Molecular genetic and biochemical characterization of the vaccinia virus I3 protein, the replicative single-stranded DNA binding protein

Poxin-deficient poxviruses are sensed by cGAS prior to genome replication

Poxviruses are dsDNA viruses infecting a wide range of cell types, where they need to contend with multiple host antiviral pathways, including DNA and RNA sensing. Accordingly, poxviruses encode a variety of immune antagonists, most of which are expressed early during infection from within virus cores before uncoating and genome release take place. Amongst these antagonists, the poxvirus immune nuclease (poxin) counteracts the cyclic 2'3'-GMP-AMP (2'3'-cGAMP) synthase (cGAS)/stimulator of interferon genes DNA sensing pathway by degrading the immunomodulatory cyclic dinucleotide 2'3'-cGAMP, the product of activated cGAS. Here, we use poxviruses engineered to lack poxin to investigate how virus infection triggers the activation of STING and its downstream transcription factor interferon-responsive factor 3 (IRF3). Our results demonstrate that poxin-deficient vaccinia virus (VACV) and ectromelia virus (ECTV) induce IRF3 activation in primary fibroblasts and differentiated macrophages, although to a lower extent in VACV compared to ECTV. In fibroblasts, IRF3 activation was detectable at 10 h post-infection (hpi) and was abolished by the DNA replication inhibitor cytosine arabinoside (AraC), indicating that the sensing was mediated by replicated genomes. In macrophages, IRF3 activation was detectable at 4 hpi, and this was not affected by AraC, suggesting that the sensing in this cell type was induced by genomes released from incoming virions. In agreement with this, macrophages expressing short hairpin RNA (shRNA) against the virus uncoating factor D5 showed reduced IRF3 activation upon infection. Collectively, our data show that the viral genome is sensed by cGAS prior to and during genome replication, but immune activation downstream of it is effectively suppressed by poxin. Our data also support the model where virus uncoating acts as an immune evasion strategy to simultaneously cloak the viral genome and allow the expression of early immune antagonists.

Birth of protein folds and functions in the virome

The rapid evolution of viruses generates proteins that are essential for infectivity and replication but with unknown functions, due to extreme sequence divergence1. Here, using a database of 67,715 newly predicted protein structures from 4,463 eukaryotic viral species, we found that 62% of viral proteins are structurally distinct and lack homologues in the AlphaFold database2,3. Among the remaining 38% of viral proteins, many have non-viral structural analogues that revealed surprising similarities between human pathogens and their eukaryotic hosts. Structural comparisons suggested putative functions for up to 25% of unannotated viral proteins, including those with roles in the evasion of innate immunity. In particular, RNA ligase T-like phosphodiesterases were found to resemble phage-encoded proteins that hydrolyse the host immune-activating cyclic dinucleotides 3',3'- and 2',3'-cyclic GMP-AMP (cGAMP). Experimental analysis showed that RNA ligase T homologues encoded by avian poxviruses similarly hydrolyse cGAMP, showing that RNA ligase T-mediated targeting of cGAMP is an evolutionarily conserved mechanism of immune evasion that is present in both bacteriophage and eukaryotic viruses. Together, the viral protein structural database and analyses presented here afford new opportunities to identify mechanisms of virus-host interactions that are common across the virome.

The Life Cycle of the Vaccinia Virus Genome

Poxviruses, of which vaccinia virus is the prototype, are a large family of double-stranded DNA viruses that replicate exclusively in the cytoplasm of infected cells. This physical and genetic autonomy from the host cell nucleus necessitates that these viruses encode most, if not all, of the proteins required for replication in the cytoplasm. In this review, we follow the life of the viral genome through space and time to address some of the unique challenges that arise from replicating a 195-kb DNA genome in the cytoplasm. We focus on how the genome is released from the incoming virion and deposited into the cytoplasm; how the endoplasmic reticulum is reorganized to form a replication factory, thereby compartmentalizing and helping to protect the replicating genome from immune sensors; how the cellular milieu is tailored to support high-fidelity replication of the genome; and finally, how newly synthesized genomes are faithfully and specifically encapsidated into new virions. Expected final online publication date for the Annual Review of Virology, Volume 9 is September 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Structure-Function Analysis of Two Interacting Vaccinia Proteins That Are Critical for Viral Morphogenesis: L2 and A30.5

