Identification of a putative membrane-inserted segment in the alpha-toxin of Staphylococcus aureus

Mapping secondary substrate-binding sites on the GH11 xylanase from Bacillus subtilis

Xylanases are of significant interest for biomass conversion technologies. Here, we investigated the allosteric regulation of xylan hydrolysis by the Bacillus subtilis GH11 endoxylanase. Molecular dynamics simulations (MDS) in the presence of xylobiose identified binding to the active site and two potential secondary binding sites (SBS) around surface residues Asn54 and Asn151. Arabinoxylan titration experiments with single cysteine mutants N54C and N151C labeled with the thiol-reactive fluorophore acrylodan or the ESR spin-label MTSSL validated the MDS results. Ligand binding at the SBS around Asn54 confirms previous reports, and analysis of the second SBS around N151C discovered in the present study includes residues Val98/Ala192/Ser155/His156. Understanding the regulation of xylanases contributes to efforts for industrial decarbonization and to establishing a sustainable energy matrix.

Channel-forming bacterial toxins in biosensing and macromolecule delivery

To intoxicate cells, pore-forming bacterial toxins are evolved to allow for the transmembrane traffic of different substrates, ranging from small inorganic ions to cell-specific polypeptides. Recent developments in single-channel electrical recordings, X-ray crystallography, protein engineering, and computational methods have generated a large body of knowledge about the basic principles of channel-mediated molecular transport. These discoveries provide a robust framework for expansion of the described principles and methods toward use of biological nanopores in the growing field of nanobiotechnology. This article, written for a special volume on "Intracellular Traffic and Transport of Bacterial Protein Toxins", reviews the current state of applications of pore-forming bacterial toxins in small- and macromolecule-sensing, targeted cancer therapy, and drug delivery. We discuss the electrophysiological studies that explore molecular details of channel-facilitated protein and polymer transport across cellular membranes using both natural and foreign substrates. The review focuses on the structurally and functionally different bacterial toxins: gramicidin A of Bacillus brevis, α-hemolysin of Staphylococcus aureus, and binary toxin of Bacillus anthracis, which have found their "second life" in a variety of developing medical and technological applications.

Putative identification of an amphipathic alpha-helical sequence in hemolysin of Escherichia coli (HlyA) involved in transmembrane pore formation

Abstract Escherichia coli hemolysin is a pore-forming protein belonging to the RTX toxin family. Cysteine scanning mutagenesis was performed to characterize the putative pore-forming domain of the molecule. A single cysteine residue was introduced at 48 positions within the sequence spanning residues 170-400 and labeled with the polarity-sensitive dye badan. Spectrofluorimetric analyses indicated that several amino acids in this domain are inserted into the lipid bilayer during pore formation. An amphipathic alpha-helix spanning residues 272-298 was identified that may line the aqueous pore. The importance of this sequence was highlighted by the introduction of two prolines at positions 284 and 287. Disruption of the helix structure did not affect binding properties, but totally abolished the hemolytic activity of the molecule.

Evidence that clustered phosphocholine head groups serve as sites for binding and assembly of an oligomeric protein pore

High susceptibility of rabbit erythrocytes toward the pore-forming action of staphylococcal alpha-toxin correlates with the presence of saturable, high affinity binding sites. All efforts to identify a protein or glycolipid receptor have failed, and the fact that liposomes composed solely of phosphatidylcholine are efficiently permeabilized adds to the enigma. A novel concept is advanced here to explain the puzzle. We propose that low affinity binding moieties can assume the role of high affinity binding sites due to their spatial arrangement in the membrane. Evidence is presented that phosphocholine head groups of sphingomyelin, clustered in sphingomyelin-cholesterol microdomains, serve this function for alpha-toxin. Clustering is required so that oligomerization, which is prerequisite for stable attachment of the toxin to the membrane, can efficiently occur. Outside these clusters, binding to phosphocholine is too transient for toxin monomers to find each other. The principle of membrane targeting in the absence of any genuine, high affinity receptor may also underlie the assembly of other lipid-inserted oligomers including cytotoxic peptides, protein toxins, and immune effector molecules.

High resolution structural analysis of Helicobacter pylori VacA toxin oligomers by cryo-negative staining electron microscopy

Helicobacter pylori secretes a vacuolating toxin (VacA) that can assemble into water-soluble oligomeric complexes and insert into membranes to form anion-selective channels. Previous studies have described multiple types of oligomeric VacA structures, including single-layered astral arrays, bilayered forms, and two-dimensional crystalline arrays. In the current study, vitrified VacA complexes were examined by cryo-negative staining electron microscopy, views of the different oligomeric structures in multiple orientations were classified and analyzed, and three-dimensional models of the bilayered forms of VacA were constructed with a resolution of about 19 angstroms. These bilayered forms of VacA have a "flower"-like structure, consisting of a central ring surrounded by symmetrically arranged peripheral "petals." Further structural insights were obtained by analyzing a mutant form of VacA (VacADelta6-27), which lacks a unique amino-terminal hydrophobic segment and is defective in the capacity to form membrane channels. Bilayered oligomeric complexes formed by wild-type VacA contained a visible density within the central ring, whereas bilayered complexes formed by VacADelta6-27 lacked this density. These results indicate that deletion of the VacA amino-terminal hydrophobic region causes a structural alteration in the central ring within VacA oligomers, and suggest that the central ring plays an important role in the process by which VacA forms membrane channels.

