Activation and characterization of the reovirus transcriptase: genetic analysis

Protease cleavage of reovirus capsid protein mu1/mu1C is blocked by alkyl sulfate detergents, yielding a new type of infectious subvirion particle

Mammalian reovirus virions undergo partial disassembly of the outer capsid upon exposure to proteases in vitro, producing infectious subvirion particles (ISVPs) that lack protein sigma3 and contain protein mu1/mu1C as endoprotease-generated fragments mu1delta/delta and phi. ISVPs are thought to be required for two early steps in reovirus infection: membrane penetration and activation of the particle-bound viral transcriptase complexes. Genetic and biochemical evidence implicates outer-capsid protein mu1 in both these steps. To determine whether the cleavage of mu1/mu1C is relevant to the unique properties of ISVPs, we analyzed the properties of novel subvirion particles that lacked sigma3 yet retained mu1/mu1C in an uncleaved but cleavable form. These detergent-plus-protease subvirion particles (dpSVPs) were produced by treating virions with chymotrypsin in the presence of micelle-forming concentrations of alkyl sulfate detergents. Infections with dpSVPs in murine L or canine MDCK cells provided evidence that the cleavage of mu1/mu1C during viral entry into these cells is dispensable for reovirus infection. Additionally, dpSVPs behaved like ISVPs in their capacity to permeabilize lipid bilayers and to undergo transcriptase activation in vitro, supporting the conclusion that cleavage of mu1/mu1C to mu1delta/delta and phi during viral entry is not required for either membrane penetration or transcriptase activation in cells. The capacity of alkyl sulfate detergents to inhibit the cleavage of mu1/mu1C in a reversible fashion suggests a specific association between virus particle and detergent micelles that may mimic virus particle-phospholipid membrane interactions during reovirus entry into cells.

Localization of a C-terminal region of lambda2 protein in reovirus cores

The 144-kDa lambda2 protein is a structural component of mammalian reovirus particles and contains the guanylyltransferase activity involved in adding 5' caps to reovirus mRNAs. After incubation of reovirus T3D core particles at 52 degrees C, the lambda2 protein became sensitive to partial protease degradation. Sequential treatments with heat and chymotrypsin caused degradation of a C-terminal portion of lambda2, leaving a 120K core-associated fragment. The four other proteins in cores--lambda1, lambda3, mu2, and sigma2--were not affected by the treatment. Purified cores with cleaved lambda2 were subjected to transmission cryoelectron microscopy and image reconstruction. Reconstruction analysis demonstrated that a distinctive outer region of lambda2 was missing from the modified cores. The degraded region of lambda2 corresponded to the one that contacts the base of the sigma1 protein fiber in reovirus virions and infectious subvirion particles, suggesting that the sigma1-binding region of lambda2 is near its C terminus. Cores with cleaved lambda2 were shown to retain all activities required to transcribe and cap reovirus mRNAs, indicating that the C-terminal region of lambda2 is dispensable for those functions.

Core protein mu2 is a second determinant of nucleoside triphosphatase activities by reovirus cores

NTPase activities in mammalian reovirus cores were examined under various conditions that permitted several new differences to be identified between strains type 1 Lang (T1L) and type 3 Dearing (T3D). One difference concerned the ratio (at pH 8.5) of ATP hydrolysis at 50 degrees C to that at 35 degrees C. A genetic analysis using T1L x T3D reassortant viruses implicated the L3 and M1 gene segments in this difference, with M1 influencing ATPase activity most strongly at high temperatures. L3 and M1 encode the core proteins lambda1 and mu2, respectively. Another difference concerned the absolute levels of GTP hydrolysis by cores at 45 degrees C and pH 6.5. A genetic analysis using T1L x T3D reassortants implicated the M1 gene as the sole determinant of this difference. The results of these experiments, coupled with previous findings (S. Noble and M. L. Nibert, J. Virol. 71:2182-2191, 1997), suggest either that a single type of NTPase in cores is strongly influenced by two different core proteins--lambda1 and mu2--or that cores contain two different types of NTPase influenced by the two proteins. The findings appear relevant for understanding the complex functions of reovirus cores in RNA synthesis and capping.

Characterization of the nucleoside triphosphate phosphohydrolase and helicase activities of the reovirus lambda1 protein

Previous studies have shown that the reovirus lambda1 core protein harbors a putative nucleotide-binding motif and exhibits an affinity for nucleic acids. In addition, a nucleoside triphosphate phosphohydrolase activity present in reovirus cores has been recently assigned to lambda1 using gene reassortment analysis. In this study, it was demonstrated that the recombinant lambda1 protein, expressed in the yeast Pichia pastoris, is able to hydrolyze nucleoside 5'-triphosphates or deoxynucleoside 5'-triphosphates. This activity was absolutely dependent on the presence of a divalent cation, Mg2+ or Mn2+. The protein can also unwind double-stranded nucleic acid molecules in the presence of a nucleoside 5'-triphosphate or deoxynucleoside 5'-triphosphate. These results provide the first biochemical evidence that the reovirus lambda1 protein is a nucleoside triphosphate phosphohydrolase/helicase and strongly support the idea that lambda1 participates in transcription of the viral genome.

Construction and characterization of a reovirus double temperature-sensitive mutant

The infectious reovirus RNA system was used to construct a mutant with two temperature-sensitive (ts) lesions in genome segments M2 and S2, respectively. The double mutant is about 300 times more ts than either of its parents, which are about 1,500 and 170 times more ts than their wild-type parent reovirus ST3 strain Dearing. At 39 degrees C the double mutant is essentially unable to multiply. In spite of its striking temperature sensitivity, the double mutant elicits the formation of significant amounts of neutralizing antibodies in newborn mice. Possible mechanisms responsible for this are discussed, as is the significance of this double ts mutant in relation to current searches for safe and efficient vaccine strains.

