Conditional lethal expression of the vaccinia virus L1R myristylated protein reveals a role in virion assembly

Exploring monkeypox virus proteins and rapid detection techniques

Monkeypox (mpox) is an infectious disease caused by the mpox virus and can potentially lead to fatal outcomes. It resembles infections caused by viruses from other families, challenging identification. The pathogenesis, transmission, and clinical manifestations of mpox and other Orthopoxvirus species are similar due to their closely related genetic material. This review provides a comprehensive discussion of the roles of various proteins, including extracellular enveloped virus (EEV), intracellular mature virus (IMV), and profilin-like proteins of mpox. It also highlights recent diagnostic techniques based on these proteins to detect this infection rapidly.

Neutralization Determinants on Poxviruses

Smallpox was a highly contagious disease caused by the variola virus. The disease affected millions of people over thousands of years and variola virus ranked as one of the deadliest viruses in human history. The complete eradication of smallpox in 1980, a major triumph in medicine, was achieved through a global vaccination campaign using a less virulent poxvirus, vaccinia virus. Despite this success, the herd immunity established by this campaign has significantly waned, and concerns are rising about the potential reintroduction of variola virus as a biological weapon or the emergence of zoonotic poxviruses. These fears were further fueled in 2022 by a global outbreak of monkeypox virus (mpox), which spread to over 100 countries, thereby boosting interest in developing new vaccines using molecular approaches. However, poxviruses are complex and creating modern vaccines against them is challenging. This review focuses on the structural biology of the six major neutralization determinants on poxviruses (D8, H3, A27, L1, B5, and A33), the localization of epitopes targeted by neutralizing antibodies, and their application in the development of subunit vaccines.

mRNA vaccines encoding fusion proteins of monkeypox virus antigens protect mice from vaccinia virus challenge

The recent outbreaks of mpox have raised concerns over the need for effective vaccines. However, the current approved vaccines have either been associated with safety concerns or are in limited supply. mRNA vaccines, which have shown high efficacy and safety against SARS-CoV-2 infection, are a promising alternative. In this study, three mRNA vaccines are developed that encode monkeypox virus (MPXV) proteins A35R and M1R, including A35R extracellular domain -M1R fusions (VGPox 1 and VGPox 2) and a mixture of encapsulated full-length mRNAs for A35R and M1R (VGPox 3). All three vaccines induce early anti-A35R antibodies in female Balb/c mice, but only VGPox 1 and 2 generate detectable levels of anti-M1R antibodies at day 7 after vaccination. However, all three mRNA vaccine groups completely protect mice from a lethal dose of vaccinia virus (VACV) challenge. A single dose of VGPox 1, 2, and 3 provide protection against the lethal viral challenge within 7 days post-vaccination. Long-term immunity and protection were also observed in all three candidates. Additionally, VGPox 2 provided better passive protection. These results suggest that the VGPox series vaccines enhance immunogenicity and can be a viable alternative to current whole-virus vaccines to defend against mpox.

Near-atomic architecture of Singapore grouper iridovirus and implications for giant virus assembly

Singapore grouper iridovirus (SGIV), one of the nucleocytoviricota viruses (NCVs), is a highly pathogenic iridovirid. SGIV infection results in massive economic losses to the aquaculture industry and significantly threatens global biodiversity. In recent years, high morbidity and mortality in aquatic animals have been caused by iridovirid infections worldwide. Effective control and prevention strategies are urgently needed. Here, we present a near-atomic architecture of the SGIV capsid and identify eight types of capsid proteins. The viral inner membrane-integrated anchor protein colocalizes with the endoplasmic reticulum (ER), supporting the hypothesis that the biogenesis of the inner membrane is associated with the ER. Additionally, immunofluorescence assays indicate minor capsid proteins (mCPs) could form various building blocks with major capsid proteins (MCPs) before the formation of a viral factory (VF). These results expand our understanding of the capsid assembly of NCVs and provide more targets for vaccine and drug design to fight iridovirid infections.

Comprehensive Literature Review of Monkeypox

The current outbreak of monkeypox (MPX) infection has emerged as a global matter of concern in the last few months. MPX is a zoonosis caused by the MPX virus (MPXV), which is one of the Orthopoxvirus species. Thus, it is similar to smallpox caused by the variola virus, and smallpox vaccines and drugs have been shown to be protective against MPX. Although MPX is not a new disease and is rarely fatal, the current multi-country MPX outbreak is unusual because it is occurring in countries that are not endemic for MPXV. In this work, we reviewed the extensive literature available on MPXV to summarize the available data on the major biological, clinical and epidemiological aspects of the virus and the important scientific findings. This review may be helpful in raising awareness of MPXV transmission, symptoms and signs, prevention and protective measures. It may also be of interest as a basis for performance of studies to further understand MPXV, with the goal of combating the current outbreak and boosting healthcare services and hygiene practices.Trial registration: ClinicalTrials.gov identifier: NCT02977715..Trial registration: ClinicalTrials.gov identifier: NCT03745131..Trial registration: ClinicalTrials.gov identifier: NCT00728689..Trial registration: ClinicalTrials.gov identifier: NCT02080767..

New p35 (H3L) Epitope Involved in Vaccinia Virus Neutralization and Its Deimmunization

Vaccinia virus (VACV) is a promising oncolytic agent because it exhibits many characteristic features of an oncolytic virus. However, its effectiveness is limited by the strong antiviral immune response induced by this virus. One possible approach to overcome this limitation is to develop deimmunized recombinant VACV. It is known that VACV p35 is a major protein for B- and T-cell immune response. Despite the relevance of p35, its epitope structure remains insufficiently studied. To determine neutralizing epitopes, a panel of recombinant p35 variants was designed, expressed, and used for mice immunization. Plaque-reduction neutralization tests demonstrated that VACV was only neutralized by sera from mice that were immunized with variants containing both N- and C- terminal regions of p35. This result was confirmed by the depletion of anti-p35 mice sera with recombinant p35 variants. At least nine amino acid residues affecting the immunogenic profile of p35 were identified. Substitutions of seven residues led to disruption of B-cell epitopes, whereas substitutions of two residues resulted in the recognition of the mutant p35 solely by non-neutralizing antibodies.

Identification and screening of host proteins interacting with ORFV-ORF047 protein

BACKGROUND

Orf virus (ORFV) is a member of the genus Parapoxvirus and family Poxviridae. The virus has a worldwide distribution and infects sheep, goats, humans, and wild animals. However, due to the complex structure of the poxvirus, the underlying mechanism of the entry and infection by ORFV remains largely unknown. ORFV ORF047 encodes a protein named L1R. Poxviral L1R serves as the receptor-binding protein and blocks virus binding and entry independently of glycosaminoglycans (GAGs). The study aimed to identify the host interaction partners of ORFV ORF047.

METHODS

Yeast two-hybrid cDNA library of sheep testicular cells was applied to screen the host targets with ORF047 as the bait. ORF047 was cloned into a pBT3-N vector and expressed in the NMY51 yeast strain. Then, the expression of bait proteins was validated by Western blot analysis.

