Vaccinia virus A26 and A27 proteins form a stable complex tethered to mature virions by association with the A17 transmembrane protein

Tutorial: design, production and testing of oncolytic viruses for cancer immunotherapy

Oncolytic viruses (OVs) represent a novel class of cancer immunotherapy agents that preferentially infect and kill cancer cells and promote protective antitumor immunity. Furthermore, OVs can be used in combination with established or upcoming immunotherapeutic agents, especially immune checkpoint inhibitors, to efficiently target a wide range of malignancies. The development of OV-based therapy involves three major steps before clinical evaluation: design, production and preclinical testing. OVs can be designed as natural or engineered strains and subsequently selected for their ability to kill a broad spectrum of cancer cells rather than normal, healthy cells. OV selection is further influenced by multiple factors, such as the availability of a specific viral platform, cancer cell permissivity, the need for genetic engineering to render the virus non-pathogenic and/or more effective and logistical considerations around the use of OVs within the laboratory or clinical setting. Selected OVs are then produced and tested for their anticancer potential by using syngeneic, xenograft or humanized preclinical models wherein immunocompromised and immunocompetent setups are used to elucidate their direct oncolytic ability as well as indirect immunotherapeutic potential in vivo. Finally, OVs demonstrating the desired anticancer potential progress toward translation in patients with cancer. This tutorial provides guidelines for the design, production and preclinical testing of OVs, emphasizing considerations specific to OV technology that determine their clinical utility as cancer immunotherapy agents.

Monkeypox virus genomic accordion strategies

The 2023 monkeypox (mpox) epidemic was caused by a subclade IIb descendant of a monkeypox virus (MPXV) lineage traced back to Nigeria in 1971. Person-to-person transmission appears higher than for clade I or subclade IIa MPXV, possibly caused by genomic changes in subclade IIb MPXV. Key genomic changes could occur in the genome's low-complexity regions (LCRs), which are challenging to sequence and are often dismissed as uninformative. Here, using a combination of highly sensitive techniques, we determine a high-quality MPXV genome sequence of a representative of the current epidemic with LCRs resolved at unprecedented accuracy. This reveals significant variation in short tandem repeats within LCRs. We demonstrate that LCR entropy in the MPXV genome is significantly higher than that of single-nucleotide polymorphisms (SNPs) and that LCRs are not randomly distributed. In silico analyses indicate that expression, translation, stability, or function of MPXV orthologous poxvirus genes (OPGs), including OPG153, OPG204, and OPG208, could be affected in a manner consistent with the established "genomic accordion" evolutionary strategies of orthopoxviruses. We posit that genomic studies focusing on phenotypic MPXV differences should consider LCR variability.

The vaccinia chondroitin sulfate binding protein drives host membrane curvature to facilitate fusion

Cellular attachment of viruses determines their cell tropism and species specificity. For entry, vaccinia, the prototypic poxvirus, relies on four binding proteins and an eleven-protein entry fusion complex. The contribution of the individual virus binding proteins to virion binding orientation and membrane fusion is unclear. Here, we show that virus binding proteins guide side-on virion binding and promote curvature of the host membrane towards the virus fusion machinery to facilitate fusion. Using a membrane-bleb model system together with super-resolution and electron microscopy we find that side-bound vaccinia virions induce membrane invagination in the presence of low pH. Repression or deletion of individual binding proteins reveals that three of four contribute to binding orientation, amongst which the chondroitin sulfate binding protein, D8, is required for host membrane bending. Consistent with low-pH dependent macropinocytic entry of vaccinia, loss of D8 prevents virion-associated macropinosome membrane bending, disrupts fusion pore formation and infection. Our results show that viral binding proteins are active participants in successful virus membrane fusion and illustrate the importance of virus protein architecture for successful infection.

Functional epitopes and neutralizing antibodies of vaccinia virus

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

The prospective outcome of the monkeypox outbreak in 2022 and characterization of monkeypox disease immunobiology

A new threat to global health re-emerged with monkeypox's advent in early 2022. As of November 10, 2022, nearly 80,000 confirmed cases had been reported worldwide, with most of them coming from places where the disease is not common. There were 53 fatalities, with 40 occurring in areas that had never before recorded monkeypox and the remaining 13 appearing in the regions that had previously reported the disease. Preliminary genetic data suggest that the 2022 monkeypox virus is part of the West African clade; the virus can be transmitted from person to person through direct interaction with lesions during sexual activity. It is still unknown if monkeypox can be transmitted via sexual contact or, more particularly, through infected body fluids. This most recent epidemic's reservoir host, or principal carrier, is still a mystery. Rodents found in Africa can be the possible intermediate host. Instead, the CDC has confirmed that there are currently no particular treatments for monkeypox virus infection in 2022; however, antivirals already in the market that are successful against smallpox may mitigate the spread of monkeypox. To protect against the disease, the JYNNEOS (Imvamune or Imvanex) smallpox vaccine can be given. The spread of monkeypox can be slowed through measures such as post-exposure immunization, contact tracing, and improved case diagnosis and isolation. Final Thoughts: The latest monkeypox epidemic is a new hazard during the COVID-19 epidemic. The prevailing condition of the monkeypox epidemic along with coinfection with COVID-19 could pose a serious condition for clinicians that could lead to the global epidemic community in the form of coinfection.

LINE-1 retrotransposons facilitate horizontal gene transfer into poxviruses

There is ample phylogenetic evidence that many critical virus functions, like immune evasion, evolved by the acquisition of genes from their hosts through horizontal gene transfer (HGT). However, the lack of an experimental system has prevented a mechanistic understanding of this process. We developed a model to elucidate the mechanisms of HGT into vaccinia virus, the prototypic poxvirus. All identified gene capture events showed signatures of long interspersed nuclear element-1 (LINE-1)-mediated retrotransposition, including spliced-out introns, polyadenylated tails, and target site duplications. In one case, the acquired gene integrated together with a polyadenylated host U2 small nuclear RNA. Integrations occurred across the genome, in some cases knocking out essential viral genes. These essential gene knockouts were rescued through a process of complementation by the parent virus followed by nonhomologous recombination during serial passaging to generate a single, replication-competent virus. This work links multiple evolutionary mechanisms into one adaptive cascade and identifies host retrotransposons as major drivers for virus evolution.

