Reversible pressure dissociation of R17 bacteriophage. The physical individuality of virus particles

Stability of different influenza subtypes: How can high hydrostatic pressure be a useful tool for vaccine development?

BACKGROUND

Avian influenza A viruses can cross naturally into mammals and cause severe diseases, as observed for H5N1. The high lethality of human infections causes major concerns about the real risk of a possible pandemic of severe diseases to which human susceptibility may be high and universal. High hydrostatic pressure (HHP) is a valuable tool for studies regarding the folding of proteins and the assembly of macromolecular structures such as viruses; furthermore, HHP has already been demonstrated to promote viral inactivation.

METHODS

Here, we investigated the structural stability of avian and human influenza viruses using spectroscopic and light-scattering techniques. We found that both particles have similar structural stabilities and that HHP promotes structural changes.

RESULTS

HHP induced slight structural changes to both human and avian influenza viruses, and these changes were largely reversible when the pressure returned to its initial level. The spectroscopic data showed that H3N2 was more pressure-sensitive than H3N8. Structural changes did not predict changes in protein function, as H3N2 fusion activity was not affected, while H3N8 fusion activity drastically decreased. The fusion activity of H1N1 was also strongly affected by HHP. In all cases, HHP caused inactivation of the different influenza viruses.

CONCLUSIONS

HHP may be a useful tool for vaccine development, as it induces minor and reversible structural changes that may be associated with partial preservation of viral biological activities and may potentiate their immunogenic response while abolishing their infectivity. We also confirmed that, although pressure does not promote drastic changes in viral particle structure, it can distinctly affect viral fusion activity.

Equilibria of oligomeric proteins under high pressure - A theoretical description

High pressure methods have become a useful tool for studying protein structure and stability. Using them, various physico-chemical processes including protein unfolding, aggregation, oligomer dissociation or enzyme-activity decrease were studied on many different proteins. Oligomeric protein dissociation is a process that can perfectly utilize the potential of high-pressure techniques, as the high pressure shifts the equilibria to higher concentrations making them better observable by spectroscopic methods. This can be especially useful when the oligomeric form is highly stable at atmospheric pressure. These applications may be, however, hindered by less intensive experimental response as well as interference of the oligomerization equilibria with unfolding or aggregation of the subunits, but also by more complex theoretical description. In this study we develop mathematical models describing different kinds of oligomerization equilibria, both closed (equilibrium of monomer and the highest possible oligomer without any intermediates) and consecutive. Closed homooligomer equilibria are discussed for any oligomerization degree, while the more complex heterooligomer equilibria and the consecutive equilibria in both homo- and heterooligomers are taken into account only for dimers and trimers. In all the cases, fractions of all the relevant forms are evaluated as functions of pressure and concentration. Significant points (inflection points and extremes) of the resulting transition curves, that can be determined experimentally, are evaluated as functions of pressure and/or concentration. These functions can be further used in order to evaluate the thermodynamic parameters of the system, i.e. atmospheric-pressure equilibrium constants and volume changes of the individual steps of the oligomer-dissociation processes.

Exploring the stability limits of actin and its suprastructures

Actin is the main component of the microfilament system in eukaryotic cells and can be found in distinct morphological states. Global (G)-actin is able to assemble into highly organized, supramolecular cellular structures known as filamentous (F)-actin and bundled (B)-actin. To evaluate the structure and stability of G-, F-, and B-actin over a wide range of temperatures and pressures, we used Fourier transform infrared spectroscopy in combination with differential scanning and pressure perturbation calorimetry, small-angle x-ray scattering, laser confocal scanning microscopy, and transmission electron microscopy. Our analysis was designed to provide new (to our knowledge) insights into the stabilizing forces of actin self-assembly and to reveal the stability of the actin polymorphs, including in conditions encountered in extreme environments. In addition, we sought to explain the limited pressure stability of actin self-assembly observed in vivo. G-actin is not only the least temperature-stable but also the least pressure-stable actin species. Under abyssal conditions, where temperatures as low as 1-4°C and pressures up to 1 kbar are reached, G-actin is hardly stable. However, the supramolecular assemblies of actin are stable enough to withstand the extreme conditions usually encountered on Earth. Beyond ∼3-4 kbar, filamentous structures disassemble, and beyond ∼4 kbar, complete dissociation of F-actin structures is observed. Between ∼1 and 2 kbar, some disordering of actin assemblies commences, in agreement with in vivo observations. The limited pressure stability of the monomeric building block seems to be responsible for the suppression of actin assembly in the kbar pressure range.

A novel delivery platform based on Bacteriophage MS2 virus-like particles

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Here we reviewed Bacteriophage MS2 virus-like particles, including introduction to their structure, their potential as a delivery platform, and their expected use in medicine and other fields.

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Bacteriophage MS2 virus-like particles represent the novel delivery platform.

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Bacteriophage MS2 virus-like particles possess promising application prospect.

