Cloning, sequence analysis and expression of the major outer capsid protein gene of an aquareovirus

Production of human rotavirus and Salmonella antigens in plants and elicitation of fljB-specific humoral responses in mice

A Nicotiana benthamiana transient expression system was used to express single antigen and dimeric combinations of the human rotavirus (HRV) VP7 and a truncated VP4 (VP4Δ) proteins fused with Salmonella typhimurium's flagellin fljB subunit. Immunoblot analyses using rabbit antibodies generated against these proteins demonstrated that the constructs were successfully expressed with yields ranging from 0.85 to 31.97 μg of recombinant protein per gram of fresh leaf tissue. Expressing the single and dimeric antigens has no effect on plant growth and development except for VP7 and VP4Δ::VP7, which show mild necrotic lesions. Immunization of mice with proteins from leaves transformed with constructs bearing the fljB moiety elicited an fljB-specific humoral response. The Nicotiana benthamiana transient system is efficient to express multiple combinations of pathogen proteins and demonstrates the potential of generating a Salmonella typhimurium subunit vaccine in plants.

Aquareovirus effects syncytiogenesis by using a novel member of the FAST protein family translated from a noncanonical translation start site

As nonenveloped viruses, the aquareoviruses and orthoreoviruses are unusual in their ability to induce cell-cell fusion and syncytium formation. While an extraordinary family of fusion-associated small transmembrane (FAST) proteins is responsible for orthoreovirus syncytiogenesis, the basis for aquareovirus-induced syncytiogenesis is unknown. We now report that the S7 genome segment of an Atlantic salmon reovirus is polycistronic and uses a noncanonical CUG translation start codon to produce a 22-kDa integral membrane protein responsible for syncytiogenesis. The aquareovirus p22 protein represents a fourth distinct member of the FAST family with a unique repertoire and arrangement of structural motifs.

Molecular characterization of a major outer capsid protein encoded by the Threadfin aquareovirus (TFV) gene segment 10 (S10)

Genome segment 10 (S10) of Threadfin aquareovirus (TFV) was cloned, sequenced, analyzed and found to be 987 bp long encoding a protein of 298 aa with a predicted molecular mass of 32.0 kDa. The TFV S10 gene possesses terminal motifs, (5' GTTTTA and ATTCATC 3') which are also conserved in the S6 and S11 TFV gene segments. Sequence comparison revealed that the TFV S10 gene was similar to the Striped bass reovirus (SBR) VP7 outer capsid protein (OCP). A conserved putative zinc-finger motif, CCHC, present in the mammalian reovirus (MRV) delta3 protein, was identified in TFV and other aquareovirus VP7 protein. Phylogenetic analysis of the TFV VP7 protein indicated that TFV is closely related to SBR and Chum salmon reovirus (CSV) and possibly belong to the same species Aquareovirus A as SBR and CSV. The TFV VP7 protein was expressed in E. coli, purified and injected into mice. Serum specific antibodies were generated, however, the serum showed weak neutralizing activity. In contrast, co-incubation of this serum with another serum obtained from mice immunized with another OCP encoded by the TFV S6 gene segment resulted in a highly elevated antibody neutralization titer.

Orthoreovirus and Aquareovirus core proteins: conserved enzymatic surfaces, but not protein-protein interfaces

Orthoreoviruses and Aquareoviruses constitute two respective genera in the family Reoviridae of double-stranded RNA viruses. Orthoreoviruses infect mammals, birds, and reptiles and have a genome comprising 10 RNA segments. Aquareoviruses infect fish and have a genome comprising 11 RNA segments. Despite these differences, recent structural and nucleotide sequence evidence indicate that the proteins of Orthoreoviruses and Aquareoviruses share many similarities. The focus of this review is on the structure and function of the Orthoreovirus core proteins lambda1, lambda2, lambda3, and sigma2, for which X-ray crystal structures have been recently reported. The homologous core proteins in Aquareoviruses are VP3, VP1, VP2, and VP6, respectively. By mapping the locations of conserved residues onto the Orthoreovirus crystal structures, we have found that enzymatic surfaces involved in mRNA synthesis are well conserved between these two groups of viruses, whereas several surfaces involved in protein-protein interactions are not well conserved. Other evidence indicates that the Orthoreovirus mu2 and Aquareovirus VP5 proteins are homologous, suggesting that VP5 is a core protein as mu2 is known to be. These findings provide further evidence that Orthoreoviruses and Aquareoviruses have diverged from a common ancestor and contribute to a growing understanding of the functions of the core proteins in viral mRNA synthesis.

Viruses of lower vertebrates

Viruses of lower vertebrates recently became a field of interest to the public due to increasing epizootics and economic losses of poikilothermic animals. These were reported worldwide from both wildlife and collections of aquatic poikilothermic animals. Several RNA and DNA viruses infecting fish, amphibians and reptiles have been studied intensively during the last 20 years. Many of these viruses induce diseases resulting in important economic losses of lower vertebrates, especially in fish aquaculture. In addition, some of the DNA viruses seem to be emerging pathogens involved in the worldwide decline in wildlife. Irido-, herpes- and polyomavirus infections may be involved in the reduction in the numbers of endangered amphibian and reptile species. In this context the knowledge of several important RNA viruses such as orthomyxo-, paramyxo-, rhabdo-, retro-, corona-, calici-, toga-, picorna-, noda-, reo- and birnaviruses, and DNA viruses such as parvo-, irido-, herpes-, adeno-, polyoma- and poxviruses, is described in this review.

Trypsin-induced structural transformation in aquareovirus

Aquareovirus, a member of the family Reoviridae, is a large virus with multiple capsid layers surrounding a genome composed of 11 segments of double-stranded RNA. Biochemical studies have shown that treatment with the proteolytic agent trypsin significantly alters the infectivity of the virus. The most infectious stage of the virus is produced by a 5-min treatment with trypsin. However, prolonged trypsin treatment almost completely abolishes the infectivity. We have used three-dimensional electron cryomicroscopy to gain insight into the structural basis of protease-induced alterations in infectivity by examining the structural changes in the virion at various time intervals of trypsin treatment. Our data show that after 5 min of trypsinization, projection-like spikes made of VP7 (35 kDa), associated with the underlying trimeric subunits, are completely removed. Concurrent with the removal of VP7, conformational changes are observed in the trimeric subunit composed of putative VP5 (71 kDa). The removal of VP7 and the accompanied structural changes may expose regions in the putative VP5 important for cell entry processes. Prolonged trypsinization not only entirely removes the outer capsid layer, producing the poorly infectious core particle, but also causes significant conformational changes in the turret protein. These changes result in shortening of the turret and narrowing of its central channel. The turret, as in orthoreoviruses, is likely to play a major role in the capping and translocation of mRNA during transcription, and the observed conformational flexibility in the turret protein may have implications in rendering the particle transcriptionally active or inactive.