Essential role of a GXXXG motif for membrane channel formation by Helicobacter pylori vacuolating toxin

Helicobacter pylori secretes a toxin, VacA, that can form anion-selective membrane channels. Within a unique amino-terminal hydrophobic region of VacA, there are three tandem GXXXG motifs (defined by glycines at positions 14, 18, 22, and 26), which are characteristic of transmembrane dimerization sequences. The goals of the current study were to investigate whether these GXXXG motifs are required for membrane channel formation and cytotoxicity and to clarify the role of membrane channel formation in the biological activity of VacA. Six different alanine substitution mutations (P9A, G13A, G14A, G18A, G22A, and G26A) were introduced into the unique hydrophobic region located near the amino terminus of VacA. The effects of these mutations were first analyzed using the TOXCAT system, which permits the study of transmembrane oligomerization of proteins in a natural membrane environment. None of the mutations altered the capacity of ToxR-VacA-maltose-binding protein fusion proteins to insert into a membrane, but G14A and G18A mutations markedly diminished the capacity of the fusion proteins to oligomerize. We then introduced the six alanine substitution mutations into the vacA chromosomal gene of H. pylori and analyzed the properties of purified mutant VacA proteins. VacA-G13A, VacA-G22A, and VacA-G26A induced vacuolation of HeLa cells, whereas VacA-P9A, VacA-G14A, and VacA-G18A did not. Subsequent experiments examined the capacity of each mutant toxin to form membrane channels. In a planar lipid bilayer assay, VacA proteins containing G13A, G22A, and G26A mutations formed anion-selective membrane channels, whereas VacA proteins containing P9A, G14A, and G18A mutations did not. Similarly, VacA-G13A, VacA-G22A, and VacA-G26A induced depolarization of HeLa cells, whereas VacA-P9A, VacA-G14A, and VacA-G18A did not. These data indicate that an intact proline residue and an intact G(14)XXXG(18) motif within the amino-terminal hydrophobic region of VacA are essential for membrane channel formation, and they also provide strong evidence that membrane channel formation is essential for VacA cytotoxicity.

Beta-barrel membrane protein folding and structure viewed through the lens of alpha-hemolysin

The beta-barrel is a transmembrane structural motif commonly encountered in bacterial outer membrane proteins and pore-forming toxins (PFTs). Alpha-hemolysin (alphaHL) is a cytotoxin secreted by Staphylococcus aureus that assembles from a water-soluble monomer to form a membrane-bound heptameric beta-barrel on the surface of susceptible cells, perforating the cell membranes, leading to cell death and lysis. The mechanism of heptamer assembly, which has been studied extensively, occurs in a stepwise manner, and the structures of the initial, monomeric form and final, membrane-embedded pore are known. The toxin's ability to assemble from an aqueous, hydrophilic species to a membrane-inserted oligomer is of interest in understanding the assembly of PFTs in particular and the folding and structure of beta-barrel membrane proteins in general. Here we review the structures of the monomeric and heptamer states of LukF and alphaHL, respectively, the mechanism of toxin assembly, and the relationships between alphaHL and nontoxin beta-barrel membrane proteins.

Region of heat-stable enterotoxin II of Escherichia coli involved in translocation across the outer membrane

Heat-stable enterotoxin II of Escherichia coli (STII) is synthesized as a precursor form consisting of pre- and mature regions. The pre-region is cleaved off from the mature region during translocation across the inner membrane, and the mature region emerges in the periplasm. The mature region, composed of 48 amino acid residues, is processed in the periplasm by DsbA to form an intramolecular disulfide bond between Cys-10 and Cys-48 and between Cys-21 and Cys-36. STII formed with these disulfide bonds is efficiently secreted out of the cell through the secretory system, including TolC. However, it remains unknown which regions of STII are involved in interaction with TolC. In this study, we mutated the STII gene and examined the secretion of these STIIs into the culture supernatant. A deletion of the part covering from amino acid residue 37 to the carboxy terminal end did not markedly reduce the efficiency of secretion of STII into the culture supernatant. On the other hand, the efficiency of secretion of the peptide covering from the amino terminal end to position 18 to the culture supernatant was significantly low. These observations indicated that the central region of STII from amino acid residue 19 to that at position 36 is involved in the secretion of STII into the milieu. The experiment using a dsbA-deficient strain of E. coli showed that the disulfide bond between Cys-21 and Cys-36 by DsbA is necessary for STII to adapt to the structure that can cross the outer membrane.

Binding to sulfatide and enterotoxicity of various Escherichia coli STb mutants

Binding of the 48 amino acid polypeptide of the mature heat-stable Escherichia coli enterotoxin b (STb) to the functional receptor sulfatide (SFT) constitutes the first step in inducing secretory diarrhoea in the intestinal lumen of animals. The NMR structure of this toxin dictated the choice of amino acids for site-directed mutagenesis to delineate the binding site of STb to SFT. Amino acids facing the solvent either in the loop or the hydrophobic alpha-helix were selected. Seventeen site-specific mutants of STb toxin were produced and purified by high-pressure liquid chromatography. Enterotoxicity of the 17 mutants was determined using a rat loop assay and binding was evaluated using a microtitre plate binding assay. Both hydrophobic and electrostatic interactions are important for STb attachment. When mutations (F37K, I41S and M42S) were introduced into the hydrophobic alpha-helix to lessen hydrophobicity, binding activity and enterotoxicity decreased by more than sixfold. The loop defined by C21 and C36 also made specific contributions. Mutants generated at basic residues (K22, K23 and R29) within this region exhibited both reduced binding activities and reduced toxic activities. For all STb mutants constructed and analysed, when binding to SFT was reduced, a reduction in toxicity equivalent or greater was noted, indicating that binding to SFT is a step that precedes the toxic effect observed for STb toxin. Significantly, when the negatively charged D30 was substituted for either alanine or valine, the binding to SFT was about twice that of native STb, whereas the enterotoxicity was reduced by half.

