Expression of the major capsid protein VP6 of group C rotavirus and synthesis of chimeric single-shelled particles by using recombinant baculoviruses

Co-administration of rotavirus nanospheres VP6 and NSP4 proteins enhanced the anti-NSP4 humoral responses in immunized mice

Inconveniences associated with the efficacy and safety of the World Health Organization (WHO) approved/prequalified live attenuated rotavirus (RV) vaccines, sounded for finding alternative non-replicating modals and proper RV antigens (Ags). Herein, we report the development of a RV candidate vaccine based on the combination of RV VP6 nanospheres (S) and NSP4_112-175 proteins (VP6S + NSP4). Self-assembled VP6S protein was produced in insect cells. Analyses by western blotting and transmission electron microscopy (TEM) indicated expression of VP6 trimer structures with sizes of ≥140 kDa and presence of VP6S. Four group of mice were immunized (2-dose formulation) intra-peritoneally (IP) by either¨VP6S + NSP4¨ or each protein alone (VP6S or NSP4_112-175) emulsified in aluminium hydroxide or control. Results indicated that VP6S + NSP4 formulation induced significant anti-VP6 IgG (P < 0.001) and IgA (P < 0.05) as well as anti-NSP4 IgG (P < 0.001) and enhancement of protective immunity. Analyses of anti-VP6S and anti-NSP4 IgG subclass (IgG1 and IgG2a) showed IgG1/IgG2a ≥6 and IgG1/IgG2a ≥3 ratios, respectively indicating Th2 polarization of immune responses. The combination of VP6S + NSP4 proteins emulsified in aluminum hydroxide adjuvant might present a dual universal, efficient and cost-effective candidate vaccine against RV infection.

Whole Genome In-Silico Analysis of South African G1P[8] Rotavirus Strains Before and After Vaccine Introduction Over A Period of 14 Years

Rotavirus G1P[8] strains account for more than half of the group A rotavirus (RVA) infections in children under five years of age, globally. A total of 103 stool samples previously characterized as G1P[8] and collected seven years before and seven years after introducing the Rotarix® vaccine in South Africa were processed for whole-genome sequencing. All the strains analyzed had a Wa-like constellation (G1-P[8]-I1-R1-C1-M1-A1-N1-T1-E1-H1). South African pre- and post-vaccine G1 strains were clustered in G1 lineage-I and II while the majority (84.2%) of the P[8] strains were grouped in P[8] lineage-III. Several amino acid sites across ten gene segments with the exception of VP7 were under positive selective pressure. Except for the N147D substitution in the antigenic site of eight post-vaccine G1 strains when compared to both Rotarix® and pre-vaccine strains, most of the amino acid substitutions in the antigenic regions of post-vaccine G1P[8] strains were already present during the pre-vaccine period. Therefore, Rotarix® did not appear to have an impact on the amino acid differences in the antigenic regions of South African post-vaccine G1P[8] strains. However, continued whole-genome surveillance of RVA strains to decipher genetic changes in the post-vaccine period remains imperative.

Recombinant Rotaviruses Rescued by Reverse Genetics Reveal the Role of NSP5 Hyperphosphorylation in the Assembly of Viral Factories

The rotavirus (RV) double-stranded RNA genome is replicated and packaged into virus progeny in cytoplasmic structures termed viroplasms. The nonstructural protein NSP5, which undergoes a complex hyperphosphorylation process during RV infection, is required for the formation of these virus-induced organelles. However, its roles in viroplasm formation and RV replication have never been directly assessed due to the lack of a fully tractable reverse-genetics (RG) system for rotaviruses. Here, we show a novel application of a recently developed RG system by establishing a stable trans-complementing NSP5-producing cell line required to rescue rotaviruses with mutations in NSP5. This approach allowed us to provide the first direct evidence of the pivotal role of this protein during RV replication. Furthermore, using recombinant RV mutants, we shed light on the molecular mechanism of NSP5 hyperphosphorylation during infection and its involvement in the assembly and maturation of replication-competent viroplasms.

Rotavirus (RV) replicates in round-shaped cytoplasmic viral factories, although how they assemble remains unknown. During RV infection, NSP5 undergoes hyperphosphorylation, which is primed by the phosphorylation of a single serine residue. The role of this posttranslational modification in the formation of viroplasms and its impact on virus replication remain obscure. Here, we investigated the role of NSP5 during RV infection by taking advantage of a modified fully tractable reverse-genetics system. A trans-complementing cell line stably producing NSP5 was used to generate and characterize several recombinant rotaviruses (rRVs) with mutations in NSP5. We demonstrate that an rRV lacking NSP5 was completely unable to assemble viroplasms and to replicate, confirming its pivotal role in rotavirus replication. A number of mutants with impaired NSP5 phosphorylation were generated to further interrogate the function of this posttranslational modification in the assembly of replication-competent viroplasms. We showed that the rRV mutant strains exhibited impaired viral replication and the ability to assemble round-shaped viroplasms in MA104 cells. Furthermore, we investigated the mechanism of NSP5 hyperphosphorylation during RV infection using NSP5 phosphorylation-negative rRV strains, as well as MA104-derived stable transfectant cell lines expressing either wild-type NSP5 or selected NSP5 deletion mutants. Our results indicate that NSP5 hyperphosphorylation is a crucial step for the assembly of round-shaped viroplasms, highlighting the key role of the C-terminal tail of NSP5 in the formation of replication-competent viral factories. Such a complex NSP5 phosphorylation cascade may serve as a paradigm for the assembly of functional viral factories in other RNA viruses.

IMPORTANCE The rotavirus (RV) double-stranded RNA genome is replicated and packaged into virus progeny in cytoplasmic structures termed viroplasms. The nonstructural protein NSP5, which undergoes a complex hyperphosphorylation process during RV infection, is required for the formation of these virus-induced organelles. However, its roles in viroplasm formation and RV replication have never been directly assessed due to the lack of a fully tractable reverse-genetics (RG) system for rotaviruses. Here, we show a novel application of a recently developed RG system by establishing a stable trans-complementing NSP5-producing cell line required to rescue rotaviruses with mutations in NSP5. This approach allowed us to provide the first direct evidence of the pivotal role of this protein during RV replication. Furthermore, using recombinant RV mutants, we shed light on the molecular mechanism of NSP5 hyperphosphorylation during infection and its involvement in the assembly and maturation of replication-competent viroplasms.

