Unfolding transitions of Bacillus anthracis protective antigen

Pathogenic mechanism of congenital cataract caused by the CRYBA1/A3-G91del variant and related intervention strategies

Congenital cataracts, which are genetically heterogeneous eye disorders, lead to visual impairment in childhood. In our previous study, we identified a novel mutation in exon 4 of the CRYBA1/BA3 gene, which resulted in the deletion of a highly conserved glycine at codon 91 (G91del) and perinuclear zonular cataract. The G91del variant is one of the most frequent pathogenic mutations in CRYBA1/BA3; however, its pathogenic mechanism remains unclear. In this study, we purified βA3-crystallin and the βA3-G91del variant. βA3-G91del was prone to proteolysis and exhibited very low solubility and low structural stability. Next, we constructed a CRYBA1/BA3 mutant cell model and observed that G91del mutant proteins were more sensitive to environmental stress and prone to form aggregates. Size-exclusion chromatography and molecular dynamics simulation showed that the G91del mutation impaired the ability of βA3 to form homo-oligomers. In addition, the protein folding process of βA3-G91del was complicated and showed more intermediate states, resulting in amyloid fiber aggregation and induction of cellular apoptosis. Finally, we investigated intervention strategies for congenital cataract caused by the CRYBA1/A3-G91del variant. The addition of lanosterol reversed the negative effects of the G91del mutation under external stress. This study may help explore potential treatment strategies for related cataracts.

Structure of anthrax lethal toxin prepore complex suggests a pathway for efficient cell entry

Anthrax toxin comprises three soluble proteins: protective antigen (PA), lethal factor (LF), and edema factor (EF). PA must be cleaved by host proteases before it oligomerizes and forms a prepore, to which LF and EF bind. After endocytosis of this tripartite complex, the prepore transforms into a narrow transmembrane pore that delivers unfolded LF and EF into the host cytosol. Here, we find that translocation of multiple 90-kD LF molecules is rapid and efficient. To probe the molecular basis of this translocation, we calculated a three-dimensional map of the fully loaded (PA63)7-(LF)3 prepore complex by cryo-electron microscopy (cryo-EM). The map shows three LFs bound in a similar way to one another, via their N-terminal domains, to the surface of the PA heptamer. The model also reveals contacts between the N- and C-terminal domains of adjacent LF molecules. We propose that this molecular arrangement plays an important role in the maintenance of translocation efficiency through the narrow PA pore.

Recombinant anthrax protective antigen: Observation of aggregation phenomena by TEM reveals specific effects of sterols

Negatively stained transmission electron microscope images are presented that depict the aggregation of recombinant anthrax protective antigen (rPA83 monomer and the PA63 prepore oligomer) under varying in vitro biochemical conditions. Heat treatment (50°C) of rPA83 produced clumped fibrils, but following heating the PA63 prepore formed disordered aggregates. Freeze-thaw treatment of the PA63 prepore generated linear flexuous aggregates of the heptameric oligomers. Aqueous suspensions of cholesterol microcrystals were shown to bind small rPA83 aggregates at the edges of the planar bilayers. With PA63 a more discrete binding of the prepores to the crystalline cholesterol bilayer edges occurs. Sodium deoxycholate (NaDOC) treatment of rPA83 produced quasi helical fibrillar aggregate, similar but not identical to that produced by heat treatment. Remarkably, NaDOC treatment of the PA63 prepores induced transformation into pores, with a characteristic extended ß-barrel. The PA63 pores aggregated as dimers, that aggregated further as angular chains and closed structures in higher NaDOC concentrations. The significance of the sterol interaction is discussed in relation to its likely importance for PA action in vivo.

