Association between the sigma C protein of avian reovirus and virus-induced fusion of cells

Development and characterization of monoclonal antibodies against avian reovirus σC protein and their application in detection of avian reovirus isolates

Avian reovirus (ARV) is a non-enveloped virus with a segmented double-stranded RNA genome surrounded by a double icosahedral capsid shell. ARVs are associated with viral arthritis, immunosuppression, and enteric diseases in poultry. The sigma C protein was involved in induction of apoptosis and neutralization antibody. In the present study, sigma C-His protein was expressed in Sf9 insect cells and purified by immobilized metal affinity chromatography. Eight monoclonal antibodies (mAbs) against sigma C-His and three mAbs against His were screened from hybridoma cells produced by fusion of splenocytes from immunized mice with NS1 myeloma cells. Among the eight mAbs against sigma C protein, all belonged to the IgG isotype except three for IgM. It was discovered that all anti-His mAbs were mixtures of IgG and IgM isotypes. mAbs reacted with sigma C-His protein in a conformation-independent manner based on dot blot and western blotting assays. The competitive binding assay indicated that all mAbs recognized the same epitope on sigma C protein that was conserved in different isolates. Compared with the commercial anti-ARV S1133 polyclonal antibody, mAb (D15) had universal reactivity to all serotypes or genotypes of ARVs tested. This monoclonal antibody may therefore be useful for the development of an antigen-capture enzyme-linked immunosorbent assay for rapid detection of field isolates.

Muscovy duck reovirus sigmaC protein is atypically encoded by the smallest genome segment

Although muscovy duck reovirus (DRV) shares properties with the reovirus isolated from chicken, commonly named avian reovirus (ARV), the two virus species are antigenically different. Similar to the DRV sigmaB-encoded gene (1201 bp long) previously identified, the three other double-stranded RNA small genome segments of DRV have been cloned and sequenced. They were 1325, 1191 and 1124 bp long, respectively, and contained conserved terminal sequences common to ARVs. They coded for single expression products, except the smallest (S4), which contained two overlapping open reading frames (ORF1 and ORF2). BLAST analyses revealed that the proteins encoded by the 1325 and 1191 bp genes shared high identity levels with ARV sigmaA and sigmaNS, respectively, and to a lesser extent with other orthoreovirus counterparts. No homology was found for the S4 ORF1-encoded p10 protein. The 29.4 kDa product encoded by S4 ORF2 appeared to be 25% identical to ARV S1 ORF3-encoded sigmaC, a cell-attachment oligomer inducing type-specific neutralizing antibodies. Introduction of large gaps in the N-terminal part of the DRV protein was necessary to improve DRV and ARV sigmaC amino acid sequence alignments. However, a leucine zipper motif was conserved and secondary structure analyses predicted a three-stranded alpha-helical coiled-coil feature at this amino portion. Thus, despite extensive sequence divergence, DRV sigmaC was suggested to be structurally and probably functionally related to ARV sigmaC. This work provides evidence for the diversity of the polycistronic S class genes of reoviruses isolated from birds and raises the question of the relative classification of DRV in the Orthoreovirus genus.

Subunit composition and conformational stability of the oligomeric form of the avian reovirus cell-attachment protein sigmaC

Previous work has shown that the avian reovirus cell-attachment sigma C (sigmaC) protein is a multimer. In the first part of this study the oligomerization state of intracellularly synthesized sigmaC was analysed by different approaches, including SDS-PAGE, chemical cross-linking, sedimentation and gel filtration analysis. All these approaches indicated that protein sigmaC in its native state is a homotrimer. In the second part of the present work we investigated the effect of different factors and reagents on oligomer stability, in order to elucidate the nature of the forces that maintain the conformational stability of the homotrimer. Our results, based on the stabilizing effect conferred by reducing agents, demonstrate that the sigmaC subunits are not covalently bound via disulfide linkages. They further suggest that the formation of an intrachain disulfide bond between the two cysteine residues of the sigmaC polypeptide has a negative effect on oligomer stability. The susceptibility of the trimer to pH, temperature, ionic strength, chemical denaturants and detergents indicates that hydrophobic interactions contribute much more to oligomer stability than do ionic interactions and hydrogen bonding. Finally, our results also reveal that mammalian and avian reovirus cell attachment proteins follow different subunit dissociation pathways.

The avian reovirus genome segment S1 is a functionally tricistronic gene that expresses one structural and two nonstructural proteins in infected cells

The avian reovirus S1 gene contains three partially overlapping, out-of-phase open reading frames (ORFs) that the highly conserved in all avian reovirus strains examined to date. The three S1 ORFs of the avian reovirus strain S1133 were individually expressed in bacterial cells, and their purified translation products used as antigens to raise specific polyclonal antibodies. With these antibodies we were able to demonstrate that all three S1 ORFs from different avian reovirus strains are translatable in infected cells. Proteins p10 and p17, which are specified by ORF1 and ORF2, respectively, are nonstructural proteins which associate with cell membranes, whereas ORF3 directs the synthesis of protein sigma C, a structural oligomeric protein responsible for cell attachment. While intracellular synthesis of protein sigma C was demonstrated a long time ago and that of protein p10 was reported recently, this is the first time that expression of the S1 ORF2 has been demonstrated experimentally. Thus, the previously reported coding capacity of the avian reovirus genome is now expanded to 14 proteins, of which ten are structural (lambda A, lambda B, lambda C, microA, microB, microBC, microBN, sigma A, sigma B, and sigma C) and four are nonstructural (microNS, sigma NS, p17, and p10). Finally, protein p10, but not p17 or sigma C, induces cell-cell fusion when transiently expressed in mammalian cells, supporting a previously published observation that the polypeptide encoded by the S1 ORF1 plays an important role in the syncytial phenotype displayed by avian reoviruses.

Oligomerization and cell-binding properties of the avian reovirus cell-attachment protein sigmaC

Avian reovirus protein sigmaC, the viral cell-attachment protein, is a minor component of the outer-capsid shell of the viral particle that is synthesized in small amounts in infected cells. We cloned the sigmaC-encoding ORF in vector pIL-2f, expressed it in Escherichia coli, and partially purified the resulting recombinant protein from inclusion bodies. Rabbit polyclonal antibodies raised against the recombinant protein specifically recognized the viral polypeptide in ELISA, immunoprecipitation, and Western blotting. To study the oligomerization capacity and cell-binding affinity of protein sigmaC, the sigmaC-encoding ORF was also expressed in chicken embryo fibroblasts (CEFs) and in reticulocyte lysates. In all three systems protein sigmaC is expressed as a multimer with identical electrophoretic mobility to the naturally occurring protein. Cell-binding experiments show that both in vitro and in vivo expressed protein sigmaC display affinity for CEF receptors, and this property is exclusively associated with the oligomeric form of the protein. The fact that incubation of CEF cells with the recombinant protein expressed in bacterial cells completely blocks the binding of purified reovirions indicates both that binding of this protein to cells is specific and saturable, and that reovirions and protein sigmaC bind to the same class of cell receptor. Saturation binding experiments, performed with the recombinant protein expressed in E. coli and with purified reovirions, showed that the number of cellular receptor sites (CRSs) for avian reovirus S1133 is 1.8 x 10(4) per CEF cell, whereas the number of cellular receptor units (CRUs) for sigmaC is 2.2 x 10(5) per CEF cell. These results are consistent with previous reports on the binding of mammalian reoviruses.