Structure and interaction of PA63 and EF (edema toxin) of Bacillus anthracis with lipid membrane

Early Circulating Edema Factor in Inhalational Anthrax Infection: Does It Matter?

Anthrax toxins are critical virulence factors of Bacillus anthracis and Bacillus cereus strains that cause anthrax-like disease, composed of a common binding factor, the protective antigen (PA), and two enzymatic proteins, lethal factor (LF) and edema factor (EF). While PA is required for endocytosis and activity of EF and LF, several studies showed that these enzymatic factors disseminate within the body in the absence of PA after intranasal infection. In an effort to understand the impact of EF in the absence of PA, we used a fluorescent EF chimera to facilitate the study of endocytosis in different cell lines. Unexpectedly, EF was found inside cells in the absence of PA and showed a pole-dependent endocytosis. However, looking at enzymatic activity, PA was still required for EF to induce an increase in intracellular cAMP levels. Interestingly, the sequential delivery of EF and then PA rescued the rise in cAMP levels, indicating that PA and EF may functionally associate during intracellular trafficking, as well as it did at the cell surface. Our data shed new light on EF trafficking and the potential location of PA and EF association for optimal cytosolic delivery.

Anthrax toxin channel: What we know based on over 30 years of research

Protective antigen channel is the central component of the deadly anthrax exotoxin responsible for binding and delivery of the toxin's enzymatic lethal and edema factor components into the cytosol. The channel, which is more than three times longer than the lipid bilayer membrane thickness and has a 6-Å limiting diameter, is believed to provide a sophisticated unfoldase and translocase machinery for the foreign protein transport into the host cell cytosol. The tripartite toxin can be reengineered, one component at a time or collectively, to adapt it for the targeted cancer therapeutic treatments. In this review, we focus on the biophysical studies of the protective antigen channel-forming activity, small ion transport properties, enzymatic factor translocation, and blockage comparing it with the related clostridial binary toxin channels. We address issues linked to the anthrax toxin channel structural dynamics and lipid dependence, which are yet to become generally recognized as parts of the toxin translocation machinery.

Effect of late endosomal DOBMP lipid and traditional model lipids of electrophysiology on the anthrax toxin channel activity

Anthrax toxin action requires triggering of natural endocytic transport mechanisms whereby the binding component of the toxin forms channels (PA₆₃) within endosomal limiting and intraluminal vesicle membranes to deliver the toxin's enzymatic components into the cytosol. Membrane lipid composition varies at different stages of anthrax toxin internalization, with intraluminal vesicle membranes containing ~70% of anionic bis(monoacylglycero)phosphate lipid. Using model bilayer measurements, we show that membrane lipids can have a strong effect on the anthrax toxin channel properties, including the channel-forming activity, voltage-gating, conductance, selectivity, and enzymatic factor binding. Interestingly, the highest PA₆₃ insertion rate was observed in bis(monoacylglycero)phosphate membranes. The molecular dynamics simulation data show that the conformational properties of the channel are different in bis(monoacylglycero)phosphate compared to PC, PE, and PS lipids. The anthrax toxin protein/lipid bilayer system can be advanced as a novel robust model to directly investigate lipid influence on membrane protein properties and protein/protein interactions.

Solubilization and characterization of the anthrax toxin pore in detergent micelles

