Mutations in a CCHC zinc-binding motif of the reovirus sigma 3 protein decrease its intracellular stability

SLC39A5 dysfunction impairs extracellular matrix synthesis in high myopia pathogenesis

High myopia is one of the leading causes of visual impairment worldwide with high heritability. We have previously identified the genetic contribution of SLC39A5 to nonsyndromic high myopia and demonstrated that disease‐related mutations of SLC39A5 dysregulate the TGF‐β pathway. In this study, the mechanisms underlying SLC39A5 involvement in the pathogenesis of high myopia are determined. We observed the morphogenesis and migration abnormalities of the SLC39A5 knockout (KO) human embryonic kidney cells (HEK293) and found a significant injury of ECM constituents. RNA‐seq and qRT‐PCR revealed the transcription decrease in COL1A1, COL2A1, COL4A1, FN1 and LAMA1 in the KO cells. Further, we demonstrated that TGF‐β signalling, the regulator of ECM, was inhibited in SLC39A5 depletion situation, wherein the activation of receptor Smads (R‐Smads) via phosphorylation was greatly blocked. SLC39A5 re‐expression reversed the phenotype of TGF‐β signalling and ECM synthesis in the KO cells. The fact that TGF‐β signalling was zinc‐regulated and that SLC39A5 was identified as a zinc transporter urged us to check the involvement of intracellular zinc in TGF‐β signalling impairment. Finally, we determined that insufficient zinc chelation destabilized Smad proteins, which naturally inhibited TGF‐β signalling. Overall, the SLC39A5 depletion–induced zinc deficiency destabilized Smad proteins, which inhibited the TGF‐β signalling and downstream ECM synthesis, thus contributing to the pathogenesis of high myopia. This discovery provides a deep insight into myopic development.

Closely related reovirus lab strains induce opposite expression of RIG-I/IFN-dependent versus -independent host genes, via mechanisms of slow replication versus polymorphisms in dsRNA binding σ3 respectively

The Dearing isolate of Mammalian orthoreovirus (T3D) is a prominent model of virus-host relationships and a candidate oncolytic virotherapy. Closely related laboratory strains of T3D, originating from the same ancestral T3D isolate, were recently found to exhibit significantly different oncolytic properties. Specifically, the T3DPL strain had faster replication kinetics in a panel of cancer cells and improved tumor regression in an in vivo melanoma model, relative to T3DTD. In this study, we discover that T3DPL and T3DTD also differentially activate host signalling pathways and downstream gene transcription. At equivalent infectious dose, T3DTD induces higher IRF3 phosphorylation and expression of type I IFNs and IFN-stimulated genes (ISGs) than T3DPL. Using mono-reassortants with intermediate replication kinetics and pharmacological inhibitors of reovirus replication, IFN responses were found to inversely correlate with kinetics of virus replication. In other words, slow-replicating T3D strains induce more IFN signalling than fast-replicating T3D strains. Paradoxically, during co-infections by T3DPL and T3DTD, there was still high IRF3 phosphorylation indicating a phenodominant effect by the slow-replicating T3DTD. Using silencing and knock-out of RIG-I to impede IFN, we found that IFN induction does not affect the first round of reovirus replication but does prevent cell-cell spread in a paracrine fashion. Accordingly, during co-infections, T3DPL continues to replicate robustly despite activation of IFN by T3DTD. Using gene expression analysis, we discovered that reovirus can also induce a subset of genes in a RIG-I and IFN-independent manner; these genes were induced more by T3DPL than T3DTD. Polymorphisms in reovirus σ3 viral protein were found to control activation of RIG-I/ IFN-independent genes. Altogether, the study reveals that single amino acid polymorphisms in reovirus genomes can have large impact on host gene expression, by both changing replication kinetics and by modifying viral protein activity, such that two closely related T3D strains can induce opposite cytokine landscapes.

