Immature viral envelope formation is interrupted at the same stage by lac operator-mediated repression of the vaccinia virus D13L gene and by the drug rifampicin

In silico identification of potential phytochemical inhibitors for mpox virus: molecular docking, MD simulation, and ADMET studies

The mpox virus (MPXV), a member of the Poxviridae family, which recently appeared outside of the African continent has emerged as a global threat to public health. Given the scarcity of antiviral treatments for mpox disease, there is a pressing need to identify and develop new therapeutics. We investigated 5715 phytochemicals from 266 species available in IMMPAT database as potential inhibitors for six MPXV targets namely thymidylate kinase (A48R), DNA ligase (A50R), rifampicin resistance protein (D13L), palmytilated EEV membrane protein (F13L), viral core cysteine proteinase (I7L), and DNA polymerase (E9L) using molecular docking. The best-performing phytochemicals were also subjected to molecular dynamics (MD) simulations and in silico ADMET analysis. The top phytochemicals were forsythiaside for A48R, ruberythric acid for A50R, theasinensin F for D13L, theasinensin A for F13L, isocinchophyllamine for I7L, and terchebin for E9L. Interestingly, the binding energies of these potential phytochemical inhibitors were far lower than brincidofovir and tecovirimat, the standard drugs used against MPXV, hinting at better binding properties of the former. These findings may pave the way for developing new MPXV inhibitors based on natural product scaffolds. However, they must be further studied to establish their inhibitory efficacy and toxicity in in vitro and in vivo models.

Rendezvous with Vaccinia Virus in the Post-smallpox Era: R&D Advances

Smallpox was eradicated in less than 200 years after Edward Jenner's practice of cowpox variolation in 1796. The forty-three years of us living free of smallpox, beginning in 1979, never truly separated us from poxviruses. The recent outbreak of monkeypox in May 2022 might well warn us of the necessity of keeping up both the scientific research and public awareness of poxviruses. One of them in particular, the vaccinia virus (VACV), has been extensively studied as a vector given its broad host range, extraordinary thermal stability, and exceptional immunogenicity. Unceasing fundamental biological research on VACV provides us with a better understanding of its genetic elements, involvement in cellular signaling pathways, and modulation of host immune responses. This enables the rational design of safer and more efficacious next-generation vectors. To address the new technological advancement within the past decade in VACV research, this review covers the studies of viral immunomodulatory genes, modifications in commonly used vectors, novel mechanisms for rapid generation and purification of recombinant virus, and several other innovative approaches to studying its biology.

Targeting envelope proteins of poxviruses to repurpose phytochemicals against monkeypox: An in silico investigation

The monkeypox virus (MPXV) has become a major threat due to the increasing global caseload and the ongoing multi-country outbreak in non-endemic territories. Due to limited research in this avenue and the lack of intervention strategies, the present study was aimed to virtually screen bioactive phytochemicals against envelope proteins of MPXV via rigorous computational approaches. Molecular docking, molecular dynamic (MD) simulations, and MM/PBSA analysis were used to investigate the binding affinity of 12 phytochemicals against three envelope proteins of MPXV, viz., D13, A26, and H3. Silibinin, oleanolic acid, and ursolic acid were computationally identified as potential phytochemicals that showed strong binding affinity toward all the tested structural proteins of MPXV through molecular docking. The stability of the docked complexes was also confirmed by MD simulations and MM/PBSA calculations. Results from the iMODS server also complemented the findings from molecular docking and MD simulations. ADME analysis also computationally confirmed the drug-like properties of the phytochemicals, thereby asserting their suitability for consumption. Hence, this study envisions the candidature of bioactive phytochemicals as promising inhibitors against the envelope proteins of the MPXV, serving as template molecules that could further be experimentally evaluated for their efficacy against monkeypox.

Vaccinia virus H7-protein is required for the organization of the viral scaffold protein into hexamers

Viruses of the giant virus family are characterized by a structurally conserved scaffold-capsid protein that shapes the icosahedral virion. The vaccinia virus (VACV) scaffold protein D13, however, transiently shapes the newly assembled viral membrane in to a sphere and is absent from the mature brick-shaped virion. In infected cells D13, a 62 kDa polypeptide, forms trimers that arrange in hexamers and a honey-comb like lattice. Membrane association of the D13-lattice may be mediated by A17, an abundant 21 kDa viral membrane protein. Whether membrane binding mediates the formation of the honey-comb lattice or if other factors are involved, remains elusive. Here we show that H7, a 17 kDa protein conserved among poxviruses, mediates proper formation of D13-hexamers, and hence the honey comb lattice and spherical immature virus. Without H7 synthesis D13 trimers assemble into a large 3D network rather than the typical well organized scaffold layer observed in wild-type infection, composed of short D13 tubes of discrete length that are tightly associated with the endoplasmic reticulum (ER). The data show an unexpected role for H7 in D13 organization and imply that formation of the honey-comb, hexagonal, lattice is essential for VACV membrane assembly and production of infectious progeny. The data are discussed with respect to scaffold proteins of other giant viruses.

The vaccinia‐based Sementis Copenhagen Vector coronavirus disease 2019 vaccine induces broad and durable cellular and humoral immune responses

The ongoing coronavirus disease 2019 (COVID‐19) pandemic perpetuated by severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2) variants has highlighted the continued need for broadly protective vaccines that elicit robust and durable protection. Here, the vaccinia virus‐based, replication‐defective Sementis Copenhagen Vector (SCV) was used to develop a first‐generation COVID‐19 vaccine encoding the spike glycoprotein (SCV‐S). Vaccination of mice rapidly induced polyfunctional CD8 T cells with cytotoxic activity and robust type 1 T helper‐biased, spike‐specific antibodies, which are significantly increased following a second vaccination, and contained neutralizing activity against the alpha and beta variants of concern. Longitudinal studies indicated that neutralizing antibody activity was maintained up to 9 months after vaccination in both young and middle‐aged mice, with durable immune memory evident even in the presence of pre‐existing vector immunity. Therefore, SCV‐S vaccination has a positive immunogenicity profile, with potential to expand protection generated by current vaccines in a heterologous boost format and presents a solid basis for second‐generation SCV‐based COVID‐19 vaccine candidates incorporating additional SARS‐CoV‐2 immunogens.

Assembly mechanism of the pleomorphic immature poxvirus scaffold

In Vaccinia virus (VACV), the prototype poxvirus, scaffold protein D13 forms a honeycomb-like lattice on the viral membrane that results in formation of the pleomorphic immature virion (IV). The structure of D13 is similar to those of major capsid proteins that readily form icosahedral capsids in nucleocytoplasmic large DNA viruses (NCLDVs). However, the detailed assembly mechanism of the nonicosahedral poxvirus scaffold has never been understood. Here we show the cryo-EM structures of the D13 trimer and scaffold intermediates produced in vitro. The structures reveal that the displacement of the short N-terminal α-helix is critical for initiation of D13 self-assembly. The continuous curvature of the IV is mediated by electrostatic interactions that induce torsion between trimers. The assembly mechanism explains the semiordered capsid-like arrangement of D13 that is distinct from icosahedral NCLDVs. Our structures explain how a single protein can self-assemble into different capsid morphologies and represent a local exception to the universal Caspar-Klug theory of quasi-equivalence.