An enduring mystery in poxvirology is the mechanism by which virion morphogenesis is accomplished. A30.5 and L2 are two small regulatory proteins that are essential for this process. Previous studies have shown that vaccinia A30.5 and L2 localize to the ER and interact during infection, but how they facilitate morphogenesis is unknown. To interrogate the relationship between A30.5 and L2, we generated inducible complementing cell lines (CV1-HA-L2; CV1-3xFLAG-A30.5) and deletion viruses (vΔL2; vΔA30.5). Loss of either protein resulted in a block in morphogenesis and a significant (>100-fold) decrease in infectious viral yield. Structure-function analysis of L2 and A30.5, using transient complementation assays, identified key functional regions in both proteins. A clustered charge-to-alanine L2 mutant (L2-RRD) failed to rescue a vΔL2 infection and exhibits a significantly retarded apparent molecular weight in vivo (but not in vitro), suggestive of an aberrant posttranslational modification. Furthermore, an A30.5 mutant with a disrupted putative N-terminal α-helix failed to rescue a vΔA30.5 infection. Using our complementing cell lines, we determined that the stability of A30.5 is dependent on L2 and that wild-type L2 and A30.5 coimmunoprecipitate in the absence of other viral proteins. Further examination of this interaction, using wild-type and mutant forms of L2 or A30.5, revealed that the inability of mutant alleles to rescue the respective deletion viruses is tightly correlated with a failure of L2 to stabilize and interact with A30.5. L2 appears to function as a chaperone-like protein for A30.5, ensuring that they work together as a complex during viral membrane biogenesis. IMPORTANCE Vaccinia virus is a large, enveloped DNA virus that was successfully used as the vaccine against smallpox. Vaccinia continues to be an invaluable biomedical research tool in basic research and in gene therapy vector and vaccine development. Although this virus has been studied extensively, the complex process of virion assembly, termed morphogenesis, still puzzles the field. Our work aims to better understand how two small viral proteins that are essential for viral assembly, L2 and A30.5, function during early morphogenesis. We show that A30.5 requires L2 for stability and that these proteins interact in the absence of other viral proteins. We identify regions in each protein required for their function and show that mutations in these regions disrupt the interaction between L2 and A30.5 and fail to restore virus viability.

Oncolytic Virotherapy: From Bench to Bedside

Oncolytic viruses are naturally occurring or genetically engineered viruses that can replicate preferentially in tumor cells and inhibit tumor growth. These viruses have been considered an effective anticancer strategy in recent years. They mainly function by direct oncolysis, inducing an anticancer immune response and expressing exogenous effector genes. Their multifunctional characteristics indicate good application prospects as cancer therapeutics, especially in combination with other therapies, such as radiotherapy, chemotherapy and immunotherapy. Therefore, it is necessary to comprehensively understand the utility of oncolytic viruses in cancer therapeutics. Here, we review the characteristics, antitumor mechanisms, clinical applications, deficiencies and associated solutions, and future prospects of oncolytic viruses.

A poxvirus pseudokinase represses viral DNA replication via a pathway antagonized by its paralog kinase

Poxviruses employ sophisticated, but incompletely understood, signaling pathways that engage cellular defense mechanisms and simultaneously ensure viral factors are modulated properly. For example, the vaccinia B1 protein kinase plays a vital role in inactivating the cellular antiviral factor BAF, and likely orchestrates other pathways as well. In this study, we utilized experimental evolution of a B1 deletion virus to perform an unbiased search for suppressor mutations and identify novel pathways involving B1. After several passages of the ΔB1 virus we observed a robust increase in viral titer of the adapted virus. Interestingly, our characterization of the adapted viruses reveals that mutations correlating with a loss of function of the vaccinia B12 pseudokinase provide a striking fitness enhancement to this virus. In support of predictions that reductive evolution is a driver of poxvirus adaptation, this is clear experimental evidence that gene loss can be of significant benefit. Next, we present multiple lines of evidence demonstrating that expression of full length B12 leads to a fitness reduction in viruses with a defect in B1, but has no apparent impact on wild-type virus or other mutant poxviruses. From these data we infer that B12 possesses a potent inhibitory activity that can be masked by the presence of the B1 kinase. Further investigation of B12 attributes revealed that it primarily localizes to the nucleus, a characteristic only rarely found among poxviral proteins. Surprisingly, BAF phosphorylation is reduced under conditions in which B12 is present in infected cells without B1, indicating that B12 may function in part by enhancing antiviral activity of BAF. Together, our studies of B1 and B12 present novel evidence that a paralogous kinase-pseudokinase pair can exhibit a unique epistatic relationship in a virus, perhaps serving to enhance B1 conservation during poxvirus evolution and to orchestrate yet-to-be-discovered nuclear events during infection.