Discoupling the Ca(2+)-activation from the pore-forming function of the bi-component Panton-Valentine leucocidin in human PMNs

The consecutive cell activation, including Ca(2+)-channel opening, and pore formation leading to human neutrophil lysis were the two functions of the staphylococcal Panton-Valentine leucocidin attempted to be discoupled by site-directed mutagenesis. In a first approach consisting in deletions of the cytoplasmic extremity of the transmembranous domain, we produced a LukF-PV DeltaSer125-Leu128 with a slightly reduced Ca(2+) induction but with a significantly lowered lytic activity when combined with its synergistic protein LukS-PV. The second approach consisted in the modification of charges and/or introduction of a steric hindrance inside the pore, which also led to interesting mutated proteins: LukF-PV G131D, G131W and G130D. The latter had an intact Ca(2+) induction ability while the lytic one was 20-fold diminished. Binding properties and intrinsic pore diameters of these discoupled toxins remained comparable to the wild-type protein. The mutated proteins promoted interleukin-8 secretion, but they were rather inactive in an experimental model. New insights are brought concerning the role of the two functions in the virulence of this bi-component leucotoxin.

A stability transition at mildly acidic pH in the alpha-hemolysin (alpha-toxin) from Staphylococcus aureus

The effects of mildly acidic conditions on the free energy of unfolding (DeltaG(u)(buff)) of the pore-forming alpha-hemolysin (alphaHL) from Staphylococcus aureus were assessed between pH 5.0 and 7.5 by measuring intrinsic tryptophan fluorescence, circular dichroism and elution time in size exclusion chromatography during urea denaturation. Decreasing the pH from 7.0 to 5.0 reduced the calculated DeltaG(u)(buff) from 8.9 to 4.2 kcal mol(-1), which correlates with an increased rate of pore formation previously observed over the same pH range. It is proposed that the lowered surface pH of biological membranes reduces the stability of alphaHL thereby modulating the rate of pore formation.

Cysteine-scanning mutagenesis of an eukaryotic pore-forming toxin from sea anemone: topology in lipid membranes

Equinatoxin II is a cysteineless pore-forming protein from the sea anemone Actinia equina. It readily creates pores in membranes containing sphingomyelin. Its topology when bound in lipid membranes has been studied using cysteine-scanning mutagenesis. At approximately every tenth residue, a cysteine was introduced. Nineteen single cysteine mutants were produced in Escherichia coli and purified. The accessibility of the thiol groups in lipid-embedded cysteine mutants was studied by reaction with biotin maleimide. Most of the mutants were modified, except those with cysteines at positions 105 and 114. Mutants R144C and S160C were modified only at high concentrations of the probe. Similar results were obtained if membrane-bound biotinylated mutants were tested for avidin binding, but in this case three more mutants gave a negative result: S1C, S13C and K43C. Furthermore, mutants S1C, S13C, K20C, K43C and S95C reacted with biotin only after insertion into the lipid, suggesting that they were involved in major conformational changes occurring upon membrane binding. These results were further confirmed by labeling the mutants with acrylodan, a polarity-sensitive fluorescent probe. When labeled mutants were combined with vesicles, the following mutants exhibited blue-shifts, indicating the transfer of acrylodan into a hydrophobic environment: S13C, K20C, S105C, S114C, R120C, R144C and S160C. The overall results suggest that at least two regions are embedded within the lipid membrane: the N-terminal 13-20 region, probably forming an amphiphilic helix, and the tryptophan-rich 105-120 region. Arg144, Ser160 and residues nearby could be involved in making contacts with lipid headgroups. The association with the membrane appears to be unique and different from that of bacterial pore-forming proteins and therefore equinatoxin II may serve as a model for eukaryotic channel-forming toxins.

The structure of a Staphylococcus aureus leucocidin component (LukF-PV) reveals the fold of the water-soluble species of a family of transmembrane pore-forming toxins

BACKGROUND

Leucocidins and gamma-hemolysins are bi-component toxins secreted by Staphylococcus aureus. These toxins activate responses of specific cells and form lethal transmembrane pores. Their leucotoxic and hemolytic activities involve the sequential binding and the synergistic association of a class S and a class F component, which form hetero-oligomeric complexes. The components of each protein class are produced as non-associated, water-soluble proteins that undergo conformational changes and oligomerization after recognition of their cell targets.

RESULTS

The crystal structure of the monomeric water-soluble form of the F component of Panton-Valentine leucocidin (LukF-PV) has been solved by the multiwavelength anomalous dispersion (MAD) method and refined at 2.0 A resolution. The core of this three-domain protein is similar to that of alpha-hemolysin, but significant differences occur in regions that may be involved in the mechanism of pore formation. The glycine-rich stem, which undergoes a major rearrangement in this process, forms an additional domain in LukF-PV. The fold of this domain is similar to that of the neurotoxins and cardiotoxins from snake venom.

CONCLUSIONS

The structure analysis and a multiple sequence alignment of all toxic components, suggest that LukF-PV represents the fold of any water-soluble secreted protein in this family of transmembrane pore-forming toxins. The comparison of the structures of LukF-PV and alpha-hemolysin provides some insights into the mechanism of transmembrane pore formation for the bi-component toxins, which may diverge from that of the alpha-hemolysin heptamer.

