Phosphorylation of bluetongue virus nonstructural protein 2 is essential for formation of viral inclusion bodies

Role of NS2 specific RNA binding and phosphorylation in liquid-liquid phase separation and virus assembly

Liquid-liquid phase separation (LLPS) has assumed a prominent role in biological cell systems, where it underpins the formation of subcellular compartments necessary for cell function. We investigated the underlying mechanism of LLPS in virus infected cells, where virus inclusion bodies are formed by an RNA-binding phosphoprotein (NS2) of Bluetongue virus to serve as sites for subviral particle assembly and virus maturation. We show that NS2 undergoes LLPS that is dependent on protein phosphorylation and RNA-binding and that LLPS occurrence is accompanied by a change in protein secondary structure. Site-directed mutagenesis identified two critical arginine residues in NS2 responsible for specific RNA binding and thus for NS2-RNA complex driven LLPS. Reverse genetics identified the same residues as essential for VIB assembly in infected cells and virus viability. Our findings suggest that a specific arginine-RNA interaction in the context of a phosphorylated state drives LLPS in this, and possibly other, virus infections.

Molecular Characterization of Outer Capsid Proteins VP5 and VP7 of Grass Carp Reovirus

Aquareovirus, which is a member of the Reoviridae family, was isolated from aquatic animals. A close molecular evolutionary relationship between aquareoviruses and mammalian orthoreoviruses was revealed. However, the functions of the aquareovirus genome-encoded proteins are poorly understood. We investigated the molecular characteristics of the outer capsid proteins, namely, VP5 and VP7, of grass carp reovirus (GCRV). The peptides VP5 and VP7 were determined using in-gel tryptic digestion and mass spectrometry. Recovered peptides represented 76% and 66% of the full-length VP5 and VP7 sequences, respectively. Significantly, two-lysine acetylation, as well as two-serine and two-threonine phosphorylation modifications, were first revealed in VP5. We found that the initial amino acid in VP5 was Pro43, suggesting that a lower amount of VP5 remained uncleaved in virions at the autocleavage site (Asn42-Pro43). Further biochemical evidence showed that the cleaved VP5N/VP5C conformation was the major constituent of the particles. Moreover, early cleavage fragments of VP7 and enhanced infectivity were detected after limited tryptic digestion of GCRV, indicating that stepwise VP7 cleavage is essential for VP5 conformational rearrangement. Our results provide insights into the roles of posttranslational modifications in VP5 and its association with VP7 in the viral life cycle.

Rotavirus reverse genetics systems: Development and application

Rotaviruses (RVs) cause acute gastroenteritis in infants and young children. Since 2006, live-attenuated vaccines have reduced the number of RV-associated deaths; however, RV is still responsible for an estimated 228,047 annual deaths worldwide. RV, a member of the family Reoviridae, has an 11-segmented double-stranded RNA genome contained within a non-enveloped, triple layered virus particle. In 2017, a long-awaited helper virus-free reverse genetics system for RV was established. Since then, numerous studies have reported the generation of recombinant RVs; these studies verify the robustness of reverse genetics systems. This review provides technical insight into current reverse genetics systems for RVs, as well as discussing basic and applied studies that have used these systems.

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.

Bluetongue virus assembly and exit pathways

Bluetongue virus (BTV) is an insect-vectored emerging pathogen of wild ruminants and livestock in many parts of the world. The virion particle is a complex structure of consecutive layers of protein surrounding a genome of 10 double-stranded (ds) RNA segments. BTV has been studied extensively as a model system for large, nonenveloped dsRNA viruses. A combination of recombinant proteins and particles together with reverse genetics, high-resolution structural analysis by X-ray crystallography and cryo-electron microscopy techniques have been utilized to provide an order for the assembly of the capsid shell and the protein sequestration required for it. Further, a reconstituted in vitro assembly system and RNA-RNA interaction assay, have defined the individual steps required for the assembly and packaging of the 10-segmented RNA genome. In addition, various microscopic techniques have been utilized to illuminate the stages of virus maturation and its egress via multiple pathways. These findings have not only given an overall understanding of BTV assembly and morphogenesis but also indicated that similar assembly and egress pathways are likely to be used by related viruses and provided an informed starting point for intervention or prevention.

Differential Localization of Structural and Non-Structural Proteins during the Bluetongue Virus Replication Cycle

Members of the Reoviridae family assemble virus factories within the cytoplasm of infected cells to replicate and assemble virus particles. Bluetongue virus (BTV) forms virus inclusion bodies (VIBs) that are aggregates of viral RNA, certain viral proteins, and host factors, and have been shown to be sites of the initial assembly of transcriptionally active virus-like particles. This study sought to characterize the formation, composition, and ultrastructure of VIBs, particularly in relation to virus replication. In this study we have utilized various microscopic techniques, including structured illumination microscopy, and virological assays to show for the first time that the outer capsid protein VP5, which is essential for virus maturation, is also associated with VIBs. The addition of VP5 to assembled virus cores exiting VIBs is required to arrest transcriptionally active core particles, facilitating virus maturation. Furthermore, we observed a time-dependent association of the glycosylated non-structural protein 3 (NS3) with VIBs, and report on the importance of the two polybasic motifs within NS3 that facilitate virus trafficking and egress from infected cells at the plasma membrane. Thus, the presence of VP5 and the dynamic nature of NS3 association with VIBs that we report here provide novel insight into these previously less well-characterized processes.

Hsp90 Chaperones Bluetongue Virus Proteins and Prevents Proteasomal Degradation

The molecular chaperone machinery is important for the maintenance of protein homeostasis within the cells. The principle activities of the chaperone machinery are to facilitate protein folding and organize conformationally dynamic client proteins. Prominent among the members of the chaperone family are heat shock protein 70 (Hsp70) and 90 (Hsp90). Like cellular proteins, viral proteins depend upon molecular chaperones to mediate their stabilization and folding. Bluetongue virus (BTV), which is a model system for the Reoviridae family, is a nonenveloped arbovirus that causes hemorrhagic disease in ruminants. This constitutes a significant burden upon animals of commercial significance, such as sheep and cattle. Here, for the first time, we examined the role of chaperone proteins in the viral lifecycle of BTV. Using a combination of molecular, biochemical, and microscopic techniques, we examined the function of Hsp90 and its relevance to BTV replication. We demonstrate that Hsp70, the chaperone that is commonly usurped by viral proteins, does not influence virus replication, while Hsp90 activity is important for virus replication by stabilizing BTV proteins and preventing their degradation via the ubiquitin-proteasome pathway. To our knowledge this is the first report showing the involvement of Hsp90 as a modulator of BTV infection.IMPORTANCE Protein chaperones are instrumental for maintaining protein homeostasis, enabling correct protein folding and organization; prominent members include heat shock proteins 70 and 90. Virus infections place a large burden on this homeostasis. Identifying and understanding the underlying mechanisms that facilitate Bluetongue virus replication and spread through the usurpation of host factors is of primary importance for the development of intervention strategies. Our data identify and show that heat shock protein 90, but not heat shock protein 70, stabilizes bluetongue virus proteins, safeguarding them from proteasomal degradation.