Immature poxviruses are characterized by nonicosahedral semiordered protein scaffolds critical for morphogenesis. Here, the authors use cryo-EM structures of Vaccinia virus D13 scaffold intermediates to explain their assembly mechanism.

Structural basis for the inhibition of poxvirus assembly by the antibiotic rifampicin

Poxviruses are large DNA viruses that cause disease in animals and humans. They differ from classical enveloped viruses, because their membrane is acquired from cytoplasmic membrane precursors assembled onto a viral protein scaffold formed by the D13 protein rather than budding through cellular compartments. It was found three decades ago that the antibiotic rifampicin blocks this process and prevents scaffold formation. To elucidate the mechanism of action of rifampicin, we have determined the crystal structures of six D13-rifamycin complexes. These structures reveal that rifamycin compounds bind to a phenylalanine-rich region, or F-ring, at the membrane-proximal opening of the central channel of the D13 trimer. We show by NMR, surface plasmon resonance (SPR), and site-directed mutagenesis that A17, a membrane-associated viral protein, mediates the recruitment of the D13 scaffold by also binding to the F-ring. This interaction is the target of rifampicin, which prevents A17 binding, explaining the inhibition of viral morphogenesis. The F-ring of D13 is both conserved in sequence in mammalian poxviruses and essential for interaction with A17, defining a target for the development of assembly inhibitors. The model of the A17-D13 interaction describes a two-component system for remodeling nascent membranes that may be conserved in other large and giant DNA viruses.

Enigmatic origin of the poxvirus membrane from the endoplasmic reticulum shown by 3D imaging of vaccinia virus assembly mutants

The long-standing inability to visualize connections between poxvirus membranes and cellular organelles has led to uncertainty regarding the origin of the viral membrane. Indeed, there has been speculation that viral membranes form de novo in cytoplasmic factories. Another possibility, that the connections are too short-lived to be captured by microscopy during a normal infection, motivated us to identify and characterize virus mutants that are arrested in assembly. Five conserved vaccinia virus proteins, referred to as Viral Membrane Assembly Proteins (VMAPs), that are necessary for formation of immature virions were found. Transmission electron microscopy studies of two VMAP deletion mutants had suggested retention of connections between viral membranes and the endoplasmic reticulum (ER). We now analyzed cells infected with each of the five VMAP deletion mutants by electron tomography, which is necessary to validate membrane continuity, in addition to conventional transmission electron microscopy. In all cases, connections between the ER and viral membranes were demonstrated by 3D reconstructions, supporting a role for the VMAPs in creating and/or stabilizing membrane scissions. Furthermore, coexpression of the viral reticulon-like transmembrane protein A17 and the capsid-like scaffold protein D13 was sufficient to form similar ER-associated viral structures in the absence of other major virion proteins. Determination of the mechanism of ER disruption during a normal VACV infection and the likely participation of both viral and cell proteins in this process may provide important insights into membrane dynamics.

Production of a Chikungunya Vaccine Using a CHO Cell and Attenuated Viral-Based Platform Technology

Vaccinia-based systems have been extensively explored for the development of recombinant vaccines. Herein we describe an innovative vaccinia virus (VACV)-derived vaccine platform technology termed Sementis Copenhagen Vector (SCV), which was rendered multiplication-defective by targeted deletion of the essential viral assembly gene D13L. A SCV cell substrate line was developed for SCV vaccine production by engineering CHO cells to express D13 and the VACV host-range factor CP77, because CHO cells are routinely used for manufacture of biologics. To illustrate the utility of the platform technology, a SCV vaccine against chikungunya virus (SCV-CHIK) was developed and shown to be multiplication-defective in a range of human cell lines and in immunocompromised mice. A single vaccination of mice with SCV-CHIK induced antibody responses specific for chikungunya virus (CHIKV) that were similar to those raised following vaccination with a replication-competent VACV-CHIK and able to neutralize CHIKV. Vaccination also provided protection against CHIKV challenge, preventing both viremia and arthritis. Moreover, SCV retained capacity as an effective mouse smallpox vaccine. In summary, SCV represents a new and safe vaccine platform technology that can be manufactured in modified CHO cells, with pre-clinical evaluation illustrating utility for CHIKV vaccine design and construction.

Vaccinia virus A11 is required for membrane rupture and viral membrane assembly

Although most enveloped viruses acquire their membrane from the host by budding or by a wrapping process, collective data argue that nucleocytoplasmic large DNA viruses (NCLDVs) may be an exception. The prototype member of NCLDVs, vaccinia virus (VACV) may induce rupture of endoplasmic-reticulum-derived membranes to build an open-membrane sphere that closes after DNA uptake. This unconventional membrane assembly pathway is also used by at least 3 other members of the NCLDVs. In this study, we identify the VACV gene product of A11, as required for membrane rupture, hence for VACV membrane assembly and virion formation. By electron tomography, in the absence of A11, the site of assembly formed by the viral scaffold protein D13 is surrounded by endoplasmic reticulum cisternae that are closed. We use scanning transmission electron microscopy-electron tomography to analyse large volumes of cells and demonstrate that in the absence of A11, no open membranes are detected. Given the pivotal role of D13 in initiating VACV membrane assembly, we also analyse viral membranes in the absence of D13 synthesis and show that this protein is not required for rupture. Finally, consistent with a role in rupture, we show that during wild-type infection, A11 localises predominantly to the small ruptured membranes, the precursors of VACV membrane assembly. These data provide strong evidence in favour of the unusual membrane biogenesis of VACV and are an important step towards understanding its molecular mechanism.

Povidone Iodine Ointment Application to the Vaccination Site Does Not Alter Immunoglobulin G Antibody Response to Smallpox Vaccine

U.S. military personnel deployed to high-risk areas receive the live vaccinia virus (VACV) smallpox vaccine ACAM2000. VACV shedding from the vaccination site can result in autoinoculation and contact transmission. We previously found that the application of povidone iodine ointment (PIO) to the scarification site reduced viral shedding without altering the antibody response, as measured by plaque reduction neutralization or enzyme-linked immunosorbent assays. In this study, we used protein microarray assays to measure the amount of immunoglobulin G antibody bound to (1) ACAM2000 itself and (2) individual VACV antigens that are present within ACAM2000. We assessed antibody binding in sera from primary smallpox vaccinees who applied PIO to the scarification site beginning on day 7 (PIO group) and from those who did not apply PIO (control group). In both cohorts, the postvaccination antibody response-in terms of antibody binding, both to ACAM2000 and to 11 individual VACV antigens-was significantly greater than the prevaccination response (all p < 0.0001). The postvaccination antibody binding levels of vaccinees in the PIO group did not differ from those of control vaccinees. These findings further support the topical application of PIO, starting on day 7, to reduce the viral shedding associated with smallpox vaccination.

