Optimized Production and Purification of Bacillus anthracis Lethal Factor

Bacillus anthracis lethal factor (LF) is a 90-kDa zinc metalloprotease that plays an important role in the virulence of the organism. LF has previously been purified from Escherichia coli and Bacillus anthracis. The yields and purities of these preparations were inadequate for crystal structure determination. In this study, the genes encoding wild-type LF and a mutated, inactive LF (LF-E687C) were placed in an E. coli–Bacillus shuttle vector so that LF was produced with the protective antigen (PA) signal peptide at its N-terminus. The resulting vectors, pSJ115 and pSJ121, express wild-type and mutated LF fusion proteins, respectively. Expression of the LF genes is under the control of the PA promoter and, during secretion, the PA signal peptide is cleaved to release the 90-kDa LF proteins. The wild-type and mutated LF proteins were purified from the culture medium using three chromatographic steps (Phenyl–Sepharose, Q–Sepharose, and hydroxyapatite). The purified proteins were greater than 95% pure and yields (20–30 mg/L) were higher than those obtained in other expression systems (1–5 mg/L). These proteins have been crystallized and are being used to solve the crystal structure of LF. Their potential use in anthrax vaccines is also discussed.     

Functional mapping of anthrax toxin lethal factor by in-frame insertion mutagenesis

Linker insertion mutagenesis was employed to create structural disruptions of the lethal factor (LF) protein of anthrax toxin to map functional domains. A dodecameric linker was inserted at 17 blunt end restriction enzyme sites throughout the gene. Paired MluI restriction sites within the linker allowed the inserts to be reduced from four to two amino acids. Shuttle vectors containing the mutated genes were transformed into the avirulent Bacillus anthracis UM23C1-1 for expression and secretion of the gene products. Mutations at five sites in the central one-third of the sequence made the protein unstable, and purified protein could not be obtained. Mutated LF proteins with insertions at the other sites were purified and assessed for toxic activity in a macrophage lysis assay and for their ability to bind to the protective antigen (PA) component of anthrax toxin, the receptor binding moiety. Most insertions located in the NH2-terminal one-third of the LF protein eliminated both toxicity and binding to PA, while all four insertions in the COOH-terminal one-third of the protein eliminated toxicity without affecting binding to PA. These data support the hypothesis that the NH2-terminal domain contains the structures required for binding to PA and the COOH-terminal domain contains the putative catalytic domain of LF.     

Construction and characterization of a protective antigen-deficient Bacillus anthracis strain

The pag gene coding for protective antigen (PA), one of the three toxin components of Bacillus anthracis, has been cloned into the mobilizable shuttle vector pAT187 and transferred by conjugation from Escherichia coli to B. anthracis. Using this strategy, an insertionally mutated pag gene constructed and characterized in E. coli, was introduced into B. anthracis Sterne strain. This transconjugant was used to select a recombinant clone (RP8) carrying the inactivated pag gene on the toxin-encoding plasmid, pXO1. Strain RP8 was deficient for PA while still producing the two other toxin components, i.e. lethal factor (LF) and edema factor (EF). In contrast to spores from the wild-type Sterne strain, spores prepared from RP8 were totally non-lethal in mice. These results clearly establish the central role played by PA in B. anthracis pathogenicity.     

Sequence and analysis of the DNA encoding protective antigen of Bacillus anthracis

The nucleotide sequence of the protective antigen (PA) gene from Bacillus anthracis and the 5' and 3' flanking sequences were determined. PA is one of three proteins comprising anthrax toxin; and its nucleotide sequence is the first to be reported from B. anthracis. The open reading frame (ORF) is 2319 bp long, of which 2205 bp encode the 735 amino acids of the secreted protein. This region is preceded by 29 codons, which appear to encode a signal peptide having characteristics in common with those of other secreted proteins. A consensus TATAAT sequence was located at the putative -10 promoter site. A Shine-Dalgarno site similar to that found in genes of other Bacillus sp. was located 7 bp upstream from the ATG start codon. The codon usage for the PA gene reflected its high A + T (69%) base composition and differed from those of genes for bacterial proteins from most other sequences examined. The TAA translation stop codon was followed by an inverted repeat forming a potential termination signal. In addition, a 192-codon ORF of unknown significance, theoretically encoding a 21.6-kDa protein, preceded the 5' end of the PA gene.     