African Horse Sickness: A Review of Current Understanding and Vaccine Development

African horse sickness is a devastating disease that causes great suffering and many fatalities amongst horses in sub-Saharan Africa. It is caused by nine different serotypes of the orbivirus African horse sickness virus (AHSV) and it is spread by Culicoid midges. The disease has significant economic consequences for the equine industry both in southern Africa and increasingly further afield as the geographic distribution of the midge vector broadens with global warming and climate change. Live attenuated vaccines (LAV) have been used with relative success for many decades but carry the risk of reversion to virulence and/or genetic re-assortment between outbreak and vaccine strains. Furthermore, the vaccines lack DIVA capacity, the ability to distinguish between vaccine-induced immunity and that induced by natural infection. These concerns have motivated interest in the development of new, more favourable recombinant vaccines that utilize viral vectors or are based on reverse genetics or virus-like particle technologies. This review summarizes the current understanding of AHSV structure and the viral replication cycle and also evaluates existing and potential vaccine strategies that may be applied to prevent or control the disease.

Analysis of Conserved, Computationally Predicted Epitope Regions for VP5 and VP7 Across three Orbiviruses

Orbiviruses are double-stranded RNA viruses that have profound economic and veterinary significance, 3 of the most important being African horse sickness virus (AHSV), bluetongue virus (BTV), and epizootic hemorrhagic disease virus (EHDV). Currently, vaccination and vector control are used as preventative measures; however, there are several problems with the current vaccines. Comparing viral amino acid sequences, we obtained an AHSV-BTV-EHDV consensus sequence for VP5 (viral protein 5) and for VP7 (viral protein 7) and generated homology models for these proteins. The structures and sequences were analyzed for amino acid sequence conservation, entropy, surface accessibility, and epitope propensity, to computationally determine whether consensus sequences still possess potential epitope regions. In total, 5 potential linear epitope regions on VP5 and 11 on VP7, as well as potential discontinuous B-cell epitopes, were identified and mapped onto the homology models created. Regions identified for VP5 and VP7 could be important in vaccine design against orbiviruses.

Interaction between a Unique Minor Protein and a Major Capsid Protein of Bluetongue Virus Controls Virus Infectivity

Among the Reoviridae family of double-stranded RNA viruses, only members of the Orbivirus genus possess a unique structural protein, termed VP6, within their particles. Bluetongue virus (BTV), an important livestock pathogen, is the prototype Orbivirus. BTV VP6 is an ATP-dependent RNA helicase, and it is indispensable for virus replication. In the study described in this report, we investigated how VP6 might be recruited to the virus capsid and whether the BTV structural protein VP3, which forms the internal layer of the virus capsid core, is involved in VP6 recruitment. We first demonstrated that VP6 interacts with VP3 and colocalizes with VP3 during capsid assembly. A series of VP6 mutants was then generated, and in combination with immunoprecipitation and size exclusion chromatographic analyses, we demonstrated that VP6 directly interacts with VP3 via a specific region of the C-terminal portion of VP6. Finally, using our reverse genetics system, mutant VP6 proteins were introduced into the BTV genome and interactions between VP6 and VP3 were shown in a live cell system. We demonstrate that BTV strains possessing a mutant VP6 are replication deficient in wild-type BSR cells and fail to recruit the viral replicase complex into the virus particle core. Taken together, these data suggest that the interaction between VP3 and VP6 could be important in the packaging of the viral genome and early stages of particle formation.

IMPORTANCE The orbivirus bluetongue virus (BTV) is the causative agent of bluetongue disease of livestock, often causing significant economic and agricultural impacts in the livestock industry. In the study described in this report, we identified the essential region and residues of the unique orbivirus capsid protein VP6 which are responsible for its interaction with other BTV proteins and its subsequent recruitment into the virus particle. The nature and mechanism of these interactions suggest that VP6 has a key role in packaging of the BTV genome into the virus particle. As such, this is a highly significant finding, as this new understanding of BTV assembly could be exploited to design novel vaccines and antivirals against bluetongue disease.

Phosphoproteomic Analysis Reveals the Importance of Kinase Regulation During Orbivirus Infection

Bluetongue virus (BTV) causes infections in wild and domesticated ruminants with high morbidity and mortality and is responsible for significant economic losses in both developing and developed countries. BTV serves as a model for the study of other members of the Orbivirus genus. Previously, the importance of casein kinase 2 for BTV replication was demonstrated. To identify intracellular signaling pathways and novel host-cell kinases involved during BTV infection, the phosphoproteome of BTV infected cells was analyzed. Over 1000 phosphosites were identified using mass spectrometry, which were then used to determine the corresponding kinases involved during BTV infection. This analysis yielded protein kinase A (PKA) as a novel kinase activated during BTV infection. Subsequently, the importance of PKA for BTV infection was validated using a PKA inhibitor and activator. Our data confirmed that PKA was essential for efficient viral growth. Further, we showed that PKA is also required for infection of equid cells by African horse sickness virus, another member of the Orbivirus genus. Thus, despite their preference in specific host species, orbiviruses may utilize the same host signaling pathways during their replication.

Bluetongue virus structure and assembly

Bluetongue virus (BTV) is an insect-vectored emerging pathogen of wild ruminants and livestock in many parts of the world. The virion particle is a complex structure of consecutive layers of protein surrounding a genome of ten double-stranded (ds) RNA segments. BTV has been studied as a model system for large, non-enveloped dsRNA viruses. Several new techniques have been applied to define the virus-encoded enzymes required for RNA replication to provide an order for the assembly of the capsid shell and the protein sequestration required for it. Further, a reconstituted in vitro system has defined the individual steps of the assembly and packaging of the genomic RNA. These findings illuminate BTV assembly and indicate the pathways that related viruses might use to provide an informed starting point for intervention or prevention.

Cellular Casein Kinase 2 and Protein Phosphatase 2A Modulate Replication Site Assembly of Bluetongue Virus

A number of cytoplasmic replicating viruses produce cytoplasmic inclusion bodies or protein aggregates; however, a hallmark of viruses of the Reoviridae family is that they utilize these sites for purposes of replication and capsid assembly, functioning as viral assembly factories. Here we have used bluetongue virus (BTV) as a model system for this broad family of important viruses to understand the mechanisms regulating inclusion body assembly. Newly synthesized viral proteins interact with sequestered viral RNA molecules prior to capsid assembly and double-stranded RNA synthesis within viral inclusion bodies (VIBs). VIBs are predominantly comprised of a BTV-encoded non-structural protein 2 (NS2). Previous in vitro studies indicated that casein kinase 2 (CK2) mediated the phosphorylation of NS2, which regulated the propensity of NS2 to form larger aggregates. Using targeted pharmacological reagents, specific mutation in the viral genome by reverse genetics and confocal microscopy, here we demonstrate that CK2 activity is important for BTV replication. Furthermore, we show that a novel host cell factor, protein phosphatase 2A, is involved in NS2 dephosphorylation and that, together with CK2, it regulates VIB morphology and virus replication. Thus, these two host enzymes influence the dynamic nature of VIB assembly/disassembly, and these concerted activities may be relevant to the assembly and the release of these cores from VIBs.

