Activation and characterization of the reovirus transcriptase: genetic analysis

In situ structures of polymerase complex of mammalian reovirus illuminate RdRp activation and transcription regulation

Mammalian reovirus (reovirus) is a multilayered, turreted member of Reoviridae characterized by transcription of dsRNA genome within the innermost capsid shell. Here, we present high-resolution in situ structures of reovirus transcriptase complex in an intact double-layered virion, and in the uncoated single-layered core particles in the unloaded, reloaded, pre-elongation, and elongation states, respectively, obtained by cryo-electron microscopy and sub-particle reconstructions. At the template entry of RNA-dependent RNA polymerase (RdRp), the RNA-loading region gets flexible after uncoating resulting in the unloading of terminal genomic RNA and inactivity of transcription. However, upon adding transcriptional substrates, the RNA-loading region is recovered leading the RNAs loaded again. The priming loop in RdRp was found to play a critical role in regulating transcription, which hinders the elongation of transcript in virion and triggers the rearrangement of RdRp C-terminal domain (CTD) during elongation, resulting in splitting of template-transcript hybrid and opening of transcript exit. With the integration of these structures, a transcriptional model of reovirus with five states is proposed. Our structures illuminate the RdRp activation and regulation of the multilayered turreted reovirus.

Molecular Characterization of Outer Capsid Proteins VP5 and VP7 of Grass Carp Reovirus

Aquareovirus, which is a member of the Reoviridae family, was isolated from aquatic animals. A close molecular evolutionary relationship between aquareoviruses and mammalian orthoreoviruses was revealed. However, the functions of the aquareovirus genome-encoded proteins are poorly understood. We investigated the molecular characteristics of the outer capsid proteins, namely, VP5 and VP7, of grass carp reovirus (GCRV). The peptides VP5 and VP7 were determined using in-gel tryptic digestion and mass spectrometry. Recovered peptides represented 76% and 66% of the full-length VP5 and VP7 sequences, respectively. Significantly, two-lysine acetylation, as well as two-serine and two-threonine phosphorylation modifications, were first revealed in VP5. We found that the initial amino acid in VP5 was Pro43, suggesting that a lower amount of VP5 remained uncleaved in virions at the autocleavage site (Asn42-Pro43). Further biochemical evidence showed that the cleaved VP5N/VP5C conformation was the major constituent of the particles. Moreover, early cleavage fragments of VP7 and enhanced infectivity were detected after limited tryptic digestion of GCRV, indicating that stepwise VP7 cleavage is essential for VP5 conformational rearrangement. Our results provide insights into the roles of posttranslational modifications in VP5 and its association with VP7 in the viral life cycle.

Viral Capsid and Polymerase in Reoviridae

The members of the family Reoviridae (reoviruses) consist of 9-12 discrete double-stranded RNA (dsRNA) segments enclosed by single, double, or triple capsid layers. The outer capsid proteins of reoviruses exhibit the highest diversity in both sequence and structural organization. By contrast, the conserved RNA-dependent RNA polymerase (RdRp) structure in the conserved innermost shell in all reoviruses suggests that they share common transcriptional regulatory mechanisms. After reoviruses are delivered into the cytoplasm of a host cell, their inner capsid particles (ICPs) remain intact and serve as a stable nanoscale machine for RNA transcription and capping performed using enzymes in ICPs. Advances in cryo-electron microscopy have enabled the reconstruction at near-atomic resolution of not only the icosahedral capsid, including capping enzymes, but also the nonicosahedrally distributed complexes of RdRps within the capsid at different transcriptional stages. These near-atomic resolution structures allow us to visualize highly coordinated structural changes in the related enzymes, genomic RNA, and capsid protein during reovirus transcription. In addition, reoviruses encode their own enzymes for nascent RNA capping before RNA releasing from their ICPs.

Generation of Genetically RGD σ1-Modified Oncolytic Reovirus That Enhances JAM-A-Independent Infection of Tumor Cells

Mammalian reovirus (MRV) strain type 3 Dearing (T3D) is a naturally occurring oncolytic virus that has been developed as a potential cancer therapeutic. However, MRV treatment cannot be applied to cancer cells expressing low levels of junctional adhesion molecule A (JAM-A), which is the entry receptor of MRV. In this study, we developed a reverse genetics system for MRV strain T3D-L, which showed high oncolytic potency. To modify the cell tropism of MRV, an arginine-glycine-aspartic acid (RGD) peptide with an affinity to integrin was inserted at the C terminus or loop structures of the viral cell attachment protein σ1. The recombinant RGD σ1-modified viruses induced remarkable cell lysis in human cancer cell lines with marginal JAM-A expression and in JAM-A knockout cancer cell lines generated by a CRISPR/Cas9 system. Pretreatment of cells with anti-integrin antibody decreased cell death caused by the RGD σ1-modified virus, suggesting the infection to the cells was via a specific interaction with integrin αV. By using mouse models, we assessed virulence of the RGD σ1-modified viruses in vivo This system will open new avenues for the use of genetically modified oncolytic MRV for use as a cancer therapy.IMPORTANCE Oncolytic viruses kill tumors without affecting normal cells. A variety of oncolytic viruses are used as cancer therapeutics. Mammalian reovirus (MRV), which belongs to the genus Orthoreovirus, family Reoviridae, is one such natural oncolytic virus. The anticancer effects of MRV are being evaluated in clinical trials. Unlike other oncolytic viruses, MRV has not been genetically modified for use as a cancer therapeutic in clinical trials. Here, we used a reverse genetic approach to introduce an integrin-affinity peptide sequence into the MRV cell attachment protein σ1 to alter the natural tropism of the virus. The recombinant viruses were able to infect cancer cell lines expressing very low levels of the MRV entry receptor, junctional adhesion molecule A (JAM-A), and cause tumor cell death while maintaining its original tropism via JAM-A. This is a novel report of a genetically modified oncolytic MRV by introducing a peptide sequence into σ1.

Enhanced Killing of Triple-Negative Breast Cancer Cells by Reassortant Reovirus and Topoisomerase Inhibitors

Breast cancer is the second leading cause of cancer-related deaths in women in the United States. Triple-negative breast cancer constitutes a subset of breast cancer that is associated with higher rates of relapse, decreased survival, and limited therapeutic options for patients afflicted with this type of breast cancer. Mammalian orthoreovirus (reovirus) selectively infects and kills transformed cells, and a serotype 3 reovirus is in clinical trials to assess its efficacy as an oncolytic agent against several cancers. It is unclear if reovirus serotypes differentially infect and kill triple-negative breast cancer cells and if reovirus-induced cytotoxicity of breast cancer cells can be enhanced by modulating the activity of host molecules and pathways. Here, we generated reassortant reoviruses by forward genetics with enhanced infective and cytotoxic properties in triple-negative breast cancer cells. From a high-throughput screen of small-molecule inhibitors, we identified topoisomerase inhibitors as a class of drugs that enhance reovirus infectivity and cytotoxicity of triple-negative breast cancer cells. Treatment of triple-negative breast cancer cells with topoisomerase inhibitors activates DNA damage response pathways, and reovirus infection induces robust production of type III, but not type I, interferon (IFN). Although type I and type III IFNs can activate STAT1 and STAT2, triple-negative breast cancer cellular proliferation is only negatively affected by type I IFN. Together, these data show that reassortant viruses with a novel genetic composition generated by forward genetics in combination with topoisomerase inhibitors more efficiently infect and kill triple-negative breast cancer cells.IMPORTANCE Patients afflicted by triple-negative breast cancer have decreased survival and limited therapeutic options. Reovirus infection results in cell death of a variety of cancers, but it is unknown if different reovirus types lead to triple-negative breast cancer cell death. In this study, we generated two novel reoviruses that more efficiently infect and kill triple-negative breast cancer cells. We show that infection in the presence of DNA-damaging agents enhances infection and triple-negative breast cancer cell killing by reovirus. These data suggest that a combination of a genetically engineered oncolytic reovirus and topoisomerase inhibitors may provide a potent therapeutic option for patients afflicted with triple-negative breast cancer.

