A copper(ii) peptide helicate selectively cleaves DNA replication foci in mammalian cells

The use of copper-based artificial nucleases as potential anticancer agents has been hampered by their poor selectivity in the oxidative DNA cleavage process. An alternative strategy to solve this problem is to design systems capable of selectively damaging noncanonical DNA structures that play crucial roles in the cell cycle. We designed an oligocationic CuII peptide helicate that selectively binds and cleaves DNA three-way junctions (3WJs) and induces oxidative DNA damage via a ROS-mediated pathway both in vitro and in cellulo, specifically at DNA replication foci of the cell nucleus, where this DNA structure is transiently generated. To our knowledge, this is the first example of a targeted chemical nuclease that can discriminate with high selectivity 3WJs from other forms of DNA both in vitro and in mammalian cells. Since the DNA replication process is deregulated in cancer cells, this approach may pave the way for the development of a new class of anticancer agents based on copper-based artificial nucleases.

Nanoparticle- and Microparticle-Based Vaccines against Orbiviruses of Veterinary Importance

Bluetongue virus (BTV) and African horse sickness virus (AHSV) are widespread arboviruses that cause important economic losses in the livestock and equine industries, respectively. In addition to these, another arthropod-transmitted orbivirus known as epizootic hemorrhagic disease virus (EHDV) entails a major threat as there is a conducive landscape that nurtures its emergence in non-endemic countries. To date, only vaccinations with live attenuated or inactivated vaccines permit the control of these three viral diseases, although important drawbacks, e.g., low safety profile and effectiveness, and lack of DIVA (differentiation of infected from vaccinated animals) properties, constrain their usage as prophylactic measures. Moreover, a substantial number of serotypes of BTV, AHSV and EHDV have been described, with poor induction of cross-protective immune responses among serotypes. In the context of next-generation vaccine development, antigen delivery systems based on nano- or microparticles have gathered significant attention during the last few decades. A diversity of technologies, such as virus-like particles or self-assembled protein complexes, have been implemented for vaccine design against these viruses. In this work, we offer a comprehensive review of the nano- and microparticulated vaccine candidates against these three relevant orbiviruses. Additionally, we also review an innovative technology for antigen delivery based on the avian reovirus nonstructural protein muNS and we explore the prospective functionality of the nonstructural protein NS1 nanotubules as a BTV-based delivery platform.

Selective recognition of A/T-rich DNA 3-way junctions with a three-fold symmetric tripeptide

Non-canonical DNA structures, particularly 3-Way Junctions (3WJs) that are transiently formed during DNA replication, have recently emerged as promising chemotherapeutic targets. Here, we describe a new approach to target 3WJs that relies on the cooperative and sequence-selective recognition of A/T-rich duplex DNA branches by three AT-Hook peptides attached to a three-fold symmetric and fluorogenic 1,3,5-tristyrylbenzene core.

Rational Design of Copper(II)-Uracil Nanoprocessed Coordination Polymers to Improve Their Cytotoxic Activity in Biological Media

This work is focused on the rational structural design of two isostructural Cu(II) nano-coordination polymers (NCPs) with uracil-1-acetic acid (UAcOH) (CP1n) and 5-fluorouracil-1-acetic acid (CP2n). Suitable single crystals for X-ray diffraction studies of CP1 and CP2 were prepared under hydrothermal conditions, enabling their structural determination as 1D-CP ladder-like polymeric structures. The control of the synthetic parameters allows their processability into water colloids based on nanoplates (CP1n and CP2n). These NCPs are stable in water at physiological pHs for long periods. However, interestingly, CP1n is chemically altered in culture media. These transformations provoke the partial release of its building blocks and the formation of new species, such as [Cu(UAcO)₂(H₂O)₄]·2H₂O (Cu(II)-complex), and species corresponding to the partial reduction of the Cu(II) centers. The cytotoxic studies of CP1n versus human pancreatic adenocarcinoma and human uveal melanoma cells show that CP1n produces a decrease in the cell viability, while their UAcOH and Cu(II)-complex are not cytotoxic under similar conditions. The copper reduction species detected in the hydrolysis of CP1n are closely related to the formation of the reactive oxygen species (ROS) detected in the cytotoxic studies. These results prompted us to prepare CP2n that was designed to improve the cytotoxicity by the substitution of UAcO by 5-FUAcO, taking into account the anticancer activity of the 5-fluorouracil moiety. The new CP2n has a similar behavior to CP1n both in water and in biological media. However, its subtle structural differences are vital in improving its cytotoxic activity. Indeed, the release during the hydrolysis of species containing the 5-fluorouracil moiety provokes a remarkable increase in cellular toxicity and a significant increase in ROS species formation.

