Rotavirus SA11 genome segment 11 protein is a nonstructural phosphoprotein

Expression of two bovine rotavirus non-structural proteins (NSP2, NSP3) in the baculovirus system and production of monoclonal antibodies directed against the expressed proteins

Studies on rotavirus non-structural proteins have been hampered in the past by difficulties in obtaining monospecific reagents. To make such reagents available, we have expressed in the baculovirus system NSP2 and NSP3 (formerly called NS35 and NS34, respectively) of the bovine rotavirus RF and produced hybridomas against these proteins. Full-length DNA copies of RNA segments 7 (coding for NSP3) and 8 (coding for NSP2) of the virus strain RF were cloned and sequenced. Each cDNA was inserted in the transfer vector pVL941 and used to transfect Spodoptera frugiperda cells (Sf9). Recombinant baculoviruses encoding these proteins were obtained. Infection of Sf9 cells with these recombinant viruses resulted in a high level of expression of NSP2 and NSP3 (range of 1 microgram per 10(6) cells). Monoclonal antibodies (MAbs) were elicited by immunization of BALB/c mice with adjuvented, unpurified recombinant proteins in the rear foot pads. Fusion was performed using lymphocytes from popliteal lymph nodes with SP2/O-Ag14 myeloma line. Screening was by differential indirect immunofluorescent staining on monolayers of Sf9 cells infected with each recombinant virus. Two MAbs proved to be reactive against NSP3 and a single one against NSP2. They showed high specificity by immunofluorescence, immunoprecipitation and Western blot. The isotype of these MAbs was IgG1. Oligomeric forms of NSP3 and NSP2 proteins were detected and the existence of intra-chain disulfide bridge in NSP2 protein was suggested. The levels of synthesis and cellular localization of NSP3 and NSP2 proteins were different as shown by immunoprecipitation and immunofluorescence.

The rotavirus nonstructural protein, NS35, possesses RNA-binding activity in vitro and in vivo

Toward the goal of identifying and characterizing rotavirus RNA-binding proteins, we have used a gel retardation assay and protein-RNA cross-linking by ultraviolet (uv) light to examine cytoplasmic lysates prepared from SA11-infected cells for the presence of RNA-binding proteins. Analysis of band shifts produced in the gel retardation assay indicated that infected cells contained significant amounts of a viral protein which had affinity for both single-stranded and double-stranded RNA but lacked sequence specificity. Cross-linking of this protein to radiolabeled RNA in vitro followed by RNase treatment and immunoprecipitation with an anti-NS35 monoclonal antibody revealed that the RNA-binding activity was associated with NS35. Moreover, sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of the protein-RNA complex isolated from native gels revealed that NS35 was the only viral protein component of the complex. Since NS35 expressed by translation in rabbit reticulocyte lysates exhibited affinity for poly(U)-Sepharose, NS35 must possess intrinsic RNA-binding activity that is able to function in the absence of other viral proteins. Immunoprecipitation of RNase-treated cross-links formed in intact cells following exposure to uv light confirmed that NS35 was intimately associated with ssRNA in the infected cell. On the basis of its ability to bind RNA and given that previous studies have shown that NS35 localizes to the viroplasm in infected cells, is essential for RNA replication, and is a component of replicase particles, we propose that NS35 functions to concentrate viral mRNAs in the viroplasm and that NS35-mRNA complexes serve as substrates for genome assortment and replication.

Terminal sequence conservation among the genomic segments of a group B rotavirus (IDIR strain)

Terminal nucleic acid sequences were determined for all 11 segments of the IDIR strain of group B rotavirus. Consensus sequences were defined at both ends of the (+) RNA strands as 5' GGN(A/U)NA(A/U)(A/U)(A/U)---and---(A/U)NA(A/G)N(A/C)(C/A)CC3 '. The 5' and 3' terminal sequences of the (+) strand IDIR RNA were not complementary to one another. The IDIR terminal sequences and those of group A rotaviruses (GAR) were similar in that each of the (+) strands began with "GG" and ended with "CC." Otherwise, the IDIR terminal sequences did not match the consensus sequences that have been reported for the ends of the GAR genomic segments.

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.

Extraintestinal rotavirus infections in children with immunodeficiency

Some rotavirus strains, including vaccine candidates, have been demonstrated to cause hepatitis in immunodeficient and malnourished mice and to grow in human liver cells. To determine whether rotavirus spreads outside the intestine in naturally infected children, we examined tissues from four immunodeficient children affected with severe combined immunodeficiency disease, acquired immunodeficiency disease syndrome, or DiGeorge syndrome. Chronic rotavirus-related diarrhea, which persisted until death, had also developed in each child. Using indirect immunoperoxidase techniques, we identified rotavirus antigen in the liver and kidney with a hyperimmune guinea pig antiserum prepared to double-shelled rotavirus particles. Similar immunostaining with an antiserum to a rotavirus nonstructural protein (NS26) provided evidence of active virus replication. The observed reactivity was eliminated specifically when serial sections were immunostained with the same antiserum that had been absorbed with either double-shelled rotavirus particles or NS26. Immunostaining was not observed in the liver of children with other diseases (alpha 1-antitrypsin deficiency, inspissated bile syndrome, and acute rejection of a transplanted liver). These findings demonstrate that rotavirus infections in children can extend beyond the intestinal tract. Further studies are warranted to determine whether extraintestinal rotavirus replication occurs in children without severe immunodeficiency, such as malnourished children.