RESULTS

Sheep SERP1and PABPC4 were identified as host target proteins of ORFV ORF047, and a Co-IP assay further verified their interaction.

CONCLUSIONS

New host cell proteins SERP1and PABPC4 were found to interact with ORFV ORF047 and might involve viral mRNA translation and replication.

N-Myristoyltransferase as a Glycine and Lysine Myristoyltransferase in Cancer, Immunity, and Infections

Protein myristoylation, the addition of a 14-carbon saturated acyl group, is an abundant modification implicated in biological events as diverse as development, immunity, oncogenesis, and infections. N-Myristoyltransferase (NMT) is the enzyme that catalyzes this modification. Many elegant studies have established the rules guiding the catalysis including substrate amino acid sequence requirements with the indispensable N-terminal glycine, and a co-translational mode of action. Recent advances in technology such as the development of fatty acid analogs, small molecule inhibitors, and new proteomic strategies, allowed a deeper insight into the NMT activity and function. Here we focus on discussing recent work demonstrating that NMT is also a lysine myristoyltransferase, the enzyme's regulation by a previously unnoticed solvent channel, and the mechanism of NMT regulation by protein-protein interactions. We also summarize recent findings on NMT's role in cancer, immunity, and infections and the advances in pharmacological targeting of myristoylation. Our analyses highlight opportunities for further understanding and discoveries.

Genome Analysis of Dasineura jujubifolia Toursvirus 2, A Novel Ascovirus

So far, ascoviruses have only been identified from Lepidoptera host insects and their transmission vectors-endoparasitic wasps. Here, we reported the first finding of a complete novel ascovirus genome from a Diptera insect, Dasineura jujubifolia. Initially, sequence fragments with homology to ascoviruses were incidentally identified during metagenomic sequencing of the mitochondria of D. jujubifolia (Cecidomyiidae, Diptera) which is a major pest on Ziziphus jujuba. Then a full circular viral genome was assembled from the metagenomic data, which has an A+T percentage of 74% and contains 142,600 bp with 141 open reading frames (ORFs). Among the 141 ORFs, 37 were conserved in all sequenced ascoviruses (core genes) including proteins predicted to participate in DNA replication, gene transcription, protein modification, virus assembly, lipid metabolism and apoptosis. Multi-gene families including those encode for baculovirus repeated open reading frames (BROs), myristylated membrane proteins, RING/U-box E3 ubiquitin ligases, and ATP-binding cassette (ABC) transporters were found in the virus genome. Phylogenetic analysis showed that the newly identified virus belongs to genus Toursvirus of Ascoviridae, and is therefore named as Dasineura jujubifolia toursvirus 2 (DjTV-2a). The virus becomes the second reported species of the genus after Diadromus pulchellus toursvirus 1 (DpTV-1a). The genome arrangement of DjTV-2a is quite different from that of DpTV-1a, suggesting these two viruses separated in an early time of evolution. The results suggest that the ascoviruses may infect a much broader range of hosts than our previous knowledge, and shed lights on the evolution of ascoviruses and particularly on that of the toursviruses.

Genetic analysis of L1R myristoylated protein of Capripoxviruses reveals structural homogeneity among poxviruses

Sheeppox virus (SPPV) and goatpox virus (GTPV) are members of the genus Capripoxvirus (CaPV) of the family Poxviridae. CaPVs are responsible for important contagious diseases of small ruminants that are enzootic to the Indian sub-continent, Central and Northern Africa and the Middle East. In the present study, the sequence and phylogenetic analysis of the L1R gene of sixteen CaPV isolates (seven SPPV and nine GTPV) from India were performed along with 3D homology modeling of the L1R protein. L1R is a myristoylated protein responsible for virion assembly and being present on intracellular mature virion (IMV) surface, it is also a potent target for eliciting neutralizing antibodies. Sequence analysis of CaPV L1R gene revealed an ORF of 738bp with >99% and >96% identity within species and between species, respectively, at both nucleotide as well as amino acid levels. Phylogenetic analysis displayed distinct clusters of members of genus Capripoxvirus, as GTPV, SPPV and LSDV. L1R at the protein level showed various species-specific signature residues that may be useful for future grouping or genotyping of CaPV members. CaPV L1R was predicted to possess myristoylation motif GAAASIQTTVNTLNEKI and a potential N-glycosylation site at amino acid residue 50 (Asn). Despite of different host specificity in poxviruses, comparative sequence analysis of L1R proteins revealed highly conserved nature with presence of myristoylation motif (GXXXS) and six cysteine residues forming three disulfide bonds among all poxviruses. The conserved and immunogenic nature of the CaPV L1R gene may prove to be a potential candidate/target for developing molecular diagnostics including recombinant protein based assays and prophylactics for the control of CaPV diseases in tropical countries like India.

Three Conserved Regions in Baculovirus Sulfhydryl Oxidase P33 Are Critical for Enzymatic Activity and Function

Baculoviruses encode a conserved sulfhydryl oxidase, P33, which is necessary for budded virus (BV) production and multinucleocapsid occlusion-derived virus (ODV) formation. Here, the structural and functional relationship of P33 was revealed by X-ray crystallography, site-directed mutagenesis, and functional analysis. Based on crystallographic characterization and structural analysis, a series of P33 mutants within three conserved regions, i.e., the active site, the dimer interface, and the R127-E183 salt bridge, were constructed. In vitro experiments showed that mutations within the active site and dimer interface severely impaired the sulfhydryl oxidase activity of P33, while the mutations in the salt bridge had a relatively minor influence. Recombinant viruses containing mutated P33 were constructed and assayed in vivo Except for the active-site mutant AXXA, all other mutants produced infectious BVs, although certain mutants had a decreased BV production. The active-site mutant H114A, the dimer interface mutant H227D, and the salt bridge mutant R127A-E183A were further analyzed by electron microscopy and bioassays. The occlusion bodies (OBs) of mutants H114A and R127A-E183A had a ragged surface and contained mostly ODVs with a single nucleocapsid. The OBs of all three mutants contained lower numbers of ODVs and had a significantly reduced oral infectivity in comparison to control virus. Crystallographic analyses further revealed that all three regions may coordinate with one another to achieve optimal function of P33. Taken together, our data revealed that all the three conserved regions are involved in P33 activity and are crucial for virus morphogenesis and peroral infectivity.IMPORTANCE Sulfhydryl oxidase catalyzes disulfide bond formation of substrate proteins. P33, a baculovirus-encoded sulfhydryl oxidase, is different from other cellular and viral sulfhydryl oxidases, bearing unique features in tertiary and quaternary structure organizations. In this study, we found that three conserved regions, i.e., the active site, dimer interface, and the R127-E183 salt bridge, play important roles in the enzymatic activity and function of P33. Previous observations showed that deletion of p33 results in a total loss of budded virus (BV) production and in morphological changes in occlusion-derived virus (ODV). Our study revealed that certain P33 mutants lead to occlusion bodies (OBs) with a ragged surface, decreased embedded ODVs, and reduced oral infectivity. Interestingly, some P33 mutants with impaired ODV/OB still retained BV productivity, indicating that the impacts on BV and on ODV/OB are two distinctly different functions of P33, which are likely to be performed via different substrate proteins.

