Biochemical and biophysical characterization of the reovirus cell attachment protein sigma 1: evidence that it is a homotrimer

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.

Polycistronic Genome Segment Evolution and Gain and Loss of FAST Protein Function during Fusogenic Orthoreovirus Speciation

The Reoviridae family is the only non-enveloped virus family with members that use syncytium formation to promote cell–cell virus transmission. Syncytiogenesis is mediated by a fusion-associated small transmembrane (FAST) protein, a novel family of viral membrane fusion proteins. Previous evidence suggested the fusogenic reoviruses arose from an ancestral non-fusogenic virus, with the preponderance of fusogenic species suggesting positive evolutionary pressure to acquire and maintain the fusion phenotype. New phylogenetic analyses that included the atypical waterfowl subgroup of avian reoviruses and recently identified new orthoreovirus species indicate a more complex relationship between reovirus speciation and fusogenic capacity, with numerous predicted internal indels and 5’-terminal extensions driving the evolution of the orthoreovirus’ polycistronic genome segments and their encoded FAST and fiber proteins. These inferred recombination events generated bi- and tricistronic genome segments with diverse gene constellations, they occurred pre- and post-orthoreovirus speciation, and they directly contributed to the evolution of the four extant orthoreovirus FAST proteins by driving both the gain and loss of fusion capability. We further show that two distinct post-speciation genetic events led to the loss of fusion in the waterfowl isolates of avian reovirus, a recombination event that replaced the p10 FAST protein with a heterologous, non-fusogenic protein and point substitutions in a conserved motif that destroyed the p10 assembly into multimeric fusion platforms.

Grass carp reovirus-GD108 fiber protein is involved in cell attachment

Viral attachment to specific host receptors is the first step in viral infection and serves an essential function in the selection of target cells. In this study, structure analysis, neutralization assays, and cell attachment assays were carried out to evaluate the cell attachment functions of the outer capsid fiber protein of grass carp reovirus GD108 strain (GCRV-GD108). The GCRV-GD108 fiber protein contained 512 amino acids encoded by S7 segment and shared sequence similarities with mammalian reovirus cell attachment protein σ1 and adenovirus fiber. Structural analyses predicted the presence of a coiled-coil tail domain, three adenoviral shafts in the body domain, and a globular head domain, similar to other fiber proteins. Neutralization assays showed that polyclonal antibodies against the fiber protein could prevent viral infection in both fish and grass carp snout fibroblast cells (PSF), suggesting that the recombinant fiber protein could induce neutralized antibodies against GCRV-GD108. Cell attachment assays showed that recombinant fiber protein could bind to PSF cells, demonstrating that the fiber protein functioned as the cell attachment protein in GCRV-GD108. These results provided the basis for further studies of the pathogenesis of grass carp reovirus.

Sequence analysis of the genome of piscine orthoreovirus (PRV) associated with heart and skeletal muscle inflammation (HSMI) in Atlantic salmon (Salmo salar)

Piscine orthoreovirus (PRV) is associated with heart- and skeletal muscle inflammation (HSMI) of farmed Atlantic salmon (Salmo salar). We have performed detailed sequence analysis of the PRV genome with focus on putative encoded proteins, compared with prototype strains from mammalian (MRV T3D)- and avian orthoreoviruses (ARV-138), and aquareovirus (GCRV-873). Amino acid identities were low for most gene segments but detailed sequence analysis showed that many protein motifs or key amino acid residues known to be central to protein function are conserved for most PRV proteins. For M-class proteins this included a proline residue in μ2 which, for MRV, has been shown to play a key role in both the formation and structural organization of virus inclusion bodies, and affect interferon-β signaling and induction of myocarditis. Predicted structural similarities in the inner core-forming proteins λ1 and σ2 suggest a conserved core structure. In contrast, low amino acid identities in the predicted PRV surface proteins μ1, σ1 and σ3 suggested differences regarding cellular interactions between the reovirus genera. However, for σ1, amino acid residues central for MRV binding to sialic acids, and cleavage- and myristoylation sites in μ1 required for endosomal membrane penetration during infection are partially or wholly conserved in the homologous PRV proteins. In PRV σ3 the only conserved element found was a zinc finger motif. We provide evidence that the S1 segment encoding σ3 also encodes a 124 aa (p13) protein, which appears to be localized to intracellular Golgi-like structures. The S2 and L2 gene segments are also potentially polycistronic, predicted to encode a 71 aa- (p8) and a 98 aa (p11) protein, respectively. It is concluded that PRV has more properties in common with orthoreoviruses than with aquareoviruses.

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.