Localization and environment of tryptophans in soluble and membrane-bound states of a pore-forming toxin from Staphylococcus aureus

The location and environment of tryptophans in the soluble and membrane-bound forms of Staphylococcus aureus alpha-toxin were monitored using intrinsic tryptophan fluorescence. Fluorescence quenching of the toxin monomer in solution indicated varying degrees of tryptophan burial within the protein interior. N-Bromosuccinimide readily abolished 80% of the fluorescence in solution. The residual fluorescence of the modified toxin showed a blue-shifted emission maximum, a longer fluorescence lifetime as compared to the unmodified and membrane-bound alpha-toxin, and a 5- to 6-nm red edge excitation shift, all indicating a restricted tryptophan environment and deeply buried tryptophans. In the membrane-bound form, the fluorescence of alpha-toxin was quenched by iodide, indicating a conformational change leading to exposure of some tryptophans. A shorter average lifetime of tryptophans in the membrane-bound alpha-toxin as compared to the native toxin supported the conclusions based on iodide quenching of the membrane-bound toxin. Fluorescence quenching of membrane-bound alpha-toxin using brominated and spin-labeled fatty acids showed no quenching of fluorescence using brominated lipids. However, significant quenching was observed using 5- and 12-doxyl stearic acids. An average depth calculation using the parallax method indicated that the doxyl-quenchable tryptophans are located at an average depth of 10 A from the center of the bilayer close to the membrane interface. This was found to be in striking agreement with the recently described structure of the membrane-bound form of alpha-toxin.

alpha-Hemolysin from Staphylococcus aureus: an archetype of beta-barrel, channel-forming toxins

alpha-Hemolysin, secreted from Staphylococcus aureus as a water-soluble monomer of 33.2 kDa, assembles on cell membranes to form transmembrane, heptameric channels. The structure of the detergent-solubilized heptamer has been determined by X-ray crystallography to 1.9 A resolution. The heptamer has a mushroom-like shape and measures up to 100 A in diameter and 100 A in height. Spanning the length of the molecule and coincident with the molecular sevenfold axis is a water-filled channel that ranges in diameter from approximately 16 to approximately 46 A. A 14 strand antiparallel beta-barrel, in which two strands are contributed by each subunit, defines the transmembrane domain. On the exterior of the beta-barrel there is a hydrophobic belt approximately 30 A in width that provides a surface complementary to the nonpolar portion of the lipid bilayer. The extensive promoter-protomer interfaces are composed of both salt-links and hydrogen bonds, as well as hydrophobic interactions, and these contacts provide a molecular rationalization for the stability of the heptamer in SDS solutions up to 65 degrees C. With the structure of the heptamer in hand, we can better understand the mechanisms by which the assembled protein interacts with the membrane and can postulate mechanisms of assembly.

Tertiary structural changes of the alpha-hemolysin from Staphylococcus aureus on association with liposome membranes

The interaction of alpha-hemolysin (also called alpha-toxin) from Staphylococcus aureus with mixed egg-yolk phosphatidylcholine/cholesterol liposomes has been investigated using the intrinsic tryptophan fluorescence emission (ITFE) signal. The ITFE intensity of alpha-hemolysin, which was obtained using a novel purification protocol, showed a triphasic increase on incubation with liposomes at low protein/lipid ratios. The first, rapid phase results in an increase in ITFE of 10%, which reflects rapid conformation changes in the alpha-hemolysin on association with the liposome membrane. The second phase of the ITFE increase is associated with a red shift from 334 to 339 nm in the maximum emission wavelength, suggesting the transition to a partially unfolded intermediate in the oligomerization process. The third phase of the ITFE intensity change demonstrates a temporal correlation with the appearance of SDS-stable oligomers. The results demonstrate the feasibility of identification of intermediate protein conformations in complex membrane-associated processes by manipulation of the liposomal membrane composition.

Site-directed mutagenesis of Clostridium perfringens beta-toxin: expression of wild-type and mutant toxins in Bacillus subtilis

Recombinant beta-toxin has been expressed and secreted from Bacillus subtilis. Biological activity was tested in vivo and in vitro. The lethal dose in mice was determined. Hemolysis of rabbit and sheep erythrocytes was tested but no effect was observed. Seven mutant proteins were produced. Targets for mutagenesis were mostly selected on the basis of the similarity between beta-toxin and alpha-toxin from Staphylococcus aureus, a pore-forming toxin. Mutations of two amino acids affected the lethal dose in mice. Both residues have counterparts in the membrane binding region of alpha-toxin. Alteration of the single cysteine residue did not affect protein function, contrary to previous suggestions.

Transmembrane beta-barrel of staphylococcal alpha-toxin forms in sensitive but not in resistant cells

Staphylococcal alpha-toxin is a 293-residue, single-chain polypeptide that spontaneously assembles into a heptameric pore in target cell membranes. To identify the pore-forming domain, substitution mutants have been produced in which single cysteine residues were introduced throughout the toxin molecule. By attaching the environmentally sensitive dye acrylodan to the sulfhydryl groups, the environment of individual amino acid side chains could be probed. In liposomes, a single 23-amino acid sequence (residues 118-140) was found to move from a polar to a nonpolar environment, indicating that this sequence forms the walls of the pore. However, periodicity in side chain environmental polarity could not be detected in the liposomal system. In the present study, the fluorimetric analyses were extended to physiological target cells. With susceptible cells such as rabbit erythrocytes and human lymphocytes, the 23 central amino acids 118-140 were again found to insert into the membrane; in contrast to the previous study with liposomes, the expected periodicity was now detected. Thus, every other residue in the sequence 126-140 entered a nonpolar environment in a striking display of an amphipathic transmembrane beta-barrel. In contrast, human granulocytes were found to bind alpha-toxin to a similar extent as lymphocytes, but the heptamers forming on these cells failed to insert their pore-forming domain into the membrane. As a consequence, nonfunctional heptamers assembled and the cells remained viable. The data resolve the molecular organization of a pore-forming toxin domain in living cells and reveal that resistant cells can prevent insertion of the functional domain into the bilayer.