Development and evaluation of single domain antibodies for vaccinia and the L1 antigen

There is ongoing interest to develop high affinity, thermal stable recognition elements to replace conventional antibodies in biothreat detection assays. As part of this effort, single domain antibodies that target vaccinia virus were developed. Two llamas were immunized with killed viral particles followed by boosts with the recombinant membrane protein, L1, to stimulate the immune response for envelope and membrane proteins of the virus. The variable domains of the induced heavy chain antibodies were selected from M13 phage display libraries developed from isolated RNA. Selection via biopanning on the L1 antigen produced single domain antibodies that were specific and had affinities ranging from 4×10⁻⁹ M to 7.0×10⁻¹⁰ M, as determined by surface plasmon resonance. Several showed good ability to refold after heat denaturation. These L1-binding single domain antibodies, however, failed to recognize the killed vaccinia antigen. Useful vaccinia binding single domain antibodies were isolated by a second selection using the killed virus as the target. The virus binding single domain antibodies were incorporated in sandwich assays as both capture and tracer using the MAGPIX system yielding limits of detection down to 4×10⁵ pfu/ml, a four-fold improvement over the limit obtained using conventional antibodies. This work demonstrates the development of anti-vaccinia single domain antibodies and their incorporation into sandwich assays for viral detection. It also highlights the properties of high affinity and thermal stability that are hallmarks of single domain antibodies.

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.

The myristate moiety and amino terminus of vaccinia virus l1 constitute a bipartite functional region needed for entry

Vaccinia virus (VACV) L1 is a myristoylated envelope protein which is required for cell entry and the fusion of infected cells. L1 associates with members of the entry-fusion complex (EFC), but its specific role in entry has not been delineated. We recently demonstrated (Foo CH, et al., Virology 385:368-382, 2009) that soluble L1 binds to cells and blocks entry, suggesting that L1 serves as the receptor-binding protein for entry. Our goal is to identify the structural domains of L1 which are essential for its functions in VACV entry. We hypothesized that the myristate and the conserved residues at the N terminus of L1 are critical for entry. To test our hypothesis, we generated mutants in the N terminus of L1 and used a complementation assay to evaluate their ability to rescue infectivity. We also assessed the myristoylation efficiency of the mutants and their ability to interact with the EFC. We found that the N terminus of L1 constitutes a region that is critical for the infectivity of VACV and for myristoylation. At the same time, the nonmyristoylated mutants were incorporated into mature virions, suggesting that the myristate is not required for the association of L1 with the viral membrane. Although some of the mutants exhibited altered structural conformations, two mutants with impaired infectivity were similar in conformation to wild-type L1. Importantly, these two mutants, with changes at A4 and A5, undergo myristoylation. Overall, our results imply dual differential roles for myristate and the amino acids at the N terminus of L1. We propose a myristoyl switch model to describe how L1 functions.

SP600125 inhibits Orthopoxviruses replication in a JNK1/2 -independent manner: Implication as a potential antipoxviral

The pharmacological inhibitor SP600125 [anthra(1,9-cd)pyrazol-6(2H)-one 1,9-pyrazoloanthrone] has been largely employed as a c-JUN N-terminal kinase (JNK1/2) inhibitor. In this study, we evaluated whether pretreatment with SP600125 was able to prevent Orthopoxviruses Vaccinia virus (VACV), Cowpox virus (CPXV) and modified Vaccinia virus Ankara (MVA) replication. We found that incubation with SP600125 not only blocked virus-stimulated JNK phosphorylation, but also, significantly reduced virus production. We observed 1-3 log decline in viral yield depending on the cell line infected (A31, BSC-40 or BHK-21). The reduction in viral yield correlated with a dramatic impact on virus morphogenesis progress, intracellular mature viruses (IMV) were barely detected. Despite the fact that SP600125 can act as an efficient anti-orthopoxviral compound, we also provide evidence that this antiviral effect is not specifically exerted through JNK1/2 inhibition. This conclusion is supported by the fact that viral titers measured after infections of JNK1/2 knockout cells were not altered as compared to those of wild-type cells. In contrast, a decline in viral titers was verified when the infection of KO cells was carried out in the presence of the pharmacological inhibitor. SP600125 has been the focus of recent studies that have evaluated its action on diverse viral infections including DNA viruses. Our data support the notion that SP600125 can be regarded as a potential antipoxviral compound.

Structure of a baculovirus sulfhydryl oxidase, a highly divergent member of the erv flavoenzyme family

Genomes of nucleocytoplasmic large DNA viruses (NCLDVs) encode enzymes that catalyze the formation of disulfide bonds between cysteine amino acid residues in proteins, a function essential for the proper assembly and propagation of NCLDV virions. Recently, a catalyst of disulfide formation was identified in baculoviruses, a group of large double-stranded DNA viruses considered phylogenetically distinct from NCLDVs. The NCLDV and baculovirus disulfide catalysts are flavin adenine dinucleotide (FAD)-binding sulfhydryl oxidases related to the cellular Erv enzyme family, but the baculovirus enzyme, the product of the Ac92 gene in Autographa californica multiple nucleopolyhedrovirus (AcMNPV), is highly divergent at the amino acid sequence level. The crystal structure of the Ac92 protein presented here shows a configuration of the active-site cysteine residues and bound cofactor similar to that observed in other Erv sulfhydryl oxidases. However, Ac92 has a complex quaternary structural arrangement not previously seen in cellular or viral enzymes of this family. This novel assembly comprises a dimer of pseudodimers with a striking 40-degree kink in the interface helix between subunits. The diversification of the Erv sulfhydryl oxidase enzymes in large double-stranded DNA viruses exemplifies the extreme degree to which these viruses can push the boundaries of protein family folds.

Cytosolic disulfide bond formation in cells infected with large nucleocytoplasmic DNA viruses

Proteins that have evolved to contain stabilizing disulfide bonds generally fold in a membrane-delimited compartment in the cell [i.e., the endoplasmic reticulum (ER) or the mitochondrial intermembrane space (IMS)]. These compartments contain sulfhydryl oxidase enzymes that catalyze the pairing and oxidation of cysteine residues. In contrast, most proteins in a healthy cytosol are maintained in reduced form through surveillance by NADPH-dependent reductases and the lack of sulfhydryl oxidases. Nevertheless, one of the core functionalities that unify the broad and diverse set of nucleocytoplasmic large DNA viruses (NCLDVs) is the ability to catalyze disulfide formation in the cytosol. The substrates of this activity are proteins that contribute to the assembly, structure, and infectivity of the virions. If the last common ancestor of NCLDVs was present during eukaryogenesis as has been proposed, it is interesting to speculate that viral disulfide bond formation pathways may have predated oxidative protein folding in intracellular organelles.