Attachment and cell entry of mammalian orthoreovirus

Mammalian orthoreoviruses (reoviruses) serve as a tractable model system for studies of viral pathogenesis. Reoviruses infect virtually all mammals, but cause disease only in the very young. Prototype strains of the three reovirus serotypes differ in pathogenesis following infection of newborn mice. Reoviruses are nonenveloped, icosahedral particles that consist of ten segments of double-stranded RNA encapsidated within two protein shells, the inner core and outer capsid. High-resolution structures of individual components of the reovirus outer capsid and a single viral receptor have been solved and provide insight into the functions of these molecules in viral attachment, entry, and pathogenesis. Attachment of reovirus to target cells is mediated by the reovirus sigma1 protein, a filamentous trimer that projects from the outer capsid. Junctional adhesion molecule-A is a serotype-independent receptor for reovirus, and sialic acid is a coreceptor for serotype 3 strains. After binding to receptors on the cell surface, reovirus is internalized via receptor-mediated endocytosis. Internalization is followed by stepwise disassembly of the viral outer capsid in the endocytic compartment. Uncoating events, which require acidic pH and endocytic proteases, lead to removal of major outer-capsid protein sigma3, resulting in exposure of membrane-penetration mediator micro1 and a conformational change in attachment protein sigma1. After penetration of endosomes by uncoated particles, the transcriptionally active viral core is released into the cytoplasm, where replication proceeds. Despite major advances in defining reovirus attachment and entry mechanisms, many questions remain. Ongoing research is aimed at understanding serotype-dependent differences in reovirus tropism, viral cell-entry pathways, the individual and corporate roles of acidic pH and proteases in viral entry, and micro1 function in membrane penetration.

Reovirus outer capsid protein micro1 induces apoptosis and associates with lipid droplets, endoplasmic reticulum, and mitochondria

The mechanisms by which reoviruses induce apoptosis have not been fully elucidated. Earlier studies identified the mammalian reovirus S1 and M2 genes as determinants of apoptosis induction. However, no published results have demonstrated the capacities of the proteins encoded by these genes to induce apoptosis, either independently or in combination, in the absence of reovirus infection. Here we report that the mammalian reovirus micro1 protein, encoded by the M2 gene, was sufficient to induce apoptosis in transfected cells. We also found that micro1 localized to lipid droplets, endoplasmic reticulum, and mitochondria in both transfected cells and infected cells. Two small regions encompassing amphipathic alpha-helices within a carboxyl-terminal portion of micro1 were necessary for efficient induction of apoptosis and association with lipid droplets, endoplasmic reticulum, and mitochondria in transfected cells. Induction of apoptosis by micro1 and its association with lipid droplets and intracellular membranes in transfected cells were abrogated when micro1 was coexpressed with sigma3, with which it is known to coassemble. We propose that micro1 plays a direct role in the induction of apoptosis in infected cells and that this property may relate to the capacity of micro1 to associate with intracellular membranes. Moreover, during reovirus infection, association with sigma3 may regulate apoptosis induction by micro1.

Protective immunoglobulin A and G antibodies bind to overlapping intersubunit epitopes in the head domain of type 1 reovirus adhesin sigma1

Nonfusogenic mammalian orthoreovirus (reovirus) is an enteric pathogen of mice and a useful model for studies of how an enteric virus crosses the mucosal barrier of its host and is subject to control by the mucosal immune system. We recently generated and characterized a new murine immunoglobulin A (IgA)-class monoclonal antibody (MAb), 1E1, that binds to the adhesin fiber, sigma1, of reovirus type 1 Lang (T1L) and thereby neutralizes the infectivity of that strain in cell culture. 1E1 is produced in hybridoma cultures as a mixture of monomers, dimers, and higher polymers and is protective against peroral challenges with T1L either when the MAb is passively administered or when it is secreted into the intestines of mice bearing subcutaneous hybridoma tumors. In the present study, selection and analysis of mutants resistant to neutralization by 1E1 identified the region of T1L sigma1 to which the MAb binds. The region bound by a previously characterized type 1 sigma1-specific neutralizing IgG MAb, 5C6, was identified in the same way. Each of the 15 mutants isolated and analyzed was found to be much less sensitive to neutralization by either 1E1 or 5C6, suggesting the two MAbs bind to largely overlapping regions of sigma1. The tested mutants retained the capacity to recognize specific glycoconjugate receptors on rabbit M cells and cultured epithelial cells, even though viral binding to epithelial cells was inhibited by both MAbs. S1 sequence determinations for 12 of the mutants identified sigma1 mutations at four positions between residues 415 and 447, which contribute to forming the receptor-binding head domain. When aligned with the sigma1 sequence of reovirus type 3 Dearing (T3D) and mapped onto the previously reported crystal structure of the T3D sigma1 trimer, the four positions cluster on the side of the sigma1 head, across the interface between two subunits. Three such interface-spanning epitopes are thus present per sigma1 trimer and require the intact quaternary structure of the head domain for MAb binding. Identification of these intersubunit epitopes on sigma1 opens the way for further studies of the mechanisms of antibody-based neutralization and protection with type 1 reoviruses.

Reptilian reovirus: a new fusogenic orthoreovirus species

The fusogenic subgroup of orthoreoviruses contains most of the few known examples of non-enveloped viruses capable of inducing syncytium formation. The only unclassified orthoreoviruses at the species level represent several fusogenic reptilian isolates. To clarify the relationship of reptilian reoviruses (RRV) to the existing fusogenic and nonfusogenic orthoreovirus species, we undertook a characterization of a python reovirus isolate. Biochemical, biophysical, and biological analyses confirmed the designation of this reptilian reovirus (RRV) isolate as an unclassified fusogenic orthoreovirus. Sequence analysis revealed that the RRV S1 and S3 genome segments contain a novel conserved 5'-terminal sequence not found in other orthoreovirus species. In addition, the gene arrangement and the coding potential of the bicistronic RRV S1 genome segment differ from that of established orthoreovirus species, encoding a predicted homologue of the reovirus cell attachment protein and a unique 125 residue p14 protein. The RRV S3 genome segment encodes a homologue of the reovirus sigma-class major outer capsid protein, although it is highly diverged from that of other orthoreovirus species (amino acid identities of only 16-25%). Based on sequence analysis, biological properties, and phylogenetic analysis, we propose this python reovirus be designated as the prototype strain of a fifth species of orthoreoviruses, the reptilian reoviruses.