Assessing the Structure and Function of Vaccinia Virus Gene Products by Transient Complementation

Poxviruses are large, complex dsDNA viruses that are highly unusual in replicating solely within the cytoplasm of the infected cell. The most infamous poxvirus was variola virus, the etiological agent of smallpox; today, poxviruses remain of biomedical significance, both as pathogens and as recombinant vaccines and oncolytic therapies. Vaccinia virus is the prototypic poxvirus for experimental analysis. The 195 kb dsDNA genome contains >200 genes that encode proteins involved in such processes as viral entry, gene expression, genome replication and maturation, virion assembly, virion egress, and immune evasion.Molecular genetic analysis has been instrumental in the study of the structure and function of many viral gene products. Temperature-sensitive (ts) mutants have been especially useful in this endeavor; inducible recombinants and deletion mutants are now also important tools. Once a phenotype is observed following the repression, deletion, or inactivation of a particular gene product, the technique of transient complementation becomes central for further study.Simply put, transient complementation involves the transient expression of a variety of alleles of a given viral gene within infected cells, and the evaluation of which of these alleles can "complement" or "rescue" the phenotype caused by the loss of the endogenous allele. This analysis leads to the identification of key domains, motifs, and sites of posttranslational modification. Subcellular localization and protein:protein interactions can also be evaluated in these studies. The development of a reliable toolbox of vectors encoding viral promoters of different temporal classes, and the use of a variety of epitope tags, has greatly enhanced the utility of this experimental approach for poxvirus research.

The Vaccinia virion: Filling the gap between atomic and ultrastructure

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

Isolation and Characterization of vΔI3 Confirm that Vaccinia Virus SSB Plays an Essential Role in Viral Replication

Vaccinia virus is unusual among DNA viruses in replicating exclusively in the cytoplasm of infected cells. The single-stranded DNA (ssDNA) binding protein (SSB) I3 is among the replication machinery encoded by the 195-kb genome, although direct genetic analysis of I3 has been lacking. Herein, we describe a complementing cell line (CV1-I3) that fully supports the replication of a null virus (vΔI3) lacking the I3 open reading frame (ORF). In noncomplementing CV1-CAT cells, vΔI3 shows a severe defect in the production of infectious virus (≥200-fold reduction). Early protein synthesis and core disassembly occur normally. However, DNA replication is profoundly impaired (≤0.2% of wild-type [WT] levels), and late proteins do not accumulate. When several other noncomplementing cell lines are infected with vΔI3, the yield of infectious virus is also dramatically reduced (168- to 1,776-fold reduction). Surprisingly, the residual levels of DNA accumulation vary from 1 to 12% in the different cell lines (CV1-CAT < A549 < BSC40 < HeLa); however, any nascent DNA that can be detected is subgenomic in size. Although this subgenomic DNA supports late protein expression, it does not support the production of infectious virions. Electron microscopy (EM) analysis of vΔI3-infected BSC40 cells reveals that immature virions are abundant but no mature virions are observed. Aberrant virions characteristic of a block to genome encapsidation are seen instead. Finally, we demonstrate that a CV1 cell line encoding a previously described I3 variant with impaired ssDNA binding activity is unable to complement vΔI3. This report provides definitive evidence that the vaccinia virus I3 protein is the replicative SSB and is essential for productive viral replication.IMPORTANCE Poxviruses are of historical and contemporary importance as infectious agents, vaccines, and oncolytic therapeutics. The cytoplasmic replication of poxviruses is unique among DNA viruses of mammalian cells and necessitates that the double-stranded DNA (dsDNA) genome encode the viral replication machinery. This study focuses on the I3 protein. As a ssDNA binding protein (SSB), I3 has been presumed to play essential roles in genome replication, recombination, and repair, although genetic analysis has been lacking. Herein, we report the characterization of an I3 deletion virus. In the absence of I3 expression, DNA replication is severely compromised and viral yield profoundly decreased. The production of infectious virus can be restored in a cell line expressing WT I3 but not in a cell line expressing an I3 mutant that is defective in ssDNA binding activity. These data show conclusively that I3 is an essential viral protein and functions as the viral replicative SSB.

