Stabilization of poliovirus by capsid-binding antiviral drugs is due to entropic effects

When poliovirus attaches to its receptor or is heated in hypotonic buffers, the virion undergoes an irreversible conformational transition from the native 160 S (or N) particle to the 135 S (or A) particle, which is believed to mediate cell entry. The first-order rate constants for the thermally induced transition have been measured as a function of temperature for virus alone and for complexes of the virus with capsid-binding drugs that inhibit the receptor and thermally mediated conversion. Although the drugs have minimum inhibitory concentrations (MIC) that differ by almost three orders of magnitude, the activation energies for the N to A transition for the drug complexes (145 kcal/mol) were indistinguishable from each other or from that of the virus alone. We conclude that the antiviral activity of these drugs derives from a novel mechanism in which drug-binding stabilizes the virions through entropic effects.     

Characterization of early steps in the poliovirus infection process: receptor-decorated liposomes induce conversion of the virus to membrane-anchored entry-intermediate particles

The mechanism by which poliovirus infects the cell has been characterized by a combination of biochemical and structural studies, leading to a working model for cell entry. Upon receptor binding at physiological temperature, native virus (160S) undergoes a conformational change to a 135S particle from which VP4 and the N terminus of VP1 are externalized. These components interact with the membrane and are proposed to form a membrane pore. An additional conformational change in the particle is accompanied by release of the infectious viral RNA genome from the particle and its delivery, presumably through the membrane pore into the cytoplasm, leaving behind an empty 80S particle. In this report, we describe the generation of a receptor-decorated liposome system, comprising nickel-chelating nitrilotriacetic acid (NTA) liposomes and His-tagged poliovirus receptor, and its use in characterizing the early events in poliovirus infection. Receptor-decorated liposomes were able to capture virus and induce a temperature-dependent virus conversion to the 135S particle. Upon conversion, 135S particles became tethered to the liposome independently of receptor by a membrane interaction with the N terminus of VP1. Converted particles had lost VP4, which partitioned with the membrane. The development of a simple model membrane system provides a novel tool for studying poliovirus entry. The liposome system bridges the gap between previous studies using either soluble receptor or whole cells and offers a flexible template which can be extrapolated to electron microscopy experiments that analyze the structural biology of nonenveloped virus entry.     

The poliovirus 135S particle is infectious.

The molecular mechanism of cell entry by unenveloped viruses is poorly understood. The picornaviruses poliovirus, human rhinovirus, and coxsackievirus convert to an altered form (the 135S or A particle) upon interaction with receptors on susceptible cells at 37 degrees C. The 135S particle is thought to be a necessary intermediate because it accumulates inside susceptible cells soon after infection and drugs which inhibit conversion of the virus to this form also prevent infection. However, since a variable fraction of the altered 135S particles is reported to elute unproductively from the surface of susceptible cells, their precise role remains unclear. We have found that poliovirus 135S particles can infect Chinese hamster ovary (CHO) and murine L cells, neither of which are susceptible to infection by native poliovirus. The infectivity of the particles in tissue culture appears to be between 10(3) to 10(5) times less than that of poliovirus on HeLa cells. The 135S particle infectivity was not sensitive to RNase but was greatly reduced by proteolytic treatment. Proteolysis specifically removed the newly exposed N terminus of VP1, a feature which has previously been shown to mediate interactions of the particle with lipid membranes. These results demonstrate that although the infectivity of the 135S particle appears to be receptor independent, it nonetheless requires some property associated with the protein coat. In particular, the N terminus of VP1 plays an important role in the infection process. Our findings are consistent with the hypothesis that the 135S particle is an intermediate in the normal cell entry pathway of poliovirus infection.     

Cold-adapted poliovirus mutants bypass a postentry replication block.