Monkeypox: disease epidemiology, host immunity and clinical interventions

Monkeypox virus (MPXV), which causes disease in humans, has for many years been restricted to the African continent, with only a handful of sporadic cases in other parts of the world. However, unprecedented outbreaks of monkeypox in non-endemic regions have recently taken the world by surprise. In less than 4 months, the number of detected MPXV infections has soared to more than 48,000 cases, recording a total of 13 deaths. In this Review, we discuss the clinical, epidemiological and immunological features of MPXV infections. We also highlight important research questions and new opportunities to tackle the ongoing monkeypox outbreak.

In this Review, Ng and colleagues examine the clinical, epidemiological and immunological aspects of monkeypox virus (MPXV) infections, with a focus on mechanisms of host immunity to MPXV. The authors also consider the unique epidemiological and pathological characteristics of the current non-endemic outbreak of the virus and discuss vaccines, therapeutics and outstanding research questions.

Engineered Promoter-Switched Viruses Reveal the Role of Poxvirus Maturation Protein A26 as a Negative Regulator of Viral Spread

Vaccinia virus produces two types of virions known as single-membraned intracellular mature virus (MV) and double-membraned extracellular enveloped virus (EV). EV production peaks earlier when initial MVs are further wrapped and secreted to spread infection within the host. However, late during infection, MVs accumulate intracellularly and become important for host-to-host transmission. The process that regulates this switch remains elusive and is thought to be influenced by host factors. Here, we examined the hypothesis that EV and MV production are regulated by the virus through expression of F13 and the MV-specific protein A26. By switching the promoters and altering the expression kinetics of F13 and A26, we demonstrate that A26 expression downregulates EV production and plaque size, thus limiting viral spread. This process correlates with A26 association with the MV surface protein A27 and exclusion of F13, thus reducing EV titers. Thus, MV maturation is controlled by the abundance of the viral A26 protein, independently of other factors, and is rate limiting for EV production. The A26 gene is conserved within vertebrate poxviruses but is strikingly lost in poxviruses known to be transmitted exclusively by biting arthropods. A26-mediated virus maturation thus has the appearance to be an ancient evolutionary adaptation to enhance transmission of poxviruses that has subsequently been lost from vector-adapted species, for which it may serve as a genetic signature. The existence of virus-regulated mechanisms to produce virions adapted to fulfill different functions represents a novel level of complexity in mammalian viruses with major impacts on evolution, adaptation, and transmission. IMPORTANCE Chordopoxviruses are mammalian viruses that uniquely produce a first type of virion adapted to spread within the host and a second type that enhances transmission between hosts, which can take place by multiple ways, including direct contact, respiratory droplets, oral/fecal routes, or via vectors. Both virion types are important to balance intrahost dissemination and interhost transmission, so virus maturation pathways must be tightly controlled. Here, we provide evidence that the abundance and kinetics of expression of the viral protein A26 regulates this process by preventing formation of the first form and shifting maturation toward the second form. A26 is expressed late after the initial wave of progeny virions is produced, so sufficient viral dissemination is ensured, and A26 provides virions with enhanced environmental stability. Conservation of A26 in all vertebrate poxviruses, but not in those transmitted exclusively via biting arthropods, reveals the importance of A26-controlled virus maturation for transmission routes involving environmental exposure.

Inactivation of Genes by Frameshift Mutations Provides Rapid Adaptation of an Attenuated Vaccinia Virus

Unlike RNA viruses, most DNA viruses replicate their genomes with high-fidelity polymerases that rarely make base substitution errors. Nevertheless, experimental evolution studies have revealed rapid acquisition of adaptive mutations during serial passage of attenuated vaccinia virus (VACV). One way in which adaptation can occur is by an accordion mechanism in which the gene copy number increases followed by base substitutions and, finally, contraction of the gene copy number. Here, we show rapid acquisition of multiple adaptive mutations mediated by a gene-inactivating frameshift mechanism during passage of an attenuated VACV. Attenuation had been achieved by exchanging the VACV A8R intermediate transcription factor gene with the myxoma virus ortholog. A total of seven mutations in six different genes occurred in three parallel passages of the attenuated virus. The most frequent mutations were single-nucleotide insertions or deletions within runs of five to seven As or Ts, although a deletion of 11 nucleotides also occurred, leading to frameshifts and premature stop codons. During 10 passage rounds, the attenuated VACV was replaced by the mutant viruses. At the end of the experiment, virtually all remaining viruses had one fixed mutation and one or more additional mutations. Although nucleotide substitutions in the transcription apparatus accounted for two low-frequency mutations, frameshifts in genes encoding protein components of the mature virion, namely, A26L, G6R, and A14.5L, achieved 74% to 98% fixation. The adaptive role of the mutations was confirmed by making recombinant VACV with A26L or G6R or both deleted, which increased virus replication levels and decreased particle/PFU ratios.IMPORTANCE Gene inactivation is considered to be an important driver of orthopoxvirus evolution. Whereas cowpox virus contains intact orthologs of genes present in each orthopoxvirus species, numerous genes are inactivated in all other members of the genus. Inactivation of additional genes can occur upon extensive passaging of orthopoxviruses in cell culture leading to attenuation in vivo, a strategy for making vaccines. Whether inactivation of multiple viral genes enhances replication in the host cells or has a neutral effect is unknown in most cases. Using an experimental evolution protocol involving serial passages of an attenuated vaccinia virus, rapid acquisition of inactivating frameshift mutations occurred. After only 10 passage rounds, the starting attenuated vaccinia virus was displaced by viruses with one fixed mutation and one or more additional mutations. The high frequency of multiple inactivating mutations during experimental evolution simulates their acquisition during normal evolution and extensive virus passaging to make vaccine strains.

