Reovirus morphogenesis. Corelike particles in cells infected at 39 degrees with wild-type reovirus and temperature-sensitive mutants of groups B and G

Selection and Characterization of a Reovirus Mutant with Increased Thermostability

The environment represents a significant barrier to infection. Physical stressors (heat) or chemical agents (ethanol) can render virions noninfectious. As such, discrete proteins are necessary to stabilize the dual-layered structure of mammalian orthoreovirus (reovirus). The outer capsid participates in cell entry: (i) σ3 is degraded to generate the infectious subviral particle, and (ii) μ1 facilitates membrane penetration and subsequent core delivery. μ1-σ3 interactions also prevent inactivation; however, this activity is not fully characterized. Using forward and reverse genetic approaches, we identified two mutations (μ1 M258I and σ3 S344P) within heat-resistant strains. σ3 S344P was sufficient to enhance capsid integrity and to reduce protease sensitivity. Moreover, these changes impaired replicative fitness in a reassortant background. This work reveals new details regarding the determinants of reovirus stability.IMPORTANCE Nonenveloped viruses rely on protein-protein interactions to shield their genomes from the environment. The capsid, or protective shell, must also disassemble during cell entry. In this work, we identified a determinant within mammalian orthoreovirus that regulates heat resistance, disassembly kinetics, and replicative fitness. Together, these findings show capsid function is balanced for optimal replication and for spread to a new host.

Reovirus forms neo-organelles for progeny particle assembly within reorganized cell membranes

Most viruses that replicate in the cytoplasm of host cells form neo-organelles that serve as sites of viral genome replication and particle assembly. These highly specialized structures concentrate viral replication proteins and nucleic acids, prevent the activation of cell-intrinsic defenses, and coordinate the release of progeny particles. Despite the importance of inclusion complexes in viral replication, there are key gaps in the knowledge of how these organelles form and mediate their functions. Reoviruses are nonenveloped, double-stranded RNA (dsRNA) viruses that serve as tractable experimental models for studies of dsRNA virus replication and pathogenesis. Following reovirus entry into cells, replication occurs in large cytoplasmic structures termed inclusions that fill with progeny virions. Reovirus inclusions are nucleated by viral nonstructural proteins, which in turn recruit viral structural proteins for genome replication and particle assembly. Components of reovirus inclusions are poorly understood, but these structures are generally thought to be devoid of membranes. We used transmission electron microscopy and three-dimensional image reconstructions to visualize reovirus inclusions in infected cells. These studies revealed that reovirus inclusions form within a membranous network. Viral inclusions contain filled and empty viral particles and microtubules and appose mitochondria and rough endoplasmic reticulum (RER). Immunofluorescence confocal microscopy analysis demonstrated that markers of the ER and ER-Golgi intermediate compartment (ERGIC) codistribute with inclusions during infection, as does dsRNA. dsRNA colocalizes with the viral protein σNS and an ERGIC marker inside inclusions. These findings suggest that cell membranes within reovirus inclusions form a scaffold to coordinate viral replication and assembly.

Viruses alter the architecture of host cells to form an intracellular environment conducive to viral replication. This step in viral infection requires the concerted action of viral and host components and is potentially vulnerable to pharmacological intervention. Reoviruses form large cytoplasmic replication sites called inclusions, which have been described as membrane-free structures. Despite the importance of inclusions in the reovirus replication cycle, little is known about their formation and composition. We used light and electron microscopy to demonstrate that reovirus inclusions are membrane-containing structures and that the endoplasmic reticulum (ER) and the ER-Golgi intermediate compartment interact closely with these viral organelles. These findings enhance our understanding of the cellular machinery usurped by viruses to form inclusion organelles and complete an infectious cycle. This information, in turn, may foster the development of antiviral drugs that impede this essential viral replication step.