Our objective here is to review the novel delivery platform based on Bacteriophage MS2 virus-like particles (VLPs), including introduction to their structure, their potential as a delivery platform, and their expected use in medicine and other fields. Bacteriophage MS2 VLPs are nanoparticles devoid of viral genetic material and can self-assemble from the coat protein into an icosahedral capsid. As a novel delivery platform, they possess numerous features that make them suitable and attractive for targeted delivery of RNAs or DNAs, epitope peptides, and drugs within the protein capsid. In short, as a novel delivery platform, MS2 VLPs are suitable for delivery of targeted agents and hold promise for use in diagnostics, vaccines, and therapeutic modalities.

A hypothesis to reconcile the physical and chemical unfolding of proteins

High pressure (HP) or urea is commonly used to disturb folding species. Pressure favors the reversible unfolding of proteins by causing changes in the volumetric properties of the protein-solvent system. However, no mechanistic model has fully elucidated the effects of urea on structure unfolding, even though protein-urea interactions are considered to be crucial. Here, we provide NMR spectroscopy and 3D reconstructions from X-ray scattering to develop the "push-and-pull" hypothesis, which helps to explain the initial mechanism of chemical unfolding in light of the physical events triggered by HP. In studying MpNep2 from Moniliophthora perniciosa, we tracked two cooperative units using HP-NMR as MpNep2 moved uphill in the energy landscape; this process contrasts with the overall structural unfolding that occurs upon reaching a threshold concentration of urea. At subdenaturing concentrations of urea, we were able to trap a state in which urea is preferentially bound to the protein (as determined by NMR intensities and chemical shifts); this state is still folded and not additionally exposed to solvent [fluorescence and small-angle X-ray scattering (SAXS)]. This state has a higher susceptibility to pressure denaturation (lower p1/2 and larger ΔVu); thus, urea and HP share concomitant effects of urea binding and pulling and water-inducing pushing, respectively. These observations explain the differences between the molecular mechanisms that control the physical and chemical unfolding of proteins, thus opening up new possibilities for the study of protein folding and providing an interpretation of the nature of cooperativity in the folding and unfolding processes.

Pressure-Inactivated Virus: A Promising Alternative for Vaccine Production

In recent years, many applications in diverse scientific fields with various purposes have examined pressure as a thermodynamic parameter. Pressure studies on viruses have direct biotechnological applications. Currently, most studies that involve viral inactivation by HHP are found in the area of food engineering and focus on the inactivation of foodborne viruses. Nevertheless, studies of viral inactivation for other purposes have also been conducted. HHP has been shown to be efficient in the inactivation of many viruses of clinical importance and the use of HHP approach has been proposed for the development of animal and human vaccines. Several studies have demonstrated that pressure can result in virus inactivation while preserving immunogenic properties. Viruses contain several components that can be susceptible to the effects of pressure. HHP has been a valuable tool for assessing viral structure function relationships because the viral structure is highly dependent on protein-protein interactions. In the case of small icosahedral viruses, incremental increases in pressure produce a progressive decrease in the folding structure when moving from assembled capsids to ribonucleoprotein intermediates (in RNA viruses), free dissociated units (dimers and/or monomers) and denatured monomers. High pressure inactivates enveloped viruses by trapping their particles in a fusion-like intermediate state. The fusogenic state, which is characterized by a smaller viral volume, is the final conformation promoted by HHP, in contrast with the metastable native state, which is characterized by a larger volume. The combined effects of high pressure with other factors, such as low or subzero temperature, pH and agents in sub-denaturing conditions (urea), have been a formidable tool in the assessment of the component's structure, as well as pathogen inactivation. HHP is a technology for the production of inactivated vaccines that are free of chemicals, safe and capable of inducing strong humoral and cellular immune responses. Here we present a current overview about the pressure-induced viral inactivation and the production of inactivated viral vaccines.

Mechanical disassembly of single virus particles reveals kinetic intermediates predicted by theory

New experimental approaches are required to detect the elusive transient intermediates predicted by simulations of virus assembly or disassembly. Here, an atomic force microscope (AFM) was used to mechanically induce partial disassembly of single icosahedral T=1 capsids and virions of the minute virus of mice. The kinetic intermediates formed were imaged by AFM. The results revealed that induced disassembly of single minute-virus-of-mice particles is frequently initiated by loss of one of the 20 equivalent capsomers (trimers of capsid protein subunits) leading to a stable, nearly complete particle that does not readily lose further capsomers. With lower frequency, a fairly stable, three-fourths-complete capsid lacking one pentamer of capsomers and a free, stable pentamer were obtained. The intermediates most frequently identified (capsids missing one capsomer, capsids missing one pentamer of capsomers, and free pentamers of capsomers) had been predicted in theoretical studies of reversible capsid assembly based on thermodynamic-kinetic models, molecular dynamics, or oligomerization energies. We conclude that mechanical manipulation and imaging of simple virus particles by AFM can be used to experimentally identify kinetic intermediates predicted by simulations of assembly or disassembly.