Vaccinia viral A26 protein is a fusion suppressor of mature virus and triggers membrane fusion through conformational change at low pH

Vaccinia mature virus requires A26 envelope protein to mediate acid-dependent endocytosis into HeLa cells in which we hypothesized that A26 protein functions as an acid-sensitive membrane fusion suppressor. Here, we provide evidence showing that N-terminal domain (aa1-75) of A26 protein is an acid-sensitive region that regulates membrane fusion. Crystal structure of A26 protein revealed that His48 and His53 are in close contact with Lys47, Arg57, His314 and Arg312, suggesting that at low pH these His-cation pairs could initiate conformational changes through protonation of His48 and His53 and subsequent electrostatic repulsion. All the A26 mutant mature viruses that interrupted His-cation pair interactions of His48 and His 53 indeed have lost virion infectivity. Isolation of revertant viruses revealed that second site mutations caused frame shifts and premature termination of A26 protein such that reverent viruses regained cell entry through plasma membrane fusion. Together, we conclude that viral A26 protein functions as an acid-sensitive fusion suppressor during vaccinia mature virus endocytosis.

Vaccinia virus is a complex large DNA virus with a large number of viral membrane proteins to facilitate cell entry. Although it is well established that vaccinia mature virus uses endocytosis to enter cells, it remains unclear how it triggers membrane fusion in the acidic environment of endosomes. Recently, we hypothesized that A26 protein in vaccinia mature virus functions as an acid-sensitive membrane fusion suppressor, which suggests a novel viral regulation not present in other enveloped viruses. We postulated that conformational changes of A26 protein at low pH result in de-repression of viral fusion complex activity to trigger viral and endosomal membrane fusion. Here, we provide structural, biochemical and biological evidence demonstrating that vaccinia A26 protein does indeed function as an acid-sensitive fusion suppressor protein to regulate vaccinia mature virus membrane fusion during endocytosis. Our data reveal an important and unique “checkpoint” for vaccinia mature virus endocytosis that has not been described for other viruses. Furthermore, by isolating adaptive vaccinia mutants that escaped endocytic blockage, we discovered that mutations within the A26L gene serve as an effective strategy for switching the viral infection route from endocytosis to plasma membrane fusion, expanding viral host range.

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.

The Vaccinia virion: Filling the gap between atomic and ultrastructure

We have investigated the molecular-level structure of the Vaccinia virion in situ by protein-protein chemical crosslinking, identifying 4609 unique-mass crosslink ions at an effective FDR of 0.33%, covering 2534 unique pairs of crosslinked protein positions, 625 of which were inter-protein. The data were statistically non-random and rational in the context of known structures, and showed biological rationality. Crosslink density strongly tracked the individual proteolytic maturation products of p4a and p4b, the two major virion structural proteins, and supported the prediction of transmembrane domains within membrane proteins. A clear sub-network of four virion structural proteins provided structural insights into the virion core wall, and proteins VP8 and A12 formed a strongly-detected crosslinked pair with an apparent structural role. A strongly-detected sub-network of membrane proteins A17, H3, A27 and A26 represented an apparent interface of the early-forming virion envelope with structures added later during virion morphogenesis. Protein H3 seemed to be the central hub not only for this sub-network but also for an 'attachment protein' sub-network comprising membrane proteins H3, ATI, CAHH(D8), A26, A27 and G9. Crosslinking data lent support to a number of known interactions and interactions within known complexes. Evidence is provided for the membrane targeting of genome telomeres. In covering several orders of magnitude in protein abundance, this study may have come close to the bottom of the protein-protein crosslinkome of an intact organism, namely a complex animal virus.

Deletion of the Vaccinia Virus I2 Protein Interrupts Virion Morphogenesis, Leading to Retention of the Scaffold Protein and Mislocalization of Membrane-Associated Entry Proteins

The I2L open reading frame of vaccinia virus (VACV) encodes a conserved 72-amino-acid protein with a putative C-terminal transmembrane domain. Previous studies with a tetracycline-inducible mutant demonstrated that I2-deficient virions are defective in cell entry. The purpose of the present study was to determine the step of replication or entry that is affected by loss of the I2 protein. Fluorescence microscopy experiments showed that I2 colocalized with a major membrane protein of immature and mature virions. We generated a cell line that constitutively expressed I2 and allowed construction of the VACV I2L deletion mutant vΔI2. As anticipated, vΔI2 was unable to replicate in cells that did not express I2. Unexpectedly, morphogenesis was interrupted at a stage after immature virion formation, resulting in the accumulation of dense spherical particles instead of brick-shaped mature virions with well-defined core structures. The abnormal particles retained the D13 scaffold protein of immature virions, were severely deficient in the transmembrane proteins that comprise the entry fusion complex (EFC), and had increased amounts of unprocessed membrane and core proteins. Total lysates of cells infected with vΔI2 also had diminished EFC proteins due to instability attributed to their hydrophobicity and failure to be inserted into viral membranes. A similar instability of EFC proteins had previously been found with unrelated mutants blocked earlier in morphogenesis that also accumulated viral membranes retaining the D13 scaffold. We concluded that I2 is required for virion morphogenesis, release of the D13 scaffold, and the association of EFC proteins with viral membranes.IMPORTANCE Poxviruses comprise a large family that infect vertebrates and invertebrates, cause disease in both in humans and in wild and domesticated animals, and are being engineered as vectors for vaccines and cancer therapy. In addition, investigations of poxviruses have provided insights into many aspects of cell biology. The I2 protein is conserved in all poxviruses that infect vertebrates, suggesting an important role. The present study revealed that this protein is essential for vaccinia virus morphogenesis and that its absence results in an accumulation of deformed virus particles retaining the scaffold protein and deficient in surface proteins needed for cell entry.