Reovirus replication protein μ2 influences cell tropism by promoting particle assembly within viral inclusions

The double-stranded RNA virus mammalian reovirus displays broad cell, tissue, and host tropism. A critical checkpoint in the reovirus replication cycle resides within viral cytoplasmic inclusions, which are biosynthetic centers of genome multiplication and new-particle assembly. Replication of strain type 3 Dearing (T3) is arrested in Madin-Darby canine kidney (MDCK) cells at a step subsequent to inclusion development and prior to formation of genomic double-stranded RNA. This phenotype is primarily regulated by viral replication protein μ2. To understand how reovirus inclusions differ in productively and abortively infected MDCK cells, we used confocal immunofluorescence and thin-section transmission electron microscopy (TEM) to probe inclusion organization and particle morphogenesis. Although no abnormalities in inclusion morphology or viral protein localization were observed in T3-infected MDCK cells using confocal microscopy, TEM revealed markedly diminished production of mature progeny virions. T3 inclusions were less frequent and smaller than those formed by T3-T1M1, a productively replicating reovirus strain, and contained decreased numbers of complete particles. T3 replication was enhanced when cells were cultivated at 31°C, and inclusion ultrastructure at low-temperature infection more closely resembled that of a productive infection. These results indicate that particle assembly in T3-infected MDCK cells is defective, possibly due to a temperature-sensitive structural or functional property of μ2. Thus, reovirus cell tropism can be governed by interactions between viral replication proteins and the unique cell environment that modulate efficiency of particle assembly.

Reovirus nonstructural protein mu NS recruits viral core surface proteins and entering core particles to factory-like inclusions

Mammalian reoviruses are thought to assemble and replicate within cytoplasmic, nonmembranous structures called viral factories. The viral nonstructural protein mu NS forms factory-like globular inclusions when expressed in the absence of other viral proteins and binds to the surfaces of the viral core particles in vitro. Given these previous observations, we hypothesized that one or more of the core surface proteins may be recruited to viral factories through specific associations with mu NS. We found that all three of these proteins--lambda 1, lambda 2, and sigma 2--localized to factories in infected cells but were diffusely distributed through the cytoplasm and nucleus when each was separately expressed in the absence of other viral proteins. When separately coexpressed with mu NS, on the other hand, each core surface protein colocalized with mu NS in globular inclusions, supporting the initial hypothesis. We also found that lambda 1, lambda 2, and sigma 2 each localized to filamentous inclusions formed upon the coexpression of mu NS and mu 2, a structurally minor core protein that associates with microtubules. The first 40 residues of mu NS, which are required for association with mu 2 and the RNA-binding nonstructural protein sigma NS, were not required for association with any of the three core surface proteins. When coexpressed with mu 2 in the absence of mu NS, each of the core surface proteins was diffusely distributed and displayed only sporadic, weak associations with mu 2 on filaments. Many of the core particles that entered the cytoplasm of cycloheximide-treated cells following entry and partial uncoating were recruited to inclusions of mu NS that had been preformed in those cells, providing evidence that mu NS can bind to the surfaces of cores in vivo. These findings expand a model for how viral and cellular components are recruited to the viral factories in infected cells and provide further evidence for the central but distinct roles of viral proteins mu NS and mu 2 in this process.

Cathepsin S supports acid-independent infection by some reoviruses

In murine fibroblasts, efficient proteolysis of reovirus outer capsid protein sigma3 during cell entry by virions requires the acid-dependent lysosomal cysteine protease cathepsin L. The importance of cathepsin L for infection of other cell types is unknown. Here we report that the acid-independent lysosomal cysteine protease cathepsin S mediates outer capsid processing in macrophage-like P388D cells. P388D cells supported infection by virions of strain Lang, but not strain c43. Genetic studies revealed that this difference is determined by S4, the viral gene segment that encodes sigma3. c43-derived subvirion particles that lack sigma3 replicated normally in P388D cells, suggesting that the difference in infectivity of Lang and c43 virions is at the level of sigma3 processing. Infection of P388D cells with Lang virions was inhibited by the broad spectrum cysteine protease inhibitor trans-epoxysuccinyl-l-leucylamido-(4-guanidino)butane but not by NH(4)Cl, which raises the endocytic pH and thereby inhibits acid-dependent proteases such as cathepsins L and B. Outer capsid processing and infection of P388D cells with Lang virions were also inhibited by a cathepsin S-specific inhibitor. Furthermore, in the presence of NH(4)Cl, cell lines engineered to express cathepsin S supported infection by Lang, but not c43, virions. Our results thus indicate that differences in susceptibility to cathepsin S-mediated sigma3 processing are responsible for strain differences in reovirus infection of macrophage-like P388D cells and other cathepsin S-expressing cells. Additionally, our data suggest that the acid dependence of reovirus infections of most other cell types may reflect the low pH requirement for the activities of most other lysosomal proteases rather, than some other acid-dependent aspect of cell entry.