The hinge portion of the S. aureus alpha-toxin crosses the lipid bilayer and is part of the trans-mouth of the channel

This paper compares the functional properties of ion channels formed in planar lipid membranes by the wild and mutant Staphylococcus aureus alpha-toxin. It was shown that replacement of the amino acid Gly at position 130 by Cys in the primary structure of the toxin decreases the single-channel conductance with a concomitant decrease in the pH at which the channel becomes unable to discriminate between Cl- and K+ ions. The mutation also induced an increase in the asymmetry in the current-voltage relationship of the channel. The results of our experiments suggest that the trans-mouth of the channel is responsible for all the observed changes in channel properties. It was assumed that this entrance is built by the glycine-rich hinge portion of the toxin and is situated close to the surface of monolayer facing the trans-compartment.

Designed protein pores as components for biosensors

BACKGROUND

There is a pressing need for new sensors that can detect a variety of analytes, ranging from simple ions to complex compounds and even microorganisms. The devices should offer sensitivity, speed, reversibility and selectivity. Given these criteria, protein pores, remodeled so that their transmembrane conductances are modulated by the association of specific analytes, are excellent prospects as components of biosensors.

RESULTS

Structure-based design and a separation method that employs targeted chemical modification have been used to obtain a heteromeric form of the bacterial pore-forming protein staphylococcal alpha-hemolysin, in which one of the seven subunits contains a binding site for a divalent metal ion, M(II), which serves as a prototypic analyte. The single-channel current of the heteromer in planar bilayers is modulated by nanomolar Zn(II). Other M(II)s modulate the current and produce characteristic signatures. In addition, heteromers containing more than one mutant subunit exhibit distinct responses to M(II)s Hence, a large collection of responsive pores can be generated through subunit diversity and combinatorial assembly.

CONCLUSIONS

Engineered pores have several advantages as potential sensor elements: sensitivity is in the nanomolar range; analyte binding is rapid (diffusion limited in some cases) and reversible; strictly selective binding is not required because single-channel recordings are rich in information; and for a particular analyte, the dissociation rate constant, the extent of channel block and the voltage-dependence of these parameters are distinguishing, while the frequency of partial channel block reflects the analyte concentration. A single sensor element might, therefore, be used to quantitate more than one analyte at once. The approach described here can be generalized for additional analytes.

Membrane insertion: The strategies of toxins (review)

Protein toxins are soluble molecules secreted by pathogenic bacteria which act at the plasma membrane or in the cytoplasm of target cells. They must therefore interact with a membrane at some point, either to modify its permeability properties or to reach the cytoplasm. As a consequence, toxins have the built-in capacity to adopt two generally incompatible states: water-soluble and transmembrane. Irrespective of their origin or function, the membrane interacting domain of most protein toxins seems to have adopted one out of two structural strategies to be able to undergo this metamorphosis. In the first group of toxins the membrane interacting domain has the structural characteristics of most known membrane proteins, i.e. it contains hydrophobic and amphipathic alpha-helices long enough to span a membrane. To render this 'membrane protein' water-soluble during the initial part of its life the hydrophobic helices are sheltered from the solvent by a barrel of amphipathic helices. In the second group of toxins the opposite strategy is adopted. The toxin is an intrinsically soluble protein and is composed mainly of beta-structure. These toxins manage to become membrane proteins by oligomerizing in order to combine amphipathic beta-sheet to generate sufficient hydrophobicity for membrane insertion to occur. Toxins from this latter group are thought to perforate the lipid bilayer as a beta-barrel such as has been described for bacterial porins, and has recently been shown for staphylococcal alpha-toxin. The two groups of toxins will be described in detail through the presentation of examples. Particular attention will be given to the beta-structure toxins, since four new structures have been solved over the past year: the staphyloccocal alpha-toxin channel, the anthrax protective antigen protoxin, the anthrax protective antigen-soluble heptamer and the CytB protoxin. Structural similarities with mammalian proteins implicated in the immune response and apoptosis will be discussed. Peptide toxins will not be covered in this review.

Conformational changes due to membrane binding and channel formation by staphylococcal alpha-toxin

Conformational changes occurring upon membrane binding and subsequent insertion of staphylococcal alpha-toxin were studied using complementary spectroscopic techniques. Experimental conditions were established where binding could be uncoupled from membrane insertion but insertion and channel formation seemed to be concomitant. Binding led to changes in tertiary structure as witnessed by an increase in tryptophan fluorescence, a red shift of the tryptophan maximum emission wavelength, and a change in the near UV CD spectrum. In contrast to what was observed for the soluble form of the toxin, 78% of the tryptophan residues in the membrane-bound form were accessible to the hydrophilic quencher KI. At this stage, the tryptophan residues were not in the immediate vicinity of the lipid bilayer. Upon membrane insertion, a second conformational change occurred resulting in a dramatic drop of the near UV CD signal but an increase of the far UV signal. Tryptophan residues were no longer accessible to KI but could be quenched by brominated lipids. In the light of the available data on channel formation by alpha-toxin, our results suggest that the tryptophan residues might be dipping into the membrane in order to anchor the extramembranous part of the channel to the lipid bilayer.

Structure of staphylococcal alpha-hemolysin, a heptameric transmembrane pore

The structure of the Staphylococcus aureus alpha-hemolysin pore has been determined to 1.9 A resolution. Contained within the mushroom-shaped homo-oligomeric heptamer is a solvent-filled channel, 100 A in length, that runs along the sevenfold axis and ranges from 14 A to 46 A in diameter. The lytic, transmembrane domain comprises the lower half of a 14-strand antiparallel beta barrel, to which each protomer contributes two beta strands, each 65 A long. The interior of the beta barrel is primarily hydrophilic, and the exterior has a hydrophobic belt 28 A wide. The structure proves the heptameric subunit stoichiometry of the alpha-hemolysin oligomer, shows that a glycine-rich and solvent-exposed region of a water-soluble protein can self-assemble to form a transmembrane pore of defined structure, and provides insight into the principles of membrane interaction and transport activity of beta barrel pore-forming toxins.