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.

Structure-function analysis of reovirus binding to junctional adhesion molecule 1. Implications for the mechanism of reovirus attachment

Mammalian reoviruses are nonenveloped viruses with a long, filamentous attachment protein that dictates disease phenotypes following infection of newborn mice and is a structural homologue of the adenovirus attachment protein. Reoviruses use junctional adhesion molecule 1 (JAM1) as a serotype-independent cellular receptor. JAM1 is a broadly expressed immunoglobulin superfamily protein that forms stable homodimers and regulates tight-junction permeability and lymphocyte trafficking. We employed a series of structure-guided binding and infection experiments to define residues in human JAM1 (hJAM1) important for reovirus-receptor interactions and to gain insight into mechanisms of reovirus attachment. Binding and infection experiments using chimeric and domain deletion mutant receptor molecules indicate that the amino-terminal D1 domain of hJAM1 is required for reovirus attachment, infection, and replication. Reovirus binding to hJAM1 occurs more rapidly than homotypic hJAM1 association and is competed by excess hJAM1 in vitro and on cells. Cross-linking hJAM1 diminishes the capacity of reovirus to bind hJAM1 in vitro and on cells and negates the competitive effects of soluble hJAM1 on reovirus attachment. Finally, mutagenesis studies demonstrate that residues intimately associated with the hJAM1 dimer interface are critical for reovirus interactions with hJAM1. These results suggest that reovirus attachment disrupts hJAM1 dimers and highlight similarities between the attachment strategies of reovirus and adenovirus.

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.

Palmitoylation, membrane-proximal basic residues, and transmembrane glycine residues in the reovirus p10 protein are essential for syncytium formation

Avian reovirus and Nelson Bay reovirus are two unusual nonenveloped viruses that induce extensive cell-cell fusion via expression of a small nonstructural protein, termed p10. We investigated the importance of the transmembrane domain, a conserved membrane-proximal dicysteine motif, and an endodomain basic region in the membrane fusion activity of p10. We now show that the p10 dicysteine motif is palmitoylated and that loss of palmitoylation correlates with a loss of fusion activity. Mutational and functional analyses also revealed that a triglycine motif within the transmembrane domain and the membrane-proximal basic region were essential for p10-mediated membrane fusion. Mutations in any of these three motifs did not influence events upstream of syncytium formation, such as p10 membrane association, protein topology, or surface expression, suggesting that these motifs are more intimately associated with the membrane fusion reaction. These results suggest that the rudimentary p10 fusion protein has evolved a mechanism of inducing membrane merger that is highly dependent on the specific interaction of several different motifs with donor membranes. In addition, cross-linking, coimmunoprecipitation, and complementation assays provided no evidence for p10 homo- or heteromultimer formation, suggesting that p10 may be the first example of a membrane fusion protein that does not form stable, higher-order multimers.

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.

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.

Sites and determinants of early cleavages in the proteolytic processing pathway of reovirus surface protein sigma3

Entry of mammalian reovirus virions into target cells requires proteolytic processing of surface protein sigma3. In the virion, sigma3 mostly covers the membrane-penetration protein mu1, appearing to keep it in an inactive form and to prevent it from interacting with the cellular membrane until the proper time in infection. The molecular mechanism by which sigma3 maintains mu1 in this inactive state and the structural changes that accompany sigma3 processing and mu1 activation, however, are not well understood. In this study we characterized the early steps in sigma3 processing and determined their effects on mu1 function and particle infectivity. We identified two regions of high protease sensitivity, "hypersensitive" regions located at residues 208 to 214 and 238 to 244, within which all proteases tested selectively cleaved sigma3 as an early step in processing. Further processing of sigma3 was required for infection, consistent with the fact that the fragments resulting from these early cleavages remained bound to the particles. Reovirus type 1 Lang (T1L), type 3 Dearing (T3D), and T1L x T3D reassortant virions differed in the sites of early sigma3 cleavage, with T1L sigma3 being cleaved mainly at residues 238 to 244 and T3D sigma3 being cleaved mainly at residues 208 to 214. These virions also differed in the rates at which the early cleavages occurred, with cleavage of T1L sigma3 occurring faster than cleavage of T3D sigma3. Analyses using chimeric and site-directed mutants of recombinant sigma3 identified carboxy-proximal residues 344, 347, and 353 as the primary determinants of these strain differences. The spatial relationships between these more carboxy-proximal residues and the hypersensitive regions were discerned from the sigma3 crystal structure. The results indicate that proteolytic processing of sigma3 during reovirus disassembly is a multistep pathway with a number of molecular determinants.

