Second-generation antihistamines in asthma therapy: is there a protective effect?

Second-generation histamine H(1) receptor antagonists are recognized as being highly effective treatments for allergic-based disease and are among the most frequently prescribed drugs in the world. The newer antihistamines represent a heterogeneous group of compounds with markedly different chemical structures, a spectrum of antihistaminic properties, adverse effects, half-life, tissue distribution, metabolism and varying degrees of anti-inflammatory effects. Histamine is an important mast cell- and basophil-derived mediator that has been implicated in the pathogenesis of asthma, resulting in smooth muscle contraction, mucus hypersecretion, and increased vascular permeability leading to mucosal edema. Antihistamines should never be used as monotherapy for asthma but there is evidence that these drugs give a measure of protection in histamine-induced bronchoconstriction. Furthermore, several studies have demonstrated that the use of second-generation antihistamines, as adjunct therapy, may benefit those patients whose allergic asthma co-exists with allergic rhinitis. Indeed, many patients present with both allergic rhinitis and asthma. The link between the upper and lower respiratory airways is now well established and there is increasing evidence that allergic rhinitis is a risk factor for the development of asthma. More recently, a number of novel antihistamines have been developed which are either metabolites of active drugs or enantiomers and there is emerging evidence that at least one of these drugs, desloratadine, may give significant symptomatic benefit in some types of asthma. It is of interest to note that cetirizine provides a primary pharmacological intervention strategy to prevent the development of asthma in specifically-sensitized high risk groups of infants. Moreover, the documented anti-inflammatory activities of antihistamines may provide a novel mechanism of action for the therapeutic control of virus-induced asthma exacerbations by inhibiting the expression of intercellular adhesion molecule-1 (ICAM-1) by airway epithelial cells. Finally, several well-conducted studies suggest that combination therapy with antihistamines and antileukotrienes may be as effective as corticosteroid use in patients with allergic asthma and seasonal allergic rhinitis.

Normal mode calculations of icosahedral viruses with full dihedral flexibility by use of molecular symmetry

The study of the dynamics and thermodynamics of small icosahedral virus capsids is an active field of research. Normal mode analysis is one of the computational tools that can provide important insights into the conformational changes of the virus associated with cell entry or caused by changing of the physicochemical environment. Normal mode analysis of virus capsids has been limited due to the size of these systems, which often exceed 50,000 residues. Here we present the first normal mode calculation with full dihedral flexibility of several virus capsids, including poliovirus, rhinovirus, and cowpea chlorotic mottle virus. The calculations were made possible by applying group theoretical methods, which greatly simplified the calculations without any approximation beyond the all-atom force field representations in general use for smaller protein systems. Since a full Cartesian basis set was too large to be handled by the available computer memory, we used a basis set that includes all internal dihedral angles of the system with the exception of the peptide bonds, which were assumed rigid. The fluctuations of the normal modes are shown to correlate well with crystallographic temperature factors. The motions of the first several normal modes of each symmetry type are described. A hinge bending motion in poliovirus was found that may be involved in the mechanism by which bound small molecules inhibit conformational changes of the capsid. Fully flexible normal mode calculations of virus capsids are expected to increase our understanding of virus dynamics and thermodynamics, and can be useful in the refinement of cryo-electron microscopy structures of viruses.

Labeling of capsid proteins and genomic RNA of human rhinovirus with two different fluorescent dyes for selective detection by capillary electrophoresis