Frog virus 3 ORF 53R, a putative myristoylated membrane protein, is essential for virus replication in vitro

Although previous work identified 12 complementation groups with possible roles in virus assembly, currently only one frog virus 3 protein, the major capsid protein (MCP), has been linked with virion formation. To identify other proteins required for assembly, we used an antisense morpholino oligonucleotide to target 53R, a putative myristoylated membrane protein, and showed that treatment resulted in marked reductions in 53R levels and a 60% drop in virus titers. Immunofluorescence assays confirmed knock down and showed that 53R was found primarily within viral assembly sites, whereas transmission electron microscopy detected fewer mature virions and, in some cells, dense granular bodies that may represent unencapsidated DNA-protein complexes. Treatment with a myristoylation inhibitor (2-hydroxymyristic acid) resulted in an 80% reduction in viral titers. Collectively, these data indicate that 53R is an essential viral protein that is required for replication in vitro and suggest it plays a critical role in virion formation.

Single-shot immunization with recombinant adenovirus encoding vaccinia virus glycoprotein A27L is protective against a virulent respiratory poxvirus infection

Significant safety issues have emerged concerning the general use of DRYVAX vaccine. Vaccination with replication-defective recombinant adenovirus (rAd) vaccines may offer a safer and effective alternative to live vaccinia virus (VV) vaccination. Six individual poxvirus glycoproteins: A33R, A34R, A36R, B5R, A27L or L1R that are normally expressed on the surface of infectious vaccinia virus were encoded in rAd vaccines and tested in mice in this study. A single-shot intramuscular injection of rAd encoding A27L protected mice against a lethal intranasal challenge with VV at 4 weeks post-vaccination. By 10 weeks post-vaccination, a significant decrease in post-challenge morbidity was observed that correlated with potent neutralizing antibody responses and the emergence of specific polyfunctional T cell responses. The immunogenicity and protective efficacy of rAd-A27L immunization persisted for at least 35 weeks post-vaccination. This study is the first demonstration that a single-shot subunit vaccine encoding a poxvirus protein confers protection against the mortality and morbidity associated with poxvirus infection.

Kinetics and intracellular location of intramolecular disulfide bond formation mediated by the cytoplasmic redox system encoded by vaccinia virus

Poxviruses encode a redox system for intramolecular disulfide bond formation in cytoplasmic domains of viral proteins. Our objectives were to determine the kinetics and intracellular location of disulfide bond formation. The vaccinia virus L1 myristoylated membrane protein, used as an example, has three intramolecular disulfide bonds. Reduced and disulfide-bonded forms of L1 were distinguished by electrophoretic mobility and reactivity with monoclonal and polyclonal antibodies. Because disulfide bonds formed during 5 min pulse labeling with radioactive amino acids, a protocol was devised in which dithiothreitol was present at this step. Disulfide bond formation was detected by 2 min after removal of reducing agent and was nearly complete in 10 min. When the penultimate glycine residue was mutated to prevent myristoylation, L1 was mistargeted to the endoplasmic reticulum and disulfide bond formation failed to occur. These data suggested that viral membrane association was required for oxidation of L1, providing specificity for the process.

Transcriptome analysis of Frog virus 3, the type species of the genus Ranavirus, family Iridoviridae

Frog virus 3 is the best characterized species within the genus Ranavirus, family Iridoviridae. FV3's large ( approximately 105 kbp) dsDNA genome encodes 98 putative open reading frames (ORFs) that are expressed in a coordinated fashion leading to the sequential appearance of immediate early (IE), delayed early (DE) and late (L) viral transcripts. As a step toward elucidating molecular events in FV3 replication, we sought to identify the temporal class of viral messages. To accomplish this objective an oligonucleotide microarray containing 70-mer probes corresponding to each of the 98 FV3 ORFs was designed and used to examine viral gene expression. Viral transcription was initially monitored during the course of a productive replication cycle at 2, 4 and 9 h after infection. To confirm results of the time course assay, viral gene expression was also monitored in the presence of cycloheximide (CHX), which limits expression to only IE genes, and following infection with a temperature-sensitive (ts) mutant which at non-permissive temperatures is defective in viral DNA synthesis and blocked in late gene expression. Subsequently, microarray analyses were validated by RT-PCR and qRT-PCR. Using these approaches we identified 33 IE genes, 22 DE genes and 36 L viral genes. The temporal class of the 7 remaining genes could not be determined. Comparison of protein function with temporal class indicated that, in general, genes encoding putative regulatory factors, or proteins that played a part in nucleic acid metabolism and immune evasion, were classified as IE and DE genes, whereas those involved in DNA packaging and virion assembly were considered L genes. Information on temporal class will provide the basis for determining whether members of the same temporal class contain common upstream regulatory regions and perhaps allow us to identify virion-associated and virus-induced proteins that control viral gene expression.

Complete sequence determination of a novel reptile iridovirus isolated from soft-shelled turtle and evolutionary analysis of Iridoviridae

Background

Soft-shelled turtle iridovirus (STIV) is the causative agent of severe systemic diseases in cultured soft-shelled turtles (Trionyx sinensis). To our knowledge, the only molecular information available on STIV mainly concerns the highly conserved STIV major capsid protein. The complete sequence of the STIV genome is not yet available. Therefore, determining the genome sequence of STIV and providing a detailed bioinformatic analysis of its genome content and evolution status will facilitate further understanding of the taxonomic elements of STIV and the molecular mechanisms of reptile iridovirus pathogenesis.

Results

We determined the complete nucleotide sequence of the STIV genome using 454 Life Science sequencing technology. The STIV genome is 105 890 bp in length with a base composition of 55.1% G+C. Computer assisted analysis revealed that the STIV genome contains 105 potential open reading frames (ORFs), which encode polypeptides ranging from 40 to 1,294 amino acids and 20 microRNA candidates. Among the putative proteins, 20 share homology with the ancestral proteins of the nuclear and cytoplasmic large DNA viruses (NCLDVs). Comparative genomic analysis showed that STIV has the highest degree of sequence conservation and a colinear arrangement of genes with frog virus 3 (FV3), followed by Tiger frog virus (TFV), Ambystoma tigrinum virus (ATV), Singapore grouper iridovirus (SGIV), Grouper iridovirus (GIV) and other iridovirus isolates. Phylogenetic analysis based on conserved core genes and complete genome sequence of STIV with other virus genomes was performed. Moreover, analysis of the gene gain-and-loss events in the family Iridoviridae suggested that the genes encoded by iridoviruses have evolved for favoring adaptation to different natural host species.