Identification of Vaccinia Virus Replisome and Transcriptome Proteins by Isolation of Proteins on Nascent DNA Coupled with Mass Spectrometry

Poxviruses replicate within the cytoplasm and encode proteins for DNA and mRNA synthesis. To investigate poxvirus replication and transcription from a new perspective, we incorporated 5-ethynyl-2'-deoxyuridine (EdU) into nascent DNA in cells infected with vaccinia virus (VACV). The EdU-labeled DNA was conjugated to fluor- or biotin-azide and visualized by confocal, superresolution, and transmission electron microscopy. Nuclear labeling decreased dramatically after infection, accompanied by intense labeling of cytoplasmic foci. The nascent DNA colocalized with the VACV single-stranded DNA binding protein I3 in multiple puncta throughout the interior of factories, which were surrounded by endoplasmic reticulum. Complexes containing EdU-biotin-labeled DNA cross-linked to proteins were captured on streptavidin beads. After elution and proteolysis, the peptides were analyzed by mass spectrometry to identify proteins associated with nascent DNA. The known viral replication proteins, a telomere binding protein, and a protein kinase were associated with nascent DNA, as were the DNA-dependent RNA polymerase and intermediate- and late-stage transcription initiation and elongation factors, plus the capping and methylating enzymes. These results suggested that the replicating pool of DNA is transcribed and that few if any additional viral proteins directly engaged in replication and transcription remain to be discovered. Among the host proteins identified by mass spectrometry, topoisomerases IIα and IIβ and PCNA were noteworthy. The association of the topoisomerases with nascent DNA was dependent on expression of the viral DNA ligase, in accord with previous proteomic studies. Further investigations are needed to determine possible roles for PCNA and other host proteins detected.IMPORTANCE Poxviruses, unlike many well-characterized animal DNA viruses, replicate entirely within the cytoplasm of animal cells, raising questions regarding the relative roles of viral and host proteins. We adapted newly developed procedures for click chemistry and iPOND (Isolation of proteins on nascent DNA) to investigate vaccinia virus (VACV), the prototype poxvirus. Nuclear DNA synthesis ceased almost immediately following VACV infection, followed swiftly by the synthesis of viral DNA within discrete cytoplasmic foci. All viral proteins known from genetic and proteomic studies to be required for poxvirus DNA replication were identified in the complexes containing nascent DNA. The additional detection of the viral DNA-dependent RNA polymerase and intermediate and late transcription factors provided evidence for a temporal coupling of replication and transcription. Further studies are needed to assess the potential roles of host proteins, including topoisomerases IIα and IIβ and PCNA, which were found associated with nascent DNA.

Cytoplasmic ATR Activation Promotes Vaccinia Virus Genome Replication

In contrast to most DNA viruses, poxviruses replicate their genomes in the cytoplasm without host involvement. We find that vaccinia virus induces cytoplasmic activation of ATR early during infection, before genome uncoating, which is unexpected because ATR plays a fundamental nuclear role in maintaining host genome integrity. ATR, RPA, INTS7, and Chk1 are recruited to cytoplasmic DNA viral factories, suggesting canonical ATR pathway activation. Consistent with this, pharmacological and RNAi-mediated inhibition of canonical ATR signaling suppresses genome replication. RPA and the sliding clamp PCNA interact with the viral polymerase E9 and are required for DNA replication. Moreover, the ATR activator TOPBP1 promotes genome replication and associates with the viral replisome component H5. Our study suggests that, in contrast to long-held beliefs, vaccinia recruits conserved components of the eukaryote DNA replication and repair machinery to amplify its genome in the host cytoplasm.

