Bluetongue virus capsid protein VP5 perforates membranes at low endosomal pH during viral entry

Bluetongue virus (BTV) is a non-enveloped virus and causes substantial morbidity and mortality in ruminants such as sheep. Fashioning a receptor-binding protein (VP2) and a membrane penetration protein (VP5) on the surface, BTV releases its genome-containing core (VP3 and VP7) into the host cell cytosol after perforation of the endosomal membrane. Unlike enveloped ones, the entry mechanisms of non-enveloped viruses into host cells remain poorly understood. Here we applied single-particle cryo-electron microscopy, cryo-electron tomography and structure-guided functional assays to characterize intermediate states of BTV cell entry in endosomes. Four structures of BTV at the resolution range of 3.4-3.9 Å show the different stages of structural rearrangement of capsid proteins on exposure to low pH, including conformational changes of VP5, stepwise detachment of VP2 and a small shift of VP7. In detail, sensing of the low-pH condition by the VP5 anchor domain triggers three major VP5 actions: projecting the hidden dagger domain, converting a surface loop to a protonated β-hairpin that anchors VP5 to the core and stepwise refolding of the unfurling domains into a six-helix stalk. Cryo-electron tomography structures of BTV interacting with liposomes show a length decrease of the VP5 stalk from 19.5 to 15.5 nm after its insertion into the membrane. Our structures, functional assays and structure-guided mutagenesis experiments combined indicate that this stalk, along with dagger domain and the WHXL motif, creates a single pore through the endosomal membrane that enables the viral core to enter the cytosol. Our study unveils the detailed mechanisms of BTV membrane penetration and showcases general methods to study cell entry of other non-enveloped viruses.

Bluetongue Virus Nonstructural Protein 3 Orchestrates Virus Maturation and Drives Non-Lytic Egress via Two Polybasic Motifs

Bluetongue virus (BTV) is an arthropod-borne virus that infects domestic and wild ruminants. The virion is a non-enveloped double-layered particle with an outer capsid that encloses a core containing the segmented double-stranded RNA genome. Although BTV is canonically released by cell lysis, it also exits non-lytically. In infected cells, the BTV nonstructural glycoprotein 3 (NS3) is found to be associated with host membranes and traffics from the endoplasmic reticulum through the Golgi apparatus to the plasma membrane. This suggests a role for NS3 in BTV particle maturation and non-lytic egress. However, the mechanism by which NS3 coordinates these events has not yet been elucidated. Here, we identified two polybasic motifs (PMB1/PMB2), consistent with the membrane binding. Using site-directed mutagenesis, confocal and electron microscopy, and flow cytometry, we demonstrated that PBM1 and PBM2 mutant viruses retained NS3 either in the Golgi apparatus or in the endoplasmic reticulum, suggesting a distinct role for each motif. Mutation of PBM2 motif decreased NS3 export to the cell surface and virus production. However, both mutant viruses produced predominantly inner core particles that remained close to their site of assembly. Together, our data demonstrates that correct trafficking of the NS3 protein is required for virus maturation and release.

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.

Mapping the assembly pathway of Bluetongue virus scaffolding protein VP3

The structure of the Bluetongue virus (BTV) core and its outer layer VP7 has been solved by X-ray crystallography, but the assembly intermediates that lead to the inner scaffolding VP3 layer have not been defined. In this report, we addressed two key questions: (a) the role of VP3 amino terminus in core assembly and its interaction with the transcription complex (TC) components; and (b) the assembly intermediates involved in the construction of the VP3 shell. To do this, deletion mutants in the amino terminal and decamer-decamer interacting region of VP3 (DeltaDD) were generated, expressed in insect cells using baculovirus expression systems, and their ability to assemble into core-like particles (CLPs) and to incorporate the components of TC were investigated. Deletion of the N-terminal 5 (Delta5N) or 10 (Delta10N) amino acids did not affect the ability to assemble into CLPs in the presence of VP7 although the cores assembled using the 10 residue mutant (Delta10N) deletion were very unstable. Removal of five residues also did not effect incorporation of the internal VP1 RNA polymerase and VP4 mRNA capping enzyme proteins of the TC. Removal of the VP3-VP3 interacting domain (DeltaDD) led to failure to assemble into CLPs yet retained interaction with VP1 and VP4. In solution, purified DeltaDD mutant protein readily multimerized into dimers, pentamers, and decamers, suggesting that these oligomers are the authentic assembly intermediates of the subcore. However, unlike wild-type VP3 protein, the dimerization domain-deleted assembly intermediates were found to have lost RNA binding ability. Our study emphasizes the requirement of the N-terminus of VP3 for binding and encapsidation of the TC components, and defines the role of the dimerization domain in subcore assembly and RNA binding.

The bluetongue virus core: a nano-scale transcription machine

The replication phase of the bluetongue virus (BTV) infection cycle is initiated when the virus core is delivered into the cytoplasm of a susceptible host cell. The 10 segments of the viral genome remain packaged within the core throughout the replication cycle, helping to prevent the activation of host defence mechanisms that would be caused by direct contact between the dsRNA and the host cell cytoplasm. However, the BTV core is a biochemically active 'nano-scale' machine, which can simultaneously and repeatedly transcribe mRNA from each of the 10 genome segments, which are packaged as a liquid crystal array within a central cavity. These mRNAs, which are also capped and methylated within the core, are extruded into the cytoplasm through pores at the vertices of the icosahedral structure, where they are translated into viral proteins. One copy of each of the viral mRNAs is also assembled with these newly synthesised proteins to form nascent virus particles, which mature by a process that involves -ve RNA strand synthesis on the +ve stand template, thereby reforming dsRNA genome segments within progeny virus cores. The structure of the BTV core particle has been determined to atomic resolution by X-ray crystallography, revealing the organisation and interactions of its major protein components (VP3(T2)-subcore shell and VP7(T13) outer core layer) and important features of the packaged dsRNA. By soaking crystals of BTV cores with metal ions and substrates/products of the transcription reactions prior to analysis by X-ray crystallography, then constructing difference maps, it has been possible to identify binding sites and entry/exit routes for these ions, substrates and products. This has revealed how BTV solves the many logistical problems of multiple and simultaneous transcription from the 10 genome segments within the confined space of the core particle. The crystal structure of the BTV core has also revealed an outer surface festooned with dsRNA. This may represent a further protective strategy adopted by the virus to prevent host cell shut-off, by sequestering any dsRNA that may be released from damaged particles.

