The function of the vaccinia hemagglutinin in the proteolytic activation of infectivity

Selective modulation of cell surface proteins during vaccinia infection: A resource for identifying viral immune evasion strategies

The interaction between immune cells and virus-infected targets involves multiple plasma membrane (PM) proteins. A systematic study of PM protein modulation by vaccinia virus (VACV), the paradigm of host regulation, has the potential to reveal not only novel viral immune evasion mechanisms, but also novel factors critical in host immunity. Here, >1000 PM proteins were quantified throughout VACV infection, revealing selective downregulation of known T and NK cell ligands including HLA-C, downregulation of cytokine receptors including IFNAR2, IL-6ST and IL-10RB, and rapid inhibition of expression of certain protocadherins and ephrins, candidate activating immune ligands. Downregulation of most PM proteins occurred via a proteasome-independent mechanism. Upregulated proteins included a decoy receptor for TRAIL. Twenty VACV-encoded PM proteins were identified, of which five were not recognised previously as such. Collectively, this dataset constitutes a valuable resource for future studies on antiviral immunity, host-pathogen interaction, poxvirus biology, vector-based vaccine design and oncolytic therapy.

Vaccinia virus (VACV) is the vaccine used to eradicate smallpox and an excellent model for studying host-pathogen interactions. Many VACV-mediated immune evasion strategies are known, however how immune cells recognise VACV-infected cells is incompletely understood because of the complexity of surface proteins regulating such interactions. Here, a systematic study of proteins on the cell surface at different times during infection with VACV is presented. This shows not only the precise nature and kinetics of appearance of VACV proteins, but also the selective alteration of cellular surface proteins. The latter thereby identified potential novel immune evasion strategies and host proteins regulating immune activation. Comprehensive comparisons with published datasets provided further insight into mechanisms used to regulate surface protein expression. Such comparisons also identified proteins that are targeted by both VACV and human cytomegalovirus (HCMV), and which are therefore likely to represent host proteins regulating immune recognition and activation. Collectively, this work provides a valuable resource for studying viral immune evasion mechanisms and novel host proteins critical in host immunity.

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.

Sequence analysis of haemagglutinin gene of camelpox viruses shows deletion leading to frameshift: Circulation of diverse clusters among camelpox viruses

Orthopoxviruses (OPVs) have broad host range infecting a variety of species along with gene-specific determinants. Several genes including haemagglutinin (HA) are used for differentiation of OPVs. Among poxviruses, OPVs are sole members encoding HA protein as part of extracellular enveloped virion membrane. Camelpox virus (CMLV) causes an important contagious disease affecting mainly young camels, endemic to Indian subcontinent, Africa and the Middle East. This study describes the sequence features and phylogenetic analysis of HA gene (homologue of VACV A56R) of Indian CMLV isolates. Comparative analysis of CMLV HA gene revealed conserved nature within CMLVs but considerable variability was observed between various species of OPVs. Most Indian CMLV isolates showed 99.5%-100% and 96.3%-100% identity, at nucleotide (nt) and amino acid (aa) levels respectively, among themselves and with CMLV-M96 strain. Importantly, Indian CMLV strains along with CMLV-M96 showed deletion of seven nucleotides resulting in frameshift mutation at C-terminus of HA protein. Phylogenetic analysis displayed distinct clustering among CMLVs which might point to the circulation of diverse CMLV strains in nature. Despite different host specificity of OPVs, comparative sequence analysis of HA protein showed highly conserved N-terminal Ig V-set functional domain with tandem repeats. Understanding of molecular diversity of CMLVs and structural domains of HA protein will help in the elucidation of molecular mechanisms for immune evasion and design of novel antivirals for OPVs.

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.

