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

Influence of iron binding in the structural stability and cellular internalization of bovine lactoferrin

Lactoferrin (Lf) is an iron-binding glycoprotein and a component of many external secretions with a wide diversity of functions. Structural studies are important to understand the mechanisms employed by Lf to exert so varied functions. Here, we used guanidine hydrochloride and high hydrostatic pressure to cause perturbations in the structure of bovine Lf (bLf) in apo and holo (unsaturated and iron-saturated, respectively) forms, and analyzed conformational changes by intrinsic and extrinsic fluorescence spectroscopy. Our results showed that the iron binding promotes changes on tertiary structure of bLf and increases its structural stability. In addition, we evaluated the effects of bLf structural change on the kinetics of bLf internalization in Vero cells by confocal fluorescence microscopy, and observed that the holo form was faster than the apo form. This finding may indicate that structural changes promoted by iron binding may play a key role in the intracellular traffic of bLf. Altogether, our data improve the comprehension of bLf stability and uptake, adding knowledge to its potential use as a biopharmaceutical.

Fast and Efficient non-reduced Lys-C digest using pressure cycling technology for antibody disulfide mapping by LC-MS

Conventional sample preparation for antibody disulfide mapping often requires relatively long digestion time (from several hours to overnight) and relatively high endoproteinase concentration. These conditions are typically necessitated by the fact that antibody molecules are not sufficiently denatured under non-reduced conditions and chaotropic agents are used during digestion to achieve optimal denaturation. Disulfide scrambling can occur as artifacts of digestion as proteins are incubated for extended periods, often at neutral to slightly alkaline pH conditions. Shortening digestion time and lowering the pH during digestion frequently result in incomplete peptide cleavages or variable recoveries. Here, we report the development of a fast and efficient non-reduced Lys-C digestion method based on pressure cycling technology (PCT) and its application in determining disulfide-linkages in monoclonal antibodies (mAbs). Conditions were optimized to ensure complete digestion of the mAb with minimal sample preparation-related disulfide scrambling. The PCT-based method was able to generate up to 10-fold signal increase for some disulfide peptides in a 1h Lys-C digestion compared to the conventional bench-top digestion method. As a result of the shorter digestion time, disulfide scrambling that is seen as a major assay artifact of the conventional method was reduced to less than 0.05% in tested molecules. The results show that the PCT-based method offers fast digestion in a shorter time for all the mAbs tested.

TNFR-Associated Factor-2 (TRAF2): Not Only a Trimer

TNF receptor-associated factors (TRAFs) are characterized by an oligomeric structure that plays a fundamental role in the binding process with membrane receptors. In this work, we studied the trimer-to-monomer (T ↔ 3M) equilibrium transition of the TRAF2 C-terminal domain using both chemical (dilution/guanidinium hydrochloride) and mechanical stress (high pressure) to induce the dissociation of the native protein into subunits. The experimental results and computer simulations indicate that stable monomers exist and that their population accounts for 15% of the total TRAF2 molecules already at a physiological intracellular concentration (≈1 μM), being instead the predominant species in the nanomolar concentration range. Because the total amount of TRAF2 changes during a cell cycle, the monomer-trimer equilibrium can be crucial for regulating the activities of TRAF2 in vivo.

High-pressure SAXS study of folded and unfolded ensembles of proteins

A structural interpretation of the thermodynamic stability of proteins requires an understanding of the structural properties of the unfolded state. High-pressure small-angle x-ray scattering was used to measure the effects of temperature, pressure, denaturants, and stabilizing osmolytes on the radii of gyration of folded and unfolded state ensembles of staphylococcal nuclease. A set of variants with the internal Val-66 replaced with Ala, Tyr, or Arg was used to examine how changes in the volume and polarity of an internal microcavity affect the dimensions of the native state and the pressure sensitivity of the ensemble. The unfolded state ensembles achieved for these proteins with high pressure were more compact than those achieved at high temperature, and were all very sensitive to the presence of urea and glycerol. Substitutions at the hydrophobic core detectably altered the conformation of the protein, even in the folded state. The introduction of a charged residue, such as Arg, inside the hydrophobic interior of a protein could dramatically alter the structural properties, even those of the unfolded state. The data suggest that a charge at an internal position can interfere with the formation of transient hydrophobic clusters in the unfolded state, and ensure that the pressure-unfolded form of a protein occupies the maximum volume possible. Only at high temperatures does the radius of gyration of the unfolded state ensemble approach the value for a statistical random coil.

