Structural and functional characterization of a cell surface binding protein of vaccinia virus

Evaluation of antibody responses against the whole virions of goatpox and sheeppox viruses after subcutaneous immunization of rabbits

Antibody development is the integral process of generating and characterizing an antibody. It commences by inoculating the antigen of interest into laboratory animals, allowing the immune system develops large quantities of antibodies. This was aimed at developing antibodies against the virion of Goatpox and Sheeppox virus vaccines. The ability of Goatpox and Sheeppox vaccines was assessed. Regarding this study, the antibody titers against both Goatpox and Sheeppox viruses was increased in the same manner. The amount of IgG was determined to be 2.29 μg/μl and 2.18 μg/μl against virions of Goatpox virus and Sheeppox respectively. The purified IgG was analyzed by SDS-PAGE. Different bands of the purified antibodies were clearly visualized, and the molecular weight of IgG was estimated to be 67 kDa and 25 kDa. Additionally, antigen/antibody binding was confirmed by Western blot using GTPV A27 antigen. No significant differences in antibody titers were observed between the two groups (p < 0, 05).

Characterization of an In Vivo Neutralizing Anti-Vaccinia Virus D8 Single-Chain Fragment Variable (scFv) from a Human Anti-Vaccinia Virus-Specific Recombinant Library

A panel of potent neutralizing antibodies are protective against orthopoxvirus (OPXV) infections. For the development of OPXV-specific recombinant human single-chain antibodies (scFvs), the IgG repertoire of four vaccinated donors was amplified from peripheral B-lymphocytes. The resulting library consisted of ≥4 × 10⁸ independent colonies. The immuno-screening against vaccinia virus (VACV) Elstree revealed a predominant selection of scFv clones specifically binding to the D8 protein. The scFv-1.2.2.H9 was engineered into larger human scFv-Fc-1.2.2.H9 and IgG1-1.2.2.H9 formats to improve the binding affinity and to add effector functions within the human immune response. Similar binding kinetics were calculated for scFv-1.2.2.H9 and scFv-Fc-1.2.2.H9 (1.61 nM and 7.685 nM, respectively), whereas, for IgG1-1.2.2.H9, the Michaelis-Menten kinetics revealed an increased affinity of 43.8 pM. None of the purified recombinant 1.2.2.H9 formats were able to neutralize VACV Elstree in vitro. After addition of 1% human complement, the neutralization of ≥50% of VACV Elstree was achieved with 0.0776 µM scFv-Fc-1.2.2.H9 and 0.01324 µM IgG1-1.2.2.H9, respectively. In an in vivo passive immunization NMRI mouse model, 100 µg purified scFv-1.2.2.H9 and the IgG1-1.2.2.H9 partially protected against the challenge with 4 LD₅₀ VACV Munich 1, as 3/6 mice survived. In contrast, in the scFv-Fc-1.2.2.H9 group, only one mouse survived the challenge.

Comparative Pathogenesis, Genomics and Phylogeography of Mousepox

Ectromelia virus (ECTV), the causative agent of mousepox, has threatened laboratory mouse colonies worldwide for almost a century. Mousepox has been valuable for the understanding of poxvirus pathogenesis and immune evasion. Here, we have monitored in parallel the pathogenesis of nine ECTVs in BALB/cJ mice and report the full-length genome sequence of eight novel ECTV isolates or strains, including the first ECTV isolated from a field mouse, ECTV-MouKre. This approach allowed us to identify several genes, absent in strains attenuated through serial passages in culture, that may play a role in virulence and a set of putative genes that may be involved in enhancing viral growth in vitro. We identified a putative strong inhibitor of the host inflammatory response in ECTV-MouKre, an isolate that did not cause local foot swelling and developed a moderate virulence. Most of the ECTVs, except ECTV-Hampstead, encode a truncated version of the P4c protein that impairs the recruitment of virions into the A-type inclusion bodies, and our data suggest that P4c may play a role in viral dissemination and transmission. This is the first comprehensive report that sheds light into the phylogenetic and geographic relationship of the worldwide outbreak dynamics for the ECTV species.

The Vaccinia Virus H3 Envelope Protein, a Major Target of Neutralizing Antibodies, Exhibits a Glycosyltransferase Fold and Binds UDP-Glucose

The highly conserved H3 poxvirus protein is a major target of the human antibody response against poxviruses and is likely a key contributor to protection against infection. Here, we present the crystal structure of H3 from vaccinia virus at a 1.9-Å resolution. H3 looks like a glycosyltransferase, a family of enzymes that transfer carbohydrate molecules to a variety of acceptor substrates. Like glycosyltransferases, H3 binds UDP-glucose, as shown by saturation transfer difference (STD) nuclear magnetic resonance (NMR) spectroscopy, and this binding requires Mg(2+) Mutation of the glycosyltransferase-like metal ion binding motif in H3 greatly diminished its binding to UDP-glucose. We found by flow cytometry that H3 binds to the surface of human cells but does not bind well to cells that are deficient in surface glycosaminoglycans. STD NMR experiments using a heparin sulfate decasaccharide confirmed that H3 binds heparin sulfate. We propose that a surface of H3 with an excess positive charge may be the binding site for heparin. Heparin binding and glycosyltransferase activity may be involved in the function of H3 in the poxvirus life cycle.

IMPORTANCE

Poxviruses are under intense research because of bioterrorism concerns, zoonotic infections, and the side effects of existing smallpox vaccines. The smallpox vaccine using vaccinia virus has been highly successful, but it is still unclear why the vaccine is so effective. Studying the antigens that the immune system recognizes may allow a better understanding of how the vaccine elicits immunity and how improved vaccines can be developed. Poxvirus protein H3 is a major target of the immune system. The H3 crystal structure shows that it has a glycosyltransferase protein fold. We demonstrate that H3 binds the sugar nucleotide UDP-glucose, as do glycosyltransferases. Our experiments also reveal that H3 binds cell surface molecules that are involved in the attachment of poxviruses to cells. These structural and functional studies of H3 will help in designing better vaccines and therapeutics.

