proteins (RBP). We found that a soluble, truncated form of L1, but not A28, bound to cell surfaces independently of glycosaminoglycans (GAGs). Hence, VACV A28 is not likely to be a RBP and functions after attachment during entry. Importantly, soluble L1 inhibited both binding and entry of VACV in GAG-deficient cells, suggesting that soluble L1 blocks entry at the binding step by competing with the virions for non-GAG receptors on cells. In contrast, soluble A27, a VACV protein which attaches to GAGs but is non-essential for virus entry, inhibited binding and entry of VACV in a GAG-dependent manner. To our knowledge, this is the first report of a VACV envelope protein that blocks virus binding and entry independently of GAGs.

Expression of the A56 and K2 proteins is sufficient to inhibit vaccinia virus entry and cell fusion

Many animal viruses induce cells to fuse and form syncytia. For vaccinia virus, this phenomenon is associated with mutations affecting the A56 and K2 proteins, which form a multimer (A56/K2) on the surface of infected cells. Recent evidence that A56/K2 interacts with the entry/fusion complex (EFC) and that the EFC is necessary for syncytium formation furnishes a strong connection between virus entry and cell fusion. Among the important remaining questions are whether A56/K2 can prevent virus entry as well as cell-cell fusion and whether these two viral proteins are sufficient as well as necessary for this. To answer these questions, we transiently and stably expressed A56 and K2 in uninfected cells. Uninfected cells expressing A56 and K2 exhibited resistance to fusing with A56 mutant virus-infected cells, whereas expression of A56 or K2 alone induced little or no resistance, which fits with the need for both proteins to bind the EFC. Furthermore, transient or stable expression of A56/K2 interfered with virus entry and replication as determined by inhibition of early expression of a luciferase reporter gene, virus production, and plaque formation. The specificity of this effect was demonstrated by restoring entry after enzymatically removing a chimeric glycophosphatidylinositol-anchored A56/K2 or by binding a monoclonal antibody to A56. Importantly, the antibody disrupted the interaction between A56/K2 and the EFC without disrupting the A56-K2 interaction itself. Thus, we have shown that A56/K2 is sufficient to prevent virus entry and fusion as well as formation of syncytia through interaction with the EFC.

Vaccinia virus entry/fusion complex subunit A28 is a target of neutralizing and protective antibodies

The vaccinia virus entry/fusion complex (EFC) is comprised of at least eight transmembrane proteins that are conserved in all poxviruses. However, neither the physical structure of the EFC nor the immunogenicity of the individual components has been determined. We prepared soluble forms of two EFC components, A28 and H2, by replacing the transmembrane domain with a signal peptide and adding a polyhistidine tail. The proteins were expressed by baculoviruses, secreted from insect cells, purified by affinity chromatography and used to raise antibodies in rabbits. The antibodies recognized the viral proteins but only the antibody to recombinant A28 bound intact virions and neutralized infectivity. Analyses with a set of overlapping peptides revealed a neutralizing epitope between residues 73 and 92 of A28. Passive immunization of mice with IgG purified from the anti-A28 serum provided partial protection against a vaccinia virus intranasal challenge, whereas IgG from the anti-H2 serum did not.

The vaccinia virus fusion inhibitor proteins SPI-3 (K2) and HA (A56) expressed by infected cells reduce the entry of superinfecting virus

The orthopoxvirus SPI-3 (K2) and A56 (hemagglutinin, HA) proteins interact and together prevent cell-cell fusion. SPI-3/A56 has been proposed to prevent the superinfection of previously infected cells by reducing virus-cell fusion. Binding of mature virions of vaccinia virus (VV) to VV-infected cells was unaffected by SPI-3 or A56 on the surface of infected cells. Entry of VV into infected cells was assessed using VV-P(T7)-luc carrying the luciferase reporter under T7 control. Cells infected with VV or cowpox virus (CPV) expressing T7 RNA polymerase and lacking SPI-3 and/or A56 were superinfected with VV-P(T7)-luc, and luciferase activity was measured. Inactivation of SPI-3 or A56 from the pre-infecting virus resulted in greater luciferase expression from the superinfecting VV-P(T7)-luc. Antibody against SPI-3 present during infection with wild-type CPV-T7 increased luciferase expression from superinfecting VV-P(T7)-luc. The SPI-3/A56 complex on the infected cell surface therefore appears to reduce the entry of virions into infected cells.

