NgR1 binding to reovirus reveals an unusual bivalent interaction and a new viral attachment protein

Nogo-66 receptor 1 (NgR1) binds a variety of structurally dissimilar ligands in the adult central nervous system to inhibit axon extension. Disruption of ligand binding to NgR1 and subsequent signaling can improve neuron outgrowth, making NgR1 an important therapeutic target for diverse neurological conditions such as spinal crush injuries and Alzheimer's disease. Human NgR1 serves as a receptor for mammalian orthoreovirus (reovirus), but the mechanism of virus-receptor engagement is unknown. To elucidate how NgR1 mediates cell binding and entry of reovirus, we defined the affinity of interaction between virus and receptor, determined the structure of the virus-receptor complex, and identified residues in the receptor required for virus binding and infection. These studies revealed that central NgR1 surfaces form a bridge between two copies of viral capsid protein σ3, establishing that σ3 serves as a receptor ligand for reovirus. This unusual binding interface produces high-avidity interactions between virus and receptor to prime early entry steps. These studies refine models of reovirus cell-attachment and highlight the evolution of viruses to engage multiple receptors using distinct capsid components.

Deciphering molecular mechanisms stabilizing the reovirus-binding complex

Mammalian orthoreoviruses (reoviruses) serve as potential triggers of celiac disease and have oncolytic properties, making these viruses potential cancer therapeutics. Primary attachment of reovirus to host cells is mainly mediated by the trimeric viral protein, σ1, which engages cell-surface glycans, followed by high-affinity binding to junctional adhesion molecule-A (JAM-A). This multistep process is thought to be accompanied by major conformational changes in σ1, but direct evidence is lacking. By combining biophysical, molecular, and simulation approaches, we define how viral capsid protein mechanics influence virus-binding capacity and infectivity. Single-virus force spectroscopy experiments corroborated by in silico simulations show that GM2 increases the affinity of σ1 for JAM-A by providing a more stable contact interface. We demonstrate that conformational changes in σ1 that lead to an extended rigid conformation also significantly increase avidity for JAM-A. Although its associated lower flexibility impairs multivalent cell attachment, our findings suggest that diminished σ1 flexibility enhances infectivity, indicating that fine-tuning of σ1 conformational changes is required to successfully initiate infection. Understanding properties underlying the nanomechanics of viral attachment proteins offers perspectives in the development of antiviral drugs and improved oncolytic vectors.

Z-FA-FMK demonstrates differential inhibition of aquatic orthoreovirus (PRV), aquareovirus (CSRV), and rhabdovirus (IHNV) replication

Benzyloxycarbonyl-phenylalanyl-alanyl-fluoromethyl ketone (Z-FA-FMK) is a protease inhibitor that has been shown to strongly inhibit mammalian orthoreovirus replication. Here we explore the ability of Z-FA-FMK to inhibit three important yet genetically discrete aquatic fish viruses: chum salmon aquareovirus (CSRV), piscine orthoreovirus (PRV), and the rhabdovirus infectious hematopoietic necrosis virus (IHNV). Z-FA-FMK significantly attenuated CSRV in vitro transcription and infectious yield following low-dose (2-20μM) exposure, yet a relatively high dose (200μM) was required to completely block CSRV replication. For PRV and IHNV, no significant attenuation of in vitro viral transcription was observed following low-dose (2-20μM) exposure; and although high dose (200μM) exposure significantly attenuated both PRV and IHNV transcription, neither was completely inhibited. These transcriptional results were similarly reflected in IHNV infectious titre observed at 7days post exposure. PRV titre is currently undeterminable in vitro; however, in vivo intra-peritoneal injection of PRV into juvenile Atlantic salmon (Salmo salar) in conjunction with 1.5mg/kg Z-FA-FMK did not affect PRV replication as measured by blood associated viral transcripts at 14days post challenge. These results indicate that aquatic ortho- and aqua-reoviruses appear to possess resilience to Z-FA-FMK relative to mammalian orthoreoviruses and suggest that environmental parameters or alternative mechanisms for viral replication may affect the efficacy of Z-FA-FMK as an antireoviral compound. Further, as Z-FA-FMK has been shown to irreversibly inhibit cysteine proteases such as cathepsins B and L in vitro at concentrations of ≤100μM, continued replication of IHNV (and possibly PRV) at 200μM Z-FA-FMK suggests that replication of these viruses can occur in a cathepsin-independent manner whereas CSRV likely requires cathepsins or similar cysteine proteases for successful replication.