Temperature-mediated recombinant anthrax protective antigen aggregate development: Implications for toxin formation and immunogenicity

Anthrax vaccines containing recombinant PA (rPA) as the only antigen face a stability issue: rPA forms aggregates in solution after exposure to temperatures ⩾40°C, thus losing its ability to form lethal toxin (LeTx) with Lethal Factor. To study rPA aggregation's impact on immune response, we subjected rPA to several time and temperature combinations. rPA treated at 50°C for 30min formed high mass aggregates when analyzed by gel electrophoresis and failed to form LeTx as measured by a macrophage lysis assay (MLA). Aggregated rPA-formed LeTx was about 30 times less active than LeTx containing native rPA. Mice immunized with heat-treated rPA combined with Al(OH)3 developed antibody titers about 49 times lower than mice immunized with native rPA, as measured by a Toxicity Neutralization Assay (TNA). Enzyme Linked Immunosorbent Assay (ELISA) of the same immune sera showed anti-rPA titers only 2-7 times lower than titers elicited by native rPA. Thus, rPA's ability to form LeTx correlates with its production of neutralizing antibodies, and aggregation significantly impairs the protein's antibody response. However, while these findings suggest MLA has some value as an in-process quality test for rPA in new anthrax vaccines, they also confirm the superiority of TNA for use in vaccine potency.

Structural hot spots for the solubility of globular proteins

Natural selection shapes protein solubility to physiological requirements and recombinant applications that require higher protein concentrations are often problematic. This raises the question whether the solubility of natural protein sequences can be improved. We here show an anti-correlation between the number of aggregation prone regions (APRs) in a protein sequence and its solubility, suggesting that mutational suppression of APRs provides a simple strategy to increase protein solubility. We show that mutations at specific positions within a protein structure can act as APR suppressors without affecting protein stability. These hot spots for protein solubility are both structure and sequence dependent but can be computationally predicted. We demonstrate this by reducing the aggregation of human α-galactosidase and protective antigen of Bacillus anthracis through mutation. Our results indicate that many proteins possess hot spots allowing to adapt protein solubility independently of structure and function.

Mutations in aggregation prone regions of recombinant proteins often improve their solubility, although they might cause negative effects on their structure and function. Here, the authors identify proteins hot spots that can be exploited to optimize solubility without compromising stability.

Increasing the potency of an alhydrogel-formulated anthrax vaccine by minimizing antigen-adjuvant interactions

Aluminum salts are the most widely used vaccine adjuvants, and phosphate is known to modulate antigen-adjuvant interactions. Here we report an unexpected role for phosphate buffer in an anthrax vaccine (SparVax) containing recombinant protective antigen (rPA) and aluminum oxyhydroxide (AlOH) adjuvant (Alhydrogel). Phosphate ions bind to AlOH to produce an aluminum phosphate surface with a reduced rPA adsorption coefficient and binding capacity. However, these effects continued to increase as the free phosphate concentration increased, and the binding of rPA changed from endothermic to exothermic. Crucially, phosphate restored the thermostability of bound rPA so that it resembled the soluble form, even though it remained tightly bound to the surface. Batches of vaccine with either 0.25 mM (subsaturated) or 4 mM (saturated) phosphate were tested in a disease model at batch release, which showed that the latter was significantly more potent. Both formulations retained their potency for 3 years. The strongest aluminum adjuvant effects are thus likely to be via weakly attached or easily released native-state antigen proteins.

Thermal aggregation of recombinant protective antigen: aggregate morphology and growth rate

The thermal aggregation of the biopharmaceutical protein recombinant protective antigen (rPA) has been explored, and the associated kinetics and thermodynamic parameters have been extracted using optical and environmental scanning electron microscopies (ESEMs) and ultraviolet light scattering spectroscopy (UV-LSS). Visual observations and turbidity measurements provided an overall picture of the aggregation process, suggesting a two-step mechanism. Microscopy was used to examine the structure of aggregates, revealing an open morphology formed by the clustering of the microscopic aggregate particles. UV-LSS was used and developed to elucidate the growth rate of these particles, which formed in the first stage of the aggregation process. Their growth rate is observed to be high initially, before falling to converge on a final size that correlates with the ESEM data. The results suggest that the particle growth rate is limited by rPA monomer concentration, and by obtaining data over a range of incubation temperatures, an approach was developed to model the aggregation kinetics and extract the rate constants and the temperature dependence of aggregation. In doing so, we quantified the susceptibility of rPA aggregation under different temperature and environmental conditions and moreover demonstrated a novel use of UV spectrometry to monitor the particle aggregation quantitatively, in situ, in a nondestructive and time-resolved manner.