The thermodynamics of virus capsid assembly

Virus capsid assembly is a critical step in the viral life cycle. The underlying basis of capsid stability is key to understanding this process. Capsid subunits interact with weak individual contact energies to form a globally stable icosahedral lattice; this structure is ideal for encapsidating the viral genome and host partners and protecting its contents upon secretion, yet the unique properties of its assembly and inter-subunit contacts allow the capsid to dissociate upon entering a new host cell. The stability of the capsid can be analyzed by treating capsid assembly as an equilibrium polymerization reaction, modified from the traditional polymer model to account for the fact that a separate nucleus is formed for each individual capsid. From the concentrations of reactants and products in an equilibrated assembly reaction, it is possible to extract the thermodynamic parameters of assembly for a wide array of icosahedral viruses using well-characterized biochemical and biophysical methods. In this chapter we describe this basic analysis and provide examples of thermodynamic assembly data for several different icosahedral viruses. These data provide new insights into the assembly mechanisms of spherical virus capsids, as well as into the biology of the viral life cycle.

Different urea stoichiometries between the dissociation and denaturation of tobacco mosaic virus as probed by hydrostatic pressure

Viruses are very efficient self-assembly structures, but little is understood about the thermodynamics governing their directed assembly. At higher levels of pressure or when pressure is combined with urea, denaturation occurs. For a better understanding of such processes, we investigated the apparent thermodynamic parameters of dissociation and denaturation by assuming a steady-state condition. These processes can be measured considering the decrease of light scattering of a viral solution due to the dissociation process, and the red shift of the fluorescence emission spectra, that occurs with the denaturation process. We determined the apparent urea stoichiometry considering the equilibrium reaction of TMV dissociation and subunit denaturation, which furnished, respectively, 1.53 and 11.1 mol of urea/mol of TMV subunit. The denaturation and dissociation conditions were arrived in a near reversible pathway, allowing the determination of thermodynamic parameters. Gel filtration HPLC, electron microscopy and circular dichroism confirmed the dissociation and denaturation processes. Based on spectroscopic results from earlier papers, the calculation of the apparent urea stoichiometry of dissociation and denaturation of several other viruses resulted in similar values, suggesting a similar virus-urea interaction among these systems.

Long-lived conformational isomerism of protein dimers: the role of the free energy of subunit association

The association of protein subunits to form N-mers (N >or= 3) does not follow the dependence on the law of mass action predicted by the classical thermodynamic description used for the equilibrium of association of small molecules. For those anomalous cases, a so-called deterministic model has been previously proposed. The latter model was based on the empirical observation that the dynamics of subunit exchange between protein oligomers can be very slow, leading to the existence of long-lived conformational isomers and to a persistently heterogeneous ensemble of oligomers in solution. Contrary to the expectation for a protein dimer, we have recently shown that the subunit association of triosephosphate isomerase (TIM) could also be described as a deterministic process and that long-lived conformational isomers of TIM could be isolated in solution. Here we show that a), observation of hysteresis in pressure dissociation curves is an additional indicator of deterministic behavior; b), the extent of deviation from the classical thermodynamic behavior correlates with the free-energy change of subunit association; and c), experimental manipulation of the free energy of subunit association through the addition of a subdenaturing concentration of a chaotropic agent restores the concentration dependence of subunit association of TIM. A model that explains these features and its biological relevance is discussed.

Dissecting the role of protein-protein and protein-nucleic acid interactions in MS2 bacteriophage stability

To investigate the role of protein-protein and protein-nucleic acid interactions in virus assembly, we compared the stabilities of native bacteriophage MS2, virus-like particles (VLPs) containing nonviral RNAs, and an assembly-defective coat protein mutant (dlFG) and its single-chain variant (sc-dlFG). Physical (high pressure) and chemical (urea and guanidine hydrochloride) agents were used to promote virus disassembly and protein denaturation, and the changes in virus and protein structure were monitored by measuring tryptophan intrinsic fluorescence, bis-ANS probe fluorescence, and light scattering. We found that VLPs dissociate into capsid proteins that remain folded and more stable than the proteins dissociated from authentic particles. The proposed model is that the capsid disassembles but the protein remains bound to the heterologous RNA encased by VLPs. The dlFG dimerizes correctly, but fails to assemble into capsids, because it lacks the 15-amino acid FG loop involved in inter-dimer interactions at the viral fivefold and quasi-sixfold axes. This protein was very unstable and, when compared with the dissociation/denaturation of the VLPs and the wild-type virus, it was much more susceptible to chemical and physical perturbation. Genetic fusion of the two subunits of the dimer in the single-chain dimer sc-dlFG stabilized the protein, as did the presence of 34-bp poly(GC) DNA. These studies reveal mechanisms by which interactions in the capsid lattice can be sufficiently stable and specific to ensure assembly, and they shed light on the processes that lead to the formation of infectious viral particles.

Pressure denaturation of staphylococcal nuclease studied by neutron small-angle scattering and molecular simulation

We studied the pressure-induced folding/unfolding transition of staphylococcal nuclease (SN) over a pressure range of approximately 1-3 kilobars at 25 degrees C by small-angle neutron scattering and molecular dynamics simulations. We find that applying pressure leads to a twofold increase in the radius of gyration derived from the small-angle neutron scattering spectra, and P(r), the pair distance distribution function, broadens and shows a transition from a unimodal to a bimodal distribution as the protein unfolds. The results indicate that the globular structure of SN is retained across the folding/unfolding transition although this structure is less compact and elongated relative to the native structure. Pressure-induced unfolding is initiated in the molecular dynamics simulations by inserting water molecules into the protein interior and applying pressure. The P(r) calculated from these simulations likewise broadens and shows a similar unimodal-to-bimodal transition with increasing pressure. The simulations also reveal that the bimodal P(r) for the pressure-unfolded state arises as the protein expands and forms two subdomains that effectively diffuse apart during initial stages of unfolding. Hydrophobic contact maps derived from the simulations show that water insertions into the protein interior and the application of pressure together destabilize hydrophobic contacts between these two subdomains. The findings support a mechanism for the pressure-induced unfolding of SN in which water penetration into the hydrophobic core plays a central role.

