Insertion of anthrax protective antigen into liposomal membranes: effects of a receptor

Membrane binding and pore formation is Ca 2+ -dependent for the Clostridioides difficile binary toxin

The C. difficile binary toxin (CDT) enters host cells via endosomal delivery like many other 'AB'-type binary toxins. In this study, the cell-binding component of CDT, termed CDTb, was found to bind and form pores in lipid bilayers upon depleting free Ca 2+ ion concentrations, and not by lowering pH, as found for other binary toxins (i.e., anthrax). Cryoelectron microscopy, nuclear magnetic resonance spectroscopy, surface plasmon resonance, electrochemical impedance spectroscopy, CDT toxicity studies, and site directed mutagenesis show that dissociation of Ca 2+ from a single site in receptor binding domain 1 (RBD1) of CDTb is consistent with a molecular mechanism in which Ca 2+ dissociation from RBD1 induces a "trigger" via conformational exchange that enables CDTb to bind and form pores in endosomal membrane bilayers as free Ca 2+ concentrations decrease during CDT endosomal delivery.

Regulation of cadherin dimerization by chemical fragments as a trigger to inhibit cell adhesion

Many cadherin family proteins are associated with diseases such as cancer. Since cell adhesion requires homodimerization of cadherin molecules, a small-molecule regulator of dimerization would have therapeutic potential. Herein, we describe identification of a P-cadherin-specific chemical fragment that inhibits P-cadherin-mediated cell adhesion. Although the identified molecule is a fragment compound, it binds to a cavity of P-cadherin that has not previously been targeted, indirectly prevents formation of hydrogen bonds necessary for formation of an intermediate called the X dimer and thus modulates the process of X dimerization. Our findings will impact on a strategy for regulation of protein-protein interactions and stepwise assembly of protein complexes using small molecules.

A Fluorescence Dequenching-based Liposome Leakage Assay to Measure Membrane Permeabilization by Pore-forming Proteins

Pore-forming toxins (PFTs) have been discovered in a wide range of organisms. Their functions are essential to the survival or virulence of many species. PFTs often interact with lipid membranes. Large unilamellar vesicles (LUV), also known as liposomes, have been commonly used as reliable membrane models for testing PFTs activity. Liposomes have great adaptability in size, lipid composition, and loading cargo. Incorporating the fluorescent dye/quencher pair, 8-Aminonaphthalene-1,3,6-Trisulfonic Acid (ANTS) and p-Xylene-Bis-Pyridinium Bromide (DPX), in liposomes is an effective approach for measuring membrane leakage. When ANTS and DPX are encapsulated in a liposome, the fluorescence of ANTS is quenched by DPX. However, disruption of liposome integrity and subsequent leakage result in measurable fluorescence emitted by ANTS. Here, we report our protocol for optimal liposome preparation for measuring liposome leakage by fluorescence dequenching.

Development of an In Vitro Membrane Model to Study the Function of EsxAB Heterodimer and Establish the Role of EsxB in Membrane Permeabilizing Activity of Mycobacterium tuberculosis

EsxA and EsxB are secreted as a heterodimer and have been shown to play critical roles in phagosome rupture and translocation of Mycobacterium tuberculosis into the cytosol. Recent in vitro studies have suggested that the EsxAB heterodimer is dissociated upon acidification, which might allow EsxA insertion into lipid membranes. While the membrane permeabilizing activity (MPA) of EsxA has been well characterized in liposomes composed of di-oleoyl-phosphatidylcholine (DOPC), the MPA of EsxAB heterodimer has not been detected through in vitro assays due to its negligible activity with DOPC liposomes. In this study, we established a new in vitro membrane assay to test the MPA activity of N-terminal acetylated EsxA (N-EsxA). We established that a dose-dependent increase in anionic charged lipids enhances the MPA of N-EsxA. The MPA of both N-EsxA and EsxAB were significantly increased with this new liposome system and made it possible to characterize the MPA of EsxAB in more physiologically-relevant conditions. We tested, for the first time, the effect of temperature on the MPA of N-EsxA and EsxAB in this new system. Interestingly, the MPA of N-EsxA was lower at 37 °C than at RT, and on the contrary, the MPA of EsxAB was higher at 37 °C than at RT. Surprisingly, after incubation at 37 °C, the MPA of N-EsxA continuously decreased over time, while MPA of EsxAB remained stable, suggesting EsxB plays a key role in stabilizing N-EsxA to preserve its MPA at 37 °C. In summary, this study established a new in vitro model system that characterizes the MPA of EsxAB and the role of EsxB at physiological-relevant conditions.

Nα-Acetylation of the virulence factor EsxA is required for mycobacterial cytosolic translocation and virulence

The Mycobacterium tuberculosis virulence factor EsxA and its chaperone EsxB are secreted as a heterodimer (EsxA:B) and are crucial for mycobacterial escape from phagosomes and cytosolic translocation. Current findings support the idea that for EsxA to interact with host membranes, EsxA must dissociate from EsxB at low pH. However, the molecular mechanism by which the EsxA:B heterodimer separates is not clear. In the present study, using liposome-leakage and cytotoxicity assays, LC-MS/MS-based proteomics, and CCF-4 FRET analysis, we obtained evidence that the Nα-acetylation of the Thr-2 residue on EsxA, a post-translational modification that is present in mycobacteria but absent in Escherichia coli, is required for the EsxA:B separation. Substitutions at Thr-2 that precluded Nα-acetylation inhibited the heterodimer separation and hence prevented EsxA from interacting with the host membrane, resulting in attenuated mycobacterial cytosolic translocation and virulence. Molecular dynamics simulations revealed that at low pH, the Nα-acetylated Thr-2 makes direct and frequent "bind-and-release" contacts with EsxB, which generates a force that pulls EsxB away from EsxA. In summary, our findings provide evidence that the Nα-acetylation at Thr-2 of EsxA facilitates dissociation of the EsxA:B heterodimer required for EsxA membrane permeabilization and mycobacterial cytosolic translocation and virulence.