Nature and duration of protective immunity to bluetongue virus infection

Genetic engineering offers a variety of approaches to producing viral vaccines. An exciting advance in this field is the ability to construct virus-like particles (VLPs) that resemble their natural counterparts but lack genetic information. To develop a rationally designed vaccine for bluetongue disease of sheep that is caused by virus (BTV), we have synthesised individual BTV proteins and BTV-like particles (VLPs and CLPs) using baculovirus expression systems and insect cell cultures. A series of clinical trials were undertaken using these proteins and particles in BTV-susceptible sheep. The accumulated data obtained from these studies are: (i) the two surface proteins when used in high doses (approximately 100 microg/dose) could afford complete protection in sheep against virulent virus challenge; (ii) in contrast, only 5-10 microg of VP2 of a related virus, African horse sickness virus (AHSV) afforded protection in horses against virulent virus challenges when vaccinated in the presence of appropriate adjuvant; (iii) vaccination with as little as 10 microg VLPs (consisting of all four major proteins) gave long lasting protection (at least for 14 months) against homologous BTV challenge; (iv) cross-protection was also achieved depending on the challenge virus and amounts of antigen used for vaccination and (v) limited vaccination trials with CLPs (containing only two highly conserved internal proteins) afforded partial (with slight fever) protection against homologous and heterologous virus challenges. Since CLPs are conserved across the twenty four BTV serotypes, CLPs could have potential for a candidate vaccine that may at least mitigate the disease and inhibit virus spread. In summary, VLPs and CLPs offer completely safe and efficacious vaccines as their particles are devoid of any detectable amount of insect, baculovirus proteins or nucleic acids and thus pose no potential adverse effects.

Virus-derived tubular structure displaying foreign sequences on the surface elicit CD4+ Th cell and protective humoral responses

Particulate vector systems for the presentation of immunogenic epitopes provide an alternate and powerful approach for the delivery of immunogens of interest. In this article, we have exploited a viral protein of unknown function, bluetongue virus (BTV) nonstructural protein NS1, which forms distinct tubular aggregates in infected cells, as an immunogen delivery system. Tubules are helical assemblies of NS1 protein that present the C-terminus of the protein to the outer edge effectively displaying appended residues in a regular and repeating array akin to the coat of a filamentous phage. To assess the breadth of response induced following tubule-based immunization, two different immunodominant foreign peptides were inserted at the C-terminus of NS1 and chimeric tubules generated following expression in the baculovirus expression system. Both constructs, one carrying a peptide of foot and mouth disease virus (FMDV) (aa 135-144 of VP1) and the other, a peptide of influenza A virus (aa 186-205 of HA), effectively assembled into tubules and were easily purified. Subsequently, using in vitro assay systems, we demonstrated that each purified chimeric particle was capable of eliciting strong immune responses. Further, NS1-FMDV chimeric tubules could induce a potent immune response that could protect against disease.

IMIRS: a high-resolution 3D reconstruction package integrated with a relational image database

Recent advances in electron cryomicroscopy instrumentation and single particle reconstruction have created opportunities for high-throughput and high-resolution three-dimensional (3D) structure determination of macromolecular complexes. However, it has become impractical and inefficient to rely on conventional text file data management and command-line programs to organize and process the increasing numbers of image data required in high-resolution studies. Here, we present a distributed relational database for managing complex datasets and its integration into our high-resolution software package IMIRS (Image Management and Icosahedral Reconstruction System). IMIRS consists of a complete set of modular programs for icosahedral reconstruction organized under a graphical user interface and provides options for user-friendly, step-by-step data processing as well as automatic reconstruction. We show that the integration of data management with processing in IMIRS automates the tedious tasks of data management, enables data coherence, and facilitates information sharing in a distributed computer and user environment without significantly increasing the time of program execution. We demonstrate the applicability of IMIRS in icosahedral reconstruction toward high resolution by using it to obtain an 8-A 3D structure of an intermediate-sized dsRNA virus.

High resolution structural studies of complex icosahedral viruses: a brief overview

Structural descriptions of viral particles are key to our understanding of their assembly mechanisms and properties. We will describe the application of X-ray crystallography and electron cryomicroscopy to the structural determination of the bluetongue virus core and the herpesvirus capsid. These represent the highest resolution structural studies carried out by these techniques on such complex and large icosahedral virus particles. The bluetongue virus core consists of two layers of distinct proteins with different protein packing symmetries, while the herpes virus capsid is made up of four types of proteins with 3.3 MDa per asymmetric unit. The structural results reveal that each of these proteins has distinct folds and they are packed uniquely to form stable particles.

RGD tripeptide of bluetongue virus VP7 protein is responsible for core attachment to Culicoides cells

Bluetongue virus (BTV) is an arthropod-borne virus transmitted by Culicoides species to vertebrate hosts. The double-capsid virion is infectious for Culicoides vector and mammalian cells, while the inner core is infectious for only Culicoides-derived cells. The recently determined crystal structure of the BTV core has revealed an accessible RGD motif between amino acids 168 to 170 of the outer core protein VP7, whose structure and position would be consistent with a role in cell entry. To delineate the biological role of the RGD sequence within VP7, we have introduced point mutations in the RGD tripeptide and generated three recombinant baculoviruses, each expressing a mutant derivative of VP7 (VP7-AGD, VP7-ADL, and VP7-AGQ). Each expressed mutant protein was purified, and the oligomeric nature and secondary structure of each was compared with those of the wild-type (wt) VP7 molecule. Each mutant VP7 protein was used to generate empty core-like particles (CLPs) and were shown to be biochemically and morphologically identical to those of wt CLPs. However, when mutant CLPs were used in an in vitro cell binding assay, each showed reduced binding to Culicoides cells compared to wt CLPs. Twelve monoclonal antibodies (MAbs) was generated using purified VP7 or CLPs as a source of antigen and were utilized for epitope mapping with available chimeric VP7 molecules and the RGD mutants. Several MAbs bound to the RGD motif on the core, as shown by immunogold labeling and cryoelectron microscopy. RGD-specific MAb H1.5, but not those directed to other regions of the core, inhibited the binding activity of CLPs to the Culicoides cell surface. Together, these data indicate that the RGD motif present on BTV VP7 is responsible for Culicoides cell binding activity.

