Molecular characterization of a subgroup specificity associated with the rotavirus inner capsid protein VP2

Group A rotaviruses are classified into serotypes, based on the reactivity pattern of neutralizing antibodies to VP4 and VP7, as well as into subgroups (SGs), based on non-neutralizing antibodies directed against VP6. The inner capsid protein (VP2) has also been described as a SG antigen; however, little is known regarding the molecular determinants of VP2 SG specificity. In this study, we characterize VP2 SGs by correlating genetic markers with the immunoreactivity of the SG-specific monoclonal antibody (YO-60). Our results show that VP2 proteins similar in sequence to that of the prototypic human strain Wa are recognized by YO-60, classifying them as VP2 SG-II. In contrast, proteins not bound by YO-60 are similar to those of human strains DS-1 or AU-1 and represent VP2 SG-I. Using a mutagenesis approach, we identified residues that determine recognition by either YO-60 or the group A-specific VP2 monoclonal antibody (6E8). We found that YO-60 binds to a conformationally dependent epitope that includes Wa VP2 residue M328. The epitope for 6E8 is also contingent upon VP2 conformation and resides within a single region of the protein (Wa VP2 residues A440 to T530). Using a high-resolution structure of bovine rotavirus double-layered particles, we predicted these epitopes to be spatially distinct from each other and located on opposite surfaces of VP2. This study reveals the extent of genetic variation among group A rotavirus VP2 proteins and illuminates the molecular basis for a previously described SG specificity associated with the rotavirus inner capsid protein.     

Assembly of highly infectious rotavirus particles recoated with recombinant outer capsid proteins

Assembly of the rotavirus outer capsid is the final step of a complex pathway. In vivo, the later steps include a maturational membrane penetration that is dependent on the scaffolding activity of a viral nonstructural protein. In vitro, simply adding the recombinant outer capsid proteins VP4 and VP7 to authentic double-layered rotavirus subviral particles (DLPs) in the presence of calcium and acidic pH increases infectivity by a factor of up to 10(7), yielding particles as infectious as authentic purified virions. VP4 must be added before VP7 for high-level infectivity. Steep dependence of infectious recoating on VP4 concentration suggests that VP4-VP4 interactions, probably oligomerization, precede VP4 binding to particles. Trypsin sensitivity analysis identifies two populations of VP4 associated with recoated particles: properly mounted VP4 that can be specifically primed by trypsin, and nonspecifically associated VP4 that is degraded by trypsin. A full complement of properly assembled VP4 is not required for efficient infectivity. Minimal dependence of recoating on VP7 concentration suggests that VP7 binds DLPs with high affinity. The parameters for efficient recoating and the characterization of recoated particles suggest a model in which, after a relatively weak interaction between oligomeric VP4 and DLPs, VP7 binds the particles and locks VP4 in place. Recoating will allow the use of infectious modified rotavirus particles to explore rotavirus assembly and cell entry and could lead to practical applications in novel immunization strategies.     

Cross-linking of rotavirus outer capsid protein VP7 by antibodies or disulfides inhibits viral entry

Antibodies that neutralize rotavirus infection target outer coat proteins VP4 and VP7 and inhibit viral entry. The structure of a VP7-Fab complex (S. T. Aoki, et al., Science 324:1444-1447, 2009) led us to reclassify epitopes into two binding regions at inter- and intrasubunit boundaries of the calcium-dependent trimer. It further led us to show that antibodies binding at the intersubunit boundary inhibit uncoating of the virion outer layer. We have now tested representative antibodies for each of the defined structural epitope regions and find that antibodies recognizing epitopes in either binding region neutralize by cross-linking VP7 trimers. Antibodies that bind at the intersubunit junction neutralize as monovalent Fabs, while those that bind at the intrasubunit region require divalency. The VP7 structure has also allowed us to design a disulfide cross-linked VP7 mutant which recoats double-layered particles (DLPs) as efficiently as does wild-type VP7 but which yields particles defective in cell entry as determined both by lack of infectivity and by loss of α-sarcin toxicity in the presence of recoated particles. We conclude that dissociation of the VP7 trimer is an essential step in viral penetration into cells.     