Effects of membrane lipid composition on Mycobacterium tuberculosis EsxA membrane insertion: A dual play of fluidity and charge

As a key virulence factor of Mycobacterium tuberculosis, EsxA or 6-kDa early secreted antigenic target (ESAT-6) has been implicated in phagosome rupture and mycobacterial translocation from the phagosome to the cytosol within macrophages. Our previous studies have shown that EsxA permeabilizes liposomal membrane at acidic pH and a membrane-permeabilization defective mutant Q5K attenuates mycobacterial cytosolic translocation and virulence in macrophages. To further probe the mechanism of EsxA membrane permeabilization, here we characterized the effects of various lipid compositions, including biologically relevant phagosome-mimicking lipids and lipid rafts, on the structural stability and membrane insertion of EsxA WT and Q5K. We have found a complex dual play of membrane fluidity and charge in regulating EsxA membrane insertion. Moreover, Q5K affects the membrane insertion through a structure- and lipid composition-independent mechanism. The results of this study provide a novel insights into the mechanism of EsxA membrane interaction.

EsxA membrane-permeabilizing activity plays a key role in mycobacterial cytosolic translocation and virulence: effects of single-residue mutations at glutamine 5

EsxA is required for virulence of Mycobacterium tuberculosis (Mtb) and plays an essential role in phagosome rupture and translocation to the cytosol of macrophages. Recent biochemical studies have demonstrated that EsxA is a membrane-permeabilizing protein. However, evidence that link EsxA membrane-permeabilizing activity to Mtb cytosolic translocation and virulence is lacking. Here we found that mutations at glutamine 5 (Q5) could up or down regulate EsxA membrane-permeabilizing activity. The mutation Q5K significantly diminished the membrane-permeabilizing activity, while Q5V enhanced the activity. By taking advantage of the single-residue mutations, we tested the effects of EsxA membrane-permeabilizing activity on mycobacterial virulence and cytosolic translocation using the esxA/esxB knockout strains of Mycobacterium marinum (Mm) and Mtb. Compared to wild type (WT), the Q5K mutant exhibited significantly attenuated virulence, evidenced by intracellular survival and cytotoxicity in mouse macrophages as well as infection of zebra fish embryos. The attenuated virulence of the Q5K mutant was correlated to the impaired cytosolic translocation. On the contrary, the Q5V mutant had a significantly increased cytosolic translocation and showed an overall increased virulence. This study provides convincing evidence that EsxA contributes to mycobacterial virulence with its membrane-permeabilizing activity that is required for cytosolic translocation.

Characterization of differential pore-forming activities of ESAT-6 proteins from Mycobacterium tuberculosis and Mycobacterium smegmatis

Mycobacterium tuberculosis ESAT-6 (MtbESAT-6) plays essential roles in pathogenesis. MtbESAT-6 exhibits a unique pore-forming activity (PFA) that is not found in its ortholog from non-pathogenic Mycobacterium smegmatis (MsESAT-6). Here, we characterized the differential PFAs and found that exchange of I25-H26/T25-A26 between two proteins reciprocally affected their PFAs. MtbESAT-6(IH/TA) had ~ 40% reduction, while MsESAT-6(TA/IH) fully acquired its activity similar to MtbESAT-6. Mutations of A17E, K38T, N67L or R74Q on MtbESAT-6(IH/TA) further reduced the activity, with MtbESAT-6(IH/TA-17) being the lowest. This study suggests I25-H26 as the pH-sensor essential for MsESAT-6 to fully acquire the activity, while multiple residues contributed to MtbESAT-6 PFA.

The Disulfide Bond Cys255-Cys279 in the Immunoglobulin-Like Domain of Anthrax Toxin Receptor 2 Is Required for Membrane Insertion of Anthrax Protective Antigen Pore

Anthrax toxin receptors act as molecular clamps or switches that control anthrax toxin entry, pH-dependent pore formation, and translocation of enzymatic moieties across the endosomal membranes. We previously reported that reduction of the disulfide bonds in the immunoglobulin-like (Ig) domain of the anthrax toxin receptor 2 (ANTXR2) inhibited the function of the protective antigen (PA) pore. In the present study, the disulfide linkage in the Ig domain was identified as Cys255-Cys279 and Cys230-Cys315. Specific disulfide bond deletion mutants were achieved by replacing Cys residues with Ala residues. Deletion of the disulfide bond C255-C279, but not C230-C315, inhibited the PA pore-induced release of the fluorescence dyes from the liposomes, suggesting that C255-C279 is essential for PA pore function. Furthermore, we found that deletion of C255-C279 did not affect PA prepore-to-pore conversion, but inhibited PA pore membrane insertion by trapping the PA membrane-inserting loops in proteinaceous hydrophobic pockets. Fluorescence spectra of Trp59, a residue adjacent to the PA-binding motif in von Willebrand factor A (VWA) domain of ANTXR2, showed that deletion of C255-C279 resulted in a significant conformational change on the receptor ectodomain. The disulfide deletion-induced conformational change on the VWA domain was further confirmed by single-particle 3D reconstruction of the negatively stained PA-receptor heptameric complexes. Together, the biochemical and structural data obtained in this study provides a mechanistic insight into the role of the receptor disulfide bond C255-C279 in anthrax toxin action. Manipulation of the redox states of the receptor, specifically targeting to C255-C279, may become a novel strategy to treat anthrax.

