Molecular biology of rotaviruses

Overview of the Development, Impacts, and Challenges of Live-Attenuated Oral Rotavirus Vaccines

Safety, efficacy, and cost-effectiveness are paramount to vaccine development. Following the isolation of rotavirus particles in 1969 and its evidence as an aetiology of severe dehydrating diarrhoea in infants and young children worldwide, the quest to find not only an acceptable and reliable but cost-effective vaccine has continued until now. Four live-attenuated oral rotavirus vaccines (LAORoVs) (Rotarix^®, RotaTeq^®, Rotavac^®, and RotaSIIL^®) have been developed and licensed to be used against all forms of rotavirus-associated infection. The efficacy of these vaccines is more obvious in the high-income countries (HIC) compared with the low- to middle-income countries (LMICs); however, the impact is far exceeding in the low-income countries (LICs). Despite the rotavirus vaccine efficacy and effectiveness, more than 90 countries (mostly Asia, America, and Europe) are yet to implement any of these vaccines. Implementation of these vaccines has continued to suffer a setback in these countries due to the vaccine cost, policy, discharging of strategic preventive measures, and infrastructures. This review reappraises the impacts and effectiveness of the current live-attenuated oral rotavirus vaccines from many representative countries of the globe. It examines the problems associated with the low efficacy of these vaccines and the way forward. Lastly, forefront efforts put forward to develop initial procedures for oral rotavirus vaccines were examined and re-connected to today vaccines.

A cytoplasmic region of the NSP4 enterotoxin of rotavirus is involved in retention in the endoplasmic reticulum

The rotavirus genome encodes two glycoproteins, one structural (VP7) and one non-structural (NSP4), both of which mature and remain in the endoplasmic reticulum (ER). While three amino acids in the N terminus have been proposed to function as a retention signal for VP7, no information is yet available on how NSP4 remains associated with the ER. In this study, we have investigated the ER retention motif of NSP4 by producing various C-terminal truncations. Deleting the C terminus by 52 amino acids did not change the intracellular distribution of NSP4, but an additional deletion of 38 amino acids diminished the ER retention and resulted in the expression of NSP4 on the cell surface. Brefeldin A treatment prevented NSP4 from reaching the cell surface, suggesting that C-terminal truncated plasma membrane NSP4 is transported through the normal secretory pathway. On the basis of these results, we propose that the region between amino acids 85 and 123 in the cytoplasmic region of NSP4 are involved in ER retention.

Localization of membrane permeabilization and receptor binding sites on the VP4 hemagglutinin of rotavirus: implications for cell entry

The surface of rotavirus is decorated with 60 spike-like projections, each composed of a dimer of VP4, the viral hemagglutinin. Trypsin cleavage of VP4 generates two fragments, VP8*, which binds sialic acid (SA), and VP5*, containing an integrin binding motif and a hydrophobic region that permeabilizes membranes and is homologous to fusion domains. Although the mechanism for cell entry by this non-enveloped virus is unclear, it is known that trypsin cleavage enhances viral infectivity and facilitates viral entry. We used electron cryo-microscopy and difference map analysis to localize the binding sites for two neutralizing monoclonal antibodies, 7A12 and 2G4, which are directed against the SA-binding site within VP8* and the membrane permeabilization domain within VP5*, respectively. Fab 7A12 binds at the tips of the dimeric heads of VP4, and 2G4 binds in the cleft between the two heads of the spike. When these binding results are combined with secondary structure analysis, we predict that the VP4 heads are composed primarily of beta-sheets in VP8* and that VP5* forms the body and base primarily in beta-structure and alpha-helical conformations, respectively. Based on these results and those of others, a model is proposed for cell entry in which VP8* and VP5* mediate receptor binding and membrane permeabilization, and uncoating occurs during transfer across the lipid bilayer, thereby generating the transcriptionally active particle.

