Staphylococcal alpha toxin--recent advances

The Potential of Cyclodextrins as Inhibitors for the BM2 Protein: An In Silico Investigation

The influenza BM2 transmembrane domain (BM2TM), an acid-activated proton channel, is an attractive antiviral target due to its essential roles during influenza virus replication, whereas no effective inhibitors have been reported for BM2. In this study, we draw inspiration from the properties of cyclodextrins (CDs) and hypothesize that CDs of appropriate sizes may possess the potential to act as inhibitors of the BM2TM proton channel. To explore this possibility, molecular dynamics simulations were employed to assess their inhibitory capabilities. Our findings reveal that CD4, CD5, and CD6 are capable of binding to the BM2TM proton channel, resulting in disrupted water networks and reduced hydrogen bond occupancy between H19 and the solvent within the BM2TM channel necessary for proton conduction. Notably, CD4 completely obstructs the BM2TM water channel. Based on these observations, we propose that CD4, CD5, and CD6 individually contribute to diminishing the proton transfer efficiency of the BM2 protein, and CD4 demonstrates promising potential as an inhibitor for the BM2 proton channel.

Protein Biotoxins of Military Significance

There is a spectrum of several threat agents, ranging from nerve agents and mustard agents to natural substances, such as biotoxins and new, synthetic, bioactive molecules produced by the chemical industry, to the classical biological warfare agents. The new, emerging threat agents are biotoxins produced by animals, plants, fungi, and bacteria. Many types of organisms produce substances that are toxic to humans. Examples of such biotoxins are botulinum toxin, tetanus toxin, and ricin. Several bioactive molecules produced by the pharmaceutical industry can be even more toxic than are the classical chemical warfare agents. Such new agents, like the biotoxins and bioregulators, often are called mid-spectrum agents. The threat to humans from agents developed by modern chemical synthesis and by genetic engineering also must be considered, since such agents may be more toxic or more effective in causing death or incapacitation than classical warfare agents. By developing effective medical protection and treatment against the most likely chemical and mid-spectrum threat agents, the effects of such agents in a war scenario or following a terrorist attack can be reduced. Toxin-mediated diseases have made humans ill for millennia. Unfortunately, the use of biological agents as weapons of terror has now been realized, and separating naturally occurring disease from bioterroristic events has become an important public health goal. The key to timely identification of such attacks relies on education of primary care physicians, first responders, and public health officials.

Monolayers from Genetically Engineered Protein Pores

A selection of microscopic pores is being made by genetic manipulation of a bacterial channel protein, α-hemolysin (α-HL). It will include: pores with different internal diameters, with differential selectivity for the passage of classes of molecules, and with different gating properties. The pores will be made into monolayers and incorporated into materials such as thin films to confer novel permeability properties upon them. Such products will have several technological applications, for example as molecular filters in sensors or as components of optically gated devices in electronics.

Inhibition of S. aureus alpha-hemolysin and B. anthracis lethal toxin by beta-cyclodextrin derivatives

Many pathogens utilize the formation of transmembrane pores in target cells in the process of infection. A great number of pore-forming proteins, both bacterial and viral, are considered to be important virulence factors, which makes them attractive targets for the discovery of new therapeutic agents. Our research is based on the idea that compounds designed to block the pores can inhibit the action of virulence factors, and that the chances to find high affinity blocking agents increase if they have the same symmetry as the target pore. Recently, we demonstrated that derivatives of beta-cyclodextrin inhibited anthrax lethal toxin (LeTx) action by blocking the transmembrane pore formed by the protective antigen (PA) subunit of the toxin. To test the broader applicability of this approach, we sought beta-cyclodextrin derivatives capable of inhibiting the activity of Staphylococcus aureus alpha-hemolysin (alpha-HL), which is regarded as a major virulence factor playing an important role in staphylococcal infection. We identified several amino acid derivatives of beta-cyclodextrin that inhibited the activity of alpha-HL and LeTx in cell-based assays at low micromolar concentrations. One of the compounds was tested for the ability to block ion conductance through the pores formed by alpha-HL and PA in artificial lipid membranes. We anticipate that this approach can serve as the basis for a structure-directed drug discovery program to find new and effective therapeutics against various pathogens that utilize pore-forming proteins as virulence factors.

Advances in small animal mesentery models for in vivo flow cytometry, dynamic microscopy, and drug screening

Using animal mesentery with intravital optical microscopy is a well-established experimental model for studying blood and lymph microcirculation in vivo. Recent advances in cell biology and optical techniques provide the basis for extending this model for new applications, which should generate significantly improved experimental data. This review summarizes the achievements in this specific area, including in vivo label-free blood and lymph photothermal flow cytometry, super-sensitive fluorescence image cytometry, light scattering and speckle flow cytometry, microvessel dynamic microscopy, infrared (IR) angiography, and high-speed imaging of individual cells in fast flow. The capabilities of these techniques, using the rat mesentery model, were demonstrated in various studies; e.g., real-time quantitative detection of circulating and migrating individual blood and cancer cells, studies on vascular dynamics with a focus on lymphatics under normal conditions and under different interventions (e.g. lasers, drugs, nicotine), assessment of lymphatic disturbances from experimental lymphedema, monitoring cell traffic between blood and lymph systems, and high-speed imaging of cell transient deformability in flow. In particular, the obtained results demonstrated that individual cell transportation in living organisms depends on cell type (e.g., normal blood or leukemic cells), the cell’s functional state (e.g., live, apoptotic, or necrotic), and the functional status of the organism. Possible future applications, including in vivo early diagnosis and prevention of disease, monitoring immune response and apoptosis, chemo- and radio-sensitivity tests, and drug screening, are also discussed.