Aging, mortality, and the fast growth trade-off of Schizosaccharomyces pombe

Replicative aging has been demonstrated in asymmetrically dividing unicellular organisms, seemingly caused by unequal damage partitioning. Although asymmetric segregation and inheritance of potential aging factors also occur in symmetrically dividing species, it nevertheless remains controversial whether this results in aging. Based on large-scale single-cell lineage data obtained by time-lapse microscopy with a microfluidic device, in this report, we demonstrate the absence of replicative aging in old-pole cell lineages of Schizosaccharomyces pombe cultured under constant favorable conditions. By monitoring more than 1,500 cell lineages in 7 different culture conditions, we showed that both cell division and death rates are remarkably constant for at least 50–80 generations. Our measurements revealed that the death rate per cellular generation increases with the division rate, pointing to a physiological trade-off with fast growth under balanced growth conditions. We also observed the formation and inheritance of Hsp104-associated protein aggregates, which are a potential aging factor in old-pole cell lineages, and found that these aggregates exhibited a tendency to preferentially remain at the old poles for several generations. However, the aggregates were eventually segregated from old-pole cells upon cell division and probabilistically allocated to new-pole cells. We found that cell deaths were typically preceded by sudden acceleration of protein aggregation; thus, a relatively large amount of protein aggregates existed at the very ends of the dead cell lineages. Our lineage tracking analyses, however, revealed that the quantity and inheritance of protein aggregates increased neither cellular generation time nor cell death initiation rates. Furthermore, our results demonstrated that unusually large amounts of protein aggregates induced by oxidative stress exposure did not result in aging; old-pole cells resumed normal growth upon stress removal, despite the fact that most of them inherited significant quantities of aggregates. These results collectively indicate that protein aggregates are not a major determinant of triggering cell death in S. pombe and thus cannot be an appropriate molecular marker or index for replicative aging under both favorable and stressful environmental conditions.

Multicellular organisms universally senesce and must produce rejuvenated progenies in order to transmit life. Although similar age-related deterioration in physiological functions and reproduction is also found in unicellular organisms that divide asymmetrically to produce morphologically distinct aged and younger cells, it has been unclear whether symmetrically dividing microbes—such as fission yeast—exhibit the same traits. Using long-term live-cell microscopy combined with a microfluidic device, we monitor the growth and death of a large number of fission yeast cells and demonstrate the existence of aging-free lineages. These lineages are, however, not immortal, and the probability of death increases as the cells grow more rapidly; thus, the “live fast, die fast” trade-off exists in fission yeast. We further characterize the segregation and inheritance of protein aggregates, which are commonly thought of as “aging factors.” The aging-free lineages bear the aggregate load for some generations with no apparent adverse effects on growth. We also show that there is no threshold amount of protein aggregate above which cells are destined to death in both normal and stressed conditions: protein aggregate is thus not a direct initiation signal for cell death. Our data reveal that protein aggregation might not be an appropriate index for aging and that we should revisit its role in cell physiology.

Efficient reovirus- and measles virus-mediated pore expansion during syncytium formation is dependent on annexin A1 and intracellular calcium

Orthoreovirus fusion-associated small transmembrane (FAST) proteins are dedicated cell-cell fusogens responsible for multinucleated syncytium formation and are virulence determinants of the fusogenic reoviruses. While numerous studies on the FAST proteins and enveloped-virus fusogens have delineated steps involved in membrane fusion and pore formation, little is known about the mechanics of pore expansion needed for syncytiogenesis. We now report that RNA interference (RNAi) knockdown of annexin A1 (AX1) expression dramatically reduced both reptilian reovirus p14 and measles virus F and H protein-mediated pore expansion during syncytiogenesis but had no effect on pore formation. A similar effect was obtained by chelating intracellular calcium, which dramatically decreased syncytiogenesis in the absence of detectable effects on p14-induced pore formation. Coimmunoprecipitation revealed calcium-dependent interaction between AX1 and p14 or measles virus F and H proteins, and fluorescence resonance energy transfer (FRET) demonstrated calcium-dependent p14-AX1 interactions in cellulo. Furthermore, antibody inhibition of extracellular AX1 had no effect on p14-induced syncytium formation but did impair cell-cell fusion mediated by the endogenous muscle cell fusion machinery in C2C12 mouse myoblasts. AX1 can therefore exert diverse, fusogen-specific effects on cell-cell fusion, functioning as an extracellular mediator of differentiation-dependent membrane fusion or as an intracellular promoter of postfusion pore expansion and syncytium formation following virus-mediated cell-cell fusion.

