Synthesis of plus- and minus-strand RNA in rotavirus-infected cells

Rotavirus Particle Disassembly and Assembly In Vivo and In Vitro

Rotaviruses (RVs) are non-enveloped multilayered dsRNA viruses that are major etiologic agents of diarrheal disease in humans and in the young in a large number of animal species. The viral particle is composed of three different protein layers that enclose the segmented dsRNA genome and the transcriptional complexes. Each layer defines a unique subparticle that is associated with a different phase of the replication cycle. Thus, while single- and double-layered particles are associated with the intracellular processes of selective packaging, genome replication, and transcription, the viral machinery necessary for entry is located in the third layer. This modular nature of its particle allows rotaviruses to control its replication cycle by the disassembly and assembly of its structural proteins. In this review, we examine the significant advances in structural, molecular, and cellular RV biology that have contributed during the last few years to illuminating the intricate details of the RV particle disassembly and assembly processes.

FAST Proteins: Development and Use of Reverse Genetics Systems for Reoviridae Viruses

Reverse genetics systems for viruses, the technology used to generate gene-engineered recombinant viruses from artificial genes, enable the study of the roles of the individual nucleotides and amino acids of viral genes and proteins in infectivity, replication, and pathogenicity. The successful development of a reverse genetics system for poliovirus in 1981 accelerated the establishment of protocols for other RNA viruses important for human health. Despite multiple efforts, rotavirus (RV), which causes severe gastroenteritis in infants, was refractory to reverse genetics analysis, and the first complete reverse genetics system for RV was established in 2017. This novel technique involves use of the fusogenic protein FAST (fusion-associated small transmembrane) derived from the bat-borne Nelson Bay orthoreovirus, which induces massive syncytium formation. Co-transfection of a FAST-expressing plasmid with complementary DNAs encoding RV genes enables rescue of recombinant RV. This review focuses on methodological insights into the reverse genetics system for RV and discusses applications and potential improvements to this system.

[Rotaviruses]

Rotavirus, a member of the family Reoviridae, was identified as the leading etiological agent of severe gastroenteritis in infants and young children in 1973. The rotavirus genome is composed of 11 gene segments of double-stranded (ds)RNA. During the last 40 years, a large amount of basic research on rotavirus structure, genome, antigen, replication, pathogenesis, epidemiology, immune responses, and evolution has been accumulated. This article reviews the fundamental aspects of rotavirology including recent important achievements in research.

Regulation of rotavirus polymerase activity by inner capsid proteins

Rotavirus, a cause of pediatric gastroenteritis, has a genome consisting of 11 segments of double-stranded (ds)RNA surrounded by a triple-layered protein capsid. The rotavirus RNA-dependent RNA polymerase, VP1, synthesizes both dsRNA and plus-strand RNA (+RNA) within subviral particles. Structural analyses of the rotavirus capsid and polymerase, combined with functional studies of purified capsid proteins, indicate that the inner capsid protein controls the initiation of RNA synthesis by VP1. Whether VP1 directs dsRNA versus +RNA synthesis may be regulated by the impact of the viral RNA capping enzyme on the position of the polymerase plug, a flexible element that inserts into one of the polymerase's RNA exit tunnels. This review discusses recent findings and ideas into the mechanisms used by rotavirus capsid proteins to control the activities of its viral polymerase and to coordinate RNA synthesis with the assembly of virus particles.

Characterization of rotavirus RNAs that activate innate immune signaling through the RIG-I-like receptors