Identification of a Small Molecule That Compromises the Structural Integrity of Viroplasms and Rotavirus Double-Layered Particles

Despite the availability of two attenuated vaccines, rotavirus (RV) gastroenteritis remains an important cause of mortality among children in developing countries, causing about 215,000 infant deaths annually. Currently, there are no specific antiviral therapies available. RV is a nonenveloped virus with a segmented double-stranded RNA genome. Viral genome replication and assembly of transcriptionally active double-layered particles (DLPs) take place in cytoplasmic viral structures called viroplasms. In this study, we describe strong impairment of the early stages of RV replication induced by a small molecule known as an RNA polymerase III inhibitor, ML-60218 (ML). This compound was found to disrupt already assembled viroplasms and to hamper the formation of new ones without the need for de novo transcription of cellular RNAs. This phenotype was correlated with a reduction in accumulated viral proteins and newly made viral genome segments, disappearance of the hyperphosphorylated isoforms of the viroplasm-resident protein NSP5, and inhibition of infectious progeny virus production. In in vitro transcription assays with purified DLPs, ML showed dose-dependent inhibitory activity, indicating the viral nature of its target. ML was found to interfere with the formation of higher-order structures of VP6, the protein forming the DLP outer layer, without compromising its ability to trimerize. Electron microscopy of ML-treated DLPs showed dose-dependent structural damage. Our data suggest that interactions between VP6 trimers are essential, not only for DLP stability, but also for the structural integrity of viroplasms in infected cells.IMPORTANCE Rotavirus gastroenteritis is responsible for a large number of infant deaths in developing countries. Unfortunately, in the countries where effective vaccines are urgently needed, the efficacy of the available vaccines is particularly low. Therefore, the development of antivirals is an important goal, as they might complement the available vaccines or represent an alternative option. Moreover, they may be decisive in fighting the acute phase of infection. This work describes the inhibitory effect on rotavirus replication of a small molecule initially reported as an RNA polymerase III inhibitor. The molecule is the first chemical compound identified that is able to disrupt viroplasms, the viral replication machinery, and to compromise the stability of DLPs by targeting the viral protein VP6. This molecule thus represents a starting point in the development of more potent and less cytotoxic compounds against rotavirus infection.

Modeling of the rotavirus group C capsid predicts a surface topology distinct from other rotavirus species

Rotavirus C (RVC) causes sporadic gastroenteritis in adults and is an established enteric pathogen of swine. Because RVC strains grow poorly in cell culture, which hinders generation of virion-derived RVC triple-layered-particle (TLP) structures, we used the known Rotavirus A (RVA) capsid structure to model the human RVC (Bristol) capsid. Comparative analysis of RVA and RVC capsid proteins showed major differences at the VP7 layer, an important target region for vaccine development due to its antigenic properties. Our model predicted the presence of a surface extended loop in RVC, which could form a major antigenic site on the capsid. We analyzed variations in the glycosylation patterns among RV capsids and identified group specific conserved sites. In addition, our results showed a smaller RVC VP4 foot, which protrudes toward the intermediate VP6 layer, in comparison to that of RVA. Finally, our results showed major structural differences at the VP8* glycan recognition sites.

Formation of self-assembled triple-layered rotavirus-like particles (tlRLPs) by constitutive co-expression of VP2, VP6, and VP7 in stably transfected high-five insect cell lines

In this study, stable high-five insect cell line constitutively expressing rotavirus (RV) VP2 was co-transfected with VP6 and VP7-recombinant plasmids. The presence of RV proteins in stably transfected high-five cells was verified by molecular and protein analyses. To yield self-assembled triple-layered RV-like particles (tlRLPs), a stable insect high-five cell line was generated to produce RV VP6 and VP7 besides VP2. Self-assembled tlRLPs were observed by transmission electron microscopy (TEM), and enzyme-linked immunosorbent assay (ELISA) was used to assess their antigenicity in vivo. The results suggest that the stable transfected high-five cells are able to generate tlRLPs with the efficient antigenicity.

On the effect of thermodynamic equilibrium on the assembly efficiency of complex multi-layered virus-like particles (VLP): the case of rotavirus VLP

Previous studies have reported the production of malformed virus-like-particles (VLP) in recombinant host systems. Here we computationally investigate the case of a large triple-layered rotavirus VLP (RLP). In vitro assembly, disassembly and reassembly data provides strong evidence of microscopic reversibility of RLP assembly. Light scattering experimental data also evidences a slow and reversible assembly untypical of kinetic traps, thus further strengthening the fidelity of a thermodynamically controlled assembly. In silico analysis further reveals that under favourable conditions particles distribution is dominated by structural subunits and completely built icosahedra, while other intermediates are present only at residual concentrations. Except for harshly unfavourable conditions, assembly yield is maximised when proteins are provided in the same VLP protein mass composition. The assembly yield decreases abruptly due to thermodynamic equilibrium when the VLP protein mass composition is not obeyed. The latter effect is more pronounced the higher the Gibbs free energy of subunit association is and the more complex the particle is. Overall this study shows that the correct formation of complex multi-layered VLPs is restricted to a narrow range of association energies and protein concentrations, thus the choice of the host system is critical for successful assembly. Likewise, the dynamic control of intracellular protein expression rates becomes very important to minimize wasted proteins.

Shared and group-specific features of the rotavirus RNA polymerase reveal potential determinants of gene reassortment restriction

Rotaviruses (RVs) are nonenveloped, 11-segmented, double-stranded RNA viruses that are major pathogens associated with acute gastroenteritis. Group A, B, and C RVs have been isolated from humans; however, intergroup gene reassortment does not occur for reasons that remain unclear. This restriction might reflect the failure of the viral RNA-dependent RNA polymerase (RdRp; VP1) to recognize and replicate the RNA of a different group. To address this possibility, we contrasted the sequences, structures, and functions of RdRps belonging to RV groups A, B, and C (A-VP1, B-VP1, and C-VP1, respectively). We found that conserved amino acid residues are located within the hollow center of VP1 near the active site, whereas variable, group-specific residues are mostly surface exposed. By creating a three-dimensional homology model of C-VP1 with the A-VP1 crystallographic data, we provide evidence that these RV RdRps are nearly identical in their tertiary folds and that they have the same RNA template recognition mechanism that differs from that of B-VP1. Consistent with the structural data, recombinant A-VP1 and C-VP1 are capable of replicating one another's RNA templates in vitro. Nonetheless, the activity of both RdRps is strictly dependent upon the presence of cognate RV core shell protein A-VP2 or C-VP2, respectively. Together, the results of this study provide unprecedented insight into the structure and function of RV RdRps and support the notion that VP1 interactions may influence the emergence of reassortant viral strains.