Components of the Reovirus Capsid Differentially Contribute to Stability

The mammalian orthoreovirus (reovirus) outer capsid is composed of 200 μ1-σ3 heterohexamers and a maximum of 12 σ1 trimers. During cell entry, σ3 is degraded by luminal or intracellular proteases to generate the infectious subviral particle (ISVP). When ISVP formation is prevented, reovirus fails to establish a productive infection, suggesting proteolytic priming is required for entry. ISVPs are then converted to ISVP*s, which is accompanied by μ1 rearrangements. The μ1 and σ3 proteins confer resistance to inactivating agents; however, neither the impact on capsid properties nor the mechanism (or basis) of inactivation is fully understood. Here, we utilized T1L/T3D M2 and T3D/T1L S4 to investigate the determinants of reovirus stability. Both reassortants encode mismatched subunits. When μ1-σ3 were derived from different strains, virions resembled wild-type particles in structure and protease sensitivity. T1L/T3D M2 and T3D/T1L S4 ISVPs were less thermostable than wild-type ISVPs. In contrast, virions were equally susceptible to heating. Virion associated μ1 adopted an ISVP*-like conformation concurrent with inactivation; σ3 preserves infectivity by preventing μ1 rearrangements. Moreover, thermostability was enhanced by a hyperstable variant of μ1. Unlike the outer capsid, the inner capsid (core) was highly resistant to elevated temperatures. The dual layered architecture allowed for differential sensitivity to inactivating agents.IMPORTANCE Nonenveloped and enveloped viruses are exposed to the environment during transmission to a new host. Protein-protein and/or protein-lipid interactions stabilize the particle and protect the viral genome. Mammalian orthoreovirus (reovirus) is composed of two concentric, protein shells. The μ1 and σ3 proteins form the outer capsid; contacts between neighboring subunits are thought to confer resistance to inactivating agents. We further investigated the determinants of reovirus stability. The outer capsid was disrupted concurrent with the loss of infectivity; virion associated μ1 rearranged into an altered conformation. Heat sensitivity was controlled by σ3; however, particle integrity was enhanced by a single μ1 mutation. In contrast, the inner capsid (core) displayed superior resistance to heating. These findings reveal structural components that differentially contribute to reovirus stability.

Synthesis and Translation of Viral mRNA in Reovirus-Infected Cells: Progress and Remaining Questions

At the end of my doctoral studies, in 1988, I published a review article on the major steps of transcription and translation during the mammalian reovirus multiplication cycle, a topic that still fascinates me 30 years later. It is in the nature of scientific research to generate further questioning as new knowledge emerges. Our understanding of these fascinating viruses thus remains incomplete but it seemed appropriate at this moment to look back and reflect on our progress and most important questions that still puzzle us. It is also essential of being careful about concepts that seem so well established, but could still be better validated using new approaches. I hope that the few reflections presented here will stimulate discussions and maybe attract new investigators into the field of reovirus research. Many other aspects of the viral multiplication cycle would merit our attention. However, I will essentially limit my discussion to these central aspects of the viral cycle that are transcription of viral genes and their phenotypic expression through the host cell translational machinery. The objective here is not to review every aspect but to put more emphasis on important progress and challenges in the field.

Bacteria and bacterial envelope components enhance mammalian reovirus thermostability

Enteric viruses encounter diverse environments as they migrate through the gastrointestinal tract to infect their hosts. The interaction of eukaryotic viruses with members of the host microbiota can greatly impact various aspects of virus biology, including the efficiency with which viruses can infect their hosts. Mammalian orthoreovirus, a human enteric virus that infects most humans during childhood, is negatively affected by antibiotic treatment prior to infection. However, it is not known how components of the host microbiota affect reovirus infectivity. In this study, we show that reovirus virions directly interact with Gram positive and Gram negative bacteria. Reovirus interaction with bacterial cells conveys enhanced virion thermostability that translates into enhanced attachment and infection of cells following an environmental insult. Enhanced virion thermostability was also conveyed by bacterial envelope components lipopolysaccharide (LPS) and peptidoglycan (PG). Lipoteichoic acid and N-acetylglucosamine-containing polysaccharides enhanced virion stability in a serotype-dependent manner. LPS and PG also enhanced the thermostability of an intermediate reovirus particle (ISVP) that is associated with primary infection in the gut. Although LPS and PG alter reovirus thermostability, these bacterial envelope components did not affect reovirus utilization of its proteinaceous cellular receptor junctional adhesion molecule-A or cell entry kinetics. LPS and PG also did not affect the overall number of reovirus capsid proteins σ1 and σ3, suggesting their effect on virion thermostability is not mediated through altering the overall number of major capsid proteins on the virus. Incubation of reovirus with LPS and PG did not significantly affect the neutralizing efficiency of reovirus-specific antibodies. These data suggest that bacteria enhance reovirus infection of the intestinal tract by enhancing the thermal stability of the reovirus particle at a variety of temperatures through interactions between the viral particle and bacterial envelope components.

Potential for Improving Potency and Specificity of Reovirus Oncolysis with Next-Generation Reovirus Variants

Viruses that specifically replicate in tumor over normal cells offer promising cancer therapies. Oncolytic viruses (OV) not only kill the tumor cells directly; they also promote anti-tumor immunotherapeutic responses. Other major advantages of OVs are that they dose-escalate in tumors and can be genetically engineered to enhance potency and specificity. Unmodified wild type reovirus is a propitious OV currently in phase I–III clinical trials. This review summarizes modifications to reovirus that may improve potency and/or specificity during oncolysis. Classical genetics approaches have revealed reovirus variants with improved adaptation towards tumors or with enhanced ability to establish specific steps of virus replication and cell killing among transformed cells. The recent emergence of a reverse genetics system for reovirus has provided novel strategies to fine-tune reovirus proteins or introduce exogenous genes that could promote oncolytic activity. Over the next decade, these findings are likely to generate better-optimized second-generation reovirus vectors and improve the efficacy of oncolytic reotherapy.

A putative ATPase mediates RNA transcription and capping in a dsRNA virus

mRNA transcription in dsRNA viruses is a highly regulated process but the mechanism of this regulation is not known. Here, by nucleoside triphosphatase (NTPase) assay and comparisons of six high-resolution (2.9-3.1 Å) cryo-electron microscopy structures of cytoplasmic polyhedrosis virus with bound ligands, we show that the large sub-domain of the guanylyltransferase (GTase) domain of the turret protein (TP) also has an ATP-binding site and is likely an ATPase. S-adenosyl-L-methionine (SAM) acts as a signal and binds the methylase-2 domain of TP to induce conformational change of the viral capsid, which in turn activates the putative ATPase. ATP binding/hydrolysis leads to an enlarged capsid for efficient mRNA synthesis, an open GTase domain for His217-mediated guanylyl transfer, and an open methylase-1 domain for SAM binding and methyl transfer. Taken together, our data support a role of the putative ATPase in mediating the activation of mRNA transcription and capping within the confines of the virus.