Nucleotide sequence variation of the VP7 gene of two G3-type rotaviruses isolated from dogs

The sequence of the VP7 gene of two rotaviruses isolated from dogs in southern Italy was determined and the inferred amino acid sequence was compared with that of other rotavirus strains. There was very high nucleotide and amino acid identity between canine strain RV198/95 and other canine strains, and to the human strain HCR3A. Strain RV52/96, however, was found to have about 95% identity to the G3 serotype canine strains K9, A79-10 and CU-1 and 96% identity to strain RV198/95 and to the simian strain RRV. Therefore both of the canine strains belong to the G3 serotype. Nevertheless, detailed analysis of the VP7 variable regions revealed that RV52/96 possesses amino acid substitutions uncommon to the other canine isolates. In addition, strain RV52/96 exhibited a nucleotide divergence greater than 16% from all the other canine strains studied; however, it revealed the closest identity (90.4%) to the simian strain RRV. With only a few exceptions, phylogenetic analysis allowed clear differentiation of the G3 rotaviruses on the basis of the species of origin. The nucleotide and amino acid variations observed in strain RV52/96 could account for the existence of a canine rotavirus G3 sub-type.

Rotavirus spike protein VP4 is present at the plasma membrane and is associated with microtubules in infected cells

VP4 is an unglycosylated protein of the outer layer of the capsid of rotavirus. It forms spikes that project from the outer layer of mature virions, which is mainly constituted by glycoprotein VP7. VP4 has been implicated in several important functions, such as cell attachment, penetration, hemagglutination, neutralization, virulence, and host range. Previous studies indicated that VP4 is located in the space between the periphery of the viroplasm and the outside of the endoplasmic reticulum in rotavirus-infected cells. Confocal microscopy of infected MA104 monolayers, immunostained with specific monoclonal antibodies, revealed that a significant fraction of VP4 was present at the plasma membrane early after infection. Another fraction of VP4 is cytoplasmic and colocalizes with beta-tubulin. Flow cytometry analysis confirmed that at the early stage of viral infection, VP4 was present on the plasma membrane and that its N-terminal region, the VP8* subunit, was accessible to antibodies. Biotin labeling of the infected cell surface monolayer with a cell-impermeable reagent allowed the identification of the noncleaved form of VP4 that was associated with the glycoprotein VP7. The localization of VP4 was not modified in cells transfected with a plasmid allowing the expression of a fusion protein consisting of VP4 and the green fluorescent protein. The present data suggest that VP4 reaches the plasma membrane through the microtubule network and that other viral proteins are dispensable for its targeting and transport.

Rotavirus nonstructural glycoprotein NSP4 alters plasma membrane permeability in mammalian cells

The endoplasmic reticulum-localized transmembrane glycoprotein NSP4 of rotavirus is a key protein involved in rotavirus cytopathology. We have used a dual-recombinant vaccinia virus system to express NSP4 in monkey kidney epithelial cells at a level comparable to that observed during rotavirus infection. Expression of NSP4 results in loss of plasma membrane integrity, which can be demonstrated by release of both 51Cr and lactate dehydrogenase into the medium. The cytotoxic behavior of NSP4 is dose dependent, and morphological analysis reveals gross changes to cell ultrastructure, indicative of cell death. Thus, intracellular expression of a single rotavirus protein which localizes to the endoplasmic reticulum membrane has profound effects on the stability of the plasma membrane and cell viability. Analysis of NSP4 deletion mutants indicates that a membrane-proximal region located within the cytoplasmic domain may mediate cytotoxicity.

Details of the arrangement of the outer capsid of rice dwarf phytoreovirus, as visualized by two-dimensional crystallography

Two-dimensional crystals were obtained from purified P8, an outer capsid protein of rice dwarf phytoreovirus. A filtered image of the two-dimensional crystal, in combination with the results of biochemical analysis, revealed the unit formation of the capsid protein, a capsomere structure, which appeared to be an approximately equilateral triangle with sides of approximately 6 nm and which was composed of a trimer of P8 protein. Details of the arrangements of the outer capsid of the virus are described.