Functional dissection of the major structural protein of bluetongue virus: identification of key residues within VP7 essential for capsid assembly

A lattice of VP7 trimers forms the surface of the icosahedral bluetongue virus (BTV) core. To investigate the role of VP7 oligomerization in core assembly, a series of residues for substitution were predicted based on crystal structures of BTV type 10 VP7 molecule targeting the monomer-monomer contacts within the trimer. Seven site-specific substitution mutations of VP7 have been created using cDNA clones and were employed to produce seven recombinant baculoviruses. The effects of these mutations on VP7 solubility, ability to trimerize and formation of core-like particles (CLPs) in the presence of the scaffolding VP3 protein, were investigated. Of the seven VP7 mutants examined, three severely affected the stability of CLP, while two other mutants had lesser effect on CLP stability. Only one mutant had no apparent effect on the formation of the stable capsid. One mutant in which the conserved tyrosine at residue 271 (lower domain helix 6) was replaced by arginine formed insoluble aggregates, implying an effect in the folding of the molecule despite the prediction that such a change would be accommodated. All six soluble VP7 mutants were purified, and their ability to trimerize was examined. All mutants, including those that did not form stable CLPs, assembled into stable trimers, implying that single substitution may not be sufficient to perturb the complex monomer-monomer contacts, although subtle changes within the VP7 trimer could destabilize the core. The study highlights some of the key residues that are crucial for BTV core assembly and illustrates how the structure of VP7 in isolation underrepresents the dynamic nature of the assembly process at the biological level.

Structures of virus and virus-like particles

Virus structures continue to be the basis for mechanistic virology and serve as a paradigm for solutions to problems concerning macromolecular assembly and function in general. The use of X-ray crystallography, electron cryomicroscopy and computational and biochemical methods has provided not only details of the structural folds of individual viral components, but also insights into the structural basis of assembly, nucleic acid packaging, particle dynamics and interactions with cellular molecules.

Virus crystallography

Virus crystallography can provide atomic resolution structures for intact isometric virus particles and components thereof. The methodology is illustrated by reference to a particularly complex example, the core of the bluetongue virus (700 A).

The structure of a cypovirus and the functional organization of dsRNA viruses

Cytoplasmic polyhedrosis virus (CPV) is unique among the double-stranded RNA viruses of the family Reoviridae in having a single capsid layer. Analysis by cryo-electron microscopy allows comparison of the single shelled CPV and orthoreovirus with the high resolution crystal structure of the inner shell of the bluetongue virus (BTV) core. This suggests that the novel arrangement identified in BTV, of 120 protein subunits in a so-called 'T=2' organization, is a characteristic of the Reoviridae and allows us to delineate structural similarities and differences between two subgroups of the family--the turreted and the smooth-core viruses. This in turn suggests a coherent picture of the structural organization of many dsRNA viruses.

Quantification of recombinant core-like particles of bluetongue virus using immunosorbent electron microscopy

Immunosorbent electron microscopy was used to quantify recombinant baculovirus-generated bluetongue virus (BTV) core-like particles (CLP) in either purified preparations or lysates of recombinant baculovirus-infected cells. The capture antibody was an anti-BTV VP7 monoclonal antibody. The CLP concentration in purified preparations was determined to be 6.6 x 10(15) particles/l. CLP concentration in lysates of recombinant baculovirus-infected cells was determined at various times post-infection and shown to reach a value of 3 x 10(15) particles/l of culture medium at 96 h post-infection. The results indicated that immunosorbent electron microscopy, aided by an improved particle counting method, is a simple, rapid and accurate technique for the quantification of virus and virus-like particles produced in large scale in vitro systems.

Courageous science: structural studies of bluetongue virus core

The structure of the bluetongue virus core was recently reported and represents the largest structure determined to atomic resolution. As a biological machine capable of RNA transcription, the structure has immense biological significance.

New generation of African horse sickness virus vaccines based on structural and molecular studies of the virus particles

African horse sickness virus (AHSV) is a member of the genus Orbivirus, which also includes bluetongue virus (BTV) and epizootic haemorrhagic disease (EHDV) virus. These orbiviruses have similar morphological and biochemical properties, with distinctive pathobiological properties and host ranges. Sequencing studies of the capsid proteins have revealed evolutionary relationships between these viruses. Biochemical studies of the viruses together with the expression of individual proteins and protein complexes have resulted in the development of new generation vaccines. Baculovirus expressed AHSV VP2 provides protection against death caused by AHSV challenge. Similarly, BTV VP2 alone elicits protective neutralising antibodies against BTV in sheep, which is enhanced in the presence of VP5. Recent developments in biotechnology (multiple gene expression baculovirus systems) have made it possible to synthesise orbivirus particles that biochemically and immunologically mimic authentic virions but lack the genetic material. Particle doses as low as 10 micrograms elicit responses that are sufficient to protect sheep 15 months post vaccination, against virulent virus challenge. Moreover, knowledge of the three dimensional structure of these particles enables us to engineer them to deliver multiple foreign peptide components representing other viral epitopes (e.g. foot and mouth disease virus and influenza virus) in order to elicit protective immunity.

Projection structure of VP6, the rotavirus inner capsid protein, and comparison with bluetongue VP7

The rotavirus nucleocapsid protein (VP6) is the major structural protein of inner capsid particles (ICP). VP6 is essential for RNA transcription and binds to a virally encoded glycoprotein receptor (NSP4) involved in the rotavirus assembly pathway. To explore the structure of VP6, two-dimensional (2D) crystals of VP6 were generated and examined by electron microscopy and image processing. Fourier transforms computed from low-dose images of negatively stained 2D VP6 crystals displayed complete data to 13 A resolution for p6 plane group symmetry. To correct for the resolution dependent fall-off of the amplitudes derived from electron microscopic images, the rotavirus VP6 amplitudes were scaled to the bluetongue VP7 amplitudes derived from the atomic model by applying a B factor of -360 A-2. The unit cell (a=b=101(+/-2)A, gamma=120(+/-1) degrees) contains two VP6 trimers, each composed of three roughly circular subunits approximately 30 A in diameter. The trimeric organization of VP6 is similar to the oligomeric structure of VP6 when assembled in T=13l icosahedral inner capsid particles at 25 to 40 A resolution. However, a channel at the center of the trimer is better resolved in our map at 15 A resolution. The projection structure of rotavirus VP6 was compared to the homologous protein (VP7) of bluetongue virus, which is also a member of the family of Reoviridae. Notably, both VP6 and bluetongue VP7 assemble as 260 capsomers on the surface of the inner capsid. To compare VP6 and VP7, a projection map of bluetongue VP7 at 15 A resolution was generated using the atomic model derived by X-ray crystallography. VP6 and VP7 both exhibit a trimeric organization with a central channel, even though the alignment identity between the 45 kDa VP6 and the 38 kDa VP7 primary sequences is only 12%. The ability of VP6 to form well-ordered 2D crystals should enable a higher resolution structure analysis by cryo-electron microscopy that will extend our understanding of the icosahedral ICP structure, clarify the mechanism by which VP6 interacts with the NSP4 receptor, and allow a more detailed comparison of VP6 and VP7.