Reovirus-induced apoptosis of MDCK cells is not linked to viral yield and is blocked by Bcl-2

In this study, we investigated the relationship between reovirus-induced apoptosis and viral growth. Madin-Darby canine kidney (MDCK) epithelial cells infected with prototype reovirus strains type 1 Lang (T1L) or type 3 Dearing (T3D) were found to undergo apoptosis, and T3D induced apoptosis of MDCK cells to a substantially greater extent than T1L. By using T1L x T3D reassortant viruses, we found that differences in the capacities of these strains to induce apoptosis are determined by the viral S1 and M2 gene segments. These genes encode viral outer-capsid proteins that play important roles in viral entry into cells. T1L grew significantly better in MDCK cells than T3D, and these differences in growth segregated with the viral L1 and M1 gene segments. The L1 and M1 genes encode viral core proteins involved in viral RNA synthesis. Bcl-2 overexpression in MDCK cells inhibited reovirus-induced apoptosis but did not substantially affect reovirus growth. These findings indicate that differences in the capacities of reovirus strains to induce apoptosis and grow in MDCK cells are determined by different viral genes and that premature cell death by apoptosis does not limit reovirus growth in MDCK cells.

Characterization of an ATPase activity in reovirus cores and its genetic association with core-shell protein lambda1

A previously identified nucleoside triphosphatase activity in mammalian reovirus cores was further characterized by comparing two reovirus strains whose cores differ in their efficiencies of ATP hydrolysis. In assays using a panel of reassortant viruses derived from these strains, the difference in ATPase activity at standard conditions was genetically associated with viral genome segment L3, encoding protein lambda1, a major constituent of the core shell that possesses sequence motifs characteristic of other ATPases. The ATPase activity of cores was affected by several other reaction components, including temperature, pH, nature and concentration of monovalent and divalent cations, and nature and concentration of anions. A strain difference in the response of core ATPase activity to monovalent acetate salts was also mapped to L3/lambda1 by using reassortant viruses. Experiments with different nucleoside triphosphates demonstrated that ATP is the preferred ribonucleotide substrate for cores of both strains. Other experiments suggested that the ATPase is latent in reovirus virions and infectious subviral particles but undergoes activation during production of cores in close association with the protease-mediated degradation of outer-capsid protein mu1 and its cleavage products, suggesting that mu1 may play a role in regulating the ATPase.

Nonrandom segregation of parental alleles in reovirus reassortants

To test for nonrandom segregations among their 10 genomic RNA segments, we examined a set of 83 reassortants derived from mammalian reovirus type 1 Lang and type 3 Dearing. After confirming the genotypes of the reassortants, we performed statistical analyses on the distributions of parental alleles for each of the 10 gene segments, as well as for the 45 possible pairings of the 10 segments. The analyses revealed nonrandom associations of parental alleles in the L1-L2, L1-M1, L1-S1, and L3-S1 segment pairs, at levels indicating high statistical significance (P < 0.005). Such associations may reflect specific interactions between viral components (protein-protein, protein-RNA, or RNA-RNA) and may influence both the evolution of reoviruses in nature and their genetic analysis in the laboratory. The data may also support an hypothesis that reovirus reassortants commonly contain mutations that improve their fitness for independent replication.

Identification and characterization of a double-stranded RNA- reovirus temperature-sensitive mutant defective in minor core protein mu2

A newly identified temperature-sensitive mutant whose defect was mapped to the reovirus M1 gene (minor core protein mu2) was studied to better understand the functions of this virion protein. Sequence determination of the Ml gene of this mutant (tsH11.2) revealed a predicted methionine-to-threonine alteration at amino acid 399 and a change from proline to histidine at amino acid 414. The mutant made normal amounts of single-stranded RNA, both in in vitro transcriptase assays and in infected cells, and normal amounts of progeny viral protein at early times in a restrictive infection. However, tsH11.2 produced neither detectable progeny protein nor double-stranded RNA at late times in a restrictive infection. These studies indicate that mu2 plays a role in the conversion of reovirus mRNA to progeny double-stranded RNA.

The M1 gene is associated with differences in the temperature optimum of the transcriptase activity in reovirus core particles

The reovirus core is a multienzyme complex that contains five different structural proteins and 10 segments of double-stranded RNA. The core is responsible for transcribing mRNA from the enclosed double-stranded RNA. The reovirus transcriptase has an unusual temperature profile, with optimum transcription occurring at approximately 50 degrees C and little activity occurring below 30 or above 60 degrees C. Purified reovirus serotype 1 Lang (T1L) cores transcribed most efficiently at 48 degrees C. The transcriptase temperature optimum of purified reovirus serotype 3 Dearing (T3D) cores was 52 degrees C. In addition, T1L cores produced more mRNA per particle than did T3D cores at their respective temperature optima. Core particles were purified from T1L x T3D reassortants and were used to map these differences. The M1 gene, which encodes minor core protein mu 2, was uniquely associated with the difference in temperature optimum of transcription (P = 0.0003). The L1 gene, which encodes minor core protein lambda 3 (previously implicated as the RNA polymerase), and the M1 gene were associated with the difference in absolute amounts of transcript produced (P = 0.01 and P = 0.0002, respectively). These data suggest that minor core protein mu 2 also plays a role in reovirus transcription.

Identification of signals required for the insertion of heterologous genome segments into the reovirus genome

In cells simultaneously infected with any two of the three reovirus serotypes ST1, ST2, and ST3, up to 15% of the yields are intertypic reassortants that contain all possible combinations of parental genome segments. We have now found that not all genome segments in reassortants are wild type. In reassortants that possess more ST1 than ST3 genome segments, all ST1 genome segments appear to be wild type, but the incoming ST3 genome segments possess mutations that make them more similar to the ST1 genome segments that they replace. In reassortants resulting from crosses of the more distantly related ST3 and ST2 viruses that possess a majority of ST3 genome segments, all incoming ST2 genome segments are wild type, but the ST3 S4 genome segment possesses two mutations, G74 to A and G624 to A, that function as acceptance signals. Recognition of these signals has far-reaching implications for the construction of reoviruses with novel properties and functions.