Structural studies of MS2 bacteriophage virus particle disassembly by nuclear magnetic resonance relaxation measurements

In this article we studied, by nuclear magnetic resonance relaxation measurements, the disassembly of a virus particle-the MS2 bacteriophage. MS2 is one of the single-stranded RNA bacteriophages that infect Escherichia coli. At pH 4.5, the phage turns to a metastable state, as is indicated by an increase in the observed nuclear magnetic resonance signal intensity upon decreasing the pH from 7.0 to 4.5. Steady-state fluorescence and circular dichroism spectra at pH 4.5 show that the difference in conformation and secondary structure is not pronounced if compared with the phage at pH 7.0. At pH 4.5, two-dimensional (15)N-(1)H heteronuclear multiple quantum coherence (HMQC) spectrum shows approximately 40 crosspeaks, corresponding to the most mobile residues of MS2 coat protein at pH 4.5. The (15)N linewidth is approximately 30 Hz, which is consistent with an intermediate with a rotational relaxation time of 100 ns. The average spin lattice relaxation time (T(1)) of the mobile residues was measured at different temperatures, clearly distinguishing between the dimer and the equilibrium intermediate. The results show, for the first time, the presence of intermediates in the process of dissociation of the MS2 bacteriophage.

Observed hysteresis of virus capsid disassembly is implicit in kinetic models of assembly

For many protein multimers, association and dissociation reactions fail to reach the same end point; there is hysteresis preventing one and/or the other reaction from equilibrating. We have studied in vitro assembly of dimeric hepatitis B virus (HBV) capsid protein and dissociation of the resulting T = 4 icosahedral capsids. Empty HBV capsids composed of 120 capsid protein dimers were more resistant to dissociation by dilution or denaturants than anticipated from assembly experiments. Using intrinsic fluorescence, circular dichroism, and size exclusion chromatography, we showed that denaturants dissociate the HBV capsids without unfolding the capsid protein; unfolding of dimer only occurred at higher denaturant concentrations. The apparent energy of interaction between dimers measured in dissociation experiments was much stronger than when measured in assembly studies. Unlike assembly, capsid dissociation did not have the concentration dependence expected for a 120-subunit complex; consequently the apparent association energy systematically varied with reactant concentration. These data are evidence of hysteresis for HBV capsid dissociation. Simulations of capsid assembly and dissociation reactions recapitulate and provide an explanation for the observed behavior; these results are also applicable to oligomeric and multidomain proteins. In our calculations, we find that dissociation is impeded by temporally elevated concentrations of intermediates; this has the paradoxical effect of favoring re-assembly of those intermediates despite the global trend toward dissociation. Hysteresis masks all but the most dramatic decreases in contact energy. In contrast, assembly reactions rapidly approach equilibrium. These results provide the first rigorous explanation of how virus capsids can remain intact under extreme conditions but are still capable of "breathing." A biological implication of enhanced stability is that a triggering event may be required to initiate virus uncoating.

Role of hydration in the closed-to-open transition involved in Ca2+ binding by troponin C

Troponin C (TnC) is the Ca(2+)-binding subunit of the troponin complex of vertebrate skeletal muscle. It consists of two structurally homologous domains, N and C, connected by an exposed alpha-helix. The C-domain has two high-affinity sites for Ca(2+) that also bind Mg(2+), whereas the N-domain has two low-affinity sites for Ca(2+). Previous studies using isolated N- and C-domains showed that the C-domain apo form was less stable than the N-domain. Here we analyzed the stability of isolated N-domain (F29W/N-domain) against urea and pressure denaturation in the absence and in the presence of glycerol using fluorescence spectroscopy. Increasing the glycerol concentration promoted an increase in the stability of the protein to urea (0-8 M) in the absence of Ca(2+). Furthermore, the ability to expose hydrophobic surfaces normally promoted by Ca(2+) binding or low temperature under pressure was partially lost in the presence of 20% (v/v) glycerol. Glycerol also led to a decrease in the Ca(2+) affinity of the N-domain in solution. From the ln K(obs) versus ln a(H)2(O), we obtained the number of water molecules (63.5 +/- 8.7) involved in the transition N <=>N:Ca(2) that corresponds to an increase in the exposed surface area of 571.5 +/- 78.3 A(2). In skinned fibers, the affinity for Ca(2+) was also reduced by glycerol, although the effect was much less pronounced than in solution. Our results demonstrate quantitatively that the stability of this protein and its affinity for Ca(2+) are critically dependent on protein hydration.