Conclusion

This study has provided the complete genome sequence of STIV. Phylogenetic analysis suggested that STIV and FV3 are strains of the same viral species belonging to the Ranavirus genus in the Iridoviridae family. Given virus-host co-evolution and the phylogenetic relationship among vertebrates from fish to reptiles, we propose that iridovirus might transmit between reptiles and amphibians and that STIV and FV3 are strains of the same viral species in the Ranavirus genus.

Vaccinia virus L1 binds to cell surfaces and blocks virus entry independently of glycosaminoglycans

L1 and A28 are vaccinia virus (VACV) envelope proteins which are essential for cellular entry. However, their specific roles during entry are unknown. We tested whether one or both of these proteins might serve as receptor binding proteins (RBP). We found that a soluble, truncated form of L1, but not A28, bound to cell surfaces independently of glycosaminoglycans (GAGs). Hence, VACV A28 is not likely to be a RBP and functions after attachment during entry. Importantly, soluble L1 inhibited both binding and entry of VACV in GAG-deficient cells, suggesting that soluble L1 blocks entry at the binding step by competing with the virions for non-GAG receptors on cells. In contrast, soluble A27, a VACV protein which attaches to GAGs but is non-essential for virus entry, inhibited binding and entry of VACV in a GAG-dependent manner. To our knowledge, this is the first report of a VACV envelope protein that blocks virus binding and entry independently of GAGs.

African swine fever virus

African swine fever virus (ASFV) is a large, intracytoplasmically-replicating DNA arbovirus and the sole member of the family Asfarviridae. It is the etiologic agent of a highly lethal hemorrhagic disease of domestic swine and therefore extensively studied to elucidate the structures, genes, and mechanisms affecting viral replication in the host, virus-host interactions, and viral virulence. Increasingly apparent is the complexity with which ASFV replicates and interacts with the host cell during infection. ASFV encodes novel genes involved in host immune response modulation, viral virulence for domestic swine, and in the ability of ASFV to replicate and spread in its tick vector. The unique nature of ASFV has contributed to a broader understanding of DNA virus/host interactions.

The vaccinia virus gene I2L encodes a membrane protein with an essential role in virion entry

The previously unstudied vaccinia virus gene I2L is conserved in all orthopoxviruses. We show here that the 8-kDa I2 protein is expressed at late times of infection, is tightly associated with membranes, and is encapsidated in mature virions. We have generated a recombinant virus in which I2 expression is dependent upon the inclusion of tetracycline in the culture medium. In the absence of I2, the biochemical events of the viral life cycle progress normally, and virion morphogenesis culminates in the production of mature virions. However, these virions show an approximately 400-fold reduction in specific infectivity due to an inability to enter target cells. Several proteins that have been previously identified as components of an essential entry/fusion complex are present at reduced levels in I2-deficient virions, although other membrane proteins, core proteins, and DNA are encapsidated at normal levels. A preliminary structure/function analysis of I2 has been performed using a transient complementation assay: the C-terminal hydrophobic domain is essential for protein stability, and several regions within the N-terminal hydrophilic domain are essential for biological competency. I2 is thus yet another component of the poxvirus virion that is essential for the complex process of entry into target cells.

Vaccinia virus l1 protein is required for cell entry and membrane fusion

Genetic and biochemical studies have provided evidence for an entry/fusion complex (EFC) comprised of at least eight viral proteins (A16, A21, A28, G3, G9, H2, J5, and L5) that together with an associated protein (F9) participates in entry of vaccinia virus (VACV) into cells. The genes encoding these proteins are conserved in all poxviruses, are expressed late in infection, and are components of the mature virion membrane but are not required for viral morphogenesis. In addition, all but one component has intramolecular disulfides that are formed by the poxvirus cytoplasmic redox system. The L1 protein has each of the characteristics enumerated above except that it has been reported to be essential for virus assembly. To further investigate the role of L1, we constructed a recombinant VACV (vL1Ri) that inducibly expresses L1. In the absence of inducer, L1 synthesis was repressed and vL1Ri was unable to form plaques or produce infectious progeny. Unexpectedly, assembly and morphogenesis appeared normal and the noninfectious virus particles were indistinguishable from wild-type VACV as determined by transmission electron microscopy and analysis of the component polypeptides. Notably, the L1-deficient virions were able to attach to cells but the cores failed to penetrate into the cytoplasm. In addition, cells infected with vL1Ri in the absence of inducer did not form syncytia following brief low-pH treatment even though extracellular virus was produced. Coimmunoprecipitation experiments demonstrated that L1 interacted with the EFC and indirectly with F9, suggesting that L1 is an additional component of the viral entry apparatus.

Disparity between levels of in vitro neutralization of vaccinia virus by antibody to the A27 protein and protection of mice against intranasal challenge

Immunization with recombinant proteins may provide a safer alternative to live vaccinia virus for prophylaxis of poxvirus infections. Although antibody protects against vaccinia virus infection, the mechanism is not understood and the selection of immunogens is daunting as there are dozens of surface proteins and two infectious forms known as the mature virion (MV) and the enveloped virion (EV). Our previous studies showed that mice immunized with soluble forms of EV membrane proteins A33 and B5 and MV membrane protein L1 or passively immunized with antibodies to these proteins survived an intranasal challenge with vaccinia virus. The present study compared MV protein A27, which has a role in virus attachment to glycosaminoglycans on the cell surface, to L1 with respect to immunogenicity and protection. Although mice developed similar levels of neutralizing antibody after immunizations with A27 or L1, A27-immunized mice exhibited more severe disease upon an intranasal challenge with vaccinia virus. In addition, mice immunized with A27 and A33 were not as well protected as mice receiving L1 and A33. Polyclonal rabbit anti-A27 and anti-L1 IgG had equivalent MV-neutralizing activities when measured by the prevention of infection of human or mouse cells or cells deficient in glycosaminoglycans or by adding antibody prior to or after virus adsorption. Nevertheless, the passive administration of antibody to A27 was poorly protective compared to the antibody to L1. These studies raise questions regarding the basis for antibody protection against poxvirus disease and highlight the importance of animal models for the early evaluation of vaccine candidates.

Targeting the vaccinia virus L1 protein to the cell surface enhances production of neutralizing antibodies

The current live-orthopoxvirus vaccine is associated with minor to serious adverse affects, and is contraindicated for use in a significant portion of the population. As an alternative vaccine, we have previously shown that a DNA subunit vaccine (4pox) based on four orthopoxvirus immunogens (L1R, B5R, A27L and A33R) can produce protective immunity against lethal orthopoxvirus challenges in mice and nonhuman primates. Because antibodies are critical for protection against secondary orthopoxvirus infections, we are now interested in strategies that will enhance the humoral immune response against vaccine targets. Here, we tested the immunogenicity of an L1R construct to which a tissue plasminogen activator signal sequence was placed in frame with the full-length L1R gene. The tPA-L1R construct produced a more robust neutralizing antibody response in vaccinated mice when the DNA vaccine was administered by gene-gun as a prime/single boost. When the tPA-L1R construct was substituted for the unmodified L1R gene in the 4pox vaccine, given as a prime and single boost, animals were better protected from lethal challenge with vaccinia virus (VACV). These findings indicate that adding a tPA-leader sequence can enhance the immunogenicity of the L1R gene when given as a DNA vaccine. Furthermore, our results demonstrate that a DNA-based vaccine is capable of establishing protection from lethal orthopoxvirus challenges when administered as a prime and single boost without requiring adjuvant.