Characterization of Mycobacterium tuberculosis EsxA membrane insertion: roles of N- and C-terminal flexible arms and central helix-turn-helix motif

EsxA (ESAT-6), an important virulence factor of Mycobacterium tuberculosis, plays an essential role in phagosome rupture and bacterial cytosolic translocation within host macrophages. Our previous study showed that EsxA exhibits a unique membrane-interacting activity that is not found in its ortholog from nonpathogenic Mycobacterium smegmatis. However, the molecular mechanism of EsxA membrane insertion remains unknown. In this study, we generated truncated EsxA proteins with deletions of the N- and/or C-terminal flexible arm. Using a fluorescence-based liposome leakage assay, we found that both the N- and C-terminal arms were required for membrane disruption. Moreover, we found that, upon acidification, EsxA converted into a more organized structure with increased α-helical content, which was evidenced by CD analysis and intrinsic tryptophan fluorescence. Finally, using an environmentally sensitive fluorescent dye, we obtained direct evidence that the central helix-turn-helix motif of EsxA inserted into the membranes and formed a membrane-spanning pore. A model of EsxA membrane insertion is proposed and discussed.

Polylysine-mediated translocation of the diphtheria toxin catalytic domain through the anthrax protective antigen pore

The protective antigen (PA) moiety of anthrax toxin forms oligomeric pores in the endosomal membrane, which translocate the effector proteins of the toxin to the cytosol. Effector proteins bind to oligomeric PA via their respective N-terminal domains and undergo N- to C-terminal translocation through the pore. Earlier we reported that a tract of basic amino acids fused to the N-terminus of an unrelated effector protein (the catalytic domain diphtheria toxin, DTA) potentiated that protein to undergo weak PA-dependent translocation. In this study, we varied the location of the tract (N-terminal or C-terminal) and the length of a poly-Lys tract fused to DTA and examined the effects of these variations on PA-dependent translocation into cells and across planar bilayers in vitro. Entry into cells was most efficient with ∼12 Lys residues (K12) fused to the N-terminus but also occurred, albeit 10-100-fold less efficiently, with a C-terminal tract of the same length. Similarly, K12 tracts at either terminus occluded PA pores in planar bilayers, and occlusion was more efficient with the N-terminal tag. We used biotin-labeled K12 constructs in conjunction with streptavidin to show that a biotinyl-K12 tag at either terminus is transiently exposed to the trans compartment of planar bilayers at 20 mV; this partial translocation in vitro was more efficient with an N-terminal tag than a C-terminal tag. Significantly, our studies with polycationic tracts fused to the N- and C-termini of DTA suggest that PA-mediated translocation can occur not only in the N to C direction but also in the C to N direction.

Tetanus neurotoxin utilizes two sequential membrane interactions for channel formation

Tetanus neurotoxin (TeNT) causes neuroparalytic disease by entering the neuronal soma to block the release of neurotransmitters. However, the mechanism by which TeNT translocates its enzymatic domain (light chain) across endosomal membranes remains unclear. We found that TeNT and a truncated protein devoid of the receptor binding domain (TeNT-LHN) associated with membranes enriched in acidic phospholipids in a pH-dependent manner. Thus, in contrast to diphtheria toxin, the formation of a membrane-competent state of TeNT requires the membrane interface and is modulated by the bilayer composition. Channel formation is further enhanced by tethering of TeNT to the membrane through ganglioside co-receptors prior to acidification. Thus, TeNT channel formation can be resolved into two sequential steps: 1) interaction of the receptor binding domain (heavy chain receptor binding domain) with ganglioside co-receptors orients the translocation domain (heavy chain translocation domain) as the lumen of the endosome is acidified and 2) low pH, in conjunction with acidic lipids within the membrane drives the conformational changes in TeNT necessary for channel formation.

Expression and purification of the functional ectodomain of human anthrax toxin receptor 2 in Escherichia coli Origami B cells with assistance of bacterial Trigger Factor

The ectodomain of anthrax toxin receptor 2 (ANTXR2) is composed of a von Willebrand factor A (VWA) domain that binds to anthrax toxin protective antigen (PA) and a newly defined immunoglobulin-like (Ig) domain, in which the disulfide bonds are required for PA pore formation and for the folding of ANTXR2. While the VWA domain has been well characterized, the structure and function of the whole ectodomain (VWA-Ig) are poorly defined, which is mainly due to the limited production of the soluble recombinant protein of the ectodomain. In the present study, the ANTXR2 ectodomain was fused to the C-terminus of bacterial Trigger Factor (TF), a chaperone that mediates the ribosome-associated, co-translational folding of newly synthesized polypeptides in Escherichia coli. Under the control of a cold shock promoter, the fusion protein was overly expressed as a dominant soluble protein at a low temperature in the oxidative cytoplasm of Origami B cells, where formation of the disulfide bonds is favored. Through a series of chromatography, the ANTXR2 ectodomain was purified into homogeneity. The purified ectodomain is functional in binding to PA and mediating PA pore formation on the liposomal membranes, and the yield is applicable for future biochemical and structural characterization.

Monitoring the kinetics of the pH-driven transition of the anthrax toxin prepore to the pore by biolayer interferometry and surface plasmon resonance

Domain 2 of the anthrax protective antigen (PA) prepore heptamer unfolds and refolds during endosome acidification to generate an extended 100 Å β barrel pore that inserts into the endosomal membrane. The PA pore facilitates the pH-dependent unfolding and translocation of bound toxin enzymic components, lethal factor (LF) and/or edema factor, from the endosome to the cytoplasm. We constructed immobilized complexes of the prepore with the PA-binding domain of LF (LFN) to monitor the real-time prepore to pore kinetic transition using surface plasmon resonance and biolayer interferometry (BLI). The kinetics of this transition increased as the solution pH was decreased from 7.5 to 5.0, mirroring acidification of the endosome. Once it had undergone the transition, the LFN-PA pore complex was removed from the BLI biosensor tip and deposited onto electron microscopy grids, where PA pore formation was confirmed by negative stain electron microscopy. When the soluble receptor domain (ANTRX2/CMG2) binds the immobilized PA prepore, the transition to the pore state was observed only after the pH was lowered to early (pH 5.5) or late (pH 5.0) endosomal pH conditions. Once the pore formed, the soluble receptor readily dissociated from the PA pore. Separate binding experiments with immobilized PA pores and the soluble receptor indicate that the receptor has a weakened propensity to bind to the transitioned pore. This immobilized anthrax toxin platform can be used to identify or validate potential antimicrobial lead compounds capable of regulating and/or inhibiting anthrax toxin complex formation or pore transitions.