Single Amino Acid Differences between Closely Related Reovirus T3D Lab Strains Alter Oncolytic Potency In Vitro and In Vivo

Little is known about how genetic variations in viruses affect their success as therapeutic agents. The type 3 Dearing strain of Mammalian orthoreovirus (T3D) is undergoing clinical trials as an oncolytic virotherapy. Worldwide, studies on reovirus oncolysis use T3D stocks propagated in different laboratories. Here, we report that genetic diversification among T3D stocks from various sources extensively impacts oncolytic activity. The T3D strain from the Patrick Lee laboratory strain (TD3^PL) showed significantly stronger oncolytic activities in a murine model of melanoma than the strain from the Terence Dermody laboratory (T3D^TD). Overall in vitro replication and cytolytic properties of T3D laboratory strains were assessed by measuring virus plaque size on a panel of human and mouse tumor cells, and results were found to correlate with in vivo oncolytic potency in a melanoma model. T3D^PL produced larger plaques than T3D^TD and than the T3D strain from the ATCC (T3D^ATCC) and from the Kevin Coombs laboratory (T3D^KC). Reassortant and reverse genetics analyses were used to decipher key genes and polymorphisms that govern enhanced plaque size of T3D^PL Five single amino acid changes in the S4, M1, and L3 genome segments of reovirus were each partially correlated with plaque size and when combined were able to fully account for differences between T3D^PL and T3D^TD Moreover, polymorphisms were discovered in T3D^TD that promoted virus replication and spread in tumors, and a new T3D^PL/T3D^TD hybrid was generated with enhanced plaque size compared to that of T3D^PL Altogether, single amino acid changes acquired during laboratory virus propagation can have a large impact on reovirus therapeutic potency and warrant consideration as possible confounding variables between studies.IMPORTANCE The reovirus serotype 3 Dearing (T3D) strain is in clinical trials for cancer therapy. We find that closely related laboratory strains of T3D exhibit large differences in their abilities to replicate in cancer cells in vitro, which correlates with oncolytic activity in a in a murine model of melanoma. The study reveals that five single amino acid changes among three reovirus genes strongly impact reovirus therapeutic potency. In general, the findings suggest that attention should be given to genomic divergence of virus strains during research and optimization for cancer therapy.

HIV-1 NC-induced stress granule assembly and translation arrest are inhibited by the dsRNA binding protein Staufen1

The nucleocapsid (NC) is an N-terminal protein derived from the HIV-1 Gag precursor polyprotein, pr55^Gag NC possesses key functions at several pivotal stages of viral replication. For example, an interaction between NC and the host double-stranded RNA-binding protein Staufen1 was shown to regulate several steps in the viral replication cycle, such as Gag multimerization and genomic RNA encapsidation. In this work, we observed that the overexpression of NC leads to the induction of stress granule (SG) assembly. NC-mediated SG assembly was unique as it was resistant to the SG blockade imposed by the HIV-1 capsid (CA), as shown in earlier work. NC also reduced host cell mRNA translation, as judged by a puromycylation assay of de novo synthesized proteins, and this was recapitulated in polysome profile analyses. Virus production was also found to be significantly reduced. Finally, Staufen1 expression completely rescued the blockade to NC-mediated SG assembly, global mRNA translation as well as virus production. NC expression also resulted in the phosphorylation of protein kinase R (PKR) and eIF2α, and this was inhibited with Staufen1 coexpression. This work sheds light on an unexpected function of NC in host cell translation. A comprehensive understanding of the molecular mechanisms by which a fine balance of the HIV-1 structural proteins NC and CA act in concert with host proteins such as Staufen1 to modulate the host stress response will aid in the development of new antiviral therapeutics.