During uncoating of human rhinoviruses, the innermost capsid protein VP4 and the genomic RNA are released from the viral protein shell. This process gives rise to subviral particles that are composed of the remaining three capsid proteins VP1, VP2, and VP3. The process is believed to take place in a sequential manner in that first VP4 is expelled resulting in A-particles sedimenting at 135S followed by the RNA resulting in B-particles sedimenting at 80S. Aiming at ultimately analyzing this process in vivo, we introduced two different fluorophores into the RNA and the viral capsid proteins, respectively. Incubation of the virus with RiboGreen resulted in formation of a RNA-dye complex with lambda(ex)/lambda(em) = 500/525 nm, whereas subsequent derivatization of the viral protein shell in the same sample with AMCA-S introduced a label with lambda(ex)/lambda(em) = 345-350/440-460 nm. In this way, both viral components could be selectively detected via fluorescence in a capillary electrophoresis system. The intact virus delivers two superimposed signals in the electropherogram. Derivatization of the free amino groups of the capsid proteins partially preserved the bioaffinity of the virus toward a synthetic receptor fragment, an artificial recombinant concatemer of repeat number 3 of the very low density lipoprotein receptor. Between 10 and 20% of the infectivity were recovered after labeling when compared to native virus. In addition to analysis of factors influencing the stability of the virus by CE, double-labeled virions might be useful for the investigation of the uncoating process by real-time confocal fluorescence microscopy.

Crystal structure of complete rhinovirus RNA polymerase suggests front loading of protein primer

Picornaviruses utilize virally encoded RNA polymerase and a uridylylated protein primer to ensure replication of the entire viral genome. The molecular details of this mechanism are not well understood due to the lack of structural information. We report the crystal structure of human rhinovirus 16 3D RNA-dependent RNA polymerase (HRV16 3Dpol) at a 2.4-A resolution, representing the first complete polymerase structure from the Picornaviridae family. HRV16 3Dpol shares the canonical features of other known polymerase structures and contains an N-terminal region that tethers the fingers and thumb subdomains, forming a completely encircled active site cavity which is accessible through a small tunnel on the backside of the molecule. The small thumb subdomain contributes to the formation of a large cleft on the front face of the polymerase which also leads to the active site. The cleft appears large enough to accommodate a template:primer duplex during RNA elongation or a protein primer during the uridylylation stage of replication initiation. Based on the structural features of HRV16 3Dpo1 and the catalytic mechanism known for all polymerases, a front-loading model for uridylylation is proposed.

Structural and virological studies of the stages of virus replication that are affected by antirhinovirus compounds

Pleconaril is a broad-spectrum antirhinovirus and antienterovirus compound that binds into a hydrophobic pocket within viral protein 1, stabilizing the capsid and resulting in the inhibition of cell attachment and RNA uncoating. When crystals of human rhinovirus 16 (HRV16) and HRV14 are incubated with pleconaril, drug occupancy in the binding pocket is lower than when pleconaril is introduced during assembly prior to crystallization. This effect is far more marked in HRV16 than in HRV14 and is more marked with pleconaril than with other compounds. These observations are consistent with virus yield inhibition studies and radiolabeled drug binding studies showing that the antiviral effect of pleconaril against HRV16 is greater on the infectivity of progeny virions than the parent input viruses. These data suggest that drug integration into the binding pocket during assembly, or at some other late stage in virus replication, may contribute to the antiviral activity of capsid binding compounds.

Twelve receptor molecules attach per viral particle of human rhinovirus serotype 2 via multiple modules

The crystallographic T = 1 (pseudo T = 3) icosahedral symmetry of the human rhinovirus capsid dictates the presence of 60 identical, symmetry related surface structures that are available for antibody and receptor binding. X-ray crystallography has shown that 60 individual very-low density lipoprotein receptor (VLDLR) modules bind to HRV2. Their arrangement around the fivefold axes of the virion suggested that tandem oligomers of such modules could attach simultaneously to symmetry-related sites. By resolving virus particles carrying various numbers of artificial recombinant concatemers of VLDLR repeat 3 (V33333) by capillary electrophoresis and extrapolation of the measured mobilities to that at saturation of all binding sites, we present evidence for up to 12 molecules of the concatemer to bind one single virion.