Mycobacterium tuberculosis ESAT-6 exhibits a unique membrane-interacting activity that is not found in its ortholog from non-pathogenic Mycobacterium smegmatis

Mycobacterium tuberculosis ESAT-6 (MtbESAT-6) reportedly shows membrane/cell-lysis activity, and recently its biological roles in pathogenesis have been implicated in rupture of the phagosomes for bacterial cytosolic translocation. However, molecular mechanism of MtbESAT-6-mediated membrane interaction, particularly in relation with its biological functions in pathogenesis, is poorly understood. In this study, we investigated the pH-dependent membrane interaction of MtbESAT-6, MtbCFP-10, and the MtbESAT-6/CFP-10 heterodimer, by using liposomal model membranes that mimic phagosomal compartments. MtbESAT-6, but neither MtbCFP-10 nor the heterodimer, interacted with the liposomal membranes at acidic conditions, which was evidenced by release of K(+) ions from the liposomes. Most importantly, the orthologous ESAT-6 from non-pathogenic Mycobacterium smegmatis (MsESAT-6) was essentially inactive in release of K(+). The differential membrane interactions between MtbESAT-6 and MsESAT-6 were further confirmed in an independent membrane leakage assay using the dye/quencher pair, 8-aminonapthalene-1,3,6 trisulfonic acid (ANTS)/p-xylene-bis-pyridinium bromide (DPX). Finally, using intrinsic and extrinsic fluorescence approaches, we probed the pH-dependent conformational changes of MtbESAT-6 and MsESAT-6. At acidic pH conditions, MtbESAT-6 underwent a significant conformational change, which was featured by an increased solvent-exposed hydrophobicity, while MsESAT-6 showed little conformational change in response to acidification. In conclusion, we have demonstrated that MtbESAT-6 possesses a unique membrane-interacting activity that is not found in MsESAT-6 and established the utility of rigorous biochemical approaches in dissecting the virulence of M. tuberculosis.

Novel approaches to the treatment of systemic anthrax

Anthrax continues to generate concern as an agent of bioterrorism and as a natural cause of sporadic disease outbreaks. Despite the use of appropriate antimicrobial agents and advanced supportive care, the mortality associated with the systemic disease remains high. This is primarily due to the pathogenic exotoxins produced by Bacillus anthracis as well as other virulence factors of the organism. For this reason, new therapeutic strategies that target events in the pathogenesis of anthrax and may potentially augment antimicrobials are being investigated. These include anti-toxin approaches, such as passive immune-based therapies; non-antimicrobial drugs with activity against anthrax toxin components; and agents that inhibit binding, processing, or assembly of toxins. Adjunct therapies that target spore germination or downstream events in anthrax intoxication are also under investigation. In combination, these modalities may enhance the management of systemic anthrax.

Botulinum neurotoxins B and E translocate at different rates and exhibit divergent responses to GT1b and low pH

Botulinum neurotoxins (BoNTs, serotypes A–G) are the most deadly substances known. Here, we investigated how BoNT/E, a serotype that causes human botulism, translocates into the cytosol of neurons. Analogous to BoNT/B, BoNT/E required binding of the coreceptor, GT1b, to undergo significant secondary structural changes and transform into a hydrophobic protein at low pH. These data indicate that both serotypes act as coincidence detectors for both GT1b and low pH, to undergo translocation. However, BoNT/E translocated much more rapidly than BoNT/B. Also, BoNT/E required only GT1b, and not low pH, to oligomerize, whereas BoNT/B required both. In further contrast to the case of BoNT/B, low pH alone altered the secondary structure of BoNT/E to some degree and resulted in its premature inactivation. Hence, comparison of two BoNT serotypes revealed that these agents exhibit both convergent and divergent responses to receptor interactions, and pH, in the translocation pathway.

Receptor binding enables botulinum neurotoxin B to sense low pH for translocation channel assembly

Botulinum neurotoxins (BoNTs, serotypes A-G), elaborated by Clostridium botulinum, can induce lethal paralysis and are classified as Category A bioterrorism agents. However, how BoNTs translocate from endosomes into the cytosol of neurons to gain access to their intracellular targets remains enigmatic. We discovered that binding to the ganglioside GT1b, a toxin coreceptor, enables BoNT/B to sense low pH, undergo a significant change in secondary structure, and transform into a hydrophobic oligomeric membrane protein. Imaging of the toxin on lipid bilayers using atomic force microscopy revealed donut-shaped channel-like structures that resemble other protein translocation assemblies. Toosendanin, a drug with therapeutic effects against botulism, inhibited GT1b-dependent BoNT/B oligomerization and in parallel truncated BoNT/B single-channel conductance, suggesting that oligomerization plays a role in the translocation reaction. Thus, BoNT/B functions as a coincidence detector for receptor and low pH to ensure spatial and temporal accuracy for toxin conversion into a translocation channel.

Ultrasensitive detection of protein translocated through toxin pores in droplet-interface bilayers

Many bacterial toxins form proteinaceous pores that facilitate the translocation of soluble effector proteins across cellular membranes. With anthrax toxin this process may be monitored in real time by electrophysiology, where fluctuations in ionic current through these pores inserted in model membranes are used to infer the translocation of individual protein molecules. However, detecting the minute quantities of translocated proteins has been a challenge. Here, we describe use of the droplet-interface bilayer system to follow the movement of proteins across a model membrane separating two submicroliter aqueous droplets. We report the capture and subsequent direct detection of as few as 100 protein molecules that have translocated through anthrax toxin pores. The droplet-interface bilayer system offers new avenues of approach to the study of protein translocation.