Structure of Broadhaven virus by cryoelectron microscopy: correlation of structural and antigenic properties of Broadhaven virus and bluetongue virus outer capsid proteins

The three-dimensional structure of Broadhaven virus (BRDV) has been determined to 23 A resolution by cryoelectron microscopy and image processing. As predicted from sequence homology, the BRDV structure resembles that of bluetongue virus (BTV) with the notable exception of one of the outer shell proteins. The cores of BRDV and BTV are identical at medium resolution; they have a diameter of 710 A and the VP7 trimers are arranged on a T = 13 icosahedral lattice. The outer shell proteins, VP5 of BRDV and BTV, have roughly the same molecular weight while VP4 of BRDV is only half the molecular weight of the corresponding VP2 of BTV. This size difference allows unambiguous determination of the identity of the triskelion shape as trimers of VP4 of BRDV (VP2 of BTV). The VP4 of BRDV sits on the VP7 trimers and projects outwards 40 A, giving the capsid an overall diameter of 790 A. This contrasts with VP2 of BTV, which projects outwards 95 A to give the capsid a diameter of 900 A. The difference in accessibility of the outer shell proteins of BRDV and BTV correlates with the difference in antigenic properties of these viral proteins. The shape of the BRDV VP5 indicates that it too is a trimer, thus implying that there are 360 copies of VP5 and 180 copies of VP4 per virion.

In vitro replication of epizootic hemorrhagic disease and bluetongue viruses in white-tailed deer peripheral blood mononuclear cells and virus-cell association during in vivo infections

In vitro and in vivo infections were conducted to determine if the epizootic hemorrhagic disease (EHD) and bluetongue (BT) viruses would replicate in peripheral blood mononuclear (PBM) cells of white-tailed deer (Odocoileus virginianus). All of the North American EHD and BT viruses (EHD virus serotypes 1 and 2, and BT virus serotypes 2, 10, 11, 13, and 17) replicated in vitro in cultures of white-tailed deer PBM cells. However, this replication appeared to be monocyte-dependent and was not enhanced by lymphocyte blastogenesis induced by the addition of concanavalin A. In white-tailed deer infected with either EHD virus serotype 2 or BT virus serotype 10, virus could be isolated consistently from PBM cells only from post-infection day 4 through 8, although they remained viremic through post-infection day 21. In deer, highest viral titers were associated with the erythrocyte fraction, and in no cases did viral titers detected in the platelet, PBM cell or polymorphonuclear cell fractions approach titers observed in whole blood. In the in vitro infections of white-tailed deer erythrocytes, the EHD and BT viruses were associated with pits in the erythrocyte membrane. This association may be important in the long-term viremia observed in deer.

Applications of dot-blot, ELISA, and immunoelectron microscopy to field detection of bluetongue virus in Culicoides variipennis sonorensis: an ecological perspective

An avidin-biotin complex (ABC) dot-blot, an antigen capture enzyme-linked immunosorbent assay (ELISA), and immunoelectron microscopy (IEM) were used to detect bluetongue (BLU) virus and viral antigen in field-collected C. varriipennis sonorensis Wirth & Jones from an enzootic BLU area in northeastern Colorado. This is the 1st attempt to apply these immunodiagnostic methods to an epidemiologically relevant, large-scale ecological system. One of the 1,800 midges (0.0005%) was positive by the dot-blot procedure, 2 (0.0011%) were positive by the ELISA, and BLU virus was identified in 8 midges (0.0044%) by IEM. These data are interpreted in context of the "whole system" of the disease to provide a framework for determining the knowledge gaps in our understanding and directing future studies in these areas. Our basic model of BLU ecology suggests that the infection rates found by the diagnostic methods are within expected ranges, thus strongly supporting the proposed ecological model and the work used to parameterize the model. This integration of immunodiagnostic methods and ecology makes it evident that further investigations of daily mortality during the extrinsic incubation period are vital to a better understanding of BLU virus occurrence in Culicoides vector and vertebrate host populations.

Bluetongue virus in laboratory-reared Culicoides variipennis sonorensis: applications of dot-blot, ELISA, and immunoelectron microscopy

An avidin-biotin complex (ABC) dot-blot, an antigen capture enzyme-linked immunosorbent assay (ELISA), and immunoelectron microscopy (IEM) were used to detect bluetongue (BLU) virus or viral antigen or both in adult Culicoides variipennis sonorensis Wirth & Jones. The dot-blot and ELISA procedures detected viral antigen in 10-22% (depending on serotype) of the biting midges infected with BLU-2, BLU-10, BLU-13, and BLU-17 and approximately 68% of the midges infected with BLU-11. IEM analyses revealed BLU virus in salivary glands, fat body, and thoracic muscle tissue from infected insects. There appeared to be selective growth of the virus in salivary gland tissue.

Improved method for counting virus and virus like particles

An improved method for counting virus and virus like particles by electron microscopy (EM) was developed. The procedure involves the determination of the absolute concentration of pure or semi-pure particles once deposited evenly on EM grids using either centrifugation or antibody capture techniques. The counting of particles was done with a Microfiche unit which enlarged approximately 50 x the image of particles on a developed negative film which had been taken at a relatively low magnification (2500 x) by EM. Initially, latex particles of a known concentration were counted using this approach, to prove the accuracy of the technique. The latex particles were deposited evenly on an EM grid using centrifugation (Modified Beckmen EM-90 Airfuge technique). Subsequently, recombinant Bluetongue virus (BTV) core-like particles (CLPs) captured by a Monoclonal antibody using a novel sample loading method were counted by the Microfiche unit method and by a direct EM method. Comparison of the simplified counting method developed with a conventional method, showed good agreement. The method is simple, accurate, rapid, and reproducible when used with either pure particles or with particles from crude cell culture extracts.