Inter-segment complementarity in orbiviruses: a driver for co-ordinated genome packaging in the Reoviridae?

The process by which eukaryotic viruses with segmented genomes select a complete set of genome segments for packaging into progeny virus particles is not understood. In this study a model based on the association of genome segments through specific RNA-RNA interactions driven by base pairing was formalized and tested in the Orbivirus genus of the Reoviridae family. A strategy combining screening of the genomic sequences for inter-segment complementarity with direct functional testing of inter-segment RNA-RNA interactions using reverse genetics is described in the type species of the Orbivirus genus, Bluetongue virus (BTV). Two examples, involving four of the ten BTV genomic segments, of specific inter-segment interaction motifs whose maintenance is essential for the generation of infectious virus, were identified. Equivalent inter-segment complementarities were found between the identified regions of the orthologous genome segments of all orbiviruses, including phylogenetically distant species. Specific interaction of the participating RNA segments was confirmed in vitro using electrophoretic mobility shift assays, with the interactions inhibited using oligonucleotides complementary to the interaction motif of one of the interacting partners, and also through mutagenesis of the motifs. In each example, the base pairing rather than the absolute sequence was critical to the formation of a functional inter-segment interaction, with mutations only being tolerated in rescued virus if compensating changes were made in the interacting partner to restore uninterrupted base pairing. The absolute sequence of the complementarity motifs varied between species, indicating that this newly identified phenomenon may contribute to the observed lack of reassortment between Orbivirus species.

Development of reverse genetics for Ibaraki virus to produce viable VP6-tagged IBAV

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A reverse genetics system for Ibaraki virus (IBAV) was developed.

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The RG system was used to produce viable VP6-tagged IBAV.

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A region of VP6 (aa 34–82) is not required for IBAV replication in tissue culture.

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The insertion of tags into the nonessential VP6 region did not disrupt replication.

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IBAV VP6 quickly assembled into puncta in the cytosol of infected cells.

Ibaraki virus (IBAV) is a member of the epizootic hemorrhagic disease virus (EHDV) serogroup, which belongs to the Orbivirus genus of the Reoviridae family. Although EHDV, including IBAV, represents an ongoing threat to livestock in the world, molecular mechanisms of EHDV replication and pathogenesis have been unclear. The reverse genetics (RG) system is one of the strong tools to understand molecular mechanisms of virus replication. Here, we developed a RG system for IBAV to identify the nonessential region of a minor structural protein, VP6, by generating VP6-truncated IBAV. Moreover, several tags were inserted into the truncated region to produce VP6-tagged IBAV. We demonstrated that all VP6-tagged IBAV could replicate in BHK cells in the absence of any helper VP6 protein. Further, tagged-VP6 proteins were first assembled into puncta in cells infected with VP6-tagged IBAV. Our data suggests that, in order to initiate primary replication, IBAV VP6 is likely to accumulate in some parts of infected cells to assemble efficiently into the primary replication complex (subcore).

Influence of cellular trafficking pathway on bluetongue virus infection in ovine cells

Bluetongue virus (BTV), a non-enveloped arbovirus, causes hemorrhagic disease in ruminants. However, the influence of natural host cell proteins on BTV replication process is not defined. In addition to cell lysis, BTV also exits non-ovine cultured cells by non-lytic pathways mediated by nonstructural protein NS3 that interacts with virus capsid and cellular proteins belonging to calpactin and ESCRT family. The PPXY late domain motif known to recruit NEDD4 family of HECT ubiquitin E3 ligases is also highly conserved in NS3. In this study using a mixture of molecular, biochemical and microscopic techniques we have analyzed the importance of ovine cellular proteins and vesicles in BTV infection. Electron microscopic analysis of BTV infected ovine cells demonstrated close association of mature particles with intracellular vesicles. Inhibition of Multi Vesicular Body (MVB) resident lipid phosphatidylinositol-3-phosphate resulted in decreased total virus titre suggesting that the vesicles might be MVBs. Proteasome mediated inhibition of ubiquitin or modification of virus lacking the PPXY in NS3 reduced virus growth. Thus, our study demonstrated that cellular components comprising of MVB and exocytic pathways proteins are involved in BTV replication in ovine cells.

Aquareovirus NS80 Initiates Efficient Viral Replication by Retaining Core Proteins within Replication-Associated Viral Inclusion Bodies

Viral inclusion bodies (VIBs) are specific intracellular compartments for reoviruses replication and assembly. Aquareovirus nonstructural protein NS80 has been identified to be the major constituent for forming globular VIBs in our previous study. In this study, we investigated the role of NS80 in viral structural proteins expression and viral replication. Immunofluorescence assays showed that NS80 could retain five core proteins or inner-capsid proteins (VP1-VP4 and VP6), but not outer-capsid proteins (VP5 and VP7), within VIBs in co-transfected or infected cells. Further co-immunoprecipitation analysis confirmed that NS80 could interact with each core protein respectively. In addition, we found that newly synthesized viral RNAs co-localized with VIBs. Furthermore, time-course analysis of viral structural proteins expression showed that the expression of NS80 was detected first, followed by the detection of inner shell protein VP3, and then of other inner-capsid proteins, suggesting that VIBs were essential for the formation of viral core frame or progeny virion. Moreover, knockdown of NS80 by shRNA not only inhibited the expression of aquareovirus structural proteins, but also inhibited viral infection. These results indicated that NS80-based VIBs were formed at earlier stage of infection, and NS80 was able to coordinate the expression of viral structural proteins and viral replication.

Bluetongue virus capsid assembly and maturation

Maturation is an intrinsic phase of the viral life cycle and is often intertwined with egress. In this review we focus on orbivirus maturation by using Bluetongue virus (BTV) as a representative. BTV, a member of the genus Orbivirus within the family Reoviridae, has over the last three decades been subjected to intense molecular study and is thus one of the best understood viruses. BTV is a non-enveloped virus comprised of two concentric protein shells that encapsidate 10 double-stranded RNA genome segments. Upon cell entry, the outer capsid is shed, releasing the core which does not disassemble into the cytoplasm. The polymerase complex within the core then synthesizes transcripts from each genome segment and extrudes these into the cytoplasm where they act as templates for protein synthesis. Newly synthesized ssRNA then associates with the replicase complex prior to encapsidation by inner and outer protein layers of core within virus-triggered inclusion bodies. Maturation of core occurs outside these inclusion bodies (IBs) via the addition of the outer capsid proteins, which appears to be coupled to a non-lytic, exocytic pathway during early infection. Similar to the enveloped viruses, BTV hijacks the exocytosis and endosomal sorting complex required for trafficking (ESCRT) pathway via a non-structural glycoprotein. This exquisitely detailed understanding is assembled from a broad array of assays, spanning numerous and diverse in vitro and in vivo studies. Presented here are the detailed insights of BTV maturation and egress.