Nucleotide sequence of the gene encoding the serotype-specific antigen of human (Wa) rotavirus: comparison with the homologous genes from simian SA11 and UK bovine rotaviruses

The nucleotide sequence of human (Wa) rotavirus genome segment 9, which encodes the serotype-specific antigen VP7, has been determined. Comparison of the deduced amino acid sequence of Wa VP7 protein to the sequences of simian SA11 and UK bovine VP7 proteins shows that the majority of the amino acid differences are clustered between amino acid residues 37 through 49, 65 through 75, 87 through 105, 122 through 126, 146 through 149, 178 through 181, and 208 through 242. A hydrophilicity profile of the three proteins reveals correlations between hydrophilic peaks, potentially antigenic determinants, and certain clusters of amino acid changes.     

Protective immunity and antibody-secreting cell responses elicited by combined oral attenuated Wa human rotavirus and intranasal Wa 2/6-VLPs with mutant Escherichia coli heat-labile toxin in gnotobiotic pigs

Two combined rotavirus vaccination regimens were evaluated in a gnotobiotic pig model of rotavirus infection and disease and were compared to previously tested rotavirus vaccination regimens. The first (AttHRV/VLP2x) involved oral inoculation with one dose of attenuated (Att) Wa human rotavirus (HRV), followed by two intranasal (i.n.) doses of a rotavirus-like particle (2/6-VLPs) vaccine derived from Wa (VP6) and bovine RF (VP2) rotavirus strains. The 2/6-VLPs were coadministered with a mutant Escherichia coli heat-labile toxin, LT-R192G (mLT) adjuvant. For the second regimen (VLP2x/AttHRV), two i.n. doses of 2/6-VLPs+mLT were given, followed by one oral dose of attenuated Wa HRV. To compare the protective efficacy and immune responses induced by the combined vaccine regimens with individual rotavirus vaccine regimens, we included in the experiments the following vaccine groups: one oral dose of attenuated Wa HRV (AttHRV1x and Mock2x/AttHRV, respectively), three oral doses of attenuated Wa HRV (AttHRV3x), three i.n. doses of 2/6-VLPs plus mLT (VLP3x), three i.n. doses of purified double-layered inactivated Wa HRV plus mLT (InactHRV3x), mLT alone, and mock-inoculated pigs. The isotype, magnitude, and tissue distribution of antibody-secreting cells (ASCs) in the intestinal and systemic lymphoid tissues were evaluated using an enzyme-linked immunospot assay. The AttHRV/VLP2x regimen stimulated the highest mean numbers of intestinal immunoglobulin A (IgA) ASCs prechallenge among all vaccine groups. This regimen induced partial protection against virus shedding (58%) and diarrhea (44%) upon challenge of pigs with virulent Wa HRV. The reverse VLP2x/AttHRV regimen was less efficacious than the AttHRV/VLP2x regimen in inducing IgA ASC responses and protection against diarrhea (25% protection rate) but was more efficacious than VLP3x or InactHRV3x (no protection). In conclusion, the AttHRV/VLP2x vaccination regimen stimulated the strongest B-cell responses in the intestinal mucosal immune system at challenge and conferred a moderately high protection rate against rotavirus disease, indicating that priming of the mucosal inductive site at the portal of natural infection with a replicating vaccine, followed by boosting with a nonreplicating vaccine at a second mucosal inductive site, may be a highly effective approach to stimulate the mucosal immune system and induce protective immunity against various mucosal pathogens.     