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

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

Antibody responses to vaccinia membrane proteins after smallpox vaccination

BACKGROUND

Vaccinia virus (VV) membrane proteins are candidates for orthopoxvirus subunit vaccines and potential targets for therapeutic antibodies. Human antibody responses to these proteins after VV vaccination have not been well characterized.

METHODS

Pre- and postvaccination (day 26-30) serum specimens from 80 VV vaccine recipients were examined for immunoglobulin G antibodies specific for B5, A33, A27, and L1 by enzyme-linked immunosorbent assay (ELISA). Responses were compared between vaccinia-naive and previously vaccinated (nonnaive) recipients and between nonnaive recipients of undiluted or 1 : 10 diluted vaccine.

RESULTS

VV vaccination elicited anti-A33 and anti-A27 antibodies in nearly all vaccinia-naive subjects (100% and 93%, respectively). Preexisting antibodies were commonly detected in nonnaive subjects (for anti-B5, 68%; for anti-A33, 59%; for anti-A27, 38%; and for anti-L1, 10%). Anti-B5 antibodies were strongly boosted by undiluted vaccine (geometric mean titer [GMT], 151 vs. 1010 for pre- vs. postvaccination; P<.001), whereas anti-L1 antibody responses were less robust (detection rate, 31%; GMT, 75) in nonnaive subjects. Diluted vaccine elicited antibody responses that were similar to those elicited by undiluted vaccine.

CONCLUSIONS

Vaccination with VV elicits long-lived specific antibody responses directed against VV membrane proteins that vary by previous vaccination status but not with respect to 10-fold dilution of vaccine. B5, A33, and A27 should be considered for inclusion in future human orthopoxvirus subunit vaccines.

Potential role of N-myristoyltransferase in cancer

Colorectal cancer is the second leading cause of malignant death, and better preventive strategies are needed. The treatment of colonic cancer remains difficult because of the lack of effective chemotherapeutic agents; therefore it is important to continue to search for cellular functions that can be disrupted by chemotherapeutic drugs resulting in the inhibition of the development and progression of cancer. The current knowledge of the modification of proteins by myristoylation involving myristoyl-CoA: protein N-myristoyltransferase (NMT) is in its infancy. This process is involved in the pathogenesis of cancer. We have reported for the first time that NMT activity and protein expression were higher in human colorectal cancer, gallbladder carcinoma and brain tumors. In addition, an increase in NMT activity appeared at an early stage in colonic carcinogenesis. It is conceivable therefore that NMT can be used as a potential marker for the early detection of cancer. These observations lead to the possibility of developing NMT specific inhibitors, which may be therapeutically useful. We proposed that HSC70 and/or enolase could be used as an anticancer therapeutic target. This review summarized the status of NMT in cancer which has been carried in our laboratory.

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

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

Protein composition of the vaccinia virus mature virion

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

A trans-Golgi network resident protein, golgin-97, accumulates in viral factories and incorporates into virions during poxvirus infection

Poxviruses are the only DNA viruses known to replicate and assemble in the cytoplasm of infected cells. Poxvirus morphogenesis is a complicated process in which four distinct infectious forms of the virus are produced: intracellular mature virus, intracellular enveloped virus, cell-associated enveloped virus, and extracellular enveloped virus. The source of primary membrane wrapping the intracellular mature virus, the first infectious form, is still unknown. Although the membrane was suggested to originate from the endoplasmic reticulum-Golgi intermediate compartment, none of the marker proteins from this or any other cell compartments has been found in the intracellular mature virus. Thus, it was hypothesized that the membrane is either extensively modified by the virus or synthesized de novo. In the work described here, we demonstrate that a host cell protein residing in the trans-Golgi network membrane, golgin-97, is transported to the sites of virus replication and assembly and becomes incorporated into the virions during poxvirus infection. Inside the virion, golgin-97 is associated with the insoluble core protein fraction. Being able to adopt a long rod-like structure, the protein apparently extends through the virion envelope and protrudes from its surface. Here we discuss the potential role and functions of golgin-97 in poxvirus replication and propose two working models.

In a nutshell: structure and assembly of the vaccinia virion

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

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.

Importance of disulphide bonds for vaccinia virus L1R protein function

L1R, a myristylated late gene product of vaccinia virus, is essential for formation of infectious intracellular mature virions (IMV). In its absence, only viral particles arrested at an immature stage are detected and no infectious progeny virus is produced. Previous studies have shown that the L1R protein is exclusively associated with the IMV membrane and that myristylation is required for correct targeting. The L1R protein contains six cysteine amino acid residues that have all been shown to participate in intramolecular disulphide bonds. However, it was not clear what role, if any, the disulfide bonds play in the membrane topology of the L1R protein. To address this question, a comprehensive library of L1R mutants in which the cysteine residues have been mutated to serine (either individually or in combination) were tested for their ability to rescue a L1R conditional lethal mutant virus under non-permissive conditions. Much to our surprise, we determined that C57 was not essential for production of infectious IMV. These results suggest that protein disulphide isomerases may be involved in reorganization of disulfide bonds within the L1R protein.

Physical and immunological characterization of a recombinant secreted form of the membrane protein encoded by the vaccinia virus L1R gene

We reported that immunization with recombinant proteins derived from vaccinia virus (VV) particles could provide protection against infection. Here we describe the physical and antigenic properties of the L1R membrane protein. The recombinant protein (L1R(185t)) was secreted as a monomer and correct folding was suggested by the presence of three intramolecular disulfide bonds and binding to conformation-specific monoclonal antibodies (MAbs). Furthermore, anti-L1R(185t) rabbit antisera exhibited potent virus-neutralizing activity against the IMV form of VV. We raised six MAbs against L1R(185t). Three recognized linear epitopes (residues 118--128) and neutralized IMV infectivity. These MAbs blocked binding of each other to L1R(185t) but failed to block binding of two previously described neutralizing anti-L1R MAbs, 7D11 and 2D5. The latter two antibodies blocked each other in binding L1R(185t). Thus, two antigenic sites on L1R overlap functional domains and based on recent structural studies these are found in accessible regions of the IMV L1R protein.