Identification of the human rhinovirus serotype 1A binding site on the murine low-density lipoprotein receptor by using human-mouse receptor chimeras

Human rhinovirus serotype 1A (HRV1A) binds more strongly to the mouse low-density lipoprotein receptor (LDLR) than to the human homologue (M. Reithmayer, A. Reischl, L. Snyers, and D. Blaas, J. Virol. 76:6957-6965, 2002). Here, we used this fact to determine the binding site of HRV1A by replacing selected ligand binding modules of the human receptor with the corresponding ligand binding modules of the mouse receptor. The chimeric proteins were expressed in mouse fibroblasts deficient in endogenous LDLR and LDLR-related protein, both used by minor group HRVs for cell entry. Binding was assessed by virus overlay blots, by immunofluorescence microscopy, and by measuring cell attachment of radiolabeled virus. Replacement of ligand binding repeat 5 of the human LDLR with the corresponding mouse sequence resulted in a substantial increase in HRV1A binding, whereas substitution of repeats 3 and 4 was without effect. Replacement of human receptor repeats 1 and 2 with the murine homologues also increased virus binding. Finally, murine receptor modules 1, 2, and 5 simultaneously introduced into the human receptor resulted in HRV1A binding indistinguishable from mouse wild-type receptor. Thus, repeats 1 and/or 2 and repeat 5 are involved in HRV1A attachment. Changing CDGGPD in the acidic cluster of module 5 in the human receptor to CDGEAD present in the mouse receptor led to substantially increased binding of HRV1A, indicating an important role of the glutamate residue in HRV1A recognition.

Monitoring RNA release from human rhinovirus by dynamic force microscopy

Human rhinoviruses were imaged under physiological conditions by dynamic force microscopy. Topographical images revealed various polygonal areas on the surfaces of the 30-nm viral particles. RNA release was initiated by exposure to a low-pH buffer. The lengths of the RNAs that were released but still connected to the virus capsid varied between 40 and 330 nm, whereas RNA molecules that were completely released from the virus were observed with lengths up to 1 micro m. Fork-like structure elements with 30-nm extensions were sometimes resolved at one end of the RNA molecules. They possibly correspond to the characteristic multi-stem-loop conformation, the internal ribosomal entry site, located at the 5' region of the genome. This study demonstrates that dynamic force microscopy can be used to study viral RNA release in situ under physiological conditions.

Cryoelectron microscopy analysis of the structural changes associated with human rhinovirus type 14 uncoating

Release of the human rhinovirus (HRV) genome into the cytoplasm of the cell involves a concerted structural modification of the viral capsid. The intracellular adhesion molecule 1 (ICAM-1) cellular receptor of the major-group HRVs and the low-density lipoprotein (LDL) receptor of the minor-group HRVs have different nonoverlapping binding sites. While ICAM-1 binding catalyzes uncoating, LDL receptor binding does not. Uncoating of minor-group HRVs is initiated by the low pH of late endosomes. We have studied the conformational changes concomitant with uncoating in the major-group HRV14 and compared them with previous results for the minor-group HRV2. The structure of empty HRV14 was determined by cryoelectron microscopy, and the atomic structure of native HRV14 was used to examine the conformational changes of the capsid and its constituent viral proteins. For both HRV2 and HRV14, the transformation from full to empty capsid involves an overall 4% expansion and an iris type of movement of viral protein VP1 to open up a 10-A-diameter channel on the fivefold axis to allow exit of the RNA genome. The beta-cylinders formed by the N termini of the VP3 molecules inside the capsid on the fivefold axis all open up in HRV2, but we propose that only one opens up in HRV14. The release of VP4 is less efficient in HRV14 than in HRV2, and the N termini of VP1 may exit at different points. The N-terminal loop of VP2 is modified in both viruses, probably to detach the RNA, but it bends only inwards in HRV2.

X-ray structure of a minor group human rhinovirus bound to a fragment of its cellular receptor protein

Although many viral receptors have been identified, the ways in which they interact with their cognate viruses are not understood at the molecular level. We have determined the X-ray structure of a complex between calcium-containing modules of the very low-density lipoprotein receptor and the minor group human rhinovirus HRV2. The receptor binds close to the icosahedral five-fold vertex, with only one module per virus protomer. The binding face of this module is defined by acidic calcium-chelating residues and, in particular, by an exposed tryptophan that is highly conserved. The attachment site on the virus involves only residues from VP1, particularly a lysine strictly conserved in all minor group HRVs. The disposition of the attached ligand-binding repeats around the five-fold axis, together with the proximity of the N- and C-terminal ends of adjacent modules, suggests that more than one repeat in a single receptor molecule might attach simultaneously.