Calcium influences the stability and conformation of rotavirus SA11 glycoprotein VP7 expressed in Dictyostelium discoideum

We have previously reported expression of the rotavirus outer capsid glycoprotein, VP7, in the relatively new expression host, Dictyostelium discoideum. To optimise yields of recombinant VP7, we examined the role of Ca2+ since stability of both VP7 and mature rotavirus during a rotavirus infection are calcium-dependent. Low micromolar levels of free extracellular Ca2+ were required to maximise yields of VP7 in D. discoideum whilst levels of VP7 were reduced following depletion of intracellular Ca2+ reserves using A23187 and EGTA. Immunoblot analysis suggested that VP7 was being degraded in an intracellular compartment. Immunoprecipitation with a conformation-dependent neutralising antibody confirmed that EGTA-induced Ca2+ chelation alters the conformation of VP7. These results suggest that stability of VP7 is dependent on maintaining adequate levels of intracellular Ca2+ and that conformational changes in VP7 which occur following depletion of Ca2+ reserves induce rapid proteolysis of the protein. Since these results establish conditions for expressing optimal levels of VP7 in the correct conformation they have important implications for the development of a subunit vaccine based on recombinant VP7.

Rotavirus vaccines: an overview

Rotavirus vaccine development has focused on the delivery of live attenuated rotavirus strains by the oral route. The initial "Jennerian" approach involving bovine (RIT4237, WC3) or rhesus (RRV) rotavirus vaccine candidates showed that these vaccines were safe, well tolerated, and immunogenic but induced highly variable rates of protection against rotavirus diarrhea. The goal of a rotavirus vaccine is to prevent severe illness that can lead to dehydration in infants and young children in both developed and developing countries. These studies led to the concept that a multivalent vaccine that represented each of the four epidemiologically important VP7 serotypes might be necessary to induce protection in young infants, the target population for vaccination. Human-animal rotavirus reassortants whose gene encoding VP7 was derived from their human rotavirus parent but whose remaining genes were derived from the animal rotavirus parent were developed as vaccine candidates. The greatest experience with a multivalent vaccine to date has been gained with the quadrivalent preparation containing RRV (VP7 serotype 3) and human-RRV reassortants of VP7 serotype 1, 2, and 4 specificity. Preliminary efficacy trial results in the United States have been promising, whereas a study in Peru has shown only limited protection. Human-bovine reassortant vaccines, including a candidate that contains the VP4 gene of a human rotavirus (VP4 serotype 1A), are also being studied.

Characterization and replicase activity of double-layered and single-layered rotavirus-like particles expressed from baculovirus recombinants

Rotavirus has a capsid composed of three concentric protein layers. We coexpressed various combinations of the rotavirus structural proteins of single-layered (core) and double-layered (single-shelled) capsids from baculovirus vectors in insect cells and determined the ability of the various combinations to assemble into viruslike particles (VLPs). VLPs were purified by centrifugation, their structure was examined by negative-stain electron microscopy, their protein content was determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and GTP binding assays, and their ability to support synthesis of negative-strand RNAs on positive-sense template RNAs was determined in an in vitro replication system. Coexpression of all possible combinations of VP1, VP2, VP3, and VP6, the proteins of double-layered capsids, resulted in the formation of VP1/2/3/6, VP1/2/6, VP2/3/6, and VP2/6 double-layered VLPs. These VLPs had the structural characteristics of empty rotavirus double-layered particles and contained the indicated protein species. Only VPI/2/3/6 and VP1/2/6 particles supported RNA replication. Coexpression of all possible combinations of VPl, VP2, and VP3, the proteins of single-layered capsids, resulted in the formation of VP1/2/3, VP1/2, VP2/3, and VP2 single-layered VLPs. These VLPs had the structural characteristics of empty single-layered rotavirus particles and contained the indicated protein species. Only VP1/2/3 and VP1/2 VLPs supported RNA replication. We conclude that (i) the assembly of VP1 and VP3 into VLPs requires the presence of VP2, (ii) the role of VP2 in the assembly of VP1 and VP3 and in replicase activity is most likely structural, (iii) VP1 is required and VP3 is not required for replicase activity of VLPs, and (iv) VP1/2 VLPs constitute the minimal replicase particle in the in vitro replication system.