Glycosaminoglycans-Specific Cell Targeting and Imaging Using Fluorescent Nanodiamonds Coated with Viral Envelope Proteins

Understanding virus-host interactions is crucial for vaccine development. This study investigates such interactions using fluorescent nanodiamonds (FNDs) coated with vaccinia envelope proteins as the model system. To achieve this goal, we noncovalently conjugated 100 nm FNDs with rA27(aa 21-84), a recombinant envelope protein of vaccinia virus, for glycosaminoglycans (GAGs)-specific targeting and imaging of living cells. Another recombinant protein rDA27(aa 33-84) that removes the GAGs-binding sequences was also used for comparison. Three types of A27-coated FNDs were generated, including rA27(aa 21-84)-FND, rDA27(aa 33-84)-FND, and hybrid rA27(aa 21-84)/rDA27(aa 33-84)-FND. The specificity of these viral protein-FND conjugates toward GAGs binding was examined by flow cytometry, fluorescence microscopy, and gel electrophoresis. Results obtained for normal and GAGs-deficient cells showed that the recombinant proteins maintain their GAG-targeting activities even after immobilization on the FND surface. Our studies provide a new nanoparticle-based platform not only to target specific cell types but also to track the fates of these immobilized viral proteins in targeted cells as well as to isolate and enrich GAGs-associated proteins on cell membrane.

Antibody Recognition of Immunodominant Vaccinia Virus Envelope Proteins

Vaccinia Virus (VACV) is an enveloped double stranded DNA virus and the active ingredient of the smallpox vaccine. The systematic administration of this vaccine led to the eradication of circulating smallpox (variola virus, VARV) from the human population. As a tribute to its success, global immunization was ended in the late 1970s. The efficacy of the vaccine is attributed to a robust production of protective antibodies against several envelope proteins of VACV, which cross-protect against infection with pathogenic VARV. Since global vaccination was ended, most children and young adults do not possess immunity against smallpox. This is a concern, since smallpox is considered a potential bioweapon. Although the smallpox vaccine is considered the gold standard of all vaccines and the targeted antigens have been widely studied, the epitopes that are targeted by the protective antibodies and their mechanism of binding had been, until recently, poorly characterized. Understanding the precise interaction between the antibodies and their epitopes will be helpful in the design of better vaccines against other diseases. In this review we will discuss the structural basis of recognition of the immunodominant VACV antigens A27, A33, D8, and L1 by protective antibodies and discuss potential implications regarding their protective capacity.

Linear Epitopes in Vaccinia Virus A27 Are Targets of Protective Antibodies Induced by Vaccination against Smallpox

Vaccinia virus (VACV) A27 is a target for viral neutralization and part of the Dryvax smallpox vaccine. A27 is one of the three glycosaminoglycan (GAG) adhesion molecules and binds to heparan sulfate. To understand the function of anti-A27 antibodies, especially their protective capacity and their interaction with A27, we generated and subsequently characterized 7 murine monoclonal antibodies (MAbs), which fell into 4 distinct epitope groups (groups I to IV). The MAbs in three groups (groups I, III, and IV) bound to linear peptides, while the MAbs in group II bound only to VACV lysate and recombinant A27, suggesting that they recognized a conformational and discontinuous epitope. Only group I antibodies neutralized the mature virion in a complement-dependent manner and protected against VACV challenge, while a group II MAb partially protected against VACV challenge but did not neutralize the mature virion. The epitope for group I MAbs was mapped to a region adjacent to the GAG binding site, a finding which suggests that group I MAbs could potentially interfere with the cellular adhesion of A27. We further determined the crystal structure of the neutralizing group I MAb 1G6, as well as the nonneutralizing group IV MAb 8E3, bound to the corresponding linear epitope-containing peptides. Both the light and the heavy chains of the antibodies are important in binding to their antigens. For both antibodies, the L1 loop seems to dominate the overall polar interactions with the antigen, while for MAb 8E3, the light chain generally appears to make more contacts with the antigen.

IMPORTANCE

Vaccinia virus is a powerful model to study antibody responses upon vaccination, since its use as the smallpox vaccine led to the eradication of one of the world's greatest killers. The immunodominant antigens that elicit the protective antibodies are known, yet for many of these antigens, little information about their precise interaction with antibodies is available. In an attempt to better understand the interplay between the antibodies and their antigens, we generated and functionally characterized a panel of anti-A27 antibodies and studied their interaction with the epitope using X-ray crystallography. We identified one protective antibody that binds adjacent to the heparan sulfate binding site of A27, likely affecting ligand binding. Analysis of the antibody-antigen interaction supports a model in which antibodies that can interfere with the functional activity of the antigen are more likely to confer protection than those that bind at the extremities of the antigen.