In the current model of poliovirus entry, the initial interaction of the native virion with its cellular receptor is followed by a transition to an altered form, which then acts as an intermediate in viral entry. While the native virion sediments at 160S in a sucrose gradient, the altered particle sediments at 135S, has lost the coat protein VP4, and has become more hydrophobic. Altered particles can be found both associated with cells and in the culture medium. It has been hypothesized that the cell-associated 135S particle releases the viral genome into the cell cytoplasm and that nonproductive transitions to the 135S form are responsible for the high particle-to-PFU ratio observed for polioviruses. At 25 degrees C, a temperature at which the transition to 135S particles does not occur, the P1/Mahoney strain of poliovirus was unable to replicate, and cold-adapted (ca) mutants were selected from the population. These mutants have not gained the ability to convert to 135S particles at 25 degrees C, and the block to wild-type (wt) infection at low temperatures is not at the level of cellular entry. The particle-to-PFU ratio of poliovirus does not change at 25 degrees C in the absence of alteration. Three independent amino acid changes in the 2C coding region were identified in ca mutants, at positions 218 (Val to Ile), 241 (Arg to Ala), and 309 (Met to Val). Introduction of any of these mutations individually into wt poliovirus by site-directed mutagenesis confers the ca phenotype. All three serotypes of the Sabin vaccine strains and the P3/Leon strain of poliovirus also exhibit the ca phenotype. These results do not support a model of poliovirus entry into cells that includes an obligatory transition to the 135S particle.     

A mutation in VP4 defines a new step in the late stages of cell entry by poliovirus.

During the entry of poliovirus into cells, a conformational transition occurs within the virion that is dependent upon its binding to the cell surface receptor. This conformational rearrangement generates an altered particle of 135S, results in the extrusion of capsid protein VP4 and the amino terminus of VP1 from the virion interior, and leads to the acquisition of membrane-binding properties by the 135S particle. Although the subsequent fate of VP4 is unknown, its apparent absence from purified 135S particles has long suggested that VP4 is not directly involved during virus entry. We report here the construction by site-specific mutagenesis of a nonviable VP4 mutant that upon transfection of the cDNA appears to form mature virus particles. These particles, upon interaction with the cellular receptor, undergo the 135S conformational transition but are defective at a subsequent stage in virus entry. The results demonstrate that the participation of VP4 is required during cell entry of poliovirus. In addition, these data indicate the existence of additional stages in the cell entry process beyond receptor binding and the transition to 135S particles. These post-135S stages must include the poorly understood processes by which nonenveloped viruses cross the cell membrane, uncoat, and deliver their genomes into the cytoplasm.     

Is the 135S Poliovirus Particle an Intermediate during Cell Entry?

Poliovirus binding to its receptor (PVR) on the cell surface induces a conformational transition which generates an altered particle with a sedimentation value of 135S versus the 160S of the native virion. A number of lines of evidence suggest that the 135S particle is a cell entry intermediate. However, the low infection efficiencies of the 135S particle and the absence of detectable 135S particles during infection at 26 degrees C by the cold-adapted mutants argue against a role for the 135S particle during the cell entry process. We show here that binding of 135S-antibody complexes to the Fc receptor (CDw32) increases the infectivity of these particles by 2 to 3 orders of magnitude. Thus, the low efficiency of infection by 135S particles is due in part to the low binding affinity of these particles. In addition, we show that there is an additional stage in the entry process that is associated with RNA release. This stage occurs after formation of the 135S particle, is rate limiting during infection at 37 degrees C, but not at 26 degrees C, and is PVR independent. The data also demonstrate that during infection at 26 degrees C, the rate-limiting step is the PVR-mediated conversion of wild-type 160S particles to 135S particles. This suggests that during infection at 26 degrees C by the cold-adapted viruses, 135S particles are formed, but they fail to accumulate to detectable levels because the subsequent post-135S particle events occur at a significantly faster rate than the initial conversion of 160S to 135S particles. These data support a model in which the 135S particle is an intermediate during poliovirus entry.     

A monoclonal antibody that blocks poliovirus attachment recognizes the lymphocyte homing receptor CD44.

A monoclonal antibody, AF3, was previously shown to specifically inhibit poliovirus binding to HeLa cells and to detect a 100-kDa glycoprotein only in cell lines and tissues permissive for poliovirus infection. These results suggested that the 100-kDa protein may be involved in the pathogenesis of poliomyelitis and the cellular function of the poliovirus receptor site. To study further the role of the 100-kDa protein in poliovirus attachment, immunoaffinity purification, amino acid sequencing, and cDNA cloning were undertaken. The results demonstrate that antibody AF3 reacts with the lymphocyte homing receptor CD44, a multifunctional cell surface glycoprotein involved in the homing of circulating lymphocytes to lymph nodes and the modulation of lymphocyte adhesion and activation. Antibody AF3 reacts with a subset of CD44 molecules (AF3CD44H), which appears to be a small fraction of the heterogeneously glycosylated CD44 molecules expressed on hematopoietic and nonhematopoietic cells. Anti-CD44 monoclonal antibodies, previously reported to induce CD44-mediated modulation of lymphocyte activation and adhesion, compete with 125I-AF3 in binding assays, demonstrating functional overlap among the epitopes. The anti-CD44 monoclonal antibody A3D8, which binds to a greater molecular weight range of CD44 than does AF3, inhibits poliovirus binding to a similar degree. CD44 does not act as a poliovirus receptor, since CD44-expressing mouse L-cell transformants did not bind poliovirus. The poliovirus receptor and AF3CD44H may be noncovalently associated, or they may interact through the cytoskeleton or signal transduction pathways.     

