Virus-like particles of a fish nodavirus display a capsid subunit domain organization different from that of insect nodaviruses

Assembly and intracellular localization of the bluetongue virus core protein VP3

The bluetongue virus (BTV) core protein VP3 plays a crucial role in the virion assembly and replication process. Although the structure of the protein is well characterized, much less is known about the intracellular processing and localization of the protein in the infected host cell. In BTV-infected cells, newly synthesized viral core particles accumulate in specific locations within the host cell in structures known as virus inclusion bodies (VIBs), which are composed predominantly of the nonstructural protein NS2. However, core protein location in the absence of VIBs remains unclear. In this study, we examined VP3 location and degradation both in the absence of any other viral protein and in the presence of NS2 or the VP3 natural associate protein, VP7. To enable real-time tracking and processing of VP3 within the host cell, a fully functional enhanced green fluorescent protein (EGFP)-VP3 chimera was synthesized, and distribution of the fusion protein was monitored in different cell types using specific markers and inhibitors. In the absence of other BTV proteins, EGFP-VP3 exhibited distinct cytoplasmic focus formation. Further evidence suggested that EGFP-VP3 was targeted to the proteasome of the host cells but was dispersed throughout the cytoplasm when MG132, a specific proteasome inhibitor, was added. However, the distribution of the chimeric EGFP-VP3 protein was altered dramatically when the protein was expressed in the presence of the BTV core protein VP7, a normal partner of VP3 during BTV assembly. Interaction of EGFP-VP3 and VP7 and subsequent assembly of core-like particles was further examined by visualizing fluorescent particles and was confirmed by biochemical analysis and by electron microscopy. These data indicated the correct assembly of EGFP-VP3 subcores, suggesting that core formation could be monitored in real time. When EGFP-VP3 was expressed in BTV-infected BSR cells, the protein was not associated with proteasomes but instead was distributed within the BTV inclusion bodies, where it colocalized with NS2. These findings expand our knowledge about VP3 localization and its fate within the host cell and illustrate the assembly capability of a VP3 molecule with a large amino-terminal extension. This also opens up the possibility of application as a delivery system.

Identification of structural domains involved in astrovirus capsid biology

Coat proteins of non-enveloped, icosahedral viruses must perform a variety of functions during their life cycle such as assembly of the coat protein subunits into a closed shell, specific encapsidation of the viral nucleic acid, maturation of the capsid, interaction with host receptors, and disassembly to deliver the genetic information into the newly infected cell. A thorough understanding of the multiple capsid properties at the molecular level is required in order to identify potential targets for antiviral therapy and the prevention of viral disease. The system we have chosen for study is the astrovirus, a family of icosahedral, single-stranded RNA viruses that cause disease in mammals and birds. Very little is known about what regions of the coat protein contribute to the diverse capsid functions. This review will present novel structural predictions for the coat protein sequence of different astrovirus family members. Based on these predictions, we hypothesize that the assembly and RNA packaging functions of the astrovirus coat protein constitutes an individual domain distinct from the determinants required for receptor binding and internalization. Information derived from these structural predictions will serve as an important tool in designing experiments to understand astrovirus biology.

Early endocytosis pathways in SSN-1 cells infected by dragon grouper nervous necrosis virus

Many fish undergo betanodavirus infection. To study the infection process of dragon grouper nervous necrosis virus (DGNNV), native virus and virus-like particles (VLPs) were used to analyse the binding and internalization in SSN-1 cells. The binding of DGNNV and VLPs to SSN-1 cells was demonstrated using Western blotting and immunofluorescence microscopy. As estimated by indirect ELISA, the DGNNV particles bound SSN-1 cells in a dose-dependent manner up to 8 x 10(4) particles per cell. The binding of VLPs was sensitive to neuraminidase and tunicamycin, suggesting that cell-surface sialic acid is involved in binding. The penetration of DGNNV into cells, which was monitored by electron microscopy, appeared to occur mainly via the spherical pit and membrane ruffling pathways. Occasionally, a spherical pit was engulfed by membrane ruffling so as to form a large figure-of-eight-shaped vesicle with an open connection. Our observations suggest that DGNNV utilizes both micro- and macropinocytosis pathways to enter SSN-1 cells.

The birnavirus crystal structure reveals structural relationships among icosahedral viruses

Double-stranded RNA virions are transcriptionally competent icosahedral particles that must translocate across a lipid bilayer to function within the cytoplasm of the target cell. Birnaviruses are unique among dsRNA viruses as they have a single T = 13 icosahedral shell, lacking the characteristic inner capsid observed in the others. We determined the crystal structures of the T = 1 subviral particle (260 angstroms in diameter) and of the T = 13 intact virus particle (700 angstroms in diameter) of an avian birnavirus to 3 angstroms and 7 angstroms resolution, respectively. Our results show that VP2, the only component of the virus icosahedral capsid, is homologous both to the capsid protein of positive-strand RNA viruses, like the T = 3 nodaviruses, and to the T = 13 capsid protein of members of the Reoviridae family of dsRNA viruses. Together, these results provide important insights into the multiple functions of the birnavirus capsid and reveal unexpected structural relationships among icosahedral viruses.