Recent applications of Kirkwood-Buff theory to biological systems

The effect of cosolvents on biomolecular equilibria has traditionally been rationalized using simple binding models. More recently, a renewed interest in the use of Kirkwood-Buff (KB) theory to analyze solution mixtures has provided new information on the effects of osmolytes and denaturants and their interactions with biomolecules. Here we review the status of KB theory as applied to biological systems. In particular, the existing models of denaturation are analyzed in terms of KB theory, and the use of KB theory to interpret computer simulation data for these systems is discussed.

Pressure and Denaturants in the Unfolding of Triosephosphate Isomerase: The Monomeric Intermediates of the Enzymes from Saccharomyces cerevisiae and Entamoeba histolytica

Triosephosphate isomerase (TIM) is a dimeric enzyme formed by two identical (beta/alpha)8 barrels. In this work, we compare the unfolding and refolding of the TIMs from Entamoeba histolytica (EhTIM) and baker's yeast (yTIM). A monomeric intermediate was detected in the GdnHCl-induced unfolding of EhTIM. The thermodynamic, spectroscopic, catalytic, and hydrodynamic properties of this intermediate were found to be very similar to those previously described for a monomeric intermediate of yTIM observed in GdnHCl. Monomer unfolding was reversible for both TIMs; however, the dissociation step was reversible in yTIM and irreversible in EhTIM. Monomer unfolding induced by high pressure in the presence of GdnHCl was a reversible process. DeltaGUnf, DeltaVUnf, and P1/2 were obtained for the 0.7-1.2 M GdnHCl range. The linear extrapolation of these thermodynamic parameters to the absence of denaturant showed the same values for both intermediates. The DeltaVUnfH2O values calculated for EhTIM and yTIM monomeric intermediates are the same within experimental error (-57 +/- 10 and -76 +/- 14 mL/mol, respectively). These DeltaVUnf H2O values are smaller than those reported for the unfolding of monomeric proteins of similar size, suggesting that TIM intermediates are only partially hydrated. |DeltaVUnf| increased with denaturant concentration; this behavior is probably related to structural changes in the unfolded state induced by GdnHCl and pressure. From the thermodynamic parameters that were obtained, it is predicted that in the absence of denaturants, pressure would induce monomer unfolding (P1/2 approximately 140 MPa) prior to dimer dissociation (P1/2 approximately 580 MPa). Therefore, dimerization prevents the pressure unfolding of the monomer.

Theoretical study of the cosolvent effect on the partial molar volume change of staphylococcal nuclease associated with pressure denaturation

We explain the molecular mechanism of the effect of urea and glycerol cosolvents on the partial molar volume (PMV) change associated with the pressure denaturation of staphylococcal nuclease (SNase) protein recently observed in experiments. Native and denatured conformations of SNase are produced by using molecular dynamics simulations in water, and the PMV is obtained from the integral equation theory of molecular liquids called 3D-RISM, which is based on statistical mechanics. The PMV of the native SNase in water predicted by 3D-RISM theory is in good agreement with experiment. The PMV changes associated with pressure denaturation in water and in water-urea and water-glycerol mixtures are qualitatively reproduced. By analyzing the results obtained, we found two interesting cosolvent effects on the PMV: (1) both urea and glycerol cosolvents increase the PMVs of both native and denatured SNase compared to those in water and (2) both urea and glycerol cosolvents increase the PMV of denatured SNase more than that of native SNase. We also showed that these two observations can be explained in terms of the thermal volume, which is related to the packing effect of solvent molecules.

