Vaccinia virus A21 virion membrane protein is required for cell entry and fusion

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.

Ethacrynic and alpha-lipoic acids inhibit vaccinia virus late gene expression

Smallpox was declared eradicated in 1980. However recently, the need of agents effective against poxvirus infection has emerged again. In this paper, we report an original finding that two redox-modulating agents, the ethacrynic and α-lipoic acids (EA, LA), inhibit growth of vaccinia virus (VACV) in vitro. The effect of EA and LA was compared with those of β-mercaptoethanol, DTT and ascorbic acid, but these agents increased VACV growth in HeLa G cells. The inhibitory effects of EA and LA on the growth of VACV were further confirmed in several cell lines of different embryonic origin, in epithelial cells, fibroblasts, macrophages and T-lymphocytes.

Finally, we have analyzed the mechanism of action of the two agents. They both decreased expression of VACV late genes, as demonstrated by western blot analysis and activity of luciferase expressed under control of different VACV promoters. In contrast, they did not inhibit virus entry into the cell, expression of VACV early genes or VACV DNA synthesis.

The results suggest new directions in development of drugs effective against poxvirus infection.

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 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.

Structural basis for the binding of the neutralizing antibody, 7D11, to the poxvirus L1 protein

Medical countermeasures to prevent or treat smallpox are needed due to the potential use of poxviruses as biological weapons. Safety concerns with the currently available smallpox vaccine indicate a need for research on alternative poxvirus vaccine strategies. Molecular vaccines involving the use of proteins and/or genes and recombinant antibodies are among the strategies under current investigation. The poxvirus L1 protein, encoded by the L1R open reading frame, is the target of neutralizing antibodies and has been successfully used as a component of both protein subunit and DNA vaccines. L1-specific monoclonal antibodies (e.g., mouse monoclonal antibody mAb-7D11, mAb-10F5) with potent neutralizing activity bind L1 in a conformation-specific manner. This suggests that proper folding of the L1 protein used in molecular vaccines will affect the production of neutralizing antibodies and protection. Here, we co-crystallized the Fab fragment of mAb-7D11 with the L1 protein. The crystal structure of the complex between Fab-7D11 and L1 reveals the basis for the conformation-specific binding as recognition of a discontinuous epitope containing two loops that are held together by a disulfide bond. The structure of this important conformational epitope of L1 will contribute to the development of molecular poxvirus vaccines and also provides a novel target for anti-poxvirus drugs. In addition, the sequence and structure of Fab-7D11 will contribute to the development of L1-targeted immunotherapeutics.

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.

Resistance of a vaccinia virus A34R deletion mutant to spontaneous rupture of the outer membrane of progeny virions on the surface of infected cells

The extracellular form of vaccinia virus is referred to as an enveloped virion (EV) because it contains an additional lipoprotein membrane surrounding the infectious mature virion (MV) that must be discarded prior to cell fusion and entry. Most EVs adhere to the surface of the parent cell and mediate spread of the infection to adjacent cells. Here we show that some attached EVs have ruptured envelopes. Rupture was detected by fluorescence microscopy of unfixed and unpermeabilized cells using antibodies to the F13 and L1 proteins, which line the inner side of the EV membrane and the outer side of the MV membrane, respectively. The presence of ruptured EV membranes was confirmed by immunogold transmission electron microscopy. EVs with broken membranes were present on several cell lines examined including one deficient in glycosaminoglycans, which are thought to play a role in breakage of the EV membrane prior to fusion of the MV. No correlation was found between EVs with ruptured membranes and actin tail formation. Studies with several mutant viruses indicated that EV membranes lacking the A34 protein were unbroken. This result was consistent with other properties of A34R deletion mutants including resistance of the EV membrane to polyanions, small plaque formation and low infectivity that can be increased by disruption of the EV membrane by freezing and thawing.