Dynamic Stereoselection of Peptide Helicates and Their Selective Labeling of DNA Replication Foci in Cells*

Although largely overlooked in peptide engineering, coordination chemistry offers a new set of interactions that opens unexplored design opportunities for developing complex molecular structures. In this context, we report new artificial peptide ligands that fold into chiral helicates in the presence of labile metal ions such as Fe^II and Co^II . Heterochiral β-turn-promoting sequences encode the stereoselective folding of the peptide ligands and define the physicochemical properties of their corresponding metal complexes. Circular dichroism and NMR spectroscopy in combination with computational methods allowed us to identify and determine the structure of two isochiral ΛΛ-helicates, folded as topological isomers. Finally, in addition to the in-vitro characterization of their selective binding to DNA three-way junctions, cell-microscopy experiments demonstrated that a rhodamine-labeled Fe^II helicate was internalized and selectively stains DNA replication factories in functional cells.

MitoBlue as a tool to analyze the mitochondria-lysosome communication

MitoBlue is a fluorescent bisamidine that can be used to easily monitor the changes in mitochondrial degradation processes in different cells and cellular conditions. MitoBlue staining pattern is exceptional among mitochondrial dyes and recombinant fluorescent probes, allowing the dynamic study of mitochondrial recycling in a variety of situations in living cells. MitoBlue is a unique tool for the study of these processes that will allow the detailed characterization of communication between mitochondria and lysosomes.

Cross-protective immune responses against African horse sickness virus after vaccination with protein NS1 delivered by avian reovirus muNS microspheres and modified vaccinia virus Ankara

African horse sickness virus (AHSV) is an insect-borne pathogen that causes acute disease in horses and other equids. In an effort to improve the safety of currently available vaccines and to acquire new knowledge about the determinants of AHSV immunogenicity, new generation vaccines are being developed. In this work we have generated and tested a novel immunization approach comprised of nonstructural protein 1 (NS1) of AHSV serotype 4 (AHSV-4) incorporated into avian reovirus muNS protein microspheres (MS-NS1) and/or expressed using recombinant modified vaccinia virus Ankara vector (MVA-NS1). The protection conferred against AHSV by a homologous MS-NS1 or heterologous MS-NS1 and MVA-NS1 prime/boost was evaluated in IFNAR (-/-) mice. Our results indicate that immunization based on MS-NS1 and MVA-NS1 afforded complete protection against the infection with homologous AHSV-4. Moreover, priming with MS-NS1 and boost vaccination with MVA-NS1 (MS-MVA-NS1) triggered NS1 specific cytotoxic CD8 + T cells and prevented AHSV disease in IFNAR (-/-) mice after challenge with heterologous serotype AHSV-9. Cross-protective immune responses are highly important since AHS can be caused by nine different serotypes, which means that a universal polyvalent vaccination would need to induce protective immunity against all serotypes.

Selective G-quadruplex binding by oligoarginine-Ru(dppz) metallopeptides

We demonstrate that both the R₈ functionalization and its interplay with the ancillary ligand have and an important role in the G-quadruplex recognition process by Ru(dppz) metallopeptides.

Membrane-disrupting iridium(iii) oligocationic organometallopeptides

A series of oligoarginine peptide derivatives containing cyclometallated iridium(iii) units display remarkable cytotoxicity, comparable to that of cisplatin. In vitro studies with unilamellar vesicles support a membrane-disrupting mechanism of action.

