A single point mutation of Ala-25 to Asp in the 14,000-Mr envelope protein of vaccinia virus induces a size change that leads to the small plaque size phenotype of the virus

Species-Specific Conservation of Linear Antigenic Sites on Vaccinia Virus A27 Protein Homologs of Orthopoxviruses

The vaccinia virus (VACV) A27 protein and its homologs, which are found in a large number of members of the genus Orthopoxvirus (OPXV), are targets of viral neutralization by host antibodies. We have mapped six binding sites (epitopes #1A: aa 32–39, #1B: aa 28–33, #1C: aa 26–31, #1D: 28–34, #4: aa 9–14, and #5: aa 68–71) of A27 specific monoclonal antibodies (mAbs) using peptide arrays. MAbs recognizing epitopes #1A–D and #4 neutralized VACV Elstree in a complement dependent way (50% plaque-reduction: 12.5–200 µg/mL). Fusion of VACV at low pH was blocked through inhibition of epitope #1A. To determine the sequence variability of the six antigenic sites, 391 sequences of A27 protein homologs available were compared. Epitopes #4 and #5 were conserved among most of the OPXVs, while the sequential epitope complex #1A–D was more variable and, therefore, responsible for species-specific epitope characteristics. The accurate and reliable mapping of defined epitopes on immuno-protective proteins such as the A27 of VACV enables phylogenetic studies and insights into OPXV evolution as well as to pave the way to the development of safer vaccines and chemical or biological antivirals.

Induction of wheat straw delignification by Trametes species

Wheat straw is the major crop residue in European countries which makes it the most promising material for bioconversion into biofuels. However, cellulose and hemicellulose are protected with lignin, so delignification is an inevitable phase in lignocellulose processing. The organisms predominantly responsible for its degradation are white-rot fungi and among them Trametes species represent promising degraders due to a well-developed ligninolytic enzyme system. Although numerous studies have confirmed that low molecular weight compounds can induce the production and activity of ligninolytic enzymes it is not clear how this reflects on the extent of delignification. The aim of the study was to assess the capacity of p-anisidine and veratryl alcohol to induce the production and activity of Mn-oxidizing peroxidases and laccases, and wheat straw delignification by six Trametes species. Significant inter- and intraspecific variations in activity and features of these enzymes were found, as well as differences in the potential of lignocellulose degradation in the presence or absence of inducers. Differences in the catalytic properties of synthesized enzyme isoforms strongly affected lignin degradation. Apart from enhanced lignin degradation, the addition of p-anisidine could significantly improve the selectivity of wheat straw ligninolysis, which was especially evident for T. hirsuta strains.

Vaccinia viral protein A27 is anchored to the viral membrane via a cooperative interaction with viral membrane protein A17

The vaccinia viral protein A27 in mature viruses specifically interacts with heparan sulfate for cell surface attachment. In addition, A27 associates with the viral membrane protein A17 to anchor to the viral membrane; however, the specific interaction between A27 and A17 remains largely unclear. To uncover the active binding sites and the underlying binding mechanism, we expressed and purified the N-terminal (18-50 residues) and C-terminal (162-203 residues) fragments of A17, which are denoted A17-N and A17-C. Through surface plasmon resonance, the binding affinity of A27/A17-N (KA = 3.40 × 10(8) m(-1)) was determined to be approximately 3 orders of magnitude stronger than that of A27/A17-C (KA = 3.40 × 10(5) m(-1)), indicating that A27 prefers to interact with A17-N rather than A17-C. Despite the disordered nature of A17-N, the A27-A17 interaction is mediated by a specific and cooperative binding mechanism that includes two active binding sites, namely (32)SFMPK(36) (denoted as F1 binding) and (20)LDKDLFTEEQ(29) (F2). Further analysis showed that F1 has stronger binding affinity and is more resistant to acidic conditions than is F2. Furthermore, A27 mutant proteins that retained partial activity to interact with the F1 and F2 sites of the A17 protein were packaged into mature virus particles at a reduced level, demonstrating that the F1/F2 interaction plays a critical role in vivo. Using these results in combination with site-directed mutagenesis data, we established a computer model to explain the specific A27-A17 binding mechanism.