Grb2 mediates negative regulation of stem cell factor receptor/c-Kit signaling by recruitment of Cbl

Aberrant activation of c-Kit is involved in a number of human diseases including cancers and leukemias. Certain receptor tyrosine kinases, such as epidermal growth factor receptor, have been shown to indirectly recruit Cbl through the adapter protein Grb2, leading to receptor ubiquitination and degradation. In order to study the role of Grb2 in c-Kit degradation, a series of mutations of the Grb2 binding sites in c-Kit were generated (Y703F, Y936F, and Y703F/Y936F). Since other signal transduction molecules are also known to bind Y703 and Y936, the more selective asparagine-to-alanine (N-to-A) mutants N705A, N938A, and N705A/N938A were generated. We could clearly demonstrate that binding of Grb2 was dependent on intact phosphorylation sites Y703 and Y936. Furthermore, we could demonstrate the presence of Cbl in a complex with Grb2 and c-Kit. Thus, Grb2 is able to indirectly recruit Cbl to c-Kit. In the N-to-A mutants, Cbl phosphorylation was strongly reduced, which correlated with reduced ubiquitination of c-Kit as well as decreased internalization and degradation of the receptor. Taken together, we have demonstrated that, in addition to its role in positive signaling via the Ras/Erk pathway, Grb2 mediates c-Kit degradation through recruitment of Cbl to c-Kit, leading to ubiquitination of c-Kit followed by internalization and degradation.     

Two distinct binding affinities of poliovirus for its cellular receptor

To study the kinetics and equilibrium of poliovirus binding to the poliovirus receptor, we used surface plasmon resonance to examine the interaction of a soluble form of the receptor with poliovirus. Soluble receptor purified from mammalian cells is able to bind poliovirus, neutralize viral infectivity, and induce structural changes in the virus particle. Binding studies revealed that there are two binding sites for the receptor on the poliovirus type 1 capsid, with affinity constants at 20 degrees C of K(D)(1) = 0.67 microm and K(D)(2) = 0.11 microm. The relative abundance of the two binding sites varies with temperature. At 20 degrees C, the K(D)(2) site constitutes approximately 46% of the total binding sites on the sensor chip, and its relative abundance decreased with decreasing temperature such that at 5 degrees C, the relative abundance of the K(D)(2) site is only 12% of the total binding sites. Absolute levels of the K(D)(1) site remained relatively constant at all temperatures tested. The two binding sites may correspond to docking sites for domain 1 of the receptor on the viral capsid, as predicted by a model of the poliovirus-receptor complex. Alternatively, the binding sites may be a consequence of structural breathing, or could result from receptor-induced conformational changes in the virus.     

Interaction of Poliovirus with Its Receptor Affords a High Level of Infectivity to the Virion in Poliovirus Infections Mediated by the Fc Receptor

Poliovirus infects susceptible cells through the poliovirus receptor (PVR), which functions to bind virus and to change its conformation. These two activities are thought to be necessary for efficient poliovirus infection. How binding and conformation conversion activities contribute to the establishment of poliovirus infection was investigated. Mouse L cells expressing mouse high-affinity Fcγ receptor molecules were established and used to study poliovirus infection mediated by mouse antipoliovirus monoclonal antibodies (MAbs) (immunoglobulin G2a [IgG2a] subtypes) or PVR-IgG2a, a chimeric molecule consisting of the extracellular moiety of PVR and the hinge and Fc portion of mouse IgG2a. The antibodies and PVR-IgG2a showed the same degree of affinity for poliovirus, but the infectivities mediated by these molecules were different. Among the molecules tested, PVR-IgG2a mediated the infection most efficiently, showing 50- to 100-fold-higher efficiency than that attained with the different MAbs. A conformational change of poliovirus was induced only by PVR-IgG2a. These results strongly suggested that some specific interaction(s) between poliovirus and the PVR is required for high-level infectivity of poliovirus in this system.     