Biosensor incorporating cell barrier architectures for detecting Staphylococcus aureus alpha toxin

Alpha toxin is a common virulent factor of Staphylococcus aureus and is believed to play crucial roles in pathogenicity induced by S. aureus. Alpha toxin is also known to induce permeability to endothelial cell monolayers in vitro due to the formation of interendothelial gaps. The present study is directed towards measuring alpha toxin using a whole-cell-based biosensor. The biosensor, consisting of a confluent monolayer of human umbilical vein endothelial cells (HUVECs) on a potassium ion-selective electrode, takes advantage of cell permeability dysfunction to detect the presence of small quantities of alpha toxin. When a confluent monolayer of cells was formed on the membrane surface, the response of the electrode toward the marker ion, potassium, was inhibited. Upon exposing this sensor to varying concentrations of alpha toxin for 20 min, an increase in sensor response to potassium was observed. The response thus obtained was indirectly related to the concentration of alpha toxin. The detection limit of this sensor for alpha toxin was found to be 0.1 ng/ml. Cell monolayers were stained with silver nitrate to quantify the formation of intercellular gaps as well as to study the effect of this toxin on HUVECs morphology. A strong positive correlation was observed between the response obtained from the biosensor and the area of the intercellular gaps. Silver staining also revealed the tendency of cells to round up upon being exposed to alpha toxin.

Most cited papers in Toxicon

Citation of a published work is one of the parameters considered in the analysis of relevance and importance of scientific contributions. In 2002, for the first time the Impact Factor of Toxicon has risen above 2.0, placing it at the 17th position among 76 journals in the 'toxicology' field. The aim of this article was to identify the most cited articles in Toxicon, that have contributed to the steady increase of its Impact Factor. The number of citations, complete reference and type of all documents appearing in Toxicon in the period 1963-2003 were retrieved from the ISI Web-of-Science homepage. The documents retrieved were sorted by the number of citations received. A 'citation index', defined as the number of citations divided by the number of years since publication, was calculated for each document. It was clearly seen that reviews in Toxicon received 4.4-fold more citations than articles. Unexpectedly, it was found that recent papers were proportionally more cited than old ones. A decrease in the proportion of papers dealing on 'snake*' through out the period and the broadened range of subjects of the most cited papers recently published in Toxicon reflects an increased 'visibility' in other fields of toxinology. Research on plant toxins gained its own space in Toxicon with newer publications showing high citation indexes. It can be postulated that these facts helped to increase Toxicon's Impact Factor from 1.248 in 1999 to 2.003 in 2002. With the increased number of issues in Toxicon as well as publications of subject-dedicated volumes containing mostly reviews, the Impact Factor of Toxicon is expected to keep rising in the near future.

The role of bacterial and non-bacterial toxins in the induction of changes in membrane transport: implications for diarrhea

Bacterial toxins induce changes in membrane transport which underlie the loss of electrolyte homeostasis associated with diarrhea. Bacterial- and their secreted toxin-types which have been linked with diarrhea include: (a) Vibrio cholerae (cholera toxin, E1 Tor hemolysin and accessory cholera enterotoxin); (b) Escherichia coli (heat stable enterotoxin, heat-labile enterotoxin and colicins); (c) Shigella dysenteriae (shiga-toxin); (d) Clostridium perfringens (C. perfringens enterotoxin, alpha-toxin, beta-toxin and theta-toxin); (e) Clostridium difficile (toxins A and B); (f) Staphylococcus aureus (alpha-haemolysin); (g) Bacillus cereus (cytotoxin K and haemolysin BL); and (h) Aeromonas hydrophila (aerolysin, heat labile cytotoxins and heat stable cytotoxins). The mechanisms of toxin-induced diarrhea include: (a) direct effects on ion transport in intestinal epithelial cells, i.e. direct toxin interaction with intrinsic ion channels in the membrane and (b) indirect interaction with ion transport in intestinal epithelial cells mediated by toxin binding to a membrane receptor. These effects consequently cause the release of second messengers, e.g. the release of adenosine 3',5'-cyclic monophosphate/guanosine 3',5'-monophosphate, IP(3), Ca2+ and/or changes in second messengers that are the result of toxin-formed Ca2+ and K+ permeable channels, which increase Ca2+ flux and augment changes in Ca2+ homeostasis and cause depolarisation of the membrane potential. Consequently, many voltage-dependent ion transport systems, e.g. voltage-dependent Ca2+ influx, are affected. The toxin-formed ion channels may act as a pathway for loss of fluid and electrolytes. Although most of the diarrhea-causing toxins have been reported to act via cation and anion channel formation, the properties of these channels have not been well studied, and the available biophysical properties that are needed for the characterization of these channels are inadequate.

Cytotoxic activities of Leptospira interrogans hemolysin SphH as a pore-forming protein on mammalian cells

Leptospirosis is a spirochetal zoonosis that causes an acute febrile systemic illness in humans. Leptospira sp. hemolysins have been shown to be virulence factors for the pathogenesis of leptospirosis. Previously, we cloned a hemolysin SphH of Leptospira interrogans serovar lai, a homologue of L. borgpetersenii sphingomyelinase (SphA), from a genomic library (S. H. Lee, K. A. Kim, Y. K. Kim, I. W. Seong, M. J. Kim, and Y. J. Lee, Gene 254:19-28, 2000). Escherichia coli lysate harboring the sphH showed high hemolytic activities on sheep erythrocytes. However, it neither showed sphingomyelinase nor phospholipase activities, in contrast to SphA which was known to have sphingomyelinase activity. Interestingly, the SphH-mediated hemolysis on erythrocytes was osmotically protected by PEG 5000, suggesting that the SphH might have caused pore formation on the erythrocyte membrane. In the present study, we have prepared the Leptospira hemolysin SphH and investigated its hemolytic and cytotoxic activities on mammalian cells. SphH was shown to be a pore-forming protein on several mammalian cells: When treated with the SphH, the sheep erythrocyte membranes formed pores, which were morphologically confirmed by transmission electron microscopy. Furthermore, the SphH-mediated cytotoxicities on mammalian cells were demonstrated by the release of LDH and by inverted microscopic examinations. Finally, the immune serum against the full-length hemolysin could effectively neutralize the SphH-mediated hemolytic and cytotoxic activities. In conclusion, these results suggest that the virulence of Leptospira SphH was due to the pore formation on mammalian cell membranes.