IMPORTANCE

Numerous enveloped viruses and nonenveloped fusogenic orthoreoviruses encode membrane fusion proteins that induce syncytium formation, which has been linked to viral pathogenicity. Considerable insights into the mechanisms of membrane fusion have been obtained, but processes that drive postfusion expansion of fusion pores to generate syncytia are poorly understood. This study identifies intracellular calcium and annexin A1 (AX1) as key factors required for efficient pore expansion during syncytium formation mediated by the reptilian reovirus p14 and measles virus F and H fusion protein complexes. Involvement of intracellular AX1 in syncytiogenesis directly correlates with a requirement for intracellular calcium in p14-AX1 interactions and pore expansion but not membrane fusion and pore formation. This is the first demonstration that intracellular AX1 is involved in pore expansion, which suggests that the AX1 pathway may be a common host cell response needed to resolve virus-induced cell-cell fusion pores.

Reovirus binding determinants in junctional adhesion molecule-A

Junctional adhesion molecule-A (JAM-A) serves as a serotype-independent receptor for mammalian orthoreoviruses (reoviruses). The membrane-distal immunoglobulin-like D1 domain of JAM-A is required for homodimerization and binding to reovirus attachment protein sigma1. We employed a structure-guided mutational analysis of the JAM-A dimer interface to identify determinants of reovirus binding. We purified mutant JAM-A ectodomains for solution-phase and surface plasmon resonance binding studies and expressed mutant forms of full-length JAM-A in Chinese hamster ovary cells to assess reovirus binding and infectivity. Mutation of residues in the JAM-A dimer interface that participate in salt-bridge or hydrogen-bond interactions with apposing JAM-A monomers abolishes the capacity of JAM-A to form dimers. JAM-A mutants incapable of dimer formation form complexes with the sigma1 head that are indistinguishable from wild-type JAM-A-sigma1 head complexes, indicating that sigma1 binds to JAM-A monomers. Residues Glu(61) and Lys(63) of beta-strand C and Leu(72) of beta-strand C' in the dimer interface are required for efficient JAM-A engagement of strain type 3 Dearing sigma1. Mutation of neighboring residues alters the kinetics of the sigma1-JAM-A binding interaction. Prototype reovirus strains type 1 Lang and type 2 Jones share similar, although not identical, binding requirements with type 3 Dearing. These results indicate that reovirus engages JAM-A monomers via residues found mainly on beta-strands C and C' of the dimer interface and raise the possibility that the distinct disease phenotypes produced in mice following infection with different strains of reovirus are in part attributable to differences in contacts with JAM-A.

Atypical fusion peptide of Nelson Bay virus fusion-associated small transmembrane protein

A 10-kDa nonstructural transmembrane protein (p10) encoded by a reovirus, Nelson Bay virus, has been shown to induce syncytium formation (34). Sequence analysis and structural studies identified p10 as a type I membrane protein with a central transmembrane domain, a cytoplasmic basic region, and an N-terminal hydrophobic domain (HD) that was hypothesized to function as a fusion peptide. We performed mutational analysis on this slightly hydrophobic motif to identify possible structural requirements for fusion activity. Bulky aliphatic residues were found to be essential for optimal fusion, and an aromatic or highly hydrophobic side chain was found to be required at position 12. The requirement for hydrophilic residues within the HD was also examined: substitution of 10-Ser or 14-Ser with hydrophobic residues was found to reduce cell surface expression of p10 and delayed the onset of syncytium formation. Nonconservative substitutions of charged residues in the HD did not have an effect on fusion activity. Taken together, our results suggest that the HD is involved in both syncytium formation and in determining p10 transport and surface expression.