Anthrax toxin receptor drives protective antigen oligomerization and stabilizes the heptameric and octameric oligomer by a similar mechanism

Background

Anthrax toxin is comprised of protective antigen (PA), lethal factor (LF), and edema factor (EF). These proteins are individually nontoxic; however, when PA assembles with LF and EF, it produces lethal toxin and edema toxin, respectively. Assembly occurs either on cell surfaces or in plasma. In each milieu, PA assembles into a mixture of heptameric and octameric complexes that bind LF and EF. While octameric PA is the predominant form identified in plasma under physiological conditions (pH 7.4, 37°C), heptameric PA is more prevalent on cell surfaces. The difference between these two environments is that the anthrax toxin receptor (ANTXR) binds to PA on cell surfaces. It is known that the extracellular ANTXR domain serves to stabilize toxin complexes containing the PA heptamer by preventing premature PA channel formation—a process that inactivates the toxin. The role of ANTXR in PA oligomerization and in the stabilization of toxin complexes containing octameric PA are not understood.

Methodology

Using a fluorescence assembly assay, we show that the extracellular ANTXR domain drives PA oligomerization. Moreover, a dimeric ANTXR construct increases the extent of and accelerates the rate of PA assembly relative to a monomeric ANTXR construct. Mass spectrometry analysis shows that heptameric and octameric PA oligomers bind a full stoichiometric complement of ANTXR domains. Electron microscopy and circular dichroism studies reveal that the two different PA oligomers are equally stabilized by ANTXR interactions.

Conclusions

We propose that PA oligomerization is driven by dimeric ANTXR complexes on cell surfaces. Through their interaction with the ANTXR, toxin complexes containing heptameric and octameric PA oligomers are similarly stabilized. Considering both the relative instability of the PA heptamer and extracellular assembly pathway identified in plasma, we propose a means to regulate the development of toxin gradients around sites of infection during anthrax pathogenesis.

Evidence that histidine protonation of receptor-bound anthrax protective antigen is a trigger for pore formation

The protective antigen (PA) component of the anthrax toxin forms pores within the low pH environment of host endosomes through mechanisms that are poorly understood. It has been proposed that pore formation is dependent on histidine protonation. In previous work, we biosynthetically incorporated 2-fluorohistidine (2-FHis), an isosteric analogue of histidine with a significantly reduced pK(a) ( approximately 1), into PA and showed that the pH-dependent conversion from the soluble prepore to a pore was unchanged. However, we also observed that 2-FHisPA was nonfunctional in the ability to mediate cytotoxicity of CHO-K1 cells by LF(N)-DTA and was defective in translocation through planar lipid bilayers. Here, we show that the defect in cytotoxicity is due to both a defect in translocation and, when bound to the host cellular receptor, an inability to undergo low pH-induced pore formation. Combining X-ray crystallography with hydrogen-deuterium (H-D) exchange mass spectrometry, our studies lead to a model in which hydrogen bonds to the histidine ring are strengthened by receptor binding. The combination of both fluorination and receptor binding is sufficient to block low pH-induced pore formation.

Disulfide bonds in the ectodomain of anthrax toxin receptor 2 are required for the receptor-bound protective-antigen pore to function

Background

Cell-surface receptors play essential roles in anthrax toxin action by providing the toxin with a high-affinity anchor and self-assembly site on the plasma membrane, mediating the toxin entry into cells through endocytosis, and shifting the pH threshold for prepore-to-pore conversion of anthrax toxin protective antigen (PA) to a more acidic pH, thereby inhibiting premature pore formation. Each of the two known anthrax toxin receptors, ANTXR1 and ANTXR2, has an ectodomain comprised of an N-terminal von Willebrand factor A domain (VWA), which binds PA, and an uncharacterized immunoglobulin-like domain (Ig) that connects VWA to the membrane-spanning domain. Potential roles of the receptor Ig domain in anthrax toxin action have not been investigated heretofore.

Methodology/Principal Findings

We expressed and purified the ANTXR2 ectodomain (R2-VWA-Ig) in E. coli and showed that it contains three disulfide bonds: one in R2-VWA and two in R2-Ig. Reduction of the ectodomain inhibited functioning of the pore, as measured by K⁺ release from liposomes or Chinese hamster ovary cells or by PA-mediated translocation of a model substrate across the plasma membrane. However, reduction did not affect binding of the ectodomain to PA or the transition of ectodomain-bound PA prepore to the pore conformation. The inhibitory effect depended specifically on reduction of the disulfides within R2-Ig.

Conclusions/Significance

We conclude that disulfide integrity within R2-Ig is essential for proper functioning of receptor-bound PA pore. This finding provides a novel venue to investigate the mechanism of anthrax toxin action and suggests new strategies for inhibiting toxin action.

Role of the protective antigen octamer in the molecular mechanism of anthrax lethal toxin stabilization in plasma

Anthrax is caused by strains of Bacillus anthracis that produce two key virulence factors, anthrax toxin (Atx) and a poly-gamma-D-glutamic acid capsule. Atx is comprised of three proteins: protective antigen (PA) and two enzymes, lethal factor (LF) and edema factor (EF). To disrupt cell function, these components must assemble into holotoxin complexes, which contain either a ring-shaped homooctameric or homoheptameric PA oligomer bound to multiple copies of LF and/or EF, producing lethal toxin (LT), edema toxin, or mixtures thereof. Once a host cell endocytoses these complexes, PA converts into a membrane-inserted channel that translocates LF and EF into the cytosol. LT can assemble on host cell surfaces or extracellularly in plasma. We show that, under physiological conditions in bovine plasma, LT complexes containing heptameric PA aggregate and inactivate more readily than LT complexes containing octameric PA. LT complexes containing octameric PA possess enhanced stability, channel-forming activity, and macrophage cytotoxicity relative to those containing heptameric PA. Under physiological conditions, multiple biophysical probes reveal that heptameric PA can prematurely adopt the channel conformation, but octameric PA complexes remain in their soluble prechannel configuration, which allows them to resist aggregation and inactivation. We conclude that PA may form an octameric oligomeric state as a means to produce a more stable and active LT complex that could circulate freely in the blood.

