Structural features of the Bluetongue virus NS2 protein

Orbivirus NS4 Proteins Play Multiple Roles to Dampen Cellular Responses

Non-structural protein 4 (NS4) of insect-borne and tick-borne orbiviruses is encoded by genome segment 9, from a secondary open reading frame. Though a protein dispensable for bluetongue virus (BTV) replication, it has been shown to counter the interferon response in cells infected with BTV or African horse sickness virus. We further explored the functional role(s) of NS4 proteins of BTV and the tick-borne Great Island virus (GIV). We show that NS4 of BTV or GIV helps an E3L deletion mutant of vaccinia virus to replicate efficiently in interferon-treated cells, further confirming the role of NS4 as an interferon antagonist. Our results indicate that ectopically expressed NS4 of BTV localised with caspase 3 within the nucleus and was found in a protein complex with active caspase 3 in a pull-down assay. Previous studies have shown that pro-apoptotic caspases (including caspase 3) suppress type I interferon response by cleaving mediators involved in interferon signalling. Our data suggest that orbivirus NS4 plays a role in modulating the apoptotic process and/or regulating the interferon response in mammalian cells, thus acting as a virulence factor in pathogenesis.

A Fijivirus Major Viroplasm Protein Shows RNA-Stimulated ATPase Activity by Adopting Pentameric and Hexameric Assemblies of Dimers

Fijiviruses replicate and package their genomes within viroplasms in a process involving RNA-RNA and RNA-protein interactions. Here, we demonstrate that the 24 C-terminal residues (C-arm) of the P9-1 major viroplasm protein of the mal de Río Cuarto virus (MRCV) are required for its multimerization and the formation of viroplasm-like structures. Using an integrative structural approach, the C-arm was found to be dispensable for P9-1 dimer assembly but essential for the formation of pentamers and hexamers of dimers (decamers and dodecamers), which favored RNA binding. Although both P9-1 and P9-1ΔC-arm catalyzed ATP with similar activities, an RNA-stimulated ATPase activity was only detected in the full-length protein, indicating a C-arm-mediated interaction between the ATP catalytic site and the allosteric RNA binding sites in the (do)decameric assemblies. A stronger preference to bind phosphate moieties in the decamer was predicted, suggesting that the allosteric modulation of ATPase activity by RNA is favored in this structural conformation. Our work reveals the structural versatility of a fijivirus major viroplasm protein and provides clues to its mechanism of action. IMPORTANCE The mal de Río Cuarto virus (MRCV) causes an important maize disease in Argentina. MRCV replicates in several species of Gramineae plants and planthopper vectors. The viral factories, also called viroplasms, have been studied in detail in animal reovirids. This work reveals that a major viroplasm protein of MRCV forms previously unidentified structural arrangements and provides evidence that it may simultaneously adopt two distinct quaternary assemblies. Furthermore, our work uncovers an allosteric communication between the ATP and RNA binding sites that is favored in the multimeric arrangements. Our results contribute to the understanding of plant reovirids viroplasm structure and function and pave the way for the design of antiviral strategies for disease control.

Isolation and characterization of Wad Medani virus obtained in the tuva Republic of Russia

Wad Medani virus (WMV) belongs to the genus Orbivirus and is a poorly studied arbovirus with unclear medical significance. Presently, a limited number of WMV strains are characterized and available in NCBI GenBank, some isolated many years ago. A new WMV strain was isolated in 2012 from Dermacentor nuttalli ticks collected from sheep in the Tuva Republic, Russia, and sequenced using high-throughput methods. Complete coding sequences were obtained revealing signs of multiple intersegment reassortments. These point to a high variability potential in WMV that may lead to the formation of strains with novel properties. These new data on WMV can promote better understanding of: ecological features of its circulation; relationships within the genus Orbivirus; and the medical significance of the virus.