Carbonic anhydrase related proteins: molecular biology and evolution

The catalytically inactive isoforms of α-carbonic anhydrases are known as carbonic anhydrase related proteins (CARPs). The CARPs occur independently or as domains of other proteins in animals (both vertebrates and invertebrates) and viruses. The catalytic inactivity of CARPs is due to the lack of histidine residues required for the coordination of the zinc atom. The phylogenetic analysis shows that these proteins are highly conserved across the species. The three CARPs in vertebrates are known as CARP VIII, X and XI. CARPs orthologous to CARP VIII are found in deuterostome invertebrates, whereas protostomes only possess orthologs of CARP X. The CA-like domains of receptor-type protein tyrosine phosphatases (PTPR) are found only in PTPRG and PTPRZ. Most of these CARPs are predominantly expressed in central nervous system. Among the three vertebrate CA isoforms, CARP VIII is functionally associated with motor coordination in human, mouse and zebrafish and certain types of cancers in humans. Vertebrate expression studies show that CARP X is exclusively expressed in the brain. CARP XI is only found in tetrapods and is highly expressed in the central nervous system (CNS) of humans and mice and is also associated with several cancers. CARP VIII, PTPRZ and PTPRG have been shown to coordinate the function of other proteins by protein-protein interaction, and viral CARPs participate in attachment to host cells, but the precise biological function of CARPs X and XI is still unknown. The findings so far suggest many novel functions for the CARP subfamily, most likely related to binding to other proteins.

Measurement of antibody responses to Modified Vaccinia virus Ankara (MVA) and Dryvax(®) using proteome microarrays and development of recombinant protein ELISAs

Modified Vaccinia virus Ankara (MVA) is an attenuated strain of vaccinia virus that is being considered as a safer alternative to replicating vaccinia vaccine strains such as Dryvax(®) and ACAM2000. Its excellent safety profile and large genome also make it an attractive vector for the delivery of heterologous genes from other pathogens. MVA was attenuated by prolonged passage through chick embryonic fibroblasts in vitro. In human and most mammalian cells, production of infectious progeny is aborted in the late stage of infection. Despite this, MVA provides high-level gene expression and is immunogenic in humans and other animals. A key issue for vaccine developers is the ability to be able to monitor an immune response to MVA in both vaccinia naïve and previously vaccinated individuals. To this end we have used antibody profiling by proteome microarray to compare profiles before and after MVA and Dryvax vaccination to identify candidate serodiagnostic antigens. Six antigens with diagnostic utility, comprising three membrane and three non-membrane proteins from the intracellular mature virion, were purified and evaluated in ELISAs. The membrane protein WR113/D8L provided the best sensitivity and specificity of the six antigens tested for monitoring both MVA and Dryvax vaccination, whereas the A-type inclusion protein homolog, WR148, provided the best discrimination. The ratio of responses to membrane protein WR132/A13L and core protein WR070/I1L also provided good discrimination between primary and secondary responses to Dryvax, whereas membrane protein WR101/H3L and virion assembly protein WR118/D13L together provided the best sensitivity for detecting antibody in previously vaccinated individuals. These data will aid the development novel MVA-based vaccines.

Polyclonal antibody cocktails generated using DNA vaccine technology protect in murine models of orthopoxvirus disease

Background

Previously we demonstrated that DNA vaccination of nonhuman primates (NHP) with a small subset of vaccinia virus (VACV) immunogens (L1, A27, A33, B5) protects against lethal monkeypox virus challenge. The L1 and A27 components of this vaccine target the mature virion (MV) whereas A33 and B5 target the enveloped virion (EV).

Results

Here, we demonstrated that the antibodies produced in vaccinated NHPs were sufficient to confer protection in a murine model of lethal Orthopoxvirus infection. We further explored the concept of using DNA vaccine technology to produce immunogen-specific polyclonal antibodies that could then be combined into cocktails as potential immunoprophylactic/therapeutics. Specifically, we used DNA vaccines delivered by muscle electroporation to produce polyclonal antibodies against the L1, A27, A33, and B5 in New Zealand white rabbits. The polyclonal antibodies neutralized both MV and EV in cell culture. The ability of antibody cocktails consisting of anti-MV, anti-EV, or a combination of anti-MV/EV to protect BALB/c mice was evaluated as was the efficacy of the anti-MV/EV mixture in a mouse model of progressive vaccinia. In addition to evaluating weight loss and lethality, bioimaging technology was used to characterize the spread of the VACV infections in mice. We found that the anti-EV cocktail, but not the anti-MV cocktail, limited virus spread and lethality.

Conclusions

A combination of anti-MV/EV antibodies was significantly more protective than anti-EV antibodies alone. These data suggest that DNA vaccine technology could be used to produce a polyclonal antibody cocktail as a possible product to replace vaccinia immune globulin.

Vaccinia virus exhibits cell-type-dependent entry characteristics

Differing and sometimes conflicting data have been reported regarding several aspects of vaccinia virus (VV) entry. To address this, we used a beta-galactosidase reporter virus to monitor virus entry into multiple cell types under varying conditions. Entry into HeLa, B78H1 and L cells was strongly inhibited by heparin whereas entry into Vero and BSC-1 cells was unaffected. Bafilomycin also exhibited variable and cell-type-specific effects on VV entry. Entry into B78H1 and BSC-1 cells was strongly inhibited by bafilomycin whereas entry into Vero and HeLa cells was only partially inhibited suggesting the co-existence of both pH-dependent and pH-independent VV entry pathways in these cell types. Finally, entry into HeLa, B78H1, L and BSC-1 cells exhibited a lag of 6-9 min whereas this delay was undetectable in Vero cells. Our results suggest that VV exploits multiple cell attachment and entry pathways allowing it to infect a broad range of cells.