Proteomic Screen for Cellular Targets of the Vaccinia Virus F10 Protein Kinase Reveals that Phosphorylation of mDia Regulates Stress Fiber Formation

Vaccinia virus, a complex dsDNA virus, is unusual in replicating exclusively within the cytoplasm of infected cells. Although this prototypic poxvirus encodes >200 proteins utilized during infection, a significant role for host proteins and cellular architecture is increasingly evident. The viral B1 kinase and H1 phosphatase are known to target cellular proteins as well as viral substrates, but little is known about the cellular substrates of the F10 kinase. F10 is essential for virion morphogenesis, beginning with the poorly understood process of diversion of membranes from the ER for the purpose of virion membrane biogenesis. To better understand the function of F10, we generated a cell line that carries a single, inducible F10 transgene. Using uninduced and induced cells, we performed stable isotope labeling of amino acids in cell culture (SILAC) coupled with phosphopeptide analysis to identify cellular targets of F10-mediated phosphorylation. We identified 27 proteins that showed statistically significant changes in phosphorylation upon the expression of the F10 kinase: 18 proteins showed an increase in phosphorylation whereas 9 proteins showed a decrease in phosphorylation. These proteins participate in several distinct cellular processes including cytoskeleton dynamics, membrane trafficking and cellular metabolism. One of the proteins with the greatest change in phosphorylation was mDia, a member of the formin family of cytoskeleton regulators; F10 induction led to increased phosphorylation on Ser22 Induction of F10 induced a statistically significant decrease in the percentage of cells with actin stress fibers; however, this change was abrogated when an mDia Ser22Ala variant was expressed. Moreover, expression of a Ser22Asp variant leads to a reduction of stress fibers even in cells not expressing F10. In sum, we present the first unbiased screen for cellular targets of F10-mediated phosphorylation, and in so doing describe a heretofore unknown mechanism for regulating stress fiber formation through phosphorylation of mDia. Data are available via ProteomeXchange with identifier PXD005246.

Live-Cell Imaging of Vaccinia Virus Recombination

Recombination between co-infecting poxviruses provides an important mechanism for generating the genetic diversity that underpins evolution. However, poxviruses replicate in membrane-bound cytoplasmic structures known as factories or virosomes. These are enclosed structures that could impede DNA mixing between co-infecting viruses, and mixing would seem to be essential for this process. We hypothesize that virosome fusion events would be a prerequisite for recombination between co-infecting poxviruses, and this requirement could delay or limit viral recombination. We have engineered vaccinia virus (VACV) to express overlapping portions of mCherry fluorescent protein fused to a cro DNA-binding element. In cells also expressing an EGFP-cro fusion protein, this permits live tracking of virus DNA and genetic recombination using confocal microscopy. Our studies show that different types of recombination events exhibit different timing patterns, depending upon the relative locations of the recombining elements. Recombination between partly duplicated sequences is detected soon after post-replicative genes are expressed, as long as the reporter gene sequences are located in cis within an infecting genome. The same kinetics are also observed when the recombining elements are divided between VACV and transfected DNA. In contrast, recombination is delayed when the recombining sequences are located on different co-infecting viruses, and mature recombinants aren’t detected until well after late gene expression is well established. The delay supports the hypothesis that factories impede inter-viral recombination, but even after factories merge there remain further constraints limiting virus DNA mixing and recombinant gene assembly. This delay could be related to the continued presence of ER-derived membranes within the fused virosomes, membranes that may once have wrapped individual factories.

Recombination plays a critical role in DNA repair and also creates the genetic diversity that underpins evolution. This has important implications for viruses, since recombination may create new pathogens with new infectious properties. It has long been known that hybrids can be recovered from cells co-infected with related viruses, some of the first artificial recombinants were produced >50 years ago from variola and rabbitpox viruses. A particular property of poxviruses is that they replicate in membrane-wrapped cytoplasmic structures called “factories”, and each of these factories develops from a single infecting particle. However, if each genome is isolated inside different factories, when and how does the DNA mix to permit recombination? To examine this question, we have developed a fluorescence-based virus recombination assay. Using live cell confocal microscopy, we have timed these reactions and observed that recombinants can be quickly formed when the recombining sequences are located on the same virus genome. However, when the gene fragments are located on different viruses, there is a significant delay (and a reduction) in recombinant gene formation. This delay supports the hypothesis that factories, and the ER-derived cell membranes that surround factories, impede recombination in poxvirus-infected cells.