Vaccinia virus temperature-sensitive mutants in the A28 gene produce non-infectious virions that bind to cells but are defective in entry

The vaccinia virus temperature-sensitive mutations Cts6 and Cts9 were mapped by marker rescue and DNA sequencing to the A28 gene. Cts6 and Cts9 contain an identical 2-bp deletion truncating the A28 protein and removing the fourth conserved cysteine near the C-terminus. Cts9 mutant virions produced at 40 degrees C were non-infectious and unable to cause cytopathic effect. However, the mutant A28 protein localized to purified mature virions (MV) at 31 degrees C and 40 degrees C. MV of Cts9 produced at 40 degrees C bound to cells but did not enter cells. Low pH treatment of Cts9-infected cells at 18 h p.i. failed to produce fusion from within at 40 degrees C, but gave fusion at 31 degrees C. Adsorption of Cts9 mutant virions to cells followed by low pH treatment showed a defect in fusion from without. The Cts9 phenotype suggests that the A28 protein is involved in both virus entry and cell-cell fusion, and supports the linkage between the two processes.

Association of vaccinia virus fusion regulatory proteins with the multicomponent entry/fusion complex

The proteins encoded by the A56R and K2L genes of vaccinia virus form a heterodimer (A56/K2) and have a fusion regulatory role as deletion or mutation of either causes infected cells to form large syncytia spontaneously. Here, we showed that syncytia formation is dependent on proteins of the recently described entry fusion complex (EFC), which are also required for virus-cell fusion and low-pH-triggered cell-cell fusion. This finding led us to consider that A56/K2 might prevent fusion by direct or indirect interaction with the EFC. To test this hypothesis, we made a panel of recombinant vaccinia viruses that have a tandem affinity purification tag attached to A56, K2, or the A28 EFC protein. Interaction between A56/K2 and the EFC was demonstrated by their copurification from detergent-treated lysates of infected cells and identification by mass spectrometry or Western blotting. In addition, a purified soluble transmembrane-deleted form of A56/K2 was shown to interact with the EFC. Tagged A56 did not interact with the EFC in the absence of K2, nor did tagged K2 interact with the EFC in the absence of A56. The finding that both A56 and K2 are required for efficient binding to the EFC fits well with prior experiments showing that mutation of either A56 or K2 results in spontaneous fusion of infected cells. Because A56 and K2 are located on the surface of infected cells, they are in position to interact with the EFC of released progeny virions and prevent back-fusion and syncytia formation.

Vaccinia virus 4c (A26L) protein on intracellular mature virus binds to the extracellular cellular matrix laminin

Vaccinia virus intracellular mature virus (IMV) binds to glycosaminoglycans (GAGs) on cells via three virion proteins, H3L, A27L, and D8L. In this study, we demonstrated that binding of IMV to BSC40 cells was competitively inhibited by soluble laminin but not by fibronectin or collagen V, suggesting that this cell surface extracellular matrix (ECM) protein may play a role in vaccinia virus entry. Moreover, IMV infection of GAG(-) sog9 cells was also inhibited by laminin, demonstrating that virion binding to laminin does not involve a prior interaction with GAGs. Furthermore, comparative envelope protein analyses of wild-type vaccinia virus strain Western Reserve, which binds to laminin, and of a mutant virus, IA27L, which does not, showed that the A26L open reading frame (ORF), encoding an envelope protein, was mutated in IA27L, resulting in A26L being absent from the IMV. Expression of the wild-type A26L ORF in IA27L resulted in laminin binding activity. Moreover, recombinant A26L protein bound to laminin in vitro with a high affinity, providing direct evidence that A26L is the laminin binding protein on IMV. In summary, these results reveal a novel role for the vaccinia viral envelope protein A26L in binding to the ECM protein laminin, an association that is proposed to facilitate IMV entry.