Poxvirus membrane biogenesis

Poxviruses differ from most DNA viruses by replicating entirely within the cytoplasm. The first discernible viral structures are crescents and spherical immature virions containing a single lipoprotein membrane bilayer with an external honeycomb lattice. Because this viral membrane displays no obvious continuity with a cellular organelle, a de novo origin was suggested. Nevertheless, transient connections between viral and cellular membranes could be difficult to resolve. Despite the absence of direct evidence, the intermediate compartment (ERGIC) between the endoplasmic reticulum (ER) and Golgi apparatus and the ER itself were considered possible sources of crescent membranes. A break-through in understanding poxvirus membrane biogenesis has come from recent studies of the abortive replication of several vaccinia virus null mutants. Novel images showing continuity between viral crescents and the ER and the accumulation of immature virions in the expanded ER lumen provide the first direct evidence for a cellular origin of this poxvirus membrane.

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.

Duplication of the A17L locus of vaccinia virus provides an alternate route to rifampin resistance

Specific gene duplications can enable double-stranded DNA viruses to adapt rapidly to environmental pressures despite the low mutation rate of their high-fidelity DNA polymerases. We report on the rapid positive selection of a novel vaccinia virus genomic duplication mutant in the presence of the assembly inhibitor rifampin. Until now, all known rifampin-resistant vaccinia virus isolates have contained missense mutations in the D13L gene, which encodes a capsid-like scaffold protein required for stabilizing membrane curvature during the early stage of virion assembly. Here we describe a second pathway to rifampin resistance involving A17, a membrane protein that binds and anchors D13 to the immature virion. After one round of selection, a rifampin-resistant virus that contained a genomic duplication in the A17L-A21L region was recovered. The mutant had both C-terminally truncated and full-length A17L open reading frames. Expression of the truncated A17 protein was retained when the virus was passaged in the presence of rifampin but was lost in the absence of the drug, suggesting that the duplication decreased general fitness. Both forms of A17 were bound to the virion membrane and associated with D13. Moreover, insertion of an additional truncated or inducible full-length A17L open reading frame into the genome of the wild-type virus was sufficient to confer rifampin resistance. In summary, this report contains the first evidence of an alternate mechanism for resistance of poxviruses to rifampin, indicates a direct relationship between A17 levels and the resistance phenotype, and provides further evidence of the ability of double-stranded DNA viruses to acquire drug resistance through gene duplication.

IMPORTANCE

The present study provides the first evidence of a new mechanism of resistance of a poxvirus to the antiviral drug rifampin. In addition, it affirms the importance of the interaction between the D13 scaffold protein and the A17 membrane protein for assembly of virus particles. Resistance to rifampin was linked to a partial duplication of the gene encoding the A17 protein, similar to the resistance to hydroxyurea enabled by duplication of the gene encoding the small subunit of ribonucleotide reductase and of the K3L gene to allow adaptation to the antiviral action of protein kinase R. Gene duplication may provide a way for poxviruses and other DNA viruses with high-fidelity DNA polymerases to adjust rapidly to changes in the environment.

Human antibody responses to the polyclonal Dryvax vaccine for smallpox prevention can be distinguished from responses to the monoclonal replacement vaccine ACAM2000

Dryvax (Wyeth Laboratories, Inc., Marietta, PA) is representative of the vaccinia virus preparations that were previously used for preventing smallpox. While Dryvax was highly effective, the national supply stocks were depleted, and there were manufacturing concerns regarding sterility and the clonal heterogeneity of the vaccine. ACAM2000 (Acambis, Inc./Sanofi-Pasteur Biologics Co., Cambridge, MA), a single-plaque-purified vaccinia virus derivative of Dryvax, recently replaced the polyclonal smallpox vaccine for use in the United States. A substantial amount of sequence heterogeneity exists within the polyclonal proteome of Dryvax, including proteins that are missing from ACAM2000. Reasoning that a detailed comparison of antibody responses to the polyclonal and monoclonal vaccines may be useful for identifying unique properties of each antibody response, we utilized a protein microarray comprised of approximately 94% of the vaccinia poxvirus proteome (245 proteins) to measure protein-specific antibody responses of 71 individuals receiving a single vaccination with ACAM2000 or Dryvax. We observed robust antibody responses to 21 poxvirus proteins in vaccinated individuals, including 11 proteins that distinguished Dryvax responses from ACAM2000. Analysis of protein sequences from Dryvax clones revealed amino acid level differences in these 11 antigenic proteins and suggested that sequence variation and clonal heterogeneity may contribute to the observed differences between Dryvax and ACAM2000 antibody responses.

De novo fatty acid biosynthesis contributes significantly to establishment of a bioenergetically favorable environment for vaccinia virus infection

The poxvirus life cycle, although physically autonomous from the host nucleus, is nevertheless dependent upon cellular functions. A requirement for de novo fatty acid biosynthesis was implied by our previous demonstration that cerulenin, a fatty acid synthase inhibitor, impaired vaccinia virus production. Here we show that additional inhibitors of this pathway, TOFA and C75, reduce viral yield significantly, with partial rescue provided by exogenous palmitate, the pathway's end-product. Palmitate's major role during infection is not for phospholipid synthesis or protein palmitoylation. Instead, the mitochondrial import and β-oxidation of palmitate are essential, as shown by the impact of etomoxir and trimetazidine, which target these two processes respectively. Moreover, the impact of these inhibitors is exacerbated in the absence of exogenous glucose, which is otherwise dispensable for infection. In contrast to glucose, glutamine is essential for productive viral infection, providing intermediates that sustain the TCA cycle (anaplerosis). Cumulatively, these data suggest that productive infection requires the mitochondrial β-oxidation of palmitate which drives the TCA cycle and energy production. Additionally, infection causes a significant rise in the cellular oxygen consumption rate (ATP synthesis) that is ablated by etomoxir. The biochemical progression of the vaccinia life cycle is not impaired in the presence of TOFA, C75, or etomoxir, although the levels of viral DNA and proteins synthesized are somewhat diminished. However, by reversibly arresting infections at the onset of morphogenesis, and then monitoring virus production after release of the block, we determined that virion assembly is highly sensitive to TOFA and C75. Electron microscopic analysis of cells released into C75 revealed fragmented aggregates of viroplasm which failed to be enclosed by developing virion membranes. Taken together, these data indicate that vaccinia infection, and in particular virion assembly, relies on the synthesis and mitochondrial import of fatty acids, where their β-oxidation drives robust ATP production.