Synthesis of double-layered rotavirus-like particles using internal ribosome entry site vector system in stably-transformed <Emphasis Type="Italic">Drosophila melanogaster</Emphasis>

We established a bicistronic expression system using an encephalomyocarditis virus (EMCV)-derived internal ribosomal entry site (IRES) element to generate stably transformed Drosophila melanogaster Schneider 2 (S2) cells expressing human rotavirus Wa capsid proteins, VP2 and VP6, for the synthesis of VP2/6 double-layered virus-like particle (DVLP). The EMCV-derived IRES permitted bicistronic translation of recombinant VP6. Recombinant VP2 and VP6 were detected in extracellular fractions of stably transformed S2 cells. A wheel-like DVLP (diam ~ 50–55 nm) with short spikes was produced from the extracellular fraction of stably transformed S2 cells. A bicistronic expression system using an EMCV-derived IRES element can thus be used to express two proteins of interest in stably transformed S2 cells. The bi-or tri-cistronic expression of recombinant VP2/6/7 using stably transformed S2 cells can also be used to produce rotavirus VLPs.     

Comparative sequence analysis of rotavirus genomic segment 6--the gene specifying viral subgroups 1 and 2

Cloned DNA copies of rotavirus genomic segment 6 from simian 11 (subgroup 1) and human strain Wa (subgroup 2) rotaviruses have been used to determine the nucleotide sequences of the gene that determines viral subgroup specificity. Both genomic segments are 1,356 nucleotides in length and possess 5'- and 3'-terminal untranslated regions of 23 and 142 nucleotides, respectively. The inferred amino acid sequence reveals VP6 to be a polypeptide of 397 amino acids in which more than 90% of the amino acid sequence is conserved between the two viruses. There are 34 amino acid changes between the subgroup 1 and 2 polypeptides, most clustered in three regions of the molecule at residues 39 through 62, 80 through 122, and 281 through 315.     

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

VP6 of group C (Cowden strain) rotavirus was expressed in the baculovirus system. The recombinant protein, expressed to a high level in insect cells, was purified by ion-exchange chromatography. The purified protein was proven to be trimeric. The effect of pH on the trimer's stability was investigated. Coexpression of VP6 from group A (bovine strain RF) and VP6 from group C in the baculovirus system did not result in the formation of chimeric trimers. Coexpression of VP2 from group A rotavirus (bovine strain RF) and VP6 from group C in the baculovirus system led to the formation of chimeric, empty, single-shelled particles. These results demonstrate conservation in the domains necessary for binding to VP2 in different serogroups of VP6. The locations of the domains involved in trimerization and in the interaction with VP2 are discussed.     

Silencing the morphogenesis of rotavirus

The morphogenesis of rotaviruses follows a unique pathway in which immature double-layered particles (DLPs) assembled in the cytoplasm bud across the membrane of the endoplasmic reticulum (ER), acquiring during this process a transient lipid membrane which is modified with the ER resident viral glycoproteins NSP4 and VP7; these enveloped particles also contain VP4. As the particles move towards the interior of the ER cisternae, the transient lipid membrane and the nonstructural protein NSP4 are lost, while the virus surface proteins VP4 and VP7 rearrange to form the outermost virus protein layer, yielding mature infectious triple-layered particles (TLPs). In this work, we have characterized the role of NSP4 and VP7 in rotavirus morphogenesis by silencing the expression of both glycoproteins through RNA interference. Silencing the expression of either NSP4 or VP7 reduced the yield of viral progeny by 75 to 80%, although the underlying mechanism of this reduction was different in each case. Blocking the synthesis of NSP4 affected the intracellular accumulation and the cellular distribution of several viral proteins, and little or no virus particles (neither DLPs nor TLPs) were assembled. VP7 silencing, in contrast, did not affect the expression or distribution of other viral proteins, but in its absence, enveloped particles accumulated within the lumen of the ER, and no mature infectious virus was produced. Altogether, these results indicate that during a viral infection, NSP4 serves as a receptor for DLPs on the ER membrane and drives the budding of these particles into the ER lumen, while VP7 is required for removing the lipid envelope during the final step of virus morphogenesis.     