Characterization and role in morphogenesis of a new subviral particle (55S) isolated from poliovirus-infected cells.

A previously undetected subviral particle, designated the 55S particle because of its position in sucrose density gradients, has been found in cytoplasmic extracts of poliovirus-infected cells. It contains no RNA, is composed of equimolar amounts of the structural polypeptides P1AB, P1C, and P1D, and is stable in vitro under a variety of conditions: presence or absence of EDTA, dilution in low- or high-ionic-strength buffers, suspension in buffers up to pH 10, incubation at 37 degrees C, and centrifugation to equilibrium in CsCl gradients (where it bands at a density of 1.285 g/cm3). Conventional pulse-chase experiments show that 55S particles are the products of the assembly of 14S subunits and the precursors of virions. These data led to the formulation of a model of poliovirus morphogenesis in which the conversion of capsomers into 73S empty capsids does not occur directly, but through the formation of an intermediate structure, the 55S particle.     

An antiviral compound that blocks structural transitions of poliovirus prevents receptor binding at low temperatures

Drugs such as WIN51711 that inhibit picornavirus replication are thought to block poliovirus infectivity by binding to the capsid and preventing structural transitions required for uncoating. We examined the activity of WIN51711 at temperatures where capsid flexibility is thought to be decreased. Below 37 degrees C, WIN51711 inhibits the binding of wild-type poliovirus to cells but does not affect the binding of a poliovirus mutant which is believed to undergo structural transitions more readily. These results suggest that the poliovirus capsid must undergo structural changes to bind to its cellular receptor.     

Effects of calcium on the chick oviduct progesterone receptor

The chick oviduct cytosol progesterone receptor can be transformed to a small form (Rs = 21A, S20,w:2.9) denoted "mero-receptor" by incubation in the presence of Ca2+ [8]. In the molybdate-free cytosol all the progestin binding components could be completely transformed to mero-form by 1 h treatment with 100 mM Ca2+ at 0 degrees C. If EDTA was secondarily added, the ligand was rapidly released. If molybdate (20 mM) containing cytosol was incubated with Ca2+, no radioactivity was found in the meroposition on the Agarose A 0.5 m column, but the bound steroid sedimented at 2.9 S in sucrose gradients containing Ca2+ (and no molybdate). When 20 nM molybdate was added to cytosol containing receptor activated by 0.3 M KCl, complete mero-transformation by Ca2+ was obtained also by the gel filtration criterion, indicating that molybdate does not inhibit the mero-transforming factor. Ligand-free progesterone receptor could also be completely converted to mero-form by endogenous cytosolic transforming factor and calcium. The transforming factor was completely inactivated, when cytosol was run through Agarose A 0.5 m gel. Mero-transformation was found to be irreversible. The purified progesterone receptor subunit 110 K (B) was partially converted to smaller forms by calcium alone (100 mM, 0 degrees C, 1 h) whereas addition of a small amount of cytosol allowed complete conversion to mero-form.     

Interaction of Poliovirus with Its Purified Receptor and Conformational Alteration in the Virion

Polypeptides of amino acids 1 to 241 (PVR241) and 1 to 330 (PVR330) of the human poliovirus receptor (hPVR) were produced in a baculovirus expression system. PVR241 contained extracellular domains 1 and 2 of hPVR, and PVR330 contained extracellular domains 1, 2, and 3. These peptides were purified by immunoaffinity column chromatography with an anti-hPVR monoclonal antibody (MAb). After the purification, PVR241 and PVR330 appeared to retain their native conformation as judged by reactivity with an anti-PVR MAb that recognized domain 1 of hPVR in a conformation-dependent manner. The virulent Mahoney strain of poliovirus type 1 was mixed with the purified PVRs in various concentrations. An average of at least 43 PVR330 molecules were able to bind to one virion particle under the conditions used. The equilibrium dissociation constant between the PVR330 molecule and the PVR binding site (canyon) on the virion was determined to be 4.50 ± (0.86) × 10⁻⁸ M at 4°C. Higher rates of conformational change of the virus (160S) to 135S and 80S particles were observed as the concentration of PVR330 was increased. In this in vitro system, the ratio of the amount of the 135S particle to that of the 80S particle seemed to be always constant. After the disappearance of the 160S particle, the amount of the 80S particle was not increased by further incubation at 37°C. These results suggested that the 80S particle was not derived from the 135S particle under the conditions used in this study.     