Propagation of viruses on micropatterned host cells

We have developed a technique to characterize the in vitro propagation of viruses. Microcontact printing was used to generate linear arrays of alkanethiols on gold surfaces, which served as substrates for the patterned culture of baby hamster kidney (BHK-21) cells. Vesicular stomatitis virus (VSV) was added to unpatterned cell reservoirs adjacent to the patterned cells and incubated, setting in motion a continuously advancing viral infection into the patterned cells. At different incubation times, multiple arrays were chemically fixed to stop the viral propagation. Viral propagation distances into the patterned cells were determined by indirect immunofluorescent labeling and visualization of the VSV surface glycoprotein (G). The infection spread at approximately 50 microm/h in the 140-microm lines. Moreover, different temporal stages of the infection process were simultaneously visualized along individual lines. These stages included initiation of infection, based on G protein expression; cell-cell fusion, based on virus-induced clustering of cell nuclei; and cytoskeletal degradation, based on localized release of cells from the surface. This work sets a foundation for parallel, high-throughput characterization of viral and cellular processes.

The vaccinia virus A27L protein is needed for the microtubule-dependent transport of intracellular mature virus particles

The vaccinia virus (VV) A27L gene encodes a 14 kDa protein that is required for the formation of intracellular enveloped virus (IEV) and, consequently, normal sized plaques. Data presented here show that A27L plays an additional role in VV assembly. When cells were infected with the VV WR32-7/Ind 14K, under conditions that repress A27L expression, transport of intracellular mature virus (IMV) from virus factories was inhibited and some IMV was found in aberrant association with virus crescents. In contrast, other VV mutants (vDeltaB5R and vDeltaF13L) that are defective in IEV formation produce IMV particles that are transported out of virus factories. This indicated a specific role for A27L in IMV transport. Induction of A27L expression at 10 h post-infection promoted the dispersal of clustered IMV particles, but only when microtubules were intact. Formation of IEV particles was also impaired when cells were infected with WR32-7/14K, a VV strain expressing a mutated form of the A27L protein; however, this mutation did not inhibit intracellular transport of IMV particles. Collectively, these data define two novel aspects of VV morphogenesis. Firstly, A27L is required for both IMV transport and the process of envelopment that leads to IEV formation. Secondly, movement of IMV particles between the virus factory and the site of IEV formation is microtubule-dependent.

Identification of functional domains in the 14-kilodalton envelope protein (A27L) of vaccinia virus

The mechanism of entry of vaccinia virus (VV) into cells is still a poorly understood process. A 14-kDa protein (encoded by the A27L gene) in the envelope of intracellular mature virus (IMV) has been implicated in virus-cell attachment, virus-cell fusion, and virus release from cells. We have previously described the structural organization of the VV 14-kDa protein, consisting of a triple-stranded coiled-coil region responsible for oligomer formation and a predicted Leu zipper-like third alpha helix with an important role in the interaction with a 21-kDa membrane protein (encoded by the A17L gene) thought to anchor the 14-kDa protein to the envelope of IMV (M.-I. Vázquez, G. Rivas, D. Cregut, L. Serrano, and M. Esteban, J. Virol. 72:10126-10137, 1998). To identify the functional domains important for virus entry and release, we have generated VV recombinants containing a copy of the A27L gene regulated by the lacI operator-repressor system of Escherichia coli (VVIndA27L) in the thymidine kinase locus and a mutant form of the A27L gene in the hemagglutinin locus but expressed constitutively under the control of an early-late VV promoter. Cells infected with a VV recombinant that expresses a mutant 14-kDa form lacking the first 29 amino acids at the N terminus failed to form extracellular enveloped virus (EEV). Fusion-from-without assays with purified virus confirmed that the fusion process was mediated by the 14-kDa protein and the fusion domain to be contained within amino acids 29 to 43 of the N-terminal region. Competitive inhibition of the infection process with soluble heparin and synthetic peptides and in vitro experiments with purified mutant proteins identified the heparin binding domain within amino acids 21 to 33, suggesting that this domain is involved in virus-cell binding via heparan sulfate. Thus, the N terminus of the 14-kDa protein contains a heparin binding domain, a fusion domain, and a domain responsible for interacting with proteins or lipids in the Golgi stacks for EEV formation and virus spread.