The sea anemone toxin Bc2 induces continuous or transient exocytosis, in the presence of sustained levels of high cytosolic Ca2+ in chromaffin cells

We have isolated and characterized a new excitatory toxin from the venom of the sea anemone Bunodosoma caissarum, named Bc2. We investigated the mechanism of action of the toxin on Ca(2+)-regulated exocytosis in single bovine adrenal chromaffin cells, monitoring simultaneously fura-2 fluorescence measurements and electrochemical recordings using a carbon fiber microelectrode. Bc2 induced quantal release of catecholamines in a calcium-dependent manner. This release was associated with a sustained rise in cytosolic Ca(2+) and displayed two different patterns of response: a continuous discharge of prolonged duration that changed to a transient burst as the toxin concentration (or incubation time) increased. Continuous secretion was dependent on the activity of native voltage-dependent Ca(2+) channels and showed a pattern similar to that of alpha-latrotoxin; however, its kinetics adjusted better to that of continuous cell depolarization with high K(+) concentration. In contrast, transient secretion was independent of Ca(2+) entry through native voltage-dependent Ca(2+) channels and showed inhibition of late vesicle fusion that was accompanied by "freezing" of F-actin disassembly. These new features make Bc2 a promising new tool for studying the machinery of neurotransmitter release.

Senescent erythrocytes exhibit a single-hit response to staphylococcal alpha toxin

In vitro aged rabbit erythrocytes are 2- to 3-fold less sensitive than fresh erythrocytes to lysis by the alpha toxin of Staphylococcus aureus. Previously, we correlated the loss in hemolytic sensitivity as cells aged with the proteolytic degradation of Band 3, a putative binding site for staphylococcal alpha toxin. Here we separated young and old erythrocytes by density gradient centrifugation and compared their patterns of lysis by alpha toxin. Derivative plots of lytic-time curves exhibited two lytic peaks for young erythrocytes versus a single peak for the aged erythrocytes. Hit analysis showed that the first peak in the young cells was generated by the monomeric form of the toxin while the second peak involved the polymeric form. Similar analysis of the single peak in the aged erythrocytes showed that only a monomeric form of the toxin is involved in the lysis. Furthermore, analysis of alpha toxin receptor (Band 3) indicated that the receptor is gradually lost as erythrocytes age. This loss in Band 3 is proportional to the decrease in the rate of lysis by alpha toxin. Based on these comparisons we suggest that the intact Band 3 promotes polymeric assembly of the alpha toxin which becomes disrupted by the age-related changes occurring in the receptor.

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.

Analysis of ectatomin action on cell membranes

Ectatomin (m = 7928 Da) is a toxic component from the Ectatomma tuberculatum ant venom containing two homologous polypeptide chains (37 and 34 residues) linked to each other by a disulfide bond. In aqueous solution it forms a four alpha-helix bundle. At concentrations of 0.05-0.1 microm, ectatomin forms channels in cellular and artificial bilayer membranes. Immunochemical analysis of the intracellular distribution of ectatomin showed that the toxin gets efficiently inserted into the plasma membrane at a concentration of 5 x 10-7 m and does not penetrate inside the cell. The effect of ectatomin on cardiac L-type calcium current was studied. Calcium currents (ICa) in isolated rat cardiac ventricular myocytes were measured using the whole-cell perforated patch-clamp technique. It was shown that ectatomin at concentrations of 0.01-10 nm inhibited ICa after a latency of few seconds. ICa was decreased twofold by 10 nm ectatomin. However, the most prominent effect of ectatomin was observed after stimulation of ICa by isoproterenol, an agonist of beta-adrenoreceptors, or forskolin, a stimulator of adenylate cyclase. At a concentration of 1 nm, ectatomin abolished the isoproterenol- and forskolin-sensitive components of ICa. The inhibitory effect of ectatomin was partially reversed by subsequent application of 2 microm of forskolin, whereas subsequent isoproterenol application did not produce the same effect.

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.

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.

Variation in the agr-dependent expression of alpha-toxin and protein A among clinical isolates of Staphylococcus aureus from patients with septicaemia

In Staphylococcus aureus synthesis of many virulence factors is regulated by the agr locus. The regulatory molecule RNAIII, induced by agr, activates transcription of the alpha-toxin gene, hla, while it acts as a repressor of the protein A gene, spa. Forty clinical strains of S. aureus from human blood cultures were analysed for alpha-toxin and protein A production. An inverse correlation between alpha-toxin and protein A production was found in most strains. The levels of alpha-toxin and protein A production varied significantly among strains, which indicates various levels of the regulator, RNAIII. This was confirmed by selecting strains producing different amounts of alpha-toxin, showing that the variations in toxin production are due to the variations of RNAIII transcript. However, in one of the selected strains which produced high levels of alpha-toxin, OR153, although RNAIII is also strongly expressed, the specific hla mRNA was unexpectedly low. One partial explanation for the high alpha-toxin production by this clinical isolate might be its lack of extracellular proteases.