Proteolytically activated Protective Antigen (PA) moiety of anthrax toxin self-associates to form a heptameric ring-shaped oligomer (the prepore). Acidic pH within the endosome converts the prepore to a pore that serves as a passageway for the toxin's enzymatic moieties to cross the endosomal membrane. Prepore is stable in solution under mildly basic conditions, and lowering the pH promotes a conformational transition to an insoluble pore-like state. N-tetradecylphosphocholine (FOS14) was the only detergent among 110 tested that prevented aggregation without dissociating the multimer into its constituent subunits. FOS14 maintained the heptamers as monodisperse, insertion-competent 440-kDa particles, which formed channels in planar phospholipid bilayers with the same unitary conductance and ability to translocate a model substrate protein as channels formed in the absence of detergent. Electron paramagnetic resonance analysis detected pore-like conformational changes within PA on solubilization with FOS14, and electron micrograph images of FOS14-solubilized pore showed an extended, mushroom-shaped structure. Circular dichroïsm measurements revealed an increase in alpha helix and a decrease in beta structure in pore formation. Spectral changes caused by a deletion mutation support the hypothesis that the 2beta2-2beta3 loop transforms into the transmembrane segment of the beta-barrel stem of the pore. Changes caused by selected point mutations indicate that the transition to alpha structure is dependent on residues of the luminal 2beta11-2beta12 loop that are known to affect pore formation. Stabilizing the PA pore in solution with FOS14 may facilitate further structural analysis and a more detailed understanding of the folding pathway by which the pore is formed.

Sizing the Bacillus anthracis PA63 channel with nonelectrolyte poly(ethylene glycols)

Nonelectrolyte polymers of poly(ethylene glycol) (PEG) were used to estimate the diameter of the ion channel formed by the Bacillus anthracis protective antigen 63 (PA(63)). Based on the ability of different molecular weight PEGs to partition into the pore and reduce channel conductance, the pore appears to be narrower than the one formed by Staphylococcus aureus alpha-hemolysin. Numerical integration of the PEG sample mass spectra and the channel conductance data were used to refine the estimate of the pore's PEG molecular mass cutoff (approximately 1400 g/mol). The results suggest that the limiting diameter of the PA(63) pore is <2 nm, which is consistent with an all-atom model of the PA(63) channel and previous experiments using large ions.

Molecular basis for improved anthrax vaccines

The current vaccine for anthrax has been licensed since 1970 and was developed based on the outcome of human trials conducted in the 1950s. This vaccine, known as anthrax vaccine adsorbed (AVA), consists of a culture filtrate from an attenuated strain of Bacillus anthracis adsorbed to aluminum salts as an adjuvant. This vaccine is considered safe and effective, but is difficult to produce and is associated with complaints about reactogenicity among users of the vaccine. Much of the work in the past decade on generating a second generation vaccine is based on the observation that antibodies to protective antigen (PA) are crucial in the protection against exposure to virulent anthrax spores. Antibodies to PA are thought to prevent binding to its cellular receptor and subsequent binding of lethal factor (LF) and edema factor (EF), which are required events for the action of the two toxins: lethal toxin (LeTx) and edema toxin (EdTx). The bacterial capsule as well as the two toxins are virulence factors of B. anthracis. The levels of antibodies to PA must exceed a certain minimal threshold in order to induce and maintain protective immunity. Immunity can be generated by vaccination with purified PA, as well as spores and DNA plasmids that express PA. Although antibodies to PA address the toxemia component of anthrax disease, antibodies to additional virulence factors, including the capsule or somatic antigens in the spore, may be critical in development of complete, sterilizing immunity to anthrax exposure. The next generation anthrax vaccines will be derived from the thorough understanding of the interaction of virulence factors with human and animal hosts and the role the immune response plays in providing protective immunity.

Anthrax toxins

Bacillus anthracis, the etiological agent of anthrax, secretes three polypeptides that assemble into toxic complexes on the cell surfaces of the host it infects. One of these polypeptides, protective antigen (PA), binds to the integrin-like domains of ubiquitously expressed membrane proteins of mammalian cells. PA is then cleaved by membrane endoproteases of the furin family. Cleaved PA molecules assemble into heptamers, which can then associate with the two other secreted polypeptides: edema factor (EF) and/or lethal factor (LF). The heptamers of PA are relocalized to lipid rafts where they are quickly endocytosed and routed to an acidic compartment. The low pH triggers a conformational change in the heptamers, resulting in the formation of cation-specific channels and the translocation of EF/LF. EF is a calcium- and calmodulin-dependent adenylate cyclase that dramatically raises the intracellular concentration of cyclic adenosine monophosphate (cAMP). LF is a zinc-dependent endoprotease that cleaves the amino terminus of mitogen-activated protein kinase kinases (Meks). Cleaved Meks cannot bind to their substrates and have reduced kinase activity, resulting in alterations of the signaling pathways they govern. The structures of PA, PA heptamer, EF, and LF have been solved and much is now known about the molecular details of the intoxication mechanism. The in vivo action of the toxins, on the other hand, is still poorly understood and hotly debated. A better understanding of the toxins will help in the design of much-needed anti-toxin drugs and the development of new toxin-based medical applications.