MitoBlue: a nontoxic and photostable blue-emitting dye that selectively labels functional mitochondria

We report the discovery of a fluorogenic dye, N¹,N³-di(2-aminidonaphthalen-6-yl) propane-1,3-diamine, MitoBlue, which selectively stains functional mitochondria while displaying low toxicity, bright blue emission, and high resistance to photobleaching. Additionally, we show that a biotin-labeled MitoBlue derivative can be used as a handle for the delivery of streptavidin-tagged species to the mitochondria.

Avian reovirus-triggered apoptosis enhances both virus spread and the processing of the viral nonstructural muNS protein

Avian reovirus non-structural protein muNS is partially cleaved in infected chicken embryo fibroblast cells to produce a 55-kDa carboxyterminal protein, termed muNSC, and a 17-kDa aminoterminal polypeptide, designated muNSN. In this study we demonstrate that muNS processing is catalyzed by a caspase 3-like protease activated during the course of avian reovirus infection. The cleavage site was mapped by site directed mutagenesis between residues Asp-154 and Ala-155 of the muNS sequence. Although muNS and muNSC, but not muNSN, are able to form inclusions when expressed individually in transfected cells, only muNS is able to recruit specific ARV proteins to these structures. Furthermore, muNSC associates with ARV factories more weakly than muNS, sigmaNS and lambdaA. Finally, the inhibition of caspase activity in ARV-infected cells does not diminish ARV gene expression and replication, but drastically reduces muNS processing and the release and dissemination of progeny viral particles.

•

Avian reovirus-triggered apoptosis promotes cleavage of the viral nonstructural muNS protein.

•

muNS processing is catalyzed by a caspase 3-like protease activated during avian reovirus infection.

•

Cleavage occurs between residues Asp-154 and Ala-155 of the muNS sequence.

•

Avian reovirus-induced apoptosis enhances the release and dissemination of progeny viral particles.

Programmed stereoselective assembly of DNA-binding helical metallopeptides

We have applied solid phase peptide synthesis methods for the construction of peptide ligands that coordinate Fe(ii) ions and fold into chiral peptide helicates that show great affinity and chiral selectivity for three-way DNA junctions and promising cell-internalization properties.

Custom-fit ruthenium(II) metallopeptides: a new twist to DNA binding with coordination compounds

A new bipyridine building block has been used for the solid-phase synthesis of dinuclear DNA-binding ruthenium(II) metallopeptides. Detailed spectroscopic studies suggest that these compounds bind to the DNA by insertion into the DNA minor groove. Moreover, the potential of the solid-phase peptide synthesis approach is demonstrated by the straightforward synthesis of an octaarginine derivative that shows effective cellular internalization and cytotoxicity linked with strong DNA interaction, as evidenced by steady-state fluorescence spectroscopy and AFM studies.

Different intracellular distribution of avian reovirus core protein sigmaA in cells of avian and mammalian origin

A comparative analysis of the intracellular distribution of avian reovirus (ARV) core protein sigmaA in cells of avian and mammalian origin revealed that, whereas the viral protein accumulates in the cytoplasm and nucleolus of avian cells, most sigmaA concentrates in the nucleoplasm of mammalian cells in tight association with the insoluble nuclear matrix fraction. Our results further showed that sigmaA becomes arrested in the nucleoplasm of mammalian cells via association with mammalian cell-specific factors and that this association prevents nucleolar targeting. Inhibition of RNA polymerase II activity, but not of RNA polymerase I activity, in infected mammalian cells induces nucleus-to-cytoplasm sigmaA translocation through a CRM1- and RanGTP-dependent mechanism, yet a heterokaryon assay suggests that sigmaA does not shuttle between the nucleus and cytoplasm. The scarcity of sigmaA in cytoplasmic viral factories of infected mammalian cells could be one of the factors contributing to limited ARV replication in mammalian cells.

In vivo light-driven DNA binding and cellular uptake of nucleic acid stains

Chemical derivatization of nucleic stains such as ethidium bromide or DAPI with tailored, photoresponsive caging groups, allows for "on demand" spatiotemporal control of their in vivo nucleic acid binding, as well as for improving their cellular uptake. This effect was particularly noteworthy for a nitro-veratryloxycarbonyl-caged derivative of ethidium bromide that, in contrast with the parent stain, is effectively internalized into living cells. The activation strategy works in light-accessible, therapeutically relevant settings, such as human retinas, and can even be applied for the release of active compounds in the eyes of living mice.