Sequential partially overlapping gene arrangement in the tricistronic S1 genome segments of avian reovirus and Nelson Bay reovirus: implications for translation initiation

Previous studies of the avian reovirus strain S1133 (ARV-S1133) S1 genome segment revealed that the open reading frame (ORF) encoding the final sigmaC viral cell attachment protein initiates over 600 nucleotides distal from the 5' end of the S1 mRNA and is preceded by two predicted small nonoverlapping ORFs. To more clearly define the translational properties of this unusual polycistronic RNA, we pursued a comparative analysis of the S1 genome segment of the related Nelson Bay reovirus (NBV). Sequence analysis indicated that the 3'-proximal ORF present on the NBV S1 genome segment also encodes a final sigmaC homolog, as evidenced by the presence of an extended N-terminal heptad repeat characteristic of the coiled-coil region common to the cell attachment proteins of reoviruses. Most importantly, the NBV S1 genome segment contains two conserved ORFs upstream of the final sigmaC coding region that are extended relative to the predicted ORFs of ARV-S1133 and are arranged in a sequential, partially overlapping fashion. Sequence analysis of the S1 genome segments of two additional strains of ARV indicated a similar overlapping tricistronic gene arrangement as predicted for the NBV S1 genome segment. Expression analysis of the ARV S1 genome segment indicated that all three ORFs are functional in vitro and in virus-infected cells. In addition to the previously described p10 and final sigmaC gene products, the S1 genome segment encodes from the central ORF a 17-kDa basic protein (p17) of no known function. Optimizing the translation start site of the ARV p10 ORF lead to an approximately 15-fold increase in p10 expression with little or no effect on translation of the downstream final sigmaC ORF. These results suggest that translation initiation complexes can bypass over 600 nucleotides and two functional overlapping upstream ORFs in order to access the distal final sigmaC start site.

Subunit composition and conformational stability of the oligomeric form of the avian reovirus cell-attachment protein sigmaC

Previous work has shown that the avian reovirus cell-attachment sigma C (sigmaC) protein is a multimer. In the first part of this study the oligomerization state of intracellularly synthesized sigmaC was analysed by different approaches, including SDS-PAGE, chemical cross-linking, sedimentation and gel filtration analysis. All these approaches indicated that protein sigmaC in its native state is a homotrimer. In the second part of the present work we investigated the effect of different factors and reagents on oligomer stability, in order to elucidate the nature of the forces that maintain the conformational stability of the homotrimer. Our results, based on the stabilizing effect conferred by reducing agents, demonstrate that the sigmaC subunits are not covalently bound via disulfide linkages. They further suggest that the formation of an intrachain disulfide bond between the two cysteine residues of the sigmaC polypeptide has a negative effect on oligomer stability. The susceptibility of the trimer to pH, temperature, ionic strength, chemical denaturants and detergents indicates that hydrophobic interactions contribute much more to oligomer stability than do ionic interactions and hydrogen bonding. Finally, our results also reveal that mammalian and avian reovirus cell attachment proteins follow different subunit dissociation pathways.

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.

Oligomerization and cell-binding properties of the avian reovirus cell-attachment protein sigmaC

Avian reovirus protein sigmaC, the viral cell-attachment protein, is a minor component of the outer-capsid shell of the viral particle that is synthesized in small amounts in infected cells. We cloned the sigmaC-encoding ORF in vector pIL-2f, expressed it in Escherichia coli, and partially purified the resulting recombinant protein from inclusion bodies. Rabbit polyclonal antibodies raised against the recombinant protein specifically recognized the viral polypeptide in ELISA, immunoprecipitation, and Western blotting. To study the oligomerization capacity and cell-binding affinity of protein sigmaC, the sigmaC-encoding ORF was also expressed in chicken embryo fibroblasts (CEFs) and in reticulocyte lysates. In all three systems protein sigmaC is expressed as a multimer with identical electrophoretic mobility to the naturally occurring protein. Cell-binding experiments show that both in vitro and in vivo expressed protein sigmaC display affinity for CEF receptors, and this property is exclusively associated with the oligomeric form of the protein. The fact that incubation of CEF cells with the recombinant protein expressed in bacterial cells completely blocks the binding of purified reovirions indicates both that binding of this protein to cells is specific and saturable, and that reovirions and protein sigmaC bind to the same class of cell receptor. Saturation binding experiments, performed with the recombinant protein expressed in E. coli and with purified reovirions, showed that the number of cellular receptor sites (CRSs) for avian reovirus S1133 is 1.8 x 10(4) per CEF cell, whereas the number of cellular receptor units (CRUs) for sigmaC is 2.2 x 10(5) per CEF cell. These results are consistent with previous reports on the binding of mammalian reoviruses.

A new class of fusion-associated small transmembrane (FAST) proteins encoded by the non-enveloped fusogenic reoviruses

The non-enveloped fusogenic avian and Nelson Bay reoviruses encode homologous 10 kDa non-structural transmembrane proteins. The p10 proteins localize to the cell surface of transfected cells in a type I orientation and induce efficient cell-cell fusion. Mutagenic studies revealed the importance of conserved sequence-predicted structural motifs in the membrane association and fusogenic properties of p10. These motifs included a centrally located transmembrane domain, a conserved cytoplasmic basic region, a small hydrophobic motif in the N-terminal domain and four conserved cysteine residues. Functional analysis indicated that the extreme C-terminus of p10 functions in a sequence-independent manner to effect p10 membrane localization, while the N-terminal domain displays a sequence-dependent effect on the fusogenic property of p10. The small size, unusual arrangement of structural motifs and lack of any homologues in previously described membrane fusion proteins suggest that the fusion-associated small transmembrane (FAST) proteins of reovirus represent a new class of membrane fusion proteins.