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

We provide the initial characterization of the product of the vaccinia virus A21L (VACWR140) gene and demonstrate that it is required for cell entry and low pH-triggered membrane fusion. The A21L open reading frame, which is conserved in all sequenced members of the poxvirus family, encodes a protein of 117 amino acids with an N-terminal hydrophobic domain and four invariant cysteines. Expression of the A21 protein occurred at late times of infection and was dependent on viral DNA replication. The A21 protein contained two intramolecular disulfide bonds, the formation of which required the vaccinia virus-encoded cytoplasmic redox pathway, and was localized on the surface of the lipoprotein membrane of intracellular mature virions. A conditional lethal mutant, in which A21L gene expression was regulated by isopropyl-beta-d-thiogalactopyranoside, was constructed. In the absence of inducer, cell-to-cell spread of virus did not occur, despite the formation of morphologically normal intracellular virions and extracellular virions with actin tails. Purified virions lacking A21 were able to bind to cells, but cores did not penetrate into the cytoplasm and synthesize viral RNA. In addition, virions lacking A21 were unable to mediate low pH-triggered cell-cell fusion. The A21 protein, like the A28 and H2 proteins, is an essential component of the poxvirus entry/fusion apparatus for both intracellular and extracellular virus particles.

Vaccinia virus H2 protein is an essential component of a complex involved in virus entry and cell-cell fusion

The vaccinia virus H2R gene (VACWR 100) is conserved in all sequenced members of the poxvirus family and encodes a protein with a predicted transmembrane domain and four invariant cysteines. A recombinant vaccinia virus, in which expression of the H2 protein is stringently regulated, was unable to replicate without inducer. However, under nonpermissive conditions, all stages of virus morphogenesis appeared normal and extracellular virions were detected at the tips of actin tails. Nevertheless, virus did not spread to neighboring cells nor did syncytia form after low-pH treatment. Purified -H2 and +H2 virions from cells infected in the absence or presence of inducer, respectively, were indistinguishable in microscopic appearance and contained the same complement of major proteins, though only +H2 virions were infectious. The -H2 virions bound to cells, but their cores did not penetrate into the cytoplasm. In addition, exogenously added -H2 virions were unable to mediate the formation of syncytia after low-pH treatment. In contrast, virions lacking the A27 (p14) protein, which was previously considered to have an essential role in fusion, penetrated cells and induced extensive syncytia. The properties of H2, however, are very similar to those recently reported for the A28 protein. Moreover, coimmunoprecipitation experiments indicated an interaction between H2 and A28. Therefore, H2 and A28 are the only proteins presently known to be specifically required for vaccinia virus entry and are likely components of a fusion complex.

Temperature-sensitive mutants in the vaccinia virus 4b virion structural protein assemble malformed, transcriptionally inactive intracellular mature virions

Two noncomplementing vaccinia virus temperature-sensitive mutants, Cts8 and Cts26, were mapped to the A3L gene, which encodes the major virion structural protein, 4b. The two ts mutants display normal patterns of gene expression, DNA replication, telomere resolution, and protein processing during infection. Morphogenesis during mutant infections is normal through formation of immature virions with nucleoids (IVN) but appears to be defective in the transition from IVN to intracellular mature virus (IMV). In mutant infections, aberrant particles that have the appearance of malformed IMV accumulate. The mutant particles are wrapped in Golgi-derived membranes and exported from cells. Purified mutant particles are indistinguishable from wt particles in protein and DNA composition; however, they are defective in a permeabilized-virion-directed transcription reaction despite containing significant (Cts8) or even normal (Cts26) levels of specific transcription enzymes. These results indicate that the 4b protein is required for proper metamorphosis of IMV from IVN and that proper organization of the IMV structure is required to produce a transcriptionally active virion particle.

MALDI-TOF mass spectroscopy detects the capsid structural instabilities created by deleting the myxoma virus cupro-zinc SOD1 homolog M131R

The myxoma virus M131R gene encodes a catalytically inactive homolog of cellular Cu-Zn superoxide dismutase (SOD1) and this 17,786 Da protein is a major virion component. We have used matrix-assisted laser desorption ionization time-of-flight mass spectroscopy (MALDI-TOF MS) to study the effect(s) of deleting the gene on virion composition and structure. This approach confirmed that the M131R gene product is an abundant virion component. This conclusion was based upon the ready detection of a 1805.3 Da peptide released from the N-terminus of the myxoma SOD1 protein by mild trypsin treatment, as well as the detection of a 17,790 Da protein in HPLC fractionated virus extracts, which subsequently yielded M131R-encoded tryptic peptides. Neither peptide nor protein was detected in particles bearing a genome encoding an M131RDelta deletion mutation. Curiously, more proteins and tryptic peptides were detected when M131RDelta mutant virions were subjected to MALDI-TOF MS analysis compared with wild-type virus particles. This suggested that particles assembled in the absence of myxoma SOD protein are structurally unstable. Plaque analysis confirmed this conjecture by showing that SOD-deficient MYX particles are unusually heat labile and trypsin sensitive. Mutant Shope fibroma virus exhibited the same phenotype. Thus a previously unappreciated feature of MALDI-TOF MS is that the method can sometimes detect alterations in virion stability.

The 1.51-Angstrom structure of the poxvirus L1 protein, a target of potent neutralizing antibodies

Although eradicated from nature more than two decades ago, the threat of smallpox has reemerged because of concerns over its use as a biological weapon. We present the structure of the poxvirus L1 protein, a molecule that is conserved throughout the poxvirus family and is nearly identical in vaccinia virus and in variola virus, which causes smallpox. L1 is a myristoylated envelope protein that is a potent target for neutralizing antibodies and an important component of current experimental vaccines. The L1 structure reveals a hydrophobic cavity located adjacent to its N terminus. The cavity would be capable of shielding the myristate moiety, which is essential for virion assembly. The structure of L1 is a step in the elucidation of molecular mechanisms common to all poxviruses that may stimulate the design of safer vaccines and new antipoxvirus drugs.

Vaccinia virus morphogenesis: a13 phosphoprotein is required for assembly of mature virions

The 70-amino-acid A13L protein is a component of the vaccinia virus membrane. We demonstrate here that the protein is expressed at late times of infection, undergoes phosphorylation at a serine residue(s), and becomes encapsidated in a monomeric form. Phosphorylation is dependent on Ser40, which lies within the proline-rich motif SPPP. Because phosphorylation of the A13 protein is only minimally affected by disruption of the viral F10 kinase or H1 phosphatase, a cellular kinase is likely to be involved. We generated an inducible recombinant in which A13 protein expression is dependent upon the inclusion of tetracycline in the culture medium. Repression of the A13L protein spares the biochemical progression of the viral life cycle but arrests virion morphogenesis. Virion assembly progresses through the formation of immature virions (IVs); however, these virions do not acquire nucleoids, and DNA crystalloids accumulate in the cytoplasm. Further development into intracellular mature virions is blocked, causing a 1,000-fold decrease in the infectious virus yield relative to that obtained in the presence of the inducer. We also determined that the temperature-sensitive phenotype of the viral mutant Cts40 is due to a nucleotide transition within the A13L gene that causes a Thr(48)-->Ile substitution. This substitution disrupts the function of the A13 protein but does not cause thermolability of the protein; at the nonpermissive temperature, virion morphogenesis arrests at the stage of IV formation. The A13L protein, therefore, is part of a newly recognized group of membrane proteins that are dispensable for the early biogenesis of the virion membrane but are essential for virion maturation.