Vaccinia virus, the prototypic poxvirus, is closely related to variola virus, the etiological agent of smallpox. A full understanding of the poxviral life cycle is imperative for the development of novel antiviral therapies, the design of new vaccines, and the effective and safe use of these viruses as oncolytic agents. Metabolomic studies have shed light on the novel mechanisms used by viruses to replicate efficiently within their hosts. de novo fatty acid biosynthesis has been shown to be of relevance for numerous viral infections as well as for the development of cancer. Here we describe an important role for de novo fatty acid biosynthesis during vaccinia infection. Ongoing synthesis of palmitate is needed to fuel the production of energy within mitochondria. The biochemical events of viral DNA replication and protein synthesis are minimally affected by inhibition of this pathway, but viral assembly is disrupted more dramatically. Further exploration of this pathway will provide additional insight into the infectious cycle and inform new therapeutic strategies for this important class of pathogen.

Association of the vaccinia virus A11 protein with the endoplasmic reticulum and crescent precursors of immature virions

The apparent de novo formation of viral membranes within cytoplasmic factories is a mysterious, poorly understood first step in poxvirus morphogenesis. Genetic studies identified several viral proteins essential for membrane formation and the assembly of immature virus particles. Their repression results in abortive replication with the accumulation of dense masses of viroplasm. In the present study, we further characterized one of these proteins, A11, and investigated its association with cellular and viral membranes under normal and abortive replication conditions. We discovered that A11 colocalized in cytoplasmic factories with the endoplasmic reticulum (ER) and L2, another viral protein required for morphogenesis. Confocal microscopy and subcellular fractionation indicated that A11 was not membrane associated in uninfected cells, whereas L2 still colocalized with the ER. Cell-free transcription and translation experiments indicated that both A11 and L2 are tail-anchored proteins that associate posttranslationally with membranes and likely require specific cytoplasmic targeting chaperones. Transmission electron microscopy indicated that A11, like L2, associated with crescent membranes and immature virions during normal infection and with vesicles and tubules near masses of dense viroplasm during abortive infection in the absence of the A17 or A14 protein component of viral membranes. When the synthesis of A11 was repressed, "empty" immature-virion-like structures formed in addition to masses of viroplasm. The immature-virion-like structures were labeled with antibodies to A17 and to the D13 scaffold protein and were closely associated with calnexin-labeled ER. These studies revealed similarities and differences between A11 and L2, both of which may be involved in the recruitment of the ER for virus assembly.

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.

Biogenesis of the vaccinia virus membrane: genetic and ultrastructural analysis of the contributions of the A14 and A17 proteins

Vaccinia virus membrane biogenesis requires the A14 and A17 proteins. We show here that both proteins can associate with membranes co- but not posttranslationally, and we perform a structure function analysis of A14 and A17 using inducible recombinants. In the absence of A14, electron-dense virosomes and distinct clusters of small vesicles accumulate; in the absence of A17, small vesicles form a corona around the virosomes. When the proteins are induced at 12 h postinfection (hpi), crescents appear at the periphery of the electron-dense virosomes, with the accumulated vesicles likely contributing to their formation. A variety of mutant alleles of A14 and A17 were tested for their ability to support virion assembly. For A14, biologically important motifs within the N-terminal or central loop region affected crescent maturation and the immature virion (IV)→mature virion (MV) transition. For A17, truncation or mutation of the N terminus of A17 engendered a phenotype consistent with the N terminus of A17 recruiting the D13 scaffold protein to nascent membranes. When N-terminal processing was abrogated, virions attempted to undergo the IV-to-MV transition without removing the D13 scaffold and were therefore noninfectious and structurally aberrant. Finally, we show that A17 is phosphorylated exclusively within the C-terminal tail and that this region is a direct substrate of the viral F10 kinase. In vivo, the biological competency of A17 was reduced by mutations that prevented its serine-threonine phosphorylation and restored by phosphomimetic substitutions. Precleavage of the C terminus or abrogation of its phosphorylation diminished the IV→MV maturation; a block to cleavage spared virion maturation but compromised the yield of infectious virus.

Orthopoxvirus targets for the development of new antiviral agents

Investments in the development of new drugs for orthopoxvirus infections have fostered new avenues of research, provided an improved understanding of orthopoxvirus biology and yielded new therapies that are currently progressing through clinical trials. These broad-based efforts have also resulted in the identification of new inhibitors of orthopoxvirus replication that target many different stages of viral replication cycle. This review will discuss progress in the development of new anti-poxvirus drugs and the identification of new molecular targets that can be exploited for the development of new inhibitors. The prototype of the orthopoxvirus group is vaccinia virus and its replication cycle will be discussed in detail noting specific viral functions and their associated gene products that have the potential to serve as new targets for drug development. Progress that has been achieved in recent years should yield new drugs for the treatment of these infections and might also reveal new approaches for antiviral drug development with other viruses.

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.

Insights into the evolution of a complex virus from the crystal structure of vaccinia virus D13

The morphogenesis of poxviruses such as vaccinia virus (VACV) sees the virion shape mature from spherical to brick-shaped. Trimeric capsomers of the VACV D13 protein form a transitory, stabilizing lattice on the surface of the initial spherical immature virus particle. The crystal structure of D13 reveals that this major scaffolding protein comprises a double β barrel "jelly-roll" subunit arranged as pseudo-hexagonal trimers. These structural features are characteristic of the major capsid proteins of a lineage of large icosahedral double-stranded DNA viruses including human adenovirus and the bacteriophages PRD1 and PM2. Structure-based phylogenetic analysis confirms that VACV belongs to this lineage, suggesting that (analogously to higher organism embryogenesis) early poxvirus morphogenesis reflects their evolution from a lineage of viruses sharing a common icosahedral ancestor.

Participation of vaccinia virus l2 protein in the formation of crescent membranes and immature virions

Morphogenesis of vaccinia virus begins with the appearance of crescent-shaped membrane precursors of immature virions in cytoplasmic factories. During the initial characterization of the product of the L2R reading frame, we discovered that it plays an important role in crescent formation. The L2 protein was expressed early in infection and was associated with the detergent-soluble membrane fraction of mature virions, consistent with two potential membrane-spanning domains. All chordopoxviruses have L2 homologs, suggesting an important function. Indeed, we were unable to isolate an infectious L2R deletion mutant. Consequently, we constructed an inducible mutant with a conditional lethal phenotype. When L2 expression was repressed, proteolytic processing of the major core proteins and the A17 protein, which is an essential component of the immature virion membrane, failed to occur, suggesting an early block in viral morphogenesis. At 8 h after infection in the presence of inducer, immature and mature virions were abundantly seen by electron microscopy. In contrast, those structures were rare in the absence of inducer and were replaced by large, dense aggregates of viroplasm. A minority of these aggregates had short spicule-coated membranes, which resembled the beginnings of crescent formation, at their periphery. These short membrane segments at the edge of the dense viroplasm increased in number at later times, and some immature virions were seen. Although the L2 protein was not detected under nonpermissive conditions, minute amounts could account for stunted and delayed viral membrane formation. These findings suggested that L2 is required for the formation or elongation of crescent membranes.