Piscine orthoreovirus (PRV) ơ3 protein binds dsRNA

Piscine orthoreovirus (PRV) has a double-stranded, segmented RNA genome and belongs to the family Reoviridae. PRV is associated with heart and skeletal muscle inflammation (HSMI) in farmed Atlantic salmon (Salmo salar L.) and cause intraerythrocytic inclusions. The virus is widespread in both wild and farmed salmonid fish in Europe, North- and South America. In mammalian orthoreovirus (MRV), the outer capsid protein ơ3 has dsRNA binding properties, which serve to inhibit the early innate immune response of the host. Important structural motifs and key amino acid residues are conserved between MRV ơ3 and the homologous PRV protein, and we hypothesized that PRV ơ3 binds dsRNA. Gene regions and amino acid residues predicted to be important for dsRNA binding were determined through bioinformatic analysis and investigated functionally following site-directed mutagenesis and the generation of truncated ơ3 variants. Our results provide evidence that the PRV protein ơ3 binds dsRNA in a sequence independent manner, thus sharing this function with MRV ơ3. Although no specific domain solely responsible for dsRNA binding was determined, the results point to residues within a predominantly basic region to be important for this functional property. We conclude that multiple sites are involved in the dsRNA binding activity of PRV ơ3.

Molecular characterization of turkey enteric reovirus S3 gene

The molecular diversity in S3 gene sequences of turkey reovirus (TRV) was determined in poult enteritis syndrome (PES)-affected and apparently healthy turkey poults. Twenty-nine TRV-positive samples (15 from PES-affected flocks and 14 from apparently healthy flocks) were tested using self-designed primers for the S3 gene. Phylogenetic analysis revealed that the TRV S3 sequences of this study clustered in clade III and formed two different groups in this clade. The avian reoviruses from duck and goose formed clade I and those from chickens formed clade II. The clade III TRV sequences had a nucleotide percent identity of 88.9 to 100% among themselves but only of 59.5 to 63.5% and 69.2 to 72.6% with clades I and II, respectively. More amino acid substitutions were present in TRVs from PES-affected flocks than in those from apparently healthy flocks using ATCC VR-818 (AY444912) as a benchmark. All TRVs of this study showed substitutions at positions 244 and 285. The impact of these changes on the virulence of the virus, if any, needs to be studied.

Isolation and genomic characterization of a classical Muscovy duck reovirus isolated in Zhejiang, China

A classical Muscovy reovirus was isolated from a sick Muscovy duck with white necrotic foci in its liver in Zhejiang, China, in 2000. This classical reovirus was propagated in a chicken fibroblast cell line (DF-1) with obvious cytopathic effects. Its genome was 22,967 bp in length, with approximately 51.41% G+C content and 10 dsRNA segments encoding 11 proteins, which formed a 3/3/4 electrophoretic PAGE profile pattern. The length of the genomic segments was similar to those of avian orthoreoviruses (ARV and N-MDRV), ranging from 3959 nt (L1) to 1191nt (S4). All of the segments have the conserved terminal sequences 5'-GCUUUU--UUCAUC-3', and with the exception of the S4 segment, all the genome segments apparently encode one single primary translation product. The genome analysis revealed that the S4 segment of classical MDRV is a bicistronic gene, encoding the overlapping ORFs for p10 and σC but distinct from ARV and N-MDRV/N-GRV, which codes for p10, p18 and σC via the tricistronic S1 segment. A comparative sequence analysis provided evidence indicating extensive sequence divergence between classical MDRV and other avian orthoreoviruses. A phylogenetic analysis based on the RNA-dependent RNA polymerase (RdRp) and the major outer capsid proteins σC was performed. Members of the DRVs in the Avian orthoreovirus species were clustered into two genetic groups (classical MDRV and N-MDRV genotype), and the classical MDRV isolates formed distinct lineages (China and Europe lineages), suggesting that the classical MDRVs isolated in restricted geographical region are evolving by different and independent pathways.