Structural and immunological analysis of anthrax recombinant protective antigen adsorbed to aluminum hydroxide adjuvant

New anthrax vaccines currently under development are based on recombinant protective antigen (rPA) and formulated with aluminum adjuvant. Because long-term stability is a desired characteristic of these vaccines, an understanding of the effects of adsorption to aluminum adjuvants on the structure of rPA is important. Using both biophysical and immunological techniques, we compared the structure and immunogenicity of freshly prepared rPA-Alhydrogel formulations to that of formulations stored for 3 weeks at either room temperature or 37°C in order to assess the changes in rPA structure that might occur upon long-term storage on aluminum adjuvant. Intrinsic fluorescence emission spectra of tryptophan residues indicated that some tertiary structure alterations of rPA occurred during storage on Alhydrogel. Using anti-PA monoclonal antibodies to probe specific regions of the adsorbed rPA molecule, we found that two monoclonal antibodies that recognize epitopes located in domain 1 of PA exhibited greater reactivity to the stored formulations than to freshly prepared formulations. Immunogenicity of rPA-Alhydrogel formulations in mice was assessed by measuring the induction of toxin-neutralizing antibodies, as well as antibodies reactive to 12-mer peptides spanning the length of PA. Mice immunized with freshly prepared formulations developed significantly higher toxin-neutralizing antibody titers than mice immunized with the stored preparations. In contrast, sera from mice immunized with stored preparations exhibited increased reactivity to nine 12-mer peptides corresponding to sequences located throughout the rPA molecule. These results demonstrate that storage of rPA-Alhydrogel formulations can lead to structural alteration of the protein and loss of the ability to elicit toxin-neutralizing antibodies.

Biophysical characterisation of thermal-induced precipitates of recombinant anthrax protective antigen: evidence for kinetically trapped unfolding domains in solid-state

Insoluble aggregation or precipitation is one of the most common degradation pathways observed for biotherapeutics; despite this, the structural mechanisms by which this occurs remain poorly understood due to difficulties associated with biophysical characterisation of protein particulates. To address this knowledge gap, we developed a solid-state circular dichroism (CD) technique, which allows in situ measurements of the secondary and tertiary structural changes associated with the formation of visible therapeutic protein aggregates. We demonstrate how solid-state CD, in conjunction with other biophysical and computational methods can aid in gaining valuable insights into the mechanisms and pathways of thermal-induced precipitation of Bacillus anthracis recombinant protective antigen (rPA), the primary immunogen of anthrax subunit vaccine. Using these methods, we show the domains d3 and d4 are the most labile of the four structurally distinct domains of rPA and play the critical role in nucleating the cascade of unfolding and aggregation. During the assembly process, the domains d1 and d2 become kinetically trapped within the insoluble aggregate and reveal previously intractable distinct tertiary structural elements of the rPA native structure. These findings reveal a uniquely detailed insight into the role of rPA domains on protein stability and provide a mechanistic framework for thermal-induced unfolding and precipitation. It also shows that solid-state CD provides a novel approach in characterising protein precipitation that may facilitate rational improvements to the stability of biopharmaceuticals.