The μ1 72-96 loop controls conformational transitions during reovirus cell entry

The reovirus outer capsid protein μ1 forms a lattice surrounding the viral core. In the native state, μ1 determines the environmental stability of the viral capsid. Additionally, during cell entry, μ1 undergoes structural rearrangements that facilitate delivery of the viral cores across the membrane. To determine how the capsid-stabilizing functions of μ1 impinge on the capacity of μ1 to undergo conformational changes required for cell entry, we characterized viruses with mutations engineered at charged residues within the μ1 loop formed by residues 72 to 96 (72-96 loop). This loop is proposed to stabilize the capsid by mediating interactions between neighboring μ1 trimers and between trimers and the core. We found that mutations at Glu89 (E89) within this loop produced viruses with compromised efficiency for completing their replication cycle. ISVPs of E89 mutants converted to ISVP*s more readily than those of wild-type viruses. The E89 mutants yielded revertants with second-site substitutions within regions that mediate interaction between μ1 trimers at a site distinct from the 72-96 loop. These viruses also contained changes in regions that control interactions within μ1 trimers. Viruses containing these second-site changes displayed restored plaque phenotypes and were capable of undergoing ISVP-to-ISVP* conversion in a regulated manner. These findings highlight regions of μ1 that stabilize the reovirus capsid and demonstrate that an enhanced propensity to form ISVP*s in an unregulated manner compromises viral fitness.

Cell entry-associated conformational changes in reovirus particles are controlled by host protease activity

Membrane penetration by reovirus requires successive formation of two cell entry intermediates, infectious subvirion particles (ISVPs) and ISVP*s. In vitro incubation of reovirus virions with high concentration of chymotrypsin (CHT) results in partial digestion of the viral outer capsid to form ISVPs. When virions are instead digested with low concentrations of chymotrypsin, the outer capsid is completely proteolyzed to form cores. We investigated the basis for the inverse relationship between CHT activity and protease susceptibility of the reovirus outer capsid. We report that core formation following low-concentration CHT digestion proceeds via formation of particles that contain a protease-sensitive form of the μ1C protein, a characteristic of ISVP*s. In addition, we found that both biochemical features and viral genetic requirements for ISVP* formation and core formation following low-concentration CHT digestion are identical, suggesting that core formation proceeds via a particle resembling ISVP*s. Furthermore, we determined that intermediates generated following low-concentration CHT digestion are distinct from ISVPs and convert to ISVP*-like particles much more readily than ISVPs. These results suggest that the activity of host proteases used to generate ISVPs can influence the efficiency with which the next step in reovirus cell entry, namely, ISVP-to-ISVP* conversion, occurs.

Src kinase mediates productive endocytic sorting of reovirus during cell entry

Reovirus cell entry is initiated by viral attachment to cell surface glycans and junctional adhesion molecule A. Following receptor engagement, reovirus is internalized into cells by receptor-mediated endocytosis using a process dependent on β1 integrin. Endocytosed virions undergo stepwise disassembly catalyzed by cathepsin proteases, followed by endosomal membrane penetration and delivery of transcriptionally active core particles into the cytoplasm. Cellular factors that mediate reovirus endocytosis are poorly defined. We found that both genistein, a broad-spectrum tyrosine kinase inhibitor, and PP2, a specific Src-family kinase inhibitor, diminish reovirus infectivity by blocking a cell entry step. Although neither inhibitor impedes internalization of reovirus virions, both inhibitors target virions to lysosomes. Reovirus colocalizes with Src during cell entry, and reovirus infection induces phosphorylation of Src at the activation residue, tyrosine 416. Diminished Src expression by RNA interference reduces reovirus infectivity, suggesting that Src is required for efficient reovirus entry. Collectively, these data provide evidence that Src kinase is an important mediator of signaling events that regulate the appropriate sorting of reovirus particles in the endocytic pathway for disassembly and cell entry.

From touchdown to transcription: the reovirus cell entry pathway

Mammalian orthoreoviruses (reoviruses) are prototype members of the Reoviridae family of nonenveloped viruses. Reoviruses contain ten double-stranded RNA gene segments enclosed in two concentric protein shells, outer capsid and core. These viruses serve as a versatile experimental system for studies of virus cell entry, innate immunity, and organ-specific disease. Reoviruses engage cells by binding to cell-surface carbohydrates and the immunoglobulin superfamily member, junctional adhesion molecule-A (JAM-A). JAM-A is a homodimer formed by extensive contacts between its N-terminal immunoglobulin-like domains. Reovirus attachment protein σ1 disrupts the JAM-A dimer, engaging a single JAM-A molecule by virtually the same interface used for JAM-A homodimerization. Following attachment to JAM-A and carbohydrate, reovirus internalization is promoted by β1 integrins, most likely via clathrin-dependent endocytosis. In the endocytic compartment, reovirus outer-capsid protein σ3 is removed by cathepsin proteases, which exposes the viral membrane-penetration protein, μ1. Proteolytic processing and conformational rearrangements of μ1 mediate endosomal membrane rupture and delivery of transcriptionally active reovirus core particles into the host cell cytoplasm. These events also allow the φ cleavage fragment of μ1 to escape into the cytoplasm where it activates NF-κB and elicits apoptosis. This review will focus on mechanisms of reovirus cell entry and activation of innate immune response signaling pathways.

Backbone model of an aquareovirus virion by cryo-electron microscopy and bioinformatics

Grass carp reovirus (GCRV) is a member of the aquareovirus genus in the Reoviridae family and has a capsid with two shells-a transcription-competent core surrounded by a coat. We report a near-atomic-resolution reconstruction of the GCRV virion by cryo-electron microscopy and single-particle reconstruction. A backbone model of the GCRV virion, including seven conformers of the five capsid proteins making up the 1500 molecules in both the core and the coat, was derived using cryo-electron microscopy density-map-constrained homology modeling and refinement. Our structure clearly showed that the amino-terminal segment of core protein VP3B forms an approximately 120-A-long alpha-helix-rich extension bridging across the icosahedral 2-fold-symmetry-related molecular interface. The presence of this unique structure across this interface and the lack of an external cementing molecule at this location in GCRV suggest a stabilizing role of this extended amino-terminal density. Moreover, part of this amino-terminal extension becomes invisible in the reconstruction of transcription-competent core particles, suggesting its involvement in endogenous viral RNA transcription. Our structure of the VP1 turret represents its open state, and comparison with its related structures at the closed state suggests hinge-like domain movements associated with the mRNA-capping machinery. Overall, this first backbone model of an aquareovirus virion provides a wealth of structural information for understanding the structural basis of GCRV assembly and transcription.