Association of reovirus outer capsid proteins sigma 3 and mu 1 causes a conformational change that renders sigma 3 protease sensitive

Association of the reovirus proteins sigma 3 and mu 1 influences viral entry, initiation of outer capsid assembly, and modulation of the effect of sigma 3 on cellular translation. In this study, we have addressed whether structural changes occur in sigma 3 as a result of its interaction with mu 1. Using differences in protease sensitivity to detect conformationally distinct forms of sigma 3, we showed that association of sigma 3 with mu 1 caused a conformational change in sigma 3 that converted it from a protease-resistant to a protease-sensitive structure and occurred posttranslationally. The effect of mu 1 on the structure of sigma 3 was stoichiometric. Our results are consistent with a model in which sigma 3's association with mu 1 shifts its function from translational control to assembly of an outer capsid in which sigma 3 is folded into the protease-sensitive conformation that is required for its cleavage during the next round of infection.

Infectious subvirion particles of reovirus type 3 Dearing exhibit a loss in infectivity and contain a cleaved sigma 1 protein

Mammalian reoviruses exhibit differences in the capacity to grow in intestinal tissue: reovirus type 1 Lang (T1L), but not type 3 Dearing (T3D), can be recovered in high titer from intestinal tissue of newborn mice after oral inoculation. We investigated whether in vitro protease treatment of virions of T1L and T3D, using conditions to generate infectious subvirion particles (ISVPs) as occurs in the intestinal lumen of mice (D. K. Bodkin, M. L. Nibert, and B. N. Fields, J. Virol. 63:4676-4681, 1989), affects viral infectivity. Chymotrypsin treatment of T1L was associated with a 2-fold increase in viral infectivity, whereas identical treatment of T3D resulted in a 10-fold decrease in infectivity. Using sodium dodecyl sulfate-polyacrylamide gel electrophoresis, we found that loss of T3D infectivity was correlated with cleavage of its sigma 1 protein. We used reassortant viruses to identify viral determinants of infectivity loss and sigma 1 cleavage and found that both phenotypes segregate with the sigma 1-encoding S1 gene. Comparable results were obtained when trypsin treatment of virions of T1L and T3D was used. In experiments to determine the fate of sigma 1 fragments following cleavage, the capacity of anti-sigma 1 monoclonal antibody G5 to neutralize infectivity of T3D ISVPs was significantly decreased in comparison with its capacity to neutralize infectivity of virions, suggesting that a sigma 1 domain bound by G5 is lost from viral particles after proteolytic digestion. In contrast to the decrease in infectivity, chymotrypsin treatment of T3D virions leading to generation of ISVPs resulted in a 10-fold increase in their capacity to produce hemagglutination, indicating that a domain of sigma 1 important for binding to sialic acid remains associated with viral particles after sigma 1 cleavage. Neuraminidase treatment of L cells substantially decreased the yield of T3D ISVPs in comparison with the yield of virions, indicating that a sigma 1 domain important for binding sialic acid also can mediate attachment of T3D ISVPs to L cells and lead to productive infection. These results suggest that cleavage of T3D sigma 1 protein following oral inoculation of newborn mice is at least partly responsible for the decreased growth of T3D in the intestine and provide additional evidence that T3D sigma 1 contains more than a single receptor-binding domain.

Multiple viral core proteins are determinants of reovirus-induced acute myocarditis

Previously, we showed that the M1 gene (encoding a viral core protein, mu 2, whose function is unknown) was associated with the efficiently myocarditic phenotype of a reovirus variant, 8B. Here, we have extended our genetic analysis of 8B and conducted genetic analyses of two other reovirus strains (T1L [serotype 1 strain Lang] and Abney). Our results demonstrate that multiple viral core proteins are determinants of reovirus-induced myocarditis. In contrast to our previous association of mu 2 with induction of myocarditis, this provides strong evidence that a core function achieved through the interaction of multiple core proteins is responsible for induction of the disease.

Characterization and structural localization of the reovirus lambda 3 protein

The putative reovirus RNA polymerase, protein lambda 3, was characterized using antiserum prepared against a TrpE-lambda 3 fusion protein synthesized in Escherichia coli. Immunofluorescence microscopy showed that lambda 3 accumulated in perinuclear inclusion bodies in reovirus-infected cells. Analysis of lambda 3 accumulation in infected cells indicates that, once synthesized, lambda 3 is quite stable throughout the course of infection. Anti-lambda 3 serum did not immunoprecipitate virions, core particles or iodinated surface proteins of either virions or cores. These results indicate that lambda 3 is located in the inner part of the core. Experiments involving urea denaturation of purified reovirus cores indicate that lambda 3 cannot be selectively removed from the core without total denaturation of the core structure. When the dsRNA genome was eliminated from the core, lambda 3 remained associated with the other viral proteins in the core. Thus, lambda 3 appears to be a stable, structural component of the reovirus core, not bound to genomic dsRNA or free in soluble form inside the core.

Sequence diversity within the reovirus S2 gene: reovirus genes reassort in nature, and their termini are predicted to form a panhandle motif

To better understand genetic diversity within mammalian reoviruses, we determined S2 nucleotide and deduced sigma 2 amino acid sequences of nine reovirus strains and compared these sequences with those of prototype strains of the three reovirus serotypes. The S2 gene and sigma 2 protein are highly conserved among the four type 1, one type 2, and seven type 3 strains studied. Phylogenetic analyses based on S2 nucleotide sequences of the 12 reovirus strains indicate that diversity within the S2 gene is independent of viral serotype. Additionally, we found marked topological differences between phylogenetic trees generated from S1 and S2 gene nucleotide sequences of the seven type 3 strains. These results demonstrate that reovirus S1 and S2 genes have distinct evolutionary histories, thus providing phylogenetic evidence for lateral transfer of reovirus genes in nature. When variability among the 12 sigma 2-encoding S2 nucleotide sequences was analyzed at synonymous positions, we found that approximately 60 nucleotides at the 5' terminus and 30 nucleotides at the 3' terminus were markedly conserved in comparison with other sigma 2-encoding regions of S2. Predictions of RNA secondary structures indicate that the more conserved S2 sequences participate in the formation of an extended region of duplex RNA interrupted by a pair of stem-loops. Among the 12 deduced sigma 2 amino acid sequences examined, substitutions were observed at only 11% of amino acid positions. This finding suggests that constraints on the structure or function of sigma 2, perhaps in part because of its location in the virion core, have limited sequence diversity within this protein.