Anthrax sub-unit vaccine: the structural consequences of binding rPA83 to Alhydrogel®

An anthrax sub-unit vaccine, comprising recombinant Protective Antigen (rPA83) and aluminium hydroxide adjuvant (Alhydrogel®) is currently being developed. Here, a series of biophysical techniques have been applied to free and adjuvant bound antigen. Limited proteolysis and fluorescence identified no changes in rPA83 tertiary structure following binding to Alhydrogel and the bound rPA83 retained two structurally important calcium ions. For adsorbed rPA83, differential scanning calorimetry revealed a small reduction in unfolding temperature but a large decrease in unfolding enthalpy whilst urea unfolding demonstrated unchanged stability but a loss of co-operativity. Overall, these results demonstrate that interactions between rPA83 and Alhydrogel have a minimal effect on the folded protein structure and suggest that antigen destabilisation is not a primary mechanism of Alhydrogel adjuvancy. This study also shows that informative structural characterisation is possible for adjuvant bound sub-unit vaccines.

Feasibility of the use of ELISA in an immunogenicity-based potency test of anthrax vaccines

Complexities of lethal challenge animal models have prompted the investigation of immunogenicity assays as potency tests of anthrax vaccines. An ELISA was used to measure the antibody response to protective antigen (PA) in mice immunized once with a commercially available (AVA) or a recombinant PA vaccine (rPAV) formulated in-house with aluminum hydroxide. Results from the anti-PA ELISA were used to select a single dose appropriate for the development of a potency test. Immunization with 0.2 mL of AVA induced a measurable response in the majority of animals. This dose was located in the linear range of the vaccine dose-antibody response curve. In the case of rPAV, practical limitations prevented the finding of the best single dose for the potency testing of purified vaccines. In additional immunogenicity experiments neither the magnitude of the response to a single dose of vaccine, nor the estimation of the dose necessary to induce a measurable response were able to consistently detect brief exposure of vaccines to potentially damaging temperatures. However, differences detected for rPAV in the proportion of mice responding to the same dose of treated and untreated vaccine suggested that further assay development to increase the sensitivity of the latter design may be warranted.

Metal ion binding to anticoagulation factor II from the venom of Agkistrodon acutus: stabilization of the structure and regulation of the binding affinity to activated coagulation factor X

Anticoagulation factor II (ACF II) isolated from the venom of Agkistrodon acutus is an activated coagulation factor X (FXa)-binding protein with both anticoagulant and hypotensive activities. The thermodynamics of the binding of alkaline earth metal ions to ACF II and their effects on the stability of ACF II and the binding of ACF II to FXa were investigated by isothermal titration calorimetry, fluorescence, differential scanning calorimetry, and surface plasmon resonance. The binding of ACF II to FXa does not have an absolute requirement for Ca(2+). Mg(2+), Sr(2+), and Ba(2+) can induce the binding of ACF II to FXa. The radii of the cations bound in ACF II crucially affect the binding affinity of ACF II for cations and the structural stability of ACF II against guanidine hydrochloride and thermal denaturation, whereas the radii of cations bound in FXa markedly affect the binding affinity between ACF II and FXa. The binding affinities of ACF II for cations and the capacities of metal-induced stabilization of ACF II follow the same trend: Ca(2+) > Sr(2+) > Ba(2+). The metal-induced binding affinities of ACF II for FXa follow the trend Mg(2+) > Ca(2+) > Sr(2+) > Ba(2+). Although Mg(2+) shows significantly low binding affinity with ACF II, Mg(2+) is the most effective to induce the binding of ACF II with FXa. Our observations suggest that in blood the bindings of Ca(2+) in two sites of ACF II increase the structural stability of ACF II, but these bindings are not essential for the binding of ACF II with FXa, and that the binding of Mg(2+) and Ca(2+) to FXa may be essential for the recognition between FXa and ACF II. Like Ca(2+), the abundant Mg(2+) in blood also plays an important role in the anticoagulation of ACF II.