Requirements for the formation of membrane pores by the reovirus myristoylated micro1N peptide

The outer capsid of the nonenveloped mammalian reovirus contains 200 trimers of the micro1 protein, each complexed with three copies of the protector protein sigma3. Conformational changes in micro1 following the proteolytic removal of sigma3 lead to release of the myristoylated N-terminal cleavage fragment micro1N and ultimately to membrane penetration. The micro1N fragment forms pores in red blood cell (RBC) membranes. In this report, we describe the interaction of recombinant micro1 trimers and synthetic micro1N peptides with both RBCs and liposomes. The micro1 trimer mediates hemolysis and liposome disruption under conditions that promote the micro1 conformational change, and mutations that inhibit micro1 conformational change in the context of intact virus particles also prevent liposome disruption by particle-free micro1 trimer. Autolytic cleavage to form micro1N is required for hemolysis but not for liposome disruption. Pretreatment of RBCs with proteases rescues hemolysis activity, suggesting that micro1N cleavage is not required when steric barriers are removed. Synthetic myristoylated micro1N peptide forms size-selective pores in liposomes, as measured by fluorescence dequenching of labeled dextrans of different sizes. Addition of a C-terminal solubility tag to the peptide does not affect activity, but sequence substitution V13N or L36D reduces liposome disruption. These substitutions are in regions of alternating hydrophobic residues. Their locations, the presence of an N-terminal myristoyl group, and the full activity of a C-terminally extended peptide, along with circular dichroism data that indicate prevalence of beta-strand secondary structure, suggest a model in which micro1N beta-hairpins assemble in the membrane to form a beta-barrel pore.

Formation of the factory matrix is an important, though not a sufficient function of nonstructural protein mu NS during reovirus infection

Genome replication of mammalian orthoreovirus (MRV) occurs in cytoplasmic inclusion bodies called viral factories. Nonstructural protein microNS, encoded by genome segment M3, is a major constituent of these structures. When expressed without other viral proteins, microNS forms cytoplasmic inclusions morphologically similar to factories, suggesting a role for microNS as the factory framework or matrix. In addition, most other MRV proteins, including all five core proteins (lambda1, lambda2, lambda3, micro2, and sigma2) and nonstructural protein sigmaNS, can associate with microNS in these structures. In the current study, small interfering RNA targeting M3 was transfected in association with MRV infection and shown to cause a substantial reduction in microNS expression as well as, among other effects, a reduction in infectious yields by as much as 4 log(10) values. By also transfecting in vitro-transcribed M3 plus-strand RNA containing silent mutations that render it resistant to the small interfering RNA, we were able to complement microNS expression and to rescue infectious yields by ~100-fold. We next used microNS mutants specifically defective at forming factory-matrix structures to show that this function of microNS is important for MRV growth; point mutations in a C-proximal, putative zinc-hook motif as well as small deletions at the extreme C terminus of microNS prevented rescue of viral growth while causing microNS to be diffusely distributed in cells. We furthermore confirmed that an N-terminally truncated form of microNS, designed to represent microNSC and still able to form factory-matrix structures, is unable to rescue MRV growth, localizing one or more other important functions to an N-terminal region of microNS known to be involved in both micro2 and sigmaNS association. Thus, factory-matrix formation is an important, though not a sufficient function of microNS during MRV infection; microNS is multifunctional in the course of viral growth.

Conformational changes accompany activation of reovirus RNA-dependent RNA transcription

Many critical biologic processes involve dynamic interactions between proteins and nucleic acids. Such dynamic processes are often difficult to delineate by conventional static methods. For example, while a variety of nucleic acid polymerase structures have been determined at atomic resolution, the details of how some multi-protein transcriptase complexes actively produce mRNA, as well as conformational changes associated with activation of such complexes, remain poorly understood. The mammalian reovirus innermost capsid (core) manifests all enzymatic activities necessary to produce mRNA from each of the 10 encased double-stranded RNA genes. We used rapid freezing and electron cryo-microscopy to trap and visualize transcriptionally active reovirus core particles and compared them to inactive core images. Rod-like density centered within actively transcribing core spike channels was attributed to exiting nascent mRNA. Comparative radial density plots of active and inactive core particles identified several structural changes in both internal and external regions of the icosahedral core capsid. Inactive and transcriptionally active cores were partially digested with trypsin and identities of initial tryptic peptides determined by mass spectrometry. Differentially-digested peptides, which also suggest transcription-associated conformational changes, were placed within the known three-dimensional structures of major core proteins.

Silencing and complementation of reovirus core protein mu2: functional correlations with mu2-microtubule association and differences between virus- and plasmid-derived mu2

A low-copy component of mammalian reovirus particles is mu2, an 83-kDa protein encoded by the M1 viral genome segment and packaged within the viral core. Previous studies have identified mu2 as a nucleoside triphosphate phosphohydrolase (NTPase) as well as an RNA 5'-triphosphate phosphohydrolase (RTPase), putatively involved in reovirus RNA synthesis and/or 5'-capping. Other studies have identified mu2 as a microtubule-binding protein, which also associates with the viral factory matrix protein muNS and thereby anchors the factories to cellular microtubules during infections by most reovirus strains. To extend studies of mu2 functions during infection, we tested a small interfering RNA (siRNA) directed against the M1 plus-strand RNAs of reovirus strains Type 1 Lang (T1L) and Type 3 Dearing (T3D). The siRNA strongly suppressed mu2 expression by either strain and reduced infectious yields in a strain-dependent manner. This first strain difference was genetically mapped to the M1 genome segment and tentatively assigned to a single mu2 sequence polymorphism, Pro/Ser208, which also determines a T1L-T3D strain difference in microtubule association. The siRNA-based defect in mu2 expression was rescued by plasmids, containing silent mutations in the siRNA-targeted sequence, which encoded either T1L or T3D mu2, but the growth defect was rescued only by T1L mu2. This second strain difference was also mapped to Pro/Ser208, in that swapping this one residue between T1L and T3D mu2 reversed the rescue phenotypes. Thus, the T1L-T3D strain difference in mu2-microtubule association was correlated not only with the extent of reduction in infectious yields by the siRNA but also with the extent of rescue by plasmid-derived mu2. In addition, the rescue capacity of T1L mu2 was abrogated by nocodazole treatment, providing independent evidence for the importance of mu2-microtubule association in plasmid-based rescue. In two separate cases, the results revealed functional differences between virus- and plasmid-derived mu2. Ala substitutions within the NTP-binding motif of T1L mu2 also abrogated its rescue capacity, suggesting that the NTPase or RTPase activity of mu2 is additionally required for effective viral growth.

Mammalian reovirus, a nonfusogenic nonenveloped virus, forms size-selective pores in a model membrane

During cell entry, reovirus particles with a diameter of 70-80 nm must penetrate the cellular membrane to access the cytoplasm. The mechanism of penetration, without benefit of membrane fusion, is not well characterized for any such nonenveloped animal virus. Lysis of RBCs is an in vitro assay for the membrane perforation activity of reovirus; however, the mechanism of lysis has been unknown. In this report, osmotic-protection experiments using PEGs of different sizes revealed that reovirus-induced lysis of RBCs occurs osmotically, after formation of small size-selective lesions or "pores." Consistent results were obtained by monitoring leakage of fluorophore-tagged dextrans from the interior of resealed RBC ghosts. Gradient fractionations showed that whole virus particles, as well as the myristoylated fragment mu1N that is released from particles, are recruited to RBC membranes in association with pore formation. We propose that formation of small pores is a discrete, intermediate step in the reovirus membrane-penetration pathway, which may be shared by other nonenveloped animal viruses.