Mode of action of 2-furylmercury chloride, an anti-rhinovirus compound

2-Furylmercury chloride (2-FMC), an organic mercury derivative, has been found to inhibit the replication of all tested human rhinovirus (HRV) serotypes belonging to the antiviral group B and a limited number of HRV serotypes belonging to the antiviral group A. The mechanism of action of 2-FMC was tested against HRV-2 (antiviral group B, minor receptor group), and compared with an antiviral compound for which the viral target was already determined (enviroxime). 2-FMC was found to bind reversibly to virus particles. However, time-dependent plaque reduction assays revealed that 2-FMC did not interfere with early events of HRV-2 replication. Using a quantitative RT-PCR ELISA assay, we were able to prove that 2-FMC inhibits the synthesis of viral RNA. However, the mode of action of 2-FMC is not identical to that of enviroxime, another inhibitor of viral RNA synthesis. Time-of-addition and time-of-withdrawal experiments demonstrated that 2-FMC acted during a broader time interval than enviroxime.

Human rhinovirus capsid dynamics is controlled by canyon flexibility

Quantitative enzyme accessibility experiments using nano liquid chromatography electrospray mass spectrometry combined with limited proteolysis and isotope-labeling was used to examine the dynamic nature of the human rhinovirus (HRV) capsid in the presence of three antiviral compounds, a neutralizing Fab, and drug binding cavity mutations. Using these methods, it was found that the antivirals WIN 52084 and picovir (pleconaril) stabilized the capsid, while dansylaziridine caused destabilization. Site-directed mutations in the drug-binding cavity were found to stabilize the HRV14 capsid against proteolytic digestion in a manner similar to WIN 52084 and pleconaril. Antibodies that bind to the NIm-IA antigenic site and penetrate the canyon were also observed to protect the virion against proteolytic cleavage. These results demonstrate that quantifying the effects of antiviral ligands on protein "breathing" can be used to compare their mode of action and efficacy. In this case, it is apparent that hydrophobic antiviral agents, antibodies, or mutations in the canyon region block viral breathing. Therefore, these studies demonstrate that mobility in the canyon region is a major determinant in capsid breathing.

Dehydron: a structurally encoded signal for protein interaction

We introduce a quantifiable structural motif, called dehydron, that is shown to be central to protein-protein interactions. A dehydron is a defectively packed backbone hydrogen bond suggesting preformed monomeric structure whose Coulomb energy is highly sensitive to binding-induced water exclusion. Such preformed hydrogen bonds are effectively adhesive, since water removal from their vicinity contributes to their stability. At the structural level, a significant correlation is established between dehydrons and sites for protein complexation, with the HIV-1 capsid protein P24 complexed with antibody light-chain FAB25.3 providing the most dramatic correlation. Furthermore, the number of dehydrons in homologous similar-fold proteins from different species is shown to be a signature of proteomic complexity. The techniques are then applied to higher levels of organization: The formation of the capsid and its organization in picornaviruses correlates strongly with the distribution of dehydrons on the rim of the virus unit. Furthermore, antibody contacts and crystal contacts may be assigned to dehydrons still prevalent after the capsid has been assembled. The implications of the dehydron as an encoded signal in proteomics, bioinformatics, and inhibitor drug design are emphasized.