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.

Orthoreovirus and Aquareovirus core proteins: conserved enzymatic surfaces, but not protein-protein interfaces

Orthoreoviruses and Aquareoviruses constitute two respective genera in the family Reoviridae of double-stranded RNA viruses. Orthoreoviruses infect mammals, birds, and reptiles and have a genome comprising 10 RNA segments. Aquareoviruses infect fish and have a genome comprising 11 RNA segments. Despite these differences, recent structural and nucleotide sequence evidence indicate that the proteins of Orthoreoviruses and Aquareoviruses share many similarities. The focus of this review is on the structure and function of the Orthoreovirus core proteins lambda1, lambda2, lambda3, and sigma2, for which X-ray crystal structures have been recently reported. The homologous core proteins in Aquareoviruses are VP3, VP1, VP2, and VP6, respectively. By mapping the locations of conserved residues onto the Orthoreovirus crystal structures, we have found that enzymatic surfaces involved in mRNA synthesis are well conserved between these two groups of viruses, whereas several surfaces involved in protein-protein interactions are not well conserved. Other evidence indicates that the Orthoreovirus mu2 and Aquareovirus VP5 proteins are homologous, suggesting that VP5 is a core protein as mu2 is known to be. These findings provide further evidence that Orthoreoviruses and Aquareoviruses have diverged from a common ancestor and contribute to a growing understanding of the functions of the core proteins in viral mRNA synthesis.

Identification and characterization of a baboon reovirus-specific nonstructural protein encoded by the bicistronic s4 genome segment

All characterized orthoreoviruses encode a characteristic spike-like protein on their polycistronic S1 genome segments that mediates virus cell attachment. In the case of baboon reovirus (BRV), the polycistronic S-class genome segment corresponds to the smallest S4 segment. We recently determined that the 5'-proximal open reading frame (ORF) of the bicistronic S4 segment encodes a nonstructural protein responsible for virus-induced syncytium formation. Current analysis indicates that the p16 protein encoded by the 3'-proximal ORF of the BRV S4 genome segment shows no sequence similarity to any other protein encoded by the orthoreoviruses, including the well-characterized sigma1/sigmaC reovirus cell attachment protein. Results indicate that p16 is a BRV-specific nonstructural protein that is not required for virus infection in cell culture and is not involved in viral cell attachment. In conjunction with previous studies of the BRV S1, S2, and S3 genome segments, the current results indicate that, unlike all other orthoreoviruses, BRV does not encode a cell attachment protein in its S-class genome segments. Furthermore, cell binding and infectivity studies suggested BRV may not utilize a functional homolog of the prototypical reovirus sigma1/sigmaC cell receptor-binding protein to mediate endocytic uptake by cells.

The S4 genome segment of baboon reovirus is bicistronic and encodes a novel fusion-associated small transmembrane protein

We demonstrate that the S4 genome segment of baboon reovirus (BRV) contains two sequential partially overlapping open reading frames (ORFs), both of which are functional in vitro and in virus-infected cells. The 15-kDa gene product (p15) of the 5"-proximal ORF induces efficient cell-cell fusion when expressed by itself in transfected cells, suggesting that p15 is the only viral protein required for induction of syncytium formation by BRV. The p15 protein is a small, hydrophobic, basic, integral membrane protein, properties shared with the p10 fusion-associated small transmembrane (FAST) proteins encoded by avian reovirus and Nelson Bay reovirus. As with p10, the BRV p15 protein is also a nonstructural protein and, therefore, is not involved in virus entry. Sequence analysis indicates that p15 shares no significant sequence similarity with the p10 FAST proteins and contains a unique repertoire and arrangement of sequence-predicted structural and functional motifs. These motifs include a functional N-terminal myristylation consensus sequence, an N-proximal proline-rich motif, two potential transmembrane domains, and an intervening polybasic region. The unique structural properties of p15 suggest that this protein is a novel member of the new family of FAST proteins.