Physico-chemical properties of molten dimer ascorbate oxidase

The possible presence of dimeric unfolding intermediates might offer a clue to understanding the relationship between tertiary and quaternary structure formation in dimers. Ascorbate oxidase is a large dimeric enzyme that displays such an intermediate along its unfolding pathway. In this study the combined effect of high pressure and denaturing agents gave new insight on this intermediate and on the mechanism of its formation. The transition from native dimer to the dimeric intermediate is characterized by the release of copper ions forming the tri-nuclear copper center located at the interface between domain 2 and 3 of each subunit. This transition, which is pH-dependent, is accompanied by a decrease in volume, probably associated to electrostriction due to the loosening of intra-subunit electrostatic interactions. The dimeric species is present even at 3 x 10(8) Pa, providing evidence that mechanically or chemically induced unfolding lead to a similar intermediate state. Instead, dissociation occurs with an extremely large and negative volume change (DeltaV approximately -200 mL.mol(-1)) by pressurization in the presence of moderate amounts of denaturant. This volume change can be ascribed to the elimination of voids at the subunit interface. Furthermore, the combination of guanidine and high pressure uncovers the presence of a marginally stable (DeltaG approximately 2 kcal.mol(-1)) monomeric species (which was not observed in previous equilibrium unfolding measurements) that might be populated in the early folding steps of ascorbate oxidase. These findings provide new aspects of the protein folding pathway, further supporting the important role of quaternary interactions in the folding strategy of large dimeric enzymes.

The contribution of the residues from the main hydrophobic core of ribonuclease A to its pressure-folding transition state

The role of hydrophobic interactions established by the residues that belong to the main hydrophobic core of ribonuclease A in its pressure-folding transition state was investigated using the Phi-value method. The folding kinetics was studied using pressure-jump techniques both in the pressurization and depressurization directions. The ratio between the folding activation volume and the reaction volume (beta p-value), which is an index of the compactness or degree of solvation of the transition state, was calculated. All the positions analyzed presented fractional Phi f-values, and the lowest were those corresponding to the most critical positions for the ribonuclease A stability. The structure of the transition state of the hydrophobic core of ribonuclease A, from the point of view of formed interactions, is a relatively, uniformly expanded form of the folded structure with a mean Phi f-value of 0.43. This places it halfway between the folded and unfolded states. On the other hand, for the variants, the average of beta p-values is 0.4, suggesting a transition state that is 40% native-like. Altogether the results suggest that the pressure-folding transition state of ribonuclease A looks like a collapsed globule with some secondary structure and a weakened hydrophobic core. A good correlation was found between the Phi f-values and the Deltabeta p-values. Although the nature of the transition state inferred from pressure-induced folding studies and the results of the protein engineering method have been reported to be consistent for other proteins, to the best of our knowledge this is the first direct comparison using a set of mutants.

Pressure as a tool to study protein-unfolding/refolding processes: The case of ribonuclease A

This paper gives an overview of the application of high-pressure to study the folding/unfolding processes of proteins using Ribonuclease A as a model protein. A particular focus is the study of pressure-equilibrium unfolding and folding kinetics using variants and the information obtained by comparing these with the wild-type enzyme.

The yeast prion protein Ure2: Structure, function and folding

The Saccharomyces cerevisiae protein Ure2 functions as a regulator of nitrogen metabolism and as a glutathione-dependent peroxidase. Ure2 also has the characteristics of a prion, in that it can undergo a heritable conformational change to an aggregated state; the prion form of Ure2 loses the regulatory function, but the enzymatic function appears to be maintained. A number of factors are found to affect the prion properties of Ure2, including mutation and expression levels of molecular chaperones, and the effect of these factors on structure and stability are being investigated. The relationship between structure, function and folding for the yeast prion Ure2 are discussed.