The vaccinia virus A56 protein: a multifunctional transmembrane glycoprotein that anchors two secreted viral proteins

The vaccinia virus A56 protein was one of the earliest-described poxvirus proteins with an identifiable activity. While originally characterized as a haemagglutinin protein, A56 has other functions as well. The A56 protein is capable of binding two viral proteins, a serine protease inhibitor (K2) and the vaccinia virus complement control protein (VCP), and anchoring them to the surface of infected cells. This is important; while both proteins have biologically relevant functions at the cell surface, neither one can locate there on its own. The A56-K2 complex reduces the amount of virus superinfecting an infected cell and also prevents the formation of syncytia by infected cells; the A56-VCP complex can protect infected cells from complement attack. Deletion of the A56R gene results in varying effects on vaccinia virus virulence. In addition, since the gene encoding the A56 protein is non-essential, it can be used as an insertion point for foreign genes and has been deleted in some viruses that are in clinical development as oncolytic agents.

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.

Interactions between vaccinia virus IEV membrane proteins and their roles in IEV assembly and actin tail formation

The intracellular enveloped form of vaccinia virus (IEV) induces the formation of actin tails that are strikingly similar to those seen in Listeria and Shigella infections. In contrast to the case for Listeria and Shigella, the vaccinia virus protein(s) responsible for directly initiating actin tail formation remains obscure. However, previous studies with recombinant vaccinia virus strains have suggested that the IEV-specific proteins A33R, A34R, A36R, B5R, and F13L play an undefined role in actin tail formation. In this study we have sought to understand how these proteins, all of which are predicted to have small cytoplasmic domains, are involved in IEV assembly and actin tail formation. Our data reveal that while deletion of A34R, B5R, or F13L resulted in a severe reduction in IEV particle assembly, IEVs formed by the DeltaB5R and DeltaF13L deletion strains, but not DeltaA34R, were still able to induce actin tails. The DeltaA36R deletion strain produced normal amounts of IEV particles, although these were unable to induce actin tails. Using several different approaches, we demonstrated that A36R is a type Ib membrane protein with a large, 195-amino-acid cytoplasmic domain exposed on the surface of IEV particles. Finally, coimmunoprecipitation experiments demonstrated that A36R interacts with A33R and A34R but not with B5R and that B5R forms a complex with A34R but not with A33R or A36R. Using extracts from DeltaA34R- and DeltaA36R-infected cells, we found that the interaction of A36R with A33R and that of A34R with B5R are independent of A34R and A36R, respectively. We conclude from our observations that multiple interactions between IEV membrane proteins exist which have important implications for IEV assembly and actin tail formation. Furthermore, these data suggest that while A34R is involved in IEV assembly and organization, A36R is critical for actin tail formation.

The genomic sequence analysis of the left and right species-specific terminal region of a cowpox virus strain reveals unique sequences and a cluster of intact ORFs for immunomodulatory and host range proteins

Sequencing and computer analysis of the left (52,283 bp) and right (49,649 bp) variable DNA regions of the cowpox virus strain GRI-90 (CPV-GRI) has revealed 51 and 37 potential open reading frames (ORFs), respectively. Comparison of the structure-function organization of these DNA regions of CPV-GRI with those previously published for corresponding regions of genomes of vaccinia virus, strains Copenhagen (VAC-COP) and Western Reserve (VAC-WR); and variola major virus, strains India-1967 (VAR-IND), Bangladesh-1975 (VAR-BSH); and alastrim variola minor virus, strain Garcia-1966 (VAR-GAR), was performed. Within the left terminal region under study, an extended DNA sequence (14,171 bp), unique to CPV, has been found. Within the right region of the CPV-GRI genome two segments, which are unique to CPV DNA (1579 and 3585 bp) have been found. Numerous differences have been revealed in the genetic structure of CPV-GRI DNA regions, homologous to fragments of the genomes of the above-mentioned orthopoxvirus strains. A cluster of ORFs with structural similarity ot immunomodulatory and host range function of other poxviruses have also been detected. A comparison of the sequences of ORF B, crmA, crmB, crmC, IMP, and CHO hr genes of CPV Brighton strain (CPV-BRI) with the corresponding genes in strain GRI-90 have revealed an identity at the amino acid level ranging from 82 to 96% between the two strains. The findings are significant in light of the recent demonstration of CPV as an important poxvirus model system to probe the precise in vivo role(s) of the unique virally encoded immunomodulatory proteins. Also, the presence of a complete and intact repertoire of immunomodulatory proteins, ring canal proteins family, and host range genes indicates that CPV may have been the most ancient of all studied orthopoxviruses.