Identification of carbohydrate-binding domains in the attachment proteins of type 1 and type 3 reoviruses

The reovirus attachment protein, sigma1, is responsible for strain-specific patterns of viral tropism in the murine central nervous system and receptor binding on cultured cells. The sigma1 protein consists of a fibrous tail domain proximal to the virion surface and a virion-distal globular head domain. To better understand mechanisms of reovirus attachment to cells, we conducted studies to identify the region of sigma1 that binds cell surface carbohydrate. Chimeric and truncated sigma1 proteins derived from prototype reovirus strains type 1 Lang (T1L) and type 3 Dearing (T3D) were expressed in insect cells by using a baculovirus vector. Assessment of expressed protein susceptibility to proteolytic cleavage, binding to anti-sigma1 antibodies, and oligomerization indicates that the chimeric and truncated sigma1 proteins are properly folded. To assess carbohydrate binding, recombinant sigma1 proteins were tested for the capacity to agglutinate mammalian erythrocytes and to bind sialic acid presented on glycophorin, the cell surface molecule bound by type 3 reovirus on human erythrocytes. Using a panel of two wild-type and ten chimeric and truncated sigma1 proteins, the sialic acid-binding domain of type 3 sigma1 was mapped to a region of sequence proposed to form the more amino terminal of two predicted beta-sheet structures in the tail. This unit corresponds to morphologic region T(iii) observed in computer-processed electron micrographs of sigma1 protein purified from virions. In contrast, the homologous region of T1L sigma1 sequence was not implicated in carbohydrate binding; rather, sequences in the distal portion of the tail known as the neck were required. Results of these studies demonstrate that a functional receptor-binding domain, which uses sialic acid as its ligand, is contained within morphologic region T(iii) of the type 3 sigma1 tail. Furthermore, our findings indicate that T1L and T3D sigma1 proteins contain different arrangements of receptor-binding domains.

Characterization of avian reovirus non structural protein sigmaNS synthesized in Escherichia coli

The coding region of avian reovirus S1133 genomic segment S4, encoding the non structural protein sigmaNS, was inserted into expression vector pET28a and the protein was expressed in Escherichia coli BL21(DE3) as a fusion protein containing a C-terminal peptide with six tandem histidines (His-tag). The expressed protein (esigmaNS) consistent with the expected molecular size of the avian reovirus protein sigmaNS synthesized in infected cells was readily purified by His-Bind Resin. The esigmaNS was further confirmed to be indistinguishable from viral sigmaNS by immunoblot analysis. The esigmaNS binds 32P-labeled ssRNA probe produced by run-off transcription of clone pGEM-3Zf(+)S4. The binding activity is blocked by heterologous yeast rRNA, but not by homologous avian reovirus dsRNA and heterologous infectious bursal disease virus dsRNA and salmon sperm dsDNA. Therefore, the ssRNA-binding activity of the expressed protein sigmaNS is non sequence-specific, similar to that previously described for viral sigmaNS purified from avian reovirus infected cell extracts. In addition, the recent data also show that the optimal salt (NaCl) concentration and pH for its binding are 100-150 mM and 7.0, respectively, in terms of the UV cross-linking and RNase A treatment of the reaction mixtures prior to the denaturing gel analysis.

Synthesis in Escherichia coli of avian reovirus core protein varsigmaA and its dsRNA-binding activity

The genome segment S2 of p6ian reovirus (ARV) S1133 was cloned and sequenced. The entire S2 nucleotide sequence is 1325 bp long with one long open reading frame that encodes a protein of 415 amino acids, corresponding to varsigmaA, a major core protein of ARV. S2 possesses a pentanucleotide, TCATC, at the 3'-terminus of its plus strand, common to other known genome segments of ARV and to 10 genome segments of mammalian reovirus. Amino acid sequence analysis revealed that varsigmaA contains a carboxy-terminal region (one-fourth of the protein) that is formed from alpha-helices and beta-turns, and the remainder (three-fourths of the protein) is formed predominantly from beta-strands and beta-turns. Analysis of binding activity to poly(rI)-poly(rC)-agarose suggested that ARV protein A present in total virus-infected chicken embryo fibroblasts (CEF) had dsRNA-binding activity. To further characterize the binding activity, protein varsigmaA was subsequently expressed in Escherichia coli BL21(DE3) cells as a fusion protein and isolated by metal chelate affinity chromatography. The expressed protein evarsigmaA was further purified through a Superdex 75 HR 10/30 column after digestion of the purified fusion peptide with enterokinase. The expressed protein evarsigmaA has the same molecular weight as virion protein varsigmaA purified from ARV-infected CEF and is indistinguishable from virion protein varsigmaA by immunoblot analysis. The evarsigmaA binds cooperatively alpha (32)P-labeled dsRNA probe produced by run-off transcription of clone pGEM-3Zf(+)S4. The binding reaction is blocked by homologous ARV dsRNA or heterologous infectious bursal disease virus dsRNA and poly(rI)-poly(rC), but not by salmon sperm DNA. The results indicate that the expressed protein evarsigmaA has dsRNA-binding activity similar to that of varsigmaA obtained from infected cells, and its binding is sequence-independent.