Parenteral administration of RF 8-2/6/7 rotavirus-like particles in a one-dose regimen induce protective immunity in mice

Rotavirus virus-like particles (RV-VLPs) represent a novel strategy for development of a rotavirus subunit vaccine. In this study, RF 8-2/6/7-VLPs with rotavirus VP8 protein (amino acid 1-241 of VP4) fused to the amino terminal end of a truncated VP2, were evaluated for their immunogenic and protective properties. A single intramuscular dose of, either 2 or 20 microg, RF 8-2/6/7-VLPs alone or combined with a potent adjuvant poly[di(carboxylatophenoxy)]phosphazene] (PCPP) induced rotavirus-specific serum IgG and IgA, fecal IgG titers that were enhanced 5-90-fold by adjuvant. Passive protective immunity was achieved in offspring to dams vaccinated with 2 and 20 microg RV-VLPs in presence of adjuvant and 20 microg RV-VLP without adjuvant.

Geometric mismatches within the concentric layers of rotavirus particles: a potential regulatory switch of viral particle transcription activity

Rotaviruses are prototypical double-stranded RNA viruses whose triple-layered icosahedral capsid constitutes transcriptional machinery activated by the release of the external layer. To understand the molecular basis of this activation, we studied the structural interplay between the three capsid layers by electron cryo-microscopy and digital image processing. Two viral particles and four virus-like particles containing various combinations of inner (VP2)-, middle (VP6)-, and outer (VP7)-layer proteins were studied. We observed that the absence of the VP2 layer increases the particle diameter and changes the type of quasi-equivalent icosahedral symmetry, as described by the shift in triangulation number (T) of the VP6 layer (from T = 13 to T = 19 or more). By fitting X-ray models of VP6 into each reconstruction, we determined the quasi-atomic structures of the middle layers. These models showed that the VP6 lattices, i.e., curvature and trimer contacts, are characteristic of the particle composition. The different functional states of VP6 thus appear as being characterized by trimers having similar conformations but establishing different intertrimeric contacts. Remarkably, the external protein VP7 reorients the VP6 trimers located around the fivefold axes of the icosahedral capsid, thereby shrinking the channel through which mRNA exits the transcribing rotavirus particle. We conclude that the constraints arising from the different geometries imposed by the external and internal layers of the rotavirus capsid constitute a potential switch regulating the transcription activity of the viral particles.

Modeling rotavirus-like particles production in a baculovirus expression vector system: Infection kinetics, baculovirus DNA replication, mRNA synthesis and protein production

Rotavirus is the most common cause of severe diarrhoea in children worldwide, responsible for more than half a million deaths in children per year. Rotavirus-like particles (Rota VLPs) are excellent vaccine candidates against rotavirus infection, since they are non-infectious, highly immunogenic, amenable to large-scale production and safer to produce than those based on attenuated viruses. This work focuses on the analysis and modeling of the major events taking place inside Spodoptera frugiperda (Sf-9) cells infected by recombinant baculovirus that may be critical for the expression of rotavirus viral proteins (VPs). For model validation, experiments were performed adopting either a co-infection strategy, using three monocistronic recombinant baculovirus each one coding for viral proteins VP(2), VP(6) and VP(7), or single-infection strategies using a multigene baculovirus coding for the three proteins of interest. A characteristic viral DNA (vDNA) replication rate of 0.19+/-0.01 h(-1) was obtained irrespective of the monocistronic or multigene vector employed, and synthesis of progeny virus was found to be negligible in comparison to intracellular vDNA concentrations. The timeframe for vDNA, mRNA and VP synthesis tends to decrease with increasing multiplicity of infection (MOI) due to the metabolic burden effect. The protein synthesis rates could be ranked according to the gene size in the multigene experiments but not in the co-infection experiments. The model exhibits acceptable prediction power of the dynamics of intracellular vDNA replication, mRNA synthesis and VP production for the three proteins involved. This model is intended to be the basis for future Rota VLPs process optimisation and also a means to evaluating different baculovirus constructs for Rota VLPs production.

Production of rotavirus-like particles in tomato (Lycopersicon esculentum L.) fruit by expression of capsid proteins VP2 and VP6 and immunological studies

A number of different antigens have been successfully expressed in transgenic plants, and some are currently being evaluated as orally delivered vaccines. Here we report the successful expression of rotavirus capsid proteins VP2 and VP6 in fruits of transgenic tomato plants. By western blot analysis, using specific antibodies, we determined that the VP2 and VP6 produced in plants have molecular weights similar to those found in native rotavirus. The plant-synthesized VP6 protein retained the capacity to form trimers. We were able to recover rotavirus virus-like particles from tomato fruit (i.e., tomatoes) by centrifugation on a sucrose cushion and to visualize them by electron microscopy. This result indicated that VP2/VP6 can self-assemble into virus-like particles (VLPs) in plant cells, even though only a small proportion of VP2/VP6 assembled into VLPs. To investigate immunogenicity, adult mice were immunized intraperitoneally (i.p.) three times with a protein extract from a transgenic tomatoes in adjuvant. We found that the transgenic tomato extract induced detectable levels of anti-rotavirus antibodies in serum; however, we did not determine the contribution of either the free rotavirus proteins or the VLPs to the induction of the antibody response. These results suggest the potential of plant-based rotavirus VLPs for the development of a vaccine against rotavirus infection.

Intrarectal immunization with rotavirus 2/6 virus-like particles induces an antirotavirus immune response localized in the intestinal mucosa and protects against rotavirus infection in mice

Rotavirus (RV) is the main etiological agent of severe gastroenteritis in infants, and vaccination seems the most effective way to control the disease. Recombinant rotavirus-like particles composed of the viral protein 6 (VP6) and VP2 (2/6-VLPs) have been reported to induce protective immunity in mice when administered by the intranasal (i.n.) route. In this study, we show that administration of 2/6-VLPs by the intrarectal (i.r.) route together with either cholera toxin (CT) or a CpG-containing oligodeoxynucleotide as the adjuvant protects adult mice against RV infection. Moreover, when CT is used, RV shedding in animals immunized by the i.r. route is even reduced in comparison with that in animals immunized by the i.n. route. Humoral and cellular immune responses induced by these immunization protocols were analyzed. We found that although i.r. immunization with 2/6-VLPs induces lower RV-specific immunoglobulin G (IgG) and IgA levels in serum, intestinal anti-RV IgA production is higher in mice immunized by the i.r. route. Cellular immune response has been evaluated by measuring cytokine production by spleen and Peyer's patch cells (PPs) after ex vivo restimulation with RV. Mice immunized by the i.n. and i.r. routes display higher gamma interferon production in spleen and PPs, respectively. In conclusion, we demonstrate that i.r. immunization with 2/6-VLPs protects against RV infection in mice and is more efficient than i.n. immunization in inducing an anti-RV immune response in intestinal mucosa.