From crescent to mature virion: vaccinia virus assembly and maturation

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

A highly specific monoclonal antibody against monkeypox virus detects the heparin binding domain of A27

The eradication of smallpox and the cessation of global vaccination led to the increased prevalence of human infections in Central Africa. Serologic and protein-based diagnostic assay for MPXV detection is difficult due to cross-reactive antibodies that do not differentiate between diverse orthopoxvirus (OPXV) species. A previously characterized monoclonal antibody (mAb 69-126-3-7) against MPXV [1] was retested for cross-reactivity with various OPXVs. The 14.5 kDa band protein that reacted with mAb 69-126-3 was identified to be MPXV A29 protein (homolog of vaccinia virus Copenhagen A27). Amino acid sequence analysis of the MPXV A29 with other OPXV homologs identified four amino acid changes. Peptides corresponding to these regions were designed and evaluated for binding to mAb 69-126-3 by ELISA and BioLayer Interferometry (BLI). Further refinement and truncations mapped the specificity of this antibody to a single amino acid difference in a 30-mer peptide compared to other OPXV homologs. This particular residue is proposed to be essential for heparin binding by VACV A27 protein. Despite this substitution, MPXV A29 bound to heparin with similar affinity to that of VACV A27 protein, suggesting flexibility of this motif for heparin binding. Although binding of mAb 69-126-3-7 to MPXV A29 prevented interaction with heparin, it did not have any effect on the infectivity of MPXV. Characterization of 69-126-3-7 mAb antibody allows for the possibility of the generation of a serological based species-specific detection of OPXVs despite high proteomic homology.

Elimination of A-type inclusion formation enhances cowpox virus replication in mice: implications for orthopoxvirus evolution

Some orthopoxviruses including cowpox virus embed virus particles in dense bodies, comprised of the A-type inclusion (ATI) protein, which may provide long-term environmental protection. This strategy could be beneficial if the host population is sparse or spread is inefficient or indirect. However, the formation of ATI may be neutral or disadvantageous for orthopoxviruses that rely on direct respiratory spread. Disrupted ATI open reading frames in orthopoxviruses such as variola virus, the agent of smallpox, and monkeypox virus suggests that loss of this feature provided positive selection. To test this hypothesis, we constructed cowpox virus mutants with deletion of the ATI gene or another gene required for embedding virions. The ATI deletion mutant caused greater weight loss and higher replication in the respiratory tract than control viruses, supporting our hypothesis. Deletion of the gene for embedding virions had a lesser effect, possibly due to known additional functions of the encoded protein.

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.

Direct formation of vaccinia virus membranes from the endoplasmic reticulum in the absence of the newly characterized L2-interacting protein A30.5

Crescents consisting of a single lipoprotein membrane with an external protein scaffold comprise the initial structural elements of poxvirus morphogenesis. Crescents enlarge to form spherical immature virions, which enclose viroplasm consisting of proteins destined to form the cores of mature virions. Previous studies suggest that the L2 protein participates in the recruitment of endoplasmic reticulum (ER)-derived membranes to form immature virions within assembly sites of cytoplasmic factories. Here we show that L2 interacts with the previously uncharacterized 42-amino-acid A30.5 protein. An open reading frame similar in size to the one encoding A30.5 is at the same genome location in representatives of all chordopoxvirus genera. A30.5 has a putative transmembrane domain and colocalized with markers of the endoplasmic reticulum and with L2. By constructing a complementing cell line expressing A30.5, we isolated a deletion mutant virus that exhibits a defect in morphogenesis in normal cells. Large electron-dense cytoplasmic inclusions and clusters of scaffold protein-coated membranes that resemble crescents and immature virions devoid of viroplasm were seen in place of normal structures. Crescent-shaped membranes were continuous with the endoplasmic reticulum membrane and oriented with the convex scaffold protein-coated side facing the lumen, while clusters of completed spherical immature-virion-like forms were trapped within the expanded lumen. Immature-virion-like structures were more abundant in infected RK-13 cells than in BS-C-1 or HeLa cells, in which cytoplasmic inclusions were decorated with scaffold protein-coated membrane arcs. We suggest that the outer surface of the poxvirus virion is derived from the luminal side of the ER membrane.

Crystal structure of vaccinia viral A27 protein reveals a novel structure critical for its function and complex formation with A26 protein

Vaccinia virus envelope protein A27 has multiple functions and is conserved in the Orthopoxvirus genus of the poxvirus family. A27 protein binds to cell surface heparan sulfate, provides an anchor for A26 protein packaging into mature virions, and is essential for egress of mature virus (MV) from infected cells. Here, we crystallized and determined the structure of a truncated form of A27 containing amino acids 21-84, C71/72A (tA27) at 2.2 Å resolution. tA27 protein uses the N-terminal region interface (NTR) to form an unexpected trimeric assembly as the basic unit, which contains two parallel α-helices and one unusual antiparallel α-helix; in a serpentine way, two trimers stack with each other to form a hexamer using the C-terminal region interface (CTR). Recombinant tA27 protein forms oligomers in a concentration-dependent manner in vitro in gel filtration. Analytical ultracentrifugation and multi-angle light scattering revealed that tA27 dimerized in solution and that Leu47, Leu51, and Leu54 at the NTR and Ile68, Asn75, and Leu82 at the CTR are responsible for tA27 self-assembly in vitro. Finally, we constructed recombinant vaccinia viruses expressing full length mutant A27 protein defective in either NTR, CTR, or both interactions; the results demonstrated that wild type A27 dimer/trimer formation was impaired in NTR and CTR mutant viruses, resulting in small plaques that are defective in MV egress. Furthermore, the ability of A27 protein to form disulfide-linked protein complexes with A26 protein was partially or completely interrupted by NTR and CTR mutations, resulting in mature virion progeny with increased plasma membrane fusion activity upon cell entry. Together, these results demonstrate that A27 protein trimer structure is critical for MV egress and membrane fusion modulation. Because A27 is a neutralizing target, structural information will aid the development of inhibitors to block A27 self-assembly or complex formation against vaccinia virus infection.