In mammalian cells, the first line of defense against viral pathogens is the innate immune response, which is characterized by induction of type I interferons (IFN) and other pro-inflammatory cytokines that establish an antiviral milieu both in infected cells and in neighboring uninfected cells. Rotavirus, a double-stranded RNA virus of the Reoviridae family, is the primary etiological agent of severe diarrhea in young children worldwide. Previous studies demonstrated that rotavirus replication induces a MAVS-dependent type I IFN response that involves both RIG-I and MDA5, two cytoplasmic viral RNA sensors. This study reports the isolation and characterization of rotavirus RNAs that activate IFN signaling. Using an in vitro approach with purified rotavirus double-layer particles, nascent single-stranded RNA (ssRNA) transcripts (termed in vitro ssRNA) were found to be potent IFN inducers. In addition, large RNAs isolated from rotavirus-infected cells six hours post-infection (termed in vivo 6 hr large RNAs), also activated IFN signaling, whereas a comparable large RNA fraction isolated from cells infected for only one hour lacked this stimulatory activity. Experiments using knockout murine embryonic fibroblasts showed that RIG-I is required for and MDA5 partly contributes to innate immune signaling by both in vitro ssRNA and in vivo 6 hr large RNAs. Enzymatic studies demonstrated that in vitro ssRNA and in vivo 6 hr large RNA samples contain uncapped RNAs with exposed 5’ phosphate groups. RNAs lacking 2’-O-methylated 5’ cap structures were also detected in the in vivo 6 hr large RNA sample. Taken together, our data provide strong evidence that the rotavirus VP3 enzyme, which encodes both guanylyltransferase and methyltransferase activities, is not completely efficient at either 5’ capping or 2’-O-methylation of the 5’ cap structures of viral transcripts, and in this way produces RNA patterns that activate innate immune signaling through the RIG-I-like receptors.

Human milk oligosaccharides inhibit rotavirus infectivity in vitro and in acutely infected piglets

Human milk (HM) is rich in oligosaccharides (HMO) that exert prebiotic and anti-infective activities. HM feeding reduces the incidence of rotavirus (RV) infection in infants. Herein, the anti-RV activity of oligosaccharides was tested in an established in vitro system for assessing cellular binding and viral infectivity/replication, and also tested in a newly developed, acute RV infection, in situ piglet model. For the in vitro work, crude HMO isolated from pooled HM, neutral HMO (lacto-N-neotetraose, LNnT; 2'-fucosyllactose) and acidic HMO (aHMO, '-sialyllactose, 3'-SL; -sialyllactose, -SL) were tested against the porcine OSU strain and human RV Wa strain. The RV Wa strain was not inhibited by any oligosaccharides. However, the RV OSU strain infectivity was dose-dependently inhibited by sialic acid (SA)-containing HMO. 3'-SL and 6'-SL concordantly inhibited (125)I-radiolabelled RV cellular binding and infectivity/replication. For the in situ study, a midline laparotomy was performed on 21-d-old formula-fed piglets and six 10 cm loops of ileum were isolated in situ. Briefly, 2 mg/ml of LNnT, aHMO mixture (40% 6'-SL/10 % 3'-SL/50 % SA) or media with or without the RV OSU strain (1 x 10(7) focus-forming units)were injected into the loops and maintained for 6 h. The loops treated with HMO treatments þ RV had lower RV replication, as assessed by non-structural protein-4 (NSP4) mRNA expression, than RV-treated loops alone. In conclusion, SA-containing HMO inhibited RV infectivity in vitro; however, both neutral HMO and SA with aHMO decreased NSP4 replication during acute RV infection in situ.

Assortment and packaging of the segmented rotavirus genome

The rotavirus (RV) genome comprises 11 segments of double-stranded RNA (dsRNA) and is contained within a non-enveloped, icosahedral particle. During assembly, a highly coordinated selective packaging mechanism ensures that progeny RV virions contain one of each genome segment. Cis-acting signals thought to mediate assortment and packaging are associated with putative panhandle structures formed by base-pairing of the ends of RV plus-strand RNAs (+RNAs). Viral polymerases within assembling core particles convert the 11 distinct +RNAs to dsRNA genome segments. It remains unclear whether RV +RNAs are assorted before or during encapsidation, and the functions of viral proteins during these processes are not resolved. However, as reviewed here, recent insights gained from the study of RV and two other segmented RNA viruses, influenza A virus and bacteriophage Φ6, reveal potential mechanisms of RV assortment and packaging.