The acidic C-terminus of vaccinia virus I3 single-strand binding protein promotes proper assembly of DNA-protein complexes

The vaccinia virus I3L gene encodes a single-stranded DNA binding protein (SSB) that is essential for virus DNA replication and is conserved in all Chordopoxviruses. The I3 protein contains a negatively charged C-terminal tail that is a common feature of SSBs. Such acidic tails are critical for SSB-dependent replication, recombination and repair. We cloned and purified variants of the I3 protein, along with a homolog from molluscum contagiosum virus, and tested how the acidic tail affected DNA-protein interactions. Deleting the C terminus of I3 enhanced the affinity for single-stranded DNA cellulose and gel shift analyses showed that it also altered the migration of I3-DNA complexes in agarose gels. Microinjecting an antibody against I3 into vaccinia-infected cells also selectively inhibited virus replication. We suggest that this domain promotes cooperative binding of I3 to DNA in a way that would maintain an open DNA configuration around a replication site.

Vaccinia Virus B1 Kinase Is Required for Postreplicative Stages of the Viral Life Cycle in a BAF-Independent Manner in U2OS Cells

The vaccinia virus B1R gene encodes a highly conserved protein kinase that is essential for the poxviral life cycle. As demonstrated in many cell types, B1 plays a critical role during viral DNA replication when it inactivates the cellular host defense effector barrier to autointegration factor (BAF or BANF1). To better understand the role of B1 during infection, we have characterized the growth of a B1-deficient temperature-sensitive mutant virus (Cts2 virus) in U2OS osteosarcoma cells. In contrast to all other cell lines tested to date, we found that in U2OS cells, Cts2 viral DNA replication is unimpaired at the nonpermissive temperature. However, the Cts2 viral yield in these cells was reduced more than 10-fold, thus indicating that B1 is required at another stage of the vaccinia virus life cycle. Our results further suggest that the host defense function of endogenous BAF may be absent in U2OS cells but can be recovered through either overexpression of BAF or fusion of U2OS cells with mouse cells in which the antiviral function of BAF is active. Interestingly, examination of late viral proteins during Cts2 virus infection demonstrated that B1 is required for optimal processing of the L4 protein. Finally, execution point analyses as well as electron microscopy studies uncovered a role for B1 during maturation of poxviral virions. Overall, this work demonstrates that U2OS cells are a novel model system for studying the cell type-specific regulation of BAF and reveals a role for B1 beyond DNA replication during the late stages of the viral life cycle.

IMPORTANCE

The most well characterized role for the vaccinia virus B1 kinase is to facilitate viral DNA replication by phosphorylating and inactivating BAF, a cellular host defense responsive to foreign DNA. Additional roles for B1 later in the viral life cycle have been postulated for decades but are difficult to examine directly due to the importance of B1 during DNA replication. Here, we demonstrate that in U2OS cells, a B1 mutant virus escapes the block in DNA replication observed in other cell types and, instead, this mutant virus exhibits impaired late protein accumulation and incomplete maturation of new virions. These data provide the clearest evidence to date that B1 is needed for multiple critical junctures in the poxviral life cycle in a manner that is both dependent on and independent of BAF.

