Native Poliovirus Research Articles

Structure of the Fab-Labeled “Breathing” State of Native Poliovirus

At 37°C, the structure of poliovirus is dynamic, and internal polypeptides VP4 and N terminus of VP1 (residues 1 to 53) externalize reversibly. An Fab fragment of a monospecific antibody, which binds to residues 39 to 55 of VP1, was utilized to locate the N termini of VP1 in native (160S) particles in this "breathing" state. Fab and virus were mixed and imaged via cryogenic electron microscopy. The resulting reconstruction showed the capsid expands similarly to the irreversibly altered cell entry intermediate (135S) particle, but the N terminus of VP1 is located near the 2-fold axes, instead of the "propeller tip" as in 135S particles.

An Externalized Polypeptide Partitions between Two Distinct Sites on Genome-Released Poliovirus Particles

During cell entry, native poliovirus (160S) converts to a cell-entry intermediate (135S) particle, resulting in the externalization of capsid proteins VP4 and the amino terminus of VP1 (residues 1 to 53). Externalization of these entities is followed by release of the RNA genome (uncoating), leaving an empty (80S) particle. The antigen-binding fragment (Fab) of a monospecific peptide 1 (P1) antibody, which was raised against a peptide corresponding to amino-terminal residues 24 to 40 of VP1, was utilized to track the location of the amino terminus of VP1 in the 135S and 80S states of poliovirus particles via cryogenic electron microscopy (cryo-EM) and three-dimensional image reconstruction. On 135S, P1 Fabs bind to a prominent feature on the external surface known as the "propeller tip." In contrast, our initial 80S-P1 reconstruction showed P1 Fabs also binding to a second site, at least 50 Å distant, at the icosahedral 2-fold axes. Further analysis showed that the overall population of 80S-P1 particles consisted of three kinds of capsids: those with P1 Fabs bound only at the propeller tips, P1 Fabs bound only at the 2-fold axes, or P1 Fabs simultaneously bound at both positions. Our results indicate that, in 80S particles, a significant fraction of VP1 can deviate from icosahedral symmetry. Hence, this portion of VP1 does not change conformation synchronously when switching from the 135S state. These conclusions are compatible with previous observations of multiple conformations of the 80S state and suggest that movement of the amino terminus of VP1 has a role in uncoating. Similar deviations from icosahedral symmetry may be biologically significant during other viral transitions.

Kinetic analysis of the effect of poliovirus receptor on viral uncoating: the receptor as a catalyst.

We examined the role of soluble poliovirus receptor on the transition of native poliovirus (160S or N particle) to an infectious intermediate (135S or A particle). The viral receptor behaves as a classic transition state theory catalyst, facilitating the N-to-A conversion by lowering the activation energy for the process by 50 kcal/mol. In contrast to earlier studies which demonstrated that capsid-binding drugs inhibit thermally mediated N-to-A conversion through entropic stabilization alone, capsid-binding drugs are shown to inhibit receptor-mediated N-to-A conversion through a combination of enthalpic and entropic effects.

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.

Characterization of Poliovirus Conformational Alteration Mediated by Soluble Cell Receptors

Soluble extracts of Spodoptera frugiperda cells expressing the poliovirus receptor (PVR) induce the native poliovirus (PV) to "A" particle conformational change ( J. Virol. 64, 4697-4702). We describe the variables that regulate this passage and study the suitability of solubilized PVR both for use as an in vitro system to characterize the receptor-mediated conformational alteration and for the production of large amounts of altered virus for structural analysis. PVR seems to function in a stoichiometric fashion and the A particles produced appear as intact, stain excluding, spherical structures by electron microscopy, regardless of the extensive proteolysis of the capsid protein VP 1, which takes place during the conversion. The products obtained, time course, and temperature and ionic strength dependence of the alteration of PV by the solubilized PVR are indistinguishable from those of the alteration that leads to productive infection in cultured cells. Therefore, solubilized PVR may provide a convenient in vitro system for further characterization of the cell entry process.

Wall effects in sucrose density gradient centrifugation of viruses

The depletion of sedimenting particles to be expected from wall effects in the course of sucrose gradient centrifugation, was computed for various types of swing-out rotors on the assumption that particles hitting the tube wall are lost from the gradient. This assumption was roughly verified for heat-denatured, but not for native poliovirus. Tween 20 reduced the depletion, and prevented tight sticking of heat-denatured virus particles to tube walls.

Monospecific Antisera Against Capsid Polypeptides of Poliovirus Type 1 Distinguish Antigenic Structures of Poliovirus Proteins

Pure poliovirus polypeptides, namely VP1, VP2, VP3 and VP4, have been isolated by isoelectric focusing after dissociation of poliovirus by urea. When injected into rabbits, all four polypeptides produced monospecific antisera which were used for the characterization of poliovirus particles and poliovirus-infected cells. The specificity of these antisera was determined by immunoprecipitation of polypeptides obtained by dissociation of poliovirus with SDS, followed by characterization by polyacrylamide gel electrophoresis. The antisera revealed differences in the antigenic sites of native poliovirus particles, heated poliovirus particles and naturally occurring empty capsids. Only VP3 antiserum reacted with native poliovirus and showed some neutralization, whereas all antisera precipitated heated virus and empty capsids. These antisera reacted also with the appropriate precursors of the capsid polypeptides demonstrating their usefulness for an analysis of the cleavage pathway by monospecific antibodies and revealed a second protomer (90 kilodalton) polypeptide for the capsid proteins of poliovirus particles.

Relationship between poliovirus and echovirus 6 antigens. I. Immunization experiments in guinea pigs.

The relationship between antigens of polioviruses 1–3 and echovirus 6 was studied by immunization of guinea pigs. Antibody activity against virion (N) and empty capsid (H) antigens of these viruses was assayed by immunodiffusion and complement fixation. Intergroup activity was not encountered but intragroup activity within the poliovirus group was demonstrable after immunization using adjuvant. Heterologous anti N activity was encountered within the poliovirus group at a low magnitude and frequency after immunization with native poliovirus preparations; low heterologous anti H activity was more frequently demonstrable after immunization with native as well as heated virus preparations. Heterologous antibody titres were lower than the homologous ones. Activity common to all poliovirus H antigens was demonstrable. Formation of antibodies active to N antigens of two poliovirus types were provoked in at least one animal immunized with one type and booster injected with a second type.

Die Bestimmung der Antikörperbindung von infektiösem Poliovirus in vitro durch ein chromatographisches Verfahren mit 32P-markiertem Indikatorvirus (CAB - Test)

A simple in-vitro-technique for determining the antibody-combining-capacity of native poliovirus antigens is described. After primary incubation of the antigen with homologous reference serum in a suitable dilution the residual antibody activity is titrated chromatographically with aluminiumhydroxide and radioactive indicator virus. The antibody-combining-capacity of two antigens may be compared. The antibody-combining-capacity of the reference antigen may be expressed in terms of chromatographically determined antibody units, which themselves are related to the adsorptive quality of a virus strain.