Antigenic analysis monoclonal antibodies against different epitopes of σB protein of Muscovy duck reovirus

σB is one of the major structural proteins of Muscovy duck reovirus (DRV), which is able to induce protective immune response in target birds. Four anti-DRV σB MAbs were identified belong to two distinct epitopes, designated A (1E5, 4E3, and 5D8) and B (2F7) (Liu et al., 2010). To understand antigenic determinants of the σB protein, a set of 20 (P1-P20), partially overlapping and consecutive peptides spanning σB were expressed and then screened by MAbs. With Western blot and enzyme-linked immunosorbent assay (ELISA), two minimal units of the linear epitopes, 19YIRAPACWD27 (epitope B) and 65TDGVCFPHHK74 (epitope A), were identified within N-terminal region of the σB protein. The epitope B was highly conserved among DRV and avian reovirus (ARV) strains through sequence alignment analysis. Immunofluorescence assays (IFA) and ELISA, confirmed that epitope B is a broad group-specific epitope among DRV and ARV. Epitope A could only react with chicken embyonated fibroblast cells (CEF) infected with DRV, but not ARV. However, both peptides have good immunogenicity and could induce antibodies against DRV in BALB/c mice. This report documents the first identification of σB epitopes in the precise locations. The two probes would be useful in the development of discriminating diagnostic kits for DRV and ARV infection.

Characterization of the σB-encoding genes of muscovy duck reovirus: σC–σB-ELISA for antibodies against duck reovirus in ducks

The sigmaB/sigmaC-encoding genes of muscovy duck reovirus (DRV) S12 strain were cloned, sequenced, and expressed in Escherichia coli. The sigmaC-encoding gene of DRV showed only 21-22% identity to that of avian reovirus (ARV) at both nucleotide and amino acid level. The sigmaB-encoding gene of DRV comprised 1163bp with one open reading frame (ORF). The ORF comprised 1104bp and encoded 367 amino acids with a predicted molecular mass of 40.44 kDa. A zinc-binding motif and a basic amino acid motif were found within the predicted amino acid sequence of sigmaB. The identities between the S12 and ARV were 59.3-64.0% and 60.9-62.5%, respectively, at the nucleotide and deduced amino acid levels. Phylogenetic analysis of the sigmaB-encoding gene sequence indicated that S12 separated as a distinct virus relative to other avian strains. The expressed sigmaB/sigmaC fusion proteins in E. coli could be detected, approximately 45 and 50kDa, respectively, by duck anti-reovirus polyclonal serum. In addition, an ELISA (sigmaB-sigmaC-ELISA) using the expressed sigmaB-sigmaC proteins as coating antigen for detection of antibodies to DRV in ducks was developed. In comparison with the virus neutralization test and agar gel immuno-diffusion test (AGID), the sigmaB-sigmaC-ELISA showed perfect specificity and sensitivity. The sigmaB-sigmaC-ELISA did not react with the antisera to other duck pathogens, implying that these two proteins were specific in recognition of DRV antibodies. Taken together, the results demonstrated that sigmaB-sigmaC-ELISA was a sensitive and accurate method for detecting antibodies to DRV.

The use of monoreassortants and reverse genetics to map reovirus lysis of a ras-transformed cell line

Reovirus has been shown to lyse most transformed cells while establishing a persistent or abortive infection in non-transformed cells. Developing methods to identify the reovirus genes associated with oncolysis is an important step toward understanding the mechanisms involved. This report is the first to develop and apply the use of monoreassortants and reverse genetics to identify an individual reovirus gene associated with reovirus oncolysis. Infection with reovirus serotypes 1/Lang, 2/Jones or 3/Dearing of cells transformed with a normal copy of c-Ha-RAS (N1 cells) or with a normal copy of c-Myc (Myc-3 cells), produces large amounts of progeny virus of all three serotypes and results in lysis of both these cell lines. Infection of cells transformed with a mutant c-Ha-RAS gene (T1 cells) with either serotype 1/Lang and 2/Jones results in the production of large amounts of virus and lysis of the cells. In sharp contrast, serotype 3/Dearing virus infection of these cells produced small amounts of virus and resulted in limited lysis of these cells. Using monoreassortants and reverse genetics we exploited this phenotypic difference between the three serotypes to identify a single reovirus gene linked to the preferential lysis of the T1 cells, the S4 gene.