Coding assignments of the genome of adult diarrhea rotavirus

Adult diarrhea rotavirus (ADRV) has caused epidemics of diarrhea in China since 1982 and remains the only group B rotavirus associated with widespread disease in humans. We recently characterized the proteins of ADRV and have now proceeded to identify the gene segments encoding each protein. Viral RNA transcripts were synthesized in vitro with the endogenous viral RNA polymerase and separated by electrophoresis in agarose. The individual transcripts were translated in a cell-free system using nuclease-treated rabbit reticulocyte lysates. The translation products were compared with polypeptides found in purified virus and were characterized by SDS-PAGE, immunoprecipitation, and Western blot analysis using antisera to double- and single-shelled virions, virus cores, and monoclonal antibodies. Furthermore, individual RNA transcripts were hybridized to total dsRNA to determine their genomic origin. Based on this analysis, the core polypeptides VP1, VP2 and VP3 are encoded by segments 1, 2, and 3, respectively. The main polypeptides in the inner capsid, VP6, and the outer capsid, VP4 and VP7, are encoded by segments 6, 4, and 8 respectively. Segments 5, 7, and 9 code for 60, 45, and 30 kDa nonstructural polypeptides. Two other nonstructural polypeptides (24 and 25 kDa) are derived from gene segment 11. Gene segment 10 codes for a 26 kDa polypeptide that is precipitated with serum to ADRV and may be a structural protein VP9. With this exception, gene coding assignments of ADRV are comparable to those of the group A rotaviruses. Our results have clear implications for further work in cloning, sequencing, and expression genes of ADRV and can provide direction towards understanding the origin and the evolution of this virus.

Rotavirus NS26 is modified by addition of single O-linked residues of N-acetylglucosamine

We studied the post-translational modification of NS26, the protein product of rotavirus gene 11 segment. Based on the presence of a putative N-glycosylation site and the high content of serine and threonine residues in gene 11 amino acid sequence we investigated whether NS26 is modified by carbohydrate addition. Specific antibodies raised against the gene 11 product expressed in Escherichia coli recognized in infected cells two polypeptides with apparent molecular weight of 26,000 (26-kDa polypeptide) and 28,000 (28-kDa polypeptide). Pulse-chase experiments demonstrated that the 26-kDa product was processed to the 28-kDa polypeptide. Both polypeptides were metabolically labeled with [3H]glucosamine, indicating the presence of a carbohydrate moiety on the protein. NS26 was found to be resistant to endo-beta-N-acetylglucosaminidase H and endo-beta-N-acetylglucosaminidase F/peptide:N-glycosidase F treatment, but sensitive to removal by alkali-induced beta-elimination, suggesting that the saccharide chain was attached to the protein via an O-glycosidic linkage. Chromatographic analysis of total acid hydrolysates of [3H]glucosamine-labeled NS26-bound carbohydrate indicated the presence of N-acetylglucosamine. In addition, mild alkaline treatment of NS26 in the presence of NaB3H4 identified the O-linked carbohydrate moiety as N-acetylglucosamine. Taken together, these data demonstrate that NS26 is processed to a 28-kDa polypeptide by addition of O-linked monosaccharide residues of N-acetylglucosamine.

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.

Biochemical characterization of the structural and nonstructural polypeptides of a porcine group C rotavirus

Purified virions or radiolabeled lysates of infected MA104 cells were used to characterize the structural and nonstructural polypeptides of a porcine group C rotavirus. At least six structural proteins were identified from purified group C rotavirus by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Of these, two (37,000- and 33,000-molecular-weight polypeptides) were associated with the outer shell, as demonstrated by the ability of EDTA to remove them from the purified virion. The other four polypeptides (molecular weights, 125,000, 93,000, 74,000, and 41,000) were located in the inner shell. The structural or nonstructural nature of a 25,000-molecular-weight protein identified in our studies was unclear. Glycosylation inhibition studies with tunicamycin in infected cells demonstrated that the 37,000- and 25,000-molecular-weight proteins were glycosylated and contained mannose-rich oligosaccharides identified by radiolabeling of the infected cells with [3H]mannose. The 37,000-molecular-weight outer shell glycoprotein was shown by pulse-chase experiments to be posttranslationally processed. The kinetics of viral polypeptide synthesis in infected cells were also studied, and maximal synthesis occurred at 6 to 9 h postinfection. The 41,000-molecular-weight inner capsid polypeptide was the most abundant and was the subunit structure of a 165,000-molecular-weight protein aggregate. Two polypeptides (molecular weights, 39,000 and 35,000) appeared to be nonstructural, as determined by comparison of the protein pattern of radiolabeled infected cell lysates with that of purified virions. Radioimmunoprecipitation was used to examine the serologic cross-reactions between the viral polypeptides of a group C rotavirus with those of a group A rotavirus. No serologic cross-reactivities were detected. The polypeptides of group A and C rotaviruses are compared and discussed.