Avian and mammalian reoviruses use different molecular mechanisms to synthesize their {micro}NS isoforms

Previous reports revealed that the M3 gene of both avian and mammalian reoviruses express two isoforms of the non-structural protein μNS in infected cells. The larger isoforms initiate translation at the AUG codon closest to the 5' end of their respective m3 mRNAs, and were therefore designated μNS. In this study we have performed experiments to identify the molecular mechanisms by which the smaller μNS isoforms are generated. The results of this study confirmed the previous findings indicating that the smaller mammalian reovirus μNS isoform is a primary translation product, the translation of which is initiated at the internal AUG-41 codon of mammalian reovirus m3 mRNA. Our results further revealed that the smaller avian reovirus μNS isoform originates from a specific post-translational cleavage site near the amino terminus of μNS. This cleavage produces a 55 kDa carboxy-terminal protein, termed μNSC, and a 17 kDa amino-terminal polypeptide, designated μNSN. These results allowed us to extend the known avian reovirus protein-encoding capacity to 18 proteins, 12 of which are structural proteins and six of which are non-structural proteins. Our finding that avian and mammalian reoviruses use different mechanisms to express their μNSC isoforms suggests that these isoforms are important for reovirus replication.

Crystallization of the avian reovirus double-stranded RNA-binding and core protein sigmaA

The avian reovirus protein sigmaA plays a dual role: it is a structural protein forming part of the transcriptionally active core, but it has also been implicated in the resistance of the virus to interferon by strongly binding double-stranded RNA and thus inhibiting the double-stranded RNA-dependent protein kinase. The sigmaA protein has been crystallized from solutions containing ammonium sulfate at pH values around 6. Crystals belonging to space group P1, with unit-cell parameters a = 103.2, b = 129.9, c = 144.0 A, alpha = 93.8, beta = 105.1, gamma = 98.2 degrees were grown and a complete data set has been collected to 2.3 A resolution. The self-rotation function suggests that sigmaA may form symmetric arrangements in the crystals.

Early steps in avian reovirus morphogenesis

Avian reoviruses are important pathogens that may cause considerable economic losses in poultry farming. Their genome expresses at least eight structural and four nonstructural proteins, three of them encoded by the S1 gene. These viruses enter cells by receptor-mediated endocytosis, and acidification of virus-containing endosomes is necessary for the virus to uncoat and release transcriptionally active cores into the cytosol. Avian reoviruses replicate within cytoplasmic inclusions of globular morphology, termed viral factories, which are not microtubule-associated, and which are formed by the nonstructural protein muNS. This protein also mediates the association of some viral proteins (but not of others) with inclusions, suggesting that the recruitment of viral proteins into avian reovirus factories has specificity. Avian reovirus morphogenesis is a complex and temporally controlled process that takes place exclusively within viral factories of infected cells. Core assembly takes place within the first 30 min after the synthesis of their protein components, and fully formed cores are then coated by outer-capsid polypeptides over the next 30 min to generate mature infectious reovirions. Based on data from avian reovirus studies and on results reported for other members of the Reoviridae family, we present a model for avian reovirus gene expression and morphogenesis.

Structure of the carboxy-terminal receptor-binding domain of avian reovirus fibre sigmaC

Avian reovirus fibre, a homo-trimer of the sigmaC protein, is responsible for primary host cell attachment. The protein expressed in bacteria forms elongated fibres comprised of a carboxy-terminal globular head domain and a slender shaft, and partial proteolysis yielded a carboxy-terminal protease-stable domain that was amenable to crystallisation. Here, we show that this fragment retains receptor-binding capability and report its structure, solved using two-wavelength anomalous diffraction and refined using data collected from three different crystal forms at 2.1 angstroms, 2.35 angstroms and 3.0 angstroms resolution. The carboxy-terminal globular domain has a beta-barrel fold with the same overall topology as the mammalian reovirus fibre (sigma1). However, the monomers of the sigmaC trimer show a more splayed-out arrangement than in the sigma1 structure. Also resolved are two triple beta-spiral repeats of the shaft or stalk domain. The presence in the sequence of heptad repeats amino-terminal to these triple beta-spiral repeats suggests that the unresolved portion of the shaft domain contains a triple alpha-helical coiled-coil structure. Implications for the function and stability of the sigmaC protein are discussed.