Production of biologically active Bacillus anthracis edema factor in Escherichia coli

Anthrax is caused by the gram-positive spore-forming bacterium Bacillus anthracis. The anthrax toxin consists of three proteins, protective antigen (PA), lethal factor (LF), and edema factor (EF). PA facilitates the translocation of LF and EF into the cytosol of mammalian cells. LF is thought to be a zinc-dependent metalloprotease that results in death. EF is a calmodulin- and calcium-dependent adenylate cyclase that causes edema upon entrance into the cytosol by elevating the cAMP levels in cells. Previous efforts to produce recombinant EF (rEF) in Escherichia coli yielded only 2.5 mg of rEF per liter of culture. In this work, we produced soluble rEF in large quantities in both the periplasm and cytoplasm of E. coli from shake flasks and fermentors. The rEF protein was purified by standard chromatography and yielded >97% pure, biologically active rEF. Yields of purified rEF from medium cell density fermentations resulted in up to 2.38 g/L of highly pure, biologically active rEF protein. These results exhibit the ability to generate gram quantities of active rEF from E. coli.     

Nucleotide sequence and analysis of the lethal factor gene (lef) from Bacillus anthracis

The nucleotide sequence of the Bacillus anthracis lethal factor (LF) gene (lef) has been determined. LF is part of the tripartite protein exotoxin of B. anthracis along with protective antigen (PA) and edema factor (EF). The apparent ATG start codon, which is located immediately upstream from codons which specify the first 16 amino acids (aa) of the mature secreted LF, is preceded by an AAAGGAG sequence, which is its probable ribosome-binding site. This ATG codon begins a continuous 2427-bp open reading frame which encodes the 809-aa LF-precursor protein with an Mr of 93,798. The mature secreted protein (776 aa; Mr 90,237) was preceded by a 33-aa signal peptide which has characteristics in common with leader peptides for other secreted proteins of the Bacillus species. The codon usage of the LF gene reflects its high (70%) A + T content. The N-terminus of LF (first 300 aa) shared extensive homology with the N-terminus of the anthrax EF protein. Since LF and EF each bind PA at the same site, these homologous regions probably represent their common PA-binding domains.     

High-level heterologous expression and secretion in Streptomyces lividans of two major antigenic proteins from Mycobacterium tuberculosis

Two major antigens from Mycobacterium tuberculosis were produced by Streptomyces lividans as secreted extracellular proteins. An expression-secretion vector had been constructed that contained the promoter of xylanase A and the signal sequence of cellulase A. The latter contained two initiation codons preceded by a Shine-Dalgarno sequence plus eight nucleotides complementary to the 16S rRNA. The genes encoding the 38-kDa (Rv0934) and 19-kDa (Rv3763) proteins, respectively, were amplified by polymerase chain reaction and cloned into that vector. The recombinant proteins were then purified from the culture supernatants of the clones. The yields after purification were 80 mg/L for the 38-kDa protein and 200 mg/L for the 19-kDa protein. Sequence analysis of the N-terminal sequences showed a deletion of seven or eight amino acids for the 38-kDa protein, while in the 19-kDa protein 22 or 23 amino acids were lost, as compared with the respective wild-type proteins. However, the 19 kDa recombinant protein had the same N-terminal sequence as the one recovered from the M. tuberculosis culture supernatant. The high yields obtained for these two proteins demonstrated the potential of S. lividans as an alternative host for the production of recombinant proteins from M. tuberculosis. The culture conditions have yet to be worked out to minimize proteolytic degradation and to recover intact products.     

The carboxyl-terminal end of protective antigen is required for receptor binding and anthrax toxin activity.

Anthrax toxin consists of three separate proteins produced by Bacillus anthracis: protective antigen (PA), lethal factor (LF), and edema factor (EF). Previous work showed that the process by which these proteins damage eukaryotic cells begins with binding of PA (83 kDa) to cell surface receptors. PA is then cleaved by a cell surface protease so as to expose a high-affinity binding site for LF or EF on the COOH-terminal, receptor-bound, 63-kilodalton fragment. In this report we more closely define a region of PA involved in receptor binding. The gene encoding PA was mutagenized so as to delete 3, 5, 7, 12, or 14 amino acids from the carboxyl terminus of the protein, and the truncated PA variants were purified from Bacillus subtilis or Escherichia coli. Deletion of 3, 5, or 7 amino acids reduced the binding of PA to cells and the subsequent toxicity of the PA.LF complex to J774A.1 cells and also the ability to cause EF binding to cells. Deletion of 12 or 14 amino acids completely eliminated all these activities. These results show that the carboxy terminus comprises or is part of the receptor-binding domain of PA.     