The hydrophilic amino-terminal arm of reovirus core shell protein lambda1 is dispensable for particle assembly

The reovirus core particle is a molecular machine that mediates synthesis, capping, and export of the viral plus strand RNA transcripts. Its assembly and structure-function relationships remain to be well understood. Following the lead of previous studies with other Reoviridae family members, most notably orbiviruses and rotaviruses, we used recombinant baculoviruses to coexpress reovirus core proteins lambda1, lambda2, and sigma2 in insect cells. The resulting core-like particles (CLPs) were purified and characterized. They were found to be similar to cores with regard to their sizes, morphologies, and protein compositions. Like cores, they could also be coated in vitro with the two major outer-capsid proteins, micro 1 and sigma3, to produce virion-like particles. Coexpression of core shell protein lambda1 and core nodule protein sigma2 was sufficient to yield CLPs that could withstand purification, whereas expression of lambda1 alone was not, indicating a required role for sigma2 as a previous study also suggested. In addition, CLPs that lacked lambda2 (formed from lambda1 and sigma2 only) could not be coated with micro 1 and sigma3, indicating a required role for lambda2 in the assembly of these outer-capsid proteins into particles. To extend the use of this system for understanding the core and its assembly, we addressed the hypothesis that the hydrophilic amino-terminal region of lambda1, which adopts an extended arm-like conformation around each threefold axis in the reovirus core crystal structure, plays an important role in assembling the core shell. Using a series of lambda1 deletion mutants, we showed that the amino-terminal 230 residues of lambda1, including its zinc finger, are dispensable for CLP assembly. Residues in the 231-to-259 region of lambda1, however, were required. The core crystal structure suggests that residues in the 231-to-259 region are necessary because they affect the interaction of lambda1 with the threefold and/or fivefold copies of sigma2. An effective system for studies of reovirus core structure, assembly, and functions is hereby established.

Loss of activities for mRNA synthesis accompanies loss of lambda2 spikes from reovirus cores: an effect of lambda2 on lambda1 shell structure

The 144-kDa lambda2 protein, a component of the transcriptionally active reovirus core particle, catalyzes the last three enzymatic activities for formation of the 5' cap 1 structure on the viral plus-strand transcripts. Limited evidence suggests it may also play a role in transcription per se. Particle-associated lambda2 forms pentameric turrets ("spikes") around the fivefold axes of the icosahedral core. To address the requirements for lambda2 in core functions other than the known functions in RNA capping, particles depleted of lambda2 were generated from cores in vitro by a series of treatments involving heat, protease, and ionic detergent. The resulting particles contained less than 5% of pretreatment levels of lambda2 but showed negligible loss of the other four core proteins or the 10 double-stranded RNA genome segments. Transmission cryo-electron microscopy (cryo-TEM) and scanning cryo-electron microscopy demonstrated loss of the lambda2 spikes from these otherwise intact particles. In functional analyses, the "spikeless cores" showed greatly reduced activities not only for RNA capping but also for transcription and nucleoside triphosphate hydrolysis, suggesting enzymatic or structural roles for lambda2 in all these activities. Comparison of the core and spikeless core structures obtained by cryo-TEM and three-dimensional image reconstruction revealed changes in the lambda1 core shell that accompany lambda2 loss, most notably the elimination of small pores that span the shell near the icosahedral fivefold axes. Changes in the shell may explain the reductions in transcriptase-related activities by spikeless cores.