Vaccination of BALB/c mice with Escherichia coli-expressed vaccinia virus proteins A27L, B5R, and D8L protects mice from lethal vaccinia virus challenge

The potential threat of smallpox use in a bioterrorist attack has heightened the need to develop an effective smallpox vaccine for immunization of the general public. Vaccination with the current smallpox vaccine, Dryvax, produces protective immunity but may result in adverse reactions for some vaccinees. A subunit vaccine composed of protective vaccinia virus proteins should avoid the complications arising from live-virus vaccination and thus provide a safer alternative smallpox vaccine. In this study, we assessed the protective efficacy and immunogenicity of a multisubunit vaccine composed of the A27L and D8L proteins from the intracellular mature virus (IMV) form and the B5R protein from the extracellular enveloped virus (EEV) form of vaccinia virus. BALB/c mice were immunized with Escherichia coli-produced A27L, D8L, and B5R proteins in an adjuvant consisting of monophosphoryl lipid A and trehalose dicorynomycolate or in TiterMax Gold adjuvant. Following immunization, mice were either sacrificed for analysis of immune responses or lethally challenged by intranasal inoculation with vaccinia virus strain Western Reserve. We observed that three immunizations either with A27L, D8L, and B5R or with the A27L and B5R proteins alone induced potent neutralizing antibody responses and provided complete protection against lethal vaccinia virus challenge. Several linear B-cell epitopes within the three proteins were recognized by sera from the immunized mice. In addition, protein-specific cellular responses were detected in spleens of immunized mice by a gamma interferon enzyme-linked immunospot assay using peptides derived from each protein. Our data suggest that a subunit vaccine incorporating bacterially expressed IMV- and EEV-specific proteins can be effective in stimulating anti-vaccinia virus immune responses and providing protection against lethal virus challenge.

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.

Reconstitution of carbonic anhydrase activity of the cell-surface-binding protein of vaccinia virus

The N-terminal region of a 32 kDa cell-surface-binding protein, encoded by the D8L gene of vaccinia virus, shows sequence homology to CAs (carbonic anhydrases; EC 4.2.1.1). The active CAs catalyse the reversible hydration of CO2 to bicarbonate participating in many physiological processes. The CA-like domain of vaccinia protein [vaccCA (vaccinia virus CA-like protein)] contains one of the three conserved histidine residues required for co-ordination to the catalytic zinc ion and for enzyme activity. In the present study, we report the engineering of catalytically active vaccCA mutants by introduction of the missing histidine residues into the wild-type protein. The wild-type vaccCA was inactive as a catalyst and does not bind sulfonamide CA inhibitors. Its position on a phylogram with other hCAs (human CAs) shows a relationship with the acatalytic isoforms CA X and XI, suggesting that the corresponding viral gene was acquired from the human genome by horizontal gene transfer. The single mutants (vaccCA N92H/Y69H) showed low enzyme activity and low affinity for acetazolamide, a classical sulfonamide CA inhibitor. The activity of the double mutant, vaccCA N92H/Y69H, was much higher, of the same order of magnitude as that of some human isoforms, namely CA VA and CA XII. Moreover, its affinity for acetazolamide was high, comparable with that of the most efficient human isoenzyme, CA II (in the low nanomolar range). Multiplication of vaccinia virus in HeLa cells transfected with the vaccCA N92H/Y69H double mutant was approx. 2-fold more efficient than in wild-type vaccCA transfectants, suggesting that the reconstitution of the enzyme activity improved the virus life cycle.

Generalized fatal Cowpox virus infection in a cat with transmission to a human contact case

A 4-month-old female domestic shorthair cat was infected by a virus of the Poxvirus family. The animal developed a severe pneumonia and generalized ulcerating lesions of the skin. Histologically, typical eosinophilic intracytoplasmic inclusion bodies indicative of an Orthopoxvirus (OPV) infection were present. The lung showed grey-white to haemorrhagic nodular lesions with a central zone of complete necrosis of alveolar and bronchial tissue. Electron microscopy from skin and lung nodules revealed typical square-shaped OPV particles. Cultivation of the virus on chorio-allantoic membranes of embryonated chicken eggs resulted in haemorrhagic plaques. Restriction enzyme analysis, PCR and sequencing of the D8L gene identified the OPV isolate as a typical Cowpox virus. It was transmitted by the cat to a human contact person who developed a local nodular dermatitis at the inoculation site in association with signs of general infection and had an increase of OPV-specific neutralizing antibodies in paired serum samples.

Immunogenicity and protection efficacy of monovalent and polyvalent poxvirus vaccines that include the D8 antigen

Recent studies have established the feasibility of subunit-based experimental vaccines to protect animals from lethal poxvirus infection. Individual outer membrane proteins from intracellular and extracellular virions of vaccinia virus, when delivered in the form of either DNA vaccines or recombinant protein vaccines produced from baculovirus-infected insect cells, were able to protect mice from the vaccinia virus challenge and rhesus macaques from the monkeypox virus challenge. The polyvalent formulations with various combinations of the four poxvirus antigens (A27, L1, B5 and A33) achieved better protection than the monovalent formulation using only one of these antigens. However, it is not clear whether any of the remaining outer membrane poxvirus proteins can further improve the efficacy of the current polyvalent formulations. In this study, we conducted detailed analysis on the immunogenicity of D8, a previously reported protective antigen from intracellular mature virions. Our results indicated that D8 induced strong protective antibody responses and was effective in improving the efficacy of previously reported polyvalent poxvirus vaccine formulations. Therefore, D8 is an excellent candidate antigen to be included in the final polyvalent subunit-based poxvirus vaccines.

Predicting the subcellular localization of viral proteins within a mammalian host cell

Background

The bioinformatic prediction of protein subcellular localization has been extensively studied for prokaryotic and eukaryotic organisms. However, this is not the case for viruses whose proteins are often involved in extensive interactions at various subcellular localizations with host proteins.

Results

Here, we investigate the extent of utilization of human cellular localization mechanisms by viral proteins and we demonstrate that appropriate eukaryotic subcellular localization predictors can be used to predict viral protein localization within the host cell.

Conclusion

Such predictions provide a method to rapidly annotate viral proteomes with subcellular localization information. They are likely to have widespread applications both in the study of the functions of viral proteins in the host cell and in the design of antiviral drugs.

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.