[Reverse genetics systems for orbiviruses reveal the essential mechanisms in their replication]

The members of Orbivirus genus within the family Reoviridae cause severe arthropod-born diseases mainly in ruminants and equids. In addition, the orbiviruses, which can infect humans, have been reported. In the last decade, the molecular and structural studies for orbiviruses, including Bluetongue virus (BTV), has made a great progress. Especially, a reverse genetics system (RG) for BTV, developed soon after Orhoreovirus and Rotavirus, is a major breakthrough. Here, I introduced the recent findings in orbivirus replication, especially the function of an enzymatic protein, VP6.

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.

Cellular phosphoinositides and the maturation of bluetongue virus, a non-enveloped capsid virus

Background

Bluetongue virus (BTV), a member of Orbivirus genus in the Reoviridae family is a double capsid virus enclosing a genome of 10 double-stranded RNA segments. A non-structural protein of BTV, NS3, which is associated with cellular membranes and interacts with outer capsid proteins, has been shown to be involved in virus morphogenesis in infected cells. In addition, studies have also shown that during the later stages of virus infection NS3 behaves similarly to HIV protein Gag, an enveloped viral protein. Since Gag protein is known to interact with membrane lipid phosphatidylinositol (4,5) bisphosphate [PI(4,5)P₂] and one of the known binding partners of NS3, cellular protein p11 also interacts with annexin a PI(4,5)P₂ interacting protein, this study was designed to understand the role of this negatively charged membrane lipid in BTV assembly and maturation.

Methods

Over expression of cellular enzymes that either depleted cells of PI(4,5)P₂ or altered the distribution of PI(4,5)P₂, were used to analyze the effect of the lipid on BTV maturation at different times post-infection. The production of mature virus particles was monitored by plaque assay. Microscopic techniques such as confocal microscopy and electron microscopy (EM) were also undertaken to study localization of virus proteins and virus particles in cells, respectively.

Results

Initially, confocal microscopic analysis demonstrated that PI(4,5)P₂ not only co-localized with NS3, but it also co-localized with VP5, one of the outer capsid proteins of BTV. Subsequently, experiments involving depletion of cellular PI(4,5)P₂ or its relocation demonstrated an inhibitory effect on normal BTV maturation and it also led to a redistribution of BTV proteins within the cell. The data was supported further by EM visualization showing that modulation of PI(4,5)P₂ in cells indeed resulted in less particle production.

Conclusion

This study to our knowledge, is the first report demonstrating involvement of PI(4,5)P₂ in a non-enveloped virus assembly and release. As BTV does not have lipid envelope, this finding is unique for this group of viruses and it suggests that the maturation of capsid and enveloped viruses may be more closely related than previously thought.

Minimum requirements for bluetongue virus primary replication in vivo

The replication mechanism of bluetongue virus (BTV) has been studied by an in vivo reverse genetics (RG) system identifying the importance of certain BTV proteins for primary replication of the virus. However, a unique in vitro cell-free virus assembly system was subsequently developed, showing that it did not require the same set of viral components, which is indicative of differences in these two systems. Here, we studied the in vivo primary replicase complex more in-depth to determine the minimum components of the complex. We showed that while NS2 is an essential component of the primary replication stage during BTV infection, NS1 is not an essential component but may play a role in enhancing BTV protein synthesis. Furthermore, we demonstrated that VP7, a major structural protein of the inner core, is not required for primary replication but appears to stabilize the replicase complex. In contrast, VP3, the other major structural core protein, is an essential component of the complex, together with the three minor enzymatic proteins (VP1, VP4, and VP6) of the core. In addition, our data have demonstrated that the smallest minor protein, VP6, which is known to possess an RNA-dependent helicase activity, may also act as an RNA translocator during assembly of the primary replicase complex.

Bluetongue virus genetic and phenotypic diversity: towards identifying the molecular determinants that influence virulence and transmission potential

Bluetongue virus (BTV) is the prototype member of the Orbivirus genus in the family Reoviridae and is the aetiological agent of the arthropod transmitted disease bluetongue (BT) that affects both ruminant and camelid species. The disease is of significant global importance due to its economic impact and effect on animal welfare. Bluetongue virus, a dsRNA virus, evolves through a process of quasispecies evolution that is driven by genetic drift and shift as well as intragenic recombination. Quasispecies evolution coupled with founder effect and evolutionary selective pressures has over time led to the establishment of genetically distinct strains of the virus in different epidemiological systems throughout the world. Bluetongue virus field strains may differ substantially from each other with regards to their phenotypic properties (i.e. virulence and/or transmission potential). The intrinsic molecular determinants that influence the phenotype of BTV have not clearly been characterized. It is currently unclear what contribution each of the viral genome segments have in determining the phenotypic properties of the virus and it is also unknown how genetic variability in the individual viral genes and their functional domains relate to differences in phenotype. In order to understand how genetic variation in particular viral genes could potentially influence the phenotypic properties of the virus; a closer understanding of the BTV virion, its encoded proteins and the evolutionary mechanisms that shape the diversity of the virus is required. This review provides a synopsis of these issues and highlights some of the studies that have been conducted on BTV and the closely related African horse sickness virus (AHSV) that have contributed to ongoing attempts to identify the molecular determinants that influence the virus' phenotype. Different strategies that can be used to generate BTV mutants in vitro and methods through which the causality between particular genetic modifications and changes in phenotype may be determined are also described. Finally examples are highlighted where a clear understanding of the molecular determinants that influence the phenotype of the virus may have contributed to risk assessment and mitigation strategies during recent outbreaks of BT in Europe.

Casein kinase 2 regulates vaccinia virus actin tail formation

Casein kinase 2 (CK2) is a pleiotropic serine/threonine kinase that regulates numerous cellular processes and is essential to the infectious cycle of several viruses. Here we investigated the potential role of CK2 in vaccinia virus (VACV) infection. We used the CK2 inhibitor TBB and found that CK2 inactivation impaired VACV dissemination and actin tail formation. We used RNAi and confirmed that CK2 depletion impaired VACV actin tail formation. Furthermore, we designed a recombinant virus that allowed us to specifically detect cell-associated enveloped viruses (CEVs) at the plasma membrane and demonstrated that CK2 inactivation does not affect CEV formation. Finally, we showed that CK2 depletion impaired the recruitment of Src to CEVs. We discuss the possibility that CK2 may stimulate the A36-dependent recruitment of Src through A36 phosphorylation.