Production of Two Vaccinating Recombinant Rotavirus Proteins in the Milk of Transgenic Rabbits

Rotaviruses are the main cause of infantile viral gastroenteritis worldwide leading to approximately 500,000 deaths each year mostly in the developing world. For unknown reasons, live attenuated viruses used in classical vaccine strategies were shown to be responsible for intussusception (a bowel obstruction). New strategies allowing production of safe recombinant non-replicating rotavirus candidate vaccine are thus clearly needed. In this study we utilized transgenic rabbit milk as a source of rotavirus antigens. Individual transgenic rabbit lines were able to produce several hundreds of micrograms per ml of secreted recombinant VP2 and VP6 proteins in their milk. Viral proteins expressed in our model were immunogenic and were shown to induce a significant reduction in viral antigen shedding after challenge with virulent rotavirus in the adult mouse model. To our knowledge, this is the first report of transgenic mammal bioreactors allowing the rapid co-production of two recombinant viral proteins in milk to be used as a vaccine.

Molecular characterization of the major capsid protein VP6 of bovine group B rotavirus and its use in seroepidemiology

The major inner capsid protein (VP6) gene of the bovine group B rotavirus (GBR) Nemuro strain is 1269 nt in length and contains one open reading frame encoding 391 aa. Nucleotide and amino acid sequence identities of the Nemuro VP6 gene compared with the published corresponding human and rodent GBR genes were respectively 66–67 and 70–72 %, which are notably lower than those between human and rodent viruses (72–73 and 83–84 %, respectively). Overall identities of VP6 genes among GBRs were substantially lower than those among both group A rotaviruses (GARs) and group C rotaviruses (GCRs) derived from different species of mammals. These results demonstrate that bovine GBR is remarkably distinct from other GBRs and that GBRs from different species may have had a longer period of divergence than GARs and GCRs. Recombinant VP6 was generated with a baculovirus expression system and used for an ELISA to detect GBR antibodies. All 13 paired sera from adult cows with GBR-induced diarrhoea in the field showed antibody responses in the ELISA. In serological surveys of GBR infection using the ELISA, 47 % of cattle sera were positive for GBR antibodies, with a higher antibody prevalence in adults than in young cattle. In pigs, a high prevalence of GBR antibodies (97 %) was detected in sera from sows. These results suggest that GBR infection is common in cattle and pigs, notwithstanding the scarcity of reports of GBR detection in these species to date.

Transcapsidation and the conserved interactions of two major structural proteins of a pair of phytoreoviruses confirm the mechanism of assembly of the outer capsid layer

The strongly conserved amino acid sequences of the P8 outer capsid proteins of Rice dwarf virus (RDV) and Rice gall dwarf virus (RGDV) and the distribution of electrostatic potential on the proteins at the interfaces between structural proteins suggested the possibility that P8-trimers of RGDV might bind to the 3-fold symmetrical axes of RDV core particles, with vertical interaction between heterologous P3 and P8 proteins and lateral binding of homologous P8 proteins, thereby allowing formation of the double-layered capsids that are characteristic of viruses that belong to the family Reoviridae. We proved this hypothesis using chimeric virus-like particles composed of the P3 core capsid protein of RDV and the P8 outer capsid protein of RGDV, which were co-expressed in a baculovirus expression system. This is the first report on the molecular biological proof of the mechanism of the assembly of the double-layered capsids with disparate icosahedral lattices.

Virus-like particles: models for assembly studies and foreign epitope carriers

Virus‐like particles (VLPs), formed by the structural elements of viruses, have received considerable attention over the past two decades. The number of reports on newly obtained VLPs has grown proportionally with the systems developed for the expression of these particles. The chapter outlines the recent achievements in two important fields of research brought about by the availability of VLPs produced in a foreign host. These are: (1) The requirements for VLP assembly and (2) the use of VLPs as carriers for foreign epitopes. VLP technology is a rapidly advancing domain of molecular and structural biology. Extensive progress in VLP studies was achieved as the insect cell based protein production system was developed. This baculovirus expression system has many advantages for the synthesis of viral structural proteins resulting in the formation of VLPs. It allows production of large amounts of correctly folded proteins while also providing cell membranes that can serve as structural elements for enveloped viruses. These features give us the opportunity to gain insights into the interactions and requirements accompanying VLP formation that are similar to the assembly events occurring in mammalian cells. Other encouraging elements are the ability to easily scale up the system and the simplicity of purification of the assembled VLPs. The growing number of VLPs carrying foreign protein fragments on their surface and studies on the successful assembly of these chimeric molecules is a promising avenue towards the development of a new technology, in which the newly designed VLPs will be directed to particular mammalian cell types by exposing specific binding domains. The progress made in modeling the surface of VLPs makes them to date the best candidates for the design of delivery systems that can efficiently reach their targets.

A zinc ion controls assembly and stability of the major capsid protein of rotavirus

The recent determination of the crystal structure of VP6, the major capsid protein of rotavirus, revealed a trimer containing a central zinc ion coordinated by histidine 153 from each of the three subunits. The role of the zinc ion in the functions of VP6 was investigated by site-directed mutagenesis. The mutation of histidine 153 into a serine (H153S and H153S/S339H) did not prevent the formation of VP6 trimers. At pH <7.0, about the pK of histidine, wild-type and mutated VP6 proteins display similar properties, giving rise to identical tubular and spherical assemblies. However, at pH >7.0, histidine 153 mutant proteins did not assemble into the characteristic 45-nm-diameter tubes, in contrast to wild-type VP6. These observations showed that under conditions in which histidine residues are not charged, the properties of VP6 depended on the presence of the centrally coordinated zinc atom in the trimer. Indeed, wild-type VP6 depleted of the zinc ion by a high concentration (100 mM) of a metal-chelating agent behaved like the H153 mutant proteins. The susceptibility of wild-type VP6 to proteases is greatly increased in the absence of zinc. NH(2)-terminal sequencing of the proteolytic fragments showed that they all contained the beta-sheet-rich VP6 head domain, which appeared to be less sensitive to protease activity than the alpha-helical basal domain. Finally, the mutant proteins assembled well on cores, as demonstrated by both electron microscopy and rescue of transcriptase activity. Zinc is thus not necessary for the transcription activity. All of these observations suggest that, in solution, VP6 trimers present a structural flexibility that is controlled by the presence of a zinc ion.