Sensitive detection of group A rotaviruses by immunomagnetic separation and reverse transcription-polymerase chain reaction

An immunomagnetic separation (IMS) method was developed for concentrating rotaviruses from environmental samples, as well as a reverse transcription-polymerase chain reaction (RT-PCR) for the sensitive and specific detection of group A rotaviruses. Magnetic beads were coated with monoclonal antibodies directed against the group-specific, inner capsid protein (VP6) and subsequently used to capture and purify the virus with the help of a magnet. The genome was made available for RT by heat-disrupting the viral particles. A single 40-cycle PCR was as sensitive as a nested PCR, both detecting 0.005 PFU of the Wa strain, corresponding to approximately 5 particles as indicated by EM. The nested PCR was positive for all the group A strains tested, but negative for group C rotaviruses and other RNA viruses. The IMS-RT-PCR method functioned satisfactorily with virus seeded out in fresh water samples; with sea water, the IMS removed most, but not all, inhibiting activity.

Membrane binding and endoplasmic reticulum retention sequences of rotavirus VP7 are distinct: role of carboxy-terminal and other residues in membrane binding

The sequences responsible for binding rotavirus glycoprotein VP7 to the membrane of the endoplasmic reticulum (ER) have not been identified. Here we show that the sequences which promote membrane binding in vitro are distinct from the N-terminal sequences which promote retention of VP7 in the ER in vivo. The role of the C-terminal region in membrane binding was also examined by using truncation mutants. Membrane binding in vitro was reduced but not abolished by removing up to 102 residues from the C terminus. The data suggest that the last 36 residues of VP7 may be present in the membrane or translocation pore, possibly with the C terminus protruding into the cytoplasm, since these residues contribute to, but do not account for, membrane binding. Surprisingly, modified forms of VP7 which are secreted from transfected cells showed the same membrane-binding properties in vitro as the protein retained in the ER membrane. Thus, secreted VP7 may not be present as a soluble polypeptide in the ER. A model to explain these results is presented. Previously published data are consistent with the idea that the highly conserved C terminus of nascent VP7 could have a cytoplasmic orientation which is important for assembly of mature virus particles.

Rotaviruses: immunological determinants of protection against infection and disease

Although studies of rotavirus immunity in experimental animals and humans have often yielded conflicting data, a preponderance of evidence supports the following answers to the questions initially posed. 1. What is the importance of virus serotype in formulating an optimal vaccine? Both vp4 and vp7 induce virus-neutralizing antibodies after either natural infection or immunization; the capacity of vp4 to induce rotavirus-specific neutralizing antibodies is probably greater than that of vp7. However, protection against disease after immunization of infants and young children is induced by strains heterotypic to the challenge virus (e.g., immunization with WC3 induces protection against disease induced by serotypically distinct human G1 strains). In addition, oral inoculation of infants with primate or bovine reassortant rotaviruses containing genes that encode human vp7 has not consistently induced a higher level of protection against challenge than that induced by parent animal rotaviruses (see Table I). Therefore, although vp4 or vp7 or both are probably important in inducing protection against challenge, it has not been clearly demonstrated that inclusion of the epidemiologically important human (as distinct from animal) P or G type is important in protection against human disease. 2. Which immunological effector arm most likely protects against rotavirus disease? No immunological effector arm clearly explains protection against heterotypic challenge. Protection against disease is not predicted by rotavirus-specific neutralizing antibodies in serum. Rotavirus-specific, binding sIgA in feces [detected by enzyme-linked immunosorbent assay (ELISA)] induced after natural infection does correlate with protection against disease induced by subsequent infection. However, protection after immunization with WC3 may occur in the absence of a detectable fecal sIgA response. The relationship between rotavirus-binding sIgA and sIgA-mediated neutralizing activity directed against the challenge virus remains to be determined. Binding rotavirus-specific sIgA in feces detected by ELISA may only be a correlate of other events occurring at the intestinal mucosal surface. The presence of broadly cross-reactive, rotavirus-specific CTLs at the intestinal mucosal surface of mice acutely after infection is intriguing. It would be of interest to determine the degree to which the presence of cross-reactive, rotavirus-specific CTLs in the circulation is predictive of the presence of virus-specific CTLs among intestinal lymphocytes and protection against challenge. Unfortunately, studies of virus-specific CTLs are difficult to perform in children. 3. By what means is virus antigen best presented to the host to elicit a protective immune response? Oral inoculation may not be necessary to induce a protective, virus-specific immune response at the intestinal mucosal surface.(ABSTRACT TRUNCATED AT 400 WORDS)