Mutational analysis of the cellular receptor for poliovirus.

To identify sequences of the cellular poliovirus receptor (PVR) involved in viral infection, mutant PVR cDNAs were constructed and assayed for biological activity in mouse L cells. To confirm that mutant PVRs reached the cell surface, an immunological tag, consisting of part of CH3 from human immunoglobulin G1, was engineered into the PVR. Deletion of PVR amino acids 256 to 320 or 385 to the carboxy terminus yielded receptors that were able to support poliovirus infection. PVRs lacking amino acids 40 to 136 or 137 to 256 were expressed at the cell surface but were not active as receptors for poliovirus. The results show that immunoglobulin-type domain 3 and the extreme carboxy terminus of the PVR are not required for viral receptor function, but sequences within the two amino-terminal domains contribute to the initiation of poliovirus infection.     

Homolog-scanning mutagenesis reveals poliovirus receptor residues important for virus binding and replication.

Poliovirus initiates infection of primate cells by binding to the poliovirus receptor, Pvr. Mouse cells do not bind poliovirus but express a Pvr homolog, Mph, that does not function as a poliovirus receptor. Previous work has shown that the first immunoglobulin-like domain of the Pvr protein contains the virus binding site. To further identify sequences of Pvr important for its interaction with poliovirus, stable cell lines expressing mutated Pvr molecules were examined for their abilities to bind virus and support virus replication. Substitution of the amino-terminal domain of Mph with that of Pvr yields a molecule that can function as a poliovirus receptor. Cells expressing this chimeric receptor have normal binding affinity for poliovirus, yet the kinetics of virus replication are delayed. Results of virus alteration assays indicate that this chimeric receptor is defective in converting native virus to 135S altered particles. This defect is not observed with cells expressing receptor recombinants that include Pvr domains 1 and 2. Because altered particles are believed to be an intermediate in poliovirus entry, these findings suggest that Pvr domains 2 and 3 participate in early stages of infection. Additional mutants were made by substituting variant Mph residues for the corresponding residues in Pvr. The results were interpreted by using a model of Pvr predicted from the known structures of other immunoglobulin-like V-type domains. Analysis of stable cell lines expressing the mutant proteins revealed that virus binding is influenced by mutations in the predicted C'-C" loop, the C" beta-strand, the C"-D loop, and the D-E loop. Mutations in homologous regions of the immunoglobulin-like CD4 molecule alter its interaction with gp120 of human immunodeficiency virus type 1. Cells expressing Pvr mutations on the predicted C" edge do not develop cytopathic effect during poliovirus infection, suggesting that poliovirus-induced cytopathic effect may be induced by the virus-receptor interaction.     

Soluble receptor-resistant poliovirus mutants identify surface and internal capsid residues that control interaction with the cell receptor.

Poliovirus initiates infection by binding to its cell receptor and undergoing a receptor-mediated conformational alteration. To identify capsid residues that control these interactions, we have isolated and characterized poliovirus mutants that are resistant to neutralization by a soluble form of the poliovirus receptor. Twenty one soluble receptor-resistant (srr) mutants were identified which still use the poliovirus receptor to infect cells. All but one srr mutant contain a single amino acid change at one of 13 different positions, either on the surface or in the interior of the virion. The results of binding and alteration assays demonstrate that both surface and internal capsid residues regulate attachment to the receptor and conformational change of the virus. Mutations that reduce alteration also affect receptor binding, suggesting a common structural basis for early events in poliovirus infection.     

Correlation of the 4-5 S form and the 8 S form of the cytosolic androgen receptor in murine skeletal muscle

Binding experiments with the cytosolic androgen receptor from murine skeletal muscle yield with testosterone a biphasic saturation curve and a biphasic Scatchard plot. These binding characteristics result from the conversion of 8 S receptor (KD = 1,4 X 10(-10) M) into 4-5 S receptor (KD = 1,2 X 10(-9) M). This conversion is androgen dependent and is facilitated in vitro by either UV-irradiation or by methods known to activate steroid hormone receptor complexes to a nuclear binding form (e.g. high ionic strength or elevated temperature). The measured data show that both receptor forms are in a complex dissociation equilibrium. The reassociation of the 4-5 S receptor to form the 8 S complex is inhibited by RNase.