Cell surface proteoglycans are necessary for A27L protein-mediated cell fusion: identification of the N-terminal region of A27L protein as the glycosaminoglycan-binding domain

We previously showed that vaccinia virus infection of BSC40 cells was blocked by soluble heparin, suggesting that cell surface heparan sulfate mediates vaccinia virus binding (C.-S. Chung, J.-C. Hsiao, Y. -S. Chang, and W. Chang, J. Virol. 72:1577-1585, 1998). In this study, we extended our previous work and demonstrated that soluble A27L protein bound to heparan sulfate on cells and interfered with vaccinia virus infection at a postbinding step. In addition, we investigated the structure of A27L protein that provides for its binding to heparan sulfate on cells. A mutant of A27L protein, named D-A27L, devoid of a cluster of 12 amino acids rich in basic residues, was constructed. In contrast to the soluble A27L protein, purified D-A27L protein was inactive in all of our assays, including binding to heparin in vitro, binding to heparan sulfate on cells, and the ability to block virus infection. These data demonstrated that the N-terminal region acts as a glycosaminoglycan (GAG)-binding domain critical for A27L protein binding to cells. Previously A27L protein was thought to be involved in fusion of virus-infected cells induced by acid treatment. When we investigated whether cell surface GAGs also participate in A27L-dependent fusion, our results indicated that soluble A27L protein blocked cell fusion, whereas D-A27L protein did not. Taken together, the results therefore demonstrated that A27L-mediated cell fusion is triggered by its interaction with cell surface GAGs through the N-terminal domain.

A27L protein mediates vaccinia virus interaction with cell surface heparan sulfate

Vaccinia virus has a wide host range and infects mammalian cells of many different species. This suggests that the cell surface receptors for vaccinia virus are ubiquitously expressed and highly conserved. Alternatively, different receptors are used for vaccinia virus infection of different cell types. Here we report that vaccinia virus binds to heparan sulfate, a glycosaminoglycan (GAG) side chain of cell surface proteoglycans, during virus infection. Soluble heparin specifically inhibits vaccinia virus binding to cells, whereas other GAGs such as condroitin sulfate or dermantan sulfate have no effect. Heparin also blocks infections by cowpox virus, rabbitpox virus, myxoma virus, and Shope fibroma virus, suggesting that cell surface heparan sulfate could be a general mediator of the entry of poxviruses. The biochemical nature of the heparin-blocking effect was investigated. Heparin analogs that have acetyl groups instead of sulfate groups also abolish the inhibitory effect, suggesting that the negative charges on GAGs are important for virus infection. Furthermore, BSC40 cells treated with sodium chlorate to produce undersulfated GAGs are more refractory to vaccinia virus infection. Taken together, the data support the notion that cell surface heparan sulfate is important for vaccinia virus infection. Using heparin-Sepharose beads, we showed that vaccinia virus virions bind to heparin in vitro. In addition, we demonstrated that the recombinant A27L gene product binds to the heparin beads in vitro. This recombinant protein was further shown to bind to cells, and such interaction could be specifically inhibited by soluble heparin. All the data together indicated that A27L protein could be an attachment protein that mediates vaccinia virus binding to cell surface heparan sulfate during viral infection.

Attenuation of B5R mutants of rabbitpox virus in vivo is related to impaired growth and not an enhanced host inflammatory response

The rabbitpox virus (RPV) B5R protein, synthesized late in infection, is found as a 45-kDa membrane-associated protein of the envelope of infectious extracellular enveloped virus (EEV) and as a 38-kDa protein secreted from the cell by a process independent of morphogenesis. The protein is not found associated with intracellular mature virus (IMV). Deletion of the gene attenuates the virus (RPV delta B5R) in animals (mice and rabbits), has relatively little effect on formation of IMV, prevents EEV formation in some but not all cells, and leads to a reduced host range. Analysis of the sequence of the protein suggests relatedness to factor H of the complement cascade. Collectively, these observations suggest that attenuation of the virus in vivo could be linked to an inhibition of the inflammatory response, a deficiency in growth, or both. In this report we have analyzed the behavior of RPV delta B5R in infected mice and rabbits and conclude that attenuation of the mutant virus likely results from simple failure to grow within the infected animal and that the inflammatory response probably contributes little to the observed attenuation.