Characterization of diphtheria toxin's catalytic domain interaction with lipid membranes

In response to a low environmental pH and with the help of the B fragment (DTB) the catalytic domain of diphtheria toxin (DTA) crosses the endosomal membrane to inhibit protein synthesis. In this study, we investigated the interaction of DTA with lipid membranes by biochemical and biophysical approaches. Data obtained from proteinase K and trypsin digestion experiments of membrane-inserted DTA suggested that residues 134-157 may adopt a transmembrane orientation and residues 77-100 could be membrane-associated, adopting either a surface or a transmembrane orientation. Fourier transform infrared spectroscopy analysis (FTIR) was used to characterize the secondary and tertiary structure of DTA along its pathway, from the native secreted form at pH 7.2 to the refolded structure at neutral pH after interaction with and desorption from a lipid membrane. We found that the association of DTA with lipid membranes at low pH was characterized by an increase of beta-sheet structures and that the refolded structure at neutral pH after interaction with the membrane was identical to the native structure at the same pH. We also investigated the desorption of DTA from the membrane at neutral pH as a function of temperature. Although a complete desorption was observed at 37 degrees C, no desorption took place at 4 degrees C. A model of translocation involving the possibility that DTA might insert one or several transient transmembrane domains during translocation is discussed.

Attenuated total reflection IR spectroscopy as a tool to investigate the orientation and tertiary structure changes in fusion proteins

Membrane fusion proceeds via a merging of two lipid bilayers and a redistribution of aqueous contents and bilayer components. It involves transition states in which the phospholipids are not arranged in bilayers and in which the monolayers are highly curved. Such transition states are energetically unfavourable since biological membranes are submitted to strong repulsive hydration electrostatic and steric barriers. Viral membrane proteins can help to overcome these barriers. Viral proteins involved in membrane fusion are membrane associated and the presence of lipids restricts drastically the potential of methods (RMN, X-ray crystallography) that have been used successfully to determine the tertiary structure of soluble proteins. We describe here how IR spectroscopy allows to solve some of the problems related to the lipid environment. The principles of the method, the experimental setup and the preparation of the samples are briefly described. A few examples illustrate how attenuated total reflection Fourier-transform IR (ATR-FTIR) spectroscopy can be used to gain information on the orientation and the accessibility to the water phase of the fusogenic domain of viral proteins. Recent developments suggest that the method could also be used to detect changes located in the membrane domains and to identify intermediate structural states involved in the fusion process.

Structure and function of anthrax toxin

Anthrax toxin is a binary A-B toxin comprised of protective antigen (PA) and two enzymatic moieties, edema factor (EF) and lethal factor (LF). In the presence of a host cell-surface receptor, PA can mediate the delivery of EF and LF from the extracellular milieu into the host cell cytosol to effect toxicity. In this delivery, PA undergoes multiple structural changes--from a monomer to a heptameric prepore to a membrane-spanning heptameric pore. The catalytic factors also undergo dramatic structural changes as they unfold to allow for their translocation across the endosomal membrane and refold to preserve their catalytic activity within the cytosol. In addition to these gross structural changes, the intoxication mechanism depends on the ability of PA to form specific interactions with the host cell receptor, EF, and LF. This chapter presents a review of experiments probing these structural interactions and rearrangements in the hopes of gaining a molecular understanding of toxin action.