African swine fever virus protein p17 is essential for the progression of viral membrane precursors toward icosahedral intermediates

The first morphological evidence of African swine fever virus (ASFV) assembly is the appearance of precursor viral membranes, thought to derive from the endoplasmic reticulum, within the assembly sites. We have shown previously that protein p54, a viral structural integral membrane protein, is essential for the generation of the viral precursor membranes. In this report, we study the role of protein p17, an abundant transmembrane protein localized at the viral internal envelope, in these processes. Using an inducible virus for this protein, we show that p17 is essential for virus viability and that its repression blocks the proteolytic processing of polyproteins pp220 and pp62. Electron microscopy analyses demonstrate that when the infection occurs under restrictive conditions, viral morphogenesis is blocked at an early stage, immediately posterior to the formation of the viral precursor membranes, indicating that protein p17 is required to allow their progression toward icosahedral particles. Thus, the absence of this protein leads to an accumulation of these precursors and to the delocalization of the major components of the capsid and core shell domains. The study of ultrathin serial sections from cells infected with BA71V or the inducible virus under permissive conditions revealed the presence of large helicoidal structures from which immature particles are produced, suggesting that these helicoidal structures represent a previously undetected viral intermediate.

Cidofovir inhibits genome encapsidation and affects morphogenesis during the replication of vaccinia virus

Cidofovir (CDV) is one of the most effective antiorthopoxvirus drugs, and it is widely accepted that viral DNA replication is the main target of its activity. In the present study, we report a detailed analysis of CDV effects on the replicative cycles of distinct vaccinia virus (VACV) strains: Cantagalo virus, VACV-IOC, and VACV-WR. We show that despite the approximately 90% inhibition of production of virus progeny, virus DNA accumulation was reduced only 30%, and late gene expression and genome resolution were unaltered. The level of proteolytic cleavage of the major core proteins was diminished in CDV-treated cells. Electron microscopic analysis of virus-infected cells in the presence of CDV revealed reductions as great as 3.5-fold in the number of mature forms of virus particles, along with a 3.2-fold increase in the number of spherical immature particles. A detailed analysis of purified virions recovered from CDV-treated cells demonstrated the accumulation of unprocessed p4a and p4b and nearly 67% inhibition of DNA encapsidation. However, these effects of CDV on virus morphogenesis resulted from a primary effect on virus DNA synthesis, which led to later defects in genome encapsidation and virus assembly. Analysis of virus DNA by atomic force microscopy revealed that viral cytoplasmic DNA synthesized in the presence of CDV had an altered structure, forming aggregates with increased strand overlapping not observed in the absence of the drug. These aberrant DNA aggregations were not encapsidated into virus particles.

Vaccinia virus H7 protein contributes to the formation of crescent membrane precursors of immature virions

Crescent membranes are the first viral structures that can be discerned during poxvirus morphogenesis. The crescents consist of a lipoprotein membrane and an outer lattice scaffold, which provides uniform curvature. Relatively little is known regarding the composition of the crescent membrane or its mode of formation. Here, we show that the H7 protein, which is conserved in all vertebrate poxviruses but has no discernible functional motifs or nonpoxvirus homologs, contributes to the formation of crescents and immature virions. Synthesis of the 17-kDa H7 protein was dependent on DNA replication and occurred late during vaccinia virus infection. Unlike many late proteins, however, H7 was not incorporated into mature virions or localized in cellular organelles. To gain insight into the role of H7, an inducible mutant was constructed and shown to have a conditional lethal phenotype: H7 expression and infectious virus formation were dependent on isopropyl-beta-D-thiogalactopyranoside. In the absence of inducer, viral late proteins were made, but membrane and core proteins were not processed by the I7 protease. A block in morphogenesis was demonstrated by transmission electron microscopy: neither typical crescents nor immature virions were detected in the absence of inducer. Instead, factory areas of the cytoplasm contained large, electron-dense inclusions, some of which had partially coated membrane segments at their surfaces. Separate, lower-density inclusions containing the D13 scaffold protein and endoplasmic reticulum membranes were also present. These features are most similar to those previously seen when expression of A11, another conserved nonvirion protein, is repressed.

Assembly and disassembly of the capsid-like external scaffold of immature virions during vaccinia virus morphogenesis

Infectious poxvirus particles are unusual in that they are brick shaped and lack symmetry. Nevertheless, an external honeycomb lattice comprised of a capsid-like protein dictates the spherical shape and size of immature poxvirus particles. In the case of vaccinia virus, trimers of 63-kDa D13 polypeptides form the building blocks of the lattice. In the present study, we addressed two questions: how D13, which has no transmembrane domain, associates with the immature virion (IV) membrane to form the lattice structure and how this scaffold is removed during the subsequent stage of morphogenesis. Interaction of D13 with the A17 membrane protein was demonstrated by immunoaffinity purification and Western blot analysis. In addition, the results of immunogold electron microscopy indicated a close association of A17 and D13 in crescents, as well as in vesicular structures when crescent formation was prevented. Further studies indicated that binding of A17 to D13 was abrogated by truncation of the N-terminal segment of A17. The N-terminal region of A17 was also required for the formation of crescent and IV structures. Disassembly of the D13 scaffold correlated with the processing of A17 by the I7 protease. When I7 expression was repressed, D13 was retained on aberrant virus particles. Furthermore, the morphogenesis of IVs to mature virions was blocked by mutation of the N-terminal but not the C-terminal cleavage site on A17. Taken together, these data indicate that A17 and D13 interactions regulate the assembly and disassembly of the IV scaffold.

Expression of the highly conserved vaccinia virus E6 protein is required for virion morphogenesis

The vaccinia virus E6R gene (VACVWR062) is conserved in all members of the poxvirus family and encodes a protein associated with the mature virion. We confirmed this association and provided evidence for an internal location. An inducible mutant that conditionally expresses E6 was constructed. In the absence of inducer, plaque formation and virus production were severely inhibited in several cell lines, whereas some replication occurred in others. This difference could be due to variation in the stringency of repression, since we could not isolate a stable deletion mutant even in the more "permissive" cells. Under non-permissive conditions, viral late proteins were synthesized but processing of core proteins was inefficient, indicative of an assembly block. Transmission electron microscopy of sections of cells infected with the mutant in the absence of inducer revealed morphogenetic defects with crescents and empty immature virions adjacent to dense inclusions of viroplasm. Mature virions were infrequent and cores appeared to have lucent centers.