Putative autocleavage of reovirus mu1 protein in concert with outer-capsid disassembly and activation for membrane permeabilization

Capsid proteins of several different families of non-enveloped animal viruses with single-stranded RNA genomes undergo autocatalytic cleavage (autocleavage) as a maturation step in assembly. Similarly, the 76 kDa major outer-capsid protein mu1 of mammalian orthoreoviruses (reoviruses), which are non-enveloped and have double-stranded RNA genomes, undergoes putative autocleavage between residues 42 and 43, yielding N-terminal N-myristoylated fragment mu1N and C-terminal fragment mu1C. Cleavage at this site allows release of mu1N, which is thought to be critical for penetration of the host-cell membrane during cell entry. Most previous studies have suggested that cleavage at the mu1N/mu1C junction precedes addition to the outer capsid during virion assembly, such that only a small number of the mu1 subunits in mature virions remain uncleaved at that site (approximately 5%). In this study, we varied the conditions for disruption of virions before running the proteins on denaturing gels and in several circumstances recovered much higher levels of uncleaved mu1 (up to approximately 60%). Elements of the disruption conditions that allowed greater recovery of uncleaved protein were increased pH, absence of reducing agent, and decreased temperature. These same elements allowed comparably higher levels of the mu1delta protein, in which cleavage at the mu1N/delta junction has not occurred, to be recovered from particle uncoating intermediates in which mu1 had been previously cleaved by chymotrypsin in a distinct protease-sensitive region near residue 580. The capacity to recover higher levels of mu1delta following disruption of these particles for electrophoresis was lost, however, in concert with a series of structural changes that activate the particles for membrane permeabilization, suggesting that the putative autocleavage is itself one of these changes.

Increased ubiquitination and other covariant phenotypes attributed to a strain- and temperature-dependent defect of reovirus core protein mu2

Reovirus replication and assembly are thought to occur within cytoplasmic inclusion bodies, which we call viral factories. A strain-dependent difference in the morphology of these structures reflects more effective microtubule association by the mu2 core proteins of some viral strains, which form filamentous factories, than by those of others, which form globular factories. For this report, we identified and characterized another strain-dependent attribute of the factories, namely, the extent to which they colocalized with conjugated ubiquitin (cUb). Among 16 laboratory strains and field isolates, the extent of factory costaining for cUb paralleled factory morphology, with globular strains exhibiting higher levels by far. In reassortant viruses, factory costaining for cUb mapped primarily to the mu2-encoding M1 genome segment, although contributions by the lambda3- and lambda2-encoding L1 and L2 genome segments were also evident. Immunoprecipitations revealed that cells infected with globular strains contained higher levels of ubiquitinated mu2 (Ub-mu2). In M1-transfected cells, cUb commonly colocalized with aggregates formed by mu2 from globular strains but not with microtubules coated by mu2 from filamentous strains, and immunoprecipitations revealed that mu2 from globular strains displayed higher levels of Ub-mu2. Allelic changes at mu2 residue 208 determined these differences. Nocodazole treatment of cells infected with filamentous strains resulted in globular factories that still showed low levels of costaining for cUb, indicating that higher levels of costaining were not a direct result of decreased microtubule association. The factories of globular strains, or their mu2 proteins expressed in transfected cells, were furthermore shown to gain microtubule association and to lose colocalization with cUb when cells were grown at reduced temperature. From the sum of these findings, we propose that mu2 from globular strains is more prone to temperature-dependent misfolding and as a result displays increased aggregation, increased levels of Ub-mu2, and decreased association with microtubules. Because so few of the viral strains formed factories that were regularly associated with ubiquitinated proteins, we conclude that reovirus factories are generally distinct from cellular aggresomes.

Comparisons of the M1 genome segments and encoded mu2 proteins of different reovirus isolates

Background

The reovirus M1 genome segment encodes the μ2 protein, a structurally minor component of the viral core, which has been identified as a transcriptase cofactor, nucleoside and RNA triphosphatase, and microtubule-binding protein. The μ2 protein is the most poorly understood of the reovirus structural proteins. Genome segment sequences have been reported for 9 of the 10 genome segments for the 3 prototypic reoviruses type 1 Lang (T1L), type 2 Jones (T2J), and type 3 Dearing (T3D), but the M1 genome segment sequences for only T1L and T3D have been previously reported. For this study, we determined the M1 nucleotide and deduced μ2 amino acid sequences for T2J, nine other reovirus field isolates, and various T3D plaque-isolated clones from different laboratories.

Results

Determination of the T2J M1 sequence completes the analysis of all ten genome segments of that prototype. The T2J M1 sequence contained a 1 base pair deletion in the 3' non-translated region, compared to the T1L and T3D M1 sequences. The T2J M1 gene showed ~80% nucleotide homology, and the encoded μ2 protein showed ~71% amino acid identity, with the T1L and T3D M1 and μ2 sequences, respectively, making the T2J M1 gene and μ2 proteins amongst the most divergent of all reovirus genes and proteins. Comparisons of these newly determined M1 and μ2 sequences with newly determined M1 and μ2 sequences from nine additional field isolates and a variety of laboratory T3D clones identified conserved features and/or regions that provide clues about μ2 structure and function.

Conclusions

The findings suggest a model for the domain organization of μ2 and provide further evidence for a role of μ2 in viral RNA synthesis. The new sequences were also used to explore the basis for M1/μ2-determined differences in the morphology of viral factories in infected cells. The findings confirm the key role of Ser/Pro208 as a prevalent determinant of differences in factory morphology among reovirus isolates and trace the divergence of this residue and its associated phenotype among the different laboratory-specific clones of type 3 Dearing.

Putative autocleavage of outer capsid protein micro1, allowing release of myristoylated peptide micro1N during particle uncoating, is critical for cell entry by reovirus

Several nonenveloped animal viruses possess an autolytic capsid protein that is cleaved as a maturation step during assembly to yield infectious virions. The 76-kDa major outer capsid protein micro1 of mammalian orthoreoviruses (reoviruses) is also thought to be autocatalytically cleaved, yielding the virion-associated fragments micro1N (4 kDa; myristoylated) and micro1C (72 kDa). In this study, we found that micro1 cleavage to yield micro1N and micro1C was not required for outer capsid assembly but contributed greatly to the infectivity of the assembled particles. Recoated particles containing mutant, cleavage-defective micro1 (asparagine --> alanine substitution at amino acid 42) were competent for attachment; processing by exogenous proteases; structural changes in the outer capsid, including micro1 conformational change and sigma1 release; and transcriptase activation but failed to mediate membrane permeabilization either in vitro (no hemolysis) or in vivo (no coentry of the ribonucleotoxin alpha-sarcin). In addition, after these particles were allowed to enter cells, the delta region of micro1 continued to colocalize with viral core proteins in punctate structures, indicating that both elements remained bound together in particles and/or trapped within the same subcellular compartments, consistent with a defect in membrane penetration. If membrane penetration activity was supplied in trans by a coinfecting genome-deficient particle, the recoated particles with cleavage-defective micro1 displayed much higher levels of infectivity. These findings led us to propose a new uncoating intermediate, at which particles are trapped in the absence of micro1N/micro1C cleavage. We additionally showed that this cleavage allowed the myristoylated, N-terminal micro1N fragment to be released from reovirus particles during entry-related uncoating, analogous to the myristoylated, N-terminal VP4 fragment of picornavirus capsid proteins. The results thus suggest that hydrophobic peptide release following capsid protein autocleavage is part of a general mechanism of membrane penetration shared by several diverse nonenveloped animal viruses.

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.