Toxins of Bacillus anthracis

Bacillus anthracis, a gram positive bacterium, is the causative agent of anthrax. This organism is capsulogen and toxinogenic. It secretes two toxins which are composed of three proteins: the protective antigen (PA), the lethal factor (LF) and the edema factor (EF). The lethal toxin (PA+LF) provokes a subit death in animals, the edema toxin (PA+EF) induces edema. The edema and the lethal factors are internalised into the eukaryotic target cells via the protective antigen. EF and LF exert a calmoduline dependent adenylate cyclase and a metalloprotease activity respectively. Progress in the structure-function relationship of these three proteins, their regulation mechanisms and their roles in pathogenesis and immunoprotection will be exposed.

Attenuated total reflection IR spectroscopy as a tool to investigate the structure, orientation and tertiary structure changes in peptides and membrane proteins

During the last few years, attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) has become one of the most powerful methods to determine the structure of biological materials and in particular of components of biological membranes, like proteins that cannot be studied by x-ray crystallography and NMR. ATR-FTIR requires a little amount of material (1-100 microg) and spectra are recorded in a matter of minutes. The environment of the molecules can be modulated so that their conformation can be studied as a function of temperature, pressure, pH, as well as in the presence of specific ligands. For instance, replacement of amide hydrogen by deuterium is extremely sensitive to environmental changes and the kinetics of exchange can be used to detect tertiary conformational changes in the protein structure. Moreover, in addition to the conformational parameters that can be deduced from the shape of the infrared spectra, the orientation of various parts of the molecule can be estimated with polarized IR. This allows more precise analysis of the general architecture of the membrane molecules within the biological membranes. The present review focuses on ATR-IR as an experimental approach of special interest for the study of the structure, orientation, and tertiary structure changes in peptides and membrane proteins.

Membrane topology of VacA cytotoxin from H. pylori

The interaction of VacA with membranes involves: (i) a low pH activation that induces VacA monomerization in solution, (ii) binding of the monomers to the membrane, (iii) oligomerization and (iv) channel formation. To better understand the structure-activity relationship of VacA, we determined its topology in a lipid membrane by a combination of proteolytic, structural and fluorescence techniques. Residues 40-66, 111-169, 205-266, 548-574 and 723-767 were protected from proteolysis because of their interaction with the membrane. This last peptide was shown to most probably adopt a surface orientation. Both alpha-helices and beta-sheets were found in the structure of the protected peptides.

Translocation of Bacillus anthracis lethal and oedema factors across endosome membranes

The two exotoxins of Bacillus anthracis, the causative agent of anthrax, are the oedema toxin (PA-EF) and the lethal toxin (PA-LF). They exert their catalytic activities within the cytosol. The internalization process requires receptor-mediated endocytosis and passage through acidic vesicles. We investigated the translocation of EF and LF enzymatic moieties across the target cell membrane. By selective permeabilization of the plasma membrane with Clostridium perfringens delta-toxin, we observed free full-size lethal factor (LF) within the cytosol, resulting from specific translocation from early endosomes. In contrast, oedema factor (EF) remained associated with the membranes of vesicles.

Proteolytic activation of receptor-bound anthrax protective antigen on macrophages promotes its internalization

Immunofluorescence and other methods have been used to probe the self-assembly and internalization of the binary toxin, anthrax lethal toxin (LeTx), in primary murine macrophages. Proteolytic activation of protective antigen (PA; 83 kDa, the B moiety of the toxin) by furin was the rate-limiting step in internalization of LeTx and promoted clearance of PA from the cell surface. A furin-resistant form of PA remained at the cell surface for at least 90 min. Oligomerization of receptor-bound PA63, the 63 kDa active fragment of PA, was manifested by its conversion to a pronase-resistant state, characteristic of the heptameric prepore form in solution. That oligomerization of PA63 triggers toxin internalization is supported by the observation that PA20, the complementary 20 kDa fragment of PA, inhibited clearance of nicked PA. The PA63 prepore, with or without lethal factor (LF), cleared slowly from the cell surface. These studies show that proteolytic cleavage of PA, in addition to permitting oligomerization and LF binding, also promotes internalization of the protein. The relatively long period of activation and internalization of PA at the cell surface may reflect adaptation of this binary toxin that maximizes self-assembly.