Production of hybrid double- or triple-layered virus-like particles of group A and C rotaviruses using a baculovirus expression system

Dual infections by group A and group C rotaviruses have been reported, but no reassortants between group A and group C rotaviruses have been described. The VP6 major inner capsid protein of group A and C rotaviruses shares common antigens detected by monoclonal antibodies and also shares 40-43% amino acid identity. Coinfection of Spodoptera frugiperda (Sf9) insect cells with different combinations of the recombinant baculoviruses encoding either group A [RF VP2 (A-VP2), IND VP6 (A-VP6), and VP7 (A-VP7[IND]), 2292B VP7 (A-VP7[2292B])] or C [Shintoku VP6 (C-VP6) and VP7 (C-VP7)] bovine rotavirus proteins produced hybrid group A/C triple-layered VP2/6/7 virus-like particles (TLPs) composed of A-VP2/C-VP6/C-VP7, A-VP2/C-VP6/A-VP7(IND), A-VP2/C-VP6/A-VP7(2292B), and A-VP2/A-VP6/C-VP7. To our knowledge, this is the first report that the inner capsid VP6 of group A or group C rotavirus can support attachment of the heterologous, antigenically distinct group A (G6, IND or G10, 2292B) or group C rotavirus outer capsid VP7 to produce hybrid TLPs in vitro.

Heterologous protection induced by the inner capsid proteins of rotavirus requires transcytosis of mucosal immunoglobulins

Protective immunization against rotavirus (RV) can be achieved with heterologous RV, i.e., virus isolated from another species, and with heterologous inner core VP2 and VP6 proteins assembled as virus-like particles (VLP). Although the antigenically conserved VP6 protein does not induce in vitro-neutralizing antibodies, it may, however, elicit immunoglobulins (Ig) involved in heterologous protection, as some IgA against VP6 prevent RV infection in a backpack mouse model. The protective role of Ig directed to the RV inner core proteins VP2 and VP6 was investigated in J-chain-deficient mice (J chain(-/-)), which have a defect in the polymeric Ig receptor (pIgR)-mediated transcytosis of IgA and IgM. J chain(-/-) mice and wild-type (WT) mice were intranasally vaccinated with bovine RV-derived VLP2/6 and then challenged with highly infectious murine ECw RV. Whereas WT mice were totally protected, immunized J chain(-/-) mice shed RV for several days. In addition, naïve J chain(-/-) mice exhibited a 2-day delay in clearing RV compared with WT mice. The immunized J chain(-/-) mice displayed unaltered VLP2/6-specific B-cell numbers in spleen and in mesenteric nodes and similar levels of serum anti-VLP2/6 Ig, confirming that the adaptive B-cell response is preserved in J chain(-/-) mice. These results indicate that J-chain-mediated transcytosis of Ig participates in the clearance of RV and that epithelial pIgR-mediated transport of Ig is involved in the heterologous protection induced by VLP2/6.

Identification of rotavirus VP6 residues located at the interface with VP2 that are essential for capsid assembly and transcriptase activity

Rotavirus has a complex triple-layered icosahedral capsid. The external layer consists of VP7 and VP4, the intermediate layer consists of VP6 trimers, and the internal layer consists of VP2. Double-layered particles (DLP) derived from the virus by solubilization of VP4 and VP7 are transcriptionally competent and extrude capped mRNA from their vertices. Analysis of the pseudoatomic model of the VP6 layer, obtained by placing the atomic structure of VP6 into electron microscopy reconstructions of the DLP, has identified the regions of the protein involved in interactions with the internal layer. To study the role of VP6 both in the assembly of DLP and in transcription, 13 site-specific substitution mutations of VP6, targeting the contacts between the two inner layers, were constructed and expressed in the baculovirus system. The effects of these mutations on VP6 expression, trimerization, and formation of macromolecular assemblies were investigated. Using either in vitro reconstituted DLP derived from purified viral cores and recombinant VP6 or in vivo self-assembled virus-like particles resulting from the coexpression of VP2 and VP6 in the baculovirus-Sf9 system (VLP2/6), we have identified the amino acids essential for recovery of transcription or assembly. All VP6 mutants formed stable trimers which, like wild-type VP6, assembled into tubular structures. The ability of VP6 to interact with VP2 was examined by several assays, including electron microscopy, coimmunoprecipitation, purification of VLP2/6, and monitoring of the transcriptase activity of reconstituted DLP. Of the 13 VP6 mutants examined, 3 were unable to assemble with VP2 and 3 others partially assembled. These mutants either did not rescue the transcriptase activity of core particles or did so only marginally. Four mutants as well as the wild-type VP6 assembled and transcribed very well. Three mutants assembled well on cores but, surprisingly, did not rescue the transcriptase activity of reconstituted DLP. Our results indicate that hydrophobic interactions between VP6 and VP2 residues are responsible for the stability of the DLP. They also show that subtle electrostatic interactions between VP6 and the underlying transcriptase machinery can be essential for mRNA synthesis.

Rotavirus 2/6 virus-like particles administered intranasally in mice, with or without the mucosal adjuvants cholera toxin and Escherichia coli heat-labile toxin, induce a Th1/Th2-like immune response

We investigated the rotavirus-specific lymphocyte responses induced by intranasal immunization of adult BALB/c mice with rotavirus 2/6 virus-like particles (2/6-VLPs) of the bovine RF strain, by assessing the profile of cytokines produced after in vitro restimulation and serum and fecal antibody responses. The cytokines produced by splenic cells were first evaluated. Intranasal immunization with 50 microg of 2/6-VLPs induced a high serum antibody response, including immunoglobulin G1 (IgG1) and IgG2a, a weak fecal antibody response, and a mixed Th1/Th2-like profile of cytokines characterized by gamma interferon and interleukin 10 (IL-10) production and very low levels of IL-2, IL-4, and IL-5. Intranasal immunization with 10 microg of 2/6-VLPs coadministered with the mucosal adjuvants cholera toxin and Escherichia coli heat-labile toxin (LT) considerably enhanced the Th1/Th2-like response; notably, significant levels of IL-2, IL-4, and IL-5 were observed. Since rotavirus is an enteric pathogen, we next investigated the production of IL-2 and IL-5, as being representative of Th1 and Th2 responses, by Peyer's patch and mesenteric lymph node cells from mice immunized intranasally with 2/6-VLPs and LT. The results were compared to those obtained from splenic and cervical lymph node cells. We found that both cytokines were produced by cells from each of these lymphoid tissues. These results confirm the Th1/Th2-like response observed at the systemic level and show, on the assumption that T cells are the primary cells producing the cytokines after in vitro restimulation, that rotavirus-specific T lymphocytes are present in the intestine after intranasal immunization with 2/6-VLPs and LT.