Molecular characterization of a major outer capsid protein encoded by the Threadfin aquareovirus (TFV) gene segment 10 (S10)

Genome segment 10 (S10) of Threadfin aquareovirus (TFV) was cloned, sequenced, analyzed and found to be 987 bp long encoding a protein of 298 aa with a predicted molecular mass of 32.0 kDa. The TFV S10 gene possesses terminal motifs, (5' GTTTTA and ATTCATC 3') which are also conserved in the S6 and S11 TFV gene segments. Sequence comparison revealed that the TFV S10 gene was similar to the Striped bass reovirus (SBR) VP7 outer capsid protein (OCP). A conserved putative zinc-finger motif, CCHC, present in the mammalian reovirus (MRV) delta3 protein, was identified in TFV and other aquareovirus VP7 protein. Phylogenetic analysis of the TFV VP7 protein indicated that TFV is closely related to SBR and Chum salmon reovirus (CSV) and possibly belong to the same species Aquareovirus A as SBR and CSV. The TFV VP7 protein was expressed in E. coli, purified and injected into mice. Serum specific antibodies were generated, however, the serum showed weak neutralizing activity. In contrast, co-incubation of this serum with another serum obtained from mice immunized with another OCP encoded by the TFV S6 gene segment resulted in a highly elevated antibody neutralization titer.

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.

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.

Incorporation of epitope-tagged viral σ3 proteins to reovirus virions

Tagging of viral capsid proteins is a powerful tool to study viral assembly; it also raises the possibility of using viral particles to present exogenous epitopes in vaccination or gene therapy strategies. The ability of reoviruses to induce strong mucosal immune response and their large host range and low pathogenicity in humans are some of the advantages of using reoviruses in such applications. In the present study, the feasibility of introducing foreign epitopes, "tags", to the σ3 protein, a major component of the reovirus outer capsid, was investigated. Among eight different positions, the amino-terminal end of the protein appeared as the best location to insert exogenous sequences. Additional amino acids at this position do not preclude interaction with the µ1 protein, the other major constituent of the viral outer capsid, but strongly interfere with µ1 to µ1C cleavage. Nevertheless, the tagged σ3 protein was still incorporated to virions upon recoating of infectious subviral particles to which authentic σ3 protein was removed by proteolysis, indicating that µ1 cleavage is not a prerequisite for outer capsid assembly. The recently published structure of the σ3-µ1 complex suggests that the amino-terminally inserted epitope could be exposed at the outer surface of viral particles.Key words: reovirus, recombinant viruses, epitope tagging, vaccination vectors, virus assembly.

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.

Sequence analysis of the S3 gene from a turkey reovirus

The deduced sigma-2 protein sequence from the S3 gene segment of a novel turkey reovirus, designated NC98, isolated from the bursa of birds exhibiting poult enteritis and mortality syndrome was determined. The isolate, serologically distinct from other avian reoviruses, was isolated in turkey embryo kidney cells and RNA was purified for cDNA synthesis. Oligonucleotide primers were designed based on conserved avian S3 nucleotide sequence data. The NC98 S3 open reading frame comprised 1,101 base pairs and encoded 366 amino acids with a predicated molecular mass of 40.5 kDa. Although the S3 nucleotide sequence from several chicken isolates share at least 86% identity, they share only 64% with the NC98 turkey isolate. Interestingly, the S3 nucleotide sequence from a muscovy duck reovirus shares 55% identity with NC98 and 53% identity with chicken isolates. As observed in other avian reovirus sigma2 protein sequences, a zinc-binding motif and double-stranded RNA binding domain were found within the predicted amino acid sequence of NC98. Phylogenetic analysis of the deduced sigma2 sequence demonstrated that NC98 separated as a distinct virus relative to other avian strains. The results of this study indicate that NC98 is a novel turkey reovirus that shares limited genomic sequence identity to isolates of chicken and duck origin and should be considered a separate virus species within subgroup 2 of the Orthoreovirus genus.