In vitro reconstitution of Bluetongue virus infectious cores

Bluetongue virus (BTV) is a vector-borne, nonenveloped icosahedral particle that is organized in two capsids, an outer capsid of two proteins, VP2 and VP5, and an inner capsid (or core) composed of two major proteins, VP7 and VP3, in two layers. The VP3 layer (subcore) encloses viral transcription complex (VP1 polymerase, VP4 capping enzyme, VP6 helicase) and a 10-segmented double-stranded (dsRNA) genome. Although much is known about the BTV capsids, the order of the core assembly and the mechanism of genome packaging remain unclear. Here, we established a cell-free system to reconstitute subcore and core structures with the proteins and ssRNAs, demonstrating that reconstituted cores are infectious in insect cells. Furthermore, we showed that the BTV ssRNAs are essential to drive the assembly reaction and that there is a distinct order of internal protein recruitment during the assembly process. The in vitro engineering of infectious BTV cores is unique for any member of the Reoviridae and will facilitate future studies of RNA-protein interactions during BTV core assembly.

Phosphorylation of RIG-I by casein kinase II inhibits its antiviral response

RIG-I is an intracellular RNA virus sensor that mediates a signaling pathway that triggers the alpha/beta interferon (IFN-α/β) immune defenses. However, the mechanism for regulation of RIG-I activity remains largely unknown. Here we show that RIG-I activity is regulated by phosphorylation and dephosphorylation in its repressor domain (RD). Threonine at amino acid (aa) 770 and serine at aa 854 to 855 of RIG-I are phosphorylated by casein kinase II (CK2) in the resting state of the cell and dephosphorylated when cells are infected by RNA virus. Mutation at aa position 770 or 854 to 855 of RIG-I renders it constitutively active. Pharmacological inhibition of CK2 enhances virus-induced expression of IFN-β and suppresses virus proliferation, while inhibition of phosphatase reduces virus-induced expression of IFN-β. Overexpression of CK2 suppresses RIG-I-mediated signaling, while silencing of CK2 results in the increased suppression of virus proliferation. Our results reveal a novel mechanism of the regulation of RIG-I activity during RNA virus infection.

Role of lipids on entry and exit of bluetongue virus, a complex non-enveloped virus

Non-enveloped viruses such as members of Picornaviridae and Reoviridae are assembled in the cytoplasm and are generally released by cell lysis. However, recent evidence suggests that some non-enveloped viruses exit from infected cells without lysis, indicating that these viruses may also utilize alternate means for egress. Moreover, it appears that complex, non-enveloped viruses such as bluetongue virus (BTV) and rotavirus interact with lipids during their entry process as well as with lipid rafts during the trafficking of newly synthesized progeny viruses. This review will discuss the role of lipids in the entry, maturation and release of non-enveloped viruses, focusing mainly on BTV.

Bluetongue virus VP6 acts early in the replication cycle and can form the basis of chimeric virus formation

A minor core protein, VP6, of bluetongue virus (BTV) possesses nucleoside triphosphatase, RNA binding, and helicase activities. Although the enzymatic functions of VP6 have been documented in vitro using purified protein, its definitive role in BTV replication remains unclear. In this study, using a recently developed T7 transcript-based reverse genetics system for BTV, we examined the importance of VP6 in virus replication. We show that VP6 is active early in replication, consistent with a role as part of the transcriptase or packaging complex, and that its action can be provided in trans by a newly developed complementary cell line. Furthermore, the genomic segment encoding VP6 was mutated to reveal the cis-acting sequences required for replication or packaging, which subsequently enabled the construction of a chimeric BTV expressing enhanced green fluorescent protein. These data confirm that one of the 10 genome segments of BTV can be replaced with a chimeric RNA containing the essential packaging and replication signals of BTV and the coding sequence of a foreign gene.

Bluetongue virus outer capsid protein VP5 interacts with membrane lipid rafts via a SNARE domain

Bluetongue virus (BTV) is a nonenveloped double-stranded RNA virus belonging to the family Reoviridae. The two outer capsid proteins, VP2 and VP5, are responsible for virus entry. However, little is known about the roles of these two proteins, particularly VP5, in virus trafficking and assembly. In this study, we used density gradient fractionation and methyl beta cyclodextrin, a cholesterol-sequestering drug, to demonstrate not only that VP5 copurifies with lipid raft domains in both transfected and infected cells, but also that raft domain integrity is required for BTV assembly. Previously, we showed that BTV nonstructural protein 3 (NS3) interacts with VP2 and also with cellular exocytosis and ESCRT pathway proteins, indicating its involvement in virus egress (A. R. Beaton, J. Rodriguez, Y. K. Reddy, and P. Roy, Proc. Natl. Acad. Sci. USA 99:13154-13159, 2002; C. Wirblich, B. Bhattacharya, and P. Roy J. Virol. 80:460-473, 2006). Here, we show by pull-down and confocal analysis that NS3 also interacts with VP5. Further, a conserved membrane-docking domain similar to the motif in synaptotagmin, a protein belonging to the SNARE (soluble N-ethylmaleimide-sensitive fusion attachment protein receptor) family was identified in the VP5 sequence. By site-directed mutagenesis, followed by flotation and confocal analyses, we demonstrated that raft association of VP5 depends on this domain. Together, these results indicate that VP5 possesses an autonomous signal for its membrane targeting and that the interaction of VP5 with membrane-associated NS3 might play an important role in virus assembly.

Bluetongue virus: dissection of the polymerase complex

Bluetongue is a vector-borne viral disease of ruminants that is endemic in tropical and subtropical countries. Since 1998 the virus has also appeared in Europe. Partly due to the seriousness of the disease, bluetongue virus (BTV), a member of genus Orbivirus within the family Reoviridae, has been a subject of intense molecular study for the last three decades and is now one of the best understood viruses at the molecular and structural levels. BTV is a complex non-enveloped virus with seven structural proteins arranged in two capsids and a genome of ten double-stranded (ds) RNA segments. Shortly after cell entry, the outer capsid is lost to release an inner capsid (the core) which synthesizes capped mRNAs from each genomic segment, extruding them into the cytoplasm. This requires the efficient co-ordination of a number of enzymes, including helicase, polymerase and RNA capping activities. This review will focus on our current understanding of these catalytic proteins as derived from the use of recombinant proteins, combined with functional assays and the in vitro reconstitution of the transcription/replication complex. In some cases, 3D structures have complemented this analysis to reveal the fine structural detail of these proteins. The combined activities of the core enzymes produce infectious transcripts necessary and sufficient to initiate BTV infection. Such infectious transcripts can now be synthesized wholly in vitro and, when introduced into cells by transfection, lead to the recovery of infectious virus. Future studies thus hold the possibility of analysing the consequence of mutation in a replicating virus system.