Entry of vaccinia virus and cell-cell fusion require a highly conserved cysteine-rich membrane protein encoded by the A16L gene

The vaccinia virus A16L open reading frame encodes a 378-amino-acid protein with a predicted C-terminal transmembrane domain and 20 invariant cysteine residues that is conserved in all sequenced members of the poxvirus family. The A16 protein was expressed late in infection and incorporated into intracellular virus particles with the N-terminal segment of the protein exposed on the surface. The cysteine residues were disulfide bonded via the poxvirus cytoplasmic redox system. Unsuccessful attempts to isolate a mutant virus with the A16L gene deleted suggested that the protein is essential for replication. To study the role of the A16 protein, we made a recombinant vaccinia virus that has the Escherichia coli lac operator system regulating transcription of the A16L gene. In the absence of inducer, A16 synthesis was repressed and plaque size and virus yield were greatly reduced. Nevertheless, virus morphogenesis occurred and normal-looking intracellular and extracellular virus particles formed. Purified virions made in the presence and absence of inducer were indistinguishable, though the latter had 60- to 100-fold-lower specific infectivity. A16-deficient virions bound to cells, but their cores did not penetrate into the cytoplasm. Furthermore, A16-deficient virions were unable to induce low-pH-triggered syncytium formation. The phenotype of the inducible A16L mutant was similar to those of mutants in which synthesis of the A21, A28, H2, or L5 membrane protein was repressed, indicating that at least five conserved viral proteins are required for entry of poxviruses into cells as well as for cell-cell fusion.

Immunity and immunological memory following smallpox vaccination

The smallpox vaccine consists of live vaccinia virus and is generally considered the gold standard of vaccines, since it is the only one that has led to the complete eradication of an infectious disease from the human population. Renewed fears that smallpox might be deliberately released in an act of bioterrorism have led to resurgence in the study of immunity and immunological memory to vaccinia virus and other poxviruses. Here we review our current understanding of memory T-cell, memory B-cell, and antibody responses to vaccinia and related poxviruses, both in animal models and human subjects. Of particular interest are recent advances in understanding protective immunity to poxviruses, quantifying immunological memory to the smallpox vaccine in humans, and identifying major vaccinia-specific T-cell and B-cell epitopes. In addition, potential mechanisms for maintenance of immunological memory are discussed.

Vaccinia virus G9 protein is an essential component of the poxvirus entry-fusion complex

The vaccinia virus G9R gene (VACWR087) encodes a protein of 340 amino acids with the following structural features that are conserved in all poxviruses: a site for N-terminal myristoylation, 14 cysteines, and a C-terminal transmembrane domain. Previous studies showed that G9 is one of eight proteins associated in a putative entry-fusion complex. Our attempt to isolate a mutant without the G9R gene was unsuccessful, suggesting that it is essential for virus replication. To further investigate its role, we constructed a recombinant vaccinia virus in which G9R is regulated by addition of an inducer. Induced G9 protein was associated with mature infectious virions and could be labeled with a membrane-impermeant biotinylation reagent, indicating surface exposure. Omission of inducer reduced the infectious-virus yield by about 1.5 logs; nevertheless, all stages of virus morphogenesis appeared normal and extracellular virions were present on the cell surface. Purified virions assembled without inducer had a specific infectivity of less than 5% of the normal level and a comparably small amount of G9, whereas their overall polypeptide composition, including other components of the entry-fusion complex, was similar to that of virions made in the presence of inducer or of wild-type virions. G9-deficient virions bound to cells, but penetration of cores into the cytoplasm and early viral RNA synthesis were barely detected, and cell-cell fusion was not triggered by low pH. Of the identified components of the multiprotein complex, G9 is the sixth that has been shown to be required for entry and membrane fusion.

Vaccinia virus F9 virion membrane protein is required for entry but not virus assembly, in contrast to the related L1 protein

All sequenced poxviruses encode orthologs of the vaccinia virus L1 and F9 proteins, which are structurally similar and share about 20% amino acid identity. We found that F9 further resembles L1 as both proteins are membrane components of the mature virion with similar topologies and induce neutralizing antibodies. In addition, a recombinant vaccinia virus that inducibly expresses F9, like a previously described L1 mutant, had a conditional-lethal phenotype: plaque formation and replication of infectious virus were dependent on added inducer. However, only immature virus particles are made when L1 is repressed, whereas normal-looking intracellular and extracellular virions formed in the absence of F9. Except for the lack of F9, the polypeptide components of such virions were indistinguishable from those of wild-type virus. These F9-deficient virions bound to cells, but their cores did not penetrate into the cytoplasm. Furthermore, cells infected with F9-negative virions did not fuse after a brief low-pH treatment, as did cells infected with virus made in the presence of inducer. In these respects, the phenotype associated with F9 deficiency was identical to that produced by the lack of individual components of a previously described poxvirus entry/fusion complex. Moreover, F9 interacted with proteins of that complex, supporting a related role. Thus, despite the structural relationships of L1 and F9, the two proteins have distinct functions in assembly and entry, respectively.