High Affinity, Sequence Specific DNA Binding by Synthetic Tripyrrole-Peptide Conjugates

Linking the basic region of a bZIP transcription factor to a distamycin-like tripyrrole peptide by means of a nitrogen-containing tether produces a hybrid capable of high-affinity recognition of specific, designated DNA sequences. The importance of the nitrogen in the tether is shown by the considerable reduction in affinity (more than 10-fold) caused by its replacement with an ether linkage. Attachment of an aminopropyl chain on the pyrrole adjacent to the pyrrole bearing the nitrogen-containing tether increases affinity approximately one order of magnitude. These results confirm that a suitable location of protonated amine groups on designed DNA-binding peptides provides for higher affinities, most probably because of the generation of salt bridged contacts with the phosphodiester backbone.

Crystallization of the C-terminal globular domain of avian reovirus fibre

Avian reovirus fibre, a homotrimer of the sigmaC protein, is responsible for primary host-cell attachment. Using the protease trypsin, a C-terminal sigmaC fragment containing amino acids 156-326 has been generated which was subsequently purified and crystallized. Two different crystal forms were obtained, one grown in the absence of divalent cations and belonging to space group P6(3)22 (unit-cell parameters a = 75.6, c = 243.1 A) and one grown in the presence of either zinc or cadmium sulfate and belonging to space group P321 (unit-cell parameters a = 74.7, c = 74.5 A and a = 73.1, c = 69.9 A for the Zn(II)- and Cd(II)-grown crystals, respectively). The first crystal form diffracted synchrotron radiation to 3.0 A resolution and the second form to 2.2-2.3 A. Its closest related structure, the C-terminal fragment of mammalian reovirus fibre, has only 18% sequence identity and molecular-replacement attempts were unsuccessful. Therefore, a search is under way for suitable heavy-atom derivatives and attempts are being made to grow protein crystals containing selenomethionine instead of methionine.

Characterization of the nucleic acid-binding activity of the avian reovirus non-structural protein σNS

The avian reovirus non-structural proteinσNS has previously been shown to bind single-stranded (ss) RNAin vitroin a sequence-independent manner. The results of the present study further reveal thatσNS binds poly(A), poly(U) and ssDNA, but not poly(C), poly(G) or duplex nucleic acids, suggesting thatσNS has some nucleotide-sequence specificity for ssRNA binding. The current findings also show thatσNS is present in large ribonucleoprotein complexes in the cytoplasm of avian reovirus-infected cells, indicating that it exists in intimate association with ssRNAsin vivo. Removal of RNA from the complexes generates aσNS protein form that sediments between 4·5 and 7 S, suggesting that RNA-freeσNS associates into small oligomers. Expression and purification of recombinantσNS in insect cells allowed us to generate specific antibodies and to perform a variety of assays. The results of these assays revealed that: (i) RNA-freeσNS exists as homodimers and homotrimers; (ii) the minimum RNA size forσNS binding is between 10 and 20 nt; (iii)σNS does not have a preference for viral mRNA sequences; and (iv) its RNA-binding activity is conformation-dependent. Baculovirus expression of point and deletionσNS mutants in insect cells showed that the five conserved basic amino acids that are important for RNA binding and ribonucleoprotein-complex formation are dispersed throughout the entireσNS sequence, suggesting that this protein binds ssRNA through conformational domains. Finally, the properties of the avian reovirus proteinσNS are compared with those of its mammalian reovirus counterpart.