Retention by the endoplasmic reticulum of rotavirus VP7 is controlled by three adjacent amino-terminal residues

The rotavirus outer capsid glycoprotein, VP7, is an endoplasmic reticulum (ER) membrane-associated glycoprotein in both infected and transfected cells. It was previously demonstrated in this laboratory and by others that both the cleaved signal sequence (H2) and the first NH2-terminal 61 amino acids of VP7 are sufficient and necessary for ER retention of this molecule. Using site-specific mutagenesis and transfection techniques, we show that residues Ile-9, Thr-10, and Gly-11 were specifically necessary for ER retention. These results further define the ER retention sequence of VP7 and demonstrate that conservative changes, apparently innocuous in only three adjacent amino acids, can lead to major solubility and compartmentalization changes. It was found that placement of the first 31 mature NH2-terminal residues of VP7, in addition to the cleaved ER translocation signal sequence, was sufficient to retain the enzymatically active chimeric alpha-amylase in the ER; this enzyme is normally secreted. Deletions of the residues Ile-9, Thr-10, and Gly-11 within the amylase chimera containing 31 VP7 amino acids resulted in secretion of enzymatically active protein. It was also observed that the residues of VP7 presented in certain chimeras were able to abolish alpha-amylase enzymatic activity. These chimeras are presumably misfolded since it was demonstrated by pulse-chase experiments that these molecules are degraded in the ER. We surmise that a favorable conformation is necessary for retention since ER retention and activity of the chimeras depend on the primary sequence context.

Biosynthesis and morphogenesis of group C rotavirus in swine testicular cells

Polypeptide synthesis and morphogenesis of a group C rotavirus (AmC-1) adapted to a continuous swine testicular cell line was examined. SDS-PAGE analysis of 35S methionine labeled infected cell lysates revealed 9 viral polypeptides (122, 98, 79, 78, 43, 41, 35, 24, and 20 kD). Viral polypeptide synthesis appeared to be maximal at 7-10h post infection. Purified group C virus grown in the presence of trypsin was found to contain seven structural polypeptides (122, 98, 79, 53, 43, 41, and 30 kD) by protein blotting and five polypeptides (98, 79, 78, 43, and 41 kD) by immunoprecipitation with a hyperimmune rabbit antisera. Tunicamycin treatment, Concanavalin A binding, protein blotting, endo-H treatment and 2,6H-mannose labeling suggested that group C rotavirus contains one structural glycoprotein (41 kD) with a corresponding precursor mol. wt. of 37 kD and one not previously identified nonstructural glycoprotein (24 kD) with a corresponding precursor mol. wt. of < or = 20 kD. Electron microscopy of infected swine testicular cells revealed an assembly process for group C rotavirus similar to group A, with single-shelled particles budding through the rough endoplasmic reticulum with concomitant acquisition of a transient membrane.