Palmitylation of the vaccinia virus 37-kDa major envelope antigen. Identification of a conserved acceptor motif and biological relevance

Computer-assisted alignment of known palmitylproteins was used to identify a potential peptide motif, TMDX1-12AAC(C)A (TMD, transmembrane domain; X, any amino acid; C, cysteine acceptor residues; A, aliphatic residue) responsible for directing internal palmitylation of the vaccinia virus 37-kDa major envelope antigen, p37. Site-directed mutagenesis was used to confirm this motif as the site of modification and to produce a nonpalmitylated version of the p37 protein. Comparative phenotypic analysis of the wild-type and mutant p37 alleles confirmed that the p37 protein is involved in viral envelopment and egress, and suggested that attachment of the palmitate moiety was essential for correct intracellular targeting and protein function.

Functional organization of variola major and vaccinia virus genomes

Comparison of the genomic organization of variola and vaccinia viruses has been carried out. Molecular factors of virulence of these viruses is the focus of this review. Possible roles of the genes of soluble cytokine receptors, complement control proteins, factors of virus replication, and dissemination in vivo for variola virus pathogenesis are discussed. The existence of "buffer" genes in the vaccinia virus genome is proposed.

Analysis of the nucleotide sequence of 53 kbp from the right terminus of the genome of variola major virus strain India-1967

Sequencing and computer analysis of a variola major virus strain India-1967 (VAR-IND) genome segment (53,018 bp) from the right terminal region has been carried out. Fifty-nine potential open reading frames (ORFs) of over 60 amino acid residues were identified. Structure-function organization of the VAR-IND DNA segment was compared with the previously reported sequences from the analogous genomic regions of vaccinia virus strains Copenhagen (VAC-COP) and Western Reserve (VAC-WR) and variola virus strain Harvey (VAR-HAR). Multiple differences between VAR-IND and the strains of VAC but the high identity of VAR-IND with VAR-HAR in the genetic maps are revealed. Possible functions of the predicted viral proteins and the effect of their differences on the features of orthopoxviruses are discussed.

Identification and functional analysis of the fowlpox virus homolog of the vaccinia virus p37K major envelope antigen gene

A fowlpox virus (FPV) gene with homology to the vaccinia virus p37K major envelope antigen gene was identified and sequenced. The predicted product has a molecular weight of 43,018 Da (p43K). The FPV p43K gene has 37.5% identity with its vaccinia counterpart and higher homology with a molluscum contagiosum virus gene (42.6% identity). Based on upstream sequences, p43K appears to be regulated as a late gene. Recombinant FPV were generated in which a large portion of p43K was replaced by the Escherichia coli lacZ gene. These recombinants failed to produce visible plaques under standard conditions. After prolonged incubation the microplaques developed into small macroscopic plaques. Plaques were purified on the basis of lacZ expression. Single-cycle growth curves comparing the p43K-deleted recombinant (designated fJd43Z) with parental FPV showed that the two viruses produce identical amounts of intracellular virions, but that fJd43Z released 20-fold fewer infectious particles into the medium. CsCl gradient centrifugation of [3H]thymidine-labeled virus was employed to examine differences in the production of physical particles. The two viruses produced equivalent levels of intracellular virions, but fJd43Z failed to produce detectable levels of released particles. FPV p43K is therefore involved in the release of virions from infected cells.