Regulation of hepatitis C virion production via phosphorylation of the NS5A protein

Hepatitis C virus (HCV) is a significant pathogen, infecting some 170 million people worldwide. Persistent virus infection often leads to cirrhosis and liver cancer. In the infected cell many RNA directed processes must occur to maintain and spread infection. Viral genomic RNA is constantly replicating, serving as template for translation, and being packaged into new virus particles; processes that cannot occur simultaneously. Little is known about the regulation of these events. The viral NS5A phosphoprotein has been proposed as a regulator of events in the HCV life cycle for years, but the details have remained enigmatic. NS5A is a three-domain protein and the requirement of domains I and II for RNA replication is well documented. NS5A domain III is not required for RNA replication, and the function of this region in the HCV lifecycle is unknown. We have identified a small deletion in domain III that disrupts the production of infectious virus particles without altering the efficiency of HCV RNA replication. This deletion disrupts virus production at an early stage of assembly, as no intracellular virus is generated and no viral RNA and nucleocapsid protein are released from cells. Genetic mapping has indicated a single serine residue within the deletion is responsible for the observed phenotype. This serine residue lies within a casein kinase II consensus motif, and mutations that mimic phosphorylation suggest that phosphorylation at this position regulates the production of infectious virus. We have shown by genetic silencing and chemical inhibition experiments that NS5A requires casein kinase II phosphorylation at this position for virion production. A mutation that mimics phosphorylation at this position is insensitive to these manipulations of casein kinase II activity. These data provide the first evidence for a function of the domain III of NS5A and implicate NS5A as an important regulator of the RNA replication and virion assembly of HCV. The ability to uncouple virus production from RNA replication, as described herein, may be useful in understanding HCV assembly and may be therapeutically important.

Regulation of hepatitis C virion production via phosphorylation of the NS5A protein

Hepatitis C virus (HCV) is a significant pathogen, infecting some 170 million people worldwide. Persistent virus infection often leads to cirrhosis and liver cancer. In the infected cell many RNA directed processes must occur to maintain and spread infection. Viral genomic RNA is constantly replicating, serving as template for translation, and being packaged into new virus particles; processes that cannot occur simultaneously. Little is known about the regulation of these events. The viral NS5A phosphoprotein has been proposed as a regulator of events in the HCV life cycle for years, but the details have remained enigmatic. NS5A is a three-domain protein and the requirement of domains I and II for RNA replication is well documented. NS5A domain III is not required for RNA replication, and the function of this region in the HCV lifecycle is unknown. We have identified a small deletion in domain III that disrupts the production of infectious virus particles without altering the efficiency of HCV RNA replication. This deletion disrupts virus production at an early stage of assembly, as no intracellular virus is generated and no viral RNA and nucleocapsid protein are released from cells. Genetic mapping has indicated a single serine residue within the deletion is responsible for the observed phenotype. This serine residue lies within a casein kinase II consensus motif, and mutations that mimic phosphorylation suggest that phosphorylation at this position regulates the production of infectious virus. We have shown by genetic silencing and chemical inhibition experiments that NS5A requires casein kinase II phosphorylation at this position for virion production. A mutation that mimics phosphorylation at this position is insensitive to these manipulations of casein kinase II activity. These data provide the first evidence for a function of the domain III of NS5A and implicate NS5A as an important regulator of the RNA replication and virion assembly of HCV. The ability to uncouple virus production from RNA replication, as described herein, may be useful in understanding HCV assembly and may be therapeutically important.

Functional mapping of bluetongue virus proteins and their interactions with host proteins during virus replication

Bluetongue virus (BTV) is a double-stranded RNA (dsRNA) virus which is transmitted by blood-feeding gnats to wild and domestic ruminants, causing high morbidity and often high mortality. Partly due to this BTV has been in the forefront of molecular studies for last three decades and now represents one of the best understood viruses at the molecular and structural levels. BTV, like the other members of the Reoviridae family is a complex non-enveloped virus with seven structural proteins and a RNA genome consisting of 10 dsRNA segments of different sizes. In virus infected cells, three other virus encoded nonstructural proteins are synthesized. Significant recent advances have been made in understanding the structure-function relationships of BTV proteins and their interactions during virus assembly. By combining structural and molecular data it has been possible to make progress on the fundamental mechanisms used by the virus to invade, replicate in, and escape from, susceptible host cells. Data obtained from studies over a number of years have defined the key players in BTV entry, replication, assembly and egress. Specifically, it has been possible to determine the complex nature of the virion through three dimensional structure reconstructions; atomic structure of proteins and the internal capsid; the definition of the virus encoded enzymes required for RNA replication; the ordered assembly of the capsid shell and the protein sequestration required for it; and the role of three NS proteins in virus replication, assembly and release. Overall, this review demonstrates that the integration of structural, biochemical and molecular data is necessary to fully understand the assembly and replication of this complex RNA virus.

Impaired hyperphosphorylation of rotavirus NSP5 in cells depleted of casein kinase 1alpha is associated with the formation of viroplasms with altered morphology and a moderate decrease in virus replication

The rotavirus (RV) non-structural protein 5, NSP5, is encoded by the smallest of the 11 genomic segments and localizes in 'viroplasms', cytoplasmic inclusion bodies in which viral RNA replication and packaging take place. NSP5 is essential for the replicative cycle of the virus because, in its absence, viroplasms are not formed and viral RNA replication and transcription do not occur. NSP5 is produced early in infection and undergoes a complex hyperphosphorylation process, leading to the formation of proteins differing in electrophoretic mobility. The role of hyperphosphorylation of NSP5 in the replicative cycle of rotavirus is unknown. Previous in vitro studies have suggested that the cellular kinase CK1alpha is responsible for the NSP5 hyperphosphorylation process. Here it is shown, by means of specific RNA interference, that in vivo, CK1alpha is the enzyme that initiates phosphorylation of NSP5. Lack of NSP5 hyperphosphorylation affected neither its interaction with the virus VP1 and NSP2 proteins normally found in viroplasms, nor the production of viral proteins. In contrast, the morphology of viroplasms was altered markedly in cells in which CK1alpha was depleted and a moderate decrease in the production of double-stranded RNA and infectious virus was observed. These data show that CK1alpha is the kinase that phosphorylates NSP5 in virus-infected cells and contribute to further understanding of the role of NSP5 in RV infection.