The delta region of outer-capsid protein micro 1 undergoes conformational change and release from reovirus particles during cell entry

Cell entry by reoviruses requires a large, transcriptionally active subvirion particle to gain access to the cytoplasm. The features of this particle have been the subject of debate, but three primary candidates-the infectious subvirion particle (ISVP), ISVP*, and core particle forms-that differ in whether putative membrane penetration protein micro 1 and adhesin sigma1 remain particle bound have been identified. Experiments with antibody reagents in this study yielded new information about the steps in particle disassembly during cell entry. Monoclonal antibodies specific for the delta region of micro 1 provided evidence for a conformational change in micro 1 and for release of the delta proteolytic fragment from entering particles. Antiserum raised against cores provided evidence for entry-related changes in particle structure and identified entering particles that largely lack the delta fragment inside cells. Antibodies specific for sigma1 showed that it is also largely shed from entering particles. Limited coimmunostaining with markers for late endosomes and lysosomes indicated the particles lacking delta and sigma1 did not localize to those subcellular compartments, and other observations suggested that both the particles and free delta were released into the cytoplasm. Essentially equivalent findings were obtained with native ISVPs and highly infectious recoated particles containing wild-type proteins. Poorly infectious recoated particles containing a hyperstable mutant form of micro 1, however, showed no evidence for the in vitro and intracellular changes in particle structure normally detected by antibodies, and these particles instead accumulated in late endosomes or lysosomes. Recoated particles with hyperstable micro 1 were also ineffective at mediating erythrocyte lysis in vitro and promoting alpha-sarcin coentry and intoxication of cells in cultures. Based on these and other findings, we propose that ISVP* is a transient intermediate in cell entry which mediates membrane penetration and is then further uncoated in the cytoplasm to yield particles, resembling cores, that largely lack the delta fragment of micro 1.

Nucleoside and RNA triphosphatase activities of orthoreovirus transcriptase cofactor mu2

The mammalian Orthoreovirus (mORV) core particle is an icosahedral multienzyme complex for viral mRNA synthesis and provides a delimited system for mechanistic studies of that process. Previous genetic results have identified the mORV mu2 protein as a determinant of viral strain differences in the transcriptase and nucleoside triphosphatase activities of cores. New results in this report provided biochemical and genetic evidence that purified mu2 is itself a divalent cation-dependent nucleoside triphosphatase that can remove the 5' gamma-phosphate from RNA as well. Alanine substitutions in a putative nucleotide binding region of mu2 abrogated both functions but did not affect the purification profile of the protein or its known associations with microtubules and mORV microNS protein in vivo. In vitro microtubule binding by purified mu2 was also demonstrated and not affected by the mutations. Purified mu2 was further demonstrated to interact in vitro with the mORV RNA-dependent RNA polymerase, lambda3, and the presence of lambda3 mildly stimulated the triphosphatase activities of mu2. These findings confirm that mu2 is an enzymatic component of the mORV core and may contribute several possible functions to viral mRNA synthesis.

Reovirus polymerase lambda 3 localized by cryo-electron microscopy of virions at a resolution of 7.6 A

Reovirus is an icosahedral, double-stranded (ds) RNA virus that uses viral polymerases packaged within the viral core to transcribe its ten distinct plus-strand RNAs. To localize these polymerases, the structure of the reovirion was refined to a resolution of 7.6 A by cryo-electron microscopy (cryo-EM) and three-dimensional (3D) image reconstruction. X-ray crystal models of reovirus proteins, including polymerase lambda 3, were then fitted into the density map. Each copy of lambda 3 was found anchored to the inner surface of the icosahedral core shell, making major contacts with three molecules of shell protein lambda 1 and overlapping, but not centering on, a five-fold axis. The overlap explains why only one copy of lambda 3 is bound per vertex. lambda 3 is furthermore oriented with its transcript exit channel facing a small channel through the lambda 1 shell, suggesting how the nascent RNA is passed into the large external cavity of the pentameric capping enzyme complex formed by protein lambda 2.

Disulfide bonding among micro 1 trimers in mammalian reovirus outer capsid: a late and reversible step in virion morphogenesis

We examined how a particular type of intermolecular disulfide (ds) bond is formed in the capsid of a cytoplasmically replicating nonenveloped animal virus despite the normally reducing environment inside cells. The micro 1 protein, a major component of the mammalian reovirus outer capsid, has been implicated in penetration of the cellular membrane barrier during cell entry. A recent crystal structure determination supports past evidence that the basal oligomer of micro 1 is a trimer and that 200 of these trimers surround the core in the fenestrated T=13 outer capsid of virions. We found in this study that the predominant forms of micro 1 seen in gels after the nonreducing disruption of virions are ds-linked dimers. Cys679, near the carboxyl terminus of micro 1, was shown to form this ds bond with the Cys679 residue from another micro 1 subunit. The crystal structure in combination with a cryomicroscopy-derived electron density map of virions indicates that the two subunits that contribute a Cys679 residue to each ds bond must be from adjacent micro 1 trimers in the outer capsid, explaining the trimer-dimer paradox. Successful in vitro assembly of the outer capsid by a nonbonding mutant of micro 1 (Cys679 substituted by serine) confirmed the role of Cys679 and suggested that the ds bonds are not required for assembly. A correlation between micro 1-associated ds bond formation and cell death in experiments in which virions were purified from cells at different times postinfection indicated that the ds bonds form late in infection, after virions are exposed to more oxidizing conditions than those in healthy cells. The infectivity measurements of the virions with differing levels of ds-bonded micro 1 showed that these bonds are not required for infection in culture. The ds bonds in purified virions were susceptible to reduction and reformation in situ, consistent with their initial formation late in morphogenesis and suggesting that they may undergo reduction during the entry of reovirus particles into new cells.

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.

The role of interferon regulatory factors in the cardiac response to viral infection

Reovirus-induced murine myocarditis provides an excellent model for the human disease. Cardiac tissue damage varies between reovirus strains, and is caused by a direct viral cytopathogenic effect. One determinant of virus-induced cardiac tissue damage is the cardiac interferon-beta (IFN-beta) response to viral infection. Nonmyocarditic reoviruses induce more IFN-beta and/or are more sensitive to the antiviral effects of IFN-beta in cardiac cells than myocarditis reoviruses. The roles of interferon regulatory factors (IRFs) in the cardiac response to viral infection are reviewed, and results suggest possible cardiac-specific variations in IRF-3 and IRF-1 function. In addition, data are presented indicating that the role of IRF-2 in regulation of IFN-beta expression is cell type-specific and differs between skeletal and cardiac muscle cells. Together, results suggest that the heart may provide a unique environment for IRF function, critical for protection against virus-induced cardiac damage.