Rotavirus replication requires a functional proteasome for effective assembly of viroplasms

The ubiquitin-proteasome system has been shown to play an important role in the replication cycle of different viruses. In this study, we describe a strong impairment of rotavirus replication upon inhibition of proteasomal activity. The effect was evidenced at the level of accumulation of viral proteins, viral RNA, and yield of infective particles. Kinetic studies revealed that the early steps of the replicative cycle following attachment, entry, and uncoating were clearly more sensitive to proteasome inhibition. We ruled out a direct inhibition of the viral polymerase activities and stability of viral proteins and found that the crucial step that was impaired by blocking proteasome activity was the assembly of new viroplasms. This was demonstrated by using chemical inhibitors of proteasome and by gene silencing using small interfering RNAs (siRNAs) specific for different proteasomal subunits and for the ubiquitin precursor RPS27A. In addition, we show that the effect of proteasome inhibition on virus infection is not due to increased levels of beta interferon (IFN-β).

Analysis of the kinetics of transcription and replication of the rotavirus genome by RNA interference

Rotaviruses have a genome composed of 11 segments of double-stranded RNA (dsRNA) surrounded by three protein layers. The virus contains an RNA-dependent RNA polymerase that synthesizes RNA transcripts corresponding to all segments of the viral genome. These transcripts direct the synthesis of the viral proteins and also serve as templates for the synthesis of the complementary strand to form the dsRNA genome. In this work, we analyzed the kinetics of transcription and replication of the viral genome throughout the replication cycle of the virus using quantitative reverse transcription-PCR. The role of the proteins that form double-layered particles ([DLPs] VP1, VP2, VP3, and VP6) in replication and transcription of the viral genome was analyzed by silencing their expression in rotavirus-infected cells. All of them were shown to be essential for the replication of the dsRNA genome since in their absence there was little synthesis of viral mRNA and dsRNA. The characterization of the kinetics of RNA transcription and replication of the viral genome under conditions where these proteins were silenced provided direct evidence for a second round of transcription during the replication of the virus. Interestingly, despite the decrease in mRNA accumulation when any of the four proteins was silenced, the synthesis of viral proteins decreased when VP2 and VP6 were knocked down, whereas the absence of VP1 and VP3 did not have a severe impact on viral protein synthesis. Characterization of viral particle assembly in the absence of VP1 and VP3 showed that while the formation of triple-layered particles and DLPs was decreased, the amount of assembled lower-density particles, often referred to as empty particles, was not different from the amount in control-infected cells, suggesting that viral particles can assemble in the absence of either VP1 or VP3.

Association of rotavirus viroplasms with microtubules through NSP2 and NSP5

Rotavirus replication and virus assembly take place in electrodense spherical structures known as viroplasms whose main components are the viral proteins NSP2 and NSP5. The viroplasms are produced since early times after infection and seem to grow by stepwise addition of viral proteins and by fusion, however, the mechanism of viropIasms formation is unknown. In this study we found that the viroplasms surface colocalized with microtubules, and seem to be caged by a microtubule network. Moreover inhibition of microtubule assembly with nocodazole interfered with viroplasms growth in rotavirus infected cells. We searched for a physical link between viroplasms and microtubules by co-immunoprecipitation assays, and we found that the proteins NSP2 and NSP5 were co-immunoprecipitated with anti-tubulin in rotavirus infected cells and also when they were transiently co-expressed or individually expressed. These results indicate that a functional microtubule network is needed for viroplasm growth presumably due to the association of viroplasms with microtubules via NSP2 and NSP5.