Specific roles of alpha-toxin and beta-toxin during Staphylococcus aureus corneal infection

Staphylococcus aureus corneal infection results in extensive inflammation and tissue damage. Our previous studies of bacterial mutants have demonstrated a role for alpha-toxin in corneal virulence. This study analyzes, by genetic rescue experiments, the virulence of mutants affecting alpha-toxin and beta-toxin activity and demonstrates the ocular toxicity of these purified staphylococcal proteins. Three types of isogenic mutants were analyzed: (i) mutants specifically deficient in alpha-toxin (Hla) or beta-toxin (Hlb), (ii) a mutant deficient in both Hla and Hlb, and (iii) a regulatory mutant, deficient in the accessory gene regulator (agr), that produces reduced quantities of multiple exoproteins, including alpha- and beta-toxins. Plasmids coding for Hla and Hlb (pDU1212 and pCU1hlb, respectively) were used to restore toxin activity to mutants specifically deficient in each of these toxins. Either corneas were injected intrastromally with logarithmic-phase S. aureus or purified alpha- or beta-toxins were administered to normal eyes. Ocular pathology was evaluated by slit lamp examination and myeloperoxidase activity of infiltrating polymorphonuclear leukocytes. Corneal homogenates were cultured to determine the CFU per cornea. Eyes infected with the wild-type strain developed significantly greater corneal damage than eyes infected with Agr-, Hlb-, or Hla- strains. Epithelial erosions produced by parent strains were not produced by Agr- or Hla- strains. Hlb+ strains, unlike Hlb- strains, caused scleral edema. Plasmid pDU1212 restored corneal virulence to strain DU1090 (Hla-), and plasmid pCU1hlb restored corneal virulence to strain DU5719 (Hlb-). Application of purified alpha-toxin produced corneal epithelial erosions and iritis, while application of beta-toxin caused scleral inflammation. These studies confirm the role of alpha-toxin as a major virulence factor during S. aureus keratitis and implicate beta-toxin, a mediator of edema, as a lesser contributor to ocular damage.

Group B streptococcal beta-hemolysin expression is associated with injury of lung epithelial cells

Group B streptococci (GBS) are the leading cause of serious bacterial infection in newborns. Early-onset disease is heralded by pneumonia and lung injury, and the lung may serve as a portal of entry for GBS into the bloodstream. To examine a potential role for GBS beta-hemolysin in lung epithelial injury, five wild-type strains varying in beta-hemolysin expression were chosen, along with five nonhemolytic (NH) and five hyperhemolytic (HH) variants of these strains derived by chemical or transposon mutagenesis. Monolayers of A549 alveolar epithelial cells were exposed to log-phase GBS or stabilized hemolysin extracts of GBS cultures, and cellular injury was assessed by lactate dehydrogenase (LDH) release and trypan blue nuclear staining. Whereas NH strains produced no detectable injury beyond baseline (medium alone), hemolysin-producing strains induced LDH release from A549 cells in direct correlation to their ability to lyse sheep erythrocytes. HH strains were also associated with marked increases in trypan blue nuclear staining of A549 monolayers. The extent of LDH release produced by HH strains was significantly reduced in the presence of dipalmitoyl phosphatidylcholine, a known inhibitor of hemolysin and the major phospholipid component of human surfactant. Electron microscopic studies of A549 cell monolayers exposed to HH GBS mutants revealed global loss of microvillus architecture, disruption of cytoplasmic and nuclear membranes, and marked swelling of the cytoplasm and organelles. We conclude that GBS hemolysin expression correlates with lung epithelial cell injury and may be important in the initial pathogenesis of early-onset disease, particularly when pulmonary surfactant is deficient.

Molecular mechanisms of action of bacterial exotoxins

Toxins are one of the inventive strategies that bacteria have developed in order to survive. As virulence factors, they play a major role in the pathogenesis of infectious diseases. Recent discoveries have once more highlighted the effectiveness of these precisely adjusted bacterial weapons. Furthermore, toxins have become an invaluable tool in the investigation of fundamental cell processes, including regulation of cellular functions by various G proteins, cytoskeletal dynamics and neural transmission. In this review, the bacterial toxins are presented in a rational classification based on the molecular mechanisms of action.

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.

alpha-Toxin perfusion: a new method for selective impairment of endothelial function in isolated vessels or intact vascular beds

We describe a method for selectively permeabilizing endothelial cells, using the membrane pore forming exoprotein Staphylococcus aureus alpha-toxin. Experiments were performed in rabbit central ear artery or its main side branch under isometric conditions, on the isolated perfused kidney, or in cannulated pressurized renal arteries. In presence of alpha-toxin, endothelial-dependent vasodilator responses elicited by acetylcholine or A23187 were abolished, whereas the sensitivity of smooth muscle cells to constrictors (norepinephrine, phenylephrine, or KCl) or dilators (sodium nitroprusside) was not affected. The results indicate that restricting the alpha-toxin to the luminal surface induces selective impairment of vascular endothelial function. This method of eliminating endothelium-dependent vasodilator responses may prove to be useful in the study of endothelial-smooth muscle interactions of isolated small arteries and intact vascular beds.

Low conductance states of a single ion channel are not 'closed'

We have used a polymer-exclusion method to estimate the sizes of the high- and low-conductance states of Staphylococcus aureus alpha-toxin channels across planar lipid bilayers. Despite a > 10-fold difference in conductance between high- and low-conductance states, the size differs by < 2-fold. We conclude that factors other than the dimensions have a strong influence on the conductance of alpha-toxin channels. We also show that the high conductance state is destabilized by the presence of high molecular weight polymers outside the channel, compatible with the removal of channel water as the high conductance state "shrinks" to the low conductance state.