Human rhinovirus 2 2Apro recognition of eukaryotic initiation factor 4GI. Involvement of an exosite

The 2A proteinase (2Apro) of human rhinovirus 2 is a cysteine proteinase with a unique chymotrypsin-like fold. During viral replication, 2Apro performs self-processing by cleaving between its own N terminus and the C terminus of the preceding protein, VP1. Subsequently, 2Apro cleaves the two isoforms of the cellular protein, eukaryotic initiation factor (eIF) 4G. We have previously shown that HRV2 2Apro can directly bind to eIF4G isoforms. Here we demonstrate using deletion mutants of eIF4GI that HRV2 2Apro requires eIF4GI amino acids 600-674 for binding; however, the amino acids at the cleavage site, Arg681 downward arrow Gly, are not required. The HRV2 2Apro binding domain for eIF4GI was identified by site-directed mutagenesis. Specifically, mutations Leu17 --> Arg and Asp35 --> Glu severely impaired HRV2 2Apro binding and thus processing of eIF4GI in rabbit reticulocyte lysates; self-processing, however, was not affected. Alanine scanning analysis further identified the loop containing residues Tyr32, Ser33, and Ser34 as important for eIF4GI binding. Although Asp35 is part of the catalytic triad, most of the eIF4GI binding domain lies in a unique exosite structure absent from other chymotrypsin-like enzymes and is distinct from the substrate binding cleft. The exosite represents a novel virulence determinant that may allow the development of specific inhibitors for HRV2 2Apro.

A cellular receptor of human rhinovirus type 2, the very-low-density lipoprotein receptor, binds to two neighboring proteins of the viral capsid

The very-low-density lipoprotein receptor (VLDL-R) is a receptor for the minor-group human rhinoviruses (HRVs). Only two of the eight binding repeats of the VLDL-R bind to HRV2, and their footprints describe an annulus on the dome at each fivefold axis. By studying the complex formed between a selection of soluble fragments of the VLDL-R and HRV2, we demonstrate that it is the second and third repeats that bind. We also show that artificial concatemers of the same repeat can bind to HRV2 with the same footprint as that for the native receptor. In a 16-A-resolution cryoelectron microscopy map of HRV2 in complex with the VLDL-R, the individual repeats are defined. The third repeat is strongly bound to charged and polar residues of the HI and BC loops of viral protein 1 (VP1), while the second repeat is more weakly bound to the neighboring VP1. The footprint of the strongly bound third repeat extends down the north side of the canyon. Since the receptor molecule can bind to two adjacent copies of VP1, we suggest that the bound receptor "staples" the VP1s together and must be detached before release of the RNA can occur. When the receptor is bound to neighboring sites on HRV2, steric hindrance prevents binding of the second repeat.

Sequence and structure of human rhinoviruses reveal the basis of receptor discrimination

The sequences of the capsid protein VP1 of all minor receptor group human rhinoviruses were determined. A phylogenetic analysis revealed that minor group HRVs were not more related to each other than to the nine major group HRVs whose sequences are known. Examination of the surface exposed amino acid residues of HRV1A and HRV2, whose X-ray structures are available, and that of three-dimensional models computed for the remaining eight minor group HRVs indicated a pattern of positively charged residues within the region, which, in HRV2, was shown to be the binding site of the very-low-density lipoprotein (VLDL) receptor. A lysine in the HI loop of VP1 (K224 in HRV2) is strictly conserved within the minor group. It lies in the middle of the footprint of a single repeat of the VLDL receptor on HRV2. Major group virus serotypes exhibit mostly negative charges at the corresponding positions and do not bind the negatively charged VLDL receptor, presumably because of charge repulsion.

Structural analysis of human rhinovirus complexed with ICAM-1 reveals the dynamics of receptor-mediated virus uncoating

Intercellular adhesion molecule 1 (ICAM-1) functions as the cellular receptor for the major group of human rhinoviruses, being not only the target of viral attachment but also the mediator of viral uncoating. The configurations of HRV3-ICAM-1 complexes prepared both at 4 degrees C and physiological temperature (37 degrees C) were analyzed by cryoelectron microscopy and image reconstruction. The particle diameters of two complexes (with and without RNA) representing uncoating intermediates generated at 37 degrees C were each 4% larger than that of those prepared at 4 degrees C. The larger virus particle arose by an expansive movement of the capsid pentamers along the fivefold axis, which loosens interprotomer contacts, particularly at the canyon region where the ICAM-1 receptor bound. Particle expansion required receptor binding and preceded the egress of the viral RNA. These observations suggest that receptor-mediated uncoating could be a consequence of restrained capsid motion, where the bound receptors maintain the viral capsid in an expanded open state for subsequent genome release.