Development of candidate rotavirus vaccines

Candidate rotavirus vaccines tested to date have been developed using a 'Jennerian' approach. Strains of bovine and simian rotaviruses that are naturally attenuated for humans have been assessed and found to confer immunity that is serotype specific in a varying proportion of recipients. The spectrum of protection has been widened by developing reassortants in which the bovine or simian gene coding for VP7 (the major outer capsid protein) has been replaced by the corresponding gene from human VP7 types 1, 2, 3 or 4. Once the protective antigen(s) are identified it may be possible to develop subunit vaccines that eliminate side effects sometimes observed with live vaccine candidates.

Cell biology of viruses that assemble along the biosynthetic pathway

In this review we discuss five groups of viruses that bud into, or assemble from, different compartments along the biosynthetic pathway. These are herpes-, rota-, corona-, bunya- and pox-viruses. Our main emphasis will be on the virally-encoded membrane glycoproteins that are responsible for determining the site of virus assembly. In a number of cases these proteins have been well characterized and appear to serve as resident markers of the budding compartments. The assembly and dissemination of these viruses raises many questions of cell biological interest.

Characterization of an oligomerization domain and RNA-binding properties on rotavirus nonstructural protein NS34

Intermolecular interactions between polypeptide chains often play essential roles in such biological phenomena as replication, transcription, translation, transport, ligand binding, and assembly. We have initiated studies of the functions of the rotavirus SA114F gene 7 product by sequence analysis and expression in insect cells. This nonstructural protein, NS34, is a slightly acidic protein, and its secondary structure is predicted to be 78% alpha-helix, with several heptad repeats of hydrophobic amino acids being present in its carboxy half. NS34 was found in oligomers when analyzed in insect cells, in SA11-infected MA104 cells, and in cell-free translation reactions. Investigation of the multiple electrophoretically distinct forms of NS34 showed they were all composed of homooligomers. Deletion mutants constructed and tested for oligomerization showed that the carboxy terminus of the protein, containing the predicted heptad repeats, was responsible for oligomerization. A basic region present in NS34 of group A rotaviruses, found to be 40% conserved in NS34 of group C rotavirus, is a candidate for a functional domain of this protein. NS34, which was found to be associated with the cytoskeleton fraction of cells, also interacts with viral RNA. These results make it likely that NS34 plays a central role in the replication and assembly of genomic RNA structures.

Transient expression and mutational analysis of the rotavirus intracellular receptor: the C-terminal methionine residue is essential for ligand binding

Maturation of rotavirus involves an intracellular membrane budding event in which the single-shelled icosahedral particle interacts with a virus-encoded receptor glycoprotein, NS28, that is located in the rough endoplasmic reticulum membrane. The receptor is a tetramer and is oriented with the C-terminal 131 amino acids on the cytoplasmic side of the membrane (A.R. Bellamy and G.W. Both, Adv. Virus Res. 38:1-48, 1990). We have used the T7-vaccinia virus transient expression system to deliver mutant variants of the NS28 gene to CV1 cells in order to assess the effects of site-specific modifications on receptor function. Three types of mutant proteins have been constructed by altering the extreme C-terminal methionine, cysteine residues within the third hydrophobic domain, and internal residues located within the cytoplasmic portion of the receptor, respectively. Deletion or conservative substitution of the C-terminal methionine completely abolishes receptor activity. Substitution of cysteine residues has no effect on receptor activity or on the ability of the receptor to adopt its native oligomeric state. Internal deletions result only in a reduction in the level of binding. An N-terminally truncated form of the receptor, containing only the cytoplasmic domain, retains full receptor activity and can form membrane-associated tetramers.