Molecular dynamics simulations of complexes between wild-type and mutant anthrax protective antigen variants and a model anthrax toxin receptor

Bacillus anthracis, a spore-forming infectious bacterium, produces a toxin consisting of three proteins: lethal factor (LF), edema factor (EF), and protective antigen (PA). LF and EF possess intracellular enzymatic functions, the net effect of which is to severely compromise host innate immunity. During an anthrax infection PA plays the critical role of facilitating entry of both EF and LF toxins into host cell cytoplasm. Crystal structures of all three of the anthrax toxins have been determined, as well as the crystal structure of the (human) von Willebrand factor A (integrin VWA/I domain) -- an anthrax toxin receptor. A theoretical structure of the complex between VWA/I and PA has also been reported. Here we report on the results of 1,000 psec molecular dynamics (MD) simulations carried out on complexes between the Anthrax Protective Antigen Domain 4 (PA-D4) and the von Willebrand Factor A (VWA/I). MD simulations (using Insight II software) were carried out for complexes containing wild-type (WT) PA-D4, as well as for complexes containing three different mutants of PA-D4, one containing three substitutions in the PA-D4 "small loop" (residues 679-693) (D683A/L685E/Y688C), one containing a single substitution at a key site at the PA-D4 -- receptor interface (K679A) and another containing a deletion of eleven residues at the C-terminus of PA (Delta724-735). All three sets of PA mutations have been shown experimentally to result in serious deficiencies in PA function. Our MD results are consistent with these findings. Major disruptions in interactions were observed between the mutant PA-D4 domains and the anthrax receptor during the MD simulations. Many secondary structural features in PA-D4 are also severely compromised when VWA complexes with mutant variants of PA-D4 are subjected to MD simulations. These MD simulation results clearly indicate the importance of the mutated PA-D4 residues in both the "small loop" and at the carboxyl terminus in maintaining a PA conformation that is capable of effective interaction with the anthrax toxin receptor.     

Constitutive expression of protective antigen gene of Bacillus anthracis in Escherichia coli

The fatal bacterial infection caused by inhalation of the Bacillus anthracis spores results from the synthesis of protein toxins-protective antigen (PA), lethal factor (LF), and edema factor (EF)--by the bacterium. PA is the target-cell binding protein and is common to the two effector molecules, LF and EF, which exert their toxic effects once they are translocated to the cytosol by PA. PA is the major component of vaccines against anthrax since it confers protective immunity. The large-scale production of recombinant protein-based anthrax vaccines requires overexpression of the PA protein. We have constitutively expressed the protective antigen protein in E. coli DH5alpha strain. We have found no increase in degradation of PA when the protein is constitutively expressed and no plasmid instability was observed inside the expressing cells. We have also scaled up the expression by bioprocess optimization using batch culture technique in a fermentor. The protein was purified using metal-chelate affinity chromatography. Approximately 125 mg of recombinant protective antigen (rPA) protein was obtained per liter of batch culture. It was found to be biologically and functionally fully active in comparison to PA protein from Bacillus anthracis. This is the first report of constitutive overexpression of protective antigen gene in E. coli.     

A Bacillus anthracis strain deleted for six proteases serves as an effective host for production of recombinant proteins

Bacillus anthracis produces a number of extracellular proteases that impact the integrity and yield of other proteins in the B. anthracis secretome. In this study we show that anthrolysin O (ALO) and the three anthrax toxin proteins, protective antigen (PA), lethal factor (LF), and edema factor (EF), produced from the B. anthracis Ames 35 strain (pXO1?, pXO2?), are completely degraded at the onset of stationary phase due to the action of proteases. An improved Cre-loxP gene knockout system was used to sequentially delete the genes encoding six proteases (InhA1, InhA2, camelysin, TasA, NprB, and MmpZ). The role of each protease in degradation of the B. anthracis toxin components and ALO was demonstrated. Levels of the anthrax toxin components and ALO in the supernatant of the sporulation defective, pXO1? A35HMS mutant strain deleted for the six proteases were significantly increased and remained stable over 24 h. A pXO1-free variant of this six-protease mutant strain, designated BH460, provides an improved host strain for the preparation of recombinant proteins. As an example, BH460 was used to produce recombinant EF, which previously has been difficult to obtain from B. anthracis. The EF protein produced from BH460 had the highest in vivo potency of any EF previously purified from B. anthracis or Escherichia coli hosts. BH460 is recommended as an effective host strain for recombinant protein production, typically yielding greater than 10mg pure protein per liter of culture.     