Analysis of viral membranes formed in cells infected by a vaccinia virus L2-deletion mutant suggests their origin from the endoplasmic reticulum

Assembly of the poxvirus immature virion (IV) membrane is a poorly understood event that occurs within the cytoplasm. At least eight viral proteins participate in formation of the viral membrane. Of these, A14, A17, and D13 are structural components whereas A6, A11, F10, H7, and L2 participate in membrane biogenesis. L2, the object of this study, is conserved in all chordopoxviruses, expressed early in infection, and associated with the endoplasmic reticulum (ER) throughout the cell and at the edges of crescent-shaped IV precursors. Previous studies with an inducible L2 mutant revealed abortive formation of the crescent membrane. However, possible low-level L2 synthesis under nonpermissive conditions led to ambiguity in interpretation. Here, we constructed a cell line that expresses L2, which allowed the creation of an L2-deletion mutant. In noncomplementing cells, replication was aborted prior to formation of mature virions and two types of aberrant structures were recognized. One consisted of short crescents, at the surface of dense masses of viroplasm, which were labeled with antibodies to the A11, A14, A17, and D13 proteins. The other structure consisted of "empty" IV-like membranes, also labeled with antibodies to the viral proteins, which appeared to be derived from adjacent calnexin-containing ER. A subset of 25 proteins examined, exemplified by components of the entry-fusion complex, were greatly diminished in amount. The primary role of L2 may be to recruit ER and modulate its transformation to viral membranes in juxtaposition with the viroplasm, simultaneously preventing the degradation of viral proteins dependent on viral membranes for stability.

Orthopoxvirus species and strain differences in cell entry

Vaccinia virus (VACV) enters cells by a low pH endosomal route or by direct fusion with the plasma membrane. We previously found differences in entry properties of several VACV strains: entry of WR was enhanced by low pH, reduced by bafilomycin A1 and relatively unaffected by heparin, whereas entry of IHD-J, Copenhagen and Elstree were oppositely affected. Since binding and entry modes may have been selected by specific conditions of in vitro propagation, we now examined the properties of three distinct, recently isolated cowpox viruses and a monkeypox virus as well as additional VACV and cowpox virus strains. The recent isolates were more similar to WR than to other VACV strains, underscoring the biological importance of endosomal entry by orthopoxviruses. Sequence comparisons, gene deletions and gene swapping experiments indicated that viral determinants, other than or in addition to the A26 and A25 "fusion-suppressor" proteins, impact entry properties.

The role of white spot syndrome virus (WSSV) VP466 protein in shrimp antiviral phagocytosis

Widespread evidence indicates that the structural proteins of virus play very important roles in virus-host interactions. However, the effect of viral proteins on host immunity has not been addressed. Our previous studies revealed that the host shrimp Rab6 (termed as PjRab previously), tropomyosin, β-actin and the white spot syndrome virus (WSSV) envelope protein VP466 formed a complex. In this study, the VP466 protein was shown to be able to bind host Rab6 protein and increase its GTPase activity in vivo and vitro. Thus, VP466 could function as a GTPase-activating protein (GAP) of Rab6. In the VP466-Rab-actin pathway, the increase of the Rab6 activity induced rearrangements of the actin cytoskeleton, resulting in the formation of actin stress fibers which promoted the phagocytosis against virus. Therefore our findings revealed that a viral protein could be employed by host to initiate the host immunity, representing a novel molecular mechanism in the virus-host interaction. Our study would help to better understand the molecular events in immune response against virus infection in invertebrates.

Poxvirus cell entry: how many proteins does it take?

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

Formation of orthopoxvirus cytoplasmic A-type inclusion bodies and embedding of virions are dynamic processes requiring microtubules

In cells infected with some orthopoxviruses, numerous mature virions (MVs) become embedded within large, cytoplasmic A-type inclusions (ATIs) that can protect infectivity after cell lysis. ATIs are composed of an abundant viral protein called ATIp, which is truncated in orthopoxviruses such as vaccinia virus (VACV) that do not form ATIs. To study ATI formation and occlusion of MVs within ATIs, we used recombinant VACVs that express the cowpox full-length ATIp or we transfected plasmids encoding ATIp into cells infected with VACV, enabling ATI formation. ATI enlargement and MV embedment required continued protein synthesis and an intact microtubular network. For live imaging of ATIs and MVs, plasmids expressing mCherry fluorescent protein fused to ATIp were transfected into cells infected with VACV expressing the viral core protein A4 fused to yellow fluorescent protein. ATIs appeared as dynamic, mobile bodies that enlarged by multiple coalescence events, which could be prevented by disrupting microtubules. Coalescence of ATIs was confirmed in cells infected with cowpox virus. MVs were predominantly at the periphery of ATIs early in infection. We determined that coalescence contributed to the distribution of MVs within ATIs and that microtubule-disrupting drugs abrogated coalescence-mediated MV embedment. In addition, MVs were shown to move from viral factories at speeds consistent with microtubular transport to the peripheries of ATIs, whereas disruption of microtubules prevented such trafficking. The data indicate an important role for microtubules in the coalescence of ATIs into larger structures, transport of MVs to ATIs, and embedment of MVs within the ATI matrix.

Vaccinia virus L2 protein associates with the endoplasmic reticulum near the growing edge of crescent precursors of immature virions and stabilizes a subset of viral membrane proteins

The initial step in poxvirus morphogenesis, the formation of crescent membranes, occurs within cytoplasmic factories. L2 is one of several vaccinia virus proteins known to be necessary for formation of crescents and the only one synthesized early in infection. Virus replication was unaffected when the L2R open reading frame was replaced by L2R containing an N-terminal epitope tag while retaining the original promoter. L2 colocalized with the endoplasmic reticulum (ER) protein calnexin throughout the cytoplasm of infected and transfected cells. Topological studies indicated that the N terminus of L2 is exposed to the cytoplasm with the hydrophobic C terminus anchored in the ER. Using immunogold labeling and electron microscopy, L2 was detected in tubular membranes outside factories and inside factories near crescents and close to the edge or rim of crescents; a similar labeling pattern was found for the ER luminal protein disulfide isomerase (PDI). The phenotype of L2 conditional lethal mutants and the localization of L2 suggest that it participates in elongation of crescents by the addition of ER membrane to the growing edge. Small amounts of L2 and PDI were detected within immature and mature virions, perhaps trapped during assembly. The repression of L2, as well as A11 and A17, two other proteins that are required for viral crescent formation, profoundly decreased the stability of a subset of viral membrane proteins including those comprising the entry-fusion complex. To avoid degradation, these unstable membrane proteins may need to directly insert into the viral membrane or be rapidly shunted there from the ER.