Membrane-penetrating domain of streptolysin O identified by cysteine scanning mutagenesis

Streptolysin O (SLO), a polypeptide of 571 amino acids, belongs to a family of highly homologous toxins that bind to cell membranes containing cholesterol and then polymerize to form large transmembrane pores. A conserved region close to the C terminus contains the single cysteine residue of SLO and has been implicated in membrane binding, which has been the only clear assignment of function to a part of the sequence. We have used a cysteine-less active mutant of SLO to introduce single cysteine residues at 19 positions distributed throughout the sequence. The cysteines were derivatized with the polarity-sensitive fluorophore acrylodan, and the fluorescence emission of the label was examined at the different stages of SLO pore assembly. With several mutants, oligomerization on membranes was accompanied by emission blue-shifts, indicating movement of the label into a more hydrophobic environment. These effects were essentially confined to the range of amino acids 213-305. With oligomeric mutants L274C, S286C, and S305C, additional environmental alterations were induced when different nondenaturing detergents were used to dislodge the membrane lipids from the oligomers. The corresponding amino acid residues thus insert into the lipid bilayer during pore formation. Conversely, the spectra of oligomeric mutants A213C and T245C were not affected by detergents. Devoid of contact with the lipid bilayer, these amino acid residues probably participate in the interaction of SLO molecules within the oligomer.

Tumor protease-activated, pore-forming toxins from a combinatorial library

We describe a library of two-chain molecular complementation mutants of staphylococcal alpha-hemolysin that features a combinatorial cassette encoding thousands of protease recognition sites in the central pore-forming domain. The cassette is flanked by a peptide extension that inactivates the protein. We screened the library to identify alpha-hemolysins that are highly susceptible to activation by cathepsin B, a protease that is secreted by certain metastatic tumor cells. Toxins obtained by this procedure should be useful for the permeabilization of malignant cells thereby leading directly to cell death or permitting destruction of the cells with drugs that are normally membrane impermeant.

Partial C-terminal unfolding is required for channel formation by staphylococcal alpha-toxin

The pore-forming alpha-toxin from Staphylococcus aureus is secreted as a soluble monomeric protein. In order to form a transmembrane channel, the protein has to undergo oligomerization and membrane insertion. Previous studies have shown that channel formation is favored by acidic pH. We have analyzed the effect of pH on the kinetics of channel formation as well as on the conformation of the toxin. Using a variety of spectroscopic probes for protein structure, we have shown that alpha-toxin unfolded upon acidification and that the unfolding process occurred in at least three steps. The various steps could be selectively affected by modifying the salt concentration or the temperature. This unfolding was, however, only partial as the secondary structure remained native-like as witnessed by far UV CD measurements. The first unfolding step, corresponding to a region of the C-terminal half of the toxin, is of particular importance as it coincided with the exposure of hydrophobic patches on the surface of the protein as well as with the onset of channel formation. Our observations strongly suggest that transition of the C-terminal half of alpha-toxin to a molten globule-like state is required for channel formation.

Expression of active streptolysin O in Escherichia coli as a maltose-binding-protein--streptolysin-O fusion protein. The N-terminal 70 amino acids are not required for hemolytic activity

Streptolysin 0 (SLO) is the prototype of a family of cytolysins that consists of proteins which bind to cholesterol and form very large transmembrane pores. Structure/function studies on the pore-forming cytolysin SLO have been complicated by the proteolytic inactivation of a substantial portion of recombinant SLO (rSLO) expressed in Escherichia coli. To overcome this problem, translational fusions between the E. coli maltose-binding protein (MBP) gene and SLO were constructed, using the vectors pMAL-p2 and pMAL-c2. MBP-SLO fusion proteins were degraded if secreted into the E. coli periplasm, but intact, soluble MBP-SLO fusion proteins were produced at high levels in the cytoplasm. Active SLO with the expected N-terminus was separated from the MBP carrier by cleavage with factor Xa. Cleavage with plasmin or trypsin also yielded active, but slightly smaller forms of SLO. Surprisingly, uncleaved MBP-SLO was also hemolytic and cytotoxic to human fibroblasts and keratinocytes. The MBP-SLO fusion protein displayed equal activities to SLO. Sucrose density gradient analyses showed that the fusion protein assembled into polymers, and no difference in structure was discerned compared with polymers formed by native SLO. These studies show that the N-terminal 70 residues of mature (secreted) SLO are not required for pore formation and that the N-terminus of the molecule is probably not inserted into the bilayer. In addition, they provide a simple means for producing mutants for structure/function studies and highly purified SLO for use as a permeabilising reagent in cell biology research.

Key residues for membrane binding, oligomerization, and pore forming activity of staphylococcal alpha-hemolysin identified by cysteine scanning mutagenesis and targeted chemical modification

The alpha-hemolysin (alpha HL) polypeptide is secreted by Staphylococcus aureus as a water-soluble monomer that assembles into lipid bilayers to form cylindrical heptameric pores 1-2 nm in effective internal diameter. We have individually replaced each charged residue (79 of 293 amino acids) and four neutral residues in alpha HL with cysteine, which is not found in the wild-type protein. The properties of these mutants have been examined before and after modification with the 450-Da dianionic sulfhydryl reagent 4-acetamido-4'-((iodoacetyl)amino)stilbene-2,2'-disulfonate (IASD). This modification was highly informative as 28 of 83 modified polypeptides showed substantially reduced pore forming activity on rabbit erythrocytes (rRBC), while only five of the unmodified cysteine mutants were markedly affected. Through detailed examination of the phenotypes of the mutant and modified hemolysins, we have pinpointed residues and regions in the alpha HL polypeptide chain that are important for binding to rRBC, oligomer formation and pore activity. Residues in both the N-terminal (Arg-66 and Glu-70) and C-terminal (Arg-200, Asp-254, Asp-255, and Asp-276) thirds of the protein are implicated in binding to cells. The His-35 replacement mutant modified with IASD was the only polypeptide in this study that failed to form SDS-resistant oligomers on rRBC. Altered hemolysins that formed oligomers but failed to lyse rRBC represented the most common defect. These alterations were clustered in the central glycine-rich loop, which has previously been implicated as a component of the lumen of the membrane-spanning channel, and in the regions flanking the loop. Alterations in mutant and modified hemolysins with the same defect were also scattered between the N terminus and His-48, in keeping with previous suggestions that an N-terminal segment and the central loop cooperate in the final step of pore assembly.