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

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

The reovirus M1 gene determines the relative capacity of growth of reovirus in cultured bovine aortic endothelial cells

Since blood-borne viruses often interact with endothelial cells before tissue invasion, the interaction between viruses and endothelial cells is likely to be important in viral pathogenicity. Two reovirus isolates (type 1 Lang and type 3 Dearing) differ in their capacity to grow in cultured bovine aortic endothelial cells. The mammalian reoviruses have 10 double-stranded RNA gene segments in their genome. By using 24 reassortant viruses, observed differences in the capacity of different strains to grow in cultured endothelial cells were mapped to the M1 gene (P = 0.00019), which encodes the viral core protein mu 2. No differences were detected in binding or proteolytic processing of viral outer capsid proteins of parental virions between the two reovirus isolates. Northern blot analysis showed a decreased production of viral mRNA in endothelial cells infected with type 3 Dearing reovirus, but not type 1 Lang. Thus, we have identified a viral gene (the M1 gene) responsible for determining the difference in growth capacity of the two reovirus isolates in cultured endothelial cells. Reovirus is an attractive model in which to study the interaction of viruses with endothelial cells at a molecular genetic level.

Ion channels induced in lipid bilayers by subvirion particles of the nonenveloped mammalian reoviruses

Mechanisms by which nonenveloped viruses penetrate cell membranes as an early step in infection are not well understood. Current ideas about the mode for cytosolic penetration by nonenveloped viruses include (i) formation of a membrane-spanning pore through which viral components enter the cell and (ii) local breakdown of the cellular membrane to provide direct access of infecting virus to the cell's interior. Here we report that of the three viral particles of nonenveloped mammalian reoviruses: virions, infectious subvirion particles, and cores (the last two forms generated from intact reovirus virions by proteolysis), only the infectious subvirion particles induced the formation of anion-selective, multisized channels in planar lipid bilayers under the experimental conditions used in this study. The value for the smallest size conductance varied depending on the lipid composition of the bilayer between 90 pS (Asolectin) and 300 pS (phosphatidylethanolamine:phosphatidylserine) and was found to be voltage independent. These findings are consistent with a proposal that the proteolytically activated infectious subviral particles mediate the interaction between virus and the lipid bilayer of a cell membrane during penetration. In addition, the findings indicate that the "penetration proteins" of some enveloped and nonenveloped viruses share similarities in the way they interact with bilayers.

Reovirus M2 gene is associated with chromium release from mouse L cells

In this study, we investigated the interaction of reovirus particles with cell membranes by using a 51Cr release assay. We confirmed prior observations (J. Borsa, B. D. Morash, M. D. Sargent, T. P. Copps, P. A. Lievaart, and J. G. Szekely, J. Gen. Virol. 45:161-170, 1979) that intermediate subviral particles (ISVPs) of reovirus type 3 strain Abney (T3A) induced the release of 51Cr from preloaded L cells and showed that the intact virion and core forms did not. Reovirus type 1 strain Lang (T1L) ISVPs were found to be less efficient at 51Cr release than T3A ISVPs. Reassortants between these strains indicated that the 51Cr release phenotype segregates with the M2 gene segment. Biochemical studies indicated that the ISVPs' acquisition of the capacity to induce 51Cr release followed the cleavage of the viral M2 gene product mu 1/mu 1C to fragments delta and phi during virion conversion to ISVP but did not directly correlate with this cleavage. These studies suggest that the reovirus M2 gene product (in its cleaved form) plays a role in interacting with cell membranes.

Isolation and genetic characterization of ethanol-resistant reovirus mutants

To better understand the mechanism(s) by which viruses respond to chemical or physical treatments, we isolated a series of mutant strains of reovirus type 3 Dearing that exhibit increased ethanol resistance. Following exposure to 33% ethanol for 20 min, the parental strain exhibited a 5 log10 decrease in infectivity. The mutant strains, however, exhibited a 2 to 3 log10 decrease in titer following identical treatment. Through the use of reassortant viruses, we mapped this increased ethanol resistance mutation to the M2 gene segment, which encodes a major outer capsid protein, mu1C. Sequence analysis of mutant M2 genes revealed that six of seven unique mutants possessed single-point mutations in this gene. In addition, the change in six of seven mutants caused a predicted amino acid change in a 35-amino-acid region of the gene product between amino acids 425 and 459. The identification of ethanol resistance mutations within a discrete region of this outer capsid protein identifies that portion of the protein as important in reovirus stability. The presence of viral particles possessing altered stability also suggests that subpopulations of viruses may possess altered environmental stability, which, in turn, could affect viral transmission.

A carboxy-terminal fragment of protein mu 1/mu 1C is present in infectious subvirion particles of mammalian reoviruses and is proposed to have a role in penetration

Penetration of a cell membrane as an early event in infection of cells by mammalian reoviruses appears to require a particular type of viral particle, the infectious subvirion particle (ISVP), which is generated from an intact virion by proteolytic cleavage of the outer capsid proteins sigma 3 and mu 1/mu 1C. Characterizations of the structural components and properties of ISVPs are thus relevant to attempts to understand the mechanism of penetration by reoviruses. In this study, a novel, approximately 13-kDa carboxy-terminal fragment (given the name phi) was found to be generated from protein mu 1/mu 1C during in vitro treatments of virions with trypsin or chymotrypsin to yield ISVPs. With trypsin treatment, both the carboxy-terminal fragment phi and the amino-terminal fragment mu 1 delta/delta were shown to be generated and to remain attached to ISVPs in stoichiometric quantities. Sites of protease cleavage were identified in the deduced amino acid sequence of mu 1 by determining the amino-terminal sequences of phi proteins: trypsin cleaves between arginine 584 and isoleucine 585, and chymotrypsin cleaves between tyrosine 581 and glycine 582. Findings in this study indicate that sequences in the phi portion of mu 1/mu 1C may participate in the unique functions attributed to ISVPs. Notably, the delta-phi cleavage junction was predicted to be flanked by a pair of long amphipathic alpha-helices. These amphipathic alpha-helices, together with the myristoyl group at the extreme amino terminus of mu 1/mu 1N, are proposed to interact directly with the lipid bilayer of a cell membrane during penetration by mammalian reoviruses.