Development of a differential scanning fluorimetry based high throughput screening assay for the discovery of affinity binders against an anthrax protein

The anthrax protein protective antigen (PA) is responsible for cell-surface recognition and aids the delivery of the toxic anthrax enzymes into host cells. By targeting PA and preventing it from binding to host cells, it is hoped that the delivery of toxins into the cell will be inhibited. The current assay reported for PA is a low throughput functional assay. Here, the high throughput screening method using differential scanning fluorimetry (DSF) was developed and optimized to screen a number of libraries from various sources including a selection of FDA-approved drugs as well as hits selected by a virtual screening campaign. DSF is a rapid technique that uses fluorescence to monitor the thermal unfolding of proteins using a standard QPCR instrument. A positive shift in the calculated melting temperature (Tm), of the protein in the presence of a compound, relative to the Tm of the unbound protein, indicates that stabilization of the protein by ligand binding may have occurred. Optimization of the melting assay showed SYPRO Orange to be an ideal dye as a marker and lead to the reduction of DMSO concentration to <1% (v/v) in the final assay. The final assay volume was minimized to 25 L with 5 g protein per well of 96-well plate. In addition, a buffer, salt and additive screen lead to the selection of 10 mM HEPES-NaOH pH 7.5, 100 mM NaCl as the assay buffer. This method has been shown here to be useful as a primary method for the detection of small-molecule PA ligands, giving a hit rate of approximately 7%. These ligands can then be studied further using PA functional assays to confirm their biological activities before being selected as lead compounds for the treatment of anthrax.

Simple detection of protein soft structure changes

We present a rapid method for protein tertiary structure analysis which avoids the need for techniques such as circular dichroism and differential scanning calorimetry. Small changes to a protein's noncovalent "soft" structure are detected by exploiting differences in thermal stability and fluorescent reporter binding. It can detect subtle stability differences using micrograms of protein in 2 microL volumes within minutes.

Effect of metal ion substitutions in anticoagulation factor I from the venom of Agkistrodon acutus on the binding of activated coagulation factor X and on structural stability

Anticoagulation factor I (ACF I) isolated from the venom of Agkistrodon acutus is an activated coagulation factor X (FXa)-binding protein that binds in a Ca(2+)-dependent fashion with marked anticoagulant activity. The thermodynamics of the binding of alkaline earth metal ions to ACF I and the effects of alkaline earth metal ions on the guanidine hydrochloride (GdnHCl)-induced unfolding of ACF I and the binding of ACF I to FXa were studied by isothermal titration calorimetry, fluorescence, circular dichroism, and surface plasmon resonance, respectively. The results indicate that the ionic radii of the cations occupying Ca(2+)-binding sites in ACF I crucially affect the binding affinity of ACF I for alkaline earth metal ions as well as the structural stability of ACF I against GdnHCl denaturation. Sr(2+) and Ba(2+), with ionic radii larger than the ionic radius of Ca(2+), can bind to Ca(2+)-free ACF I (apo-ACF I), while Mg(2+), with an ionic radius smaller than that of Ca(2+), shows significantly low affinity for the binding to apo-ACF I. All bindings of Ca(2+), Sr(2+), and Ba(2+) ions in two sites of ACF I are mainly enthalpy-driven and the entropy is unfavorable for them. Sr(2+)-stabilized ACF I exhibits slightly lower resistance to GdnHCl denaturation than Ca(2+)-ACF I, while Ba(2+)-stabilized ACF I exhibits much lower resistance to GdnHCl denaturation than Ca(2+)-ACF I. Mg(2+) and Sr(2+), with ionic radii close to that of Ca(2+), can bind to FXa and therefore also induce the binding of ACF I to FXa, whereas Ba(2+), with a much larger ionic radius than Ca(2+), cannot support the binding of ACF I with FXa. Our observations suggest that bindings of Ca(2+), Sr(2+), and Ba(2+) ions in two sites of ACF I increase the structural stability of ACF I, but these bindings are not essential for the binding of ACF I with FXa, and that the binding of Mg(2+), Ca(2+), and Sr(2+) ions to FXa may be essential for the recognition between FXa and ACF I.