Interactions between residues in staphylococcal alpha-hemolysin revealed by reversion mutagenesis

alpha-Hemolysin (alpha HL), a pore-forming polypeptide of 293 amino acids, is secreted by Staphylococcus aureus as a water-soluble monomer. Residues that play key roles in the formation of functional heptameric pores on rabbit red blood cells (rRBC) have been identified previously by site-directed mutagenesis. alpha HL-H35N, in which the histidine at position 35 of the wild-type sequence is replaced with asparagine, is nonlytic and is arrested in assembly as a heptameric prepore. In this study, second-site revertants of H35N that have the ability to lyse rRBC were generated by error-prone PCR under conditions designed to produce single base changes. The analysis of 22 revertants revealed new codons clustered predominantly in three distinct regions of the H35N gene. One cluster includes amino acids 107-111 (four revertants) and another residues 144-155 (five revertants). These two clusters flank the central glycine-rich loop of alpha HL, which previously has been implicated in formation of the transmembrane channel, and encompass residues Lys-110 and Asp-152 that, like His-35, are crucial for lytic activity. The third cluster lies in the region spanning amino acids 217-228 (eight revertants), a region previously unexplored by mutagenesis. Single revertants were found at amino acid positions 84 and 169. When compared with H35N, the heptameric prepores formed by the revertants underwent more rapid conversion to fully assembled pores, as determined by conformational analysis by limited proteolysis. The rate of conversion to the fully assembled pore was strongly correlated with hemolytic activity. Previous work has suggested that the N terminus of alpha HL and the central loop cooperate in the final step of assembly. The present study suggests that the key N-terminal residue His-35 operates in conjunction with residues flanking the loop and C-terminal residues in the region 217-228. Hence, reversion mutagenesis extends the linear analysis that has been provided by direct point mutagenesis.

Correct oligomerization is a prerequisite for insertion of the central molecular domain of staphylococcal alpha-toxin into the lipid bilayer

Staphylococcal alpha-toxin is a primarily hydrophilic molecule that binds as a monomer to target membranes and then aggregates to form amphiphilic oligomers that represent water-filled transmembrane channels. Current evidence indicates that a region located in the center of the molecule inserts deeply into the bilayer. In the present study, we sought to determine whether membrane insertion was triggered by the oligomerization process, and whether insertion correlated with pore formation. Double mutants of alpha-toxin were prepared in which His-35 was replaced by Arg, and cysteine residues were introduced at positions 69, 130 and 186. Substitution of His-35 with Arg rendered the toxin molecules incapable of proper oligomerization, so that they remained in nonlytic form after binding to membranes. The sulfhydryl groups were labelled with the polarity-sensitive fluorescent dye acrylodan. Functionally intact, single mutant toxins containing only the cysteine residues were utilized as controls. Measurements of the fluorescence emission spectrum of acrylodan were performed for the active and inactive alpha-toxin mutants in free solution and in membrane-bound form. The collective results demonstrate that proper oligomerization is required for membrane insertion of the central region in the alpha-toxin molecule, and that lack of insertion correlates with absence of pore formation.

A photogenerated pore-forming protein

BACKGROUND

The permeabilization of cells with bacterial pore-forming proteins is an important technique in cell biology that allows the exchange of small reagents into the cytoplasm of a cell. Another notable technology is the use of caged molecules whose activities are blocked by addition of photoremovable protecting groups. This allows the photogeneration of reagents on or in cells with spatial and temporal control. Here, we combine these approaches to produce a caged pore-forming protein for the controlled permeabilization of cells.

RESULTS

2-Bromo-2-(2-nitrophenyl)acetic acid (BNPA), a water-soluble cysteine-directed reagent for caging peptides and proteins with the alpha-carboxy-2-nitrobenzyl (CNB) protecting group, was synthesized. Glutathione (gamma-Glu-Cys-Gly) was released in high yield from gamma-Glu-CysCNB-Gly by irradiation at 300 nm. Based on this finding, scanning mutagenesis was used to find a single-cysteine mutant of the pore-forming protein staphylococcal alpha-hemolysin (alpha HL) suitable for caging. When alpha HL-R104C was derivatized with BNPA, pore-forming activity toward rabbit erythrocytes was lost. Near UV irradiation led to regeneration of the cysteine sulfhydryl group and the restoration of pore-forming activity.

CONCLUSIONS

Caged pore-forming proteins are potentially useful for permeabilizing one cell in a collection of cells or one region of the plasma membrane of a single cell. Therefore, alpha HL-R104C-CNB and other caged proteins designed to create pores of various diameters should be useful for many purposes. For example, the ability to introduce reagents into one cell of a network or into one region of a single cell could be used in studies of neuronal modulation. Further, BNPA should be generally useful for caging cysteine-containing peptides and single-cysteine mutant proteins to study, for example, cell signaling or structural changes in proteins.