The formation and function of extracellular enveloped vaccinia virus

Vaccinia virus produces four different types of virion from each infected cell called intracellular mature virus (IMV), intracellular enveloped virus (IEV), cell-associated enveloped virus (CEV) and extracellular enveloped virus (EEV). These virions have different abundance, structure, location and roles in the virus life-cycle. Here, the formation and function of these virions are considered with emphasis on the EEV form and its precursors, IEV and CEV. IMV is the most abundant form of virus and is retained in cells until lysis; it is a robust, stable virion and is well suited to transmit infection between hosts. IEV is formed by wrapping of IMV with intracellular membranes, and is an intermediate between IMV and CEV/EEV that enables efficient virus dissemination to the cell surface on microtubules. CEV induces the formation of actin tails that drive CEV particles away from the cell and is important for cell-to-cell spread. Lastly, EEV mediates the long-range dissemination of virus in cell culture and, probably, in vivo. Seven virus-encoded proteins have been identified that are components of IEV, and five of them are present in CEV or EEV. The roles of these proteins in virus morphogenesis and dissemination, and as targets for neutralizing antibody are reviewed. The production of several different virus particles in the VV replication cycle represents a coordinated strategy to exploit cell biology to promote virus spread and to aid virus evasion of antibody and complement.

Analysis of the monkeypox virus genome

Monkeypox virus (MPV) belongs to the orthopoxvirus genus of the family Poxviridae, is endemic in parts of Africa, and causes a human disease that resembles smallpox. The 196,858-bp MPV genome was analyzed with regard to structural features and open reading frames. Each end of the genome contains an identical but oppositely oriented 6379-bp terminal inverted repetition, which similar to that of other orthopoxviruses, includes a putative telomere resolution sequence and short tandem repeats. Computer-assisted analysis was used to identify 190 open reading frames containing >/=60 amino acid residues. Of these, four were present within the inverted terminal repetition. MPV contained the known essential orthopoxvirus genes but only a subset of the putative immunomodulatory and host range genes. Sequence comparisons confirmed the assignment of MPV as a distinct species of orthopoxvirus that is not a direct ancestor or a direct descendent of variola virus, the causative agent of smallpox.

Microtubule-dependent organization of vaccinia virus core-derived early mRNAs into distinct cytoplasmic structures

Vaccinia virus (vv) early transcription can be reconstituted in vitro from purified virions; in this assay mRNAs are made inside the viral core and subsequently extruded. Although the in vitro process has been extensively characterized, relatively little is known about vv early transcription in vivo. In the present study the fate of vv early mRNAs in infected HeLa cells was followed by BrUTP transfection and confocal and electron microscopy. The extruded vv early mRNAs were found to be organized into unique granular cytoplasmic structures that reached a size up to 1 microm. By EM these structures appeared as amorphous electron-dense cytoplasmic aggregates that were surrounded by ribosomes. Confocal images showed that the RNA structures were located some distance away from intracellular cores and that both structures appeared to be aligned on microtubules (MTs), implying that MT tracks connected mRNAs and cores. Accordingly, intact MTs were found to be required for the typical punctate organization of viral mRNAs. Biochemical evidence supported the notion that vv mRNAs were MT associated and that MT depletion severely affected viral (but not cellular) mRNA synthesis and stability. By confocal microscopy the viral mRNA structures appeared to be surrounded by molecules of the translation machinery, showing that they were active in protein synthesis. Finally, our data suggest a role for a MT and RNA-binding viral protein of 25 kDa (gene L4R), in mRNA targeting away from intracellular cores to their sites of cytoplasmic accumulation.

The A17L gene product of vaccinia virus is exposed on the surface of IMV

The p21 membrane protein of vaccinia virus (VV), encoded by the A17L gene, has been reported to localize on the inner of the two membranes of the intracellular mature virus (IMV). It has also been shown that p21 acts as a membrane anchor for the externally located fusion protein p14 (A27L gene). Since p14 is located on the surface of IMVs, it is hard to envision that p21 should be located only on the inner membrane. Our results from (i) immunoelectron microscopy, (ii) biotinylation, and (iii) protease treatment of purified IMVs showed that the N-terminus of p21 is exposed on the surface of virus particles, while the C-terminus is embedded in the membrane. Mono-specific antibodies to the N-terminus of p21 neutralize infection of VV while antibodies to the C-terminal domain do not. We suggest that p21 molecules are located both in the inner and in the outer membrane of IMV.

The vaccinia virus superoxide dismutase-like protein (A45R) is a virion component that is nonessential for virus replication

A characterization of the A45R gene from vaccinia virus (VV) strain Western Reserve is presented. The open reading frame is predicted to encode a 125-amino-acid protein (M(r), of 13,600) with 39% amino acid identity to copper-zinc superoxide dismutase (Cu-Zn SOD). Sequencing of the A45R gene from other orthopoxviruses, here and by others, showed that the protein is highly conserved in all viruses sequenced, including 16 strains of VV, 2 strains of cowpox virus, camelpox virus, and 4 strains of variola virus. In all cases the protein lacks key residues involved in metal ion binding that are important for the catalytic activity. The A45R protein was expressed in Escherichia coli, purified, and tested for SOD activity, but neither enzymatic nor inhibitory SOD activity was detected. Additionally, no virus-encoded SOD activity was detected in infected cells or purified virions. A monoclonal antibody raised against the A45R protein expressed in E. coli identified the A45R gene product as a 13.5-kDa protein that is expressed late during VV infection. Confocal microscopy of VV-infected cells indicated that the A45R protein accumulated predominantly in cytoplasmic viral factories. Electron microscopy and biochemical analyses showed that the A45R protein is incorporated into the virion core. A deletion mutant lacking the majority of the A45R gene and a revertant virus in which the deleted gene was restored were constructed and characterized. The growth properties of the deletion mutant virus were indistinguishable from those of wild-type and revertant viruses in all cell lines tested, including macrophages. Additionally, the virulence and pathogenicity of the three viruses were also comparable in murine and rabbit models of infection. A45R is unusual in being the first VV core protein described that affects neither virus replication nor virulence.

Antibody neutralization of the extracellular enveloped form of vaccinia virus

The extracellular enveloped form (EEV) of vaccinia virus (VV) is important for the long-range dissemination of the virus inside the host. Early work suggested that both IMV and EEV infectivity could be inhibited by antibodies, although two other studies reported that EEV was resistant to neutralization. Here, we readdressed this question, using four VV-immune antisera and their purified IgG, and showed that EEV infectivity can be inhibited by antibody produced from a live infection in plaque-reduction assays, although EEV is more resistant to neutralization by convalescent antibodies than is IMV. In parallel, indirect immunofluorescent staining and confocal microscopy showed that antibody aggregated EEV and prevented it from binding to cells. Using the IgG and Fab fragments prepared from this antiserum, we tested whether EEV made by VV mutants lacking genes A33R, A34R, A36R, A56R, B5R, F12L, or F13L can be inhibited in plaque-reduction assays. Although vDeltaB5R was slightly more resistant than other mutants, none of these mutants escaped neutralization completely, suggesting that multiple virus proteins are involved in the inhibition. Using an antibody specific to B5R protein and B5R mutants with consecutive short consensus repeat (SCR) domains deleted, the neutralization epitopes on B5R were mapped to within the SCR domain 1.