Gene transfer facilitated by a cellular targeting molecule, reovirus protein sigma1

To facilitate eventual genetic vaccination of mucosal tissues, a receptor-mediated gene transfer system was devised using the reovirus adhesin, protein sigma1. Highly efficient uptake and internalization of protein sigma1 polylysine (PL) DNA complexes could be demonstrated by fluorescent microscopy. Successful cellular transfection of rodent and human cell lines was obtained with the recombinant protein sigma1 as a PL-DNA complex, and could be shown to be receptor-specific. Transfection efficiency was dependent upon the ratio of DNA complexed to protein sigma1-PL and chloroquine treatment improved transfection efficiency dramatically. To test its ability to bind a mucosal inductive tissue, recombinant protein sigma1 was specifically bound to the nasal-associated lymphoid tissues (NALT). Thus, recombinant protein sigma1-PL-DNA conjugates can efficiently bind and transfect cells that express the receptor for protein sigma1. Gene Therapy (2000) 7, 61-69.

Unfolding studies of human adenovirus type 2 fibre trimers. Evidence for a stable domain

Adenovirus fibres are trimeric proteins that protrude from the 12 fivefold vertices of the virion and are the cell attachment organelle of the virus. They consist of three segments: an N-terminal tail, which is noncovalently attached to the penton base, a thin shaft carrying 15 amino acid pseudo repeats, and a C-terminal globular head (or knob) which recognizes the primary cell receptor. Due to their exceptional stability, which allows easy distinction of native trimers from unfolded forms and folding intermediates, adenovirus fibres are a very good model system for studying folding in vivo and in vitro. To understand the folding and stability of the trimeric fibres, the unfolding pathway of adenovirus 2 fibres induced by SDS and temperature has been investigated. Unfolding starts from the N-terminus and a stable intermediate accumulates that has the C-terminal head and part of the shaft structure (shown by electron microscopy). The unfolded part can be digested away using limited proteolysis, and the precise digestion sites have been determined. The remaining structured fragment is recognized by monoclonal antibodies that are specific for the trimeric globular head and therefore retains a native trimeric structure. Taken together, our results indicate that adenovirus fibres carry a stable C-terminal domain, consisting of the knob with five shaft-repeats.

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.

A glycosyl hydrolase activity of mammalian reovirus sigma1 protein can contribute to viral infection through a mucus layer

The mammalian reovirus sigma1 protein is responsible for viral attachment to host cells and hemagglutination properties of the virus. In the present study, sequence similarity between sigma1 and chicken-type lysozymes prompted us to investigate additional functions of the sigma1 protein. Expression in Pichia pastoris yeast cells showed that sigma1 can actually cleave lysozyme substrates, including complex sugars found in bacterial cell walls. Replacement by site-directed mutagenesis of acidic amino acid residues in sigma1 by their respective isosteric, uncharged, amino acid residues has allowed us to identify Glu36 and Asp54 as the catalytic pair involved in sigma1-mediated glycosidase activity. The enzyme appears inactive in virions but its activity is unmasked upon generation of infectious subviral particles (ISVPs) by partial proteolytic removal of the outer capsid proteins. Purified sigma1 protein and ISVPs can also hydrolyze mucins, heavily glycosylated glycoproteins that are a major component of the mucus layer overlaying the intestinal epithelium. Furthermore, reovirus infection of epithelial Madin Darby canine kidney cells was inhibited tenfold in cells expressing mucin at their apical surface, while this inhibition was overcome by ISVPs. Unmasking of sigma1 mucinolytic activity in the intestine, consecutive to proteolytic cleavage of virions to ISVPs, thus likely contributes to the known increase in infectivity of reovirus ISVPs compared to complete virions. This work presents the first evidence that some mammalian viruses have evolved mechanisms to facilitate their penetration through the protective barrier of the mucus layer in the intestinal tract.

Cleavage susceptibility of reovirus attachment protein sigma1 during proteolytic disassembly of virions is determined by a sequence polymorphism in the sigma1 neck

A requisite step in reovirus infection of the murine intestine is proteolysis of outer-capsid proteins to yield infectious subvirion particles (ISVPs). When converted to ISVPs by intestinal proteases, virions of reovirus strain type 3 Dearing (T3D) lose 90% of their original infectivity due to cleavage of viral attachment protein sigma1. In an analysis of eight field isolate strains of type 3 reovirus, we identified one additional strain, type 3 clone 31 (T3C31), that loses infectivity and undergoes sigma1 cleavage upon conversion of virions to ISVPs. We examined the sigma1 deduced amino acid sequences of T3D and the eight field isolate strains for a correlation between sequence variability and sigma1 cleavage. The sigma1 proteins of T3D and T3C31 contain a threonine at amino acid position 249, whereas an isoleucine occurs at this position in the sigma1 proteins of the remaining strains. Thr249 occupies the d position of a heptad repeat motif predicted to stabilize sigma1 oligomers through alpha-helical coiled-coil interactions. This region of sequence comprises a portion of the fibrous tail domain of sigma1 known as the neck. Substitution of Thr249 with isoleucine or leucine resulted in resistance to cleavage by trypsin, whereas replacement with asparagine did not affect cleavage susceptibility. These results demonstrate that amino acid position 249 is an independent determinant of T3D sigma1 cleavage susceptibility and that an intact heptad repeat is required to confer cleavage resistance. We performed amino-terminal sequence analysis on the sigma1 cleavage product released during trypsin treatment of T3D virions to generate ISVPs and found that trypsin cleaves sigma1 after Arg245. Thus, the sequence polymorphism at position 249 controls cleavage at a nearby site in the neck region. The relevance of these results to reovirus infection in vivo was assessed by treating virions with the contents of a murine intestinal wash under conditions that result in generation of ISVPs. The pattern of sigma1 cleavage susceptibility generated by using purified protease was reproduced in assays using the intestinal wash. These results provide a mechanistic explanation for sigma1 cleavage during exposure of virions to intestinal proteases and may account for certain strain-dependent patterns of reovirus pathogenesis.