Photolabeling of a pore-forming toxin with the hydrophobic probe 2-[3H]diazofluorene. Identification of membrane-inserted segments of Staphylococcus aureus alpha-toxin

The identification of membrane-inserted segments of pore-forming soluble proteins is crucial to understanding the action of these proteins at the molecular level. A distinct member of this class of proteins is alpha-toxin, a 293-amino acid-long 33-kDa hemolytic toxin secreted by Staphylococcus aureus that can form pores in both artificial and natural membranes. We have studied the interaction of alpha-toxin with single bilayer vesicles prepared from asolectin using a hydrophobic photoactivable reagent, 2-[3H]diazofluorene ([3H]DAF) (Pradhan, D., and Lala, A. K. (1987) J. Biol. Chem. 262, 8242-8251). This reagent readily partitions into the membrane hydrophobic core and on photolysis labels the lipid and protein segments that penetrate the membrane. Current models on the mode of action of alpha-toxin indicate that, on interaction with membranes, alpha-toxin forms an oligomer, which represents the active pore. In keeping with these models, we observe that [3H]DAF photolabels the membrane-bound alpha-toxin oligomer. Cyanogen bromide fragmentation of [3H]DAF-labeled alpha-toxin gave several fragments, which were subjected to Edman degradation. We could thus sequence residues 1-19, 35-60, 114-139, 198-231, and 235-258. Radioactive analysis and phenylthiohydantoin-derivative analysis during sequencing permitted analysis of DAF insertion sites. The results obtained indicated that the N and C termini (residues 235-258) have been extensively labeled. The putative pore-forming glycine-rich central hinge region was poorly labeled, indicating that the apposing side of the lumen of the pore does not form the lipid-protein interface. The DAF labeling pattern indicated that the major structural motif in membrane-bound alpha-toxin was largely beta-sheet.

An intermediate in the assembly of a pore-forming protein trapped with a genetically-engineered switch

BACKGROUND

Studies of the mechanisms by which certain water-soluble proteins can assemble into lipid bilayers are relevant to several areas of biology, including the biosynthesis of membrane and secreted proteins, virus membrane fusion and the action of immune proteins such as complement and perforin. The alpha-hemolysin (alpha HL) protein, an exotoxin secreted by Staphylococcus aureus that forms heptameric pores in lipid bilayers, is a useful model for studying membrane protein assembly. In addition, modified alpha HL might be useful as a component of biosensors or in drug delivery. We have therefore used protein engineering to produce variants of alpha HL that contain molecular triggers and switches with which pore-forming activity can be modulated at will. Previously, we showed that the conductance of pores formed by the mutant hemolysin alpha HL-H5, which contains a Zn(II)-binding pentahistidine sequence, is blocked by Zn(II) from either side of the lipid bilayer, suggesting that residues from the pentahistidine sequence line the lumen of the transmembrane channel.

RESULTS

Here we show that Zn(II) can arrest the assembly of alpha HL-H5 before pore formation by preventing an impermeable oligomeric prepore from proceeding to the fully assembled state. The prepore is a heptamer. Limited proteolysis shows that, unlike the functional pore, the prepore contains sites near the amino terminus of the polypeptide chain that are exposed to the aqueous phase. Upon removal of the bound Zn(II) with EDTA, pore formation is completed and the sites near the amino terminus become occluded. Conversion of the prepore to the active pore is the rate-determining step in assembly and cannot be reversed by the subsequent addition of excess Zn(II).

CONCLUSIONS

The introduction of a simple Zn(II)-binding motif into a pore-forming protein has allowed the isolation of a defined intermediate in assembly. Genetically-engineered switches for trapping and releasing intermediates that are actuated by metal coordination or other chemistries might be generally useful for analyzing the assembly of membrane proteins and other supramolecular structures.

Subunit stoichiometry of staphylococcal alpha-hemolysin in crystals and on membranes: a heptameric transmembrane pore

Elucidation of the accurate subunit stoichiometry of oligomeric membrane proteins is fraught with complexities. The interpretations of chemical cross-linking, analytical ultracentrifugation, gel filtration, and low-resolution electron microscopy studies are often ambiguous. Staphylococcal alpha-hemolysin (alpha HL), a homooligomeric toxin that forms channels in cell membranes, was believed to possess six subunits arranged around a sixfold axis of symmetry. Here, we report that analysis of x-ray diffraction data and chemical modification experiments indicate that the alpha HL oligomer is a heptamer. Self-rotation functions calculated using x-ray diffraction data from single crystals of alpha HL oligomers show a sevenfold axis of rotational symmetry. The alpha HL pore formed on rabbit erythrocyte membranes was determined to be a heptamer by electrophoretic separation of alpha HL heteromers formed from subunits with the charge of wild-type alpha HL and subunits with additional negative charge generated by targeted chemical modification of a single-cysteine mutant. These data establish the heptameric oligomerization state of the alpha HL transmembrane pore both in three-dimensional crystals and on a biological membrane.

Surface labeling of key residues during assembly of the transmembrane pore formed by staphylococcal alpha-hemolysin

Structural changes in staphylococcal alpha-hemolysin (alpha HL) that occur during oligomerization and pore formation on membranes have been examined by using a simple gel-shift assay to determine the rate of modification of key single-cysteine mutants with the hydrophilic sulfhydryl reagent, 4-acetamido-4'-((iodoacetyl)amino)stilbene-2,2'-disulfonate (IASD). The central glycine-rich loop of alpha HL lines the lumen of the transmembrane channel. A residue in the loop remains accessible to IASD after assembly, in keeping with the ability of the pore to pass molecules of approximately 1000 Da. By contrast, residues near the N-terminus, which are critical for pore function, become deeply buried during oligomerization, while a residue at the extreme C-terminus increases in reactivity after assembly, consistent with a location in the part of the pore that projects from the surface of the lipid bilayer.