Structural features of the Bluetongue virus NS2 protein

Bluetongue virus (BTV) non-structural protein 2 (NS2) belongs to a class of highly conserved proteins found in members of the orbivirus genus of the reoviridae. NS2 forms large multimeric complexes, localizes to cytoplasmic inclusion bodies in the infected cells and binds non-sequence specifically single-stranded RNA (ssRNA). Due to its ability to bind ssRNA, it has been suggested that the protein is involved in the selection and condensation of the BTV ssRNA segments prior to genome encapsidation. We have previously determined the crystal structure of the 177 amino acid N-terminal domain, sufficient for ssRNA binding ability of NS2, to 2.4A resolution. The C-terminal domain, as determined at low resolution using small-angle X-ray scattering, is an elongated dimer. This domain expressed in insect cells is phosphorylated at S249 and S259. Electron microscopy of the full-length protein shows a variety of species with the largest having a ring-like appearance. Based on the electron micrographs, the crystal structure of the N-terminal domain and the structure of the C-terminal domain reported here, we propose a model for a decamer of the full-length protein. This decamer changes conformation upon binding of a non-hydrolysable ATP analogue.

The formation of viroplasm-like structures by the rotavirus NSP5 protein is calcium regulated and directed by a C-terminal helical domain

The rotavirus NSP5 protein directs the formation of viroplasm-like structures (VLS) and is required for viroplasm formation within infected cells. In this report, we have defined signals within the C-terminal 21 amino acids of NSP5 that are required for VLS formation and that direct the insolubility and hyperphosphorylation of NSP5. Deleting C-terminal residues of NSP5 dramatically increased the solubility of N-terminally tagged NSP5 and prevented NSP5 hyperphosphorylation. Computer modeling and analysis of the NSP5 C terminus revealed the presence of an amphipathic alpha-helix spanning 21 C-terminal residues that is conserved among rotaviruses. Proline-scanning mutagenesis of the predicted helix revealed that single-amino-acid substitutions abolish NSP5 insolubility and hyperphosphorylation. Helix-disrupting NSP5 mutations also abolished localization of green fluorescent protein (GFP)-NSP5 fusions into VLS and directly correlate VLS formation with NSP5 insolubility. All mutations introduced into the hydrophobic face of the predicted NSP5 alpha-helix disrupted VLS formation, NSP5 insolubility, and the accumulation of hyperphosphorylated NSP5 isoforms. Some NSP5 mutants were highly soluble but still were hyperphosphorylated, indicating that NSP5 insolubility was not required for hyperphosphorylation. Expression of GFP containing the last 68 residues of NSP5 at its C terminus resulted in the formation of punctate VLS within cells. Interestingly, GFP-NSP5-C68 was diffusely dispersed in the cytoplasm when calcium was depleted from the medium, and after calcium resupplementation GFP-NSP5-C68 rapidly accumulated into punctate VLS. A potential calcium switch, formed by two tandem pseudo-EF-hand motifs (DxDxD), is present just upstream of the predicted alpha-helix. Mutagenesis of either DxDxD motif abolished the regulatory effect of calcium on VLS formation and resulted in the constitutive assembly of GFP-NSP5-C68 into punctate VLS. These results reveal specific residues within the NSP5 C-terminal domain that direct NSP5 hyperphosphorylation, insolubility, and VLS formation in addition to defining residues that constitute a calcium-dependent trigger of VLS formation. These studies identify functional determinants within the C terminus of NSP5 that regulate VLS formation and provide a target for inhibiting NSP5-directed VLS functions during rotavirus replication.

Bluetongue virus entry into cells

Bluetongue virus (BTV) is a member of the Orbivirus genus within the Reoviridae family. Like those of other members of the family, BTV particles are nonenveloped and contain two distinct capsids, namely, an outer capsid and an inner capsid or core. The two outer capsid proteins, VP2 and VP5, are involved in BTV entry into cells and in the delivery of the transcriptionally active core to the target cell cytoplasm. However, very little is known about the precise mechanism of BTV entry. In this report, using RNA interference, we demonstrate that inhibition of the clathrin-dependent endocytic pathway correlates with reduced BTV internalization and subsequent replication. Furthermore, by using the ATPase inhibitor bafilomycin A1, we show that exposure of the virus to acidic pH is required for productive infection. Moreover, microscopic analysis of cells incubated with BTV indicated that the virus is internalized into early endosomes, where separation of the outer capsid and inner core occurs. Together, our data indicate that BTV undergoes low-pH-induced penetration in early endosomes following clathrin-mediated endocytosis from the plasma membrane, supporting a stepwise model for BTV entry and penetration.

Bluetongue virus RNA binding protein NS2 is a modulator of viral replication and assembly

Background

Bluetongue virus (BTV) particles consist of seven structural proteins that are organized into two capsids. In addition, BTV also encodes three non-structural (NS) proteins of which protein 2 (NS2) is the RNA binding protein and is also the major component of virus encoded inclusion bodies (VIBs), which are believed to be virus assembly sites. To investigate the contribution of NS2 in virus replication and assembly we have constructed inducible mammalian cell lines expressing full-length NS2. In addition, truncated NS2 fragments were also generated in an attempt to create dominant negative mutants for NS2 function.

Results

Our data revealed that expression of full-length NS2 was sufficient for the formation of inclusion bodies (IBs) that were morphologically similar to the VIBs formed during BTV infection. By using either, individual BTV proteins or infectious virions, we found that while the VP3 of the inner capsid (termed as "core") that surrounds the transcription complex was closely associated with both NS2 IBs and BTV VIBs, the surface core protein VP7 co-localized with NS2 IBs only in the presence of VP3. In contrast to the inner core proteins, the outer capsid protein VP2 was not associated with either IBs or VIBs. Like the core proteins, newly synthesized BTV RNAs also accumulated in VIBs. Unlike full-length NS2, neither the amino-, nor carboxyl-terminal fragments formed complete IB structures and each appeared to interfere in overall virus replication when similarly expressed.

Conclusion

Together, these data demonstrate that NS2 is sufficient and necessary for IB formation and a key player in virus replication and core assembly. Perturbation of NS2 IB formation resulted in reduced virus synthesis and both the N terminal (NS2-1) and C terminal (NS2-2) fragments act as dominant negative mutants of NS2 function.

Interaction between Bluetongue virus outer capsid protein VP2 and vimentin is necessary for virus egress

BACKGROUND

The VP2 outer capsid protein Bluetongue Virus (BTV) is responsible for receptor binding, haemagglutination and eliciting host-specific immunity. However, the assembly of this outer capsid protein on the transcriptionally active viral core would block transcription of the virus. Thus assembly of the outer capsid on the core particle must be a tightly controlled process during virus maturation. Earlier studies have detected mature virus particles associated with intermediate filaments in virus infected cells but the viral determinant for this association and the effect of disrupting intermediate filaments on virus assembly and release are unknown.