Inactivation of classical swine fever virus: association of hydrostatic pressure and ultraviolet irradiation

Reversible pressure-induced disassembly of several viruses has suggested the idea of using hydrostatic pressure to suppress virus infectivity. In this study, the effects of high hydrostatic pressure and ultraviolet (UV) irradiation were investigated on classical swine fever virus (CSFV) in an attempt to eliminate residual infectivity. The structural modifications were followed by intrinsic fluorescence and biological activity assays. The kinetics of CSFV inactivation showed that pressure-induced inactivation was not enough to eliminate viral infectivity. However, when pressure was applied in association with UV irradiation no infectious focus was observed. The application of these two methods against CSFV can be an attractive inactivation strategy for the development of a vaccine.

Protein-RNA interactions and virus stability as probed by the dynamics of tryptophan side chains

The correlation between dynamics and stability of icosahedral viruses was studied by steady-state and time-resolved fluorescence approaches. We compared the environment and dynamics of tryptophan side chains of empty capsids and ribonucleoprotein particles of two icosahedral viruses from the comovirus group: cowpea mosaic virus (CPMV) and bean pod mottle virus (BPMV). We found a great difference between tryptophan fluorescence emission spectra of the ribonucleoprotein particles and the empty capsids of BPMV. For CPMV, time-resolved fluorescence revealed differences in the tryptophan environments of the capsid protein. The excited-state lifetimes of tryptophan residues were significantly modified by the presence of RNA in the capsid. More than half of the emission of the tryptophans in the ribonucleoprotein particles of CPMV originates from a single exponential decay that can be explained by a similar, nonpolar environment in the local structure of most of the tryptophans, even though they are physically located in different regions of the x-ray structure. CPMV particles without RNA lost this discrete component of emission. Anisotropy decay measurements demonstrated that tryptophans rotate faster in empty particles when compared with the ribonucleoprotein particles. The increased structural breathing facilitates the denaturation of the empty particles. Our studies bring new insights into the intricate interactions between protein and RNA where part of the missing structural information on the nucleic acid molecule is compensated for by the dynamics.

Expanding the pressure technique: insights into protein folding from combined use of pressure and chemical denaturants

The fundamental principles derived from in vitro protein folding experiments have practical application in understanding the pathology of diseases of protein misfolding and for the development of industrial processes to produce proteins as pharmaceuticals and biotechnological reagents. High pressure as a tool to denature or disaggregate proteins offers a number of unique advantages. The emphasis of this review is on how low concentrations of chemical denaturants can be used in combination with high pressure to extend the range and scope of this useful technique. This approach has already been used in a number of studies, which are discussed here in the context of the questions they address. These include: the origin of the volume change observed on protein unfolding, pressure-induced formation of partially structured intermediates, pressure-induced dissociation of oligomeric and aggregated proteins, and the use of volume changes to probe the structure of the transition state. Wider use of hydrostatic pressure as a denaturation tool, facilitated by combination with chemical denaturants, is likely to bring significant advances to our understanding of protein structure, stability and folding, particularly in relation to proteins associated with the amyloid and prion diseases.

PABMB Lecture. Protein dynamics, folding and misfolding: from basic physical chemistry to human conformational diseases

Proteins exhibit a variety of motions ranging from amino acid side-chain rotations to the motions of large domains. Recognition of their conformational flexibility has led to the view that protein molecules undergo fast dynamic interconversion between different conformational substates. This proposal has received support from a wide variety of experimental techniques and from computer simulations of protein dynamics. More recently, studies of the subunit dissociation of oligomeric proteins induced by hydrostatic pressure have shown that the characteristic times for subunit exchange between oligomers and for interconversion between different conformations may be rather slow (hours or days). In such cases, proteins cannot be treated as an ensemble of rapidly interconverting conformational substates, but rather as a persistently heterogeneous population of different long-lived conformers. This is reminiscent of the deterministic behavior exhibited by macroscopic bodies, and may have important implications for our understanding of protein folding and biological functions. Here, we propose that the deterministic behavior of proteins may be closely related to the genesis of conformational diseases, a class of pathological conditions that includes transmissible spongiform encephalopathies, Alzheimer's disease and other amyloidosis.

Local heterogeneity in the pressure denaturation of the coiled-coil tropomyosin because of subdomain folding units

Coiled-coil domains mediate the oligomerization of many proteins. The assembly of long coiled coils, such as tropomyosin, presupposes the existence of intermediates. These intermediates are not well-known for tropomyosin. Hydrostatic pressure affects the equilibrium between denatured and native forms in the direction of the form that occupies a smaller volume. The hydrophobic core is the region more sensitive to pressure, which leads in most cases to the population of intermediates. Here, we used N-(1-pyrenyl)iodoacetamide covalently bound to cysteine residues of tropomyosin (PIATm) and high hydrostatic pressure to assess the chain interaction and the inherent instability of the coiled-coil molecule. The native and denatured states of tropomyosin were determined from the pyrene excimer fluorescence. The combination of low temperature and high pressure permitted the attainment of the full denaturation of tropomyosin without the separation of the subunits. High-temperature denaturation of Tm leads to a great exchange between labeled and unlabeled Tm subunits, indicating subunit dissociation linked to unfolding. In contrast, under high pressure, unlabeled and labeled tropomyosin molecules do not exchange, demonstrating that the denatured species are dimeric. The decrease of the concentration dependence of PIATm corroborates the idea that pressure produces subdomain denaturation and that the polypeptide chains do not separate. Substantial unfolding of tropomyosin was also verified by measurements of tyrosine fluorescence and bis-ANS binding. Our results indicate the presence of independent folding subdomains with different susceptibilities to pressure along the length of the coiled-coil structure of tropomyosin.