Characterization of the vaccinia virus H3L envelope protein: topology and posttranslational membrane insertion via the C-terminal hydrophobic tail

The vaccinia virus H3L open reading frame encodes a 324-amino-acid immunodominant membrane component of virus particles. Biochemical and microscopic studies demonstrated that the H3L protein was expressed late in infection, accumulated in the cytoplasmic viral factory regions, and associated primarily with amorphous material near immature virions and with intracellular virion membranes. Localization of the H3L protein on the surfaces of viral particles and anchorage via the hydrophobic tail were consistent with its extraction by NP-40 in the absence of reducing agents, its trypsin sensitivity, its reactivity with a membrane-impermeable biotinylation reagent, and its immunogold labeling with an antibody to a peptide comprising amino acids 247 to 259. The H3L protein, synthesized in a coupled in vitro transcription/translation system, was tightly anchored to membranes as determined by resistance to Na(2)CO(3) (pH 11) extraction and cytoplasmically oriented as shown by sensitivity to proteinase K digestion. Further studies demonstrated that membrane insertion of the H3L protein occurred posttranslationally and that the C-terminal hydrophobic domain was necessary and sufficient for this to occur. These data indicated that the H3L protein is a member of the C-terminal anchor family and supported a model in which it is synthesized on free ribosomes and inserts into the membranes of viral particles during their maturation.

The genome of fowlpox virus

Here we present the genomic sequence, with analysis, of a pathogenic fowlpox virus (FPV). The 288-kbp FPV genome consists of a central coding region bounded by identical 9.5-kbp inverted terminal repeats and contains 260 open reading frames, of which 101 exhibit similarity to genes of known function. Comparison of the FPV genome with those of other chordopoxviruses (ChPVs) revealed 65 conserved gene homologues, encoding proteins involved in transcription and mRNA biogenesis, nucleotide metabolism, DNA replication and repair, protein processing, and virion structure. Comparison of the FPV genome with those of other ChPVs revealed extensive genome colinearity which is interrupted in FPV by a translocation and a major inversion, the presence of multiple and in some cases large gene families, and novel cellular homologues. Large numbers of cellular homologues together with 10 multigene families largely account for the marked size difference between the FPV genome (260 to 309 kbp) and other known ChPV genomes (178 to 191 kbp). Predicted proteins with putative functions involving immune evasion included eight natural killer cell receptors, four CC chemokines, three G-protein-coupled receptors, two beta nerve growth factors, transforming growth factor beta, interleukin-18-binding protein, semaphorin, and five serine proteinase inhibitors (serpins). Other potential FPV host range proteins included homologues of those involved in apoptosis (e.g., Bcl-2 protein), cell growth (e.g., epidermal growth factor domain protein), tissue tropism (e.g., ankyrin repeat-containing gene family, N1R/p28 gene family, and a T10 homologue), and avian host range (e.g., a protein present in both fowl adenovirus and Marek's disease virus). The presence of homologues of genes encoding proteins involved in steroid biogenesis (e.g., hydroxysteroid dehydrogenase), antioxidant functions (e.g., glutathione peroxidase), vesicle trafficking (e.g., two alpha-type soluble NSF attachment proteins), and other, unknown conserved cellular processes (e.g., Hal3 domain protein and GSN1/SUR4) suggests that significant modification of host cell function occurs upon viral infection. The presence of a cyclobutane pyrimidine dimer photolyase homologue in FPV suggests the presence of a photoreactivation DNA repair pathway. This diverse complement of genes with likely host range functions in FPV suggests significant viral adaptation to the avian host.

Vaccinia virus envelope H3L protein binds to cell surface heparan sulfate and is important for intracellular mature virion morphogenesis and virus infection in vitro and in vivo

An immunodominant antigen, p35, is expressed on the envelope of intracellular mature virions (IMV) of vaccinia virus. p35 is encoded by the viral late gene H3L, but its role in the virus life cycle is not known. This report demonstrates that soluble H3L protein binds to heparan sulfate on the cell surface and competes with the binding of vaccinia virus, indicating a role for H3L protein in IMV adsorption to mammalian cells. A mutant virus defective in expression of H3L (H3L(-)) was constructed; the mutant virus has a small plaque phenotype and 10-fold lower IMV and extracellular enveloped virion titers than the wild-type virus. Virion morphogenesis is severely blocked and intermediate viral structures such as viral factories and crescents accumulate in cells infected with the H3L(-) mutant virus. IMV from the H3L(-) mutant virus are somewhat altered and less infectious than wild-type virions. However, cells infected by the mutant virus form multinucleated syncytia after low pH treatment, suggesting that H3L protein is not required for cell fusion. Mice inoculated intranasally with wild-type virus show high mortality and severe weight loss, whereas mice infected with H3L(-) mutant virus survive and recover faster, indicating that inactivation of the H3L gene attenuates virus virulence in vivo. In summary, these data indicate that H3L protein mediates vaccinia virus adsorption to cell surface heparan sulfate and is important for vaccinia virus infection in vitro and in vivo. In addition, H3L protein plays a role in virion assembly.

Alastrim smallpox variola minor virus genome DNA sequences

Alastrim variola minor virus, which causes mild smallpox, was first recognized in Florida and South America in the late 19th century. Genome linear double-stranded DNA sequences (186,986 bp) of the alastrim virus Garcia-1966, a laboratory reference strain from an outbreak associated with 0.8% case fatalities in Brazil in 1966, were determined except for a 530-bp fragment of hairpin-loop sequences at each terminus. The DNA sequences (EMBL Accession No. Y16780) showed 206 potential open reading frames for proteins containing >/=60 amino acids. The amino acid sequences of the putative proteins were compared with those reported for vaccinia virus strain Copenhagen and the Asian variola major strains India-1967 and Bangladesh-1975. About one-third of the alastrim viral proteins were 100% identical to correlates in the variola major strains and the remainder were >/=95% identical. Compared with variola major virus DNA, alastrim virus DNA has additional segments of 898 and 627 bp, respectively, within the left and right terminal regions. The former segment aligns well with sequences in other orthopoxviruses, particularly cowpox and vaccinia viruses, and the latter is apparently alastrim-specific.