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.

Domain 4 of the anthrax protective antigen maintains structure and binding to the host receptor CMG2 at low pH

Domain 4 of the anthrax protective antigen (PA) plays a key role in cellular receptor recognition as well as in pH-dependent pore formation. We present here the 1.95 A crystal structure of domain 4, which adopts a fold that is identical to that observed in the full-length protein. We have also investigated the structural properties of the isolated domain 4 as a function of pH, as well as the pH-dependence on binding to the von Willebrand factor A domain of capillary morphogenesis protein 2 (CMG2). Our results provide evidence that the isolated domain 4 maintains structure and interactions with CMG2 at pH 5, a pH that is known to cause release of the receptor on conversion of the heptameric prepore (PA(63))(7) to a membrane-spanning pore. Our results suggest that receptor release is not driven solely by a pH-induced unfolding of domain 4.

The protective antigen component of anthrax toxin forms functional octameric complexes

The assembly of bacterial toxins and virulence factors is critical to their function, but the regulation of assembly during infection has not been studied. We begin to address this question using anthrax toxin as a model. The protective antigen (PA) component of the toxin assembles into ring-shaped homooligomers that bind the two other enzyme components of the toxin, lethal factor (LF) and edema factor (EF), to form toxic complexes. To disrupt the host, these toxic complexes are endocytosed, such that the PA oligomer forms a membrane-spanning channel that LF and EF translocate through to enter the cytosol. Using single-channel electrophysiology, we show that PA channels contain two populations of conductance states, which correspond to two different PA pre-channel oligomers observed by electron microscopy-the well-described heptamer and a novel octamer. Mass spectrometry demonstrates that the PA octamer binds four LFs, and assembly routes leading to the octamer are populated with even-numbered, dimeric and tetrameric, PA intermediates. Both heptameric and octameric PA complexes can translocate LF and EF with similar rates and efficiencies. Here, we report a 3.2-A crystal structure of the PA octamer. The octamer comprises approximately 20-30% of the oligomers on cells, but outside of the cell, the octamer is more stable than the heptamer under physiological pH. Thus, the PA octamer is a physiological, stable, and active assembly state capable of forming lethal toxins that may withstand the hostile conditions encountered in the bloodstream. This assembly mechanism may provide a novel means to control cytotoxicity.

Functions of phenylalanine residues within the beta-barrel stem of the anthrax toxin pore

Background

A key step of anthrax toxin action involves the formation of a protein-translocating pore within the endosomal membrane by the Protective Antigen (PA) moiety. Formation of this transmembrane pore by PA involves interaction of the seven 2β2–2β3 loops of the heptameric precursor to generate a 14-strand transmembrane β barrel.

Methodology/Principal Findings

We examined the effects on pore formation, protein translocation, and cytotoxicity, of mutating two phenylalanines, F313 and F314, that lie at the tip the β barrel, and a third one, F324, that lies part way up the barrel.

Conclusions/Significance

Our results show that the function of these phenylalanine residues is to mediate membrane insertion and formation of stable transmembrane channels. Unlike F427, a key luminal residue in the cap of the pore, F313, F314, and F324 do not directly affect protein translocation through the pore. Our findings add to our knowledge of structure-function relationships of a key virulence factor of the anthrax bacillus.

Membrane translocation by anthrax toxin

Much attention has been focused on anthrax toxin recently, both because of its central role in the pathogenesis of Bacillus anthracis and because it has proven to be one of the most tractable toxins for studying how enzymic moieties of intracellularly acting toxins traverse membranes. The Protective Antigen (PA) moiety of the toxin, after being proteolytically activated at the cell surface, self-associates to form a heptameric pore precursor (prepore). The prepore binds up to three molecules of Edema Factor (EF), Lethal Factor (LF), or both, forming a series of complexes that are then endocytosed. Under the influence of acidic pH within the endosome, the prepore undergoes a conformational transition to a mushroom-shaped pore, with a globular cap and 100A-long stem that spans the membrane. Electrophysiological studies in planar bilayers indicate that EF and LF translocate through the pore in unfolded form and in the N- to C-terminal direction. The pore serves as an active transporter, which translocates its proteinaceous cargo across the endosomal membrane in response to DeltapH and perhaps, to a degree, Deltapsi. A ring of seven Phe residues (Phe427) in the lumen of the pore forms a seal around the translocating polypeptide and blocks the passage of ions, presumably preserving the pH gradient. A charge state-dependent Brownian ratchet mechanism has been proposed to explain how the pore translocates EF and LF. This transport mechanism of the pore may function in concert with molecular chaperonins to effect delivery of effector proteins in catalytically active form to the cytosolic compartment of host cells.