Mechanism of genome transcription in segmented dsRNA viruses

Genome transcription is a critical stage in the life cycle of a virus, as this is the process by which the viral genetic information is presented to the host cell protein synthesis machinery for the production of the viral proteins needed for genome replication and progeny virion assembly. Viruses with dsRNA genomes face a particular challenge in that host cells do not produce proteins which can transcribe from a dsRNA template. Therefore, dsRNA viruses contain all of the necessary enzymatic machinery to synthesize complete mRNA transcripts within the core without the need for disassembly. Indeed one of the more striking observations about genome transcription in dsRNA viruses is that this process occurs efficiently only when the transcriptionally competent particle is fully intact. This observation suggests that all of the components of the TCP, including the viral genome, the transcription enzymes, and the viral capsid, function together to produce and release mRNA transcripts and that each component has a specific and critical role to play in promoting the efficiency of this process. This review has examined the process of genome transcription in dsRNA viruses from the perspective of rotavirus as a model system. However, despite numerous architectural and organizational differences among the families of dsRNA viruses, numerous studies suggest that the basic mechanism of mRNA production may be similar in most, if not all, viruses having dsRNA genomes. Important functional similarities include (1) the presence of a capsid-bound RNA-dependent RNA polymerase, which produces single-stranded mRNA transcripts from the dsRNA genome and regenerates the dsRNA genome from single-stranded RNA templates; (2) in viruses infecting eukaryotic hosts, the presence of all the enzymatic activities needed to generate the 5' cap required by the eukaryotic translation machinery; (3) the high degree of structural order present in the packaged genome, suggesting the requirement for organization in the viral core; (4) the role of the innermost capsid protein as a scaffold on which the core components of the transcription apparatus are assembled; and (5) the release of nascent mRNA transcripts through channels at the icosahedral vertices. The process of genome transcription in dsRNA viruses will become better understood as structural studies progress to higher resolution and as more viruses become amenable to study using site-directed mutagenesis coupled with viral reconstitution to generate recombinant particles having precise functional and structural changes. Future studies will dissect important intermolecular interactions required for efficient mRNA synthesis and will shed further light on the reasons for which the viral core must be structurally intact in order for transcription to occur efficiently. Structural studies of the capping enzymes at atomic resolution will reveal how multiple enzyme activities reside within a single polypeptide and how they act in concert to synthesize the 5' cap on the end of each mature transcript. Perhaps most interestingly, high resolution structural studies of actively transcribing virions will provide insight into the conformational changes that occur within the core during mRNA synthesis. Together, these studies will clarify the function of this complex macromolecular machine and will also shed additional light on the basic principles of virus architecture and assembly, as well as provide avenues for the design of antiviral therapies.

cis-Acting signals that promote genome replication in rotavirus mRNA

A previous study has shown that rotavirus cores have an associated replicase activity which can direct the synthesis of double-stranded RNA from viral mRNA in a cell-free system (D. Y. Chen, C. Q.-Y. Zeng, M. J. Wentz, M. Gorziglia, M. K. Estes, and R. F. Ramig, J. Virol. 68:7030-7039, 1994). To define the cis-acting signals in rotavirus mRNA that are important for RNA replication, gene 8 transcripts which contained internal and terminal deletions and chimeric transcripts which linked gene 8-specific 3'-terminal sequences to the ends of nonviral sequences were generated. Analysis of these RNAs in the cell-free system led to the identification of a cis-acting signal in the gene 8 mRNA which is essential for RNA replication and two cis-acting signals which, while not essential for replication, serve to enhance the process. The sequence of the essential replication signal is located at the extreme 3' end of the gene 8 mRNA and, because of its highly conserved nature, is probably a common feature of all 11 viral mRNAs. By site-specific mutagenesis of the gene 8 mRNA, residues at positions -1, -2, -5, -6, and -7 of the 3' essential signal were found to be particularly important for promoting RNA replication. One of the cis-acting signals shown to enhance the replication in the cell-free system was located near the 5' end of the 3' untranslated region (UTR) of the gene 8 mRNA, while remarkably the other was located in the 5' UTR of the message. The existence of an enhancement signal in the 5' UTR raises the possibility that the 5' and 3' ends of the rotavirus mRNA may interact with each other and/or with the viral replicase during genome replication.

Detection of dengue virus from mosquito cell cultures inoculated with human serum in the presence of actinomycin D

We report the use of cultures of mosquito cells (TRA-284) to detect dengue virus in serum from cases of dengue fever in the state of Puebla, México. Using the conventional procedure 56 of 171 samples (32.7%) were positive. The negative sera (67.3%) were passaged 'blind' in mosquito cell cultures but no virus was detected. However, when these sera were incubated in the presence of actinomycin D (an inhibitor of deoxyribonucleic acid transcription) 20 of the 115 samples (17.4%) became positive. This procedure increased the virus detection rate from 32.7% to 44.4%. Serotypes 1 and 4 were identified for the first time in the state of Puebla, where the transmission of dengue virus is increasing. The addition of actinomycin D to mosquito cell cultures may improve the detection of dengue virus and could be a useful tool for virological surveillance in endemic countries.