A Calcium Sensor Discovered in Bluetongue Virus Nonstructural Protein 2 Is Critical for Virus Replication

After entering the host cells, viruses use cellular host factors to ensure a successful virus replication process. For replication in infected cells, members of the Reoviridae family form inclusion body-like structures known as viral inclusion bodies (VIB) or viral factories. Bluetongue virus (BTV) forms VIBs in infected cells through nonstructural protein 2 (NS2), a phosphoprotein. An important regulatory factor critical for VIB formation is phosphorylation of NS2. In our study, we discovered a characteristic calcium-binding EF-hand-like motif in NS2 and found that the calcium binding preferentially affects phosphorylation level of the NS2 and has a role in regulating VIB assembly.

Many viruses use specific viral proteins to bind calcium ions (Ca²⁺) for stability or to modify host cell pathways; however, to date, no Ca²⁺ binding protein has been reported in bluetongue virus (BTV), the causative agent of bluetongue disease in livestock. Here, using a comprehensive bioinformatics screening, we identified a putative EF-hand-like Ca²⁺ binding motif in the carboxyl terminal region of BTV nonstructural phosphoprotein 2 (NS2). Subsequently, using a recombinant NS2, we demonstrated that NS2 binds Ca²⁺ efficiently and that Ca²⁺ binding was perturbed when the Asp and Glu residues in the motif were substituted by alanine. Using circular dichroism analysis, we found that Ca²⁺ binding by NS2 triggered a helix-to-coil secondary structure transition. Further, cryo-electron microscopy in the presence of Ca²⁺ revealed that NS2 forms helical oligomers which, when aligned with the N-terminal domain crystal structure, suggest an N-terminal domain that wraps around the C-terminal domain in the oligomer. Further, an in vitro kinase assay demonstrated that Ca²⁺ enhanced the phosphorylation of NS2 significantly. Importantly, mutations introduced at the Ca²⁺ binding site in the viral genome by reverse genetics failed to allow recovery of viable virus, and the NS2 phosphorylation level and assembly of viral inclusion bodies (VIBs) were reduced. Together, our data suggest that NS2 is a dedicated Ca²⁺ binding protein and that calcium sensing acts as a trigger for VIB assembly, which in turn facilitates virus replication and assembly.

IMPORTANCE After entering the host cells, viruses use cellular host factors to ensure a successful virus replication process. For replication in infected cells, members of the Reoviridae family form inclusion body-like structures known as viral inclusion bodies (VIB) or viral factories. Bluetongue virus (BTV) forms VIBs in infected cells through nonstructural protein 2 (NS2), a phosphoprotein. An important regulatory factor critical for VIB formation is phosphorylation of NS2. In our study, we discovered a characteristic calcium-binding EF-hand-like motif in NS2 and found that the calcium binding preferentially affects phosphorylation level of the NS2 and has a role in regulating VIB assembly.

Evaluation of Bluetongue Virus (BTV) Antibodies for the Immunohistochemical Detection of BTV and Other Orbiviruses

The detection of bluetongue virus (BTV) antigens in formalin-fixed tissues has been challenging; therefore, only a limited number of studies on suitable immunohistochemical approaches have been reported. This study details the successful application of antibodies for the immunohistochemical detection of BTV in BSR variant baby hamster kidney cells (BHK-BSR) and infected sheep lungs that were formalin-fixed and paraffin-embedded (FFPE). BTV reactive antibodies raised against non-structural (NS) proteins 1, 2, and 3/3a and viral structural protein 7 (VP7) were first evaluated on FFPE BTV-infected cell pellets for their ability to detect BTV serotype 1 (BTV-1). Antibodies that were successful in immunolabelling BTV-1 infected cell pellets were further tested, using similar methods, to determine their broader immunoreactivity against a diverse range of BTV and other orbiviruses. Antibodies specific for NS1, NS2, and NS3/3a were able to detect all BTV isolates tested, and the VP7 antibody cross-reacted with all BTV isolates, except BTV-15. The NS1 antibodies were BTV serogroup-specific, while the NS2, NS3/3a, and VP7 antibodies demonstrated immunologic cross-reactivity to related orbiviruses. These antibodies also detected viral antigens in BTV-3 infected sheep lung. This study demonstrates the utility of FFPE-infected cell pellets for the development and validation of BTV immunohistochemistry.