Comparative effects of bluetongue virus infection of ovine and bovine endothelial cells

Bluetongue virus (BTV) infection results in disparate clinical syndromes among ruminant species. An in vitro model system of BTV/target cell interaction was developed using umbilical vein endothelial cells (EC)from fetal lambs and calves. These cells had microscopic, ultrastructural, and immunocytochemical features typical of EC. BTV infection in these cells was examined using virus binding assays, plaque assays, a whole-cell enzyme-linked immunosorbent assay, flow cytometry, electron microscopy, and a bioassay for interferon activity. EC from both species supported cytopathic BTV infections. Ovine EC bound more BTV initially and produced more virus over time, whereas bovine EC underwent more rapid lysis subsequent to infection. An ultrastructural comparison of BTV-infected ovine and bovine EC, grown as differentiated capillary-like cords on a laminin-rich matrix or as monolayers, revealed no significant interspecies differences in viral morphogenesis between 1 minute and 24 hours after infection. The intracellular distribution of BTV nonstructural protein 1, which localized to virus inclusion bodies and tubules, was identical for ovine and bovine endothelial cells. Ovine and bovine EC produced a soluble mediator of interferon activity in response to BTV infection; however, ovine EC produced higher levels of interferon activity at lower levels of infection. These findings indicate differences in BTV-EC interaction that may contribute to the pathogenesis of the severe inflammatory disease that is characteristic of clinical bluetongue disease in sheep.

Enhanced infectivity of modified bluetongue virus particles for two insect cell lines and for two Culicoides vector species

Previous studies (Mertens et al., Virology 157, 375-386, 1987) have shown that removal of the outer capsid layer from bluetongue virus (BTV) significantly reduces (approximately x 10(-4)) the infectivity of the resultant core particle for mammalian cells (BHK 21 cells). In contrast, the studies reported here, using a cell line (KC cells) derived from a species of Culicoides that can act as a vector for BTV (Culicoides variipennis), demonstrated a much higher infectivity of core particles than that in mammalian cells (approximately x 10(3)). This increase resulted in a specific infectivity for cores that was only 20-fold less than that of purified disaggregated virus particles (stored in the presence of 0.1% sodium-N-lauroylsarcosine (NLS)). Removal of this detergent caused intact virus particle aggregation and (as previously reported) resulted in an approximately 1 log10 drop in the specific infectivity of those virus particles which remained in suspension. In consequence the specific infectivity of core particles for the KC cells was directly comparable to that of the intact but aggregated virus. These data are compared with the results from oral infectivity studies using two vector species (C. variipennis and Culicoides nubeculosus), which showed similar infection rates at comparable concentrations of purified cores, or of the intact but aggregated virus particles (NLS was toxic to adult flies). The role of the outer core proteins (VP7) in cell attachment and penetration, as an alternative route of initiation of infection, is discussed. Previous studies (Mertens et al., Virology 157, 375-386, 1987) also showed that the outer capsid layer of BTV can be modified by proteases (including trypsin or chymotrypsin), thereby generating infectious subviral particles (ISVP). The specific infectivity of ISVP for mammalian cells (BHK21 cells) was shown to be similar to that of disaggregated virus particles. In contrast, we report a significantly higher specific infectivity of ISVP but not of the intact virus (approximately x 100) for two insect cell lines (KC cells and C6/36 mosquito cells (derived from Aedes albopictus)). In oral infection studies with adults of the two vector species, ISVP produced the same infection rate at approximately 100-fold lower concentrations than either core particles or the intact but aggregated virus particles. The importance of mammalian host serum proteases, or insect gut proteases, in modification of the intact virus particle to form ISVP and their role in initiation of infection and the vector status of the insect is discussed.

Orbivirus structure and assembly

Orbiviruses (Reoviridae family) are complex nonenveloped RNA viruses with seven structural proteins and a RNA genome consisting of 10 variously sized double-stranded RNA segments. Significant advances in orbivirus research have been made in recent years through the use of gene manipulation techniques coupled with the baculovirus expression system. Several orbivirus proteins have yielded to crystallization and X-ray crystallographic structure determination and, when combined with the three-dimensional image reconstruction of virion particles and cores obtained by cryoelectron microscopy, considerable insight has been gained into the intricate organization and topography of the individual viral components. Formal identification of the sites of interaction has been obtained through protein-protein interaction studies on the components of the virion particle, including those that are involved in capsid assembly. Finally, a beginning of the understanding of the sequence of assembly events has also been obtained.

Topography and immunogenicity of bluetongue virus VP7 epitopes

The core of bluetongue virus (BTV) consists of ten dsRNA viral genome segments and five proteins, including two major (VP7 and VP3) and three minor (VP1, VP4 and VP6) components. The major core protein VP7 is believed to be an important structural constituent because it interacts, not only with the underlying core protein VP3, but also with two outer capsid proteins (VP2 and VP5). In this communication we summarise data on the mapping of at least six different epitopes of VP7 distributed along the molecule. Two of the six epitopes have not been mapped previously. The accessibility of these epitopes in intact virions and core particles was analysed using immunoelectron microscopy. The epitope located near the N-terminus of VP7 was accessible at the surface of intact virions and core particles. Epitopes in other parts of the VP7 molecule were detected weakly in core particles but not in intact virions. These results support the proposal that VP7 molecules are orientated with their N-terminus accessible on the surface of either the particle or at least one of the three different channels observed by cryoelectron microscopy in the outer capsid layer. Analysis of the immune response to BTV-infected or -immunised sheep and rabbits to three selected epitopes, which are located in different regions of the VP7 molecule, demonstrated that all of them were recognised by the animals tested. These results provided further molecular evidence suggesting that VP7 is indeed a major immunogenic antigen ideal for BTV antibody detection.

Crystallization and preliminary X-ray analysis of the core particle of bluetongue virus

Core particles of bluetongue virus serotype 1 (South Africa) have been crystallized. The crystals, which grow up to 0.8 mm in diameter, belong to a primitive orthorhombic space group and have point group symmetry 222. The unit cell dimensions are 754 x 796 x 823 A3 and the crystallographic asymmetric unit contains one-half of a core particle. The best crystals diffract strongly to 4.8 A Bragg spacings, which is the maximum resolution to which we can measure data with the detectors available, suggesting that useful diffraction extends well beyond this. Core particles of serotype 10 have also been crystallized but the crystals have yet to be analyzed by X-ray diffraction.