Protonation dynamics of the alpha-toxin ion channel from spectral analysis of pH-dependent current fluctuations

To probe protonation dynamics inside the fully open alpha-toxin ion channel, we measured the pH-dependent fluctuations in its current. In the presence of 1 M NaCl dissolved in H2O and positive applied potentials (from the side of protein addition), the low frequency noise exhibited a single well defined peak between pH 4.5 and 7.5. A simple model in which the current is assumed to change by equal amounts upon the reversible protonation of each of N identical ionizable residues inside the channel describes the data well. These results, and the frequency dependence of the spectral density at higher frequencies, allow us to evaluate the effective pK = 5.5, as well as the rate constants for the reversible protonation reactions: kon = 8 x 10(9) M-1 s-1 and koff = 2.5 x 10(4) s-1. The estimate of kon is only slightly less than the diffusion-limited values measured by others for protonation reactions for free carboxyl or imidazole residues. Substitution of H2O by D2O caused a 3.8-fold decrease in the dissociation rate constant and shifted the pK to 6.0. The decrease in the ionization rate constants caused by H2O/D2O substitution permitted the reliable measurement of the characteristic relaxation time over a wide range of D+ concentrations and voltages. The dependence of the relaxation time on D+ concentration strongly supports the first order reaction model. The voltage dependence of the low frequency spectral density suggests that the protonation dynamics are virtually insensitive to the applied potential while the rate-limiting barriers for NaCl transport are voltage dependent. The number of ionizable residues deduced from experiments in H2O (N = 4.2) and D2O (N = 4.1) is in good agreement.

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.

Evidence for the formation of endothelin by lysed red blood cells from endogenous precursor

The release of endothelin from various blood cell fractions was investigated. Human as well as rat blood cell fractions homogenized by sonification were incubated in buffer for up to 60 min. Neither in platelet nor leukocyte homogenates from either species could immunoreactive endothelin be detected. In contrast, homogenates of red blood cells from both species showed a rapid and time-dependent rise of immunoreactive endothelin levels, reaching a peak at 15 min and decreasing thereafter. However, at time point 0 no immunoreactive endothelin could be detected. Reverse phase high performance liquid chromatography showed immunoreactive endothelin to consist of endothelin-1 as well as big endothelin-1. The release of immunoreactive endothelin in human and rat homogenates was concentration-dependently inhibited by the protease inhibitors, leupeptin, phosphoramidon, chymostatin and pepstatin A in order of increasing potency. Intact red blood cells did not incorporate [125I]endothelin-1 nor did they transform exogenous big endothelin-1 to endothelin-1. However, haemolysis of red blood cells with hypotonic saline (0.2%) or incubation with pore-forming staphylococcal alpha-toxin induced the release of immunoreactive endothelin into the buffer samples. Thus, apart from the indirect vasoconstrictor, haemoglobin, red blood cells can also liberate the direct vasoconstrictor, endothelin, a finding expected to be of considerable pathophysiological significance.

Characterization of neutralizing monoclonal antibodies directed against Staphylococcus aureus alpha-toxin

A panel of neutralizing murine monoclonal antibodies (MAbs) against Staphylococcus aureus alpha-toxin has been established, using formaline-inactivated alpha-toxin as an immunogen. Five independent groups of neutralizing epitopes have been identified representing five functionally important structures in the toxin molecule. Because none of the antibodies binds to overlapping decapeptides representing the toxin sequence or to bromocyanogen cleavage products of alpha-toxin, they may all bind to conformational epitopes. Nevertheless, they all bind to monomeric alpha-toxin in a Western blot. Three of the antibodies bind to the toxin monomer in an enzyme-linked immunosorbent assay (ELISA) in the presence, but not in the absence, of detergent. These epitopes are not accessible in hexameric toxin; two of them may represent the contact sites of the toxin monomers upon hexamerization and one is related to a structurally important glycine-rich central hinge region. Two different antibodies bind to monomeric toxin in an ELISA in the presence and absence of detergent and their epitopes are present more than once on oligomeric toxin; they bind strongly to hexameric toxin in a Western blot. The binding properties of the antibodies to alpha-toxin in different assay systems are summarized in an epitope model, which describes the presence of neutralizing domains in the different conformational steps required for pore formation.

Corneal virulence of Staphylococcus aureus: roles of alpha-toxin and protein A in pathogenesis

Staphylococcus aureus produces a variety of proteins, including alpha-toxin and protein A, that could contribute to corneal tissue damage during keratitis. We examined corneal infections produced by intrastromal injection of four S. aureus strains--three isogenic mutants, one lacking alpha-toxin (Hly- Spa+), one lacking protein A (Hly+ Spa-), and one lacking both alpha-toxin and protein A (Hly- Spa-), and the wild type (Hly+ Spa+)--in a rabbit model of experimental keratitis. Rabbit corneas were injected intrastromally with 100 CFU of one of the four strains, and the eyes were examined by slit lamp biomicroscopy over a 25-h period. Corneal homogenates were used for determination of CFU and neutrophil myeloperoxidase activity at 5-h intervals. All strains had the same logarithmic growth curve from 0 to 10 h postinfection, after which CFU remained constant at 10(7) CFU per cornea. By 15 h postinfection, slit lamp examination scores were significantly higher for eyes infected with Hly+ strains than for Hly(-)-infected eyes. At this time, distinct epithelial erosions were seen in Hly(+)-infected eyes but not in Hly(-)-infected eyes. Myeloperoxidase activity was significantly greater for Hly(+)-infected corneas than for Hly(-)-infected corneas at both 20 and 25 h postinfection. Spa(+)- and Spa(-)-infected eyes showed no differences in slit lamp examination scores or myeloperoxidase activities. These results suggest that alpha-toxin, but not protein A, is a major virulence factor in staphylococcal keratitis, mediating the destruction of corneal tissue in eyes infected with this bacterial pathogen.