Attenuation of Bluetongue Virus (BTV) in an in ovo Model Is Related to the Changes of Viral Genetic Diversity of Cell-Culture Passaged BTV

The embryonated chicken egg (ECE) is routinely used for the laboratory isolation and adaptation of Bluetongue virus (BTV) in vitro. However, its utility as an alternate animal model has not been fully explored. In this paper, we evaluated the pathogenesis of BTV in ovo using a pathogenic isolate of South African BTV serotype 3 (BTV-3) derived from the blood of an infected sheep. Endothelio- and neurotropism of BTV-3 were observed by immunohistochemistry of non-structural protein 1 (NS1), NS3, NS3/3a, and viral protein 7 (VP7) antigens. In comparing the pathogenicity of BTV from infectious sheep blood with cell-culture-passaged BTV, including virus propagated through a Culicoides-derived cell line (KC) or ECE, we found virus attenuation in ECE following cell-culture passage. Genomic analysis of the consensus sequences of segments (Seg)-2, -5, -6, -7, -8, -9, and -10 identified several nucleotide and amino-acid mutations among the cell-culture-propagated BTV-3. Deep sequencing analysis revealed changes in BTV-3 genetic diversity in various genome segments, notably a reduction of Seg-7 diversity following passage in cell culture. Using this novel approach to investigate BTV pathogenicity in ovo, our findings support the notion that pathogenic BTV becomes attenuated in cell culture and that this change is associated with virus quasispecies evolution.

Stability of local secondary structure determines selectivity of viral RNA chaperones

To maintain genome integrity, segmented double-stranded RNA viruses of the Reoviridae family must accurately select and package a complete set of up to a dozen distinct genomic RNAs. It is thought that the high fidelity segmented genome assembly involves multiple sequence-specific RNA-RNA interactions between single-stranded RNA segment precursors. These are mediated by virus-encoded non-structural proteins with RNA chaperone-like activities, such as rotavirus (RV) NSP2 and avian reovirus σNS. Here, we compared the abilities of NSP2 and σNS to mediate sequence-specific interactions between RV genomic segment precursors. Despite their similar activities, NSP2 successfully promotes inter-segment association, while σNS fails to do so. To understand the mechanisms underlying such selectivity in promoting inter-molecular duplex formation, we compared RNA-binding and helix-unwinding activities of both proteins. We demonstrate that octameric NSP2 binds structured RNAs with high affinity, resulting in efficient intramolecular RNA helix disruption. Hexameric σNS oligomerizes into an octamer that binds two RNAs, yet it exhibits only limited RNA-unwinding activity compared to NSP2. Thus, the formation of intersegment RNA-RNA interactions is governed by both helix-unwinding capacity of the chaperones and stability of RNA structure. We propose that this protein-mediated RNA selection mechanism may underpin the high fidelity assembly of multi-segmented RNA genomes in Reoviridae.

Immuno-reactivity of recombinant non-structural protein 3 N-terminus (rNS3Nt) in indirect-ELISA for detection of bluetongue viral antibodies in serum samples

Bluetongue, an arthropod borne non-contagious disease of ruminants especially sheep, is caused by bluetongue virus (BTV). Detection of BTV antibodies in susceptible hosts is considered to be of significance in disease diagnosis and differentiation. In the present study, a partial NS3 gene encoding for non-structural protein-3 N-terminus (1MT117 aa) of BTV-23, produced as purified recombinant NS3Nt fusion protein (~32 kDa) using prokaryotic expression system (Escherichia coli), was evaluated as a candidate antigen in an indirect-ELISA (rNS3Nt-ELISA) to measure the serologic response to NS3 protein in small ruminants. The rNS3Nt fusion protein obtained in sufficient quantity and quality has good reactivity in detecting NS3 specific antibodies in field serum samples by indirect-ELISA. As NS3 protein is highly conserved, rNS3Nt-ELISA has potential for NS3 specific detection of antibodies in BTV affected animals irrespective of different viral serotypes. In comparison to structural protein (VP7) based c-ELISA kit and i-ELISA kit, the diagnostic sensitivity (85.1%, 86.2%) and specificity (92.5%, 93.2%) of rNS3Nt-ELISA were found to be relatively lower, respectively. Nevertheless, the study indicated the potential utility of rNS3Nt-ELISA as an alternate assay in routine sero-diagnosis of BTV infection and possible sero-surveillance of ruminants under DIVA strategy.