Rotavirus protein NSP3 (NS34) is bound to the 3' end consensus sequence of viral mRNAs in infected cells

Interaction between viral proteins and RNAs has been studied in rotavirus-infected cells. The use of UV cross-linking followed by immunoprecipitation and labeling with T4 polynucleotide kinase allowed us to detect interactions between RNA and nonstructural viral proteins. The RNAs linked to the nonstructural protein NSP3 have been identified as rotavirus mRNAs, and the sequences of the RNase T1-protected fragments have been established. These sequences correspond to the 3' end sequence common to all rotavirus group A genes. We also show that the last 3' nucleotide is cross-linked to the protein and that monomeric and multimeric forms of NSP3 are bound to rotavirus mRNA. The role of NSP3 in rotavirus replication is discussed in the light of our results and by comparison with other RNA-binding proteins of members of the Reoviridae family.

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.

The stimulatory effect of actinomycin D on avian reovirus replication in L cells suggests that translational competition dictates the fate of the infection

Indirect immunostaining of avian reovirus S1133-infected L-cell monolayers showed that most of the cells can support viral replication. However, the number of cells in which the virus was actually replicating depended on the multiplicity of virus infection. The presence of actinomycin D during infection increased viral protein synthesis, viral growth, and the number of actively infected cells at late infection times. The antibiotic elicited these effects by triggering viral replication in cells that already contained unproductive cytoplasmic virus but that would not get productively infected in the absence of the drug. From these results, we propose a model for the interaction between L cells and avian reovirus S1133 in which viral versus host mRNA competition for the translational machinery determines the fate of the virus infection.

Evidence for equimolar synthesis of double-strand RNA and minus-strand RNA in rotavirus-infected cells

The genome of the rotaviruses consists of eleven segments of double-strand RNA (dsRNA). Each segment is replicated asymmetrically with viral plus-strand RNA, i.e. messenger (m)RNA, serving as the template for the synthesis of minus-strand RNA to produce dsRNA. To examine the relative frequency of replication of each of the eleven genome segments, MA104 cells were infected with low (3rd) and high (12th) passage stocks of simian rotavirus SA11. The total cytoplasmic RNA of the infected cell was radiolabeled either by maintaining the infected cells in the presence [3H]uridine prior to harvest or by 3'-endlabeling the purified RNA with [32P]pCp and T4 RNA ligase. The RNA was then analyzed for the presence of 3H- and 32P-labeled dsRNA by electrophoresis on 10% polyacrylamide gels. Total cytoplasmic RNA from infected cells was also 3'-end-labeled with [32P]pCp and T4 RNA ligase and examined for the presence of minus-strand RNA by electrophoresis on low pH agarose-urea gels. Bands representing dsRNAs and minus-strand RNAs on autoradiographs of the gels were analyzed for intensity by densitometry. The results showed that the eleven segments of viral dsRNA were present in equimolar concentrations in cells either infected with low or high passage stocks of virus. Like intracellular dsRNAs, full-length minus-strand RNAs were also present in equimolar concentration in cells either infected with low or high passage rotavirus. These data indicate that, despite the non-equimolar levels of viral RNAs in the cell, the eleven genome segments of rotavirus are replicated with equal frequencies in vivo.

