Biosynthesis and morphogenesis of group C rotavirus in swine testicular cells

Genetic divergence of rotavirus nonstructural protein 4 results in distinct serogroup-specific viroporin activity and intracellular punctate structure morphologies

Nonstructural protein 4 (NSP4) viroporin activity is critical for the replication and assembly of serogroup A rotavirus (RVA); however, the dramatic primary sequence divergence of NSP4s across serogroups raises the possibility that viroporin activity is not a common feature among RVs. We tested for NSP4 viroporin activity from divergent strains, including RVA (EC and Ty-1), RVB (IDIR), and RVC (Cowden). Canonical viroporin motifs were identified in RVA, RVB, and RVC NSP4s, but the arrangement of basic residues and the amphipathic α-helices was substantially different between serogroups. Using Escherichia coli and mammalian cell expression, we showed that each NSP4 tested had viroporin activity, but serogroup-specific viroporin phenotypes were identified. Only mammalian RVA and RVC NSP4s induced BL21-pLysS E. coli cell lysis, a classical viroporin activity assay. In contrast, RVA, RVB, and RVC NSP4 expression was universally cytotoxic to E. coli and disrupted reduction-oxidation activities, as measured by a new redox dye assay. In mammalian cells, RVB and RVC NSP4s were initially localized in the endoplasmic reticulum (ER) and trafficked into punctate structures that were mutually exclusive with RVA NSP4. The punctate structures partially localized to the ER-Golgi intermediate compartment (ERGIC) but primarily colocalized with punctate LC3, a marker for autophagosomes. Similar to RVA NSP4, expression of RVB and RVC NSP4s significantly elevated cytosolic calcium levels, demonstrating that despite strong primary sequence divergence, RV NSP4 has maintained viroporin activity across serogroups A to C. These data suggest that elevated cytosolic calcium is a common critical process for all rotavirus strains.

Efficient translation of rotavirus mRNA requires simultaneous interaction of NSP3 with the eukaryotic translation initiation factor eIF4G and the mRNA 3' end

In contrast to the vast majority of cellular proteins, rotavirus proteins are translated from capped but nonpolyadenylated mRNAs. The viral nonstructural protein NSP3 specifically binds the 3'-end consensus sequence of viral mRNAs and interacts with the eukaryotic translation initiation factor eIF4G. Here we show that expression of NSP3 in mammalian cells allows the efficient translation of virus-like mRNA. A synergistic effect between the cap structure and the 3' end of rotavirus mRNA was observed in NSP3-expressing cells. The enhancement of viral mRNA translation by NSP3 was also observed in a rabbit reticulocyte lysate translation system supplemented with recombinant NSP3. The use of NSP3 mutants indicates that its RNA- and eIF4G-binding domains are both required to enhance the translation of viral mRNA. The results reported here show that NSP3 forms a link between viral mRNA and the cellular translation machinery and hence is a functional analogue of cellular poly(A)-binding protein.

Antibody prevalence and specificity to group C rotavirus in Swedish sera

Of 160 sera collected from different age groups throughout Sweden, 38% were found to be antibody positive for group C rotavirus. The highest antibody prevalence rate was found in individuals aged 11-30 years (45%). An immunoprecipitation assay revealed that the antibodies were directed against VP2, VP4, VP6, VP7, and NSP2, with VP6 being the most immunogenic protein. Neutralising antibodies against a cultivable porcine group C rotavirus (strain AmC-1/Cowden) were detected in 16/19 individuals at titres from 160 to 5,120. The results indicate that group C rotavirus infections are relatively common in older Swedish children and adults but appear to be less common in children younger than 5 years of age. It is concluded that porcine and human group C rotaviruses share epitopes critical for stimulation of neutralising antibodies.

Assembly of viroplasm and virus-like particles of rotavirus by a Semliki Forest virus replicon

In this study we have used an expression system based on Semliki Forest virus (SFV) to study assembly and intracellular localization of certain capsid proteins of rotavirus in neurons and mammalian epithelial cells. The complete genes of vp2 (vp2A) and vp6 (vp6A) of group A rotavirus (SA-11) and gene 5 encoding vp6 (vp6C) of porcine group C rotavirus (strain Cowden/AmC-1) were inserted into an SFV expression replicon. Transfection of BHK-21 cells with in vitro-made SFV transcripts resulted in a high level of expression of the heterologous genes. Cotransfection with helper RNA encoding the SFV structural proteins, but lacking the genomic RNA packing signal, resulted in production of recombinant infectious virus. Immunological and biochemical analysis revealed that vp6 was expressed to high levels in primary neurons and mammalian epithelial cells and that vp6 was retained as an authentic homotrimer, stabilized by noncovalent interactions with native antigenic determinants. Thin section electron microscopy analysis revealed that vp6 alone assembled into viroplasm-like structures in the cytoplasm. While coexpression of vp2 and vp6 of group A rotavirus resulted in formation of single-shelled-like particles, no evidence of intracellular assembly was found, suggesting that other viral proteins are required for intracellular formation of single-shelled particles. A notable observation was that the vp6 proteins of group A and C rotaviruses showed different immunofluorescence patterns in BHK-21 cells; vp6C displayed an intense punctate immunofluorescence pattern, while vp6A was characterized by a pronounced filamentous staining in close vicinity to the cytoskeleton.

Molecular characterization of the 11th RNA segment from human group C rotavirus

The complete nucleotide sequence of genome segment 11 from the noncultivatable, human group C rotavirus (Bristol strain) was determined. Comparison of the nucleotide sequence of the segment termini with the consensus 5' and 3' terminal noncoding sequences of the human group C rotavirus genome revealed characteristic 5' and 3' sequences. Human group C rotavirus genome segment 11 is 613 bp long and encodes a single open reading frame of 450 nucleotides (150 amino acids) starting at nucleotide 39 and terminating at nucleotide 489, leaving a long 3' untranslated region of 124 nucleotides. The predicted translation product has a calculated molecular weight of 17.7 kD and contains four potential N-linked glycosylation sites. No significant homologies to other viral proteins were found in database searches. Hydropathy analysis predicted the human group C rotavirus genome segment 11 translation product has a hydrophilic carboxy terminus (amino acids 54-150) and a hydrophobic amino terminus (amino acids 1-53) that can be further subdivided into three short hydrophobic sequences--H1, H2, and H3. These features are analogous to the integral membrane glycoprotein NSP4 encoded by group A rotavirus gene 10.