Nasal immunisation with Salmonella typhimurium producing rotavirus VP2 and VP6 antigens stimulates specific antibody response in serum and milk but fails to protect offspring

Rotavirus specifically infects the small intestine of young infants resulting in severe diarrhoea. Mucosal antibody responses are required to cure the infection, and mucosal administration of rotavirus-like particles induces protective immunity without requiring a mucosal adjuvant such as cholera toxin. In addition, the rotavirus protein VP6 has been defined as a protective antigen in an adult mouse rotavirus infection model. Salmonella typhimurium is an epithelium-invasive bacterium that induces specific immune responses in mucosal tissues against itself and carried antigens. In this work, we investigated the capacity of a live recombinant S. typhimurium vaccine to stimulate antibody responses against rotavirus. We constructed an attenuated S. typhimurium strain simultaneously producing VP6 and VP2 rotavirus proteins in the cytoplasm. In contrast to expression in eukaryotic cells, VP6 and VP2 did not form virus-like particles in our bacterial system. After nasal administration of female mice, the live recombinant Salmonella were able to elicit an antibody response specific to both VP2 and VP6 in serum and milk. However, these antibodies failed to passively protect the offspring against rotavirus-induced diarrhoea.

Structural polymorphism of the major capsid protein of rotavirus

Rotaviruses are important human pathogens with a triple-layered icosahedral capsid. The major capsid protein VP6 is shown here to self-assemble into spherical or helical particles mainly depending upon pH. Assembly is inhibited either by low pH (<3.0) or by a high concentration (>100 mM) of divalent cations (Ca(2+) and Zn(2+)). The structures of two types of helical tubes were determined by electron cryomicroscopy and image analysis to a resolution of 2.0 and 2.5 nm. In both reconstructions, the molecular envelope of VP6 fits the atomic model determined by X-ray crystallography remarkably well. The 3-fold symmetry of the VP6 trimer, being incompatible with the helical symmetry, is broken at the level of the trimer contacts. One type of contact is maintained within all VP6 particles (tubes and virus), strongly suggesting that VP6 assemblies arise from different packings of a unique dimer of trimers. Our data show that the protonation state and thus the charge distribution are important switches governing the assembly of macromolecular assemblies.

Purified dengue 2 virus envelope glycoprotein aggregates produced by baculovirus are immunogenic in mice

The full-length dengue 2 virus envelope glycoprotein (Egp) was expressed in insect cells by recombinant (r) baculovirus and found to form multimeric aggregates that were recovered in the void volume of gel filtration columns and by ultracentrifugation. An immunoblot confirmed that rEgp aggregrates disrupted with SDS sample buffer released a monomeric form that migrated with a molecular weight similar to native dengue 2 virus Egp on polyacrylamide gels. The rEgp aggregates reacted strongly with a panel of monoclonal antibodies specific for the native Egp and which identify critical structural and functional epitopes. The rEgp aggregates were purified by ultracentrifugation through 30% sucrose, and were shown to be the major protein band on a polyacrylamide gel and corresponding immunoblot. Purified rEgp aggregates in combination with aluminum hydroxide induced high titer neutralizing antibodies in adult mice. The generation of full-length dengue 2 rEgp aggregates in insect cells facilitated development of a simple, effective procedure for purification of the recombinant protein, and represents a good approach for producing highly immunogenic dengue 2 rEgp as a component of a subunit vaccine.

Crystallization and preliminary X-Ray analysis of rotavirus protein VP6

As a first step to gain insight into the structure of the rotavirus virion at atomic resolution, we report here the expression, purification, and crystallization of recombinant rotavirus protein VP6. This protein has the property of polymerizing in the form of tubular structures in solution which have hindered crystallization thus far. Using a combination of electron microscopy and small-angle X-ray scattering, we found that addition of Ca2+ at concentrations higher than 100 mM results in depolymerization of the tubes, leading to an essentially monodisperse solution of trimeric VP6 even at high protein concentrations (higher than 10 mg/ml), thereby enabling us to search for crystallization conditions. We have thus obtained crystals of VP6 which diffract to better than 2.4 A resolution and belong to the cubic space group P4132 with a cell dimension a of 160 A. The crystals contain a trimer of VP6 lying along the diagonal of the cubic unit cell, resulting in one VP6 monomer per asymmetric unit and a solvent content of roughly 70%.

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.

Immunity to rotavirus in T cell deficient mice

Rotavirus infection was studied in adult nude mice (BALB/c background), alpha beta or gamma delta and alpha beta/gamma delta T cell receptor (TCR) knockout (-/-) mice (C57BL/6 and C57BL/6 x 129 backgrounds), and SCID mice (C57BL/6 background). The gamma delta TCR -/- mice cleared infection just like control mice. All of the nude mice, alpha beta, and alpha beta/gamma delta TCR -/- mice cleared primary rotavirus infection, with a short delay, compared to immunocompetent control mice and developed a rotavirus-specific intestinal IgA measured by ELISA. Elispot analysis with spleen and lamina propia cells showed that the virus-specific intestinal IgA response in immunocompetent C57BL/6 mice was similar to the gamma delta TCR -/- mice and 7- to 60-fold higher than in the alpha beta TCR -/- and alpha beta/gamma delta TCR -/- mice. Likewise, the response of nude +/- mice was 20 times greater than that of nude -/- littermates. While the intestinal IgA antibodies of C57BL/6 mice, gamma delta TCR -/- mice, and nude +/- mice recognized insect cells infected with recombinant baculovirus expressing rotavirus VP6 and VP4 proteins, those of the alpha beta TCR -/-, alpha beta/gamma delta TCR -/-, and nude -/- mice recognized only VP6. Immunocompetent C57BL/6 mice depleted of CD4+ T cell developed similar levels of rotavirus-specific intestinal IgA as the alpha beta TCR -/- mice, suggesting that this T cell-independent IgA response is present in normal mice. In contrast to previously published results with BALB/c SCID and RAG 2 -/- (C57BL/6 x 129 background) mice, all of which become chronically infected with murine rotavirus, 40% of the C57BL/6 SCID mice cleared primary rotavirus infection. These results suggest that both a T cell-independent antibody response and innate mechanisms can contribute to immunity to murine rotavirus and show that gamma delta T cells are not necessary for efficient clearance of primary rotavirus infection in mice.

