alpha-Hemolysin, gamma-hemolysin, and leukocidin from Staphylococcus aureus: distant in sequence but similar in structure

Vaccine protection against Staphylococcus aureus pneumonia

Staphylococcus aureus pneumonia causes significant mortality in hospitalized or healthy individuals, and recent increases in morbidity are attributed to the rapid spread of methicillin-resistant S. aureus (MRSA) strains, which are often not susceptible to antibiotic therapy. α-Hemolysin (Hla), a secreted pore-forming toxin, is an essential virulence factor of MRSA in a mouse model of S. aureus pneumonia. We show that the level of Hla expression by independent S. aureus strains directly correlates with their virulence. Active immunization with a mutant form of Hla (Hla_H35L), which cannot form pores, generates antigen-specific immunoglobulin G responses and affords protection against staphylococcal pneumonia. Moreover, transfer of Hla-specific antibodies protects naive animals against S. aureus challenge and prevents the injury of human lung epithelial cells during infection. Thus, Hla vaccination or immunotherapy may prevent S. aureus pneumonia in humans.

Surface proteins and exotoxins are required for the pathogenesis of Staphylococcus aureus pneumonia

A model of Staphylococcus aureus-induced pneumonia in adult, immunocompetent C57BL/6J mice is described. This model closely mimics the clinical and pathological features of pneumonia in human patients. Using this system, we defined a role for S. aureus strain Newman surface proteins and secreted exotoxins in pneumonia-related mortality.

The properties of Bacillus cereus hemolysin II pores depend on environmental conditions

Hemolysin II (HlyII), one of several cytolytic proteins encoded by the opportunistic human pathogen Bacillus cereus, is a member of the family of oligomeric beta-barrel pore-forming toxins. This work has studied the pore-forming properties of HlyII using a number of biochemical and biophysical approaches. According to electron microscopy, HlyII protein interacts with liposomes to form ordered heptamer-like macromolecular assemblies with an inner pore diameter of 1.5-2 nm and an outer diameter of 6-8 nm. This is consistent with inner pore diameter obtained from osmotic protection assay. According to the 3D model obtained, seven HlyII monomers might form a pore, the outer size of which has been estimated to be slightly larger than by the other method, with an inner diameter changing from 1 to 4 nm along the channel length. The hemolysis rate has been found to be temperature-dependent, with an explicit lag at lower temperatures. Temperature jump experiments have indicated the pore structures formed at 37 degrees C and 4 degrees C to be different. The channels formed by HlyII are anion-selective in lipid bilayers and show a rising conductance as the salt concentration increases. The results presented show for the first time that at high salt concentration HlyII pores demonstrate voltage-induced gating observed at low negative potentials. Taken together we have found that the membrane-binding properties of hemolysin II as well as the properties of its pores strongly depend on environmental conditions. The study of the properties together with structural modeling allows a better understanding of channel functioning.

Site-directed mutagenesis to assess the binding capacity of class s protein of Staphylococcus aureus leucotoxins to the surface of polymorphonuclear cells

Staphylococcal leucotoxins result from the association of class S components and class F component inducing the activation and the permeabilization of the target cells. Like alpha-toxin, the leucotoxins are pore-forming toxins with more than 70% beta-sheet. This was confirmed by attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. In addition, threonine 28 of a predicted and conserved beta-sheet at the N-terminal extremity of class S proteins composing leucotoxins aligns with histidine 35 of alpha-toxin, which has a key role in oligomerization of the final pore. Flow cytometry was used to study different aminoacid substitutions of the threonine 28 in order to evaluate its role in the biological activity of these class S proteins. Finally, results show that threonine 28 of the leucotoxin probably plays a role similar to that of histidine 35 of alpha-toxin. Mutations on this threonin largely influenced the secondary interaction of the class F component and led to inactive toxin.