RESULTS

In this study it is demonstrated that BTV VP2 associates with vimentin in both virus infected cells and in the absence of other viral proteins. Further, the determinants of vimentin localisation are mapped to the N-terminus of the protein and deletions of amino acids between residues 65 and 114 are shown to disrupt VP2-vimentin association. Site directed mutation also reveals that amino acid residues Gly 70 and Val 72 are important in the VP2-vimentin association. Mutation of these amino acids resulted in a soluble VP2 capable of forming trimeric structures similar to unmodified protein that no longer associated with vimentin. Furthermore, pharmacological disruption of intermediate filaments, either directly or indirectly through the disruption of the microtubule network, inhibited virus release from BTV infected cells.

CONCLUSION

The principal findings of the research are that the association of mature BTV particles with intermediate filaments are driven by the interaction of VP2 with vimentin and that this interaction contributes to virus egress. Furthermore, i) the N-terminal 118 amino acids of VP2 are sufficient to confer vimentin interaction. ii) Deletion of amino acids 65-114 or mutation of amino acids 70-72 to DVD abrogates vimentin association. iii) Finally, disruption of vimentin structures results in an increase in cell associated BTV and a reduction in the amount of released virus from infected cells.

Bluetongue virus assembly and morphogenesis

Like other members of the Reoviridae, bluetongue virus faces the same constraints on structure and assembly that are imposed by a large dsRNA genome. However, since it is arthropod-transmitted, BTV must have assembly pathways that are sufficiently flexible to allow it to replicate in evolutionarily distant hosts. With this background, it is hardly surprising that BTV interacts with highly conserved cellular pathways during morphogenesis and trafficking. Indeed, recent studies have revealed striking parallels between the pathways involved in the entry and egress of nonenveloped BTV and those used by enveloped viruses. In addition, recent studies with the protein that is the major component of the BTV viroplasm have revealed how the assembly and, as importantly, the disassembly of this structure may be achieved. This is a first step towards resolving the interactions that occur in these virus 'assembly factories'. Overall, this review demonstrates that the integration of structural, biochemical and molecular data is necessary to fully understand the assembly and replication of this complex RNA virus.

Pns4 of rice dwarf virus is a phosphoprotein, is localized around the viroplasm matrix, and forms minitubules

Rice dwarf virus (RDV), a member of the family Reoviridae, has a 12-segmented dsRNA genome. Seven segments, designated S1, S2, S3, S5, S7, S8, and S9, encode structural proteins, while the remainder encode nonstructural proteins. One of the nonstructural proteins, Pns4, which is encoded by S4, was characterized. Pns4 was a phosphorylatable substrate in a phosphorylation assay in vivo; it associated with large cytoplasmic fibrils and formed novel minitubules in infected cultured cells of its leafhopper insect vector, as revealed by immunofluorescence and immunoelectron microscopy. Early in infection, Pns4 was detected at the periphery of the viroplasm, and it was then observed on amorphous or fibrillar inclusions, which were identified as bundles of minitubules, at later stages of infection. Since viroplasms are believed to be the site of RDV replication, the intracellular location of Pns4 suggests that this protein might be involved in the process of assembly of the RDV virion.

Pns12 protein of Rice dwarf virus is essential for formation of viroplasms and nucleation of viral-assembly complexes

Cytoplasmic inclusion bodies, known as viroplasms or viral factories, are assumed to be the sites of replication of members of the family Reoviridae. Immunocytochemical and biochemical analyses were carried out to characterize the poorly understood viroplasms of the phytoreovirus Rice dwarf virus (RDV). Within 6 h of inoculation of cells, viroplasms, namely discrete cytoplasmic inclusions, were formed that contained the non-structural proteins Pns6, Pns11 and Pns12 of RDV, which appeared to be the constituents of the inclusions. Formation of similar inclusions in non-host insect cells upon expression of Pns12 in a baculovirus system and the association of molecules of Pns12 in vitro suggested that the inclusions observed in RDV-infected cells were composed basically of Pns12. Core proteins P1, P3, P5 and P7 and core virus particles were identified in the interior region of the inclusions. In contrast, accumulation of the outer capsid proteins P2, P8 and P9 and of intact virus particles was evident in the peripheral regions of the inclusions. These observations suggest that core particles were constructed inside the inclusions, whereas outer capsid proteins were assembled at the periphery of the inclusions. Viral inclusions were shown to be the sites of viral RNA synthesis by labelling infected cells with 5-bromouridine 5'-triphosphate. The number of viroplasms decreased with time post-inoculation as their sizes increased, suggesting that inclusions might fuse with one another during the virus-propagation process. Our results are consistent with a model, proposed for vertebrate reoviruses, in which viroplasms play a pivotal role in virus assembly.

PPSP: prediction of PK-specific phosphorylation site with Bayesian decision theory

Background

As a reversible and dynamic post-translational modification (PTM) of proteins, phosphorylation plays essential regulatory roles in a broad spectrum of the biological processes. Although many studies have been contributed on the molecular mechanism of phosphorylation dynamics, the intrinsic feature of substrates specificity is still elusive and remains to be delineated.

Results

In this work, we present a novel, versatile and comprehensive program, PPSP (Prediction of PK-specific Phosphorylation site), deployed with approach of Bayesian decision theory (BDT). PPSP could predict the potential phosphorylation sites accurately for ~70 PK (Protein Kinase) groups. Compared with four existing tools Scansite, NetPhosK, KinasePhos and GPS, PPSP is more accurate and powerful than these tools. Moreover, PPSP also provides the prediction for many novel PKs, say, TRK, mTOR, SyK and MET/RON, etc. The accuracy of these novel PKs are also satisfying.

Conclusion

Taken together, we propose that PPSP could be a potentially powerful tool for the experimentalists who are focusing on phosphorylation substrates with their PK-specific sites identification. Moreover, the BDT strategy could also be a ubiquitous approach for PTMs, such as sumoylation and ubiquitination, etc.

PPSP: prediction of PK-specific phosphorylation site with Bayesian decision theory

BACKGROUND

As a reversible and dynamic post-translational modification (PTM) of proteins, phosphorylation plays essential regulatory roles in a broad spectrum of the biological processes. Although many studies have been contributed on the molecular mechanism of phosphorylation dynamics, the intrinsic feature of substrates specificity is still elusive and remains to be delineated.

RESULTS

In this work, we present a novel, versatile and comprehensive program, PPSP (Prediction of PK-specific Phosphorylation site), deployed with approach of Bayesian decision theory (BDT). PPSP could predict the potential phosphorylation sites accurately for approximately 70 PK (Protein Kinase) groups. Compared with four existing tools Scansite, NetPhosK, KinasePhos and GPS, PPSP is more accurate and powerful than these tools. Moreover, PPSP also provides the prediction for many novel PKs, say, TRK, mTOR, SyK and MET/RON, etc. The accuracy of these novel PKs are also satisfying.

CONCLUSION

Taken together, we propose that PPSP could be a potentially powerful tool for the experimentalists who are focusing on phosphorylation substrates with their PK-specific sites identification. Moreover, the BDT strategy could also be a ubiquitous approach for PTMs, such as sumoylation and ubiquitination, etc.