Mammalian reovirus L2 gene and lambda2 core spike protein sequences and whole-genome comparisons of reoviruses type 1 Lang, type 2 Jones, and type 3 Dearing

The reovirus L2 genome segment encodes the core spike protein lambda2, which mediates enzymatic reactions in 5' capping of the viral plus-strand transcripts. Complete nucleotide-sequence determinations were made for the L2 genome segments of eight mammalian reoviruses, including the prototype isolates of serotypes 1 and 2: Lang (T1L) and Jones (T2J), respectively. Each L2 segment was found to be 3912 or 3915 bases in length. Partial nucleotide-sequence determinations were also made for the 3916-base L2 segment of reovirus type 3 Dearing (T3D), the prototype isolate of serotype 3. The whole-genome sequence of reovirus T3D was reported previously. The T1L L2 analysis represents completion of the whole-genome sequence of that isolate as well. The T2J L2 analysis leaves only the sequence of the M1 segment yet to be reported from the genome of that isolate. The T2J M1 sequence made available from analysis in another lab was used for initiating whole-genome comparisons of reoviruses T1L, T2J, and T3D in this report. The nine L2 gene sequences and deduced lambda2 protein sequences were used to gain further insights into the biological variability, structure, and functions of lambda2 through comparisons of the sequences and reference to the crystal structure of core-bound lambda2. Phylogenetic comparisons suggest the presence of three evolutionary lines of divergent L2 alleles among the nine isolates. Localized regions of conserved amino acids in the lambda2 crystal structure include active-site clefts of the RNA capping enzyme domains, sites of interactions between lambda2 domains within the pentameric spike structure, and sites of interaction between lambda2 subunits and other proteins in viral particles.

Structure of the reovirus outer capsid and dsRNA-binding protein sigma3 at 1.8 A resolution

The crystallographically determined structure of the reovirus outer capsid protein sigma3 reveals a two-lobed structure organized around a long central helix. The smaller of the two lobes includes a CCHC zinc-binding site. Residues that vary between strains and serotypes lie mainly on one surface of the protein; residues on the opposite surface are conserved. From a fit of this model to a reconstruction of the whole virion from electron cryomicroscopy, we propose that each sigma3 subunit is positioned with the small lobe anchoring it to the protein mu1 on the surface of the virion, and the large lobe, the site of initial cleavages during entry-related proteolytic disassembly, protruding outwards. The surface containing variable residues faces solvent. The crystallographic asymmetric unit contains two sigma3 subunits, tightly associated as a dimer. One broad surface of the dimer has a positively charged surface patch, which extends across the dyad. In infected cells, sigma3 binds dsRNA and inhibits the interferon response. The location and extent of the positively charged surface patch suggest that the dimer is the RNA-binding form of sigma3.

Reovirus nonstructural protein muNS binds to core particles but does not inhibit their transcription and capping activities

Previous studies provided evidence that nonstructural protein muNS of mammalian reoviruses is present in particle assembly intermediates isolated from infected cells. Morgan and Zweerink (Virology 68:455-466, 1975) showed that a subset of these intermediates, which can synthesize the viral plus strand RNA transcripts in vitro, comprise core-like particles plus large amounts of muNS. Given the possible role of muNS in particle assembly and/or transcription implied by those findings, we tested whether recombinant muNS can bind to cores in vitro. The muNS protein bound to cores, but not to two particle forms, virions and intermediate subvirion particles, that contain additional outer-capsid proteins. Incubating cores with increasing amounts of muNS resulted in particle complexes of progressively decreasing buoyant density, approaching the density of protein alone when very large amounts of muNS were bound. Thus, the muNS-core interaction did not exhibit saturation or a defined stoichiometry. Negative-stain electron microscopy of the muNS-bound cores revealed that the cores were intact and linked together in large complexes by an amorphous density, which we ascribe to muNS. The muNS-core complexes retained the capacity to synthesize the viral plus strand transcripts as well as the capacity to add methylated caps to the 5' ends of the transcripts. In vitro competition assays showed that mixing muNS with cores greatly reduced the formation of recoated cores by stoichiometric binding of outer-capsid proteins mu1 and sigma3. These findings are consistent with the presence of muNS in transcriptase particles as described previously and suggest that, by binding to cores in the infected cell, muNS may block or delay outer-capsid assembly and allow continued transcription by these particles.