Role of lipid structure in the humoral immune response in mice to covalent lipid-peptides from the membrane proximal region of HIV-1 gp41

The membrane proximal region (MPR) of HIV-1 gp41 is a desirable target for development of a vaccine that elicits neutralizing antibodies since the patient-derived monoclonal antibodies, 2F5 and 4E10, bind to the MPR and neutralize primary HIV isolates. The 2F5 and 4E10 antibodies cross-react with lipids and structural studies suggest that MPR immunogens may be presented in a membrane environment. We hypothesized that covalent attachment of lipid anchors would enhance the humoral immune response to MPR-derived peptides presented in liposomal bilayers. In a comparison of eight lipids conjugated to an extended 2F5 epitope peptide, a sterol, cholesterol hemisuccinate (CHEMS), was found to promote the strongest anti-peptide IgG titers (6.4 x 10(4)) in sera of BALB/C mice. Two lipid anchors, palmitic acid and phosphatidylcholine, failed to elicit a detectable serum anti-peptide IgG response. Association with the liposomal vehicle contributed to the ability of a lipopeptide to elicit anti-peptide antibodies, but no other single factor, such as position of the lipid anchor, peptide helical content, lipopeptide partition coefficient, or presence of phosphate on the anchor clearly determined lipopeptide potency. Conjugation to CHEMS also rendered a 4E10 epitope peptide immunogenic (5.6 x 10(2) IgG titer in serum). Finally, attachment of CHEMS to a peptide spanning both the 2F5 and 4E10 epitopes elicited serum IgG antibodies that bound to each of the individual epitopes as well as to recombinant gp140. Further research into the mechanism of how structure influences the immune response to the MPR may lead to immunogens that could be useful in prime-boost regimens for focusing the immune response in an HIV vaccine.

An approach to characterizing single-subunit mutations in multimeric prepores and pores of anthrax protective antigen

Heptameric pores formed by the protective antigen (PA) moiety of anthrax toxin translocate the intracellular effector moieties of the toxin across the endosomal membrane to the cytosol of mammalian cells. We devised a protocol to characterize the effects of individual mutations in a single subunit of heptameric PA prepores (pore precursors) or pores. We prepared monomeric PA containing a test mutation plus an innocuous Cys-replacement mutation at a second residue (Lys563, located on the external surface of the prepore). The introduced Cys was biotinylated, and the protein was allowed to cooligomerize with a 20-fold excess of wild-type PA. Finally, biotinylated prepores were freed from wild-type prepores by avidin affinity chromatography. For the proof of principle, we examined single-subunit mutations of Asp425 and Phe427, two residues where Ala replacements have been shown to cause strong inhibitory effects. The single-subunit D425A mutation inhibited pore formation by >10(4) and abrogated activity of PA almost completely in our standard cytotoxicity assay. The single-subunit F427A mutation caused approximately 100-fold inhibition in the cytotoxicity assay, and this effect was shown to result from a combination of strong inhibition of translocation and smaller effects on pore formation and ligand affinity. Our results show definitively that replacing a single residue in one subunit of the heptameric PA prepore can inhibit the transport activity of the oligomer almost completely-and by different mechanisms, depending on the specific residue mutated.

Preventing voltage-dependent gating of anthrax toxin channels using engineered disulfides

The channel-forming component of anthrax toxin, (PA(63))(7), is a heptameric water-soluble protein at neutral pH, but under acidic conditions it spontaneously inserts into lipid bilayers to form a 14-stranded beta-barrel ion-conducting channel. This channel plays a vital role in anthrax pathogenesis because it serves as a conduit for the membrane translocation of the two enzymatic components of anthrax toxin, lethal factor and edema factor. Anthrax channels open and close in response to changes in transmembrane voltage, a property shared by several other pore-forming toxins. We have discovered an unexpected phenomenon in cysteine-substituted channels that provides a window into this gating process: their normal voltage-dependent gating can be abolished by reaction with methanethiosulfonate (MTS) reagents or exposure to oxidizing conditions. Remarkably, this perturbation is seen with cysteines substituted at sites all along the approximately 100 A length of the channel's beta-barrel. In contrast, reaction with N-ethylmaleimide, a thiol-reactive compound that does not form a mixed disulfide, does not affect gating at any of the sites tested. These findings, coupled with our biochemical detection of dimers, have led us to conclude that MTS reagents are catalyzing the formation of intersubunit disulfide bonds that lock channels in a conducting state, and that voltage gating requires a conformational change that involves the entire beta-barrel.

Template-directed self-assembly enhances RTK catalytic domain function

Receptor tyrosine kinases have become important therapeutic targets because of their involvement in diseases, including cancer. Kinase domains, which are soluble and easily purified, have found widespread use in enzyme inhibitor assays, but these domains do not exhibit full function because they are isolated from the membrane. To address this shortcoming, the authors developed a simple method to restore biologically relevant function by assembling kinase domains on a nanometer-scale template, which imitates the membrane surface. Autophosphorylation of template-assembled tyrosine kinase domains from the insulin, EphB2, and Tie2 receptors led to substantially larger phosphorylation levels compared with domains assayed under conventional conditions. Template-directed assembly increased the total substrate phosphorylation of the insulin and EphB2 receptor kinase domains as much as 60-fold and 15-fold, respectively. In contrast, substrate phosphorylation by template-assembled Tie2 was much lower than conventional conditions. The lower activity observed with the template is more biologically relevant because autophosphorylation of Tie2 is self-inhibitory. These results, as well as the underlying similarity between the organization of template-assembled and natural membrane signaling environments, suggest that template-directed assembly of signaling proteins will provide widespread benefit to basic and applied signal transduction research, especially drug discovery.

Amino acid residues involved in membrane insertion and pore formation of Clostridium botulinum C2 toxin

The actin-ADP-ribosylating Clostridium botulinum C2 toxin consists of the enzymatic component C2I and the binding component C2II. C2II forms heptameric channels involved in translocation of the enzymatic component into the target cell. On the basis of the heptameric toxin channel, we studied functional consequences of mutagenesis of amino acid residues probably lining the lumen of the toxin channel. Substitution of glutamate-399 of C2II with alanine blocked channel formation and cytotoxicity of the holotoxin. Although cytotoxicity and rounding up of cells by C2I were completely blocked by exchange of phenylalanine-428 with alanine, the mutation increased potassium conductance caused by C2II in artificial membranes by about 2-3-fold over that of wild-type toxin. In contrast to its effects on single-channel potassium conductance in artificial membranes, the F428A mutation delayed the kinetics of pore formation in lipid vesicles and inhibited the activity of C2II in promoting (86)Rb (+) release from preloaded intact cells after pH shift of the medium. Moreover, F428A C2II exhibited delayed and diminished formation of C2II aggregates at low pH, indicating major changes of the biophysical properties of the toxin. The data indicate that phenylalanine-428 of C2II plays a major role in conformational changes occurring during pore formation of the binding component of C2II.