Effects of poliovirus 2A(pro) on vaccinia virus gene expression

The effects of transient expression of poliovirus 2A(pro) on p220 cleavage in COS cells have been analyzed. When 2A(pro) was cloned in plasmid pTM1 and transiently expressed in COS cells, efficient cleavage of p220 occurred after infection of these cells with a recombinant vaccinia virus bearing phage T7 RNA polymerase. High numbers of COS cells were transfected with pTM1-2A, as judged by p220 cleavage, thereby allowing an analysis of the effects of poliovirus 2A(pro) on vaccinia virus gene expression. A 40-50% cleavage of p220 by transfected poliovirus 2A(pro) was observed ten hours post infection and cleavage was almost complete (80-90%) 20-25 hours post infection with vaccinia virus. Profound inhibition of vaccinia virus protein synthesis was detectable ten hours post infection and was maximal 20-25 hours post infection. This inhibition resulted from neither a blockade of transcription of vaccinia virus nor a lack of translatability of the mRNAs present in cells that synthesize poliovirus 2A(pro). Addition of ara-C inhibited the replication of vaccinia virus and allowed the continued synthesis of cellular proteins. Under these conditions, 2A(pro) is expressed and blocks cellular translation. Finally, p220 cleavage by 2A(pro) did not inhibit the translation of a mRNA encoding poliovirus protein 2C, as directed by the 5' leader sequences of encephalomiocarditis virus. Therefore, these findings show a correlation between p220 cleavage and inhibition of translation from newly made mRNAs. Our results are discussed in the light of present knowledge of p220 function, and new approaches are considered that might provide further insights into the function(s) of initiation factor eIF-4F.

Vaccinia virus nucleoside triphosphate phosphohydrolase I controls early and late gene expression by regulating the rate of transcription

We have carried out a detailed analysis of viral mRNAs and proteins produced in cultured cells infected with a temperature-sensitive vaccinia virus mutant (ts36) containing a modified nucleoside triphosphate phosphohydrolase I (NPH-I), a nucleic acid-dependent ATPase. Using a recombinant virus (ts36LUC) which expresses the luciferase marker, we showed in seven different cell lines that early expression of the receptor gene is strongly inhibited (73.8 to 98.7%) at the nonpermissive temperature. The steady-state levels of different early viral polypeptides were also severely reduced. Analysis of steady-state mRNA levels for two early genes (DNA polymerase and D5) showed that inhibition of early polypeptide synthesis correlated with a reduction in the levels of mRNA accumulated at the nonpermissive temperature. Analysis of steady-state levels of late viral polypeptides and of mRNAs indicated that NPH-I regulation of intermediate and late gene expression is direct and not simply a consequence of its role in inhibiting early gene expression. Characterization of a rescued virus (R36) demonstrated that the temperature-sensitive phenotype of ts36 is due solely to the point mutation in the NPH-I gene. The mutant phenotype is not due to reduced levels of NPH-I present in ts36 virions or to the differential stability of this enzyme in cells infected at the nonpermissive temperature but to inhibition of normal enzymatic activity for this protein. Measurement of viral transcriptional activity in permeabilized purified virions demonstrated that NPH-I is required for normal rates of transcription in vaccinia virus. Our findings show ts36 to be a strongly defective early mutant of vaccinia virus and prove that NPH-I plays a key role in the control of early and late virus gene expression, possibly by way of an auxiliary function which regulates mRNA transcription during the virus growth cycle.

Recombinant vaccinia viruses expressing GP46/M-2 protect against Leishmania infection

Leishmania is a genus of parasitic protozoa capable of causing a spectrum of human diseases. The GP46/M-2 membrane glycoprotein has been demonstrated in a murine model system to elicit a protective immune response against infection with Leishmania amazonensis; in highly susceptible BALB/c mice, immunization leads to significant protection against infection. In the present study, for induction of long-term immunological effects, two recombinant vaccinia viruses, derived from the wild type and attenuated variant 48-7 and expressing the GP46/M-2 protein, were constructed; to ensure safety, we used the attenuated vaccinia virus mutant (48-7) as a live vector. Susceptible BALB/c mice immunized with either GP46/M-2-recombinant vaccinia virus were significantly protected against infection with L. amazonensis; 45 to 76% of the animals were completely protected (sterile) against a challenge inoculum of 10(3) infective organisms. The protectively immunized animals demonstrated T- and B-cell-dependent immunological responses; both lymphokine responses as well as antibody responses and long-term memory are indicative of T-cell activation. This first report of the use of a recombinant vaccinia virus to induce protection against a Leishmania infection indicates that recombinant vaccinia viruses should be of value in the design of a safe and effective vaccine against this parasitic disease.