The expressed VP4 protein of bluetongue virus binds GTP and is the candidate guanylyl transferase of the virus

A minor core protein, VP4, of bluetongue virus serotype 10 (BTV-10) has been synthesized in insect cells infected with a genetically manipulated recombinant baculovirus. When insect cells were coinfected by this recombinant virus and a recombinant baculovirus expressing the two major core proteins (VP3 and VP7) of the virus, core-like particles (CLPs) consisting of all three proteins were formed. Purified CLPs reacted with [32P]GTP which was covalently bound to VP4 only. Similarly reconstituted CLPs with VP1 or VP6 did not form covalent complexes with [32P]GTP. The virion-derived VP4 was also shown to have GTP-binding activity. The covalent binding of GTP indicates that expressed VP4 not only is biologically active but also is the candidate guanylyl transferase of the virus. The optimum reaction conditions for GTP binding by VP4 have been investigated.

The S2 gene nucleotide sequences of prototype strains of the three reovirus serotypes: characterization of reovirus core protein sigma 2

The S2 gene nucleotide sequences of prototype strains of the three reovirus serotypes were determined to gain insight into the structure and function of the S2 translation product, virion core protein sigma 2. The S2 sequences of the type 1 Lang, type 2 Jones, and type 3 Dearing strains are 1,331 nucleotides in length and contain a single large open reading frame that could encode a protein of 418 amino acids, corresponding to sigma 2. The deduced sigma 2 amino acid sequences of these strains are very conserved, being identical at 94% of the sequence positions. Predictions of sigma 2 secondary structure and hydrophobicity suggest that the protein has a two-domain structure. A larger domain is suggested to be formed from the amino-terminal three-fourths of sigma 2 sequence, which is separated from a smaller carboxy-terminal domain by a turn-rich hinge region. The carboxy-terminal domain includes sequences that are more hydrophilic than those in the rest of the protein and contains sequences which are predicted to form an alpha-helix. A region of striking similarity was found between amino acids 354 and 374 of sigma 2 and amino acids 1008 and 1031 of the beta subunit of the Escherichia coli DNA-dependent RNA polymerase. We suggest that the regions with similar sequence in sigma 2 and the beta subunit form amphipathic alpha-helices which may play a related role in the function of each protein. We have also performed experiments to further characterize the double-stranded RNA-binding activity of sigma 2 and found that the capacity to bind double-stranded RNA is a property of the sigma 2 protein of prototype strains and of the S2 mutant tsC447.

Avian reovirus S1133 can replicate in mouse L cells: effect of pH and cell attachment status on viral infection

Previous reports have suggested that avian reovirus S1133 fails to replicate in mouse L cells. In this article, we report that replication does occur under certain culture conditions. The avian reovirus was found to grow in mouse L cells at pH 6.4 and 7.2 but not at pH 8.2. Culture medium with a basic pH directly inhibited viral transcription and genome replication. As a result, viral protein synthesis was also affected. At permissive pH levels, avian reovirus grew better in monolayers than in suspension cultures of L cells because of the influence of cell attachment status on viral macromolecular synthesis. Our results not only show that avian reovirus can replicate in mouse L cells but also help to explain why it did not in previous studies.

Identification of the viral genes responsible for growth of strains of reovirus in cultured mouse heart cells

Viral growth in specific tissue is usually required in order to lead to pathology. Two reovirus isolates (type 1 Lang and type 3 Dearing) differ in their capacity to grow in cultured mouse heart cells. The mammalian reoviruses contain a genome of 10 double-stranded RNA gene segments. By the use of 37 reassortant viruses (consisting of viruses with different combinations of genes derived from the two parents), difference in capacity of different strains to grow in heart cells was mapped to three different genes, all of which encode viral core proteins: the M1 gene (P less than 0.000044); the L1 gene (P = 0.00094); and the L3 gene (P = 0.019). Using the same set of reassortant viruses, the L1 (P = 0.00015) and L3 (P = 0.0065) genes were involved in differences of the ability of viral strains to grow in mouse L cells (fibroblasts), but the M1 gene (P = 0.12) was not. These findings suggest that the M1 gene plays an important and specific role in determining the relative capacity of certain viral strains to grow in the heart. Thus, we have identified viral genes responsible for differing growth capacity in heart muscle cells in culture. These findings provide a novel system for studies of viral myocarditis at a molecular genetic level.

Mammalian reoviruses contain a myristoylated structural protein

The structural protein mu 1 of mammalian reoviruses was noted to have a potential N-myristoylation sequence at the amino terminus of its deduced amino acid sequence. Virions labeled with [3H]myristic acid were used to demonstrate that mu 1 is modified by an amide-linked myristoyl group. A myristoylated peptide having a relative molecular weight (Mr) of approximately 4,000 was also shown to be a structural component of virions and was concluded to represent the 4.2-kDa amino-terminal fragment of mu 1 which is generated by the same proteolytic cleavage that yields the carboxy-terminal fragment and major outer capsid protein mu 1C. The myristoylated 4,000-Mr peptide was found to be present in reovirus intermediate subviral particles but to be absent from cores, indicating that it is a component of the outer capsid. A distinct large myristoylated fragment of the intact mu 1 protein was also identified in intermediate subviral particles, but no myristoylated mu-region proteins were identified in cores, consistent with the location of mu 1 in the outer capsid. Similarities between amino-terminal regions of the reovirus mu 1 protein and the poliovirus capsid polyprotein were noted. By analogy with other viruses that contain N-myristoylated structural proteins (particularly picornaviruses), we suggest that the myristoyl group attached to mu 1 and its amino-terminal fragments has an essential role in the assembly and structure of the reovirus outer capsid and in the process of reovirus entry into cells.