X-ray Crystal Structure of the Rotavirus Inner Capsid Particle at 3.8 Å Resolution

The rotavirus inner capsid particle, known as the “double-layered particle” (DLP), is the “payload” delivered into a cell in the process of viral infection. Its inner and outer protein layers, composed of viral protein (VP) 2 and VP6, respectively, package the 11 segments of the double-stranded RNA (dsRNA) of the viral genome, as well as about the same number of polymerase molecules (VP1) and capping-enzyme molecules (VP3). We have determined the crystal structure of the bovine rotavirus DLP. There is one full particle (outer diameter ∼ 700 Å) in the asymmetric unit of the P2₁2₁2₁ unit cell of dimensions a = 740 Å, b = 1198 Å, and c = 1345 Å. A three-dimensional reconstruction from electron cryomicroscopy was used as a molecular replacement model for initial phase determination to about 18.5 Å resolution, and the 60-fold redundancy of icosahedral particle symmetry allowed phases to be extended stepwise to the limiting resolution of the data (3.8 Å). The structure of a VP6 trimer (determined previously by others) fits the outer layer density with very little adjustment. The T = 13 triangulation number of that layer implies that there are four and one-third VP6 trimers per icosahedral asymmetric unit. The inner layer has 120 copies of VP2 and thus 2 copies per icosahedral asymmetric unit, designated VP2A and VP2B. Residues 101-880 fold into a relatively thin principal domain, comma-like in outline, shaped such that only rather modest distortions (concentrated at two “subdomain” boundaries) allow VP2A and VP2B to form a uniform layer with essentially no gaps at the subunit boundaries, except for a modest pore along the 5-fold axis. The VP2 principal domain resembles those of the corresponding shells and homologous proteins in other dsRNA viruses: λ1 in orthoreoviruses and VP3 in orbiviruses. Residues 1-80 of VP2A and VP2B fold together with four other such pairs into a “5-fold hub” that projects into the DLP interior along the 5-fold axis; residues 81-100 link the 10 polypeptide chains emerging from a 5-fold hub to the N-termini of their corresponding principal domains, clustered into a decameric assembly unit. The 5-fold hub appears to have several distinct functions. One function is to recruit a copy of VP1 (or of a VP1-VP3 complex), potentially along with a segment of plus-strand RNA, as a decamer of VP2 assembles. The second function is to serve as a shaft around which can coil a segment of dsRNA. The third function is to guide nascent mRNA, synthesized in the DLP interior by VP1 and 5′-capped by the action of VP3, out through a 5-fold exit channel. We propose a model for rotavirus particle assembly, based on known requirements for virion formation, together with the structure of the DLP and that of VP1, determined earlier.     

Comparative amino acid sequence analysis of VP4 for VP7 serotype 6 bovine rotavirus strains NCDV, B641, and UK.

In a previous study (S. Zheng, G. N. Woode, D. R. Melendy, and R. F. Ramig, J. Clin. Microbiol. 27:1939-1945, 1989), it was predicted that the VP7 serotype 6 bovine rotavirus strains NCDV and B641 do not share antigenically similar VP4s. In this study, gene 4 and the VP7 gene of B641 were sequenced, and the amino acid sequences were deduced and compared with those of NCDV and bovine rotavirus strain UK. Amino acid sequence homology in VP7 between the three strains was greater than 94%, confirming their relationship as VP7 serotype 6 viruses. VP4 of B641 showed amino acid homology to UK of 94% but only 73% homology to NCDV. Sequence comparison of a variable region of VP8 demonstrated amino acid homology of 53% between B641 and NCDV, whereas B641 and UK were 89% homologous in this region. These results confirm the earlier prediction that although the same serotype by VP7 reactivity, B641 and NCDV represent different VP4 serotypes. This difference in VP4 may have contributed to the lack of homotypic protection observed in calves, implicating VP4 as an important antigen in the active immune response to rotavirus infection in bovines.     