Characterization and modification of the carboxy-terminal sequences of bluetongue virus type 10 NS1 protein in relation to tubule formation and location of an antigenic epitope in the vicinity of the carboxy terminus of the protein

Bluetongue virus produces large numbers of tubules during infection. The tubules are formed from a 552-amino-acid, 64-kDa NS1 protein encoded by the viral double-stranded RNA segment M6. A series of deletion and extension mutants of bluetongue virus serotype 10 NS1 has been generated and expressed in insect cells in order to identify the carboxy-terminal components of the protein which are important for tubule formation. The mutants AcCT5 and AcCT10, lacking 5 and 10 of the carboxy-terminal residues, respectively, were prepared. By analyzing their abilities to form tubules, it was shown that AcCT5 was capable of this function whereas AcCT10 was not, indicating that the last five amino acids are not strongly involved in NS1 tubule formation. Extension mutants including foreign antigenic sequences involving up to 16 amino acids added to the C terminus of NS1 were shown to form tubules, although an extension of 19 amino acids inhibited tubule formation. Analysis of a panel of monoclonal antibodies has established that an NS1 antigenic site is located near the carboxy terminus of the protein. It appears to be exposed on the surface of tubules. The opportunities to develop new vaccines using recombinant NS1 to deliver foreign epitopes are discussed.

Use of a gene-targeted phage display random epitope library to map an antigenic determinant on the bluetongue virus outer capsid protein VP5

We describe the use of a gene-targeted random epitope library for the mapping of antigenic determinants. A DNA clone encoding the target antigen was digested randomly with DNase I to generate a population of DNA fragments of different sizes and sequences. After size fractionation, small DNA fragments (100-200 bp) were isolated and cloned into the phage expression vector fUSE2 to form an expression library displaying random polypeptide sequences as fusion proteins at the N terminus of the phage gene III protein. This library, termed a gene-targeted random epitope library to distinguish it from totally random synthetic epitope libraries, was then screened by affinity selection for recombinant phages which were specifically bound by the antibody of interest. Using this approach, we have mapped a monoclonal antibody (mAb)-defined epitope on the bluetongue virus outer capsid protein VP5. This epitope is not accessible on the intact virus surface, but is recognised by the immune system of sheep and cattle during virus infection. Although the example given here utilised a DNA fragment of known sequence and the library was screened for a mAb-defined epitope, the strategy described should be equally applicable to genes of unknown sequence and for screening of epitopes using polyclonal antibodies. The approach can also be extended to identify immunodominant epitope from much more complex genome-targeted random epitope library for virus, bacteria and eukaryotic organisms. Other applications of recombinant phages expressing defined immunodominant epitopes include serodiagnosis and vaccine development.

The pathogenesis of bluetongue virus infection of bovine blood cells in vitro: ultrastructural characterization

Cattle are proposed to be reservoir hosts of bluetongue virus (BTV) because infected animals typically have a prolonged cell-associated viremia. Enriched populations of bovine monocytes, erythrocytes and lymphocytes were inoculated with BTV serotype 10 (BTV 10) and the infected cells then were examined by transmission electron microscopy to characterize the interaction of BTV with bovine blood cells. Replication of BTV 10 in monocytes and stimulated (replicating) lymphocytes was morphologically similar to that which occurred in Vero cells, with formation of viral inclusion bodies and virus-specific tubules. In contrast, BTV 10 infection of unstimulated (non-replicating) lymphocytes and erythrocytes did not progress beyond adsorption, after which virus particles persisted in invaginations of the cell membrane. Studies with core particles and neutralizing monoclonal antibodies established that outer capsid protein VP2 is necessary for attachment of BTV 10 to erythrocytes. These in vitro virus-cell interactions provide a cogent explanation for the pathogenesis of BTV infection of cattle, especially the prolonged cell associated viremia that occurs in BTV-infected cattle.

Isolation of bluetongue virus from sheep in Rajasthan State, India

A cytopathic agent was isolated in a baby hamster kidney (BHK)-21 cell-line from blood samples of cross-bred sheep showing typical bluetongue symptoms at Avikanagar in Rajasthan State, India. The cytopathic agent was identified as a bluetongue virus (BTV) by immunofluorescence, the immunoperoxidase test and electron microscopy of BHK-21 cells infected with the new isolate. The new isolate was typed as BTV serotype 1.

Identification of domains in bluetongue virus VP3 molecules essential for the assembly of virus cores

Bluetongue virus (BTV) cores consist of the viral genome and five proteins, including two major components (VP3 and VP7) and three minor components (VP1, VP4, and VP6). VP3 proteins form an inner scaffold for the deposition on the core of the surface layer of VP7. VP3 also encapsidates and interacts with the three minor proteins. The BTV VP3 protein consists of 901 amino acids and has a sequence that is a highly conserved among BTV serotypes and other orbiviruses (e.g., epizootic hemorrhagic disease virus and African horse sickness virus). To locate sites of interaction between VP3 and the other structural proteins, we have analyzed the effects of a number of VP3 deletion mutants representing conserved regions of the protein, using as an assay the formation of core-like particles (CLPs) expressed by recombinant baculoviruses. Five of the VP3 deletion mutants interacted with the coexpressed VP7 and made CLPs. These CLPs also incorporated the three minor proteins. One mutant, lacking VP3 amino acid residues 499 to 508, failed to make CLPs. Further mutational analyses have demonstrated that a methionine at residue 500 of VP3 and an arginine at residue 502 were both required for CLP formation.

Structure of correctly self-assembled bluetongue virus-like particles

Bluetongue virus-like particles (VLPs), synthesized by coexpression of VP2, VP3, VP5, and VP7 using recombinant baculoviruses, have been examined by cryoelectron microscopy and image analysis. The 3-D reconstruction of these VLPs reveals an icosahedral structure 86 nm in diameter with essentially the same features as for the native Bluetongue virus (BTV) particle. The VLP is thus shown to contain the four constituent proteins as the native virus particle, with each of the protein positions highly occupied. Since the BTV core-like particle formed by coexpression of VP3 and VP7 lacks five VP7 trimers around each of the five-fold axes, it appears that the presence of the outer capsid proteins VP2 and VP5 is necessary for the adhesion of these VP7 trimers around the five-fold axes. The observed spontaneous formation of complete VLP in the absence of the BTV nonstructural proteins implies that the nonstructural proteins are not necessary for the formation of the double-shelled viral capsid. However, the nonstructural proteins may be involved in different aspects of genome replication and packaging.