NYVAC: a highly attenuated strain of vaccinia virus

A highly attenuated vaccinia virus strain, NYVAC (vP866), was derived from a plaque-cloned isolate of the Copenhagen vaccine strain by the precise deletion of 18 open reading frames (ORFs) from the viral genome. Among the ORFs deleted from NYVAC (vP866) are two genes involved in nucleotide metabolism, the thymidine kinase (ORF J2R) and the large subunit of the ribonucleotide reductase (ORF I4L); the gene encoding the viral hemagglutinin (ORF A56R); the remnant (ORF A26L) of a highly expressed gene responsible for the formation of A-type inclusion bodies; the disrupted gene (ORFs B13R/B14R) normally encoding a serine protease inhibitor; and a block of 12 ORFs bounded by two known viral host range regulatory functions (ORFs C7L through K1L). Within this block a secretory protein (ORF N1L) implicated in viral virulence and a functional complement 4b binding protein (ORF C3L) are encoded. The ORFs were deleted in a manner which prevents the synthesis of undesirable novel gene products. The attenuation characteristics of the derived NYVAC strain were compared in in vitro and in vivo studies with those of the Western Reserve (WR) laboratory strain, the New York City Board of Health vaccine strain (Wyeth), the parental plaque-cloned isolate (VC-2) of the Copenhagen vaccine strain used to derive NYVAC, and the avipox virus canarypox (ALVAC), which is naturally restricted for replication to avian species. The NYVAC strain was demonstrated to be highly attenuated by the following criteria: (a) no detectable induration or ulceration at the site of inoculation on rabbit skin; (b) rapid clearance of infectious virus from the intradermal site of inoculation on rabbit skin; (c) absence of testicular inflammation in nude mice; (d) greatly reduced virulence as demonstrated by the results of intracranial challenge of both 3-week-old or newborn mice; (e) greatly reduced pathogenicity and failure to disseminate in immunodeficient (nude or cyclophosphamide treated) mice; and (f) dramatically reduced ability to replicate on a variety of human tissue culture cells. Despite these highly attenuated characteristics, the NYVAC strain, as a vector, retains the ability to induce strong immune responses to extrinsic antigens.

An orthopoxvirus serpinlike gene controls the ability of infected cells to fuse

Most orthopoxviruses encode a functional hemagglutinin (HA), which is nonessential for virus growth in cell culture. However, inactivation of the HA gene leads to the formation of polykaryocytes (syncytia) by fusion of infected cells at neutral pH. Fusion is not observed when a functional HA gene is present. Deletion of open reading frames (ORFs) K2, K3, and K4 within the HindIII K fragment of the HA-positive (HA+) vaccinia virus strain WR also led to fusion of cells upon infection at neutral pH. A novel ORF inactivation procedure utilizing the polymerase chain reaction was used to specifically implicate the K2 ORF in this phenomenon. The K2 ORF (the viral SPI-3 gene) encodes a protein resembling serine protease inhibitors (serpins). Inactivation of the SPI-3 gene in any of the HA+ orthopoxviruses tested caused infected cells to fuse in a manner which appeared identical to that seen for HA- mutants, although fusion was most pronounced with cowpox virus. SPI-3-negative strains fused despite the fact that the HA was expressed and processed normally, i.e., cells infected with SPI-3 mutants remained functionally hemadsorption positive, and analysis of the HA protein by Western immunoblot suggested that posttranslational modifications of the HA protein appeared normal. Fusion triggered by SPI-3 mutants, like that for HA- mutants, was inhibited by the monoclonal antibody C3 directed against the vaccinia virus 14-kDa envelope protein. Therefore SPI-3- and HA-mediated fusion share a requirement for the 14-kDa protein, suggesting linkage of the seemingly disparate SPI-3 and HA genes through a common pathway which normally acts to prevent fusion of cells infected with wild-type virus.