The vaccinia virus gene I2L encodes a membrane protein with an essential role in virion entry

The previously unstudied vaccinia virus gene I2L is conserved in all orthopoxviruses. We show here that the 8-kDa I2 protein is expressed at late times of infection, is tightly associated with membranes, and is encapsidated in mature virions. We have generated a recombinant virus in which I2 expression is dependent upon the inclusion of tetracycline in the culture medium. In the absence of I2, the biochemical events of the viral life cycle progress normally, and virion morphogenesis culminates in the production of mature virions. However, these virions show an approximately 400-fold reduction in specific infectivity due to an inability to enter target cells. Several proteins that have been previously identified as components of an essential entry/fusion complex are present at reduced levels in I2-deficient virions, although other membrane proteins, core proteins, and DNA are encapsidated at normal levels. A preliminary structure/function analysis of I2 has been performed using a transient complementation assay: the C-terminal hydrophobic domain is essential for protein stability, and several regions within the N-terminal hydrophilic domain are essential for biological competency. I2 is thus yet another component of the poxvirus virion that is essential for the complex process of entry into target cells.

Vaccinia virus l1 protein is required for cell entry and membrane fusion

Genetic and biochemical studies have provided evidence for an entry/fusion complex (EFC) comprised of at least eight viral proteins (A16, A21, A28, G3, G9, H2, J5, and L5) that together with an associated protein (F9) participates in entry of vaccinia virus (VACV) into cells. The genes encoding these proteins are conserved in all poxviruses, are expressed late in infection, and are components of the mature virion membrane but are not required for viral morphogenesis. In addition, all but one component has intramolecular disulfides that are formed by the poxvirus cytoplasmic redox system. The L1 protein has each of the characteristics enumerated above except that it has been reported to be essential for virus assembly. To further investigate the role of L1, we constructed a recombinant VACV (vL1Ri) that inducibly expresses L1. In the absence of inducer, L1 synthesis was repressed and vL1Ri was unable to form plaques or produce infectious progeny. Unexpectedly, assembly and morphogenesis appeared normal and the noninfectious virus particles were indistinguishable from wild-type VACV as determined by transmission electron microscopy and analysis of the component polypeptides. Notably, the L1-deficient virions were able to attach to cells but the cores failed to penetrate into the cytoplasm. In addition, cells infected with vL1Ri in the absence of inducer did not form syncytia following brief low-pH treatment even though extracellular virus was produced. Coimmunoprecipitation experiments demonstrated that L1 interacted with the EFC and indirectly with F9, suggesting that L1 is an additional component of the viral entry apparatus.

A novel cellular protein, VPEF, facilitates vaccinia virus penetration into HeLa cells through fluid phase endocytosis

Vaccinia virus is a large DNA virus that infects many cell cultures in vitro and animal species in vivo. Although it has been used widely as a vaccine, its cell entry pathway remains unclear. In this study, we showed that vaccinia virus intracellular mature virions bound to the filopodia of HeLa cells and moved toward the cell body and entered the cell through an endocytic route that required a dynamin-mediated pathway but not a clathrin- or caveola-mediated pathway. Moreover, virus penetration required a novel cellular protein, vaccinia virus penetration factor (VPEF). VPEF was detected on cell surface lipid rafts and on vesicle-like structures in the cytoplasm. Both vaccinia virus and dextran transiently colocalized with VPEF, and, importantly, knockdown of VPEF expression blocked vaccinia virus penetration as well as intracellular transport of dextran, suggesting that VPEF mediates vaccinia virus entry through a fluid uptake endocytosis process in HeLa cells. Intracellular VPEF-containing vesicles did not colocalize with Rab5a or caveolin but partially colocalized with Rab11, supporting the idea that VPEF plays a role in vesicle trafficking and recycling in HeLa cells. In summary, this study characterized the mechanism by which vaccinia virus enters HeLa cells and identified a cellular factor, VPEF, that is exploited by vaccinia virus for cell entry through fluid phase endocytosis.

A conserved sequence within the H2 subunit of the vaccinia virus entry/fusion complex is important for interaction with the A28 subunit and infectivity

The recently described vaccinia virus entry/fusion complex (EFC) comprises at least eight polypeptides that are conserved in all poxviruses. Neither the structure of the complex nor the roles of individual subunits are known. Here we provide evidence for an interaction between the H2 and A28 subunits in the context of a virus infection as well as in uninfected cells transfected with plasmids expressing the corresponding genes. We focused on a highly conserved 21-amino acid-segment in H2 that is flanked by cysteine residues. The effect of amino acid substitutions within the 21-amino-acid segment was determined by an infectivity complementation assay using a conditional H2-null mutant of vaccinia virus. Mutations that had no, moderate, or large negative effects on complementation were found. The latter group included glutamic acid substitutions of leucine and individual glycines and alanine substitution of both glycines within a LGYSG sequence. Mutations with the most pronounced effect on infectivity disrupted the interaction of H2 with A28 to the greatest extent in both infected and uninfected cells. These data indicate that the LGYSG sequence is important for the interaction of H2 with A28 and suggest that this sequence is buried within the EFC complex.