Functional characterization of the vaccinia virus I5 protein

The I5L gene is one of ~90 genes that are conserved throughout the chordopoxvirus family, and hence are presumed to play vital roles in the poxvirus life cycle. Previous work had indicated that the VP13 protein, a component of the virion membrane, was encoded by the I5L gene, but no additional studies had been reported. Using a recombinant virus that encodes an I5 protein fused to a V5 epitope tag at the endogenous locus (vI5V5), we show here that the I5 protein is expressed as a post-replicative gene and that the ~9 kDa protein does not appear to be phosphorylated in vivo. I5 does not appear to traffic to any cellular organelle, but ultrastructural and biochemical analyses indicate that I5 is associated with the membranous components of assembling and mature virions. Intact virions can be labeled with anti-V5 antibody as assessed by immunoelectron microscopy, indicating that the C' terminus of the protein is exposed on the virion surface. Using a recombinant virus which encodes only a TET-regulated copy of the I5V5 gene (vΔindI5V5), or one in which the I5 locus has been deleted (vΔI5), we also show that I5 is dispensable for replication in tissue culture. Neither plaque size nor the viral yield produced in BSC40 cells or primary human fibroblasts are affected by the absence of I5 expression.

Antibiotic-Mediated Inhibition of Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) Infection: A Novel Quinolone Function Which Potentiates the Antiviral Cytokine Response in MARC-145 Cells and Pig Macrophages

Porcine reproductive and respiratory syndrome virus (PRRSV) is an economically significant agent for which there currently are no effective treatments. Development of antiviral agents for PRRSV as well as many other viruses has been limited by toxicity of known antiviral compounds. In contrast, antibiotics for non-virus microbial infections have been widely useful, in part because of their acceptable toxicity in animals. We report here the discovery that the quinolone-containing compound Plasmocin™, as well as the quinolones nalidixic acid and ciprofloxacin, have potent anti-PRRSV activity in vitro. PRRSV replication was inhibited by these antibiotics in both cultured MARC-145 cells and cultured primary alveolar porcine macrophages (PAMs). Furthermore, sub-optimal concentrations of nalidixic acid synergized with antiviral cytokines (AK-2 or IFN-γ) to quantitatively and qualitatively inhibit PRRSV replication in MARC-145 cells or PAMs. The antiviral activity of Plasmocin and nalidixic acid correlated with reduced actin expression in MARC-145 cells. Replication of the related lactate dehydrogenase-elevating virus (LDV) was also inhibited in primary mouse macrophages by Plasmocin. These results are significant to the development of antiviral strategies with potentially reduced toxicity, and provide a model system to better understand regulation of arterivirus replication.

The structure of a putative scaffolding protein of immature poxvirus particles as determined by electron microscopy suggests similarity with capsid proteins of large icosahedral DNA viruses

Orf virus, the prototype parapoxvirus, is responsible for contagious ecthyma in sheep and goats. The central region of the viral genome codes for proteins highly conserved among vertebrate poxviruses and which are frequently essential for viral proliferation. Analysis of the recently published genome sequence of orf virus revealed that among such essential proteins, the protein orfv075 is an orthologue of D13, the rifampin resistance gene product critical for vaccinia virus morphogenesis. Previous studies showed that D13, arranged as "spicules," is necessary for the formation of vaccinia virus immature virions, a mandatory intermediate in viral maturation. We have determined the three-dimensional structure of recombinant orfv075 at approximately 25-A resolution by electron microscopy of two-dimensional crystals. orfv075 organizes as trimers with a tripod-like main body and a propeller-like smaller domain. The molecular envelope of orfv075 shows unexpectedly good agreement to that of a distant homologue, VP54, the major capsid protein of Paramecium bursaria Chlorella virus type 1. Our structural analysis suggests that orfv075 belongs in the double-barreled capsid protein family found in many double-stranded DNA icosahedral viruses and supports the hypothesis that the nonicosahedral poxviruses and the large icosahedral DNA viruses are evolutionarily related.

Amino acid substitutions at multiple sites within the vaccinia virus D13 scaffold protein confer resistance to rifampicin

D13 protein trimers, which form an external lattice providing curvature to the membrane of vaccinia virus immature virions, are the target of the drug rifampicin. We obtained 63 rifampicin-resistant mutants following random PCR mutagenesis of the D13L gene and 5 that arose spontaneously. Sequencing indicated that 26 mutants contained a single, unique, amino acid substitution whereas others contained 2 or more. The single mutations, including 6 previously identified, mapped to 24 different amino acids that were distributed over the length of the protein with the majority clustered between amino acids 17 to 33, 222 to 243 and 480 to 488. Two or three different substitutions occurred in six of the 24 amino acids. Representative mutant viruses of each cluster replicated to wild-type levels in the absence of rifampicin and nearly two logs higher than wild-type in the presence of drug. The large number and fitness of the mutations are remarkable in view of the extreme sequence conservation (99-100% amino acid identity amongst all orthopoxviruses). Clustering of mutations could suggest the presence of a rifampicin-binding pocket comprised of discontinuous regions of D13.

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.

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.

External scaffold of spherical immature poxvirus particles is made of protein trimers, forming a honeycomb lattice

During morphogenesis, poxviruses undergo a remarkable transition from spherical immature forms to brick-shaped infectious particles lacking helical or icosahedral symmetry. In this study, we show that the transitory honeycomb lattice coating the lipoprotein membrane of immature vaccinia virus particles is formed from trimers of a 62-kD protein encoded by the viral D13L gene. Deep-etch electron microscopy demonstrated that anti-D13 antibodies bound to the external protein coat and that lattice fragments were in affinity-purified D13 preparations. Soluble D13 appeared mostly trimeric by gel electrophoresis and ultracentrifugation, which is consistent with structural requirements for a honeycomb. In the presence or absence of other virion proteins, a mutated D13 with one amino acid substitution formed stacks of membrane-unassociated flat sheets that closely resembled the curved honeycombs of immature virions except for the absence of pentagonal facets. A homologous domain that is present in D13 and capsid proteins of certain other lipid-containing viruses support the idea that the developmental stages of poxviruses reflect their evolution from an icosahedral ancestor.

Cell biological and functional characterization of the vaccinia virus F10 kinase: implications for the mechanism of virion morphogenesis

The vaccinia virus F10 protein is one of two virally encoded protein kinases. A phenotypic analysis of infections involving a tetracycline-inducible recombinant (vDeltaiF10) indicated that F10 is involved in the early stages of virion morphogenesis, as previously reported for the mutants ts28 and ts15. The proteins encoded by ts28 and ts15 have primary defects in enzymatic activity and thermostability, respectively. Using a transient complementation assay, we demonstrated that the enzymatic activity of F10 is essential for its biological function and that both its enzymatic and biological functions depend upon N-terminal sequences that precede the catalytic domain. An execution point analysis indicated that in addition to its role at the onset of morphogenesis, F10 is also required at later stages, when membrane crescents surround virosomal contents and develop into immature virions. The F10 protein is phosphorylated in vivo, appears to be tightly associated with intracellular membranes, and can bind to specific phosphoinositides in vitro. When F10 is repressed or impaired, the phosphorylation of several cellular and viral proteins appears to increase in intensity, suggesting that F10 may normally intersect with cellular signaling cascades via the activation of a phosphatase or the inhibition of another kinase. These cascades may drive the F10-induced remodeling of membranes that accompanies virion biogenesis. Upon the release of ts28-infected cultures from a 40 degrees C-induced block, a synchronous resumption of morphogenesis that culminates in the production of infectious virus can be observed. The pharmacological agents H89 and cerulenin, which are inhibitors of endoplasmic reticulum exit site formation and de novo lipid synthesis, respectively, block this recovery.