Production of recombinant non-structural protein-3 hydrophobic domain deletion (NS3ΔHD) protein of bluetongue virus from prokaryotic expression system as an efficient diagnostic reagent

Serological diagnostics for bluetongue (BT), which is an infectious, non-contagious and arthropod-borne virus disease of ruminants, are primarily dependent on availability of high quality native or recombinant antigen(s) based on either structural/non-structural proteins in sufficient quantity. Non-structural proteins (NS1-NS4) of BT virus are presumed candidate antigens in development of DIVA diagnostics. In the present study, NS3 fusion gene encoding for NS3 protein containing the N- and C-termini with a deletion of two hydrophobic domains (118A to S141 aa and 162S to A182 aa) and intervening variable central domain (142D to K161 aa) of bluetongue virus 23 was constructed, cloned and over-expressed using prokaryotic expression system. The recombinant NS3ΔHD fusion protein (∼38 kDa) including hexa-histidine tag on its both termini was found to be non-cytotoxic to recombinant Escherichia coli cells and purified by affinity chromatography. The purified rNS3ΔHD fusion protein was found to efficiently detect BTV-NS3 specific antibodies in indirect-ELISA format with diagnostic sensitivity (DSn = 94.4%) and specificity (DSp = 93.9%). The study indicated the potential utility of rNS3ΔHD fusion protein as candidate diagnostic reagent in developing an indirect-ELISA for sero-surveillance of animals for BTV antibodies under DIVA strategy, wherever monovalent/polyvalent killed BT vaccine formulations devoid of NS proteins are being practiced for immunization.

A coiled-coil motif in non-structural protein 3 (NS3) of bluetongue virus forms an oligomer

Bluetongue, an arthropod-borne non-contagious hemorrhagic disease of small ruminants, is caused by bluetongue virus (BTV). Several structural and non-structural proteins encoded by BTV have been associated with virulence mechanisms. In the present study, the NS3 protein sequences of bluetongue viral serotypes were analyzed for the presence of heptad regions and oligomer formation. Bioinformatic analysis of NS3 sequences of all 26 BTV serotypes revealed the presence of at least three coiled-coil motifs (CCMs). A conserved α-helical heptad sequence was identified at 14-26 aa (CCM-I), 185-198aa (CCM-II), and 94-116 aa (CCM-III). Among these, CCM-I occurs close to the N-terminus of NS3 and was presumed to be involved in oligomerization. Furthermore, the N-terminus of NS3 (1M-R117 aa) was over-expressed as a recombinant fusion protein in a prokaryotic expression system. Biochemical characterization of recombinant NS3Nt protein revealed that it forms SDS-resistant dimers and high-order oligomers (hexamer and/or octamer) under reducing or non-reducing conditions. Coiled-coil motifs are believed to be critical for NS protein oligomerization and have potential roles in the formation of viroporin ring/pore either with six/eight subunits and this is the first study toward characterization of CCMs in NS3 of bluetongue virus.

Evidence that avian reovirus σNS is an RNA chaperone: implications for genome segment assortment

Reoviruses are important human, animal and plant pathogens having 10-12 segments of double-stranded genomic RNA. The mechanisms controlling the assortment and packaging of genomic segments in these viruses, remain poorly understood. RNA-protein and RNA-RNA interactions between viral genomic segment precursors have been implicated in the process. While non-structural viral RNA-binding proteins, such as avian reovirus σNS, are essential for virus replication, the mechanism by which they assist packaging is unclear. Here we demonstrate that σNS assembles into stable elongated hexamers in vitro, which bind single-stranded nucleic acids with high affinity, but little sequence specificity. Using ensemble and single molecule fluorescence spectroscopy, we show that σNS also binds to a partially double-stranded RNA, resulting in gradual helix unwinding. The hexamer can bind multiple RNA molecules and exhibits strand-annealing activity, thus mediating conversion of metastable, intramolecular stem-loops into more stable heteroduplexes. We demonstrate that the ARV σNS acts as an RNA chaperone facilitating specific RNA-RNA interactions between genomic precursors during segment assortment and packaging.