Sequences of the four larger proteins of a porcine group C rotavirus and comparison with the equivalent group A rotavirus proteins

The sequences of the four larger proteins of rotavirus group C (Cowden strain) are presented and compared with the sequences of the corresponding group A proteins. They exhibit a significant level of homology, allowing gene coding assignment for the group C rotavirus. The coding strategy of the group C virus RNA segment is the same as that for the group A large segments as one long open reading frame is present in each segment. The genome segment 1 encodes the structural protein VP1 which presents the RNA-dependent RNA polymerase consensus motifs. The VP1 protein is the most highly conserved between the rotaviruses of groups A and C. The genome segment 2 encodes the VP2 protein. The deduced protein sequence does not present the putative leucine zippers identified in the group A protein but its amino terminal is hydrophilic and highly charged as previously noted for the group A protein. The genome segment 3 encodes for a protein homologous to the group A outer capsid protein VP4. As observed among the various group A sequences, the amino terminal is the region presenting the fewest similarities. A cleavage region and a putative fusion motif similar to those present in the group A viruses have been identified. For this protein the comparison has been extended to the IDIRV [corrected] VP3 previously sequenced and indicates that groups A and C VP4 proteins are much more related to each other than to the group B equivalent. The genome segment 4 encodes for a protein showing an approximate 40% sequence identity to the minor core protein, VP3, of the group A rotavirus. This remarkable conservation of primary structures argues for severe functional constraint on the evolution of these proteins.

Rotavirus spike structure and polypeptide composition

Negatively stained preparations of rotavirus imaged with a low dose of electrons provide sufficient contrast to reveal surface projections or spikes. The number of spikes found projecting from different particles indicates that not all 60 peripentonal sites are occupied. Treatment at pH 11.2 with 250 mM ammonium hydroxide specifically removes the spikes, yielding smooth double-shelled particles of the same diameter as that of the native virus. Protein analysis confirms that the released spikes are composed of polypeptide VP4 (or its two cleavage products VP5* and VP8*) and that the smooth particle retains the other major outer shell protein VP7. Spikeless particles can be decorated by a monoclonal antibody specific for the major immunodominant neutralizing domain of VP7, implying that removal of the spikes does not denature the VP7 that is retained on the surface of the smooth particle.

Expression of rotavirus proteins encoded by alternative open reading frames of genome segment 11

The nucleotide sequence of rotavirus genome segment 11 shows that this gene contains three potential open reading frames. We used several approaches to determine whether any polypeptides other than NS26, the primary protein product, are expressed. In particular, we sought to determine whether the strong out-of-phase start codon present at nucleotides 80-82, which would encode a protein of 92 amino acids, is used in vivo or in cell-free systems. Several modifications of gene 11 were made and found to produce proteins from the different initiation codons in cell-free transcription-translation systems. The protein from the out-of-phase open reading frame was shown to be expressed in rotavirus-infected MA104 cells; this was demonstrated using monospecific sera prepared to this protein expressed in Spodoptera frugiperda insect cells infected with a baculovirus recombinant containing only the out-of-phase open reading frame. The origin of some of the lower-molecular-weight bands serologically related to the primary product of gene 11, NS26, was also studied by selective immunoprecipitation using two different sera made from recombinant baculovirus lysates. All of these polypeptides are present in infected cells in a complex which is still incompletely defined.

Completion of the genomic sequence of the simian rotavirus SA11: nucleotide sequences of segments 1, 2, and 3

The nucleotide sequences for gene segments 1, 2, and 3 of the simian rotavirus SA11 genome, coding for the structural polypeptides VP1, VP2, and VP3, respectively, have been determined. Comparison of the VP1 and VP2 amino acid sequences with those determined for other strains indicates that certain features of these proteins are conserved. The possible functions of the viral polypeptides VP1, VP2, and VP3 are discussed in the light of enzyme functions known to be present in the rotavirus particle. The complete sequence of the entire SA11 genome, which consists of 11 segments of dsRNA totaling 18,555 nucleotides, has now been determined. This is the first complete sequence available for a rotavirus genome. Each genome segment appears to code for only one primary product; there are no significant, alternative open reading frames which are conserved between strains. Relevant data for each genome segment are tabulated.