Production and proteolytic assay of lethal factor from Bacillus anthracis

Bacillus anthracis is the causative agent of anthrax. The major virulence factors are a poly-D-glutamic acid capsule and three-protein component exotoxin, protective antigen (PA, 83 kDa), lethal factor (LF, 90 kDa), and edema factor (EF, 89 kDa), respectively. These three proteins individually have no known toxic activities, but in combination with PA form two toxins (lethal toxin or edema toxin), causing different pathogenic responses in animals and cultured cells. In this study, we constructed and produced rLF as a form of GST fusion protein in Escherichia coli. rLF was rapidly purified through a single affinity purification step to near homogeneity. Furthermore, we developed an in vitro immobilized proteolytic assay of LF under the condition containing full-length native substrate, MEK1, rather than short synthetic peptide. The availability of full-length substrate and of an immobilized LF assay could facilitate not only the in-depth investigation of structure-function relationship of the enzyme toward its substrate but also wide spectrum screening of inhibitor collections based on the 96-well plate system.     

The assessment of antitoxic anti-anthrax immunity

In experiments on guinea pigs immunized with avirulent noncapsular strains STI, Sterne (34F2) and the avirulent mutant of Bacillus anthracis strain 228/8 the relationship between the titers of serum antibodies to the preparations of purified protective antigens (PA) and purified lethal factor (LF) of B. anthracis toxin and the level of the antitoxic activity (ATA) of blood sera, as well as acquired resistance, was analyzed. The ATA of sera was evaluated in the primary culture of peritoneal macrophages affected by the mixture of PA and LF. The level of relationship (r) between individual ATA values and the titers of antibodies to PA and LF was shown to vary over a wide range, depending on the group of the animals and did not exceed, on the average, 0.19-0.37. At the same time the mean values of these characteristics, followed in their dynamics depending on the immunogenic properties of vaccine strains or the time elapsed after vaccination, were highly correlated (r = 0.76-0.87). The possibility of using these characteristics for the evaluation of acquired resistance are discussed.     

Purification and Properties of In Vitro–produced Anthrax Toxin Components

The three components of the toxin of Bacillus anthracis, edema factor (EF), protective antigen (PA), and lethal factor (LF), were purified 197-, 156-, and 1,025- fold, with 38, 78, and 11% recovery, respectively. Each purified component was serologically active, distinct, and free from the other components. The purified EF produced edema when mixed with PA, and the purified PA was an active immunogen. The components did not appear to be simple proteins by spectrophotometric analysis. As they were purified, the pH range in which they were most stable narrowed, centering between pH 7.4 and 7.8. Heat readily destroyed the biological activity of the components but not their serological activity. The rat lethality test showed that, with a constant amount of LF and an increasing amount of PA, the time to death reached a minimum and then was extended. When an increasing amount of LF was added to a constant amount of PA, the time to death became shorter as more LF was added. The biological, immunological, and serological properties of the components were shown to vary independently with storage and extent of purification so that serological activity was not always directly correlated with biological activity. Evidence is presented that the components can exist in different molecular configurations or as aggregates, and that this property is influenced by the state of component purity and by the environment.     

Role of CypA and Hsp90 in membrane translocation mediated by anthrax protective antigen

Bacillus anthracis lethal toxin consists of the protective antigen (PA) and the metalloprotease lethal factor (LF). During cellular uptake PA forms pores in membranes of endosomes, and unfolded LF translocates through the pores into the cytosol. We have investigated whether host cell chaperones facilitate translocation of LF and the fusion protein LF(N)DTA. LF(N) mediates uptake of LF(N)DTA into the cytosol, where DTA, the catalytic domain of diphtheria toxin, ADP-ribosylates elongation factor-2, allowing for detection of small amounts of translocated LF(N)DTA. Cyclosporin A, which inhibits peptidyl-prolyl cis/trans isomerase activity of cyclophilins, and radicicol, which inhibits Hsp90 activity, prevented uptake of LF(N)DTA into the cytosol of CHO-K1 cells and protected cells from intoxication by LF(N)DTA/PA. Both inhibitors, as well as an antibody against cyclophilin A blocked the release of active LF(N)DTA from endosomal vesicles into the cytosol in vitro. In contrast, the inhibitors did not inhibit cellular uptake of LF. In vitro, cyclophilin A and Hsp90 bound to LF(N)DTA and DTA but not to LF, implying that DTA determines this interaction. In conclusion, cyclophilin A and Hsp90 facilitate translocation of LF(N)DTA, but not of LF, across endosomal membranes, and thus they function selectively in promoting translocation of certain proteins, but not of others.     