Mammalian reovirus M3 gene sequences and conservation of coiled-coil motifs near the carboxyl terminus of the microNS protein

Nucleotide sequences of the mammalian orthoreovirus (reovirus) type 1 Lang and type 2 Jones M3 gene segments were newly determined. The nucleotide sequence of the reovirus type 3 Dearing M3 segment also was determined to compare with a previously reported M3 sequence for that isolate. Comparisons showed Lang and Dearing M3 to be more closely related than either was to Jones M3, consistent with previous findings for other reovirus gene segments. The microNS protein sequences deduced from each M3 segment were shown to be related in a similar pattern as the respective nucleotide sequences and to contain several regions of greater or less than average variability among the three isolates. Identification of conserved methionine codons near the 5' ends of the Lang, Jones, and Dearing M3 plus strands lent support to the hypothesis that microNSC, a smaller protein also encoded by M3, arises by translation initiation from a downstream methionine codon within the same open reading frame as microNS. Other analyses of the deduced protein sequences indicated that regions within the carboxyl-terminal third of microNS and microNSC from each isolate have a propensity to form alpha-helical coiled coils, most likely coiled-coil dimers. The new sequences will augment further studies on microNS and microNSC structure and function.

In vitro recoating of reovirus cores with baculovirus-expressed outer-capsid proteins mu1 and sigma3

Reovirus outer-capsid proteins mu1, sigma3, and sigma1 are thought to be assembled onto nascent core-like particles within infected cells, leading to the production of progeny virions. Consistent with this model, we report the in vitro assembly of baculovirus-expressed mu1 and sigma3 onto purified cores that lack mu1, sigma3, and sigma1. The resulting particles (recoated cores, or r-cores) closely resembled native virions in protein composition (except for lacking cell attachment protein sigma1), buoyant density, and particle morphology by scanning cryoelectron microscopy. Transmission cryoelectron microscopy and image reconstruction of r-cores confirmed that they closely resembled virions in the structure of the outer capsid and revealed that assembly of mu1 and sigma3 onto cores had induced rearrangement of the pentameric lambda2 turrets into a conformation approximating that in virions. r-cores, like virions, underwent proteolytic conversion to particles resembling native ISVPs (infectious subvirion particles) in protein composition, particle morphology, and capacity to permeabilize membranes in vitro. r-cores were 250- to 500-fold more infectious than cores in murine L cells and, like virions but not ISVPs or cores, were inhibited from productively infecting these cells by the presence of either NH4Cl or E-64. The latter results suggest that r-cores and virions used similar routes of entry into L cells, including processing by lysosomal cysteine proteinases, even though the former particles lacked the sigma1 protein. To examine the utility of r-cores for genetic dissections of mu1 functions in reovirus entry, we generated r-cores containing a mutant form of mu1 that had been engineered to resist cleavage at the delta:phi junction during conversion to ISVP-like particles by chymotrypsin in vitro. Despite their deficit in delta:phi cleavage, these ISVP-like particles were fully competent to permeabilize membranes in vitro and to infect L cells in the presence of NH4Cl, providing new evidence that this cleavage is dispensable for productive infection.

Reovirus virion-like particles obtained by recoating infectious subvirion particles with baculovirus-expressed sigma3 protein: an approach for analyzing sigma3 functions during virus entry