The second open reading frame of the avian reovirus S1 gene encodes a transcription-dependent and CRM1-independent nucleocytoplasmic shuttling protein

It was previously shown that the second open reading frame of the avian reovirus S1 gene encodes a 146-amino-acid nonstructural protein, designated p17, which has no known function and no sequence similarity to other known proteins. The results presented in this report demonstrate that p17 accumulates in the nucleoplasm of infected and transfected cells. An examination of the deduced amino acid sequence of p17 revealed the presence of a putative monopartite nuclear localization signal (NLS) between residues 119 and 128. Mutagenesis analysis revealed both that this sequence is indeed a functional NLS and that two of its basic residues are critical for the normal nuclear distribution of p17. An interspecies heterokaryon assay further showed that p17 shuttles continuously between the nucleus and the cytoplasm and that this activity is restricted to its NLS-containing C-terminal tail. Finally, an analysis of the intracellular distribution of p17 in the presence of inhibitors of both RNA polymerase II and CRM1 further revealed that the nucleocytoplasmic distribution of p17 is coupled to transcriptional activity and that the viral protein exits the nucleus via a CRM1-independent pathway.

Avian reovirus morphogenesis occurs within viral factories and begins with the selective recruitment of sigmaNS and lambdaA to microNS inclusions

We have recently shown that the avian reovirus non-structural protein microNS forms cytoplasmic inclusions in transfected cells and recruits sigmaNS to these structures. In the present study we further demonstrate that microNS mediates the association of the major core protein lambdaA, but not of sigmaA or sigmaC, with inclusions, indicating that the recruitment of viral proteins into avian reovirus factories has specificity. Thus, some proteins appear to be initially recruited to factories by association with microNS, whereas others are recruited subsequently through interaction with as-yet-unknown factors. We next used metabolic pulse-chase radiolabeling combined with cell fractionation and antibody immunoprecipitation to study the recruitment of newly synthesized viral polypeptides into viral factories and virus particles. The results of this combined approach revealed that avian reovirus morphogenesis is a complex and temporally controlled process that takes place exclusively within globular viral factories that are not microtubule-associated. Our findings further suggest that cores are assembled within the first 30 minutes after the synthesis of their polypeptide components, and that reovirion morphogenesis is completed over the next 30 minutes by the subsequent addition of outer capsid proteins.

Avian reovirus nonstructural protein microNS forms viroplasm-like inclusions and recruits protein sigmaNS to these structures

The M3 genome segment of avian reovirus 1733, which encodes the nonstructural protein microNS, is 1996 nucleotides long and contains a long open reading frame that is predicted to encode a polypeptide of 635 amino acid residues. Examination of the deduced amino acid sequence of microNS revealed the presence of two regions near its carboxyl terminus with a high probability of forming alpha-helical coiled coils. Expression of the M3 gene in both infected and transfected cells revealed that this gene specifies two protein isoforms that are recognized by a microNS-specific antiserum. Only the larger microNS isoform, but not the smaller one, interacts with the nonstructural protein sigmaNS in infected cells, suggesting that the two isoforms play different roles during avian reovirus infection. In the second part of this study, we show that microNS and the nonstructural protein sigmaNS colocalize throughout the viral life cycle in large and small phase-dense globular cytoplasmic inclusions, which are believed to be the sites of viral replication and assembly. Individual expression of these proteins in transfected cells of avian and mammalian origin revealed that while microNS is able to form inclusions in the absence of other viral proteins, sigmaNS distributes diffusely throughout the cytoplasm in the absence of microNS. These data suggest that microNS is the minimal viral factor required for inclusion formation during avian reovirus infection. On the other hand, our findings that sigmaNS associates with microNS in infected cells, and that sigmaNS colocalizes with microNS in viroplasm-like inclusions when the two proteins are coexpressed in transfected cells, suggest that microNS mediates the association of sigmaNS to inclusions in avian reovirus-infected cells.