Large-scale structural changes accompany binding of lethal factor to anthrax protective antigen: a cryo-electron microscopic study

Anthrax toxin (AT), secreted by Bacillus anthracis, is a three-protein cocktail of lethal factor (LF, 90 kDa), edema factor (EF, 89 kDa), and the protective antigen (PA, 83 kDa). Steps in anthrax toxicity involve (1) binding of ligand (EF/LF) to a heptamer of PA63 (PA63h) generated after N-terminal proteolytic cleavage of PA and, (2) following endocytosis of the complex, translocation of the ligand into the cytosol by an as yet unknown mechanism. The PA63h.LF complex was directly visualized from analysis of images of specimens suspended in vitrified buffer by cryo-electron microscopy, which revealed that the LF molecule, localized to the nonmembrane-interacting face of the oligomer, interacts with four successive PA63 monomers and partially unravels the heptamer, thereby widening the central lumen. The observed structural reorganization in PA63h likely facilitates the passage of the large 90 kDa LF molecule through the lumen en route to its eventual delivery across the membrane bilayer.     

Western blot analysis of the exotoxin components from Bacillus anthracis separated by isoelectric focusing gel electrophoresis

The components of the Bacillus anthracis exotoxins, protective antigen (PA), lethal factor (LF), and edema factor (EF), from 24 isolates were separated by isoelectric focusing gel electrophoresis and detected by Western blot with monoclonal antibodies. Only two isoforms each were observed for PA and EF. Four isoforms were identified for LF. The biological activities of both lethal toxin and edema toxin were measured by using in vitro cell-based assays. This study provides another method of characterizing various isolates of B. anthracis by determining the isoelectric points of the exotoxin components and may be useful in the development of protective vaccines against B. anthracis infection.     

A deleted variant of Bacillus anthracis protective antigen is non-toxic and blocks anthrax toxin action in vivo

Anthrax toxin is the only protein secreted by Bacillus anthracis that contributes to the virulence of this bacterium. An obligatory step in the action of anthrax toxin on eukaryotic cells is cleavage of the receptor-bound protective antigen (PA) protein (83 kilodaltons) to produce a 63-kilodalton, receptor-bound COOH-terminal fragment. A similar fragment can be obtained by limited treatment with trypsin. This proteolytic processing event exposes a site with high affinity for the other two anthrax toxin proteins, lethal factor and edema factor. Terminal sequencing of the purified fragment showed that the activating cleavage occurred in the sequence Arg164-Lys165-Lys166-Arg167. The gene encoding PA was mutagenized to delete residues 163-168, and the deleted PA was purified from a Bacillus subtilis host. The deleted PA was not cleaved by either trypsin or the cell-surface protease, and was non-toxic when administered with lethal factor. Purified, deleted PA protected rats when administered 90 min before injection of 20 minimum lethal doses of toxin. This mutant PA may be useful as a replacement for the PA that is the major active ingredient in the current human anthrax vaccine, because deleted PA is expected to have normal immunogenicity, but would not combine with trace amounts of LF and EF to cause toxicity.     

炭疽杆菌保护性抗原在大肠杆菌中的表达及其纯化

利用基因重组技术获取炭疽杆菌保护性抗原(PA)。将炭疽杆菌保护性抗原编码基因pag与pET载体连接构建重组质粒,转化大肠杆菌DE3株,诱导表达炭疽杆菌保护性抗原,并经亲和层析及凝胶过滤纯化此抗原。实验成功构建了表达炭疽杆菌保护性抗原的重组菌株,纯化后PA纯度达90%,且经检测纯化产物具有天然PA的生物学活性。同时表明从大肠杆菌中纯化PA较以往从炭疽杆菌中获取PA简便易行。