G5型人A组轮状病毒LL36755株的VP4、VP6和NSP4编码基因的分子特征

最近在亚洲首次发现并报道了感染人的G5型人A组轮状病毒LL36755株,为进一步探讨其进化来源,克隆了G5型人A组轮状病毒LL36755株的VP4、VP6、NSP4编码基因,并分析其基因序列的分子特征。结果发现卢龙株LL36755为罕见的G5P[6]型,其VP6的亚群为SGⅡ型,NSP4的基因型为B型。系统进化树分析表明,卢龙株LL36755的VP7、VP4编码基因与猪来源的毒株关系密切,而VP6、NSP4编码基因与人来源的毒株紧密相联系。可以推断新的人腹泻A组轮状病毒LL36755株是猪的VP7,VP4编码基因与人的VP6,NSP4编码基因的自然重组;而且该毒株不是G5的原型,很可能是人类轮状病毒与猪轮状病毒毒株的自然重组后逐步进化而来。     

C terminus of infectious bursal disease virus major capsid protein VP2 is involved in definition of the T number for capsid assembly

Infectious bursal disease virus (IBDV), a member of the Birnaviridae family, is a double-stranded RNA virus. The IBDV capsid is formed by two major structural proteins, VP2 and VP3, which assemble to form a T=13 markedly nonspherical capsid. During viral infection, VP2 is initially synthesized as a precursor, called VPX, whose C end is proteolytically processed to the mature form during capsid assembly. We have computed three-dimensional maps of IBDV capsid and virus-like particles built up by VP2 alone by using electron cryomicroscopy and image-processing techniques. The IBDV single-shelled capsid is characterized by the presence of 260 protruding trimers on the outer surface. Five classes of trimers can be distinguished according to their different local environments. When VP2 is expressed alone in insect cells, dodecahedral particles form spontaneously; these may be assembled into larger, fragile icosahedral capsids built up by 12 dodecahedral capsids. Each dodecahedral capsid is an empty T=1 shell composed of 20 trimeric clusters of VP2. Structural comparison between IBDV capsids and capsids consisting of VP2 alone allowed the determination of the major capsid protein locations and the interactions between them. Whereas VP2 forms the outer protruding trimers, VP3 is found as trimers on the inner surface and may be responsible for stabilizing functions. Since elimination of the C-terminal region of VPX is correlated with the assembly of T=1 capsids, this domain might be involved (either alone or in cooperation with VP3) in the induction of different conformations of VP2 during capsid morphogenesis.     

Immunization with baculovirus-expressed recombinant rotavirus proteins VP1, VP4, VP6, and VP7 induces CD8+ T lymphocytes that mediate clearance of chronic rotavirus infection in SCID mice.

Clearance of chronic murine rotavirus infection in SCID mice can be demonstrated by adoptive transfer of immune CD8+ T lymphocytes from histocompatible donor mice immunized with a murine homotypic rotavirus (T. Dharakul, L. Rott, and H.B. Greenberg, J. Virol 64:4375-4382, 1990). The present study focuses on the protein specificity and heterotypic nature of cell-mediated clearance of chronic murine rotavirus infection in SCID mice. Heterotypic cell-mediated clearance was demonstrated in SCID mice infected with EDIM (murine) rotavirus after adoptive transfer of CD8+ T lymphocytes from BALB/c mice that were immunized with a variety of heterologous (nonmurine) rotaviruses including Wa (human, serotype 1), SA11 and RRV (simian, serotype 3), and NCDV and RF (bovine, serotype 6). This finding indicates the serotypic independence of T-cell-mediated rotavirus clearance. To further identify the rotavirus proteins that are capable of generating CD8+ T cells that mediate virus clearance, donor mice were immunized with SF-9 cells infected with a baculovirus recombinant expressing one of the following rotavirus proteins: VP1, VP2, NS53 (from RF), VP4, VP7, NS35 (from RRV), VP6, and NS28 (from SA11). SCID mice stopped shedding rotavirus after receiving CD8+ T cells from mice immunized with VP1, VP4, VP6, and VP7 but not with VP2, NS53, NS35, NS28, or wild-type baculovirus. These results suggest that heterotypic cell-mediated clearance of rotavirus in SCID mice is mediated by three of the major rotavirus structural proteins and by a putative polymerase protein.