Strategy for nonenveloped virus entry: a hydrophobic conformer of the reovirus membrane penetration protein micro 1 mediates membrane disruption

The mechanisms employed by nonenveloped animal viruses to penetrate the membranes of their host cells remain enigmatic. Membrane penetration by the nonenveloped mammalian reoviruses is believed to deliver a partially uncoated, but still large ( approximately 70-nm), particle with active transcriptases for viral mRNA synthesis directly into the cytoplasm. This process is likely initiated by a particle form that resembles infectious subvirion particles (ISVPs), disassembly intermediates produced from virions by proteolytic uncoating. Consistent with that idea, ISVPs, but not virions, can induce disruption of membranes in vitro. Both activities ascribed to ISVP-like particles, membrane disruption in vitro and membrane penetration within cells, are linked to N-myristoylated outer-capsid protein micro 1, present in 600 copies at the surfaces of ISVPs. To understand how micro 1 fulfills its role as the reovirus penetration protein, we monitored changes in ISVPs during the permeabilization of red blood cells induced by these particles. Hemolysis was preceded by a major structural transition in ISVPs, characterized by conformational change in micro 1 and elution of fibrous attachment protein sigma 1. The altered conformer of micro 1 was required for hemolysis and was markedly hydrophobic. The structural transition in ISVPs was further accompanied by derepression of genome-dependent mRNA synthesis by the particle-associated transcriptases. We propose a model for reovirus entry in which (i) primed and triggered conformational changes, analogous to those in enveloped-virus fusion proteins, generate a hydrophobic micro 1 conformer capable of inserting into and disrupting cell membranes and (ii) activation of the viral particles for membrane interaction and mRNA synthesis are concurrent events. Reoviruses provide an opportune system for defining the molecular details of membrane penetration by a large nonenveloped animal virus.

Reovirus core protein mu2 determines the filamentous morphology of viral inclusion bodies by interacting with and stabilizing microtubules

Cells infected with mammalian reoviruses often contain large perinuclear inclusion bodies, or "factories," where viral replication and assembly are thought to occur. Here, we report a viral strain difference in the morphology of these inclusions: filamentous inclusions formed in cells infected with reovirus type 1 Lang (T1L), whereas globular inclusions formed in cells infected with our laboratory's isolate of reovirus type 3 Dearing (T3D). Examination by immunofluorescence microscopy revealed the filamentous inclusions to be colinear with microtubules (MTs). The filamentous distribution was dependent on an intact MT network, as depolymerization of MTs early after infection caused globular inclusions to form. The inclusion phenotypes of T1L x T3D reassortant viruses identified the viral M1 genome segment as the primary genetic determinant of the strain difference in inclusion morphology. Filamentous inclusions were seen with 21 of 22 other reovirus strains, including an isolate of T3D obtained from another laboratory. When the mu2 proteins derived from T1L and the other laboratory's T3D isolate were expressed after transfection of their cloned M1 genes, they associated with filamentous structures that colocalized with MTs, whereas the mu2 protein derived from our laboratory's T3D isolate did not. MTs were stabilized in cells infected with the viruses that induced filamentous inclusions and after transfection with the M1 genes derived from those viruses. Evidence for MT stabilization included bundling and hyperacetylation of alpha-tubulin, changes characteristically seen when MT-associated proteins (MAPs) are overexpressed. Sequencing of the M1 segments from the different T1L and T3D isolates revealed that a single-amino-acid difference at position 208 correlated with the inclusion morphology. Two mutant forms of mu2 with the changes Pro-208 to Ser in a background of T1L mu2 and Ser-208 to Pro in a background of T3D mu2 had MT association phenotypes opposite to those of the respective wild-type proteins. We conclude that the mu2 protein of most reovirus strains is a viral MAP and that it plays a key role in the formation and structural organization of reovirus inclusion bodies.

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.

Detection of mammalian reovirus RNA by using reverse transcription-PCR: sequence diversity within the lambda3-encoding L1 gene

Reoviruses infect virtually all mammalian species, and infection of humans is associated with mild gastrointestinal or upper respiratory illnesses. To improve reovirus detection strategies, we developed a reverse transcription-PCR technique to amplify a fragment of the reovirus L1 gene segment. This assay was capable of detecting 44 of 44 reovirus field isolate strains and was sufficiently sensitive to detect nearly a single viral particle (1.16 +/- 0.13) per PCR of prototype strain type 3 Dearing. Pairwise comparisons of the 44 partial L1 gene sequences revealed that nucleotide variability ranged from 0 to 24.7%, with most of the nucleotide polymorphism occurring at synonymous positions. Phylogenetic trees generated from amplified L1 gene sequences suggest that multiple alleles of the L1 gene cocirculate in nature and that genetic diversity of the L1 gene is largely independent of the host species, geographic locale, or date of isolation. Phylogenetic trees constructed from the L1 gene sequences are distinct from those constructed from the four reovirus S-class gene segments, which supports the hypothesis that reovirus gene segments reassort in nature. This study establishes a new sensitive and specific technique for the identification of mammalian reoviruses and enhances our understanding of reovirus evolution.

Evidence of nucleotidyl phosphatase activity associated with core protein sigma A of avian reovirus S1133

Both avian reovirus core protein sigma A purified from virus-infected cell extracts and the purified bacterially expressed protein sigma A (e sigma A) were characterized for their nucleoside triphosphate (NTP) hydrolysis activity by thin-layer chromotography. Protein sigma A from both preparations has a nonspecific nucleotidyl phosphatase activity that hydrolyzes four types of NTP to their corresponding nucleoside di- and monophosphates and free phosphate. The divalent cation requirement for this activity of e sigma A was further examined by the addition of Mn(2+), Mg(2+), Ca(2+), and Zn(2+) ions. NTP hydrolysis by e sigma A was maximal when Mn(2+), Mg(2+), or Ca(2+) concentrations were 5, 4, or 1 mM, respectively. Addition of Mn(2+) or Mg(2+) stimulated the reactions up to 4- or 3-fold, respectively, higher than Ca(2+) (2.2-fold). However, Zn(2+) ion inhibited this activity of e sigma A. The results suggest that nucleotidyl phosphatase activity of e sigma A is absolutely dependent on the divalent cations Mn(2+), Mg(2+), or Ca(2+), but not Zn(2+). Similar results were obtained from the analysis of divalent cation requirements for the protein sigma A nucleotidyl phosphatase activity. Optimal pH for nucleotidyl phosphatase activity of protein sigma A from both preparations was determined using reaction mixtures buffered at different pH. The results show that the optimal activities of both proteins were similar and were achieved between pH 7.5 and 8.5.

Mammalian reovirus core protein micro 2 initiates at the first start codon and is acetylated

Mammalian reovirus is an enteric virus that contains a double-stranded RNA genome. The genome consists of ten RNA segments that encode eight structural and three non-structural proteins. The structural proteins form a double-layered structure. The innermost layer, called the core, consists of five proteins (lambda1, lambda2, lambda3, micro 2, and sigma2). Protein lambda3 is the RNA-dependent RNA polymerase (RdRp) and micro 2 is thought to be an RdRp cofactor. Translation of most reovirus proteins is known to commence at the first start codon. However, the translation initiation site of the viral core protein micro 2, encoded by the M1 RNA segment, has been in dispute. Although the theoretical molecular weight of micro 2 is 83 267 Da the actual molecular weight is unknown because micro 2 runs aberrantly in SDS-PAGE and has resisted characterization by Edman degradation, indicating that the amino terminus is post-translationally modified. In this study, we used proteolysis coupled with MALDI-Qq-TOFMS to determine that translation of micro 2 initiates at the first AUG codon, that its actual molecular weight approximates the theoretical value of 83 kDa, that the amino terminal methionine residue is removed, and that the next amino acid (alanine) is post-translationally acetylated.