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

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

The Cys(3)-His(1) motif of the respiratory syncytial virus M2-1 protein is essential for protein function

The M2 gene of respiratory syncytial (RS) virus has two open reading frames (ORFs). ORF1 encodes a 22-kDa protein termed M2-1. The M2-1 protein contains a Cys(3)-His(1) motif (C-X(7)-C-X(5)-C-X(3)-H) near the amino terminus. This motif is conserved in all human, bovine, and ovine strains of RS virus. A similar motif found in the mammalian transcription factor Nup475 has been shown to bind zinc. The M2-1 protein of human RS virus functions as a transcription factor which increases polymerase processivity, and it enhances readthrough of intergenic junctions during RS virus transcription, thereby acting as a transcription antiterminator. The M2-1 protein also interacts with the nucleocapsid protein. We examined the effects of mutations of cysteine and histidine residues predicted to coordinate zinc in the Cys(3)-His(1) motif on transcription antitermination and N protein binding. We found that mutating the predicted zinc-coordinating residues, the cysteine residues at amino acid positions 7 and 15 and the histidine residue at position 25, prevented M2-1 from enhancing transcriptional readthrough. In contrast, mutations of amino acids within this motif not predicted to coordinate zinc had no effect. Mutations of the predicted zinc-coordinating residues in the Cys(3)-His(1) motif also prevented M2-1 from interacting with the nucleocapsid protein. One mutation of a noncoordinating residue in the motif which did not affect readthrough during transcription, E10G, prevented interaction with the nucleocapsid protein. This suggests that M2-1 does not require interaction with the nucleocapsid protein in order to function during transcription. Analysis of the M2-1 protein in reducing sodium dodecyl sulfate-polyacrylamide gels revealed two major forms distinguished by their mobilities. The slower migrating form was shown to be phosphorylated, whereas the faster migrating form was not. Mutations in the Cys(3)-His(1) motif caused a change in distribution of the M2-1 protein from the slower to the faster migrating form. The data presented here show that the Cys(3)-His(1) motif of M2-1 is essential for maintaining the functional integrity of the protein.

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

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

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

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

Regulated, stable expression and nuclear presence of reovirus double-stranded RNA-binding protein sigma3 in HeLa cells

Reovirus genome segment S4 codes for polypeptide sigma3, a major outer capsid component of virions and a double-stranded RNA (dsRNA)-binding protein implicated in viral cytopathogenesis. We have constructed a stable HeLa cell line (S4tTA) that produces functional sigma3 under tetracycline transactivator control. In the absence of tetracycline, S4tTA cells synthesized stable dsRNA-binding sigma3 that accumulated in the nucleus as well as in the cytoplasm. However, in induced S4tTA cells also expressing reovirus outer shell polypeptide mu1/mu1C, migration of sigma3 into the nucleus was blocked, probably as a result of formation of a complex with mu1/mu1C which was exclusively in the cytoplasm. Mutant analyses indicated a correlation between dsRNA-binding activity and nuclear entry of sigma3, suggesting an additional role(s) for this capsid protein in virus-cell interactions.