Structure-function studies with mammalian reoviruses have been limited by the lack of a reverse-genetic system for engineering mutations into the viral genome. To circumvent this limitation in a partial way for the major outer-capsid protein sigma3, we obtained in vitro assembly of large numbers of virion-like particles by binding baculovirus-expressed sigma3 protein to infectious subvirion particles (ISVPs) that lack sigma3. A level of sigma3 binding approaching 100% of that in native virions was routinely achieved. The sigma3 coat in these recoated ISVPs (rcISVPs) appeared very similar to that in virions by electron microscopy and three-dimensional image reconstruction. rcISVPs retained full infectivity in murine L cells, allowing their use to study sigma3 functions in virus entry. Upon infection, rcISVPs behaved identically to virions in showing an extended lag phase prior to exponential growth and in being inhibited from entering cells by either the weak base NH4Cl or the cysteine proteinase inhibitor E-64. rcISVPs also mimicked virions in being incapable of in vitro activation to mediate lysis of erythrocytes and transcription of the viral mRNAs. Last, rcISVPs behaved like virions in showing minor loss of infectivity at 52 degrees C. Since rcISVPs contain virion-like levels of sigma3 but contain outer-capsid protein mu1/mu1C mostly cleaved at the delta-phi junction as in ISVPs, the fact that rcISVPs behaved like virions (and not ISVPs) in all of the assays that we performed suggests that sigma3, and not the delta-phi cleavage of mu1/mu1C, determines the observed differences in behavior between virions and ISVPs. To demonstrate the applicability of rcISVPs for genetic studies of protein functions in reovirus entry (an approach that we call recoating genetics), we used chimeric sigma3 proteins to localize the primary determinants of a strain-dependent difference in sigma3 cleavage rate to a carboxy-terminal region of the ISVP-bound protein.

The reovirus mutant tsA279 L2 gene is associated with generation of a spikeless core particle: implications for capsid assembly

Previous studies which used intertypic reassortants of the wild-type reovirus serotype 1 Lang and the temperature-sensitive (ts) serotype 3 mutant clone tsA279 identified two ts lesions; one lesion, in the M2 gene segment, was associated with defective transmembrane transport of restrictively assembled virions (P. R. Hazelton and K. M. Coombs, Virology 207:46-58, 1995). In the present study we show that the second lesion, in the L2 gene segment, which encodes the lambda2 protein, is associated with the accumulation of a core-like particle defective for the lambda2 pentameric spike. Physicochemical, biochemical, and immunological studies showed that these structures were deficient for genomic double-stranded RNA, the core spike protein lambda2, and the minor core protein micro2. Core particles with the lambda2 spike structure accumulated after temperature shift-down from a restrictive to a permissive temperature in the presence of cycloheximide. These data suggest the spike-deficient, core-like particle is an assembly intermediate in reovirus morphogenesis. The existence of this naturally occurring primary core structure suggests that the core proteins lambda1, lambda3, and sigma2 interact to initiate the process of virion capsid assembly through a dodecahedral mechanism. The next step in the proposed capsid assembly model would be the association of the minor core protein mu2, either preceding or collateral to the condensation of the lambda2 pentameric spike at the apices of the primary core structure. The assembly pathway of the reovirus double capsid is further elaborated when these observations are combined with structures identified in other studies.

Characterization of the thermosensitive ts453 reovirus mutant: increased dsRNA binding of sigma 3 protein correlates with interferon resistance

The mutation harbored by the reovirus ts453 thermosensitive mutant has been assigned to the S4 gene encoding the major outer capsid protein sigma 3. Previous gene sequencing has identified a nonconservative amino acid substitution located near the zinc finger of sigma 3 protein in the mutant. Coexpression in COS cells of the sigma 3 protein presenting this amino acid substitution (N16K), together with the other major capsid protein mu 1, has also revealed an altered interaction between the two proteins; this altered interaction prevents the sigma 3-dependent cleavage of mu 1 to mu 1C. This could explain the lack of outer capsid assembly observed during ts453 virus infection at nonpermissive temperature. In the present study, we pursued the characterization of this mutant sigma 3 protein. Although the N16K mutation is located close to the zinc finger region, it did not affect the ability of the protein to bind zinc. In contrast, this mutation, as well as mutations within the zinc finger motif itself, can increase the binding of the protein to double-stranded RNA (dsRNA). It also appears that the N16K mutant protein is more efficiently transported to the nucleus than the wild-type protein, an observation consistent with the postulated role of dsRNA binding in sigma 3 nuclear presence. The lack of association with mu 1, and/or the increased dsRNA-binding activity of sigma 3, could be responsible for a partial resistance of the ts453 virus to interferon treatment and this could have important consequences in the context of protein synthesis regulation during natural reovirus infection.

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.