Synergetic effects of pressure and chemical denaturant on protein unfolding: stability of a serine-type carboxyl protease, kumamolisin

Kumamolisin, a serine carboxyl proteinase, is very stable and hardly denatured by single perturbation of a chemical denaturant (urea), pressure (<500 MPa) or temperature (<65 degrees C). In order to investigate the cooperative effects of these three denaturing agents, DSC, CD, intrinsic fluorescence, and fourth derivative UV absorbance were measured under various conditions. By application of pressure to kumamolisin in 8 M urea solution, substantial red-shift in the center of fluorescence emission spectral mass was observed, and the corresponding blue-shift was observed for two major peaks in fourth derivative UV absorbance, under the similar urea-containing conditions. The denaturation curves were analyzed on the basis of a simple two-state model in order to obtain thermodynamic parameters (DeltaV, DeltaG, and m values), and the combined effects of denaturing agents are discussed, with the special interest in the large cavity and neighboring Trp residue in kumamolisin.

Main-chain Dominated Amyloid Structures Demonstrated by the Effect of High Pressure

It has been suggested that, while the globular native forms of proteins are a side-chain-dominated compact structure evolved by pursuing a unique fold with optimal packing of amino acid residues, amyloid fibrils are a main-chain-dominated structure with an extensive hydrogen bond network. To address this issue, the effects of hydrostatic pressure on amyloid fibrils of beta2-microglobulin (beta2-m), involved in dialysis-related amyloidosis, were studied. A systematic analysis at various pressures and concentrations of guanidine hydrochloride conducted by monitoring thioflavin T fluorescence, light-scattering, and tryptophan fluorescence revealed contrasting conformational changes occurring consecutively: first, a pressure-induced reorganization of fibrils and then a pressure-induced unfolding. The changes in volume as well as the observed structural changes indicate that the beta2-m amyloid fibrils under ambient pressure are less tightly packed with a larger number of cavities, consistent with the main-chain-dominated amyloid structure. Moreover, the amyloid structure without optimal packing will enable various isoforms to form, suggesting the structural basis of multiple forms of amyloid fibrils in contrast to the unique native-fold.

Volume and energy folding landscape of prion protein revealed by pressure

The main hypothesis for prion diseases proposes that the cellular protein (PrP C) can be altered into a misfolded, ss-sheet-rich isoform, the PrP Sc (from scrapie). The formation of this abnormal isoform then triggers the transmissible spongiform encephalopathies. Here, we discuss the use of high pressure as a tool to investigate this structural transition and to populate possible intermediates in the folding/unfolding pathway of the prion protein. The latest findings on the application of high pressure to the cellular prion protein and to the scrapie PrP forms will be summarized in this review, which focuses on the energetic and volumetric properties of prion folding and conversion.

Protein volume changes on cosolvent denaturation

A thermodynamic relationship is obtained which links the effect of a cosolvent on the denaturation equilibrium of a protein to the effect of the cosolvent on the change in partial molar volume (pmv) of a protein on denaturation. The relationship uses the concept of preferential interactions and is exact for an infinitely dilute protein. Analysis of the literature data on protein volume changes suggests that many of the observed volume changes are thermodynamically inconsistent with the corresponding free energy changes, especially at low cosolvent concentrations. It is argued that the most reasonable explanation for this involves cosolvent induced changes in the degree of protein-protein association.