The molecular biology of Bluetongue virus replication

The members of Orbivirus genus within the Reoviridae family are arthropod-borne viruses which are responsible for high morbidity and mortality in ruminants. Bluetongue virus (BTV) which causes disease in livestock (sheep, goat, cattle) has been in the forefront of molecular studies for the last three decades and now represents the best understood orbivirus at a molecular and structural level. The complex nature of the virion structure has been well characterised at high resolution along with the definition of the virus encoded enzymes required for RNA replication; the ordered assembly of the capsid shell as well as the protein and genome sequestration required for it; and the role of host proteins in virus entry and virus release. More recent developments of Reverse Genetics and Cell-Free Assembly systems have allowed integration of the accumulated structural and molecular knowledge to be tested at meticulous level, yielding higher insight into basic molecular virology, from which the rational design of safe efficacious vaccines has been possible. This article is centred on the molecular dissection of BTV with a view to understanding the role of each protein in the virus replication cycle. These areas are important in themselves for BTV replication but they also indicate the pathways that related viruses, which includes viruses that are pathogenic to man and animals, might also use providing an informed starting point for intervention or prevention.

Bluetongue virus infection induces aberrant mitosis in mammalian cells

Background

Bluetongue virus (BTV) is an arbovirus that is responsible for ‘bluetongue’, an economically important disease of livestock. Although BTV is well characterised at the protein level, less is known regarding its interaction with host cells. During studies of virus inclusion body formation we observed what appeared to be a large proportion of cells in mitosis. Although the modulation of the cell cycle is well established for many viruses, this was a novel observation for BTV. We therefore undertook a study to reveal in more depth the impact of BTV upon cell division.

Methods

We used a confocal microscopy approach to investigate the localisation of BTV proteins in a cellular context with their respective position relative to cellular proteins. In addition, to quantitatively assess the frequency of aberrant mitosis induction by the viral non-structural protein (NS) 2 we utilised live cell imaging to monitor HeLa-mCherry tubulin cells transfected with a plasmid expressing NS2.

Results

Our data showed that these ‘aberrant mitoses’ can be induced in multiple cell types and by different strains of BTV. Further study confirmed multiplication of the centrosomes, each resulting in a separate mitotic spindle during mitosis. Interestingly, the BTV NS1 protein was strongly localised to the centrosomal regions. In a separate, yet related observation, the BTV NS2 protein was co-localised with the condensed chromosomes to a region suggestive of the kinetochore. Live cell imaging revealed that expression of an EGFP-NS2 fusion protein in HeLa-mCherry tubulin cells also results in mitotic defects.

Conclusions

We hypothesise that NS2 is a microtubule cargo protein that may inadvertently disrupt the interaction of microtubule tips with the kinetochores during mitosis. Furthermore, the BTV NS1 protein was distinctly localised to a region encompassing the centrosome and may therefore be, at least in part, responsible for the disruption of the centrosome as observed in BTV infected mammalian cells.

Identification of a linear B-cell epitope within the Bluetongue virus serotype 8 NS2 protein using a phage-displayed random peptide library

The NS2 protein of Bluetongue virus (BTV) is an important non-structural protein and plays important roles in viral replication and assembly. In this study, one monoclonal antibody (mAb), 4D4, was raised against BTV8 NS2. Phage display technology was used and identified the consensus binding motif SNYD recognized by mAb 4D4. To define the minimal region required for antibody binding, a panel of synthetic peptides encompassing SNYD derived from the BTV8 NS2 was then used to more specifically define the 4D4 epitope as (149)RSNYDV(154). Furthermore, amino acid sequence alignments of different BTV serotypes and other orbiviruses suggested that this epitope is highly conserved among the BTV serotypes. The mAb reagent generated in this study may be applied to the development of BTV diagnosis and surveillance programs and the epitope defined here can lead to important insights into how BTV might interact with the sheep's immune system.