Intracellular RNA synthesis directed by temperature-sensitive mutants of simian rotavirus SA11

The kinetics of intracellular synthesis of single-stranded (ss) RNA and double-stranded (ds) RNA directed by prototype temperature-sensitive (ts) mutants representing the 10 mutant groups of rotavirus SA11 were examined. Cells were infected with individual mutants or wild type under one-step growth conditions and maintained at permissive temperature (31 degrees) or nonpermissive temperature (39 degrees). At various times postinfection, infected cells were pulse-labeled, ssRNA and dsRNA were purified, RNA species were resolved by electrophoresis and autoradiography, and RNA synthesis was quantitated by computer-assisted densitometry. The mutants representing all groups synthesized significantly less ssRNA and dsRNA at both 31 degrees and 39 degrees, when compared to wild type. When the ratio of synthesis at 39 degrees/31 degrees was determined for ssRNA and dsRNA of each mutant, three RNA synthesis phenotypes were evident. The tsB(339), tsC(606), and tsE(1400) mutants synthesized both ssRNA and dsRNA in a temperature-dependent manner. The group G mutant, tsG(2130), synthesized ssRNA in temperature-independent fashion but was temperature-dependent for the synthesis of dsRNA. The remaining mutants, tsA(778), tsD(975), tsF(2124), tsH(2384), tsI(2403), and tsJ(2131), synthesized both ssRNA and dsRNA in a temperature-independent fashion. The RNA synthesis phenotypes of the ts mutants are discussed in terms of what is known of the function(s) of the protein species to which ts lesions have been assigned.

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.

Characterization of rotavirus replication intermediates: a model for the assembly of single-shelled particles

The segmented double-stranded (ds)RNA genome of the rotaviruses is replicated asymmetrically with viral mRNA serving as the template for minus-strand RNA synthesis. To identify intermediate structures in rotavirus replication, subviral particles (SVPs) purified from the cytoplasm of simian rotavirus SA11-infected cells were assayed for RNA polymerase activity in a cell-free system that supports viral RNA replication. Intact SVPs containing newly made RNA were resolved by electrophoresis under nondenaturing conditions on 0.6% agarose gels (50 mM Tris-glycine, pH 8.8). This gel system was found to separate without disrupting SA11 single- and double-shelled virions and virion-derived core particles. SVPs from the cell-free system that contained newly made dsRNA migrated in the agarose gels at positions between virion-derived cores and intermediate of single- and double-shelled virions. SVPs containing newly made dsRNA were eluted from the gel and analyzed for protein content by electrophoresis on polyacrylamide gels. The results showed that three distinct types of replication intermediates (RIs) were present in SA11-infected cells. The smallest intermediate (precore RI, 45 nm, 220 S) contained the structural proteins VP1, VP3, and VP9 and the nonstructural proteins NS53, NS35, and NS34. A second intermediate (core RI, 60 nm, 310 S) contained the core proteins VP1, VP2, and VP3 and the proteins VP9, NS35 and NS34. The largest RI (single-shelled RI, 75 nm, 420 S) contained the inner shell proteins VP1, VP2, VP3, and VP6 and the proteins VP9, NS35 and NS34. Analysis of the formation and turnover of RIs in infected cells pulse-labeled with 35S-amino acids supports a hypothesis that rotavirus single-shelled particles are assembled in vivo by the sequential addition of VP2 and VP6 to precore RIs consisting of VP1, VP3, VP9, NS35, and NS34.

Structure and protein composition of the rotavirus replicase particle

To understand the role of viral proteins in the replication of rotavirus RNA, we have characterized the structure of subviral particles (SVPs) that synthesize double-stranded RNA (DS RNA). Pulse-labeling of newly made RNA in infected cells showed that rotavirus DS RNA was synthesized either in single-shell (SS)-like particles or in precursor particles that rapidly mature into SS particles. Experiments using a cell-free system demonstrated that most replicase particles containing newly made DS RNA were of greater density in CsCl than single-shelled (SS) particles. However, this was partly due to the presence of single-stranded RNAs as the treatment of replicase particles with micrococcal nuclease reduced their density to between core particles and SS particles. Electrophoretic analysis indicated that replicase particles, purified by centrifugation on CsCl and glycerol gradients, were similar to SS particles, containing the structural proteins VP1, VP2, and VP6. Rotavirus replicase particles were also found to contain the nonstructural proteins NS34 and NS35 and possible host components. The presence of VP6 in enzymatically active replicase particles suggests that, like transcription, this protein may be required for rotavirus RNA replication.