Vaccinia Virus Inhibits the Maturation of Human Dendritic Cells: A Novel Mechanism of Immune Evasion

Vaccinia virus employs multiple mechanisms to evade the immune system, yet is highly immunogenic. We studied the interaction between vaccinia and human dendritic cells (DCs), potent APCs. DCs develop from precursor cells in two stages: an immature stage in which Ag uptake and processing occur, and a mature stage in which there is up-regulation of costimulatory and HLA molecules and efficient T cell activation. Vaccinia virus undergoes an abortive replication in both stages of DCs and induces apoptotic cell death. Furthermore, maturation of immature DCs and consequently T cell activation are inhibited. Obstruction of DC maturation may constitute a novel mechanism by which vaccinia attempts to evade the immune response.

Vaccinia virus envelope D8L protein binds to cell surface chondroitin sulfate and mediates the adsorption of intracellular mature virions to cells

We previously showed that an envelope A27L protein of intracellular mature virions (IMV) of vaccinia virus binds to cell surface heparan sulfate during virus infection. In the present study we identified another viral envelope protein, D8L, that binds to chondroitin sulfate on cells. Soluble D8L protein interferes with the adsorption of wild-type vaccinia virions to cells, indicating a role in virus entry. To explore the interaction of cell surface glycosaminoglycans and vaccinia virus, we generated mutant viruses from a control virus, WR32-7/Ind14K (A27L(+) D8L(+)) to be defective in expression of either the A27L or the D8L gene (A27L(+) D8L(-) or A27L(-) D8L(+)) or both (A27L(-) D8L(-)). The A27L(+) D8L(+) and A27L(-) D8L(+) mutants grew well in BSC40 cells, consistent with previous observations. However, the IMV titers of A27L(+) D8L(-) and A27L(-) D8L(-) viruses in BSC40 cells were reduced, reaching only 10% of the level for the control virus. The data suggested an important role for D8L protein in WR32-7/Ind14K virus growth in cell cultures. A27L protein, on the other hand, could not complement the functions of D8L protein. The low titers of the A27L(+) D8L(-) and A27L(-) D8L(-) mutant viruses were not due to defects in the morphogenesis of IMV, and the mutant virions demonstrated a brick shape similar to that of the control virions. Furthermore, the infectivities of the A27L(+) D8L(-) and A27L(-) D8L(-) mutant virions were 6 to 10% of that of the A27L(+) D8L(+) control virus. Virion binding assays revealed that A27L(+) D8L(-) and A27L(-) D8L(-) mutant virions bound less well to BSC40 cells, indicating that binding of viral D8L protein to cell surface chondroitin sulfate could be important for vaccinia virus entry.

An unconventional role for cytoplasmic disulfide bonds in vaccinia virus proteins

Previous data have shown that reducing agents disrupt the structure of vaccinia virus (vv). Here, we have analyzed the disulfide bonding of vv proteins in detail. In vv-infected cells cytoplasmically synthesized vv core proteins became disulfide bonded in the newly assembled intracellular mature viruses (IMVs). vv membrane proteins also assembled disulfide bonds, but independent of IMV formation and to a large extent on their cytoplasmic domains. If disulfide bonding was prevented, virus assembly was only partially impaired as shown by electron microscopy as well as a biochemical assay of IMV formation. Under these conditions, however, the membranes around the isolated particles appeared less stable and detached from the underlying core. During the viral infection process the membrane proteins remained disulfide bonded, whereas the core proteins were reduced, concomitant with delivery of the cores into the cytoplasm. Our data show that vv has evolved an unique system for the assembly of cytoplasmic disulfide bonds that are localized both on the exterior and interior parts of the IMV.

The complete genomic sequence of the modified vaccinia Ankara strain: comparison with other orthopoxviruses

The complete genomic DNA sequence of the highly attenuated vaccinia strain modified vaccinia Ankara (MVA) was determined. The genome of MVA is 178 kb in length, significantly smaller than that of the vaccinia Copenhagen genome, which is 192 kb. The 193 open reading frames (ORFs) mapped in the MVA genome probably correspond to 177 genes, 25 of which are split and/or have suffered mutations resulting in truncated proteins. The left terminal genomic region of MVA contains four large deletions and one large insertion relative to the Copenhagen strain. In addition, many ORFs in this region are fragmented, leaving only eight genes structurally intact and therefore presumably functional. The inserted DNA codes for a cluster of genes that is also found in the vaccinia WR strain and in cowpox virus and includes a highly fragmented gene homologous to the cowpox virus host range gene, providing further evidence that a cowpox-like virus was the ancestor of vaccinia. Surprisingly, the central conserved region of the genome also contains some fragmented genes, including ORF F5L, encoding a major membrane protein, and ORFs F11L and O1L, encoding proteins of 39.7 and 77.6 kDa, respectively. The right terminal genomic region carries three large deletions all classical poxviral immune evasion genes and all ankyrin-like genes located in this region are fragmented except for those encoding the interleukin-1 beta receptor and the 68-kDa ankyrin-like protein B18R. Thus, the attenuated phenotype of MVA is the result of numerous mutations, particularly affecting the host interactive proteins, including the ankyrin-like genes, but also involving some structural proteins.