Assembly of the reovirus outer capsid requires mu 1/sigma 3 interactions which are prevented by misfolded sigma 3 protein in temperature-sensitive mutant tsG453

A temperature-sensitive reovirus mutant, tsG453, whose defect was mapped to major outer capsid protein sigma 3, makes core particles but fails to assemble the outer capsid around the core at non-permissive temperature. Previous studies that made use of electron cryo-microscopy and image reconstructions showed that mu 1, the other major outer capsid protein, but not sigma 3, interact extensively with the core capsid. Although wild-type sigma 3 and mu 1 interact with each other, immunocoprecipitation studies showed that mutant sigma 3 protein was incapable of interacting with mu 1 at the non-permissive temperature. In addition, restrictively-grown mutant sigma 3 protein could not be precipitated by some sigma 3-specific monoclonal antibodies. These observations suggest that in a wild-type infection, specific sigma 3 and mu 1 interactions result in changes in mu 1 conformation which are required to allow mu 1/sigma 3 complexes to condense onto the core capsid shell during outer capsid assembly, and that sigma 3 in non-permissive tsG453 infections is misfolded such that it cannot interact with mu 1.

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.

Mutations in the zinc-binding motif of the reovirus capsid protein delta 3 eliminate its ability to associate with capsid protein mu 1

Reovirus capsid protein delta 3 binds both double-stranded RNA (dsRNA) and zinc. Previous studies have revealed that the amino-terminal zinc finger is not required for the ability of delta 3 to bind dsRNA. We expressed wild-type and mutant delta 3 molecules by in vitro transcription/translation to evaluate the importance of the zinc finger for other functions of delta 3. delta 3 molecules with mutations in the zinc finger did not form complexes with capsid protein mu 1 but bound dsRNA more efficiently than wild-type delta 3 did. In contrast, a dsRNA-binding mutant was unimpaired in its ability to associate with mu 1. Studies with delta 3 fragments support these findings and indicate that sequences critical for delta 3's interaction with mu 1 lie in the amino terminus of the molecule. Our finding that mu 1 and dsRNA do not compete for identical binding sites on delta 3 has implications for its function as a translational regulator in infected cells.

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.

Comparative sequence analysis of the reovirus S4 genes from 13 serotype 1 and serotype 3 field isolates

The reovirus sigma 3 protein is a major outer capsid protein that may function to regulate translation within infected cells. To facilitate the understanding of sigma 3 structure and functions and the evolution of mammalian reoviruses, we sequenced cDNA copies of the S4 genes from 10 serotype 3 and 3 serotype 1 reovirus field isolates and compared these sequences with sequences of prototypic strains of the three reovirus serotypes. We found that the sigma 3 proteins are highly conserved: the two longest conserved regions contain motifs proposed to function in binding zinc and double-stranded RNA. We used the 16 viral isolates to investigate the hypothesis that structural interactions between sigma 3 and the cell attachment protein, sigma 1, constrain their evolution and to identify a determinant within sigma 3 that is in close proximity to the sigma 1 hemagglutination site.

Further characterization of the ts453 mutant of mammalian orthoreovirus serotype 3 and nucleotide sequence of the mutated S4 gene

The sigma 3 protein of mammalian orthoreoviruses has multiple proven and postulated roles during viral multiplication. In this manuscript we took advantage of the availability of the ts453 thermosensitive mutant, already assigned to the S4 gene encoding sigma 3, to begin the elucidation of the relationship between the two main domains and the different roles of the sigma 3 viral protein. The alteration in the mutant appeared to affect the structural role of the protein. Nucleotide sequence determination indicated an especially significant change close to the zinc finger of the protein. These data suggest that the zinc-binding region might be especially important during the assembly of sigma 3 into the viral capsid.