The vaccinia virus 14-kilodalton fusion protein forms a stable complex with the processed protein encoded by the vaccinia virus A17L gene

The mechanism by which the 14-kDa fusion protein of vaccinia virus (VV) is anchored in the envelope of intracellular naked virions (INV) is not understood. In this investigation, we demonstrate that the 14-kDa protein interacts with another virus protein with an apparent molecular mass of 21 kDa. Microsequence analysis of the N terminus of the 21-kDa protein revealed that this protein is encoded by the VV A17L gene. The 21-kDa protein is processed from a 23-kDa precursor, by cleavage at amino acid position 16, at the consensus motif Ala-Gly-Ala, previously identified as a cleavage site for several VV structural proteins. The 21-kDa protein contains two large internal hydrophobic domains characteristic of membrane proteins. Pulse-chase analysis showed that within 1 h after synthesis, the 14-kDa protein forms a stable complex with the 21-kDa protein. Formation of the complex was not inhibited by rifampin, indicating that the interaction between these two proteins occurs prior to virion morphogenesis. Immunoprecipitation analysis of disrupted virions showed the presence of the 21-kDa protein in the viral particle. Release of the 14-kDa-21-kDa protein complex from INV required treatment with the nonionic detergent Nonidet P-40 and a reducing agent. The protein complex consisted of 14-kDa trimers and of 21-kDa dimers. Since the 14-kDa fusion protein lacks a signal sequence and a large hydrophobic domain characteristic of membrane proteins, our findings suggest that the 21-kDa protein serves to anchor the 14-kDa protein to the envelope of INV.

Characterization of the gene encoding the most abundant in vitro translation product from virus-infected Chlorella-like algae

The gene (33kDa) encoding a 33-kDa peptide from Chlorella virus, PBCV-1, was cloned and sequenced. This gene encodes the most abundant in vitro translation product synthesized from viral mRNAs isolated beginning at 20 min post-infection. The message persisted throughout the remainder of the viral life cycle. An open reading frame (ORF) of 717 bp, which encodes a polypeptide of 238 amino acids with a predicted M(r) or 26,613, was found on a 2752-bp cloned fragment from PBCV-1 HindIII restriction fragment 9. Transcriptional analysis of this ORF indicated that it was expressed both early and late, and as the viral life cycle progressed, the mRNA increased in size and abundance. Three other ORFs were also found; the largest of which (741 bp) hybridized to a low-abundance transcript which would encode a polypeptide with a predicted M(r) of 27,854.

Sequence and analysis of a portion of the genomes of Shope fibroma virus and malignant rabbit fibroma virus that is important for viral replication in lymphocytes

The 10.7-kb BamHI "C" restriction fragment of malignant rabbit fibroma virus (MV) contains genes that are important for its immunosuppressive activity. When this fragment is transferred to a related avirulent leporipoxvirus, Shope fibroma virus (SFV), recombinant viruses show clinical features characteristic of MV: they replicate in lymphocytes and alter immune function in vitro, induce disseminated tumors in recipient rabbits, and are immunosuppressive in vivo. The 10.7-kb BamHI "C" restriction fragment of MV was sequenced in its entirety. Its DNA sequence and the 14 ORF's derived from analyzing this sequence are discussed. Analysis of known open reading frames to which the ORF's from MV's Bam "C" fragment show homology permits us to identify some MV ORF's showing high degrees of similarity to known and postulated proteins produced by vaccinia virus. Functions for some of these vaccinia proteins are known, while functions for others are hypothetical or unknown. Further analysis of genetic determinants of MV's virulence has indicated that two overlapping restriction subfragments of the BamHI "C" fragment can transfer MV's virulent behavior to SFV. The 0.7-kb region in which these two subfragments overlap includes the C-terminus of MV orf C-7 and the N terminus of MV orf C-8. These correspond to the C- and N-termini, respectively, of SFV orf's D-9 and D-10 and to vaccinia orf's D-6 (early transcription factor) and D-7 (subunit of RNA polymerase). We sequenced the region of SFV's BamHI "D" fragment in this area and illustrate here the comparative sequences of this portion of SFV's genome and orf's. On the basis of comparisons between MV, SFV, and vaccinia in this area we discuss the potential significance of these observations.