Vaccinia virus 15-kilodalton (A14L) protein is essential for assembly and attachment of viral crescents to virosomes

Early stages in vaccinia virus (VV) assembly involve the recruitment of cellular membranes from the endoplasmic reticulum-Golgi intermediate compartment (ERGIC) to virus factories (or virosomes). The key viral factors involved in this process are not yet known. We have previously identified and characterized two viral proteins, of 21 kDa (A17L gene) and 15 kDa (A14L gene), that associate with tubulovesicular elements related to the ERGIC and are localized in viral membranes at all stages of virion assembly. We showed that the 21-kDa protein is not responsible for the recruitment of membranes from the ERGIC to viral factories. However, it appears to be essential for the organization of viral membranes. In this investigation we have generated a VV recombinant, VVindA14L, in which the expression of the A14L gene is inducibly regulated by the Escherichia coli lacI operator-repressor system. Repression of 15-kDa protein synthesis has a dramatic effect on virus yields and severely impairs plaque formation. Compared to wild-type VV, reduced amounts of 15-kDa protein are produced in VVindA14L-infected cells in the presence of IPTG (isopropyl-beta-D-thiogalactoside), and this correlates with a small-plaque phenotype and reduced VVindA14L yields under these conditions. In the absence of the 15-kDa protein, early and late viral protein syntheses proceed normally; however, proteolytic cleavage of the major core precursors is inhibited. Electron microscopic examination of cells infected with VVindA14L under nonpermissive conditions reveals the presence of numerous membranous elements that look like unfinished or disassembled crescents interspersed between electron-dense masses. These abnormal membrane elements are usually well separated from the surfaces of the dense structures. These findings show that the 15-kDa protein is essential for VV morphogenesis and indicate that this polypeptide is necessary both for the correct assembly of viral crescents and for their stable attachment to the surfaces of viral factories.

Structure and mechanism of carbonic anhydrase

Carbonic anhydrase (CA; carbonate hydro-lyase, EC 4.2.1.1) is a zinc-containing enzyme that catalyzes the reversible hydration of carbon dioxide: CO2+ H2O<-->HCO3(-)+H+. The enzyme is the target for drugs, such as acetazolamide, methazolamide, and dichlorphenamide, for the treatment of glaucoma. There are three evolutionarily unrelated CA families, designated alpha, beta, and gamma. All known CAs from the animal kingdom are of the alpha type. There are seven mammalian CA isozymes with different tissue distributions and intracellular locations, CA I-VII. Crystal structures of human CA I and II, bovine CA III, and murine CA V have been determined. All of them have the same tertiary fold, with a central 10-stranded beta-sheet as the dominating secondary structure element. The zinc ion is located in a cone-shaped cavity and coordinated to three histidyl residues and a solvent molecule. Inhibitors bind at or near the metal center guided by a hydrogen-bonded system comprising Glu-106 and Thr-199. The catalytic mechanism of CA II has been studied in particular detail. It involves an attack of zinc-bound OH- on a CO2 molecule loosely bound in a hydrophobic pocket. The resulting zinc-coordinated HCO3- ion is displaced from the metal ion by H2O. The rate-limiting step is an intramolecular proton transfer from the zinc-bound water molecule to His-64, which serves as a proton shuttle between the metal center and buffer molecules in the reaction medium.

The cytoplasmic and transmembrane domains of the vaccinia virus B5R protein target a chimeric human immunodeficiency virus type 1 glycoprotein to the outer envelope of nascent vaccinia virions

The outer envelope of the extracellular form of vaccinia virus (EEV) is derived from the Golgi membrane and contains at least six viral proteins. Transfection studies indicated that the EEV protein encoded by the B5R gene associates with Golgi membranes when synthesized in the absence of other viral products. A domain swapping strategy was then used to investigate the possibility that the B5R protein contains an EEV targeting signal. We constructed chimeric genes encoding the human immunodeficiency virus (HIV) type 1 glycoprotein with the cytoplasmic and transmembrane domains replaced by the corresponding 42-amino-acid C-terminal segment of the B5R protein. Recombinant vaccinia viruses that stably express a chimeric B5R-HIV protein or a control HIV envelope protein with the original cytoplasmic and transmembrane domains were isolated. Cells infected with recombinant vaccinia viruses that expressed either the unmodified or the chimeric HIV envelope protein formed syncytia with cells expressing the CD4 receptor for HIV. However, biochemical and microscopic studies demonstrated that the HIV envelope proteins with the B5R cytoplasmic and transmembrane domains were preferentially targeted to the EEV. These data are consistent with the presence of EEV localization signals in the cytoplasmic and transmembrane domains of the B5R protein.

A monoclonal blocking-ELISA for detection of orthopoxvirus antibodies in feline sera

A double sandwich blocking-ELISA using a genus-specific neutralizing monoclonal antibody (MAb) against the vaccinia virus 32 kD adsorption protein (D8L open reading frame: ORF) was developed to detect orthopoxvirus (OPV) antibodies in sera. A collection of 2173 feline serum samples was examined in an epidemiological study. The blocking-ELISA revealed 44 (2%) sera with positive titres of 1:2-1:256. ELISA results were confirmed by the plaque-reduction test. A close correlation between titres of both assays could be observed (r = 0.986). In general, the sensitivity of the blocking ELISA was two to four times higher. Neutralizing OPV-antibodies were found in nine sera with ELISA-titres > 1:4. Antibody specificity to OPV was also demonstrated by Western blotting analysis with selected feline sera. The epidemiographical distribution of the ELISA-positive sera and case histories of 37 seropositive cats available from the referring veterinarians are demonstrated. The blocking-ELISA enables a rapid serological diagnosis and can be used in veterinary and human medicine. It allows OPV-antibody screening in human and other animal species.

The carbonic anhydrase domain of receptor tyrosine phosphatase beta is a functional ligand for the axonal cell recognition molecule contactin

Receptor-type protein tyrosine phosphatase beta (RPTP beta) is expressed in the developing nervous system and contains a carbonic anhydrase (CAH) domain as well as a fibronectin type III repeat in its extracellular domain. Fusion proteins containing these domains were used to search for ligands of RPTP beta. The CAH domain bound specifically to a 140 kDa protein expressed on the surface of neuronal cells. Expression cloning in COS7 cells revealed that this protein is contactin, a GPI membrane-anchored neuronal cell recognition molecule. The CAH domain of RPTP beta induced cell adhesion and neurite growth of primary tectal neurons, and differentiation of neuroblastoma cells. These responses were blocked by antibodies against contactin, demonstrating that contactin is a neuronal receptor for RPTP beta. These experiments show that an individual domain of RPTP beta acts as a functional ligand for the neuronal receptor contactin. The interaction between contactin and RPTP beta may generate unidirectional or bidirectional signals during neural development.