The Kirkwood–Buff theory and the effect of cosolvents on biochemical reactions

Cosolvents added to aqueous solutions of biomolecules profoundly affect protein stability, as well as biochemical equilibria. Some cosolvents, such as urea and guanidine hydrochloride, denature proteins, whereas others, such as osmolytes and crowders, stabilize the native structures of proteins. The way cosolvents interact with biomolecules is crucial information required to understand the cosolvent effect at a molecular level. We present a statistical mechanical framework based upon Kirkwood-Buff theory, which enables one to extract this picture from experimental data. The combination of two experimental results, namely, the cosolvent-induced equilibrium shift and the partial molar volume change upon the reaction, supplimented by the structural change, is shown to yield the number of water and cosolvent molecules bound or released during a reaction. Previously, denaturation experiments (e.g., m-value analysis) were analyzed by empirical and stoichiometric solvent-binding models, while the effects of osmolytes and crowders were analyzed by the approximate molecular crowding approach for low cosolvent concentration. Here we synthesize these previous approaches in a rigorous statistical mechanical treatment, which is applicable at any cosolvent concentration. The usefulness and accuracy of previous approaches was also evaluated.

High hydrostatic pressure perturbs the interactions between CF(0)F(1) subunits and induces a dual effect on activity

Chloroplast ATP-synthase is an H(+)/ATP-driven rotary motor in which a hydrophobic multi-subunit assemblage rotates within a hydrophilic stator, and subunit interactions dictate alternate-site catalysis. To explore the relevance of these interactions for catalysis we use hydrostatic pressure to induce conformational changes and/or subunit dissociation, and the resulting changes in the ATPase activity and oligomer structure are evaluated. Under moderate hydrostatic pressure (up to 60-80 MPa), ATPase activity is increased by 1.5-fold. This is not related to an increase in the affinity for ATP, but seems to correlate with an enhanced turnover induced by pressure, and an activation volume for the ATPase reaction of -23.7 ml/mol. Higher pressure (up to 200 MPa) leads to dissociation of the enzyme, as shown by enzyme inactivation, increased binding of 8-anilinonaphthalene-1-sulfonate (ANS) to hydrophobic regions, and labeling of specific Cys residues on the beta and alpha subunits by N-iodoacetyl-N'-(5-sulfo-1-naphthyl)ethylene-4-diamine (IAEDANS). Compression-decompression cycles (between 0.1 and 200 MPa) inactivate CF(0)F(1) in a concentration-dependent manner, although after decompression no enzyme subunit is retained on a Sephadex-G-50 centrifuge column or is further labeled by IAEDANS. It is proposed that moderate hydrostatic pressures induce elastic compression of CF(0)F(1), leading to enhanced turnover. High pressure dissociation impairs the contacts needed for rotational catalysis.

High pressure induces scrapie-like prion protein misfolding and amyloid fibril formation

Our understanding of conformational conversion of proteins in diseases is essential for any diagnostic and therapeutic approach. Although not fully understood, misfolding of the prion protein (PrP) is implicated in the pathogenesis of prion diseases. Despite several efforts to produce the pathologically misfolded conformation in vitro from a recombinant PrP, no positive result has yet been obtained. Within the "protein-only hypothesis", the reason for this hindrance may be that the experimental conditions used did not allow selection of the pathway adopted in vivo resulting in conversion into the infectious form. Here, using a pressure perturbation approach, we show that recombinant PrP is converted to a novel misfolded conformer, which is prone to aggregate and ultimately form amyloid fibrils. A short incubation at high pressure (600 MPa) of the truncated form of hamster prion protein (SHaPrP(90-231)) resulted in the formation of pre-amyloid structures. The mostly globular aggregates were characterized by ThT and ANS binding, and by a beta-sheet-rich secondary structure. After overnight incubation at 600 MPa, amyloid fibrils were formed. In contrast to pre-amyloid structures, they showed birefringency of polarized light after Congo red staining and a strongly decreased ANS binding capacity, but enhanced ThT binding. Both aggregate types were resistant to digestion by PK, and can be considered as potential scrapie-like forms or precursors. These results may be useful for the search for compounds preventing pathogenic PrP misfolding and aggregation.