Smallpox vaccines: targets of protective immunity

The eradication of smallpox, one of the great triumphs of medicine, was accomplished through the prophylactic administration of live vaccinia virus, a comparatively benign relative of variola virus, the causative agent of smallpox. Nevertheless, recent fears that variola virus may be used as a biological weapon together with the present susceptibility of unimmunized populations have spurred the development of new-generation vaccines that are safer than the original and can be produced by modern methods. Predicting the efficacy of such vaccines in the absence of human smallpox, however, depends on understanding the correlates of protection. This review outlines the biology of poxviruses with particular relevance to vaccine development, describes protein targets of humoral and cellular immunity, compares animal models of orthopoxvirus disease with human smallpox, and considers the status of second- and third-generation smallpox vaccines.

Capturing the natural diversity of the human antibody response against vaccinia virus

The eradication of smallpox (variola) and the subsequent cessation of routine vaccination have left modern society vulnerable to bioterrorism employing this devastating contagious disease. The existing, licensed vaccines based on live vaccinia virus (VACV) are contraindicated for a substantial number of people, and prophylactic vaccination of large populations is not reasonable when there is little risk of exposure. Consequently, there is an emerging need to develop efficient and safe therapeutics to be used shortly before or after exposure, either alone or in combination with vaccination. We have characterized the human antibody response to smallpox vaccine (VACV Lister) in immunized volunteers and isolated a large number of VACV-specific antibodies that recognize a variety of different VACV antigens. Using this broad antibody panel, we have generated a fully human, recombinant analogue to plasma-derived vaccinia immunoglobulin (VIG), which mirrors the diversity and specificity of the human antibody immune response and offers the advantage of unlimited supply and reproducible specificity and activity. The recombinant VIG was found to display a high specific binding activity toward VACV antigens, potent in vitro VACV neutralizing activity, and a highly protective efficacy against VACV challenge in the mouse tail lesion model when given either prophylactically or therapeutically. Altogether, the results suggest that this compound has the potential to be used as an effective postexposure prophylaxis or treatment of disease caused by orthopoxviruses.

Interaction of poxvirus intracellular mature virion proteins with the TPR domain of kinesin light chain in live infected cells revealed by two-photon-induced fluorescence resonance energy transfer fluorescence lifetime imaging microscopy

Using two-photon-induced fluorescence lifetime imaging microscopy, we corroborate an interaction (previously demonstrated by yeast two-hybrid domain analysis) of full-length vaccinia virus (VACV; an orthopoxvirus) A36 protein with the cellular microtubule motor protein kinesin. Quenching of enhanced green fluorescent protein (EGFP), fused to the C terminus of VACV A36, by monomeric red fluorescent protein (mDsRed), fused to the tetratricopeptide repeat (TPR) domain of kinesin, was observed in live chicken embryo fibroblasts infected with either modified vaccinia virus Ankara (MVA) or wild-type fowlpox virus (FWPV; an avipoxvirus), and the excited-state fluorescence lifetime of EGFP was reduced from 2.5 ± 0.1 ns to 2.1 ± 0.1 ns due to resonance energy transfer to mDsRed. FWPV does not encode an equivalent of intracellular enveloped virion surface protein A36, yet it is likely that this virus too must interact with kinesin to facilitate intracellular virion transport. To investigate possible interactions between innate FWPV proteins and kinesin, recombinant FWPVs expressing EGFP fused to the N termini of FWPV structural proteins Fpv140, Fpv168, Fpv191, and Fpv198 (equivalent to VACV H3, A4, p4c, and A34, respectively) were generated. EGFP fusions of intracellular mature virion (IMV) surface protein Fpv140 and type II membrane protein Fpv198 were quenched by mDsRed-TPR in recombinant FWPV-infected cells, indicating that these virion proteins are found within 10 nm of mDsRed-TPR. In contrast, and as expected, EGFP fusions of the IMV core protein Fpv168 did not show any quenching. Interestingly, the p4c-like protein Fpv191, which demonstrates late association with preassembled IMV, also did not show any quenching.

Vaccinia virus A25 and A26 proteins are fusion suppressors for mature virions and determine strain-specific virus entry pathways into HeLa, CHO-K1, and L cells

Mature vaccinia virus enters cells through either fluid-phase endocytosis/macropinocytosis or plasma membrane fusion. This may explain the wide range of host cell susceptibilities to vaccinia virus entry; however, it is not known how vaccinia virus chooses between these two pathways and which viral envelope proteins determine such processes. By screening several recombinant viruses and different strains, we found that mature virions containing the vaccinia virus A25 and A26 proteins entered HeLa cells preferentially through a bafilomycin-sensitive entry pathway, whereas virions lacking these two proteins entered through a bafilomycin-resistant pathway. To investigate whether the A25 and A26 proteins contribute to entry pathway specificity, two mutant vaccinia viruses, WRDeltaA25L and WRDeltaA26L, were subsequently generated from the wild-type WR strain. In contrast to the WR strain, both the WRDeltaA25L and WRDeltaA26L viruses became resistant to bafilomycin, suggesting that the removal of the A25 and A26 proteins bypassed the low-pH endosomal requirement for mature virion entry. Indeed, WRDeltaA25L and WRDeltaA26L virus infections of HeLa, CHO-K1, and L cells immediately triggered cell-to-cell fusion at a neutral pH at 1 to 2 h postinfection (p.i.), providing direct evidence that viral fusion machinery is readily activated after the removal of the A25 and A26 proteins to allow virus entry through the plasma membrane. In summary, our data support a model that on vaccinia mature virions, the viral A25 and A26 proteins are low-pH-sensitive fusion suppressors whose inactivation during the endocytic route results in viral and cell membrane fusion. Our results also suggest that during virion morphogenesis, the incorporation of the A25 and A26 proteins into mature virions may help restrain viral fusion activity until the time of infections.