cDNA cloning of each genomic segment of the group B rotavirus ADRV: molecular characterization of the 11th RNA segment

The group B noncultivatable rotavirus, ADRV, was purified from infected stool specimens. Double-stranded RNA was extracted, polyadenylated, reverse transcribed into cDNA, and cloned into plasmid vector pAT153. Each cDNA clone hybridized to a single ADRV RNA segment and cDNA clones of each genomic RNA segment were identified. Sequencing of genomic RNA and a full-length cDNA clone (AD63) determined that ADRV RNA segment 11 is 631 bases in length and contains a single open reading frame of 170 amino acids with a calculated molecular weight of 19.9 kDa and a pl of 6.2. The RNA 5'- and 3'-termini contain untranslated regions of 58 and 63 bases, respectively, and are complementary to each other. A comparison of encoded ADRV gene 11 amino acids with the NS26 protein of group A rotaviruses demonstrates a distant similarity between the two proteins. Further analysis and use of these ADRV clones should aid in determining the gene coding assignments for group B proteins as well as for diagnostic detection of group B or ADRV-specific nucleic acids in specimens.

Rotavirus gene structure and function

Knowledge of the structure and function of the genes and proteins of the rotaviruses has expanded rapidly. Information obtained in the last 5 years has revealed unexpected and unique molecular properties of rotavirus proteins of general interest to virologists, biochemists, and cell biologists. Rotaviruses share some features of replication with reoviruses, yet antigenic and molecular properties of the outer capsid proteins, VP4 (a protein whose cleavage is required for infectivity, possibly by mediating fusion with the cell membrane) and VP7 (a glycoprotein), show more similarities with those of other viruses such as the orthomyxoviruses, paramyxoviruses, and alphaviruses. Rotavirus morphogenesis is a unique process, during which immature subviral particles bud through the membrane of the endoplasmic reticulum (ER). During this process, transiently enveloped particles form, the outer capsid proteins are assembled onto particles, and mature particles accumulate in the lumen of the ER. Two ER-specific viral glycoproteins are involved in virus maturation, and these glycoproteins have been shown to be useful models for studying protein targeting and retention in the ER and for studying mechanisms of virus budding. New ideas and approaches to understanding how each gene functions to replicate and assemble the segmented viral genome have emerged from knowledge of the primary structure of rotavirus genes and their proteins and from knowledge of the properties of domains on individual proteins. Localization of type-specific and cross-reactive neutralizing epitopes on the outer capsid proteins is becoming increasingly useful in dissecting the protective immune response, including evaluation of vaccine trials, with the practical possibility of enhancing the production of new, more effective vaccines. Finally, future analyses with recently characterized immunologic and gene probes and new animal models can be expected to provide a basic understanding of what regulates the primary interactions of these viruses with the gastrointestinal tract and the subsequent responses of infected hosts.

Receptor activity of rotavirus nonstructural glycoprotein NS28

Rotavirus morphogenesis involves the budding of subviral particles through the rough endoplasmic reticulum (RER) membrane of infected cells. During this process, particles acquire the outer capsid proteins and a transient envelope. Previous immunocytochemical and biochemical studies have suggested that a rotavirus nonstructural glycoprotein, NS28, encoded by genome segment 10, is a transmembrane RER protein and that about 10,000 Mr of its carboxy terminus is exposed on the cytoplasmic side of the RER. We have used in vitro binding experiments to examine whether NS28 serves as a receptor that binds subviral particles and mediates the budding process. Specific binding was observed between purified simian rotavirus SA11 single-shelled particles and RER membranes from SA11-infected monkey kidney cells and from SA11 gene 10 baculovirus recombinant-infected insect cells. Membranes from insect cells synthesizing VP1, VP4, NS53, VP6, VP7, or NS26 did not possess binding activity. Comparison of the binding of single-shelled particles to microsomes from infected monkey kidney cells and from insect cells indicated that a membrane-associated component(s) from SA11-infected monkey kidney cells interfered with binding. Direct evidence showing the interaction of NS28 and its nonglycosylated 20,000-Mr precursor expressed in rabbit reticulocyte lysates and single-shelled particles was obtained by cosedimentation of preformed receptor-ligand complexes through sucrose gradients. The domain on NS28 responsible for binding also was characterized. Reduced binding of single-shelled particles to membranes was seen with membranes treated with (i) a monoclonal antibody previously shown to interact with the C terminus of NS28, (ii) proteases known to cleave the C terminus of NS28, and (iii) the Enzymobead reagent. VP6 on single-shelled particles was suggested to interact with NS28 because (i) a monoclonal antibody to the subgroup I epitope on VP6 reduced particle binding, (ii) a purified polyclonal antiserum raised against recombinant baculovirus-produced VP6 reduced ligand binding, and (iii) a monoclonal antibody to a conserved epitope on VP6 augmented ligand binding. These experimental data provide support for the hypothesized receptor role of NS28 before the budding stage of rotavirus morphogenesis.