High-pressure fluorescence correlation spectroscopy

We demonstrate that a novel high-pressure cell is suitable for fluorescence correlation spectroscopy (FCS). The pressure cell consists of a single fused silica microcapillary. The cylindrical shape of the capillary leads to refraction of the excitation light, which affects the point spread function of the system. We characterize the influence of these beam distortions by FCS and photon-counting histogram (PCH) analysis and identify the optimal position for fluorescence fluctuation experiments in the capillary. At this position within the capillary, FCS and photon-counting histogram experiments are described by the same equations as used in standard FCS experiments. We report the first experimental realization of fluorescence fluctuation spectroscopy under high pressure. A fluorescent dye was used as a model system for evaluating the properties of the capillary under pressure. The autocorrelation function and the photon count distribution were measured in the pressure range from 0 to 300 MPa. The fluctuation amplitude and the diffusion coefficient show a small pressure dependence. The changes of these parameters, which are on the order of 10%, are due to the pressure changes of the viscosity and the density of the aqueous medium.

Folding intermediates of the prion protein stabilized by hydrostatic pressure and low temperature

Prion diseases are associated with conformational conversion of the cellular prion protein, PrPC, into a misfolded form, PrPSc. We have investigated the equilibrium unfolding of the structured domain of recombinant murine prion protein, comprising residues 121-231 (mPrP-(121-231)). The equilibrium unfolding of mPrP-(121-231) by urea monitored by intrinsic fluorescence and circular dichroism (CD) spectroscopies indicated a two-state transition, without detectable folding intermediates. The fluorescent probe 4,4'-dianilino-1,1'-binaphthyl-5,5-disulfonic acid (bis-ANS) binds to native mPrP-(121-231), indicating exposure of hydrophobic domains on the protein surface. Increasing concentrations of urea (up to 4 M) caused the release of bound bis-ANS, whereas changes in intrinsic fluorescence and CD of mPrP took place only above 4 M urea. This indicates the existence of a partially unfolded conformation of mPrP, characterized by loss of bis-ANS binding and preservation of the overall structure of the protein, stabilized at low concentrations of urea. Hydrostatic pressure and low temperatures were also used to stabilize partially folded intermediates that are not detectable in the presence of chemical denaturants. Compression of mPrP to 3.5 kbar at 25 degrees C and pH 7 caused a slight decrease in intrinsic fluorescence emission and an 8-fold increase in bis-ANS fluorescence. Lowering the temperature to -9 degrees C under pressure reversed the decrease in intrinsic fluorescence and caused a marked (approximately 40-fold) increase in bis-ANS fluorescence. The increase in bis-ANS fluorescence at low temperatures was similar to that observed for mPrP at 1 atm at pH 4. These results suggest that pressure-assisted cold denaturation of mPrP stabilizes a partially folded intermediate that is qualitatively similar to the state obtained at acidic pH. Compression of mPrP in the presence of a subdenaturing concentration of urea stabilized another partially folded intermediate, and cold denaturation under these conditions led to complete unfolding of the protein. Possible implications of the existence of such partially folded intermediates in the folding of the prion protein and in the conversion to the PrPSc conformer are discussed.

Structural instability and fibrillar aggregation of non-expanded human ataxin-3 revealed under high pressure and temperature

Protein misfolding and formation of structured aggregates are considered to be the earliest events in the development of neurodegenerative diseases, but the mechanism of these biological phenomena remains to be elucidated. Here, we report a study of heat- and pressure-induced unfolding of human Q26 and murine Q6 ataxin-3 using spectroscopic methods. UV absorbance and fluorescence revealed that heat and pressure induced a structural transition of both proteins to a molten globule conformation. The unfolding pathway was partly irreversible and led to a protein conformation where tryptophans were more exposed to water. Furthermore, the use of fluorescent probes (8-anilino-1-naphthalenesulfonate and thioflavin T) allowed the identification of different intermediates during the process of pressure-induced unfolding. At high temperature and pressure, human Q26, but not murine Q6, underwent concentration-dependent aggregation. Fourier transform infrared and circular dichroism spectroscopy revealed that these aggregates are characterized by an increased beta-sheet content. As revealed by electron microscopy, heat- and pressure-induced aggregates were different; high temperature treatment led to fibrillar microaggregates (8-10-nm length), whereas high pressure induced oligomeric structures of globular shape (100 nm in diameter), which sometimes aligned to higher order suprastructures. Several intermediate structures were detected in this process. Two factors appear to govern ataxin unfolding and aggregation, the length of the polyglutamine tract and its protein context.