Evidence that avian reovirus sigmaA protein is an inhibitor of the double-stranded RNA-dependent protein kinase

The results of a previous study demonstrated that avian reovirus is highly resistant to the antiviral effects of interferon and suggested that the double-stranded RNA (dsRNA)-binding sigmaA protein might play an important role in that resistance. To gather more evidence on the interferon-inhibitory activity of sigmaA protein, its gene was cloned into the prokaryotic maltose-binding protein (MBP) gene fusion vector pMalC and into the recombinant vaccinia virus WRS2. The two recombinant sigmaA proteins displayed a dsRNA-binding affinity similar to that of sigmaA protein synthesized in avian reovirus-infected cells. Interestingly, MBP-sigmaA, but not MBP, was able to relieve the translation-inhibitory activity of dsRNA in reticulocyte lysates by blocking the activation of endogenous dsRNA-dependent enzymes. In addition, transient expression of sigmaA protein in HeLa cells rescued gene expression of a vaccinia virus mutant lacking the E3L gene, and insertion of the sigmaA-encoding gene into vaccinia virus conferred protection for the virus against interferon in chicken cells. Further studies demonstrated that expression of recombinant sigmaA in mammalian cells interfered with dsRNA-dependent protein kinase (PKR) function. From these results we conclude that sigmaA is capable of reversing the interferon-induced antiviral state by down-regulating PKR activity in a manner similar to other virus-encoded dsRNA-binding proteins.

Modification of late membrane permeability in avian reovirus-infected cells: viroporin activity of the S1-encoded nonstructural p10 protein

Infection of chicken embryo fibroblasts by avian reovirus induces an increase in the permeability of the host plasma membrane at late, but not early, infection times. The absence of permeability changes at early infection times, as well as the dependence of late membrane modification on both viral protein synthesis and an active exocytic route, suggest that a virus-encoded membrane protein is required for avian reovirus to permeabilize cells. Further studies revealed that expression of nonstructural p10 protein in bacterial cells arrested cell growth and enhanced membrane permeability. Membrane leakiness was also observed following transient expression of p10 in BSC-40 monkey cells. Both its permeabilizing effect and the fact that p10 shares several structural and physical characteristics with other membrane-active viral proteins indicate that p10 is an avian reovirus viroporin. Furthermore, the fusogenic extracellular NH(2)-terminal domain of p10 appears to be dispensable for permeabilizing activity, because its deletion entirely abolished the fusogenic activity of p10, without affecting its ability to associate with cell membranes and to enhance membrane permeability. Similar properties have reported previously for immunodeficiency virus type I transmembrane glycoprotein gp41. Thus, like gp41, p10 appears to be a multifunctional protein that plays key roles in virus-host interaction.

Cloning, expression, and characterization of avian reovirus guanylyltransferase

We have cloned and sequenced the L3 genome segment of avian reovirus strain 1733, which specifies the viral guanylyltransferase protein, lambdaC. The L3 gene is 3907 nucleotides long and encodes, in a single large open-reading frame, a polypeptide of 1285 amino acid residues, with a calculated M(r) of 142.2 kDa. Expression of this gene in a baculovirus/insect cell system produced a recombinant protein that comigrated with reovirion lambdaC and reacted with anti-reovirus polyclonal serum in a Western blot assay. Incubation of recombinant lambdaC with GTP led to the formation GMP-lambdaC complex via a phosphoamide linkage. Interestingly, a 42-kDa amino-terminal proteolytic fragment of recombinant lambdaC protein also exhibited autoguanylylation activity, demonstrating both that this fragment is necessary and sufficient for autoguanylylation activity and that the 100-kDa complementary fragment is expendable for that activity. Comparison of the deduced amino acid sequence of protein lambdaC with those of the mammalian and grass carp reovirus guanylyltransferases revealed that only two of eight lysine residues within the amino-terminal 42-kDa region are conserved. Interestingly, these two lysines match with the lysine residues in the mammalian reovirus capping enzyme proposed to be essential for autoguanylylation activity. Our alignment analysis also showed that the S-adenosyl-l-methionine-binding pocket previously detected in the mammalian reovirus capping enzyme is fully conserved in its avian and grass carp reovirus counterparts, suggesting that all three enzymes have methylase activity.