Site-directed mutagenesis of the alpha-toxin gene of Staphylococcus aureus: role of histidines in toxin activity in vitro and in a murine model

Staphylococcus aureus alpha-toxin is a membrane-damaging exoprotein that oligomerizes to form transmembrane pores. Chemical modification of histidines with diethylpyrocarbonate has been shown to reduce the hemolytic activity of alpha-toxin, suggesting that one or more of the histidine residues is important for toxin function. To individually assess the functional importance of each of the four histidine residues (residues 35, 48, 144, and 259), we used oligonucleotide-directed mutagenesis of the cloned alpha-toxin gene to replace each histidine with leucine. The mutant toxins were expressed in S. aureus and evaluated for hemolytic activity in vitro and for lethality in an intraperitoneal murine model. Substitution of histidine 35 with leucine produced a mutant toxin (H35L) without hemolytic or lethal activity. Mutant toxins H48L, H144L, and H259L exhibited 7, 16, and 46%, respectively, of the hemolytic activity of wild-type toxin. Immunoblotting of purified H35L toxin incubated with liposomal membranes demonstrated intact membrane binding and hexamer formation that was clearly detectable but reduced compared with that of the wild-type toxin. This suggests that hexamer formation alone is not sufficient for the expression of alpha-toxin activity. The nature of the defect underlying the lack of activity of the H35L mutant toxin remains to be elucidated but may involve failure of the hexamer to span the lipid bilayer to form a transmembrane pore or a change in the internal surface and permeability characteristics of the pore.

Hydrophobic interaction chromatography for the purification of cytolytic bacterial toxins

The usefulness of hydrophobic interaction chromatography for the simple purification of cytolytic bacterial toxins was studied. Conditions are described for different hydrophobic interaction chromatographic media for purifying with high yields two different kinds of such haemolysins, the thiol-activated toxin listeriolysin O from Listeria monocytogenes and alpha-toxin from Staphylococcus aureus. For listeriolysin O, purification on butyl-Sepharose was followed by gel filtration chromatography. From butyl-Sepharose the recovery of 22%. Alpha-toxin was obtained by a single purification step from alkyl-Superose with 80% recovery and a specific activity of 29,000 U/mg. On sodium dodecyl sulphate polyacrylamide gel electrophoresis purified listeriolysin O and alpha-toxin showed a single band. Another thiol-activated toxin, streptolysin O from group A streptococci, showed a recovery of 38% from butyl-Sepharose. The results suggest the feasibility of using hydrophobic interaction chromatography, particularly with columns of weak hydrophobicity, for the purification of bacterial haemolysins in high yield.

Induction of muscle-relaxing factor by staphylococcal alpha-toxin

Brain tissue and serum from mice intracerebrally injected with 1 microgram of staphylococcal alpha-toxin contained elevated amounts of a naturally occurring brain tissue component(s) called muscle-relaxing factor (MRF). MRF induced reversible, generalized, flaccid paralysis of mice after intracerebral but not intraperitoneal or intravenous administration. MRF (i) was soluble in Hanks balanced salt solution and in acidified (pH 2) Hanks balanced salt solution, in which it partitions into ethyl acetate, acetone, and methanol; (ii) was separated from some pigments by thin-layer chromatography on silica gel plates; (iii) did not comigrate with prostaglandin and leukotriene standards during high-pressure liquid chromatography with a mu Bondapak fatty acid column; and (iv) did not contain amino acids, exhibit absorption maxima at a wavelength range of 210 to 600 nm, or fluoresce when exposed to UV light. MRF has been detected in rabbit brain that has been stored frozen at -70 degrees C and has been enhanced in vitro in slices of both mouse and rabbit brain following incubation of the brain slices with staphylococcal alpha-toxin. Studies to identify the chemical nature of MRF and the mechanism by which, in mice, it induces reversible, flaccid paralysis of voluntary muscle are continuing.

The accessory gene regulator (agr) controls Staphylococcus aureus virulence in a murine arthritis model

We have studied the role of the accessory gene regulator (agr) of Staphylococcus aureus as a virulence determinant in the pathogenesis of septic arthritis. At least 15 genes coding for potential virulence factors in Staphylococcus aureus are regulated by a putative multicomponent signal transduction system encoded by the agr/hld locus. agr and hld mutants show a decreased synthesis of extracellular toxins and enzymes, such as alpha-, beta-, and delta-hemolysin, leucocidin, lipase, hyaluronate lyase, and proteases, and at the same time an increased synthesis of coagulase and protein A as compared with the wild-type counterpart. We have used a recently described murine model of S. aureus-induced arthritis to study the virulence of S. aureus 8325-4 and two agr/hld mutants derived from it. Sixty percent of the mice injected with the wild-type strain developed arthritis, whereas agrA and hld mutants displayed joint involvement in only 10 and 30%, respectively. In addition, 40% of the mice inoculated with the wild-type strain displayed an erosive arthropathy; such changes were not detectable at all in mice inoculated with the agrA mutant. Serum levels of interleukin-6, a potent B-cell differentiation factor, were significantly higher (P < 0.001) in the mice inoculated with the wild-type strain than in those inoculated with the agrA mutant counterpart. Overall, our results suggest that the agr system of S. aureus is an important virulence determinant in the induction and progression of septic arthritis in mice.

Assembly and channel-forming activity of a naturally-occurring nicked molecule of Staphylococcus aureus alpha-toxin

From the culture supernatant of Staphylococcus aureus Wood 46, we obtained a naturally-occurring nicked molecule of staphylococcal alpha-toxin. The nicked alpha-toxin consisted of non-covalently-linked 8-kDa and 25-kDa polypeptides, which were derived, respectively, from the N-terminal and the C-terminal part of the toxin nicked at the peptide bond between Glu-71 and Gly-72. The nicked toxin, as well as native alpha-toxin, bound to and oligomerized in the liposome membranes composed of choline-containing phospholipids (i.e., phosphatidylcholine and sphingomyelin) and cholesterol, and formed membrane channel in the liposome membranes. However, the channel-forming activity of the nicked toxin, assessed as a toxin-induced carboxyfluorescein leakage from the liposomes, was approx. 20-fold lower than that of native alpha-toxin. Channel-forming activity of the nicked toxin as well as native toxin was inhibited by divalent cations including Zn2+, Cd2+, Ca2+ and Mg2+, and degree of the inhibitory effect of the divalent cations was in the following order: Zn2+ > Cd2+ > Ca2+ > Mg2+. Thus, although the cleavage of alpha-toxin at the position between Glu-71 and Gly-72 drastically reduced channel-forming activity of the toxin, the nicked toxin retained the ability to oligomerize in phospholipid-cholesterol membranes and the characteristics of channel-forming activity in terms of the specificity for phospholipids and the susceptibility to divalent cations.