Virus inactivation by anilinonaphthalene sulfonate compounds and comparison with other ligands

Bis-(8-anilinonaphthalene-1-sulfonate) (bis-ANS) causes inactivation of vesicular stomatitis virus (VSV) at micromolar concentrations while butyl-ANS and ANS are effective at concentrations one and two orders of magnitude higher, respectively. VSV fully inactivated by the combined effects of 10 microM bis-ANS and 2.5 kbar hydrostatic pressure elicited a high titer of neutralizing antibodies. Incubation of VSV with >/=2 M urea at atmospheric pressure caused very little virus inactivation, whereas at a pressure of 2.5 kbar, 1 M urea caused inactivation that exceeded by more than two orders of magnitude the sum of the inactivating effects produced by urea and pressure separately. Measurements of bis-ANS fluorescence showed that increasing the urea concentration reduces the pressure required to disrupt the structure. We conclude that anilinonaphthalene sulfonate compounds inactivate VSV by a mechanism similar to that produced by pressure. The most effective antiviral compound was bis-ANS which can be used for the preparation of safe viral vaccines or as an antiviral drug eventually.

Effects of hydrostatic pressure on the structure and biological activity of infectious bursal disease virus

The effects of high hydrostatic pressure on the structure and biological activity of infectious bursal disease virus (IBDV), a commercially important pathogen of chickens, were investigated. IBDV was completely dissociated into subunits at a pressure of 240 MPa and 0 degrees C revealed by the change in intrinsic fluorescence spectrum and light scattering. The dissociation of IBDV showed abnormal concentration dependence as observed for some other viruses. Electron microscopy study showed that morphology of IBDV had an obvious change after pressure treatment at 0 degrees C. It was found that elevating pressure destroyed the infectivity of IBDV, and a completely pressure-inactivated IBDV could be obtained under proper conditions. The pressure-inactivated IBDV retained the original immunogenic properties and could elicit high titers of virus neutralizing antibodies. These results indicate that hydrostatic pressure provides a potential physical means to prepare antiviral vaccine.

Conformational changes preceding decapsidation of bromegrass mosaic virus under hydrostatic pressure: a small-angle neutron scattering study

The stability of bromegrass mosaic virus (BMV) and empty shells reassembled in vitro from purified BMV coat protein was investigated under hydrostatic pressure, using solution small-angle neutron scattering. This technique allowed us to monitor directly the dissociation of the particles, and to detect conformational changes preceding dissociation. Significant dissociation rates were observed only if virions swelled upon increase of pressure, and pressure effects became irreversible at very high-pressure in such conditions. At pH 5.0, in buffers containing 0.5 M NaCl and 5 mM MgCl(2), BMV remained compact (radius 12.9 nm), dissociation was limited to approximately 10 % at 200 MPa, and pressure effects were totally reversible. At pH 5.9, BMV particles were slightly swollen under normal pressure and swelling increased with pressure. The dissociation was reversible to 90 % for pressures up to 160 MPa, where its rate reached 28 %, but became totally irreversible at 200 MPa. Pressure-induced swelling and dissociation increased further at pH 7.3, but were essentially irreversible. The presence of (2)H(2)O in the buffer strongly stabilized BMV against pressure effects at pH 5.9, but not at pH 7.3. Furthermore, the reversible changes of the scattered intensity observed at pH 5.0 and 5.9 provide evidence that pressure could induce the release of coat protein subunits, or small aggregates of these subunits from the virions, and that the dissociated components reassociated again upon return to low pressure. Empty shells were stable at pH 5.0, at pressures up to 260 MPa. They became ill-shaped at high-pressure, however, and precipitated slowly after return to normal conditions, providing the first example of a pressure-induced conformational drift in an assembled system.

Polymorphism of turnip yellow mosaic virus empty shells and evidence for conformational changes occurring after release of the viral RNA. A differential scanning calorimetric study

Turnip yellow mosaic virus (TYMV) is a small isometric plant virus which decapsidates by releasing its RNA through a hole in the capsid, leaving behind an empty shell [R. E. F. Matthews and J. Witz, (1985) Virology 144, 318-327]. Similar empty shells (artificial top component, ATC) can be obtained by submitting the virions to various treatments in vitro. We have used differential scanning calorimetry, analytical sedimentation, and electron microscopy to investigate the thermodenaturation of natural empty shells (NTC, natural top component) present in purified virus suspensions, and of several types of ATCs. ATCs divided in two major classes. Those obtained by alkaline titration, by the action of urea or butanol behaved as NTC: their thermograms contained only one peak corresponding to the irreversible dissociation of the shells and the denaturation of the coat protein. The temperature of this unique transition varied significantly with pH, from 71 degrees C at pH 4.5 to 84 degrees C at pH 8.5. The thermograms of ATCs obtained by freezing and thawing, or by the action of high pressure, contained two peaks: shells dissociated first into smaller protein aggregates at 57 degrees C (at pH 5.0) to 61 degrees C (at pH 8.5), which denatured at the temperature of the unique transition of NTC. Shells obtained by heating virions to 55 degrees C at pH 7.6, changed conformation after the release of the viral RNA, as upon continuous heating to 95 degrees C, their thermograms were similar to those of the shells obtained by freezing and thawing, whereas after purification they behaved like NTC. Structural implications of these observations are discussed.