Salmon Gill Poxvirus, the Deepest Representative of the Chordopoxvirinae

Poxviruses are large DNA viruses of vertebrates and insects causing disease in many animal species, including reptiles, birds, and mammals. Although poxvirus-like particles were detected in diseased farmed koi carp, ayu, and Atlantic salmon, their genetic relationships to poxviruses were not established. Here, we provide the first genome sequence of a fish poxvirus, which was isolated from farmed Atlantic salmon. In the present study, we used quantitative PCR and immunohistochemistry to determine aspects of salmon gill poxvirus disease, which are described here. The gill was the main target organ where immature and mature poxvirus particles were detected. The particles were detected in detaching, apoptotic respiratory epithelial cells preceding clinical disease in the form of lethargy, respiratory distress, and mortality. In moribund salmon, blocking of gas exchange would likely be caused by the adherence of respiratory lamellae and epithelial proliferation obstructing respiratory surfaces. The virus was not found in healthy salmon or in control fish with gill disease without apoptotic cells, although transmission remains to be demonstrated. PCR of archival tissue confirmed virus infection in 14 cases with gill apoptosis in Norway starting from 1995. Phylogenomic analyses showed that the fish poxvirus is the deepest available branch of chordopoxviruses. The virus genome encompasses most key chordopoxvirus genes that are required for genome replication and expression, although the gene order is substantially different from that in other chordopoxviruses. Nevertheless, many highly conserved chordopoxvirus genes involved in viral membrane biogenesis or virus-host interactions are missing. Instead, the salmon poxvirus carries numerous genes encoding unknown proteins, many of which have low sequence complexity and contain simple repeats suggestive of intrinsic disorder or distinct protein structures.

IMPORTANCE Aquaculture is an increasingly important global source of high-quality food. To sustain the growth in aquaculture, disease control in fish farming is essential. Moreover, the spread of disease from farmed fish to wildlife is a concern. Serious poxviral diseases are emerging in aquaculture, but very little is known about the viruses and the diseases that they cause. There is a possibility that viruses with enhanced virulence may spread to new species, as has occurred with the myxoma poxvirus in rabbits. Provision of the first fish poxvirus genome sequence and specific diagnostics for the salmon gill poxvirus in Atlantic salmon may help curb this disease and provide comparative knowledge. Furthermore, because salmon gill poxvirus represents the deepest branch of chordopoxvirus so far discovered, the genome analysis provided substantial insight into the evolution of different functional modules in this important group of viruses.

Genetic Confirmation that the H5 Protein Is Required for Vaccinia Virus DNA Replication

The duplication of the poxvirus double-stranded DNA genome occurs in cytoplasmic membrane-delimited factories. This physical autonomy from the host nucleus suggests that poxvirus genomes encode the full repertoire of proteins committed for genome replication. Biochemical and genetic analyses have confirmed that six viral proteins are required for efficient DNA synthesis; indirect evidence has suggested that the multifunctional H5 protein may also have a role. Here we show that H5 localizes to replication factories, as visualized by immunofluorescence and immunoelectron microscopy, and can be retrieved upon purification of the viral polymerase holoenzyme complex. The temperature-sensitive (ts) mutant Dts57, which was generated by chemical mutagenesis and has a lesion in H5, exhibits defects in DNA replication and morphogenesis under nonpermissive conditions, depending upon the experimental protocol. The H5 variant encoded by the genome of this mutant is ts for function but not stability. For a more precise investigation of how H5 contributes to DNA synthesis, we placed the ts57 H5 allele in an otherwise wild-type viral background and also performed small interfering RNA-mediated depletion of H5. Finally, we generated a complementing cell line, CV-1-H5, which allowed us to generate a viral recombinant in which the H5 open reading frame was deleted and replaced with mCherry (vΔH5). Analysis of vΔH5 allowed us to demonstrate conclusively that viral DNA replication is abrogated in the absence of H5. The loss of H5 does not compromise the accumulation of other early viral replication proteins or the uncoating of the virion core, suggesting that H5 plays a direct and essential role in facilitating DNA synthesis.

IMPORTANCE

Variola virus, the causative agent of smallpox, is the most notorious member of the Poxviridae family. Poxviruses are unique among DNA viruses that infect mammalian cells, in that their replication is restricted to the cytoplasm of the cell. This physical autonomy from the nucleus has both cell biological and genetic ramifications. Poxviruses must establish cytoplasmic niches that support replication, and the genomes must encode the repertoire of proteins necessary for genome synthesis. Here we focus on H5, a multifunctional and abundant viral protein. We confirm that H5 associates with the DNA polymerase holoenzyme and localizes to the sites of DNA synthesis. By generating an H5-expressing cell line, we were able to isolate a deletion virus that lacks the H5 gene and show definitively that genome synthesis does not occur in the absence of H5. These data support the hypothesis that H5 is a crucial participant in cytoplasmic poxvirus genome replication.