Cytolytic T-cell mediated lysis of reovirus-infected cells: requirements for infectious virus, viral particles, and viral proteins in infected target cells

The virological requirements for the recognition of infected target cells by cytolytic T lymphocytes (CTL), using reovirus, a nonenveloped, icosahedral virus has been investigated. Using mouse L cells infected at the nonpermissive temperature with ts (temperature-sensitive) mutants of reovirus in complementation groups C and G, it has been shown that the production of complete viral particles is not necessary for efficient lysis of infected cells by CTL. In addition, adsorption of purified viral particles and viral top component (TC), empty capsids lacking genome ds-RNA, to L cells just prior to use in cytolytic T cell assays is sufficient to produce target cells capable of being lysed, though target production is less efficient than with L cells infected with reovirus. Membrane fluorescence analysis of cells infected with reovirus ts mutants at the nonpermissive temperature and with adsorbed viral particles revealed the presence of the viral sigma 1 protein on the cell surface. For adsorbed particles, the degree of membrane fluorescence paralleled the capacity of CTL to lyse target cells. It is concluded that cells infected with icosahedral, nonenveloped viruses, like cells infected with enveloped viruses, express viral antigens on the cell surface even in the absence of the production of complete viral particles; adsorbed viral particles can be incorporated into the cell membrane in a manner sufficient for recognition and lysis by CTL, in the absence of actual infection of the cells.

Structure and function of the reovirus genome

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Pyridoxal phosphate as a probe of reovirus transcriptase

The ribonucleoprotein core of reovirus is a multienzyme complex that transcribes messenger ribonucleic acid (mRNA) from double-stranded RNA templates. So far, the core has resisted attempts to disassemble it and identify the polypeptide species responsible for RNA polymerase activity. As an alternative approach, we tested pyridoxal 5-phosphate (PLP) as a potential affinity labeling reagent for reovirus transcriptase in vitro; PLP has been used as an affinity reagent for cellular and viral nucleic acid polymerases. We found that PLP inhibited reovirus transcriptase reversibly (apparent Ki = 0.2 mM), but the inhibition was noncompetitive with respect to each of the four ribonucleoside triphosphates. This interaction required both the aldehyde and phosphate moieties in PLP, since pyridoxamine and pyridoxal were relatively inactive. To identify the polypeptides involved, we labeled the PLP--core complex by reductive alkylation with [3H]borohydride. At PLP concentrations close to the apparent Ki, labeling was selective for the two largest virion polypeptides, lambda 1 and lambda 2. At saturation, there were only 10 high-affinity PLP binding sites per core in each of the lambda polypeptide species. These findings implicate either or both lambda polypeptide species in viral transcription and they indicate that a special population, representing no more than 10% of the total lambda molecules in each core, participates in RNA synthesis.

Reovirus-specific enzyme(s) associated with subviral particles responds in vitro to polyribocytidylate to yield double-stranded polyribocytidylate-polyriboguanylate

In reovirus-infected cells, virus-specific particles accumulate that have associated with them a polyribocytidylate [poly(C)]-dependent polymerase. This enzyme copies in vitro poly(C) to yield the double-stranded poly(C).polyriboguanylate [poly(G)]. The particles with poly(C)-dependent polymerase were heterogeneous in size, with most sedimenting from 300S to 550S. Exponential increase in these particles began at 23 h, and maximal amounts were present by 31 h, the time of onset of exponential growth of virus at 30 degrees C. Maximal amounts of particles with active transcriptase and replicase were present at 15 and 18 h after infection. Thereafter, there was a marked decrease in particles with active transcriptase and replicase until base line levels were reached at 31 h. Thus, the increase in poly(C)-responding particles occurred coincident with the decrease in particles with active transcriptase and replicase. The requirement for poly(C) as template was specific because no RNA was synthesized in vitro in response to any other homopolymer, including 2'-O-methyl-poly(C). Synthesis was optimal in the presence of Mn(2+) as the divalent cation, and no primer was necessary for synthesis. In contrast, the dinucleotide GpG markedly stimulated synthesis in the presence of 8 mM Mg(2+). The size of the poly(C).poly(G) synthesized in vitro was dependent on the size of the poly(C) used as template. This suggested that the whole template was copied into a complementary strand of similar size. The T(m) of the product was between 100 and 130 degrees C. Hydrolysis of the product labeled in [(32)P]GMP with alkali or RNase T2 yielded GMP as the only labeled mononucleotide. This does indicate that the synthesis of the poly(G) strand in vitro did not proceed by end addition to the poly(C) template, but proceeded on a separate strand.