The purified 14-kilodalton envelope protein of vaccinia virus produced in Escherichia coli induces virus immunity in animals

Vaccinia virus (VV) was successfully used as a live vaccine to eradicate smallpox, but the nature of viral proteins involved in eliciting viral immunity has not yet been identified. A potential candidate is a 14-kDa VV envelope protein that is involved in virus penetration at the level of virus-cell fusion, in cell-cell fusion late in infection, and in virus dissemination. The 14-kDa envelope protein has been produced in Escherichia coli, with properties similar to those of the native protein found in the virus particle and in infected cells (C. Lai, S. Gong, and M. Esteban, J. Biol. Chem. 256:22174-22180, 1990). In this investigation, we showed that mice immunized with purified VV 14-kDa protein synthesized in E. coli in the form of a monomer or a trimer develop high-titer neutralizing antibodies and are protected when challenged with lethal doses of wild-type VV. Our findings demonstrate that it is possible to confer protection against VV through immunization with the 14-kDa envelope protein.

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.

Functional and structural effects of an Ala to Val mutation in the adenovirus serotype 2 fibre

H2ts125 is a fibre-defective, temperature-sensitive mutant of adenovirus serotype 2. H2ts125 fibre is unstable at the non-permissive temperature (ts phenotype), and does not migrate in the same way as the wild-type fibre in an SDS/polyacrylamide gel (elm phenotype). Sequence analysis has shown that H2ts125 carries two mutations on the fibre gene: Leu105 to Phe, and Ala434 to Val. Analysis of the structural modifications occurring in H2ts125 fibre was performed using peptide finger-printing and antipeptide sera as immunological probes. We found that all the detectable structural alterations in the mutant fibre were due to the substitution on codon 434. In addition, the ts phenotype was rescued by a wild-type DNA fragment containing the 3' moiety of the fibre gene and overlapping the 434th codon. Morphological analysis of fibre molecules observed under the electron microscope showed minor but statistically significant differences in the fibre length between mutant and wild-type. The mutant fibre was found to be slightly longer (308.8 +/- 1.9 A) than the wild-type fibre (300.1 +/- 2.1 A). Thus both ts and elm phenotypes were carried by the same Ala434 to Val mutation which probably resulted from a change in the three-dimensional structure of the fibre protein, and not from some proteolytic cleavage.

Vaccinia virus induces cell fusion at acid pH and this activity is mediated by the N-terminus of the 14-kDa virus envelope protein

The mechanism by which the large-size poxviruses enter animal cells is not known. In this investigation we show that acid pH treatment of wild-type vaccinia virus-infected cells triggers strong fusion of cells in culture, with an optimum at pH 4.8. We have identified the virus-induced fusion protein as a 14-kDa envelope protein, based on the ability of a 14-kDa specific monoclonal antibody (mAbC3) to block vaccinia virus-induced fusion-from-within and fusion-from-without. We provide genetic evidence for a role of the 14-kDa protein in cell fusion, since insertion of the 14-kDa encoding gene into the genome of nonfusogenic mutant viruses generates heterozygous viruses that now acquire acid pH-dependent fusion activity. DNA sequence analyses of the 14-kDa encoding gene of the mutant viruses, 65-16 and 101-14, reveal N-terminal deletions of 46 and 10 amino acids, respectively. These deletions remove a small hydrophobic region at the N-terminus of the 14-kDa protein and prevent fusion. Our findings demonstrate that vaccinia virus can induce strong fusion of cells in culture at acid pH implying some entry of the virus by endocytosis, that the 14-kDa virus envelope protein is the fusogenic protein, and that the N-terminal proximal region is involved in fusion.