Active participation of Hsp90 in the biogenesis of the trimeric reovirus cell attachment protein sigma1

The reovirus cell attachment protein, sigma1, is a lollipop-shaped homotrimer with an N-terminal fibrous tail and a C-terminal globular head. Biogenesis of this protein involves two trimerization events: N-terminal trimerization, which occurs cotranslationally and is Hsp70/ATP-independent, and C-terminal trimerization, which occurs posttranslationally and is Hsp70/ATP-dependent. To determine if Hsp90 also plays a role in sigma1 biogenesis, we analyzed sigma1 synthesized in rabbit reticulocyte lysate. Coprecipitation experiments using anti-Hsp90 antibodies revealed that Hsp90 was associated with immature sigma1 trimers (hydra-like intermediates with assembled N termini and unassembled C termini) but not with mature trimers. The use of truncated sigma1 further demonstrated that only the C-terminal half of sigma1 associated with Hsp90. In the presence of the Hsp90 binding drug geldanamycin, N-terminal trimerization proceeded normally, but C-terminal trimerization was blocked. Geldanamycin did not inhibit the association of Hsp90 with sigma 1 but prevented the subsequent release of Hsp90 from the immature sigma1 complex. We also examined the status of p23, an Hsp90-associated cochaperone. Like Hsp90, p23 only associated with immature sigma1 trimers, and this association was mapped to the C-terminal half of sigma1. However, unlike Hsp90, p23 was released from the sigma1 complex upon the addition of geldanamycin. These results highlight an all-or-none concept of chaperone involvement in different oligomerization domains within a single protein and suggest a possible common usage of chaperones in the regulation of general protein folding and of steroid receptor activation.

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.

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.

Persistent reovirus infections of L cells select mutations in viral attachment protein sigma1 that alter oligomer stability

During maintenance of L-cell cultures persistently infected with reovirus, mutations are selected in viruses and cells. Cells cured of persistent infection support growth of viruses isolated from persistently infected cultures (PI viruses) significantly better than that of wild-type (wt) viruses. In a previous study, the capacity of PI virus strain L/C to grow better than wt strain type 1 Lang (T1L) in cured cells was mapped genetically to the S1 gene (R. S. Kauffman, R. Ahmed, and B. N. Fields, Virology 131:79-87, 1983), which encodes viral attachment protein sigma1. To investigate mechanisms by which mutations in S1 confer growth of PI viruses in cured cells, we determined the S1 gene nucleotide sequences of L/C virus and six additional PI viruses isolated from independent persistently infected L-cell cultures. The S1 sequences of these viruses contained from one to three mutations, and with the exception of PI 2A1 mutations in each S1 gene resulted in changes in the deduced amino acid sequence of sigma1 protein. Using electrophoresis conditions that favor migration of sigma1 oligomers, we found that sigma1 proteins of L/C, PI 1A1, PI 3-1, and PI 5-1 migrated as monomers, whereas sigma1 proteins of wt reovirus and PI 2A1 migrated as oligomers. These findings suggest that mutations in sigma1 protein affecting stability of sigma1 oligomers are important for the capacity of PI viruses to infect mutant cells selected during persistent infection. Since no mutation was found in the deduced amino acid sequence of PI 2A1 sigma1 protein, we used T1L X PI 2A1 reassortant viruses to identify viral genes associated with the capacity of this PI virus to grow better than wt in cured cells. The capacity of PI 2A1 to grow better than T1L in cured cells was mapped to the S4 gene, which encodes outer-capsid protein sigma3. This finding suggests that in some cases, mutations in sigma3 protein in the absence of sigma1 mutations confer growth of PI viruses in mutant cells. To confirm the importance of the S1 gene in PI virus growth in cured cells, we used T1L X PI 3-1 reassortant viruses to genetically map the capacity of this PI virus to grow better than wt in cured cells. In contrast to our results using PI 2A1, we found that growth of PI 3-1 in cured cells was determined by the sigma1-encoding S1 gene. Given that the sigma1 and sigma3 proteins play important roles in reovirus disassembly, findings made in this study suggest that stability of the viral outer capsid is an important determinant of the capacity of reoviruses to adapt to host cells during persistent infection.

Co-translational trimerization of the reovirus cell attachment protein

The reovirus cell attachment protein, sigma1, is a trimer with a 'lollipop' structure. Recent findings indicate that the N-terminal fibrous tail and the C-terminal globular head each possess a distinct trimerization domain. The region responsible for N-terminal trimerization (formation of a triple alpha-helical coiled-coil) is located at the N-terminal one-third of sigma1. In this study, we investigated the temporality and ATP requirement of this trimerization event in the context of sigma1 biogenesis. In vitro co-synthesis of the full-length (FL) and a C-terminally truncated (d44) sigma1 protein revealed a preference for homotrimer over heterotrimer formation, suggesting that assembly at the N-terminus occurs co-translationally. This was corroborated by the observation that polysome-associated sigma1 chains were trimeric as well as monomeric. Truncated proteins (d234 and d294) with C-terminal deletions exceeding half the length of sigma1 were found to trimerize post-translationally. This trimerization did not require ATP since it proceeded normally in the presence of apyrase. In contrast, formation of stable FL sigma1 trimers was inhibited by apyrase treatment. Collectively, our data suggest that assembly of nascent sigma1 chains at the N-terminus is intrinsically ATP independent, and occurs co-translationally when the ribosomes have traversed past the midpoint of the mRNA.