Vaccinia virus entry into cells via a low-pH-dependent endosomal pathway

Previous studies established that vaccinia virus could enter cells by fusion with the plasma membrane at neutral pH. However, low pH triggers fusion of vaccinia virus-infected cells, a hallmark of viruses that enter by the endosomal route. Here, we demonstrate that entry of mature vaccinia virions is accelerated by brief low-pH treatment and severely reduced by inhibitors of endosomal acidification, providing evidence for a predominant low-pH-dependent endosomal pathway. Entry of vaccinia virus cores into the cytoplasm, measured by expression of firefly luciferase, was increased more than 10-fold by exposure to a pH of 4.0 to 5.5. Furthermore, the inhibitors of endosomal acidification bafilomycin A1, concanamycin A, and monensin each lowered virus entry by more than 70%. This reduction was largely overcome by low-pH-induced entry through the plasma membrane, confirming the specificities of the drugs. Entry of vaccinia virus cores with or without brief low-pH treatment was visualized by electron microscopy of thin sections of immunogold-stained cells. Although some virus particles fused with the plasma membrane at neutral pH, 30 times more fusions and a greater number of cytoplasmic cores were seen within minutes after low-pH treatment. Without low-pH exposure, the number of released cores lagged behind the number of virions in vesicles until 30 min posttreatment, when they became approximately equal, perhaps reflecting the time of endosome acidification and virus fusion. The choice of two distinct pathways may contribute to the ability of vaccinia virus to enter a wide range of cells.

Protein composition of the vaccinia virus mature virion

The protein content of vaccinia virus mature virions, purified by rate zonal and isopycnic centrifugations and solubilized by SDS or a solution of urea and thiourea, was determined by the accurate mass and time tag technology which uses both tandem mass spectrometry and Fourier transform-ion cyclotron resonance mass spectrometry to detect tryptic peptides separated by high-resolution liquid chromatography. Eighty vaccinia virus-encoded proteins representing 37% of the 218 genes annotated in the complete genome sequence were detected in at least three analyses. Ten proteins accounted for approximately 80% of the virion mass. Thirteen identified proteins were not previously reported as components of virions. On the other hand, 8 previously described virion proteins were not detected here, presumably due to technical reasons including small size and hydrophobicity. In addition to vaccinia virus-encoded proteins, 24 host proteins omitting isoforms were detected. The most abundant of these were cytoskeletal proteins, heat shock proteins and proteins involved in translation.

The envelope G3L protein is essential for entry of vaccinia virus into host cells

The vaccinia virus G3L/WR079 gene encodes a conserved protein with a predicted transmembrane domain. Our proteomic analyses of vaccinia virus revealed that G3L protein is incorporated into intracellular mature virus; however, the function of G3L protein in the vaccinia virus life cycle has not been investigated. In this study, a recombinant vaccinia virus, viG3L, expressing G3L protein under IPTG (isopropyl-beta-d-thiogalactopyranoside) regulation was constructed. Under permissive conditions when G3L protein was expressed, the vaccinia virus life cycle proceeded normally, resulting in plaque formation in BSC40 cells. In contrast, under nonpermissive conditions when G3L protein expression was repressed, no plaques were formed, showing that G3L protein is essential for vaccinia virus growth in cell cultures. In infected cells when G3L protein was not expressed, the formation of intracellular mature virus (IMV) and cell-associated enveloped virus occurred normally, showing that G3L protein is not required for virion morphogenesis. IMV particles containing (G3L(+)) or lacking (G3L(-)) G3L protein were purified and were found to be indistinguishable on microscopic examination. Both G3L(+) and G3L(-) IMV bound to HeLa cells; however, G3L(-) IMV failed to enter the cells, showing that G3L protein is required for IMV penetration into cells. Finally, G3L protein was required for fusion of the infected cells under low-pH treatment. Thus, our results provide direct evidence that G3L is an essential component of the vaccinia virus fusion complex, in addition to the previously reported A28, H2, L5, A21, and A16 proteins.