Phenylalanine-427 of anthrax protective antigen functions in both pore formation and protein translocation

The protective antigen (PA) moiety of anthrax toxin forms a heptameric pore in endosomal membranes of mammalian cells and translocates the enzymatic moieties of the toxin to the cytosol of these cells. Phenylalanine-427 (F427), a solvent-exposed residue in the lumen of the pore, was identified earlier as being crucial for the transport function of PA. The seven F427 residues were shown in electrophysiological studies to form a clamp that catalyzes protein translocation through the pore. Here, we demonstrate by a variety of tests that certain F427 mutations also profoundly inhibit the conformational transition of the heptameric PA prepore to the pore and thereby block pore formation in membranes. Lysine, arginine, aspartic acid, or glycine at position 427 strongly inhibited this acidic pH-induced conformational transition, whereas histidine, serine, and threonine had virtually no effect on this step, but inhibited translocation instead. Thus, it is possible to inhibit pore formation or translocation selectively, depending on the choice of the side chain at position 427; and the net inhibition of the PA transport function by any given F427 mutation is the product of its effects on both steps. Mutations inhibiting either or both steps elicited a strong dominant-negative phenotype. These findings demonstrate the dual functions of F427 and underline its central role in transporting the enzymatic moieties of anthrax toxin across membranes.

Pore formation: an ancient yet complex form of attack

Bacteria, as well as higher organisms such as sea anemones or earthworms, have developed sophisticated virulence factors such as the pore-forming toxins (PFTs) to mount their attack against the host. One of the most fascinating aspects of PFTs is that they can adopt a water-soluble form at the beginning of their lifetime and become an integral transmembrane protein in the membrane of the target cells. There is a growing understanding of the sequence of events and the various conformational changes undergone by these toxins in order to bind to the host cell surface, to penetrate the cell membranes and to achieve pore formation. These points will be addressed in this review.

Template-directed assembly of signaling proteins: a novel drug screening and research tool

A multitude of proteins reside at or near the cell membrane, which provides a unique environment for organizing and promoting assemblies of proteins that are involved in a variety of cellular signaling functions. Many of these proteins and pathways are implicated in disease. For example, strong links have been established between receptor tyrosine kinases and disease, most notably, cancer. However, a significant impediment to researchers remains: membrane-associated proteins are difficult to reconstitute and study. Template-directed assembly represents a powerful new technology that enables the assembly of membrane-associated proteins. We show that template-directed assembly restores tyrosine kinase activity and regulation, and provides a way for researchers to build multicomponent assemblies. As an example of better enzyme regulation, the Tie2 tyrosine kinase domain exhibits (biologically relevant) autoinhibitory behavior when template assembled. Also, template-assembled insulin receptor tyrosine kinase domains exhibit significant autophosphorylation (none detected without template-directed assembly) and an eightfold increase in substrate phosphorylation (compared to best solution conditions). Thus, template-directed assembly has a demonstrated ability to effectively produce more biologically relevant results using these commercial reagents. Template-directed assembly promises to be generally applicable to the signaling networks important for human health, because these pathways frequently contain membrane-associated proteins that require the organizing influence of a membrane surface.

Liposome-mediated assembly of receptor signaling complexes

The reconstitution of membrane-associated protein complexes poses significant experimental challenges. The core signaling complex in the bacterial chemotaxis system is an illustrative example: The soluble cytoplasmic signaling proteins CheW and CheA bind to heterogeneous clusters of transmembrane receptor proteins, resulting in an assembly that exhibits cooperative kinase regulation. An understanding of the basis for the cooperativity inherent in the receptor/CheW/CheA interaction, as well as other membrane phenomena, can benefit from functional studies under defined conditions. To meet this need, a simple method was developed to assemble functional complexes on lipid membranes. The method employs a receptor cytoplasmic domain fragment (CF) with a histidine tag and liposomes that contain a Ni(2+) -chelating lipid. Assemblies of CF, CheW, and CheA form spontaneously in the presence of these liposomes, which exhibit the salient biochemical functions of kinase stimulation, cooperative regulation, and CheR-mediated receptor methylation. Although ligand binding phenomena cannot be studied directly with this approach, other factors that influence kinase stimulation and receptor methylation can be explored systematically, including receptor density and competition among stimulating and inhibiting receptor domains. The template-directed assembly of proteins leads to relatively well-defined samples that are amenable to analysis by a number of methods, including light scattering, electron microscopy, and fluorescence resonance energy transfer. The approach promises to be applicable to many systems involving membrane-associated proteins.

Anthrax toxin: receptor binding, internalization, pore formation, and translocation

Anthrax toxin consists of three nontoxic proteins that self-assemble at the surface of receptor-bearing mammalian cells or in solution, yielding a series of toxic complexes. Two of the proteins, called Lethal Factor (LF) and Edema Factor (EF), are enzymes that act on cytosolic substrates. The third, termed Protective Antigen (PA), is a multifunctional protein that binds to receptors, orchestrates the assembly and internalization of the complexes, and delivers them to the endosome. There, the PA moiety forms a pore in the endosomal membrane and promotes translocation of LF and EF to the cytosol. Recent advances in understanding the entry process include insights into how PA recognizes its two known receptors and its ligands, LF and EF; how the PA:receptor interaction influences the pH-dependence of pore formation; and how the pore functions in promoting translocation of LF and EF across the endosomal membrane.