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.

Cell- and virus-mediated regulation of the barrier-to-autointegration factor's phosphorylation state controls its DNA binding, dimerization, subcellular localization, and antipoxviral activity

Barrier-to-autointegration factor (BAF) is a DNA binding protein with multiple cellular functions, including the ability to act as a potent defense against vaccinia virus infection. This antiviral function involves BAF's ability to condense double-stranded DNA and subsequently prevent viral DNA replication. In recent years, it has become increasingly evident that dynamic phosphorylation involving the vaccinia virus B1 kinase and cellular enzymes is likely a key regulator of multiple BAF functions; however, the precise mechanisms are poorly understood. Here we analyzed how phosphorylation impacts BAF's DNA binding, subcellular localization, dimerization, and antipoxviral activity through the characterization of BAF phosphomimetic and unphosphorylatable mutants. Our studies demonstrate that increased phosphorylation enhances BAF's mobilization from the nucleus to the cytosol, while dephosphorylation restricts BAF to the nucleus. Phosphorylation also impairs both BAF's dimerization and its DNA binding activity. Furthermore, our studies of BAF's antiviral activity revealed that hyperphosphorylated BAF is unable to suppress viral DNA replication or virus production. Interestingly, the unphosphorylatable BAF mutant, which is capable of binding DNA but localizes predominantly to the nucleus, was also incapable of suppressing viral replication. Thus, both DNA binding and localization are important determinants of BAF's antiviral function. Finally, our examination of how phosphatases are involved in regulating BAF revealed that PP2A dephosphorylates BAF during vaccinia infection, thus counterbalancing the activity of the B1 kinase. Altogether, these data demonstrate that phosphoregulation of BAF by viral and cellular enzymes modulates this protein at multiple molecular levels, thus determining its effectiveness as an antiviral factor and likely other functions as well.

IMPORTANCE

The barrier-to-autointegration factor (BAF) contributes to cellular genomic integrity in multiple ways, the best characterized of which are as a host defense against cytoplasmic DNA and as a regulator of mitotic nuclear reassembly. Although dynamic phosphorylation involving both viral and cellular enzymes is likely a key regulator of multiple BAF functions, the precise mechanisms involved are poorly understood. Here we demonstrate that phosphorylation coordinately regulates BAF's DNA binding, subcellular localization, dimerization, and antipoxviral activity. Overall, our findings provide new insights into how phosphoregulation of BAF modulates this protein at multiple levels and governs its effectiveness as an antiviral factor against foreign DNA.

Two distinct SSB protein families in nucleo-cytoplasmic large DNA viruses

MOTIVATION

Eukaryote-infecting nucleo-cytoplasmic large DNA viruses (NCLDVs) feature some of the largest genomes in the viral world. These viruses typically do not strongly depend on the host DNA replication systems. In line with this observation, a number of essential DNA replication proteins, such as DNA polymerases, primases, helicases and ligases, have been identified in the NCLDVs. One other ubiquitous component of DNA replisomes is the single-stranded DNA-binding (SSB) protein. Intriguingly, no NCLDV homologs of canonical OB-fold-containing SSB proteins had previously been detected. Only in poxviruses, one of seven NCLDV families, I3 was identified as the SSB protein. However, whether I3 is related to any known protein structure has not yet been established.

RESULTS

Here, we addressed the case of 'missing' canonical SSB proteins in the NCLDVs and also probed evolutionary origins of the I3 family. Using advanced computational methods, in four NCLDV families, we detected homologs of the bacteriophage T7 SSB protein (gp2.5). We found the properties of these homologs to be consistent with the SSB function. Moreover, we implicated specific residues in single-stranded DNA binding. At the same time, we found no evolutionary link between the T7 gp2.5-like NCLDV SSB homologs and the poxviral SSB protein (I3). Instead, we identified a distant relationship between I3 and small protein B (SmpB), a bacterial RNA-binding protein. Thus, apparently, the NCLDVs have the two major distinct sets of SSB proteins having bacteriophage and bacterial origins, respectively.