C-terminal trimerization, but not N-terminal trimerization, of the reovirus cell attachment protein Is a posttranslational and Hsp70/ATP-dependent process

The C-terminal globular head of the lollipop-shaped final sigma1 protein of reovirus is responsible for interaction with the host cell receptor. Like the N-terminal fibrous tail, it has its own trimerization domain. Whereas N-terminal trimerization (formation of a triple alpha-helical coiled coil) occurs at the level of polysomes (i.e. cotranslationally) and is ATP-independent, C-terminal trimerization is a posttranslational event that requires ATP. Coprecipitation experiments using anti-Hsp70 antibodies and truncated final sigma1 proteins synthesized in vitro revealed that only regions downstream of the N-terminal alpha-helical coiled coil were associated with Hsp70. Hsp70 was also found to be associated with nascent final sigma1 chains on polysomes as well as with immature postribosomal final sigma1 trimers (hydra-like intermediates with assembled N termini and unassembled C termini). These latter structures were true intermediates in the final sigma1 biogenetic pathway since they could be chased into mature final sigma1 trimers with the release of Hsp70. Thus, unlike N-terminal trimerization, C-terminal trimerization is Hsp70- and ATP-dependent. The involvement of two mechanistically distinct oligomerization events for the same molecule, one cotranslational and one posttranslational, may represent a common approach to the generation of oligomeric proteins in the cytosol.

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

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

Genetic mapping of reovirus virulence and organ tropism in severe combined immunodeficient mice: organ-specific virulence genes

We used reovirus reassortant genetics and severe combined immunodeficient (SCID) mice to define viral genes important for organ tropism and virulence in the absence of antigen-specific immunity. Adult SCID mice infected with reovirus serotype 1 strain Lang (T1L) died after 20 +/- 6 days, while infection with serotype 3 strain Dearing (T3D) was lethal after 77 +/- 22 days. One hundred forty-five adult SCID mice were infected with T1L, T3D, and 25 different T1L x T3D reassortant reoviruses, and gene segments associated with the increased virulence of T1L were identified. Gene segments S1, L2, M1, and L1 accounted for > 90% of the genetically determined increase in T1L virulence. Gene segment M1 was independently important for virulence, with S1, L2, and L1 alone or in combination also playing a role. T1L grew to higher titers in multiple organs and caused more severe hepatitis than T3D. Seventy adult SCID mice, T1L, T3D, and 15 T1L x T3D reassortant viruses were used to map genetic determinants of viral titers in the brain, intestines, and liver, as well as the severity of hepatitis. Different sets of gene segments were important for determining viral titers in different organs. Gene segments L1 (encoding a core protein) and L2 (encoding the core spike of the virion) were important in all of the organs analyzed. The M1 gene segment (encoding a core protein), but not the S1 gene segment, was a critical determinant of reovirus titer in the liver and severity of hepatitis. The S1 gene segment (encoding the viral cell attachment protein and a nonstructural protein), but not the M1 gene segment, was a critical determinant of titers in intestines and brains. These studies demonstrate that viral growth in different organs is dependent on different subsets of the genes important for virulence. The virion-associated protein products of the four gene segments (L1, L2, M1, and S1) important for virulence and organ tropism in SCID mice likely form a structural unit, the reovirus vertex. Organs (the brain and intestines versus the liver) differ in properties that determine which virulence genes, and thus which parts of this structural unit, are important.

Crystal structure of an isoleucine-zipper trimer

Subunit oligomerization in many proteins is mediated by short coiled-coil motifs. These motifs share a characteristic seven-amino-acid repeat containing hydrophobic residues at the first (a) and fourth (d) positions. Despite this common pattern, different sequences form two-, three- and four-stranded helical ropes. We have investigated the basis for oligomer choice by characterizing variants of the GCN4 leucine-zipper dimerization domain that adopt trimeric or tetrameric structures in response to mutations at the a and d positions. We now report the high-resolution X-ray crystal structure of an isoleucine-containing mutant that folds into a parallel three-stranded, alpha-helical coiled coil. In contrast to the dimer and tetramer structures, the interior packing of the trimer can accommodate beta-branched residues in the most preferred rotamer at both hydrophobic positions. Compatibility of the shape of the core amino acids with the distinct packing spaces in the two-, three- and four-stranded conformations appears to determine the oligomerization state of the GCN4 leucine-zipper variants.