Relationships among the positive strand and double-strand RNA viruses as viewed through their RNA-dependent RNA polymerases

The sequences of 50 RNA-dependent RNA polymerases (RDRPs) from 43 positive strand and 7 double strand RNA (dsRNA) viruses have been compared. The alignment permitted calculation of distances among the 50 viruses and a resultant dendrogram based on every amino acid, rather than just those amino acids in the conserved motifs. Remarkably, a large subgroup of these viruses, including vertebrate, plant, and insect viruses, forms a single cluster whose only common characteristic is exploitation of insect hosts or vectors. This similarity may be due to molecular constraints associated with a present and/or past ability to infect insects and/or to common descent from insect viruses. If common descent is important, as it appears to be, all the positive strand RNA viruses of eucaryotes except for the picornaviruses may have evolved from an ancestral dsRNA virus. Viral RDRPs appear to be inherited as modules rather than as portions of single RNA segments, implying that RNA recombination has played an important role in their dissemination.

Crystallization of the reovirus type 3 Dearing core. Crystal packing is determined by the lambda 2 protein

Core particles of reovirus type 3 Dearing (T3D) crystallized in the face-centered cubic space group F432 with dimensions of 1270 A along each edge of the unit cell. Core particles of reovirus type 1 Lang (T1L) did not crystallize. Experiments with core particles derived from 27 different T1L x T3D reassortant viruses indicated that the L2 genome segment determined the capacity of cores to crystallize. This finding indicates important differences in the surface topography of the L2-translation product, the lambda 2 protein, of these two isolates, and suggests that important crystal contacts are mediated by this protein. These data are used to generate a model of the packing of reovirus core particles within the unit cell.

Portable encapsidation signal of the L-A double-stranded RNA virus of S. cerevisiae

The (+) single-stranded RNA (ssRNA) of the L-A virus is the species packaged to form new viral particles. Empty L-A viral particles specifically bind viral (+) ssRNA, and a sequence 400 bases from the 3' end is necessary for this activity. We show that its stem-loop structure, the A residue protruding from the stem, and the loop sequence are all important for the binding, and that this 34 base region is sufficient for the binding. M1, a satellite virus of L-A, has a similar structure on its (+) strand that is likewise sufficient for the binding. Heterologous RNA with the binding sequence from L-A or M1, when expressed in vivo, was packaged in L-A viral particles. Thus, the sites necessary to bind to empty particles are encapsidation signals for the L-A virus. Since the pol domain of the 180 kd minor coat protein appears to be responsible for the binding, this result suggests that the RNA polymerase molecule recognizes the viral genome for packaging.

Intraluminal proteolytic activation plays an important role in replication of type 1 reovirus in the intestines of neonatal mice

Oral inoculation of suckling mice with reovirus serotype 1 (strain Lang) results in the conversion of intact virions to intermediate subviral particles (ISVPs) in the intestinal lumen. Digestion of virus in vitro with chymotrypsin or trypsin reveals two distinct forms of ISVPs, while the predominant species of ISVPs found in the small intestinal lumen appears to be identical to the chymotrypsin product. The in vivo conversion of virions to ISVPs was blocked by pretreatment of mice with protease inhibitors, resulting in inefficient replication of reovirus in intestinal tissue. The early inhibition of viral replication in suckling mice pretreated with protease inhibitors was not observed when suckling mice were inoculated with ISVPs generated by in vitro digestion with either chymotrypsin or trypsin. However, replication was decreased during secondary rounds of replication in mice receiving repeated doses of protease inhibitors, suggesting that luminal proteolytic digestion is important in rendering progeny virions infectious in the gut.

The function of reovirus proteins during the reovirus multiplication cycle: analysis using monoreassortants

When cultured cells are injected with mixtures of cores of two reovirus strains, a high proportion of reassortants are monoreassortants, that is, virus particles that contain one genome segment of 1 parent and 9 genome segments of the other. We have isolated two complete sets of monoreassortants, those that contain a single serotype 2 genome segment and 9 serotype 3 genome segments, and those that contain 1 serotype 3 genome segment and 9 serotype 1 genome segments. We have used the former set of monoreassortants (because reovirus serotypes 2 and 3 are less closely related than serotypes 1 and 3) to assess the effect of all 10 genome segments, or rather of the proteins that they encode, in controlling parameters of the reovirus multiplication cycle such as yield size, extent of viral ssRNA, dsRNA and protein synthesis, plaque size, and cytopathogenicity. Among the major findings are: proteins lambda 2, mu 1/mu 1C, and sigma 3 control yield size and extent of RNA and protein synthesis; proteins mu 2 and sigma 1 control severity of cytopathic effects; and proteins sigma 1, mu 1/mu 1C, and mu 2 control plaque size. Identification of monoreassortant phenotypes is useful for identifying which viral proteins are functionally involved at the various stages of the reovirus multiplication cycle.

Overlapping genes in a yeast double-stranded RNA virus

The Saccharomyces cerevisiae viruses have a large viral double-stranded RNA which encodes the major viral capsid polypeptide. We have previously shown that this RNA (L1) also encodes a putative viral RNA-dependent RNA polymerase (D. F. Pietras, M. E. Diamond, and J. A. Bruenn, Nucleic Acids Res., 16:6226, 1988). The organization and expression of the viral genome is similar to that of the gag-pol region of the retroviruses. The complete sequence of L1 demonstrates two large open reading frames on the plus strand which overlap by 129 bases. The first is the gene for the capsid polypeptide, and the second is the gene for the putative RNA polymerase. One of the products of in vitro translation of the denatured viral double-stranded RNA is a polypeptide of the size expected of a capsid-polymerase fusion protein, resulting from a -1 frameshift within the overlapping region. A polypeptide of the size expected for a capsid-polymerase fusion product was found in virions, and it was recognized in Western blots (immunoblots) by antibodies to a synthetic peptide derived from the predicted polymerase sequence.