Reovirus binding to cell surface sialic acid potentiates virus-induced apoptosis

Reovirus induces apoptosis in cultured cells and in vivo. Genetic studies indicate that the efficiency with which reovirus strains induce apoptosis is determined by the viral S1 gene, which encodes attachment protein sigma1. However, the biochemical properties of sigma1 that influence apoptosis induction are unknown. To determine whether the capacity of sigma1 to bind cell surface sialic acid determines the magnitude of the apoptotic response, we used isogenic reovirus mutants that differ in the capacity to engage sialic acid. We found that T3SA+, a virus capable of binding sialic acid, induces high levels of apoptosis in both HeLa cells and L cells. In contrast, non-sialic-acid-binding strain T3SA- induces little or no apoptosis in these cell types. Differences in the capacity of T3SA- and T3SA+ to induce apoptosis are not due to differences in viral protein synthesis or production of viral progeny. Removal of cell surface sialic acid with neuraminidase abolishes the capacity of T3SA+ to induce apoptosis. Similarly, incubation of T3SA+ with sialyllactose, a trisaccharide comprised of lactose and sialic acid, blocks apoptosis. These findings demonstrate that reovirus binding to cell surface sialic acid is a critical requirement for the efficient induction of apoptosis and suggest that virus receptor utilization plays an important role in regulating cell death.

Transcriptional activities of reovirus RNA polymerase in recoated cores. Initiation and elongation are regulated by separate mechanisms

The particle-associated reovirus polymerase synthesizes mRNA within only certain viral particle types. Reovirus cores, subviral particles lacking outer capsid proteins mu1, sigma3, and sigma1, produce mRNA and abortive transcripts. Reovirus virions, which contain complete outer capsids, cannot produce mRNA and produce few abortive transcripts. Recoated cores are virion-like particles generated by the addition of recombinant outer capsid proteins to cores. We used recoated cores to analyze transcriptional regulation by reovirus outer capsid proteins. Partially recoated particles, containing less than virion amounts of mu1 and sigma3, synthesized mRNA at levels inversely proportional to outer capsid protein levels. Fully recoated cores exhibited undetectable mRNA synthesis levels, as did virions. However, recoated cores produced high levels of abortive transcripts. Recoated core abortive transcripts remained particle-associated and appeared to inhibit further abortive transcript production. Proteolysis of recoated cores removing mu1 and sigma3 released accumulated abortive transcripts and relieved inhibition of mRNA and abortive transcript synthesis. These results suggest transcriptional elongation, but not initiation, is blocked by virion-like amounts of mu1 and sigma3. Particle-associated abortive transcripts may down-regulate transcriptional initiation. Minor outer capsid protein sigma1 had no demonstrable effect on transcriptional activities. Transcriptional regulation may ensure progeny virions do not compete with transcribing particles for ribonucleoside triphosphates.

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.

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.

The reovirus protein mu2, encoded by the M1 gene, is an RNA-binding protein

The reovirus M1, L1, and L2 genes encode proteins found at each vertex of the viral core and are likely to form a structural unit involved in RNA synthesis. Genetic analyses have implicated the M1 gene in viral RNA synthesis and core nucleoside triphosphatase activity, but there have been no direct biochemical studies of mu2 function. Here, we expressed mu2 in vitro and assessed its RNA-binding activity. The expressed mu2 binds both poly(I-C)- and poly(U)-Sepharose, and binding activity is greater in Mn2+ than in Mg2+. Heterologous RNA competes for mu2 binding to reovirus RNA transcripts as effectively as homologous reovirus RNA does, providing no evidence for sequence-specific RNA binding by mu2. Protein mu2 is now the sixth reovirus protein demonstrated to have RNA-binding activity.

Internal/structures containing transcriptase-related proteins in top component particles of mammalian orthoreovirus

The structure of mammalian orthoreovirus top component particles, which are profoundly deficient in the content of double-stranded RNA genome, was determined at 30 A resolution by transmission cryoelectron microscopy and three-dimensional image reconstruction. Previously undetected, ordered densities, appearing primarily as pentameric flowers in the reconstruction, were seen to extend 65 A inwardly from the inner capsid at the icosahedral fivefold axes. Identically positioned but lower density elements were observed in two types of partially uncoated top component particles obtained by limited proteolysis. The levels of three inner-capsid proteins-lamda 1, lamda 3, and mu 2-were reduced in concert with the internal densities during proteolytic uncoating. Since lamda 3 contains the catalytic regions of the viral RNA polymerase and since both lamda 1 and mu 2 appear to play roles in transcription or mRNA capping, the internal structures are concluded to be complexes of the viral transcriptase-related enzymes. The findings have implications for the mechanisms of transcription and mRNA capping by orthoreovirus particles.

Stoichiometry of reovirus structural proteins in virus, ISVP, and core particles

All eight reovirus structural proteins were resolved in a new tris, glycine, and urea (TGU) electrophoretic gel system. The specific identities of proteins were determined immunologically, biochemically, and genetically. Structural proteins of reovirus type 1 Lang had different mobilities in the TGU gel than did type 3 Dearing proteins. Intertypic reassortant viruses that contained various combinations of parental genes were used to identify each of the viral protein bands. Type 1 Lang virions were metabolically-labelled with either 3H-amino acids or 35S-methionine/cysteine and gradient purified. Aliquots of purified virions were treated to generate infectious subviral particles (ISVPs) and core particles. Radiolabelled virus, ISVP, and core proteins were resolved in the TGU gel and protein band intensities were used to determine copy numbers of each structural protein. These studies confirmed the copy numbers and locations of most reovirus proteins. However, important new findings include the discovery that virions contain approximately 120 copies of major core protein sigma 2 and 20 copies of the polymerase cofactor protein mu 2, and ISVP particles contain about 24 copies of mu 1 C that has not been processed to the delta peptide. These data are used to generate a new model of the arrangement of structural proteins with the reovirus particle.

Reovirus induction of and sensitivity to beta interferon in cardiac myocyte cultures correlate with induction of myocarditis and are determined by viral core proteins

Reovirus-induced acute myocarditis in mice serves as a model to investigate non-immune-mediated mechanisms of viral myocarditis. We have used primary cardiac myocyte cultures infected with a large panel of myocarditic and nonmyocarditic reassortant reoviruses to identify determinants of viral myocarditic potential. Here, we report that while both myocarditic and nonmyocarditic reoviruses kill cardiac myocytes, viral myocarditic potential correlates with viral spread through cardiac myocyte cultures and with cumulative cell death. To address the role of secreted interferon (IFN), we added anti-IFN-alpha/beta antibody to infected cardiac myocyte cultures. Antibody benefited nonmyocarditic more than myocarditic virus spread (P < 0.001), and this benefit was associated with the reovirus M1 and L2 genes. There was no benefit for a differentiated skeletal muscle cell line culture (C2C12 cells), suggesting cell type specificity. IFN-beta induction in reovirus-infected cardiac myocyte cultures correlated with viral myocarditic potential (P = 0.006) and was associated with the reovirus M1, S2, and L2 genes. Sensitivity to the antiviral effects of IFN-alpha/beta added to cardiac myocyte cultures also correlated with viral myocarditic potential (P = 0.004) and was associated with the same reovirus genes. Several reoviruses induced IFN-beta levels discordant with their myocarditic phenotypes, and for those tested, sensitivity to IFN-alpha/beta compensated for the anomalous induction levels. Thus, the combination of induction of and sensitivity to IFN-alpha/beta is a determinant of reovirus myocarditic potential. Finally, a nonmyocarditic reovirus induced cardiac lesions in mice depleted of IFN-alpha/beta, demonstrating that IFN-alpha/beta is a determinant of reovirus-induced myocarditis. This provides the first identification of reovirus genes associated with IFN induction and sensitivity and provides the first evidence that IFN-beta can be a determinant of viral myocarditis and reovirus disease.