Reovirus protein sigma 1: from cell attachment to protein oligomerization and folding mechanisms

The reovirus cell attachment protein sigma 1 is a lollipop-shaped structure with the fibrous tail anchored to the virion. Since it interacts with the cell receptor, sigma 1 is a major determinant of reovirus infectivity and tissue tropism. Studies on its structure-function relationships have been facilitated by the fact that protein sigma 1 produced in any expression system is capable of binding to cell receptors. The use of site-specific and deletion mutants has led to the identification and characterization of its virion anchorage and receptor binding domains. Studies on the oligomeric status of sigma 1 have revealed that sigma 1 is a homotrimer and that two independent trimerization events at different loci (the N- and C-terminal halves, respectively) of the protein, are involved in its generation. This also accounts for a clearly demonstrable dominant negative effect by a mutant subunit in a wild-type/mutant sigma 1 heterotrimer. Current efforts are focused on the involvement of chaperones in the generation of sigma 1 and on events that take place upon sigma 1 binding to the cell receptor. Protein sigma 1 has therefore become an excellent model system for the study of both virus attachment and protein oligomerization and folding mechanisms.

Early steps in reovirus infection are associated with dramatic changes in supramolecular structure and protein conformation: analysis of virions and subviral particles by cryoelectron microscopy and image reconstruction

Three structural forms of type 1 Lang reovirus (virions, intermediate subviral particles [ISVPs], and cores) have been examined by cryoelectron microscopy (cryoEM) and image reconstruction at 27 to 32-A resolution. Analysis of the three-dimensional maps and known biochemical composition allows determination of capsid protein location, globular shape, stoichiometry, quaternary organization, and interactions with adjacent capsid proteins. Comparisons of the virion, ISVP and core structures and examination of difference maps reveal dramatic changes in supra-molecular structure and protein conformation that are related to the early steps of reovirus infection. The intact virion (approximately 850-A diam) is designed for environmental stability in which the dsRNA genome is protected not only by tight sigma 3-mu 1, lambda 2-sigma 3, and lambda 2-mu 1 interactions in the outer capsid but also by a densely packed core shell formed primarily by lambda 1 and sigma 2. The segmented genome appears to be packed in a liquid crystalline fashion at radii < 240 A. Depending on viral growth conditions, virions undergo cleavage by enteric or endosomal/lysosomal proteases, to generate the activated ISVP (approximately 800-A diam). This transition involves the release of an outer capsid layer spanning radii from 360 to 427 A that is formed by 60 tetrameric and 60 hexameric clusters of ellipsoidal subunits of sigma 3. The vertex-associated cell attachment protein, sigma 1, also undergoes a striking change from a poorly visualized, more compact form, to an extended, flexible fiber. This conformational change may maximize interactions of sigma 1 with cell surface receptors. Transcription of viral mRNAs is mediated by the core particle (approximately 600-A diam), generated from the ISVP after penetration and uncoating. The transition from ISVP to core involves release of the 12 sigma 1 fibers and the remaining outer capsid layer formed by 200 trimers of rod-shaped mu 1 subunits that span radii from 306 to 395 A. In the virion and ISVP, flower-shaped pentamers of the lambda 2 protein are centered at the vertices. In the ISVP-to-core transition, domains of the lambda 2 subunits rotate and swing upward and outward to form a turret-like structure extending from radii 305 to 400 A, with a diameter of 184 A, and a central channel 84 A wide. This novel conformational change allows the potential diffusion of substrates for transcription and exit of newly synthesized mRNA segments.(ABSTRACT TRUNCATED AT 400 WORDS)

Three-dimensional visualization of the rotavirus hemagglutinin structure

Three-dimensional structures of a native simian and reassortant rotavirus have been determined by electron cryomicroscopy and computer image processing. The structural features of the native virus confirm that the hemagglutinin spike is a dimer of VP4, substantiated by in vivo radiolabeling studies. Exchange of native VP4 with a bovine strain equivalent results in a poorly infectious reassortant. No VP4 spikes are detected in the three-dimensional reconstruction of the reassortant. The difference map between the two structures reveals a novel large globular domain of VP4 buried within the virion that interacts extensively with the intermediate shell protein, VP6. Our results suggest that assembly of VP4 precedes that of VP7, the major outer shell protein, and that VP4 may play an important role in the receptor recognition and budding process through the rough endoplasmic reticulum during virus maturation.

The p53 protein is an unusually shaped tetramer that binds directly to DNA

We have analyzed the size and structure of native immunopurified human p53 protein. By using a combination of chemical crosslinking, gel filtration chromatography, and zonal velocity gradient centrifugation, we have determined that the predominant form of p53 in such preparations is a tetramer. The behavior of purified p53 in gels and sucrose gradients implies that the protein has an extended shape. Wild-type p53 has been shown to bind specifically to sites in cellular and viral DNA. We show in this study by Southwestern ligand blotting and by analysis of DNA-bound crosslinked p53 that p53 monomers, dimers, and tetramers can bind directly to DNA.

The reovirus cell attachment protein possesses two independently active trimerization domains: basis of dominant negative effects

The reovirus cell attachment protein, sigma 1, is a homotrimer with an N-terminal fibrous tail and a C-terminal globular head. By cotranslating full-length and various truncated sigma 1 proteins in vitro, we show that the N- and C-terminal halves of sigma 1 possess independent trimerization and folding domains. Trimerization of sigma 1 is initiated at the N-terminus by the formation of a "loose," protease-sensitive, three-stranded, alpha-helical coiled coil. This serves to bring the three unfolded C-termini into close proximity to one another, facilitating their subsequent trimerization and cooperative folding. Concomitant with, but independent of, this latter process, the N-terminal fiber further matures into a more stable and protease-resistant structure. The coordinated folding of sigma 1 trimers exemplifies the dominant negative effects of mutant subunits in oligomeric complexes.