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 48 kbp of the variola major virus strain India-1967 located on the right terminus of the conservative genome region

Computer analysis of a variola major virus (VAR) genomic fragment bounded by the open reading frames (ORFs) D1R and A33L, which is 47,961 bp long, revealed 46 potential ORFs. The VAR proteins were compared to the analogous proteins of vaccinia virus strain Copenhagen. The subunits of DNA-dependent RNA polymerase, as well as the transcription factors, mRNA-capping enzymes, and proteins necessary for the virion morphogenesis proved to be highly conservative within orthopoxviruses. The most pronounced differences between the VAR genome fragment under study and the corresponding vaccinia virus fragment were revealed in the vicinity of the gene encoding the A-type inclusion bodies protein. Possible functions of the analysed viral proteins are discussed.

Protein tyrosine phosphatases: characterization of extracellular and intracellular domains

Protein tyrosine phosphatases (PTPs) play an important role in the regulation of cell growth and differentiation. With over 30 PTPs identified, the specific functions of these enzymes are now being addressed. The identification of extracellular domain receptor-like PTP interactions and the characterization of intracellular PTP 'targeting' domains represent recent efforts in this pursuit.

A prominent antigenic surface polypeptide involved in the biogenesis and function of the vaccinia virus envelope

Polypeptides of the vaccinia virus envelope exposed on the surface were identified by means of sulfo-N-hydroxysuccinimidobiotin as a surface tag. Among surface expressed polypeptides is the 35-kDa antigen, previously designated Ag35. Both monoclonal (mAb) and monospecific affinity pure antibodies directed against Ag35 neutralized vaccinia infectiousness, indicating that this prominent surface antigen has a function during early virus-host cell interactions. The binding of several monoclonal antibodies to various regions of Ag35 was tested by reacting CNBr fragments, derived from the polypeptide, employing Western blotting. All mAbs tested reacted with the same region of Ag35. Estimation of the molecular weights (MW), based on migration of the CNBr peptides in sodium dodecyl sulfate-polyacrylamide gel electrophoresis, revealed that those partial digestion products which contained a proline-rich 99 amino acid limit digest fragment were present at a position approximately 12.5 kDa larger than that predicted from the DNA sequence. By contrast, partial and limit digest products lacking the proline-rich fragment migrated to the MW position expected from the length of the DNA sequence. This observation demonstrates that departure from a predicted 22.3 kDa to an anomalous MW of Ag35 is conferred by the proline-rich peptide. The surface location of Ag35 was confirmed by immune electron microscopy. In a competition test the binding specificity of mAb and affinity-purified antibodies at the surface of virions could be demonstrated. Evidence for an association of Ag35 with the virus envelope at various stages during biogenesis of vaccinia was obtained by immune electron microscopy of whole mounts and thin sections. Presence of Ag35 as an early component of immature and mature virions, probably residing in the bilayer membrane structure was detected. A distinction can, therefore, be made between Ag35 and several other vaccinia envelope polypeptides which are synthesized as late functions and added during late stages of envelope assembly.

Structural properties of HIV-1 Env fused with the 14-kDa vaccinia virus envelope protein

To gain insights into the structure-function relationship of the envelope (env) glycoprotein of the human immunodeficiency virus type 1 (HIV-1) we have generated a vaccinia virus (VV) recombinant (VV-14kENV) that expresses a fusion protein (14k-env) consisting of the VV 14-kDa envelope protein (110 amino acids) fused at the C-terminus with HIV-1 env protein (816 amino acids). The 14k-env protein displayed unique structural properties in virus-infected cells. This protein was recognized by 14 kDa-specific antisera as well as HIV-1 env antisera. It was not cleaved during virus infection of cultured cells of various origins, it was stable, it was not released to the medium, and it was not incorporated into virions. Instead of a predicted 174-kDa protein, two proteins of about 110 and 100 kDa were observed. The size reduction of the fusion protein was due to limited glycosylation (110 kDa) and formation of unglycosylated protein (100 kDa). The 14k-env protein formed oligomeric structures and was exposed on the cell surface after virus infection. When mice were inoculated with the recombinant virus that expresses the 14K-env fusion protein, humoral immune response against gp160 was observed. Our findings suggest that 14k-env protein might display novel immunogenic properties.

The complete DNA sequence of vaccinia virus

The complete DNA sequence of the genome of vaccinia virus has been determined. The genome consisted of 191,636 bp with a base composition of 66.6% A + T. We have identified 198 "major" protein-coding regions and 65 overlapping "minor" regions, for a total of 263 potential genes. Genes encoded by the virus were located by examination of DNA sequence characteristics and compared with existing vaccinia virus mapping analyses, sequence data, and transcription data. These genes were found to be compactly organized along the genome with relatively few regions of noncoding sequences. Whereas several similarities to proteins of known function were discerned, the function of the majority of proteins encoded by these open reading frames is as yet undetermined.

Fusion of intra- and extracellular forms of vaccinia virus with the cell membrane

The membrane fusion activities of the isolated single-envelope intracellular form of vaccinia virus (INV) and the double-envelope extracellular (EEV) form were studied by using a lipid-mixing assay based on the dilution of a fluorescent probe. Fluorescently labeled INV and EEV from both the IHD-J and WR strains of vaccinia virus fused with HeLa cells at neutral pH, suggesting that fusion occurs with the plasma membrane during virus entry. EEV fused more efficiently and with faster kinetics than INV: approximately 50% of bound EEV particles fused over the course of 1 h, compared with only 25% of the INV particles. Fusion of INV and EEV was strongly temperature dependent, being decreased by 50% at 34 degrees C and by 90% at 28 degrees C. A monoclonal antibody to a 14-kilodalton envelope protein of INV that has been implicated in the fusion reaction (J. F. Rodriguez, E. Paez, and M. Esteban, J. Virol. 61:395-404, 1987) completely suppressed the initial rate of fusion of INV but had no effect on the fusion activity of EEV, suggesting that vaccinia virus encodes two or more membrane fusion proteins. Finally, cells infected with the WR strain of vaccinia virus formed syncytia when briefly incubated at pH 6.4 or below, indicating that an acid-activated viral fusion protein is expressed on the cell surface. However, WR INV and EEV did not display increased fusion activity at acid pH, suggesting that the acid-dependent fusion factor is not incorporated into virions or that its activity there is masked.