Characterization of vaccinia virus glycoproteins by monoclonal antibody precipitation

Monoclonal antibodies were used to characterize vaccinia virus glycoproteins known to be incorporated into the envelope of extracellular enveloped vaccinia (EEV) virus. The 89K hemagglutinin, 42K, and 23-28K glycoproteins were predominantly expressed as late vaccinia proteins. The 89K glycoprotein was sulfated and phosphorylated but not acylated. 89K precursor proteins of 32K, 41.5K, and 52K were detected. The former had a molecular weight expected from the deduced amino acid sequence of the hemagglutinin gene. A 76K glycoprotein that did not contain methionine and was not sulfated or phosphorylated was precipitated late in infection by the anti-hemagglutinin monoclonal antibody. The appearance of this protein was inhibited by rifampicin and it may thus result from 89K cleavage. A 220K complex contained some or all of the hemagglutinin gene products linked by disulfide bonds. The 42K glycoprotein was not sulfated or phosphorylated but was acylated. This glycoprotein was disulfide bonded with the EEV 37K nonglycosylated envelope protein. The 23-28K glycoprotein was not sulfated but was both phosphorylated and acylated. The 23-28K glycoprotein group of five proteins had a common protein backbone that was differentially glycosylated. Pulse-chase, glycosylation inhibition with tunicamycin, and glycosidase experiments established that the precursor to the 23-28K glycoproteins was a 21K protein. Members of this protein family formed dimers of approximately 55K through disulfide bonds.

Extracellular vaccinia virus formation and cell-to-cell virus transmission are prevented by deletion of the gene encoding the 37,000-Dalton outer envelope protein

There are two types of infectious vaccinia virus particles: intracellular naked virions and extracellular enveloped virions (EEV). To determine the biological role of the enveloped form of vaccinia virus, we produced and characterized a mutant that is defective in EEV formation. The strategy involved replacement by homologous recombination of the gene F13L, encoding a 37,000-Da protein (VP37) that is specific for the outer envelope of EEV, with a selectable antibiotic resistance marker, the Escherichia coli gpt gene. Initial experiments, however, suggested that such a mutation was lethal or prevented plaque formation. By employing a protocol consisting of high-multiplicity passages of intracellular virus from the transfected cells and then limiting dilution cloning, we succeeded in isolating the desired mutant, which was defective in production of plaques and extracellular virus but made normal amounts of intracellular naked virions. Electron microscopic examination indicated that the mutant virus particles, unlike wild type, were neither wrapped with Golgi-derived membranes nor associated with the cell surface. The absence of VP37 did not prevent the transport of the viral hemagglutinin to the plasma membrane but nevertheless abrogated both low-pH- and antibody-mediated cell fusion. These results indicate that VP37 is required for EEV formation and also plays a critical role in the local cell-to-cell transmission of vaccinia virus, perhaps via enveloped virions attached to or released from the cell membrane. By contrast, a mutated virus with a deletion of the K4L open reading frame, which is a homolog of the VP37 gene, was not defective in formation of plaques or EEV.

The fusion and hemagglutinin-neuraminidase glycoproteins of human parainfluenza virus 3 are both required for fusion

Recombinant vaccinia viruses, VF and VHN, expressing the fusion (F) and hemagglutinin-neuraminidase (HN) glycoproteins of human parainfluenza virus 3 (HPIV3) were constructed. Infection of HeLa T4 cells with VF and VHN led to the synthesis of glycoproteins, with the correct apparent molecular weights, that were recognized by monoclonal antibodies specific for HPIV3F and HN. The HN glycoprotein was present on the surface of cells infected with VHN and these cells demonstrated both hemadsorbing and neuraminidase activities. The F glycoprotein was present in cleaved and uncleaved forms and was also expressed on the surface of VF-infected cells. Fusion activity, however, as evidenced by syncytium formation and lysis of human erythrocytes, could only be demonstrated when HeLa T4 cells were coinfected with VF and VHN. Fusion events that are mediated by HPIV3, therefore, require both the F and HN glycoproteins.