In a nutshell: structure and assembly of the vaccinia virion

Poxviruses comprise a large family of viruses characterized by a large, linear dsDNA genome, a cytoplasmic site of replication and a complex virion morphology. The most notorious member of the poxvirus family is variola, the causative agent of smallpox. The laboratory prototype virus used for the study of poxviruses is vaccinia, the virus that was used as a live, naturally attenuated vaccine for the eradication of smallpox. Both the morphogenesis and structure of poxvirus virions are unique among viruses. Poxvirus virions apparently lack any of the symmetry features common to other viruses such as helical or icosahedral capsids or nucleocapsids. Instead poxvirus virions appear as "brick shaped" or "ovoid" membrane-bound particles with a complex internal structure featuring a walled, biconcave core flanked by "lateral bodies." The virion assembly pathway involves a remarkable fabrication of membrane-containing crescents and immature virions, which evolve into mature virions in a process that is unparalleled in virology. As a result of significant advances in poxvirus genetics and molecular biology during the past 15 years, we can now positively identify over 70 specific gene products contained in poxvirus virions, and we can describe the effects of mutations in over 50 specific genes on poxvirus assembly. This review summarizes these advances and attempts to assemble them into a comprehensible and thoughtful picture of poxvirus structure and assembly.

The structure of G4, the poxvirus disulfide oxidoreductase essential for virus maturation and infectivity

The possibility of the release of smallpox virus into a predominantly nonimmunized population highlights the importance of understanding poxvirus biology. Poxviruses encode a conserved pathway that is required to oxidize disulfide bonds in nascent viral proteins that fold in the reducing environment of the eukaryotic host cytoplasm. We present the structure of the last enzyme of the vaccinia virus pathway, G4, which is almost identical in smallpox virus. G4 catalyzes the formation of disulfide bonds in proteins that are critical for virus maturation and host cell infection. G4 contains a thioredoxin fold and a Cys-X-X-Cys active site. In solution, G4 monomers and dimers are observed. In the crystal, G4 is found as a dimer that buries 4,500 A(2) in the interface and occludes the active site, which could protect the reactive disulfide from reduction in the cytoplasm. The structure serves as a model for drug design targeting viral disulfide bond formation.

Ligand-induced and nonfusogenic dissolution of a viral membrane

Hitherto, all enveloped viruses were thought to shed their lipid membrane during entry into cells by membrane fusion. The extracellular form of Vaccinia virus has two lipid envelopes surrounding the virus core, and consequently a single fusion event will not deliver a naked core into the cell. Here we report a previously underscribed mechanism in which the outer viral membrane is disrupted by a ligand-induced nonfusogenic reaction, followed by the fusion of the inner viral membrane with the plasma membrane and penetration of the virus core into the cytoplasm. The dissolution of the outer envelope depends on interactions with cellular polyanionic molecules and requires the virus glycoproteins A34 and B5. This discovery represents a remarkable example of how viruses manipulate biological membranes, solves the topological problem of how a double-enveloped virus enters cells, reveals a new effect of polyanions on viruses, and provides a therapeutic approach for treatment of poxvirus infections, such as smallpox.

Vaccinia virus proteome: identification of proteins in vaccinia virus intracellular mature virion particles

Vaccinia virus is a large enveloped poxvirus with more than 200 genes in its genome. Although many poxvirus genomes have been sequenced, knowledge of the host and viral protein components of the virions remains incomplete. In this study, we used gel-free liquid chromatography and tandem mass spectroscopy to identify the viral and host proteins in purified vaccinia intracellular mature virions (IMV). Analysis of the proteins in the IMV showed that it contains 75 viral proteins, including structural proteins, enzymes, transcription factors, and predicted viral proteins not known to be expressed or present in the IMV. We also determined the relative abundances of the individual protein components in the IMV. Finally, 23 IMV-associated host proteins were also identified. This study provides the first comprehensive structural analysis of the infectious vaccinia virus IMV.