Cytolysins increase intracellular calcium and induce eicosanoids release by pheochromocytoma PC12 cell cultures

Cytolysins are the most commonly occurring toxins among bacteria, plants, and animals. By distributing cell membrane, they impair ionic permeability, leading to cell death. In an attempt to investigate cytolysin action on catecholaminergic neurons, we have treated pheochromocytoma cell cultures with Streptolysin S, Staphylococcus aureus alpha and delta, Stoichatus, Parcelsin, and cobra direct lytic factor. To measure neurotoxicity, PC12 cultures were loaded with 51Cr and exposed for 1 hr at 37 degrees C to different concentrations of cytolysins. Cytotoxic dose-response curves have been generated resulting in CD50 (cytotoxic dose 50%) in the range of 1-50 micrograms toxin/culture. Using subcytotoxic concentrations of cytolysins (which are of clinical relevance), changes on intracellular calcium were measured by Fura-2 fluorescence technique. Addition of either Stoichatus toxin and tetanolysin or streptococcus and staphylococcus cytolysins to PC12 cells caused rapidly or gradually a progressive increase in [Ca2+]i, respectively. Under similar conditions, samples of PC12 culture medium were assayed for 3H-arachidonic acid released and by radioimmunoassay for the content of PGE2 (prostaglandin), TXB2 (stable metabolite of thromboxane), and 5-HETE (hydroxy acid lipoxygenase product). PLA2 was activated 4.5-6.0-fold and the levels of all three eicosanoids were increased by 2.5-9-fold (PGE2), 4-6-fold (TXB2), and over 100-fold (5-HETE) by Stoichatus and Parcelsin cytolysins. Upon treatment with Streptolysin S and staphylococcus delta toxins PLA2 (phospholipase A2) was slightly activated (1.5-fold) and the levels of PGE2 and TXB2 increased 1.3-2.0-fold and that of 5-HETE up to 30-fold.(ABSTRACT TRUNCATED AT 250 WORDS)

Structure and function of cholera toxin and the related Escherichia coli heat-labile enterotoxin

Cholera and the related Escherichia coli-associated diarrheal disease are important problems confronting Third World nations and any area where water supplies can become contaminated. The disease is extremely debilitating and may be fatal in the absence of treatment. Symptoms are caused by the action of cholera toxin, secreted by the bacterium Vibrio cholerae, or by a closely related heat-labile enterotoxin, produced by Escherichia coli, that causes a milder, more common traveler's diarrhea. Both toxins bind receptors in intestinal epithelial cells and insert an enzymatic subunit that modifies a G protein associated with the adenylate cyclase complex. The consequent stimulated production of cyclic AMP, or other factors such as increased synthesis of prostaglandins by intoxicated cells, initiates a metabolic cascade that results in the excessive secretion of fluid and electrolytes characteristic of the disease. The toxins have a very high degree of structural and functional homology and may be evolutionarily related. Several effective new vaccine formulations have been developed and tested, and a growing family of endogenous cofactors is being discovered in eukaryotic cells. The recent elucidation of the three-dimensional structure of the heat-labile enterotoxin has provided an opportunity to examine and compare the correlations between structure and function of the two toxins. This information may improve our understanding of the disease process itself, as well as illuminate the role of the toxin in studies of signal transduction and G-protein function.

A fluorimetric assay for the effects of cytolytic toxins on the transport properties of resealed erythrocyte ghosts

We prepared resealed erythrocyte ghosts loaded with SPQ and chloride. We demonstrated that these membranes were still functional, as they were capable of exchanging anions, most probably through the band-3 protein. When cytolytic toxins (Escherichia coli hemolysin and Staphylococcus aureus alpha-toxin) were offered to the resealed ghosts, the internal SPQ was released. This could be attributed to the formation of toxin-induced ion channels into the ghost membrane that were so large that SPQ could escape through them. This release was actually independent of the anion-exchanging protein, since DIDS had no inhibitory effect on it. Due to their simplicity, and because they do not lyse, erythrocyte ghosts may serve as useful models to study the action of cytolytic pore-forming toxins. To assess the validity of these model membranes we compared results obtained using RBC and resealed erythrocyte ghosts as targets for the toxin, finding complete consistency. Pre-assembled toxin channels could also be studied on the ghosts. Applying different proteolytic enzymes to the external compartment after channel formation, we found that performed E. coli hemolysin pores were at least partially destroyed by enzymatic digestion.

Structural features of the pore formed by Staphylococcus aureus alpha-toxin inferred from chemical modification and primary structure analysis

Staphylococcus aureus alpha-toxin makes cells and model membranes permeable to ions and uncharged molecules by opening oligomeric pores of uniform size. Its primary sequence reveals peculiar features which give some hints on the structure of the pore. A flexible region separating the toxin into two halves, several amphiphilic beta-strands and two amphiphilic alpha-helices long enough to span the hydrophobic core of the lipid bilayer are predicted. In analogy to bacterial porins, we propose that the inner walls of the pore are, at least in part, built by an amphiphilic beta-barrel. The model is consistent with circular dichroism data and with the electrophysiological properties of the pore. Functional information on this toxin were obtained by chemical modification of its four histidine residues. Specific carbethoxylation suggested they have different roles: one is required for specific receptor binding, one for oligomerisation and two for unspecific lipid binding. A tentative assignment of each histidine to its specific role is done on the basis of the structural predictions. A functionally related hemolysin, Aeromonas hydrophyla aerolysin, reveals remarkably similar features including the presence and location of histidines involved in receptor binding and oligomerisation.