The avian reovirus genome segment S1 is a functionally tricistronic gene that expresses one structural and two nonstructural proteins in infected cells

The avian reovirus S1 gene contains three partially overlapping, out-of-phase open reading frames (ORFs) that the highly conserved in all avian reovirus strains examined to date. The three S1 ORFs of the avian reovirus strain S1133 were individually expressed in bacterial cells, and their purified translation products used as antigens to raise specific polyclonal antibodies. With these antibodies we were able to demonstrate that all three S1 ORFs from different avian reovirus strains are translatable in infected cells. Proteins p10 and p17, which are specified by ORF1 and ORF2, respectively, are nonstructural proteins which associate with cell membranes, whereas ORF3 directs the synthesis of protein sigma C, a structural oligomeric protein responsible for cell attachment. While intracellular synthesis of protein sigma C was demonstrated a long time ago and that of protein p10 was reported recently, this is the first time that expression of the S1 ORF2 has been demonstrated experimentally. Thus, the previously reported coding capacity of the avian reovirus genome is now expanded to 14 proteins, of which ten are structural (lambda A, lambda B, lambda C, microA, microB, microBC, microBN, sigma A, sigma B, and sigma C) and four are nonstructural (microNS, sigma NS, p17, and p10). Finally, protein p10, but not p17 or sigma C, induces cell-cell fusion when transiently expressed in mammalian cells, supporting a previously published observation that the polypeptide encoded by the S1 ORF1 plays an important role in the syncytial phenotype displayed by avian reoviruses.

Possible involvement of the double-stranded RNA-binding core protein sigmaA in the resistance of avian reovirus to interferon

Treatment of primary cultures of chicken embryo fibroblasts with a recombinant chicken alpha/beta interferon (rcIFN) induces an antiviral state that causes a strong inhibition of vaccinia virus and vesicular stomatitis virus replication but has no effect on avian reovirus S1133 replication. The fact that avian reovirus polypeptides are synthesized normally in rcIFN-treated cells prompted us to investigate whether this virus expresses factors that interfere with the activation and/or the activity of the IFN-induced, double-stranded RNA (dsRNA)-dependent enzymes. Our results demonstrate that extracts of avian-reovirus-infected cells, but not those of uninfected cells, are able to relieve the translation-inhibitory activity of dsRNA in reticulocyte lysates, by blocking the activation of the dsRNA-dependent enzymes. In addition, our results show that protein sigmaA, an S1133 core polypeptide, binds to dsRNA in an irreversible manner and that clearing this protein from extracts of infected cells abolishes their protranslational capacity. Taken together, our results raise the interesting possibility that protein sigmaA antagonizes the IFN-induced cellular response against avian reovirus by blocking the intracellular activation of enzyme pathways dependent on dsRNA, as has been suggested for several other viral dsRNA-binding proteins.

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

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

Intracellular posttranslational modifications of S1133 avian reovirus proteins

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

Avian reovirus S1133 can replicate in mouse L cells: effect of pH and cell attachment status on viral infection

Previous reports have suggested that avian reovirus S1133 fails to replicate in mouse L cells. In this article, we report that replication does occur under certain culture conditions. The avian reovirus was found to grow in mouse L cells at pH 6.4 and 7.2 but not at pH 8.2. Culture medium with a basic pH directly inhibited viral transcription and genome replication. As a result, viral protein synthesis was also affected. At permissive pH levels, avian reovirus grew better in monolayers than in suspension cultures of L cells because of the influence of cell attachment status on viral macromolecular synthesis. Our results not only show that avian reovirus can replicate in mouse L cells but also help to explain why it did not in previous studies.

The stimulatory effect of actinomycin D on avian reovirus replication in L cells suggests that translational competition dictates the fate of the infection

Indirect immunostaining of avian reovirus S1133-infected L-cell monolayers showed that most of the cells can support viral replication. However, the number of cells in which the virus was actually replicating depended on the multiplicity of virus infection. The presence of actinomycin D during infection increased viral protein synthesis, viral growth, and the number of actively infected cells at late infection times. The antibiotic elicited these effects by triggering viral replication in cells that already contained unproductive cytoplasmic virus but that would not get productively infected in the absence of the drug. From these results, we propose a model for the interaction between L cells and avian reovirus S1133 in which viral versus host mRNA competition for the translational machinery determines the fate of the virus infection.