Mutation of either of two cysteine residues or deletion of the amino or carboxy terminus of nonstructural protein NS1 of bluetongue virus abrogates virus-specified tubule formation in insect cells

Virus-specific tubules are characteristic of orbivirus infections and are likely to play an important role in virus morphogenesis. It has been shown that for bluetongue virus (BTV), the prototype orbivirus in the family Reoviridae, the virus-encoded NS1 protein forms tubules in insect cells when the BTV segment M6 gene is expressed by using a baculovirus vector. To understand the function of NS1 tubules and to identify the sequences involved in their polymerization, a series of mutant NS1 genes was generated and expressed in insect cell cultures by using baculovirus vectors. Three of the mutants were deletion mutants. One (AcNS1.dNT10) lacked 10 of the amino-terminal amino acids, and the other two mutants (AcNS1.dCT20 and AcNS1.dCT43) lacked 20 or 43 of the carboxy-terminal amino acids. In addition, site-directed mutants were constructed in which various single cysteines or pairs of cysteines were changed to serines. The ability of each mutant protein to form tubules was investigated. None of the deletion mutants formed tubules. The constructs in which the cysteines at amino acid positions 337 and/or 340 were replaced by serines (e.g., AcNS1.C337S,C340S) also did not form tubules. Instead, the NS1 protein of these and the deletion mutants made ribbon-like structures which formed large aggregates. Mutations involving six other cysteines (i.e., AcNS1.C37S,C43S,AcNS1.C462S,C465S, AcNS1.C104S, and AcNS1.C364S) produced tubules. The results show that both the amino and carboxy termini of the NS1 protein molecule and the cysteines at residues 337 and 340 are essential for tubule formation.

The use of immuno-gold silver staining in bluetongue virus adsorption and neutralisation studies

The immuno-gold-silver staining (IGSS) technique was used in scanning electron microscopy for the detection and semi-quantitation of low copy antigens on the surface of cells. The methodology was exploited in experiments designed to examine the interaction of small numbers of virus particles with the surface of susceptible host cells. Using bluetongue virus (BTV) as an example, IGSS procedures confirmed that maximum adsorption occurred within 60 min and that adsorbed virus particles were distributed randomly on the surface of the cell. Neutralising antibody did not prevent binding of BTV to the plasma membrane, but abrogated virus uptake. The use of IGSS in the study of virus-cell interactions was validated by transmission electron microscopy and classical biochemical experiments utilising radioactively-labelled virus.

Release of bluetongue virus-like particles from insect cells is mediated by BTV nonstructural protein NS3/NS3A

Recombinant baculoviruses and immunoelectron microscopy have been used to demonstrate the association of virus-like particles (VLPs) of bluetongue virus (i.e., particles composed of BTV, VP2, VP3, VP5, and VP7 proteins) and the cell cytoskeleton, as well as the release of such particles from infected cells in the presence of BTV NS3/NS3A, but not when BTV NS1 protein was coexpressed. Examination of cells infected with recombinants that express core-like particles (CLPs) composed of VP3 and VP7 showed that CLPs were present within the soluble fraction of the cells and not associated with the cell cytoskeleton. The simultaneous expression of the nonstructural NS1 or NS3/NS3A proteins with CLPs did not lead to the association of such particles with the cytoskeleton, nor to their release from cells. The failure of CLPs synthesized in the presence of VP2 or VP5 to attach to the cytoskeleton indicated that both outer coat proteins are required for a stable virus-cytoskeleton interaction. The ultrastructural, immunoelectron microscopical, and biochemical examinations, showed conclusively that the presence of NS3/NS3A is required for the budding and subsequent release of VLPs from infected cells.

Characterization of virus inclusion bodies in bluetongue virus-infected cells

A combined qualitative and quantitative approach has been used to examine the role of virus inclusion bodies (VIBs) in the morphogenesis of bluetongue virus (BTV). VIBs were detected as early as 4 h post-infection (p.i.), and their number and profile areas increased significantly between 12 and 16 h, and 20 and 28 h p.i. respectively. Core- and virus-like particles were found within and at the periphery of the VIB matrix, respectively, and their numerical density (number per area of VIB matrix) decreased during the course of infection whereas the numerical density of virus particles in the cytoplasm increased. Virus-like particles had a diameter of 57 +/- 8 nm and core-like particles appeared to fall into two size ranges, 32 +/- 3 nm and 38 +/- 3 nm in diameter. Both pre- and post-embedding immunoelectron microscopy procedures were used to localize BTV structural and non-structural proteins within the VIBs. The VIB matrix was labelled with antibodies to structural proteins VP5 and VP7 and non-structural proteins NS1 and NS2. Cores within VIBs contained proteins VP5, VP7 and NS1 but not VP2. Virus-like particles at the periphery of VIBs contained VP2, VP5, VP7 and NS1. The results suggest that BTV particles are synthesized, assembled and released from the perimeter of VIBs and not from within the matrix. Cores embedded in the VIBs are likely to have been trapped there during expansion of the matrix during replication.

A single point mutation in the VP7 major core protein of bluetongue virus prevents the formation of core-like particles

To understand the assembly process of bluetongue virus (BTV), we have established a functional assay which allows us to produce and manipulate BTV core-like particles (CLPs) composed of the viral VP7 and VP3 proteins. A cDNA clone encoding the 349-amino-acid VP7 protein has been manipulated to generate deletion, extension, and site-specific mutants. Each mutant was coexpressed with the BTV VP3 protein to generate CLPs. Deletion and extension mutants involving the VP7 carboxy terminus prevented CLP formation, while an extension mutant involving an 11-amino-acid rabies virus sequence added to the amino terminus of VP7 allowed CLP formation. Substitution of either of two cysteine residues of VP7 (Cys-15 or Cys-65) by serine also did not prevent CLP formation; however, substitution of the single lysine residue of VP7 (Lys-255) by leucine abrogated CLP formation, indicating a critical role for this lysine.

From genes to complex structures of bluetongue virus and their efficacy as vaccines

Bluetongue virus-like and core-like structures consisting of multiproteins in different molar ratios, have been synthesized using baculovirus multiple expression vectors. These particles lacking genetic materials, mimic the single- and double-shelled authentic virus particles and have been shown to be highly immunogenic and protective for sheep challenged with infectious virus. The formation of virus-like particles, using this new technology, offers a novel approach to vaccine development.