Congregation of orthopoxvirus virions in cytoplasmic A-type inclusions is mediated by interactions of a bridging protein (A26p) with a matrix protein (ATIp) and a virion membrane-associated protein (A27p)

Some orthopoxviruses, e.g., the cowpox, ectromelia, and raccoonpox viruses, form large, discrete cytoplasmic inclusions within which mature virions (MVs) are embedded by a process called occlusion. These inclusions, which may protect occluded MVs in the environment, are composed of aggregates of the A-type inclusion protein (ATIp), which is truncated in orthopoxviruses such as vaccinia virus (VACV) and variola virus that fail to form inclusions. In addition to an intact ATIp, occlusion requires the A26 protein (A26p). Although VACV contains a functional A26p, determined by complementation of a cowpox virus occlusion-defective mutant, its role in occlusion was unknown. We found that restoration of the full-length ATI gene was sufficient for VACV inclusion formation and the ensuing occlusion of MVs. A26p was present in inclusions even when virion assembly was inhibited, suggesting a direct interaction of A26p with ATIp. Analysis of a panel of ATIp mutants indicated that the C-terminal repeat region was required for inclusion formation and the N-terminal domain for interaction with A26p and occlusion. A26p is tethered to MVs via interaction with the A27 protein (A27p); A27p was not required for association of A26p with ATIp but was necessary for occlusion. In addition, the C-terminal domain of A26p, which mediates A26p-A27p interactions, was necessary but insufficient for occlusion. Taken together, the data suggest a model for occlusion in which A26p has a bridging role between ATIp and A27p, and A27p provides a link to the MV membrane.

Poxvirus proteomics and virus-host protein interactions

Studies of the functional proteins encoded by the poxvirus genome provide information about the composition of the virus as well as individual virus-virus protein and virus-host protein interactions, which provides insight into viral pathogenesis and drug discovery. Widely used proteomic techniques to identify and characterize specific protein-protein interactions include yeast two-hybrid studies and coimmunoprecipitations. Recently, various mass spectrometry techniques have been employed to identify viral protein components of larger complexes. These methods, combined with structural studies, can provide new information about the putative functions of viral proteins as well as insights into virus-host interaction dynamics. For viral proteins of unknown function, identification of either viral or host binding partners provides clues about their putative function. In this review, we discuss poxvirus proteomics, including the use of proteomic methodologies to identify viral components and virus-host protein interactions. High-throughput global protein expression studies using protein chip technology as well as new methods for validating putative protein-protein interactions are also discussed.

Characterization of a newly identified 35-amino-acid component of the vaccinia virus entry/fusion complex conserved in all chordopoxviruses

The original annotation of the vaccinia virus (VACV) genome was limited to open reading frames (ORFs) of at least 65 amino acids. Here, we characterized a 35-amino-acid ORF (O3L) located between ORFs O2L and I1L. ORFs similar in length to O3L were found at the same genetic locus in all vertebrate poxviruses. Although amino acid identities were low, the presence of a characteristic N-terminal hydrophobic domain strongly suggested that the other poxvirus genes were orthologs. Further studies demonstrated that the O3 protein was expressed at late times after infection and incorporated into the membrane of the mature virion. An O3L deletion mutant was barely viable, producing tiny plaques and a 3-log reduction in infectious progeny. A mutant VACV with a regulated O3L gene had a similar phenotype in the absence of inducer. There was no apparent defect in virus morphogenesis, though O3-deficient virus had low infectivity. The impairment was shown to be at the stage of virus entry, as cores were not detected in the cytoplasm after virus adsorption. Furthermore, O3-deficient virus did not induce fusion of infected cells when triggered by low pH. These characteristics are hallmarks of a group of proteins that form the entry/fusion complex (EFC). Affinity purification experiments demonstrated an association of O3 with EFC proteins. In addition, the assembly or stability of the EFC was impaired when expression of O3 was repressed. Thus, O3 is the newest recognized component of the EFC and the smallest VACV protein shown to have a function.

Disulfide bond formation at the C termini of vaccinia virus A26 and A27 proteins does not require viral redox enzymes and suppresses glycosaminoglycan-mediated cell fusion

Vaccinia virus A26 protein is an envelope protein of the intracellular mature virus (IMV) of vaccinia virus. A mutant A26 protein with a truncation of the 74 C-terminal amino acids was expressed in infected cells but failed to be incorporated into IMV (W. L. Chiu, C. L. Lin, M. H. Yang, D. L. Tzou, and W. Chang, J. Virol 81:2149-2157, 2007). Here, we demonstrate that A27 protein formed a protein complex with the full-length form but not with the truncated form of A26 protein in infected cells as well as in IMV. The formation of the A26-A27 protein complex occurred prior to virion assembly and did not require another A27-binding protein, A17 protein, in the infected cells. A26 protein contains six cysteine residues, and in vitro mutagenesis showed that Cys441 and Cys442 mediated intermolecular disulfide bonds with Cys71 and Cys72 of viral A27 protein, whereas Cys43 and Cys342 mediated intramolecular disulfide bonds. A26 and A27 proteins formed disulfide-linked complexes in transfected 293T cells, showing that the intermolecular disulfide bond formation did not depend on viral redox pathways. Finally, using cell fusion from within and fusion from without, we demonstrate that cell surface glycosaminoglycan is important for virus-cell fusion and that A26 protein, by forming complexes with A27 protein, partially suppresses fusion.