Chicken rotavirus Ch-1 shows a second type of avian VP6 gene

VP6 protein from chicken rotavirus Ch-1 showed more than 13% amino acid differences in comparison with pigeon and turkey VP6 sequences. This difference is greater than that observed between subgroup I and II mammalian rotavirus VP6 sequences. Phylogenetic tree analysis demonstrated that RV Ch-1 VP6 is not a link between avian and mammalian VP6 sequences. RV Ch-1 showed variant aa in 17 positions which were otherwise absolutely conserved in mammalian and avian group A RVs. The 17 replacements were scattered through the entire VP6 sequence except the C-terminal part implicated in the assembly of subviral particles. In RV Ch-1 the proline residue 309, reported to be critical for the trimerization of VP6, was replaced by leucine, but VP6 trimers were still observed. The sequence and hydrophilicity analysis of avian RV VP6 do not explain the anomalous electrophoretic migration behavior of avian VP6 proteins.

Comparison of the rotavirus gene 6 from different species by sequence analysis and localization of subgroup-specific epitopes using site-directed mutagenesis

The nucleotide sequence of gene 6 encoding the rotavirus major capsid protein VP6 of EDIM strain (EW) was determined and compared to that of 20 previously reported strains with known subgroup specificities. Multiple alignments of amino acid sequences exhibited a high level of sequence conservation (87 to 99.2%). Site-specific mutagenesis experiments were undertaken to localize regions involved in subgroup specificity. Amino acid positions 305, 315, and a region 296-299 (or 301 for equine strain H-2) were identified as contributing to subgroup epitopes. A single amino acid mutation at position 305 or 315 was sufficient to change the subgroup specificity of EW VP6 protein from non I/II to subgroup I- or subgroup II-like, respectively. Mutation at these sites may be another important mechanism for subgroup variation, along with gene reassortment.

Seroepidemiology of human group C rotavirus in the UK

The gene coding for the major inner capsid protein VP6 of human group C rotavirus was cloned into baculovirus using the pBlueBac2 vector and expressed in insect cells. When cultured in High Five cells, VP6 was expressed at a high level and exported to the cell culture medium. Purified VP6 was used to immunise rabbits. Hyperimmune rabbit serum, which reacted with native human group C rotavirus in infected cells, was used to develop and optimise an EIA for the detection of antibodies to group C rotavirus using the recombinant VP6 as a source of antigen. In a local epidemiological survey of 1000 sera grouped by age, an average of 43% of samples were found to have antibodies to human group C rotavirus with the highest proportion (66%) in the 71-75 year age group. In comparison, 97% of adults and 85% of children had antibodies to recombinant VP6 from the bovine RF strain of group A rotavirus. These results suggest that infection with human group C rotavirus is a common occurrence despite the apparent rarity of reports of human group C rotavirus in clinical samples from patients with gastroenteritis.

The concentration of Ca2+ that solubilizes outer capsid proteins from rotavirus particles is dependent on the strain

It has been previously shown that rotavirus maturation and stability of the outer capsid are calcium-dependent processes. More recently, it has been hypothesized that penetration of the cell membrane is also affected by conformational changes of the capsid induced by Ca2+. In this study, we determined quantitatively the critical concentration of calcium ion that leads to solubilization of the outer capsid proteins VP4 and VP7. Since this critical concentration is below or close to trace levels of Ca2+, we have used buffered solutions based on ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) and Ca-EGTA. This method allowed us to show a very high variability of the free [Ca2+] needed to stabilize, at room temperature, the outer capsid of several rotavirus strains. This concentration is about 600 nM for the two bovine strains tested (RF and UK), 100 nM for the porcine strain OSU, and only 10 to 20 nM for the simian strain SA11. Titration of viral infectivity after incubation in buffer of defined [Ca2+] confirmed that the loss of infectivity occurs at different [Ca2+] for these three strains. For the bovine strain, the cleavage of VP4 by trypsin has no significant effect on the [Ca2+] that solubilizes outer shell proteins. The outer layer (VP7) of virus-like particles (VLP) made of recombinant proteins VP2, VP6, and VP7 (VLP2/6/7) was also solubilized by lowering the [Ca2+]. The critical concentration of Ca2+ needed to solubilize VP7 from VLP2/6/7 made of protein from the bovine strain is close to the concentration needed for the corresponding virus. Genetic analysis of this phenotype in a set of reassortant viruses from two parental strains having the phenotypes of strains OSU (porcine) and UK (bovine) confirmed that this property of viral particles is probably associated with the gene coding for VP7. The analysis of VLP by reverse genetics might allow the identification of the region(s) essential for calcium binding.

Quantification of systemic and local immune responses to individual rotavirus proteins during rotavirus infection in mice

The purpose of the present study was to develop a quantitative assay that could be used to measure the local and systemic immune responses to specific rotavirus proteins following rotavirus infection of adult mice. To measure these responses, we used an immunocytochemical staining assay of Spodoptera frugiperda (Sf-9) cells which were infected with recombinant baculovirus expressing selected rotavirus proteins. The specificity of the assay was documented by using a series of monoclonal antibodies to individual rotavirus proteins. We observed that the assay had high levels of sensitivity and specificity for a series of VP7- and VP4-specific neutralizing monoclonal antibodies which recognized conformation-dependent epitopes on their target proteins. We also studied immunoglobulin G (IgG) immune responses in serum and IgA immune responses in the stools of mice infected with wild-type murine rotavirus strain EHPw. In both sera and stools, the most immunogenic proteins were VP6 and VP4. VP2 was less immunogenic than VP6 or VP4, and the immune responses to VP7, NSP2, and NSP4 were very low in serum and undetectable in stools.