Interactions between bluetongue virus core and capsid proteins translated in vitro

To determine whether the two major core proteins (VP3 and VP7) of bluetongue virus can interact in vitro to form morphological structures, linearized VP3 and VP7 cDNA clones were transcribed using SP6 polymerase and the resultant transcripts were co-translated using rabbit reticulocyte lysates. The structures derived were isolated by sedimentation through a sucrose gradient and found to resemble VP3-VP7 core-like particles (CLPs) expressed in vivo. Reacting CLPs synthesized in vivo with outer capsid proteins translated in vitro (VP2 or VP5) indicated that each outer capsid protein has the capacity to bind to a preformed CLP. This was confirmed by in vivo expression of the appropriate genes using baculovirus vectors. The interaction of VP2 or VP5 with the CLP was analysed by electron microscopy and by using immunogold-labelled monoclonal antibody.

Structure of bluetongue virus particles by cryoelectron microscopy

The structure of the bluetongue virus (BTV) particle, determined by cryoelectron microscopy and image analysis, reveals a well-ordered outer shell which differs markedly from other known Reoviridae. The inner shell is known to have an icosahedral structure with 260 triangular spikes of VP7 trimers arranged on a T = 13,l lattice. The outer shell is seen to consist of 120 globular regions (possibly VP5), which sit neatly on each of the six-membered rings of VP7 trimers. "Sail"-shaped spikes located above 180 of the VP7 trimers form 60 triskelion-type motifs which cover all but 20 of the VP7 trimers. These spikes are possibly the hemagglutinating protein VP2 which contains a virus neutralization epitope. Thus, VP2 and VP5 together form a continuous layer around the inner shell except for holes on the 5-fold axis.

Three-dimensional reconstruction of baculovirus expressed bluetongue virus core-like particles by cryo-electron microscopy

When the viral proteins VP3 and VP7 of bluetongue virus (BTV) are expressed simultaneously in the baculovirus system, core-like particles form spontaneously. The 3-D structure of these core-like particles, determined from cryo-electron micrographs, reveals an icosahedral structure 72.5 nm in diameter with 200 triangular spikes arranged on a T = 13,I lattice; The five spikes around each of the fivefold axes are absent. This is in contrast to the native BTV core particles which have a complete T = 13,I lattice of 260 spikes. The spikes, attributed to VP7 trimers appear as triangular columns 8.0 nm in height with distinct inner and outer domains. The inner shell of the core-like particles, or subcore-like particle, has a T = 1 lattice composed of 60 copies of VP3. The subcore-like particle is noticeably thicker around the fivefold positions. Pores in the subcore-like particle are situated near each of the local sixfold axes, below each six-membered ring of spikes. These pores could allow the passage of metabolites and RNA to and from the core for RNA transcription during infection. It is possible that the synthetic core-like particles have an incomplete complement of VP7 spikes because the ratio of VP7 to VP3 produced in the dual expression system is less than the 13:1 required for complete core-like particles. Only the VP7 spikes which have the strongest affinity for the VP3 inner core and are involved in maintaining the structural integrity of the core-like particle are incorporated. The BTV core-like particle shows greater morphological similarity to the rotavirus than to the reovirus core particle.

Three-dimensional structure of single-shelled bluetongue virus

The three-dimensional structure of single-shelled bluetongue virus has been determined to a resolution of 3 nm by using electron cryomicroscopy and image-processing techniques. The single-shelled virion has a diameter of 69 nm. The three-dimensional structure of the virion has icosahedral symmetry with a triangulation number of 13 in a left-handed configuration. The three-dimensional structure can be described in terms of two concentric layers of density surrounding a central core density. Two distinctive features of the outer layer are the 260 knobby capsomeres located at all the local and strict threefold axes and the aqueous channels located at all the five- and six-coordinated positions. These protrusions extend outward from an inner radius of 28 nm. They are interconnected out to a radius of 30 nm by saddle-shaped densities across the local and strict twofold axes. The aqueous channels surrounded by these capsomeres are about 8 nm wide at the outer surface and 8 nm deep. Some of these channels extend inward, penetrating the inner layer. These channels may provide pathways for transporting the metabolites and mRNA during the transcriptase activity of the particles. The inner layer is a featureless smooth bed of density except for the indentations in register with the channels of the outer layer. We propose that the 260 capsomeres in the outer layer are made up of trimers of the major protein, VP7, and that the inner layer is composed of the second major protein, VP3. The density in the central portion of the structure at a radius of less than 21 nm is likely due to the minor proteins and the genomic RNA.

The detection of intracellular bluetongue virus particles within ovine erythrocytes

1. We report here a simplified method for detecting viruses and other antigenic agents in red blood cells (RBCs). Using a nonionic detergent to prepare cytoskeletons, the interior of RBCs can be scanned rapidly using immunoelectron microscopy. 2. In this study, RBCs from bluetongue (BLU) virus-infected sheep were adsorbed directly onto Formvar-coated, gold electron microscope grids. 3. Cytoskeletons were prepared and then probed using a monoclonal antibody to VP 7, a structural BLU-virus protein and Protein-A gold. 4. Of the ca 32,000 RBCs that were examined from BLU virus-infected sheep, 34 (0.106%) contained labelled BLU virus particles. 5. No labelled particles were observed in any of ca 8000 RBCs taken prior to BLU virus inoculation of sheep. 6. If the antigenic BLU virus particles (which may be viral cores) are in fact infectious, this method of sequestration of virus within RBCs could contribute to the prolonged viremia typical of this arboviral disease, which is known to occur concurrently with circulating neutralizing antibody.

3-D reconstruction of bluetongue virus tubules using cryoelectron microscopy

Bluetongue virus (BTV) forms tubules in infected mammalian cells. These tubules are virally encoded entities which can be formed with only one protein, NS1. The NS1 protein does not form a part of virus particles, and its function in viral infection is uncertain. Expression of the NS1 gene in insect cells by recombinant baculovirus yields high amounts of NS1 tubules (ca. 50% of cellular proteins) which are morphologically and immunologically similar to authentic BTV NS1 and can be isolated to about 90% purity. The structure of these synthetic NS1 tubules was investigated by cryoelectron microscopy. NS1 tubules are on average 52.3 nm in diameter and up to 100 nm long. The structure of their helical surface lattice has been determined using computer image processing to a resolution of 40 A. The NS1 protein is about 5.3 nm in diameter and forms a dimer-like structure, so that the tubules are composed of helically coiled ribbons of NS1 "dimers," with 21 or 22 dimers per turn. The surface lattice displays P2 symmetry and forms a one-start helix with a pitch of 9.1 nm. The NS1 tubules exist in two slightly different pH-dependent conformational states.