Staphylococcal alpha toxin: a study with chronically instrumented awake sheep

The in vivo responses to staphylococcal alpha toxin are reported for 15 chronically instrumented awake yearling sheep. The data obtained from a total of 30 experiments are grouped into four categories of response: no response, noted in seven experiments done on 5 sheep; pressor response, obtained seven times in 4 sheep; fluid and solute exchange, noted on six occasions in 3 sheep; and acute heart failure and death, which occurred in 10 of the 15 sheep. "No response" denoted no change in any of the measured outcome variables. The group of sheep labeled as showing "pressor response" responded to alpha toxin infusion with an increase in pulmonary artery pressure, unaccompanied by changes either in lung lymph flow or in lung mechanics. "Changes in lung fluid and solute exchange" involve increases in lung lymph flow. The harbinger of the last category, acute left heart failure leading to death, was a marked elevation in left atrial pressure. The threshold response dose in sheep is approximately 21 micrograms/kg. A very steep dose-response curve is observed, with only a narrow window of doses, 15 to 25 micrograms/kg, between the group showing no response and the group showing death from acute heart failure. The data obtained in these studies indicate that the lethal effects of alpha toxin in sheep include acute heart failure, which may be due to direct toxicity to heart muscle and/or the coronary vasculature endothelium.

Calcium ion-mediated regulation of the alpha-toxin pore of Staphylococcus aureus

The water-soluble alpha-toxin monomers of Staphylococcus aureus become hexamers forming the transmembrane pore when exposed to the membranes. This pore is freely permeable to small hydrophilic molecules, e.g. carboxyfluorescein, and becomes less permeable in the presence of calcium ions. Calcium ion-mediated decrease of the carboxyfluorescein leakage could not be eliminated by EDTA added in the medium, but the carboxyfluorescein could be freed by EDTA added in the intraliposomal space. This result suggests that the alpha-toxin pore changes its conformation as the calcium ion is bound and that the binding site is exposed to the intraliposomal side of the membrane. The interaction between the alpha-toxin hexamer and 8-anilino-1-naphthalene-sulfonic acid (ANS) was monitored by determining the fluorescence in the presence and absence of calcium chloride. The mean distances between the tryptophan residues of the alpha-toxin hexamer and the bound ANS were calculated to be 1.90 and 1.80 nm in the absence and presence, respectively, of calcium ions. The results showed the calcium ion mediated conformational change of the membrane-embedded alpha-toxin hexamer.

Effect of fatty acyl domain of phospholipids on the membrane-channel formation of Staphylococcus aureus alpha-toxin in liposome membrane

By use of carboxyfluorescein-loaded multilamellar liposomes prepared from synthetic phosphatidylcholine (PC) or sphingomyelin and cholesterol in a molar ratio of 1:1, we studied whether or not fatty acyl domain of the phospholipids affects the membrane-damaging action (or channel formation) of Staphylococcus aureus alpha-toxin on the phospholipid-cholesterol membranes. Our data indicated: (1) that toxin-induced carboxyfluorescein-leakage from the liposomes composed of saturated fatty acyl residue-carrying PC and cholesterol was decreased with increasing chain length of the acyl residues between 12 and 18 carbon atoms, although toxin-binding to the liposomes was not significantly affected by the length of fatty acyl residue; (2) that unsaturated fatty acyl residue in PC or sphingomyelin molecule conferred higher sensitivity to alpha-toxin on the phospholipid-cholesterol liposomes, compared with saturated fatty acyl residues; and (3) that hexamerization of alpha-toxin, estimated by SDS-polyacrylamide gel electrophoresis, occurred more efficiently on the liposomes composed of PC with shorter fatty acyl chain or unsaturated fatty acyl chain. Thus, hydrophobic domain of the phospholipids influences membrane-channel formation of alpha-toxin in the phospholipid-cholesterol membrane, perhaps by modulating packing of phospholipid, cholesterol and the toxin in membrane.

Alpha-toxin of Staphylococcus aureus

Alpha-toxin, the major cytotoxic agent elaborated by Staphylococcus aureus, was the first bacterial exotoxin to be identified as a pore former. The protein is secreted as a single-chain, water-soluble molecule of Mr 33,000. At low concentrations (less than 100 nM), the toxin binds to as yet unidentified, high-affinity acceptor sites that have been detected on a variety of cells including rabbit erythrocytes, human platelets, monocytes and endothelial cells. At high concentrations, the toxin additionally binds via nonspecific absorption to lipid bilayers; it can thus damage both cells lacking significant numbers of the acceptor and protein-free artificial lipid bilayers. Membrane damage occurs in both cases after membrane-bound toxin molecules collide via lateral diffusion to form ring-structured hexamers. The latter insert spontaneously into the lipid bilayer to form discrete transmembrane pores of effective diameter 1 to 2 nm. A hypothetical model is advanced in which the pore is lined by amphiphilic beta-sheets, one surface of which interacts with lipids whereas the other repels apolar membrane constitutents to force open an aqueous passage. The detrimental effects of alpha-toxin are due not only to the death of susceptible targets, but also to the presence of secondary cellular reactions that can be triggered via Ca2+ influx through the pores. Well-studied phenomena include the stimulation of arachidonic acid metabolism, triggering of granule exocytosis, and contractile dysfunction. Such processes cause profound long-range disturbances such as development of pulmonary edema and promotion of blood coagulation.(ABSTRACT TRUNCATED AT 250 WORDS)