Ultra-high-pressure inactivation of prion infectivity in processed meat: a practical method to prevent human infection

Bovine spongiform encephalopathy contamination of the human food chain most likely resulted from nervous system tissue in mechanically recovered meat used in the manufacture of processed meats. We spiked hot dogs with 263K hamster-adapted scrapie brain (10% wtwt) to produce an infectivity level of approximately 9 log(10) mean lethal doses (LD(50)) per g of paste homogenate. Aliquots were subjected to short pressure pulses of 690, 1,000, and 1,200 MPa at running temperatures of 121-137 degrees C. Western blots of PrPres were found to be useful indicators of infectivity levels, which at all tested pressures were significantly reduced as compared with untreated controls: from approximately 10(3) LD(50) per g at 690 MPa to approximately 10(6) LD(50) per g at 1,200 MPa. The application of commercially practical conditions of temperature and pressure could ensure the safety of processed meats from bovine spongiform encephalopathy contamination, and could also be used to study phase transitions of the prion protein from its normal to misfolded state.

Optimized overproduction, purification, characterization and high-pressure sensitivity of the prion protein in the native (PrP(C)-like) or amyloid (PrP(Sc)-like) conformation

Overproduction and purification of the prion protein is a major concern for biological or biophysical analysis as are the structural specificities of this protein in relation to infectivity. We have developed a method for the effective cloning, overexpression in Escherichia coli and purification to homogeneity of Syrian golden hamster prion protein (SHaPrP(90-231)). A high level of overexpression, resulting in the formation of inclusion bodies, was obtained under the control of the T7-inducible promoter of the pET15b plasmid. The protein required denaturation, reduction and refolding steps to become soluble and attain its native conformation. Purification was carried out by differential centrifugation, gel filtration and reverse phase chromatography. An improved cysteine oxidation protocol using oxidized glutathione under denaturing conditions, resulted in the recovery of a higher yield of chromatographically pure protein. About 10 mg of PrP protein per liter of bacterial culture was obtained. The recombinant protein was identified by monoclonal antibodies and its integrity was confirmed by electrospray mass spectrometry (ES/MS), whereas correct folding was assessed by circular dichroism (CD) spectroscopy. This protein had the structural characteristics of PrP(C) and could be converted to an amyloid structure sharing biophysical and biochemical properties of the pathologic form (PrP(Sc)). The sensitivity of these two forms to high pressure was investigated. We demonstrate the potential of using pressure as a thermodynamic parameter to rescue trapped aggregated prion conformations into a soluble state, and to explore new conformational coordinates of the prion protein conformational landscape.

Alternative prion structural changes revealed by high pressure

At high temperature, recombinant hamster prion protein (SHaPrP(90-231)) undergoes aggregation and changes from a predominantly alpha-helical to beta-sheet conformation. We then applied high pressure (200 MPa) to the beta-sheet-rich conformation. The aggregation was reversed, and the original tertiary and secondary structures were recovered at ambient pressure, after pressure release. The application of a pressure of 200 MPa thus allowed studying the heat-induced equilibrium refolding in the absence of protein aggregation. Prion protein unfolding as a function of high pressure was also investigated. Simple two-state, reversible unfolding transitions were observed, as monitored by spectral changes in the UV and fluorescence of the hydrophobic probe 8-anilino-1-naphthalene sulfonate. However, these heat- and pressure-induced conformers differed in their unfolding free energy. At pressures over 400 MPa, strong thioflavin-T binding was observed, suggesting a further structural change to a metastable oligomeric structure.