Poxvirus entry and membrane fusion

The study of poxvirus entry and membrane fusion has been invigorated by new biochemical and microscopic findings that lead to the following conclusions: (1) the surface of the mature virion (MV), whether isolated from an infected cell or by disruption of the membrane wrapper of an extracellular virion, is comprised of a single lipid membrane embedded with non-glycosylated viral proteins; (2) the MV membrane fuses with the cell membrane, allowing the core to enter the cytoplasm and initiate gene expression; (3) fusion is mediated by a newly recognized group of viral protein components of the MV membrane, which are conserved in all members of the poxvirus family; (4) the latter MV entry/fusion proteins are required for cell to cell spread necessitating the disruption of the membrane wrapper of extracellular virions prior to fusion; and furthermore (5) the same group of MV entry/fusion proteins are required for virus-induced cell-cell fusion. Future research priorities include delineation of the roles of individual entry/fusion proteins and identification of cell receptors.

Poxvirus multiprotein entry-fusion complex

Poxviruses have evolved elaborate mechanisms for cell entry, assembly, and exocytosis. Recently, four vaccinia virus membrane proteins, namely A21, A28, H2 and L5, were reported to be necessary for cell entry and virus-induced cell-cell fusion but not for virion morphogenesis or attachment of virus particles to cells. Using immunoaffinity purification followed by mass spectrometry, we now show that these four proteins as well as four additional previously uncharacterized putative membrane proteins (A16, G3, G9, and J5) form a stable complex. These proteins fall into two groups: A21, A28, G3, H2, and L5 have an N-terminal transmembrane domain, 0-2 intramolecular disulfide bonds, and no sequence similarity, whereas A16, G9, and J5 have a C-terminal transmembrane domain and 4-10 predicted disulfide bonds and are homologous. Studies with conditional-lethal null mutants indicated that the viral membrane was crucial for assembly of the complex and that the absence of individual polypeptide components profoundly decreased complex formation or stability, suggesting a complicated interaction network. Analysis of purified virions, however, demonstrated that the polypeptides of the complex trafficked independently to the viral membrane even under conditions in which the complex itself could not be isolated. All eight proteins comprising the entry-fusion complex are conserved in all poxviruses, suggesting that they have nonredundant functions and that the basic entry mechanism evolved before the division between vertebrate and invertebrate poxvirus species.

The product of the vaccinia virus L5R gene is a fourth membrane protein encoded by all poxviruses that is required for cell entry and cell-cell fusion

The L5R gene of vaccinia virus is conserved among all sequenced members of the Poxviridae but has no predicted function or recognized nonpoxvirus homolog. Here we provide the initial characterization of the L5 protein. L5 is expressed following DNA replication with kinetics typical of a viral late protein, contains a single intramolecular disulfide bond formed by the virus-encoded cytoplasmic redox pathway, and is incorporated into intracellular mature virus particles, where it is exposed on the membrane surface. To determine whether L5 is essential for virus replication, we constructed a mutant that synthesizes L5 only in the presence of an inducer. The mutant exhibited a conditional-lethal phenotype, as cell-to-cell virus spread and formation of infectious progeny were dependent on the inducer. Nevertheless, all stages of replication occurred in the absence of inducer and intracellular and extracellular progeny virions appeared morphologically normal. Noninfectious virions lacking L5 could bind to cells, but the cores did not enter the cytoplasm. In addition, virions lacking L5 were unable to mediate low-pH-triggered cell-cell fusion from within or without. The phenotype of the L5R conditional lethal mutant is identical to that of recently described mutants in which expression of the A21, A28, and H2 genes is repressed. Thus, L5 is the fourth component of the poxvirus cell entry/fusion apparatus that is required for entry of both the intracellular and extracellular infectious forms of vaccinia virus.