Mutant cells selected during persistent reovirus infection do not express mature cathepsin L and do not support reovirus disassembly

Persistent reovirus infections of murine L929 cells select cellular mutations that inhibit viral disassembly within the endocytic pathway. Mutant cells support reovirus growth when infection is initiated with infectious subvirion particles (ISVPs), which are intermediates in reovirus disassembly formed following proteolysis of viral outer-capsid proteins. However, mutant cells do not support growth of virions, indicating that these cells have a defect in virion-to-ISVP processing. To better understand mechanisms by which viruses use the endocytic pathway to enter cells, we defined steps in reovirus replication blocked in mutant cells selected during persistent infection. Subcellular localization of reovirus after adsorption to parental and mutant cells was assessed using confocal microscopy and virions conjugated to a fluorescent probe. Parental and mutant cells did not differ in the capacity to internalize virions or distribute them to perinuclear compartments. Using pH-sensitive probes, the intravesicular pH was determined and found to be equivalent in parental and mutant cells. In both cell types, virions localized to acidified intracellular organelles. The capacity of parental and mutant cells to support proteolysis of reovirus virions was assessed by monitoring the appearance of disassembly intermediates following adsorption of radiolabeled viral particles. Within 2 h after adsorption to parental cells, proteolysis of viral outer-capsid proteins was observed, consistent with formation of ISVPs. However, in mutant cells, no proteolysis of viral proteins was detected up to 8 h postadsorption. Since treatment of cells with E64, an inhibitor of cysteine-containing proteases, blocks reovirus disassembly, we used immunoblot analysis to assess the expression of cathepsin L, a lysosomal cysteine protease. In contrast to parental cells, mutant cells did not express the mature, proteolytically active form of the enzyme. The defect in cathepsin L maturation was not associated with mutations in procathepsin L mRNA, was not complemented by procathepsin L overexpression, and did not affect the maturation of cathepsin B, another lysosomal cysteine protease. These findings indicate that persistent reovirus infections select cellular mutations that affect the maturation of cathepsin L and suggest that alterations in the expression of lysosomal proteases can modulate viral cytopathicity.

Identification and characterization of RNA-binding activities of avian reovirus non-structural protein sigmaNS

Cytoplasmic extracts prepared from avian reovirus (ARV) strain S1133-infected chicken embryo fibroblasts were examined for the presence of RNA-binding proteins in order to identify and characterize ARV RNA-binding proteins. Analysis of binding activity to poly(A)-Sepharose indicated that infected cells contained significant amounts of a protein that co-migrated with ARV protein sigmaNS present in total virus-infected cell extracts. Determination of the N-terminal amino acid sequence of several peptide fragments generated by V8 protease digestion of the poly(A)-Sepharose-purified protein confirmed that this viral protein was sigmaNS. Competition assays showed that single-stranded RNA from the unrelated avian pathogen infectious bursal disease virus was able to compete for binding of sigmaNS to poly(A)-Sepharose. These data suggest that ARV sigmaNS binds to single-stranded RNA in a nucleotide sequence non-specific manner and is functionally similar to its counterpart specified by mammalian reovirus.