Vaccinia virus A56/K2 fusion regulatory protein interacts with the A16 and G9 subunits of the entry fusion complex

Deletion of the A56R or K2L gene of vaccinia virus (VACV) results in the spontaneous fusion of infected cells to form large multinucleated syncytia. A56 and K2 polypeptides bind to one another (A56/K2) and together are required for interaction with the VACV entry fusion complex (EFC); this association has been proposed to prevent the fusion of infected cells. At least eight viral polypeptides comprise the EFC, but no information has been available regarding their interactions either with each other or with A56/K2. Utilizing a panel of recombinant VACVs designed to repress expression of individual EFC subunits, we demonstrated that A56/K2 interacted with two polypeptides: A16 and G9. Both A16 and G9 were required for the efficient binding of each to A56/K2, suggesting that the two polypeptides interact with each other within the EFC. Such an interaction was established by the copurification of A16 and G9 from infected cells under conditions in which a stable EFC complex failed to assemble and from detergent-treated lysates of uninfected cells that coexpressed A16 and G9. A recombinant VACV that expressed G9 modified with an N-terminal epitope tag induced the formation of syncytia, suggesting partial interference with the functional interaction of A56/K2 with the EFC during infection. These data suggest that A16 and G9 are physically associated within the EFC and that their interaction with A56/K2 suppresses spontaneous syncytium formation and possibly "fuse-back" superinfection of cells.

Membrane cell fusion activity of the vaccinia virus A17-A27 protein complex

Vaccinia virus enters cells by endocytosis and via a membrane fusion mechanism mediated by viral envelope protein complexes. While several proteins have been implicated in the entry/fusion event, there is no direct proof for fusogenic activity of any viral protein in heterologous systems. Transient coexpression of A17 and A27 in mammalian cells led to syncytia formation in a pH-dependent manner, as ascertained by confocal fluorescent immunomicroscopy. The pH-dependent fusion activity was identified to reside in A17 amino-terminal ectodomain after overexpression in insect cells using recombinant baculoviruses. Through the use of A17 ectodomain deletion mutants, it was found that the domain important for fusion spanned between residues 18 and 34. To further characterize A17-A27 fusion activity in mammalian cells, 293T cell lines stably expressing A17, A27 or coexpressing both proteins were generated using lentivectors. A27 was exposed on the cell surface only when A17 was coexpressed. In addition, pH-dependent fusion activity was functionally demonstrated in mammalian cells by cytoplasmic transfer of fluorescent proteins, only when A17 and A27 were coexpressed. Bioinformatic tools were used to compare the putative A17-A27 protein complex with well-characterized fusion proteins. Finally, all experimental evidence was integrated into a working model for A17-A27-induced pH-dependent cell-to-cell fusion.

Exposure of ichnovirus particles to digitonin leads to enhanced infectivity and induces fusion from without in an in vitro model system

Unlike most viruses, the mature ichnovirus particle possesses two unit membrane envelopes. Following loss of the outer membrane in vivo, nucleocapsids are believed to gain entry into the cytosol via a membrane fusion event involving the inner membrane and the plasma membrane of susceptible host cells; accordingly, experimentally induced damage to the outer membrane might be expected to increase infectivity. Here, in an attempt to develop an in vitro model system for studying ichnovirus infection, we show that digitonin-induced disruption of the virion outer membrane not only increases infectivity, but also uncovers an activity not previously associated with any polydnavirus: fusion from without.

Two distinct low-pH steps promote entry of vaccinia virus

Entry of vaccinia virus into cells occurs by an endosomal route as well as through the plasma membrane. Evidence for an endosomal pathway was based on findings that treatment at a pH of <6 of mature virions attached to the plasma membrane enhances entry, whereas inhibitors of endosomal acidification reduce entry. Inactivation of infectivity by low-pH treatment of virions prior to membrane attachment is characteristic of many viruses that use the endosomal route. Nevertheless, we show here that the exposure of unattached vaccinia virus virions to low pH at 37 degrees C did not alter their infectivity. Instead, such treatment stably activated virions as indicated by their accelerated entry upon subsequent addition to cells, as measured by reporter gene expression. Moreover, the rate of entry was not further enhanced by a second low-pH treatment following adsorption to the plasma membrane. However, the entry of virions activated prior to adsorption remained sensitive to inhibitors of endosomal acidification, whereas virions treated with low pH after adsorption were resistant. Activation of virions by low pH was closely mimicked by proteinase digestion, suggesting that the two treatments operate through a related mechanism. Although proteinase cleavage of the virion surface proteins D8 and A27 correlated with activation, mutant viruses constructed by individually deleting these genes did not exhibit an activated phenotype. We propose a two-step model of vaccinia virus entry through endosomes, in which activating or unmasking the fusion complex by low pH or by proteinase is rate limiting but does not eliminate a second low-pH step mediating membrane fusion.