Vaccinia virus nonstructural protein encoded by the A11R gene is required for formation of the virion membrane

The vaccinia virus A11R gene has orthologs in all known poxvirus genomes, and the A11 protein has been previously reported to interact with the putative DNA packaging protein A32 in a yeast two-hybrid screen. Using antisera raised against A11 peptides, we show that the A11 protein was (i) expressed at late times with an apparent mass of 40 kDa, (ii) not incorporated into virus particles, (iii) phosphorylated independently of the viral F10 kinase, (iv) coimmunoprecipitated with A32, and (v) localized to the viral factory. To determine the role of the A11 protein and test whether it is indeed involved in DNA packaging, we constructed a recombinant vaccinia virus with an inducible A11R gene. This recombinant was dependent on inducer for single-cycle growth and plaque formation. In the absence of inducer, viral late proteins were produced at normal levels, but proteolytic processing and other posttranslational modifications of some proteins were inhibited, suggesting a block in virus particle assembly. Consistent with this observation, electron microscopy of cells infected in the absence of inducer showed virus factories with abnormal electron-dense viroplasms and intermediate density regions associated with membranes and containing the D13 protein. However, no viral membrane crescents, immature virions, or mature virions were produced. The requirement for nonvirion protein A11 in order to make normal viral membranes was an unexpected and exciting finding, since neither the origin of these membranes nor their mechanism of formation in the cytoplasm of infected cells is understood.

Deep-etch EM reveals that the early poxvirus envelope is a single membrane bilayer stabilized by a geodetic "honeycomb" surface coat

Three-dimensional "deep-etch" electron microscopy (DEEM) resolves a longstanding controversy concerning poxvirus morphogenesis. By avoiding fixative-induced membrane distortions that confounded earlier studies, DEEM shows that the primary poxvirus envelope is a single membrane bilayer coated on its external surface by a continuous honeycomb lattice. Freeze fracture of quick-frozen poxvirus-infected cells further shows that there is only one fracture plane through this primary envelope, confirming that it consists of a single lipid bilayer. DEEM also illustrates that the honeycomb coating on this envelope is completely replaced by a different paracrystalline coat as the poxvirus matures. Correlative thin section images of infected cells freeze substituted after quick-freezing, plus DEEM imaging of Tokuyasu-type cryo-thin sections of infected cells (a new application introduced here) all indicate that the honeycomb network on immature poxvirus virions is sufficiently continuous and organized, and tightly associated with the envelope throughout development, to explain how its single lipid bilayer could remain stable in the cytoplasm even before it closes into a complete sphere.

A vaccinia replication system for producing recombinant hepatitis C virus

AIM: To develop a cell culture system capable of producing high titer hepatitis C virus (HCV) stocks with recombinant vaccinia viruses as helpers.

METHODS: Two plasmids were used for the generation of recombinant HCV: one containing the full-length HCV cDNA cloned between T7 promoter and T7 terminator of pOCUS-T7 vector, and the other containing the HCV polyprotein open reading frame (ORF) directly linked to a vaccinia late promoter in PSC59. These two plasmids were co-transfected into BHK₂₁ cells, which were then infected with vTF7-3 recombinant vaccinia helper viruses.

RESULTS: After 5 d of incubation, approximately 3.6 × 10⁷ copies of HCV RNA were present per milliliter of cell culture supernatant, as detected by fluorescence quantitative RT-PCR (FQ-PCR). The yield of recombinant HCV using this cell system increased 100- to 1000-fold compared to in vitro-transcribed HCV genomic RNA or selective subgenomic HCV RNA molecule method.

CONCLUSION: This cell culture system is capable of producing high titer recombinant HCV.

African swine fever virus structural protein p54 is essential for the recruitment of envelope precursors to assembly sites

The assembly of African swine fever virus (ASFV) at the cytoplasmic virus factories commences with the formation of precursor membranous structures, which are thought to be collapsed cisternal domains recruited from the surrounding endoplasmic reticulum (ER). This report analyzes the role in virus morphogenesis of the structural protein p54, a 25-kDa polypeptide encoded by the E183L gene that contains a putative transmembrane domain and localizes at the ER-derived envelope precursors. We show that protein p54 behaves in vitro and in infected cells as a type I membrane-anchored protein that forms disulfide-linked homodimers through its unique luminal cysteine. Moreover, p54 is targeted to the ER membranes when it is transiently expressed in transfected cells. Using a lethal conditional recombinant, vE183Li, we also demonstrate that the repression of p54 synthesis arrests virus morphogenesis at a very early stage, even prior to the formation of the precursor membranes. Under restrictive conditions, the virus factories appeared as discrete electron-lucent areas essentially free of viral structures. In contrast, outside the assembly sites, large amounts of aberrant zipper-like structures formed by the unprocessed core polyproteins pp220 and pp62 were produced in close association to ER cisternae. Altogether, these results indicate that the transmembrane structural protein p54 is critical for the recruitment and transformation of the ER membranes into the precursors of the viral envelope.

Evidence for an essential catalytic role of the F10 protein kinase in vaccinia virus morphogenesis

Temperature-sensitive mutants of vaccinia virus, with genetic changes that map to the open reading frame encoding the F10 protein kinase, exhibit a defect at an early stage of viral morphogenesis. To further study the role of the enzyme, we constructed recombinant vaccinia virus vF10V5i, which expresses inducible V5 epitope-tagged F10 and is dependent on a chemical inducer for plaque formation and replication. In the absence of inducer, viral membrane formation was delayed and crescents and occasional immature forms were detected only late in infection. When the temperature was raised from 37 to 39 degrees C, the block in membrane formation persisted throughout the infection. The increased stringency may be explained by a mild temperature sensitivity of the wild-type F10 kinase, which reduced the activity of the very small amount expressed in the absence of inducer, or by the thermolability of an unphosphorylated kinase substrate or uncomplexed F10-interacting protein. Further analyses demonstrated that tyrosine and threonine phosphorylation of the A17 membrane component was inhibited in the absence of inducer. The phosphorylation defect could be overcome by transfection of plasmids that express wild-type F10, but not by plasmids that express F10 with single amino acid substitutions that abolished catalytic activity. Although the mutated forms of F10 were stable and concentrated in viral factories, only the wild-type protein complemented the assembly and replication defects of vF10V5i in the absence of inducer. These studies provide evidence for an essential catalytic role of the F10 kinase in vaccinia virus morphogenesis.