Human rhinovirus type 16: mutant V1210A requires capsid-binding drug for assembly of pentamers to form virions during morphogenesis

Our laboratory has previously reported isolation of human rhinovirus type 16 (HRV16) mutants which depend on WIN 52035 to grow. A rapid rise of progeny virus infectivity occurred when drug was added late in growth cycles, suggesting that the drug-dependence lesion was at the step of virus assembly (W. Wang et al., J. Virol. 72:1210-1218, 1998). Here, we report that capsid subunits, 5S protomers and 14S pentamers, of a drug-dependent mutant were produced normally in the absence of drug, but mutant 80S empty capsids and 150S provirions were not formed, maturation cleavage of provirions (VP0 --> VP2 + VP4) did not occur, and the unassembled mutant capsid subunits were degraded with a half-life of 15 min. Drug was not required by mutant virus for attachment, uncoating, RNA synthesis and protein synthesis, and polyprotein processing except maturation cleavage. The requirement of drug for assembly of mutant pentamers to form provirions and the rapid assembly of preformed subunits (synthesized in the absence of drug) after drug addition suggested that after native pentamers (P5) have been formed they must be converted to an assembly active state (P5(*)), possibly by a conformational change induced by the binding of drug. We propose that pocket factor plays the same role in wild-type virus. In addition, we also report the construction and the properties of a full-length cDNA clone of HRV16, pR16.11, which produces in vitro transcripts with infectivity similar to that of virion RNA. This cDNA clone is available at the American Type Culture Collection.

Conformational changes, plasma membrane penetration, and infection by human rhinovirus type 2: role of receptors and low pH

Human rhinovirus type 2 (HRV2) is internalized by members of the low-density lipoprotein (LDL) receptor (LDLR) family. It then progresses into late endosomes, where it undergoes conversion from D- to C-antigenicity at pH < 5.6. Upon uncoating, the viral RNA is transferred into the cytoplasm across the endsosomal membrane. However, C-antigenic particles fail to attach to LDLR; this raised the question of whether the virus remains attached to the receptors and is carried to late compartments or rather falls off at the higher pH in early endosomes. We therefore determined the pH dependence of virus-receptor dissociation and virus conversion to C-antigen under conditions preventing endocytosis. (35)S-HRV2 was attached to HeLa cells at 4 degrees C and incubated in buffers of pH 7.4 to 5.0; levels of native virus and C-antigenic particles remaining cell associated or having been released into the medium were determined by immunoprecipitation. At pH 6.0, HRV2 was readily released from plasma membrane receptors in its native form, whereas at pH < or = 5.4, it was entirely converted to C-antigen, which, however, only dissociated from the surface upon prolonged incubation. The antigenic conversion occurred at the same pH regardless of whether HRV2 was free in solution or bound to its receptors. These data suggest that, in vivo, the virus is no longer bound to its receptors when the antigenic conversion and uncoating occur in more acidic late endosomes. When virus was bound to HeLa cells at 4 degrees C, converted into C-antigen by exposure to pH 5.3, and subsequently warmed to 34 degrees C in the presence of bafilomycin (to prevent endosomal uncoating), viral de novo synthesis was detected. This study demonstrates for the first time that a nonenveloped virus such as HRV2 can infect from the plasma membrane when artificially exposed to low pH. This implies that the viral RNA can gain access to the cytoplasm from the plasma membrane.