Low temperature and pressure stability of picornaviruses: implications for virus uncoating

The family Picornaviridae includes several viruses of great economic and medical importance. Poliovirus replicates in the human digestive tract, causing disease that may range in severity from a mild infection to a fatal paralysis. The human rhinovirus is the most important etiologic agent of the common cold in adults and children. Foot-and-mouth disease virus (FMDV) causes one of the most economically important diseases in cattle. These viruses have in common a capsid structure composed of 60 copies of four different proteins, VP1 to VP4, and their 3D structures show similar general features. In this study we describe the differences in stability against high pressure and cold denaturation of these viruses. Both poliovirus and rhinovirus are stable to high pressure at room temperature, because pressures up to 2.4 kbar are not enough to promote viral disassembly and inactivation. Within the same pressure range, FMDV particles are dramatically affected by pressure, with a loss of infectivity of more than 4 log units observed. The dissociation of polio and rhino viruses can be observed only under pressure (2.4 kbar) at low temperatures in the presence of subdenaturing concentrations of urea (1-2 M). The pressure and low temperature data reveal clear differences in stability among the three picornaviruses, FMDV being the most sensitive, polio being the most resistant, and rhino having intermediate stability. Whereas rhino and poliovirus differ little in stability (less than 10 kcal/mol at 0 degrees C), the difference in free energy between these two viruses and FMDV was remarkable (more than 200 kcal/mol of particle). These differences are crucial to understanding the different factors that control the assembly and disassembly of the virus particles during their life cycle. The inactivation of these viruses by pressure (combined or not with low temperature) has potential as a method for producing vaccines.

Tobacco mosaic virus disassembly by high hydrostatic pressure in combination with urea and low temperature

We investigated the effect of low temperature and urea combined with high pressure on tobacco mosaic virus (TMV). The evaluation of its aggregation state and denaturation process was studied using gel filtration, transmission electron microscopy, and spectroscopic methods. The incubation at 2.5 kbar induced 18% dissociation, and decreasing of temperature to -19 degreesC promoted additional dissociation to 72%, with stabilization of the dissociation products. Under such conditions, extensive denaturation did not occur. The apparent enthalpy and entropy of dissociation (Delta and TDelta) were -9.04 kcal/mol subunit and -15.1 kcal/mol subunit, respectively, indicating that the TMV association is an entropicly driven process. The apparent free energy of stabilization given by the presence of RNA is at least -1.7 kcal/mol subunit. Urea-induced dissociation of TMV samples and incubation at high-pressure promoted a higher degree of dissociation. The volume change of dissociation decreased in magnitude from -16.3 to -3.1 mL/mol of dissociated subunit, respectively, in the absence and presence of 2.5 M urea, suggesting exposure of the protein-protein interface to the solvent. High-pressure induced remarkable TMV denaturation in the presence of 2.5 M urea, with a volume change of -101 mL/mol of denatured subunit. The apparent enthalpy and entropy of denaturation (Delta and TDelta) by 1.75 M urea at 2.5 kbar was -11.1 and -10.2 kcal/mol subunit, respectively, demonstrating that the TMV protein coat presents an apparent free energy of denaturation by urea close to zero. Although the processes could not be assumed to be pure equilibria, these thermodynamic parameters could be derived by assuming a steady-state condition.

Partially folded states of the capsid protein of cowpea severe mosaic virus in the disassembly pathway

The different partially folded states of the capsid protein that appear in the disassembly pathway of cowpea severe mosaic virus (CPSMV) were investigated by examining the effects of hydrostatic pressure, sub-zero temperatures and urea. The conformational states of the coat protein were analyzed by their intrinsic fluorescence, binding of bis(8-anilinonaphthalene-1-sulfonate) (bis-ANS) and susceptibility to trypsin digestion. CPSMV could be disassembled by pressure at 2.5 kbar. Intrinsic fluorescence and hydrodynamic measurements showed that pressure-induced dissociation was completely reversible. Virus pressurization in the presence of ribonuclease revealed that viral RNA was not exposed, since it was not digested by the enzyme, suggesting the maintenance of protein-nucleic acid interactions under pressure. When the temperature was decreased to -10 degrees C under pressure, CPSMV disassembly became an irreversible process and in this condition, viral RNA was completely digested by ribonuclease. These results suggest a relationship between protein-RNA interactions and CPSMV assembly. Bis-ANS binding and trypsin digestion of coat proteins revealed that they assume a different conformation when they are denatured by low temperatures under pressure or than when they are denatured by urea at atmospheric pressure. The results indicate that the coat proteins can exist in at least four states: (1) The native conformation in the virus capsid; (2) bound to RNA when the virus is dissociated by pressure at room temperature, assuming a conformation that retains the information for reassembly; (3) free subunits in a molten-globule conformation when the virus is dissociated by low temperature under pressure; and (4) free subunits completely unfolded by high concentrations of urea.