Tentative identification of RNA-dependent RNA polymerases of dsRNA viruses and their relationship to positive strand RNA viral polymerases

Amino acid sequence stretches similar to the four most conserved segments of positive strand RNA viral RNA-dependent RNA polymerases have been identified in proteins of four dsRNA viruses belonging to three families, i.e. P2 protein of bacteriophage phi 6 (Cystoviridae), RNA 2 product of infectious bursa disease virus (Birnaviridae), lambda 3 protein of reovirus, and VP1 of bluetongue virus (Reoviridae). High statistical significance of the observed similarity was demonstrated, allowing identification of these proteins as likely candidates for RNA-dependent RNA polymerases. Based on these observations, and on the previously reported sequence similarity between the RNA polymerases of a yeast dsRNA virus and those of positive strand RNA viruses, a possible evolutionary relationship between the two virus classes is discussed.

The sequences of the reovirus serotype 1, 2, and 3 L1 genome segments and analysis of the mode of divergence of the reovirus serotypes

We report the sequence of the L1 genome segment of reovirus serotype 3 strain Dearing, which encodes the minor core component protein lambda 3. It is 3854 bp long, with a long open reading frame starting at position 19 that is 1267 codons long. Protein lambda 3 is not detectably related to any other protein, nor does it appear to possess motifs indicative of recognized specialized functions. We have also sequenced the L1 genome segments of reovirus serotypes 1 and 2. The serotype 1 and 3 L1 genome segments are extremely closely related; there are only 154 mismatches (4.1%), 80% of which are in third base codon positions, so that these two lambda 3 proteins are 98.3% related (only 22 mismatches out of 1267). The serotype 2 L1 genome segment is only 75% related to the serotype 1 and 3 genome segments, and the serotype 2 lambda 3 protein is 92% related to the serotype 1 and 3 lambda 3 proteins. We have also analyzed the divergence patterns by which the various reovirus genome segments evolved into the three serotype forms. It appears that serotype 2 separated from the serotype 1/3 precursor long before serotypes 1 and 3 themselves diverged. In all cases the third base codon positions in the various genome segments have diverged about 80% toward randomness. The first and second base codon positions have diverged much less and to varying degree, depending, presumably, on each protein's ability to accept changes without significant loss of function. For the separation into the serotype 1 and 3 forms, the extent of divergence of the various genome varies over a very wide range. The S1 genome segments have again diverged most extensively, the extent of divergence in the first, second, and third base codon positions being about 50, 35 and 75%, respectively. For seven other genome segments that we examined the extent of third base codon position divergence is 56, 53, 48, 29, 22, 13, and 6%, whereas first and second base codon position divergence ranges from no more than 6 to 2 and 3 to less than 1%, respectively. The most likely explanation of these patterns is that the separation of the various genome segments into the present-day serotype 1 and 3 associated forms occurred at different times during evolution, from progenitors that were genome segment reassortants with survival rates as high as or higher than those of homologous genome segment sets.

Growth and survival of reovirus in intestinal tissue: role of the L2 and S1 genes

Reovirus serotype 1 Lang can be recovered in high titer from the intestines of neonatal mice up to day 8 after peroral inoculation. By contrast, reovirus serotype 3 Dearing cannot be recovered from intestinal tissue past day 4 after peroral inoculation. This difference between the two reoviruses was mapped by using reassortants generated from nonmutagenized laboratory stocks. When the L2 and S1 genes of reovirus serotype 3 Dearing were present in reassortants, the reassortants behaved like serotype 3 Dearing in exhibiting a decreased capacity to be recovered from intestinal tissue. Likewise, viruses which contained the L2 and S2 genes from serotype 1 Lang exhibited an enhanced capacity to grow and survive, which is characteristic of serotype 1 Lang. Thus, the capacity of reovirus to survive in intestinal tissue was determined by the L2 and S1 genes.

Evidence for genetic relationship between RNA and DNA viruses from the sequence homology of a putative polymerase gene of bluetongue virus with that of vaccinia virus: conservation of RNA polymerase genes from diverse species

The nucleotide sequence of segment 1 of the double stranded RNA genome of bluetongue virus serotype 10 (BTV-10), encoding the largest viral core protein, VP1, has been determined. Linear sequence analysis of the predicted amino acid sequence of the 149-K Da protein, a putative component of the viral RNA-directed RNA polymerase, revealed extensive homology with the vaccinia virus 147K Da DNA-directed RNA polymerase subunit. Similar homologies were detected between the VP1 polypeptide and the beta chain subunit of Escherichia coli and common tobacco chloroplast RNA polymerases, yeast RNA polymerase II and III and fruit fly polymerase II.

Complete nucleotide sequence of the M2 gene segment of reovirus type 3 dearing and analysis of its protein product mu 1

The nucleotide sequence of the M2 gene segment of the mammalian reovirus prototype strain, type 3 Dearing, was determined from a cloned full-length cDNA copy of the viral double-stranded RNA segment. The gene comprises 2203 nucleotides and has a single long open reading frame that spans bases 30 through 2154 and encodes the 708 amino acid outer capsid protein mu 1. Aminoterminal sequence analysis of mu 1C, the proteolytically cleaved form of mu 1 that is found in purified reovirions, has identified the site of mu 1 to mu 1C cleavage between residues 42 and 43 in the mu 1 sequence. Aminoterminal sequence analysis of delta, the proteolytically cleaved product of mu 1C that is found in chymotrypsin-generated intermediate subviral particles, has indicated that the mu 1C to delta cleavage occurs near the carboxyterminus of mu 1C. Lastly, stoichiometric determinations using new sequence information have suggested that approximately equimolar amounts of mu 1C and the other major outer capsid component sigma 3 are present in virions. The data presented in this study should be useful for understanding the molecular basis of the functions of the mu 1 protein in reovirus entry into cells and in pathogenesis in the host animal.