Human rhinovirus selectively modulates membranous and soluble forms of its intercellular adhesion molecule-1 (ICAM-1) receptor to promote epithelial cell infectivity

Human rhinoviruses are responsible for many upper respiratory tract infections. 90% of rhinoviruses utilize intercellular adhesion molecule-1 (ICAM-1) as their cellular receptor, which also plays a critical role in recruitment of immune effector cells. Two forms of this receptor exist; membrane-bound (mICAM-1) and soluble ICAM-1 (sICAM-1). The soluble receptor may be produced independently from the membrane-bound form or it may be the product of proteolytic cleavage of mICAM-1. The ratio of airway epithelial cell expression of mICAM-1 to the sICAM-1 form may influence cell infectivity and outcome of rhinovirus infection. We therefore investigated the effect of rhinovirus on expression of both ICAM-1 receptors in normal human bronchial epithelial cells. We observed separate distinct messenger RNA transcripts coding for mICAM-1 and sICAM-1 in these cells, which were modulated by virus. Rhinovirus induced mICAM-1 expression on epithelial cells while simultaneously down-regulating sICAM-1 release, with consequent increase in target cell infectivity. The role of protein tyrosine kinases was investigated as a potential mechanistic pathway. Rhinovirus infection induced rapid phosphorylation of intracellular tyrosine kinase, which may be critical in up-regulation of mICAM-1. Elucidation of the underlying molecular mechanisms involved in differential modulation of both ICAM-1 receptors may lead to novel therapeutic strategies.

Changes in rhinovirus protein 2C allow efficient replication in mouse cells

Rhinovirus type 16 was found to replicate in mouse L cells that express the viral receptor, human intercellular adhesion molecule 1 (ICAM-1). However, infection of these cells at a low multiplicity of infection leads to no discernible cytopathic effect, and low virus titers are produced. A variant virus, 16/L, was isolated after alternate passage of rhinovirus 16 between HeLa and ICAM-1 L cells. Infection of mouse cells with 16/L leads to higher virus titers, increased production of RNA, and total cytopathic effect. Three amino acid changes were identified in the P2 region of virus 16/L, and the adaptation phenotype mapped to two changes in protein 2C. The characterization of a rhinovirus host range mutant will facilitate the investigation of cellular proteins required for efficient viral growth and the development of a murine model for rhinovirus infection.

Unr is required in vivo for efficient initiation of translation from the internal ribosome entry sites of both rhinovirus and poliovirus

Translation of picornavirus RNAs is mediated by internal ribosomal entry site (IRES) elements and requires both standard eukaryotic translation initiation factors (eIFs) and IRES-specific cellular trans-acting factors (ITAFs). Unr, a cytoplasmic RNA-binding protein that contains five cold-shock domains and is encoded by the gene upstream of N-ras, stimulates translation directed by the human rhinovirus (HRV) IRES in vitro. To examine the role of Unr in translation of picornavirus RNAs in vivo, we derived murine embryonic stem (ES) cells in which either one (-/+) or both (-/-) copies of the unr gene were disrupted by homologous recombination. The activity of picornaviral IRES elements was analyzed in unr(+/+), unr(+/-), and unr(-/-) cell lines. Translation directed by the HRV IRES was severely impaired in unr(-/-) cells, as was that directed by the poliovirus IRES, revealing a requirement for Unr not previously observed in vitro. Transient expression of Unr in unr(-/-) cells efficiently restored the HRV and poliovirus IRES activities. In contrast, the IRES elements of encephalomyocarditis virus and foot-and-mouth-disease virus are not Unr dependent. Thus, Unr is a specific regulator of HRV and poliovirus translation in vivo and may represent a cell-specific determinant limiting replication of these viruses.

Virus-ligand interactions: identification and characterization of ligand binding by NMR spectroscopy

We demonstrate the detection and characterization of ligand binding to viruses via NMR. To illustrate the methodology, the interaction of an antiviral compound with human rhinovirus serotype 2 (HRV2) was investigated. Specific interaction of a capsid-binding inhibitor and native HRV2 was monitored utilizing saturation transfer difference (STD) NMR. STD NMR experiments at atomic resolution allowed those regions of the ligand that are involved in the interaction with the virus to be determined. The approach allows for (i) the fast and robust assessment of binding, (ii) the determination of the ligand binding epitope at atomic resolution without the necessity to crystallize virus-ligand complexes, and (iii) the reuse of the virus in subsequent assays. This methodology enables one to easily identify binding of drugs, peptides, and receptor or antibody fragments to the viral capsid.