Genomic classification of new betanodavirus isolates by phylogenetic analysis of the coat protein gene suggests a low host-fish species specificity

Viral encephalopathy and retinopathy is a devastating disease that causes neurological disorders and high mortality in a large number of cultivated marine fish species around the world. It is now established that several viral strains classified in the genus Betanodavirus of the family Nodaviridae are the aetiological agents of this disease. Betanodaviruses can be classified into four genotypes based on the coat protein gene sequence. Here, the coat protein genes of the three major strains isolated from sea bass (Dicentrarchus labrax) in France were found to be different. In addition, 21 novel strains of betanodavirus from several fish species from France, Spain, Tunisia and Tahiti were classified by using phylogenetic analysis of a partial sequence (383 nt) of the coat protein gene. Most of the isolates were grouped in the red-spotted grouper nervous necrosis virus type, which was subdivided into two subtypes, one of them containing only French isolates. Furthermore, an isolate obtained from sea bass during an outbreak at low temperature (15 °C) was classified as the barfin flounder nervous necrosis virus type. This is the first reported isolation from sea bass of such a strain, which is known to infect several cold-water marine fish species. In addition, a betanodavirus belonging to the striped jack nervous necrosis virus type was detected in Senagalese sole (Solea senegalensis) farmed in Spain, which is the first indication of the presence of this genotype outside Japan. These findings suggest that the different genotypes can infect a variety of fish species and thus have a low host-fish species specificity.

Nodavirus infections in Israeli mariculture

Viral encephalopathy and retinopathy (VER) infections were diagnosed in five fish species: Epinephelus aeneus, Dicentrarchus labrax, Sciaenops ocellatus, Lates calcarifer and Mugil cephalus cultured on both the Red Sea and Mediterranean coasts of Israel during 1998-2002. Spongiform vacuolation of nervous tissue was observed in histological sections of all examined species. With transmission electron microscopy, paracrystalline arrays and pieces of membrane-associated non-enveloped virions measuring approximately 30 nm in diameter were observed in the brain and retina of all species. At the molecular level, the nodavirus was detected by using a primer set that amplified the T4 region of the coat protein gene. When the same set of primers was used to search for VER in an additional fish species, Sparus aurata, it was found to produce non-specific amplicons, giving rise to false-positive results. This problem was overcome by using a different primer set (F1/VR3), designed on a highly conserved region of the virus gene, which amplified a fragment of 254 bp, and confirmed that S. aurata was nodavirus-free. This set was validated on all five species of infected fish, as well as clinically healthy fish. Comparison of the coat protein genes from the Israeli isolated sequences indicated that more than one viral strain was involved. No strict host-specificity was evident. Red Sea and Mediterranean isolated sequences grouped in distinct clusters, together with several foreign isolates from the Mediterranean area and the Far East, as phylogenetically close to the Epinephelus akaara RGNNV type.

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 coat protein of Rabbit hemorrhagic disease virus contains a molecular switch at the N-terminal region facing the inner surface of the capsid

To function adequately, many if not all proteins involved in macromolecular assemblies show conformational polymorphism as an intrinsic feature. This general strategy has been described for many essential cellular processes. Here we describe this structural polymorphism in a viral protein, the coat protein of Rabbit hemorrhagic disease virus (RHDV), which is required during virus capsid assembly. By combining genetic, structure modeling, and cryo-electron microscopy and image processing analysis, we have established the mechanism that allows RHDV coat protein to switch among quasi-equivalent conformational states to achieve the appropriate curvature for the formation of a closed shell. The RHDV capsid structure is based on a T = 3 lattice, containing 180 copies of identical subunits, similar to those of other caliciviruses. The quasi-equivalent interactions between the coat proteins are achieved by the N-terminal region of a subset of subunits, which faces the inner surface of the capsid shell. Mutant coat protein lacking this N-terminal sequence assembles into T = 1 capsids. Our results suggest that the polymorphism of the RHDV T = 3 capsid might bear resemblance to that of plant virus T = 3 capsids.

Identification of host-specificity determinants in betanodaviruses by using reassortants between striped jack nervous necrosis virus and sevenband grouper nervous necrosis virus

Betanodaviruses, the causal agents of viral nervous necrosis in marine fish, have bipartite positive-sense RNAs as genomes. The larger genomic segment, RNA1 (3.1 kb), encodes an RNA-dependent RNA polymerase, and the smaller genomic segment, RNA2 (1.4 kb), codes for the coat protein. Betanodaviruses have marked host specificity, although the primary structures of the viral RNAs and encoded proteins are similar among betanodaviruses. However, no mechanism underlying the host specificity has yet been reported. To evaluate viral factors that control host specificity, we first constructed a cDNA-mediated infectious RNA transcription system for sevenband grouper nervous necrosis virus (SGNNV) in addition to that for striped jack nervous necrosis virus (SJNNV), which was previously established by us. We then tested two reassortants between SJNNV and SGNNV for infectivity in the host fish from which they originated. When striped jack and sevenband grouper larvae were bath challenged with the reassortant virus comprising SJNNV RNA1 and SGNNV RNA2, sevenband groupers were killed exclusively, similar to inoculation with SGNNV. Conversely, inoculations with the reassortant virus comprising SGNNV RNA1 and SJNNV RNA2 killed striped jacks but did not affect sevenband groupers. Immunofluorescence microscopic studies using anti-SJNNV polyclonal antibodies revealed that both of the reassortants multiplied in the brains, spinal cords, and retinas of infected fish, similar to infections with parental virus inoculations. These results indicate that viral RNA2 and/or encoded coat protein controls host specificity in SJNNV and SGNNV.

Infection competition against grouper nervous necrosis virus by virus-like particles produced in Escherichia coli

Dragon grouper (Epinephelus lanceolatus) nervous necrosis virus (DGNNV) comprises 180 copies of capsid protein that encapsulate a bipartite genome of single-stranded (+)-RNAs. This study reports that virus-like particles (VLPs) are formed in Escherichia coli expressing the full-length ORF encoding the DGNNV capsid protein. Two sizes of VLPs are observed. The heavier particles resemble the native piscine nodavirus in size and stain permeability, while the lighter ones are approximately two-thirds of the full size. The recombinant VLPs block attachment of native virus to the surface of cultured fish nerve cells, blocking infection by the native virus.