Vaccinia virus entry into cells is dependent on a virion surface protein encoded by the A28L gene

The A28L gene of vaccinia virus is conserved in all poxviruses and encodes a protein that is anchored to the surface of infectious intracellular mature virions (IMV) and consequently lies beneath the additional envelope of extracellular virions. A conditional lethal recombinant vaccinia virus, vA28-HAi, with an inducible A28L gene, undergoes a single round of replication in the absence of inducer, producing IMV, as well as extracellular virions with actin tails, but fails to infect neighboring cells. We show here that purified A28-deficient IMV appeared to be indistinguishable from wild-type IMV and were competent to synthesize RNA in vitro. Nevertheless, A28-deficient virions did not induce cytopathic effects, express early genes, or initiate a productive infection. Although A28-deficient IMV bound to the surface of cells, their cores did not penetrate into the cytoplasm. An associated defect in membrane fusion was demonstrated by the failure of low pH to trigger syncytium formation when cells were infected with vA28-HAi in the absence of inducer (fusion from within) or when cells were incubated with a high multiplicity of A28-deficient virions (fusion from without). The correlation between the entry block and the inability of A28-deficient virions to mediate fusion provided compelling evidence for a relationship between these events. Because repression of A28 inhibited cell-to-cell spread, which is mediated by extracellular virions, all forms of vaccinia virus regardless of their outer coat must use a common A28-dependent mechanism of cell penetration. Furthermore, since A28 is conserved, all poxviruses are likely to penetrate cells in a similar way.

Vaccinia virus A28L gene encodes an essential protein component of the virion membrane with intramolecular disulfide bonds formed by the viral cytoplasmic redox pathway

We report the initial characterization of the product of the vaccinia virus A28L gene, which is highly conserved in all sequenced poxviruses. Our studies showed that the A28 protein is expressed at late times during the virus replication cycle and is a membrane component of the intracellular mature virion. An N-terminal hydrophobic sequence, present in all poxvirus A28 orthologs, anchors the protein in the virion surface membrane so that most of it is exposed to the cytoplasm. The cytoplasmic domain contains four conserved cysteines, which form two intramolecular disulfide bonds. Disulfide bond formation depended on the expression of three viral proteins, E10, A2.5, and G4, which together comprise a conserved cytoplasmic redox pathway. A28 is the third identified substrate of this pathway; the others are the L1 and F9 proteins. We constructed a conditional-lethal recombinant vaccinia virus with an inducible A28L gene. The recombinant virus was propagated in the presence of inducer but was unable to replicate and spread in its absence. During a single round of an abortive infection in the absence of inducer, the synthesis and processing of viral proteins, assembly of intra- and extracellular virions, and formation of actin tails occurred normally. In another paper (T. Senkevich, B. M. Ward, and B. Moss, J. Virol. 78:2357-2366, 2004), we have demonstrated that virions assembled without A28 cannot carry out a second round of infection because they are defective in cell penetration.

Vaccinia virus J1R protein: a viral membrane protein that is essential for virion morphogenesis

Vaccinia virus, a member of the poxvirus family, contains a conserved J1R open reading frame that encodes a late protein of 17.8 kDa. The 18-kDa J1R protein is associated mainly with the membrane fraction of intracellular mature virus particles. This study examines the biological function of J1R protein in the vaccinia virus life cycle. A recombinant vaccinia virus was constructed to conditionally express J1R protein in an isopropyl-beta-D-galactopyranoside (IPTG)-inducible manner. When J1R is not expressed during vaccinia virus infection, the virus titer is reduced approximately 100-fold. In contrast, J1R protein is not required for viral gene expression, as indicated by protein pulse-labeling. J1R protein is also not required for DNA processing, as the resolution of the concatemer junctions of replicated viral DNA was detected without IPTG. A deficiency of J1R protein caused a severe delay in the processing of p4a and p4b into mature core proteins 4a and 4b, indicating that J1R protein participates in virion morphogenesis. Infected cells grown in the absence of IPTG contained very few intracellular mature virions in the cytoplasm, and enlarged viroplasm structures accumulated with viral crescents attached at the periphery. Abundant intermediate membrane structures of abnormal shapes were observed, and many immature virions were either empty or partially filled, indicating that J1R protein is important for DNA packaging into immature virions. J1R protein also coimmunoprecipited with A45R protein in infected cells. In summary, these results indicate that vaccinia virus J1R is a membrane protein that is required for virus growth and plaque formation. J1R protein interacts with A45R protein and performs an important role during immature virion formation in cultured cells.

Expression of the vaccinia virus A2.5L redox protein is required for virion morphogenesis

In this article we report the initial biochemical, genetic, and electron microscopic analysis of a previously uncharacterized, 8.9-kDa, predicted thiol-redox protein. The name A2.5L was assigned to the corresponding vaccinia virus gene, which is conserved in all sequenced poxviruses. Multiple alignment analysis and secondary structure prediction indicated that the A2.5L gene product is an all-alpha-helical protein with a conserved Cxx(x)C motif in the N-terminal alpha-helix. The DNA replication requirement and kinetics of A2.5L protein accumulation in virus-infected cells were typical of a late gene product, in agreement with the predicted promoter sequence. The A2.5L protein was a monomer under reducing conditions, but was mostly associated with the vaccinia virus E10R redox protein as a heterodimer under nonreducing conditions. The A2.5L protein was detected in virus particles at various stages of assembly, suggesting that it is an integral component of intracellular virions. An inducer-dependent A2.5L null mutant was constructed: in the absence of inducer, infectious virus formation was abolished and electron microscopy revealed an assembly block with an accumulation of crescent membranes and immature virions. This stage of assembly block was similar to that occurring when the E10R and G4L redox proteins were repressed, which is compatible with the involvement of E10R, A2.5L, and G4L in the same redox pathway.

Azathioprine inhibits vaccinia virus replication in both BSC-40 and RAG cell lines acting on different stages of virus cycle

In the present study we demonstrate that azathioprine (AZA) inhibits vaccinia virus (VV) replication in both BSC-40 and RAG cell lines, acting on different stages of virus cycle. In BSC-40 cells, early protein synthesis was not significantly affected, but late gene expression was severely impaired. In RAG cells all stages of gene expression were completed during synchronous infection in the presence of the drug. The onset of DNA replication was not affected in RAG cells, but a severe inhibition was observed in BSC-40 cells. Electron microscopic analysis of VV-infected RAG cells treated with AZA revealed brick-shaped particles presenting abnormal definition of the internal structure. Purified virions from AZA-treated RAG cells presented several modifications of the protein content, a lesser amount of DNA, and a lower PFU:particle ratio. Our results suggest that in VV-infected RAG cells AZA interfered with virus morphogenesis, whereas in BSC-40 cells the replicative cycle was inhibited at the DNA replication stage.

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.