Posttranslational regulation of a Leishmania HEXXH metalloprotease (gp63). The effects of site-specific mutagenesis of catalytic, zinc binding, N-glycosylation, and glycosyl phosphatidylinositol addition sites on N-terminal end cleavage, intracellular stability, and extracellular exit

Leishmanolysin (EC 3.4.24.36) (gp63) is a HEXXH metalloprotease, encoded by multicopied genes in Leishmania and implicated in the infectivity of these parasitic protozoa. We examined posttranslational regulation of gp63 expression by site-specific mutagenesis of the predicted catalytic/zinc-binding sites in the H264EXXH motif, the potential sites of N-glycosylation and glycosyl phosphatidylinositol addition. Mutant and wild-type genes were cloned into a Leishmania-specific vector for transfecting a deficient variant, which produced gp63 approximately 20-fold less than wild-type cells. The selective conditions chosen fully restored this deficiency in transfectants with the wild-type gene. Under these conditions, all transfectants were found comparable in both the plasmid copy number per cell and elevation of gp63 transcripts. Mutant and wild-type products in the transfectants were then compared quantitatively and qualitatively by specific immunologic and protease assays. The results indicate the following. 1) Glu-265 in the HEXXH motif is indispensable for the catalytic activity of gp63. The propeptide of the inactive mutant products was cleaved, suggestive of a non-intramolecular event. 2) Substitution of either His residue in HEXXH leads to apparent intracellular degradation of the mutant products, pointing to a role for zinc binding in in vivo stability of gp63. 3) The three potential sites of N-glycosylation at Asn-300, Asn-407, and Asn-534 are all utilized and contribute to intracellular stability of gp63. 4) Substitution of Asn-577 causes release of all mutant products, indicative of its specificity as a glycosyl phosphatidylinositol addition site for membrane anchoring of gp63. It is suggested that expression of gp63 as a functional protease is regulated by these posttranslational modification pathways.

Site-directed mutagenesis of the double-stranded RNA binding domain of bacterially-expressed sigma 3 reovirus protein

The affinity of the reovirus sigma 3 protein for double-stranded RNA (dsRNA) is well established, and efforts have been made to identify the amino acids involved in this property. In the present study, we further examined the importance of two basic amino acids motifs, located in the carboxy-terminal third of the protein. Mutants, previously characterized in COS cells, were expressed in bacterial cells using the pET expression system. The capability of the different mutants to interact with dsRNA was then determined by the binding of radiolabeled dsRNA to proteins resolved by SDS-polyacrylamide gel electrophoresis and transferred to nitrocellulose filters. It appears that the most carboxy-terminal motif is absolutely required for the binding but the second motif also contributes to this property. However, only the carboxy-terminal motif is required for normal binding upon removal of the amino-terminal domain of the protein by proteolytic cleavage, a procedure previously shown to increase dsRNA-binding. The basic charges in both motifs are important, while breaking of their potential to adopt an alpha helical configuration does not affect binding efficiency. Furthermore, alanine substitution of a single basic amino acid in the carboxy-terminal motif can be sufficient to strongly reduce the binding of dsRNA to the protein. Altogether, these data suggest that basic amino acids of the sigma 3 carboxy-terminal motif are directly involved in dsRNA binding, while the other basic motif may contribute by preventing an inhibitory effect of the amino-terminal portion of the protein.

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

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

Two basic motifs of reovirus sigma 3 protein are involved in double-stranded RNA binding

It has been reported that the sigma 3 protein of reovirus can exert an inhibitory effect on the cellular double-stranded RNA (dsRNA) activated protein kinase. Activation of this kinase is thought to be a general mechanism mediating a cellular antiviral response. This enzyme can also be activated upon transfection, resulting in translational inhibition of plasmid-encoded mRNAs. sigma 3 has an affinity for dsRNA postulated to be responsible for antikinase activity. In the present study, site-directed mutagenesis was performed on two basic regions previously suggested as dsRNA-binding motifs and the mutant sigma 3 proteins were then expressed in COS cells. These experiments revealed that both motifs are involved in sigma 3 attachment to RNA. Expression of the mutants lacking RNA-binding capability is stimulated by coexpression of another dsRNA-binding protein, the E3L vaccinia virus protein. These results support a model in which the attachment to dsRNA is directly responsible for the trans-stimulating effect of sigma 3 on expression of cotransfected genes.

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

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