Atypical porcine type I interferon. Biochemical and biological characterization of the recombinant protein expressed in insect cells

A recombinant baculovirus was designed to express short porcine type I interferon (spI interferon), a novel and atypical type I interferon that was recently described as the product of a gene transcribed in pig trophoblast at the time of implantation in the uterus [Lefèvre, F. & Boulay, V.C. (1993) J. Biol. Chem. 268, 19,760-19,768]. The recombinant protein, secreted into the culture medium of Sf9 cells at 3 days post infection (60,000 IU/ml), was purified by ion-exchange and reverse-phase HPLC. N-terminal sequencing confirmed the predicted signal peptide cleavage site and therefore the size of the mature protein (149 amino acids), the shortest of all reported type I interferons. Purified spI interferon, with a specific antiviral activity using Madin-Darby bovine kidney cells of 3.7 x 10(7) IU/mg, is an N-glycosylated monomer of 19 kDa that possesses several physicochemical characteristics of interferons: (a) disulfide bonds are necessary for bioactivity; spI interferon is thermolabile, stable at pH 2, and able to renature after complete denaturation (1% 2-mercaptoethanol, 1% SDS, and 5 M urea); (b) the carbohydrate chain is not essential for bioactivity since no loss of antiviral activity is observed following complete deglycosylation. In this study, antiviral and anti-proliferation activities of spI interferon in cell culture were compared with those of other interferons, especially with porcine type 1 interferon-alpha. A major difference with porcine type 1 interferon-alpha was that spI interferon was not active on human cells in either test, and it was relatively more active on pig cells compared to bovine cells than porcine type 1 interferon-alpha. Serological cross-neutralization results obtained with anti-(spI interferon) serum confirmed that several members of interferon families are not antigenically related to spI interferon, in agreement with previous observations; this provides further evidence that spI interferon could represent a new family of type I interferon.

Parvovirus particles as platforms for protein presentation

Empty capsids of the human pathogenic parvovirus B19 can be produced in a baculovirus system. B19 capsids are composed mainly of major capsid protein (VP2) and a small amount of minor capsid protein (VP1); VP1 is identical to VP2 but contains an additional 227-aa N-terminal region ("unique" region). A portion of that region of VP1 is external to the capsid, and VP1 is not required for capsid formation. We substituted the unique region with a sequence encoding the 147 aa of hen egg white lysozyme (HEL) and constructed recombinant baculoviruses with variable amounts of retained VP1 sequence joined to the VP2 backbone. After cotransfection with VP2 baculovirus and expression in insect cells, capsids were purified by density sedimentation. Purified recombinant capsids contained HEL. External presentation of HEL was demonstrated by immunoprecipitation, ELISA, and immune electron microscopy using anti-lysozyme monoclonal antibodies or specific rabbit antisera. Empty particles showed enzymatic activity in a micrococcal cell wall digestion assay. Rabbits inoculated with capsids made antibodies to HEL. Intact heterologous protein can be incorporated in B19 particles and presented on the capsid surface, properties that may be useful in vaccine development, cell targeting, and gene therapy.

Rearrangement of the VP6 gene of a group A rotavirus in combination with a point mutation affecting trimer stability

A group A rotavirus isolated from a lamb with diarrhea in Qinhai province, China, was serially passaged in fetal calf kidney cells. In passage 96, rearrangements of RNA segments 5 and 6 of the viral genome were found. Here we report the nucleotide and predicted amino acid sequences of normal and rearranged RNA 6, coding for the major inner capsid protein VP6. In comparison with the normal gene (N6), the rearranged RNA 6 (R6) contained the normal open reading frame followed by a 473-nucleotide (nt) duplication of the gene beginning 23 nt after the termination codon. The duplicated region starts at nt 768 and runs through to the 3' end of the gene. In accordance with the nucleotide sequence of the rearranged RNA 6, a normal-length VP6 product was found in cells infected with the mutant. However, a single-amino-acid change from proline to glutamine at position 309 slightly affected the electrophoretic mobility of the VP6 monomer of the R6 mutant and reduced the stability of VP6 trimers on gels and at low pH values compared with the normal gene product. The degree of relatedness of VP6 of the Chinese lamb rotavirus Lp14 to those of other group A rotaviruses was determined.

Inhibition of in vitro reconstitution of rotavirus transcriptionally active particles by anti-VP6 monoclonal antibodies

Six monoclonal antibodies specific for the major capsid protein of rotavirus, VP6, previously characterized, were tested in a biological assay for their capacity to block the transcriptase activity associated with the single-shelled particles. The results showed that two MAbs (RV-50 and RV-133), specific for distinct antigenic sites, were able to block the transcription when they were incubated with a purified baculovirus-expressed group A VP6, prior to the reconstitution of the single-shelled particles from the cores, suggesting that at least two domains are involved in active single-shelled particle reconstitution. The results obtained previously from immunochemistry of synthetic peptides did not allow us to attribute this biological activity to a particular linear sequence of the protein, the domain involved being probably complex and dependent on the folding of the protein. However, the C-terminal end, which is necessary for binding into single-shelled particles could be necessary but not sufficient to restore the transcription, since neither of these two MAbs reacted significantly with peptides of this region. These two MAbs will be useful reagents to study the interactions between VP6 and the cores.

Studies of the major reovirus core protein sigma 2: reversion of the assembly-defective mutant tsC447 is an intragenic process and involves back mutation of Asp-383 to Asn

The reovirus group C temperature-sensitive mutant tsC447, whose defect maps to the S2 gene, which encodes the major core protein sigma 2, fails to assemble core particles at the nonpermissive temperature. To identify other proteins that may interact with sigma 2 during assembly, we generated and examined 10 independent revertants of the mutant. To determine which gene(s) carried a compensatory suppressor mutation(s), we generated intertypic reassortants between wild-type reovirus serotype 1 Lang and each revertant and determined the temperature sensitivities of the reassortants by efficiency-of-plating assays. Results of the efficiency-of-plating analyses indicated that reversion of the tsC447 defect was an intragenic process in all revertants. To identify the region(s) of sigma 2 that had reverted, we determined the nucleotide sequences of the S2 genes. In all revertant sequences examined, the G at nucleotide position 1166 in tsC447 had reverted to the A present in the wild-type sequence. This reversion leads to the restoration of a wild-type asparagine (in place of a mutant aspartic acid) at amino acid 383 in the sigma 2 sequence. These results collectively indicate that the functional lesion in tsC447 is Asp-383 and that this lesion cannot be corrected by alterations in other core proteins. These observations suggest that this region of sigma 2, which may be important in mediating assembly of the core particle, does not interact significantly with other reovirus proteins.

In vitro reconstitution of rotavirus transcriptional activity using viral cores and recombinant baculovirus expressed VP6

Purified baculovirus-expressed group A rotavirus VP6 polypeptide was shown to be active in the recovery of the transcriptase activity associated with the reconstitution of the single-shelled rotavirus particle. Recombinant VP6 polypeptide was able to restore the transcriptional activity in purified viral cores from both SA-11 and RF rotavirus strains. Recombinant group C VP 6 (Cowden strain) is capable of binding as a trimer to group A viral core particles but unable to restore the transcriptase activity, suggesting that the binding of the polypeptide to cores is not the only requirement to restore the transcriptase activity. The VP 6 group A polypeptide was shown to bind as a monomer to viral cores, indicating that trimerization of VP 6 may be not required for reconstitution of the polymerase activity.