Pressure-induced dissociation of yeast glyceraldehyde-3-phosphate dehydrogenase: heterogeneous kinetics and perturbations of subunit structure

In studies of pressure-induced subunit dissociation of oligomeric proteins, the thermodynamic dissociation constant and the dissociation volume change are derived by assuming that high pressure itself does not significantly perturb the structure of both oligomer and isolated subunit. In this report, the intrinsic phosphorescence emission of Trp reveals that high-pressure dissociation of tetrameric yeast glyceraldehyde-3-phosphate dehydrogenase results in a dramatic shortening of the phosphorescence lifetime, from 300 to less than 2 ms, that is consistent with a profound loosening of the polypeptide structure about the phosphorescence probe. On pressure release, subunit reassociation occurs readily whereas recovery of the native phosphorescence properties is a very slow, thermally activated, process which goes hand in hand with the recovery of the catalytic activity. Further, the comparison between the kinetic traces that describe the degree of dissociation and the change in phosphorescence lifetime, at various applied pressures, has established the following: (1) that high pressure plays a direct role on the structural rearrangement, the extent of which increases with pressure; (2) that the conformational change in the monomer is concomitant with, or follows closely after, the break up of the tetramer, in any case long before an apparent tetramer-monomer equilibrium is established; (3) that native tetramers are highly heterogeneous with regard to their rate of dissociation. The influence of temperature, of protein concentration, of binding of NAD+, and of the addition of 2 M urea on the dissociation/phosphorescence kinetic profiles was also examined. The complications arising from these conformational changes for the derivation of the dissociation free energy change as well as their relevance for understanding the lack of concentration dependence of the degree of dissociation are discussed.

The use of hydrostatic pressure as a tool to study viruses and other macromolecular assemblages

Recent studies on the effect of pressure on macromolecular assemblages have provided new information on protein-protein and protein-nucleic acid interactions. New findings have recently emerged on the use of hydrostatic pressure to assess intermediate states in the assembly pathways of viruses, multimeric proteins and protein-nucleic acid complexes, addressing many questions of macromolecular recognition.

Concentration dependence of the subunit association of oligomers and viruses and the modification of the latter by urea binding

A theoretical model is presented that accounts for the facilitation of the pressure dissociation of R17 phage, and for the partial restoration of the concentration dependence of the dissociation, by the presence of subdenaturing concentrations of urea. As an indifferent osmolyte urea should promote the stability of the protein aggregates under pressure, and the decrease in pressure stability with urea concentration demonstrates that such indirect solvent effects are not significant for this case, and that the progressive destabilization is the result of direct protein-urea interactions. By acting as a "homogenizer" of the properties of the phage particles, urea addition converts the pressure-induced deterministic dissociation of the phage into a limited stochastic equilibrium. The model establishes the origin of the uniform progression from the stochastic equilibrium of dimers, to the temperature-dependent and partially concentration-dependent association of tetramers, to the fully deterministic equilibrium observed in many multimers and in the virus capsids.

Inactivation of simian immunodeficiency virus by hydrostatic pressure

The inactivation of the simian immunodeficiency viruses SIVmac251 and SIVagm by pressures of 150 and 250 MPa was determined. The extent of inactivation depended on the time that the virus was subjected to compression as well as the level of the pressure and at 150 Mpa reached 5 log10 dilution units after approximately 10 hr. The inactivations, which were uniformly carried out at room temperature, were independent of the concentration of the virus. Possible applications of pressure inactivation for molecular biological and clinical use are discussed.

Hydrostatic and osmotic pressure as tools to study macromolecular recognition

Clearly, hydrostatic and osmotic pressure techniques offer unique potential in the study of fundamental problems of molecular recognition in biological systems. With the recent advances in technology such investigations are rapidly becoming commonplace. We look forward to further advances and their report in succeeding compendiums such as this volume.

Pressure denaturation of the bacteriophage P22 coat protein and its entropic stabilization in icosahedral shells

The pressure stability of bacteriophage P22 coat protein in both monomeric and polymeric forms under hydrostatic pressure was examined using light scattering, fluorescence emission, polarization, and lifetime methodology. The monomeric protein is very unstable toward pressure and undergoes significant structural changes at pressures as low as 0.5 kbar. These structural changes ultimately lead to denaturation of the subunit. Comparison of the protein denatured by pressure to that in guanidine hydrochloride suggests that pressure results in partial unfolding, perhaps by a domain mechanism. Fluorescence lifetime measurements indicate that at atmospheric pressure the local environments of the tryptophans are remarkably similar, suggesting they may be clustered. In contrast to the monomeric protein subunit, the protein when polymerized into procapsid shells is very stable to applied pressure and does not dissociate with pressure up to 2.5 kbar. However, under applied pressure the procapsid shells are cold-labile, suggesting they are entropically stabilized. The significance of these results in terms of virus assembly are discussed.