Vaccinia virus G7L protein Interacts with the A30L protein and is required for association of viral membranes with dense viroplasm to form immature virions

The vaccinia virus A30L protein is required for the association of electron-dense, granular, proteinaceous material with the concave surfaces of crescent membranes, an early step in viral morphogenesis. For the identification of additional proteins involved in this process, we used an antibody to the A30L protein, or to an epitope appended to its C terminus, to capture complexes from infected cells. A prominent 42-kDa protein was resolved and identified by mass spectrometry as the vaccinia virus G7L protein. This previously uncharacterized protein was expressed late in infection and was associated with immature virions and the cores of mature particles. In order to study the role of the G7L protein, a conditional lethal mutant was made by replacing the G7L gene with an inducible copy. Expression of G7L and formation of infectious virus was dependent on the addition of inducer. Under nonpermissive conditions, morphogenesis was blocked and viral crescent membranes and immature virions containing tubular elements were separated from the electron-dense granular viroplasm, which accumulated in large spherical masses. This phenotype was identical to that previously obtained with an inducible, conditional lethal A30L mutant. Additional in vivo and in vitro experiments provided evidence for the direct interaction of the A30L and G7L proteins and demonstrated that the stability of each one was dependent on its association with the other.

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.

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.

Assembly of vaccinia virus revisited: de novo membrane synthesis or acquisition from the host?

In 1968 it was proposed that the first membrane structures that assemble in vaccinia virus-infected cells, the crescents, are formed by a unique viral mechanism in which a single membrane bilayer is synthesized de novo. 25 years later it was suggested that the vaccinia membranes are derived from an organelle that is part of the host cell's secretory pathway, the intermediate compartment (IC), and that the viral crescents are made of two tightly apposed membranes rather than a single bilayer. Several independent studies have subsequently shown that membrane proteins of the intracellular mature virus (IMV) insert co-translationally into endoplasmic reticulum (ER) membranes, and are targeted to and retained in the IC, the compartment from which the virus acquires its membranes. Furthermore, a recent study on the entry of both the IMV and extracellular enveloped virus (EEV) suggests that these viruses do not enter by a simple fusion mechanism, consistent with the idea that both are surrounded by more than one lipid bilayer.

The major core protein P4a (A10L gene) of vaccinia virus is essential for correct assembly of viral DNA into the nucleoprotein complex to form immature viral particles

The vaccinia virus (VV) A10L gene codes for a major core protein, P4a. This polypeptide is synthesized at late times during viral infection and is proteolytically cleaved during virion assembly. To investigate the role of P4a in the virus life cycle and morphogenesis, we have generated an inducer-dependent conditional mutant (VVindA10L) in which expression of the A10L gene is under the control of the Escherichia coli lacI operator/repressor system. Repression of the A10L gene severely impairs virus growth, as observed by both the inability of the virus to form plaques and the 2-log reduction of viral yields. This defect can be partially overcome by addition of the inducer isopropyl-beta-D-thiogalactopyranoside (IPTG). Synthesis of viral proteins other than P4a occurred, although early shutoff of host protein synthesis and expression of viral late polypeptides are clearly delayed, both in the absence and in the presence of IPTG, compared with cells infected with the parental virus. Viral DNA replication and concatemer resolution appeared to proceed normally in the absence of the A10L gene product. In cells infected with VVindA10L in the absence of the inducer virion assembly is blocked, as defined by electron microscopy. Numerous spherical immature viral particles that appear devoid of dense viroplasmic material together with highly electron-dense regular structures are abundant in VVindA10L-infected cells. These regularly spaced structures can be specifically labeled with anti-DNA antibodies as well as with a DNase-gold conjugate, indicating that they contain DNA. Some images suggest that these DNA structures enter into spherical immature viral particles. In this regard, although it has not been firmly established, it has been suggested that DNA uptake occurs after formation of spherical immature particles. Overall, our results showed that P4a and/or its cleaved products are essential for the correct assembly of the nucleoprotein complex within immature viral particles.

Vaccinia virus A30L protein is required for association of viral membranes with dense viroplasm to form immature virions

The previously uncharacterized A30L gene of vaccinia virus has orthologs in all vertebrate poxviruses but no recognizable nonpoxvirus homologs or functional motifs. We determined that the A30L gene was regulated by a late promoter and encoded a protein of approximately 9 kDa. Immunoelectron microscopy of infected cells indicated that the A30L protein was associated with viroplasm enclosed by crescent and immature virion membranes. The A30L protein was also present in mature virions and was partially released by treatment with a nonionic detergent and reducing agent, consistent with a location in the matrix between the core and envelope. To determine the role of the A30L protein, we constructed a stringent conditional lethal mutant with an inducible A30L gene. In the absence of inducer, synthesis of viral early and late proteins occurred but the proteolytic processing of certain core proteins was inhibited, suggesting an assembly block. Inhibition of virus maturation was confirmed by electron microscopy. Under nonpermissive conditions, we observed aberrant large, dense, granular masses of viroplasm with clearly defined margins; viral crescent membranes that appeared normal except for their location at a distance from viroplasm; empty immature virions; and an absence of mature virions. The data indicated that the A30L protein is needed for vaccinia virus morphogenesis, specifically the association of the dense viroplasm with viral membranes.

Vaccinia virus E10R protein is associated with the membranes of intracellular mature virions and has a role in morphogenesis

This study provides the initial biochemical, microscopic, and genetic characterization of the product of the vaccinia virus E10R gene, which belongs to the ERV1/ALR family of eukaryotic proteins, is conserved in all poxviruses and has homologs in other cytoplasmic DNA viruses. DNA encoding a short epitope tag was appended to the C-terminus of the 95-amino-acid open-reading frame without affecting virus reproduction. The E10R protein was synthesized after DNA replication and was associated with purified intracellular mature virions (IMV), from which it could be extracted with a nonionic detergent. Antibody to the tag decorated the surface of IMV, consistent with the anchorage of the E10R protein to the membrane via its hydrophobic N-terminus. Immunoelectron microscopy revealed that the E10R protein was associated with crescent membranes, immature virions, IMV, and extracellular particles. To investigate the role of E10R in the virus life cycle, we constructed an inducer-dependent null mutant. In the absence of inducer, the formation of infectious virus was severely inhibited and electron microscopy revealed an assembly block with accumulation of crescent membranes and immature virions. Cysteines 43 and 46, comprising a putative redox motif common to all poxvirus E10R homologs, were essential for complementation of the mutant virus by transfected E10R DNA.