Disruption of anthrax toxin binding with the use of human antibodies and competitive inhibitors

The protective antigen (PA83) of Bacillus anthracis is integral to the mechanism of anthrax toxicity. We have isolated a human single-chain Fv antibody fragment (scFv) that blocks binding of a fluorescently tagged protective antigen (PA) moiety to cell surface receptors. Several phage-displayed scFv were isolated from a naive library biopanned against PA83. Soluble, monomeric scFv were characterized for affinity and screened for their capacity to disrupt receptor-mediated binding of PA. Four unique scFv bound to PA83, as determined by surface plasmon resonance, the tightest binder exhibiting a Kd of 50 nM. Two scFv had similar affinities for natural PA83 and a novel, recombinant, 32-kDa carboxy-terminal PA fragment (PA32). Binding of scFv to green fluorescent protein fused to the amino-terminal 32-kDa fragment of B. anthracis edema factor, EGFP-EF32, was used to confirm specificity. Fusion of EGFP to PA32 facilitated development of a novel flow cytometric assay that showed that one of the scFv disrupted PA receptor binding. This method can now be used as a rapid assay for small molecule inhibitors of PA binding to cell receptors. The combined data presented suggest the potential utility of human scFv as prophylactics against anthrax poisoning. Moreover, recombinant PA32 may also be useful as a therapeutic agent to compete with anthrax toxins for cellular receptors during active infection.

Molecular pathogenesis of Bacillus anthracis infection

This review summarizes the current knowledge pertaining to the pathogenesis of infection with Bacillus anthracis relative to the two exotoxins and the capsule. Emphasis is given to the structure and activities of the individual components of the exotoxins, their interaction with cells, and the response of macrophages to lethal toxin. Finally, results from vaccination studies are reviewed.

Phosphorylation of the human T-cell leukemia virus type 1 transactivator tax on adjacent serine residues is critical for tax activation

The Tax transactivator protein of human T-cell leukemia virus type 1 (HTLV-1) plays a central role in the activation of viral gene expression. In addition, Tax is capable of activating the expression of specific cellular genes and is involved in the transformation of T-lymphocytes resulting in the development of adult T-cell leukemia. Tax is a phosphoprotein that colocalizes in nuclear bodies with RNA polymerase II, splicing complexes, and specific transcription factors including members of the ATF/CREB and NF-kappaB families. In this study, we identified adjacent serine residues at positions 300 and 301 in the carboxy terminus of Tax as the major sites for phosphorylation. Phosphorylation of at least one of these serine residues is required for Tax localization in nuclear bodies and for Tax-mediated activation of gene expression via both the ATF/CREB and NF-kappaB pathways. Introduction of amino acid substitutions which are phosphoserine mimetics at positions 300 and 301 restored the ability of a phosphorylation-defective Tax mutant to form nuclear bodies and to activate gene expression. These studies define sites for regulatory phosphorylation events in Tax which are critical for its ability to activate gene transcription.

Unraveling the structures and modes of action of bacterial toxins

The mechanism by which a soluble protein converts into a protein that spans a membrane remains a central question in understanding the molecular mechanism of toxicity of bacterial protein toxins. Using crystallographic structures of soluble toxins as templates, the past year has seen a number of experiments that are designed to probe the membrane state using other structural methods. In addition, crystallographic information concerning the clostridial neurotoxins has emerged, suggesting a novel mechanism of pore formation and new relationships between toxin binding domains.