Protein architecture of avian reovirus S1133 and identification of the cell attachment protein

There are a number of discrepancies in the literature regarding the protein composition of the avian reoviruses. The present study demonstrates that avian reovirus S1133 contains at least 10 proteins (lambdaA, lambdaB, lambdaC, muA, muB, muBC, muBN, sigmaA, sigmaB, and sigmaC). Polypeptides muB, muBC, muBN, sigmaB, and sigmaC are components of the outer capsid layer of the virus, while lambdaA, lambdaB, muA, and sigmaA are core polypeptides. Protein lambdaC is a component of both layers, extending from the inner core to the outer capsid. The minor outer-capsid polypeptide sigmaC is shown to be the cell attachment protein, since it is the only viral polypeptide present in extracts of S1133-infected cells that binds specifically to chicken embryo fibroblasts; furthermore, its binding to avian cells was competitively inhibited by S1133 reovirions but not by mammalian reovirions. Our results also show that sigmaC is an oligomeric protein both in the virion and free in the cytoplasm, and preliminary results suggest that the multimer is made up of three monomeric units.

Intracellular posttranslational modifications of S1133 avian reovirus proteins

Avian reovirus S1133 specifies at least 10 primary translation products, eight of which are present in the viral particle and two of which are nonstructural proteins. In the work presented here, we studied the covalent modifications undergone by these translation products in the infected cell. The structural polypeptide mu2 was shown to be intracellularly modified by both myristoylation and proteolysis. The site-specific cleavage of mu2 yielded a large carboxy-terminal fragment and a myristoylated approximately 5,500-Mr peptide corresponding to the amino terminus. Both mu2 and its cleavage products were found to be structural components of the reovirion. Most avian reovirus proteins were found to be glycosylated and to have a blocking group at the amino terminus. In contrast to the mammalian reovirus system, none of the avian reovirus polypeptides was found to incorporate phosphorus during infection. Our results add to current understanding of the similarities and differences between avian and mammalian reoviruses.

Dansylcadaverine and cytochalasin D enhance rotavirus infection of murine L cells

Although murine L cells bind and internalize rotavirus as well as permissive cell lines, L cells are essentially nonpermissive for rotaviruses. In nonpermissive cell lines such as L cells, internalized rotavirus fails to uncoat and remains as infectious, double-shelled particles. This block in the infectious cycle can be overcome by direct lipofection of viral particles into the L cell cytoplasm. We hypothesized that the internalized rotavirus particles within L cells are sequestered in the endocytic pathway and are unable to initiate infection. L cells were pretreated with a variety of inhibitors of endocytosis prior to infection with rhesus rotavirus. While agents which inhibit acidification of endosomes had no effect on rotavirus infection, two potential direct inhibitors of vesicular transport, dansylcadaverine and cytochalasin D, enhanced rotavirus infection of L cells 5- to 10-fold. All of the drugs, including both inhibitors of endocytosis and lysosomotrophic agents, significantly reduced infection of L cells by serotype 1 reovirus which is known to infect L cells by the endocytic pathway. Time course studies demonstrated that the drugs were effective in promoting rotavirus infection of L cells in only the early phases of infection. Pretreatment of L cells with dansylcadaverine significantly decreased the number of intact, double-shelled rotavirus particles sequestered within the cells. Inhibition of endocytosis may increase the efficiency of infection of L cells by rotavirus by allowing an increased proportion of attached rotavirus virions to enter cells by a productive route which is probably direct membrane penetration.

Cloning, sequencing and expression of the S1 gene of avian reovirus

The S1 genome segment of avian reovirus strain S1133 was cloned and completely sequenced. The sequence comprised 1636 bp with three distinct open reading frames (ORFs), suggesting the gene was polycistronic in nature. The three ORFs from 5' to 3' were predicted to encode polypeptides of 9.8, 3.8 and 34.9 kDa, respectively. Of the three ORFs, only the third possessed the AUG initiation codon in an optimum context for translation. The third ORF-encoded protein, 326 amino acids in length, was expressed in Escherichia coli and used as antigen in immunoblots. The protein was concluded to be sigma 3 on the basis of its recognition by a chicken anti-reovirus antiserum and due to the fact that a mouse antiserum raised against it recognized specifically the viral sigma 3 polypeptide. Sequence comparison of the avian reovirus S1 gene with its mammalian counterpart did not show any significant similarity between the two. However, amino acid sequence analysis and the predicted existence of a heptapeptide repeat pattern, as well as the relatively high frequency of alpha-helix structures in the amino terminal portion of sigma 3 suggests that this protein is structurally, and probably functionally, related to mammalian reovirus sigma 1 protein.