Distinct binding sites for zinc and double-stranded RNA in the reovirus outer capsid protein sigma 3

By atomic absorption analysis, we determined that the reovirus outer capsid protein sigma 3, which binds double-stranded RNA (dsRNA), is a zinc metalloprotein. Using Northwestern blots and a novel zinc blotting technique, we localized the zinc- and dsRNA-binding activities of sigma 3 to distinct V8 protease-generated fragments. Zinc-binding activity was contained within an amino-terminal fragment that contained a transcription factor IIIA-like zinc-binding sequence, and dsRNA-binding activity was associated with a carboxy-terminal fragment. By these techniques, new zinc- and dsRNA-binding activities were also detected in reovirus core proteins. A sequence similarity was observed between the catalytic site of the picornavirus proteases and the transcription factor IIIA-like zinc-binding site within sigma 3. We suggest that the zinc- and dsRNA-binding activities of sigma 3 may be important for its proposed regulatory effects on viral and host cell transcription and translation.

Distinct binding sites for zinc and double-stranded RNA in the reovirus outer capsid protein sigma 3

By atomic absorption analysis, we determined that the reovirus outer capsid protein sigma 3, which binds double-stranded RNA (dsRNA), is a zinc metalloprotein. Using Northwestern blots and a novel zinc blotting technique, we localized the zinc- and dsRNA-binding activities of sigma 3 to distinct V8 protease-generated fragments. Zinc-binding activity was contained within an amino-terminal fragment that contained a transcription factor IIIA-like zinc-binding sequence, and dsRNA-binding activity was associated with a carboxy-terminal fragment. By these techniques, new zinc- and dsRNA-binding activities were also detected in reovirus core proteins. A sequence similarity was observed between the catalytic site of the picornavirus proteases and the transcription factor IIIA-like zinc-binding site within sigma 3. We suggest that the zinc- and dsRNA-binding activities of sigma 3 may be important for its proposed regulatory effects on viral and host cell transcription and translation.

Viral RNA polymerases

Recent progress in molecular biological techniques revealed that genomes of animal viruses are complex in structure, for example, with respect to the chemical nature (DNA or RNA), strandedness (double or single), genetic sense (positive or negative), circularity (circle or linear), and so on. In agreement with this complexity in the genome structure, the modes of transcription and replication are various among virus families. The purpose of this article is to review and bring up to date the literature on viral RNA polymerases involved in transcription of animal DNA viruses and in both transcription and replication of RNA viruses. This review shows that the viral RNA polymerases are complex in both structure and function, being composed of multiple subunits and carrying multiple functions. The functions exposed seem to be controlled through structural interconversion.

Intracellular digestion of reovirus particles requires a low pH and is an essential step in the viral infectious cycle

Lysosomotropic drugs such as NH4Cl have been useful for studying the role of low pH in early events in virus infection. NH4Cl blocks the production of infectious progeny virus in mammalian reovirus-infected cells. The inhibitory effect of NH4Cl is mediated by an inhibition of intracellular digestion of reovirus outer capsid proteins. In vitro digestion of viral outer capsid proteins produces infectious partially uncoated particles, called intermediate subviral particles, which are no longer inhibited by the presence of NH4Cl. These results indicate that proteolytic processing of reovirus outer capsid proteins takes place in a low pH compartment of the cell and is an essential step in the viral infectious cycle.

Reovirus guanylyltransferase is L2 gene product lambda 2

Reovirus guanylyltransferase, studied as a covalent enzyme-GMP intermediate, was used to guanylate appropriate acceptor molecules in vitro to produce authentic cap structures. Guanylyltransferase activity was associated with lambda 2, the 140-kilodalton product of the L2 gene segment of reovirus serotypes 1 and 3.

Ammonium inhibits processing and cytotoxicity of reovirus, a nonenveloped virus

Successful viral infection involves a series of interactions between the virus and the host cell. The outcome of viral infection is, in fact, dependent on intact cellular function; it is required for viral binding, internalization, and uncoating. To determine the potential importance of lysosomal processing on the outcome of infection with a nonenveloped virus, we have studied the effects of NH4Cl on the course of reovirus infection on a beta-cell tumor in culture. Addition of 10 mM NH4C1 to the medium inhibited viral growth by greater than 80% and reduced toxic effects of the virus on cell viability, protein, and DNA synthesis by 30-45%. In addition, synthesis of viral proteins was markedly decreased. Uptake of virus prelabeled with [35S]methionine was not affected by the ammonium; however, cleavage of mu1C, an outer capsid protein of the virus whose cleavage appears to be required for viral replication, was delayed. These results suggest that intracellular processing of reovirus is dependent on a lysosomal pathway and that disruption of this pathway can alter the course of viral infection.

Switch-on of transcriptase function in reovirus: analysis of polypeptide changes using 2-D gels

Two-dimensional gel electrophoresis (IEF and SDS-PAGE) was used to examine virion polypeptide changes associated with switch-on of transcriptase function in reovirus. Results reveal that switch-on is correlated with altered electrophoretic behavior of a specific minor polypeptide (delta 1) which is present in intermediate subviral particles. A second finding is that each of the molecular weight classes of viral polypeptides exists as a series of subspecies with different isoelectric points. This suggests that extensive posttranslational modification of progeny viral polypeptides occurs during particle morphogenesis. These findings have important theoretical and practical implications.

Reovirus RNA transcriptase: evidence for a conformational change during activation of the core particle

Reovirus cores contain an RNA transcriptase capable of synthesizing messenger RNA. When cores are suspended in 1 X SSC at 37 degrees they are quiescent and synthesize no product, but in the presence of the components of an RNA transcriptase reaction mixture they actively synthesize mRNA. Photochemical crosslinking has been used to investigate the arrangement of RNA and protein in both "quiescent" and "active" cores. Irradiation induces the formation of a noncovalent RNA:protein complex in "quiescent" but not in "active" cores. This difference is attributed to a conformational change in the reovirus core which results from the transition between the "quiescent" and "active" states of the particle.

Topological analysis of the reovirus type 3 hemagglutinin

We previously showed that the reovirus type 3 hemagglutinin (HA) has distinct functional domains. For example, we identified one group of anti-HA monoclonal antibodies which only inhibited virus-mediated hemagglutination and another group which exclusively neutralized reovirus infectivity. Using competition radioimmunoassays, we now report that these functionally discrete domains on the reovirus type 3 HA correspond to discrete antigenic regions of the protein.