The leukocidin pore: evidence for an octamer with four LukF subunits and four LukS subunits alternating around a central axis

The staphylococcal alpha-hemolysin (alphaHL) and leukocidin (Luk) polypeptides are members of a family of related beta-barrel pore-forming toxins. Upon binding to susceptible cells, alphaHL forms water-filled homoheptameric transmembrane pores. By contrast, Luk pores are formed by two classes of subunit, F and S, rendering a heptameric structure displeasing on symmetry grounds at least. Both the subunit stoichiometry and arrangement within the Luk pore have been contentious issues. Here we use chemical and genetic approaches to show that (1) the predominant, or perhaps the only, form of the Luk pore is an octamer; (2) the subunit stoichiometry is 1:1; and (3) the subunits are arranged in an alternating fashion about a central axis of symmetry, at least when a fused LukS-LukF construct is used. The experimental approaches we have used also open up new avenues for engineering the arrangement of the subunits of beta-barrel pore-forming toxins.

Purification and cytotoxic properties of Bacillus cereus hemolysin II

The hemolysin II from Bacillus cereus, HlyII, is a member of the beta-barrel pore-forming toxin family of secreted microbial proteins that includes the Staphylococcus aureus alpha-toxin. Compared with other proteins of the family, hemolysin II has 90 extra amino acids at its C-terminus. To examine more closely the cytotoxic and pore-forming properties of the protein, we have cloned and expressed it in Escherichia coli. We developed a purification procedure for the matured HlyII protein from both culture media and cell extracts using a combination of cation exchange and affinity chromatography together with gel-filtration. In both cases, the fully processed HlyII protein was purified as confirmed by N-terminal sequence analysis. The HlyII protein exhibits cytolytic activity of different extent on erythrocytes from various kinds of mammals. The results presented here show for the first time that two types of human cells are sensitive to HlyII action. In view of its broad cytotoxic activity as well as the ability to interact with artificial membranes, we assume that HlyII needs no specific receptor to bind to cell membranes.

Assembly of the Bi-component leukocidin pore examined by truncation mutagenesis

Staphylococcal leukocidin (Luk) and alpha-hemolysin (alphaHL) are members of the same family of beta barrel pore-forming toxins (betaPFTs). Although the alphaHL pore is a homoheptamer, the Luk pore is formed by the co-assembly of four copies each of the two distantly related polypeptides, LukF and LukS, to form an octamer. Here, we examine N- and C-terminal truncation mutants of LukF and LukS. LukF subunits missing up to nineteen N-terminal amino acids are capable of producing stable, functional hetero-oligomers with WT LukS. LukS subunits missing up to fourteen N-terminal amino acids perform similarly in combination with WT LukF. Further, the simultaneous truncation of both LukF and LukS is tolerated. Both Luk subunits are vulnerable to short deletions at the C terminus. Interestingly, the N terminus of the LukS polypeptide becomes resistant to proteolytic digestion in the fully assembled Luk pore while the N terminus of LukF remains in an exposed conformation. The results from this work and related experiments on alphaHL suggest that, although the N termini of betaPFTs may undergo reorganization during assembly, they are dispensable for the formation of functional pores.

Pore-forming protein toxins: from structure to function

Pore-forming protein toxins (PFTs) are one of Nature's most potent biological weapons. An essential feature of their toxicity is the remarkable property that PFTs can exist either in a stable water-soluble state or as an integral membrane pore. In order to convert from the water-soluble to the membrane state, the toxin must undergo large conformational changes. There are now more than a dozen PFTs for which crystal structures have been determined and the nature of the conformational changes they must undergo is beginning to be understood. Although they differ markedly in their primary, secondary, tertiary and quaternary structures, nearly all can be classified into one of two families based on the types of pores they are thought to form: alpha-PFTs or beta-PFTs. Recent work suggests a number of common features in the mechanism of membrane insertion may exist for each class.

Crystal Structure of Leucotoxin S Component

Staphylococcal leucocidins and gamma-hemolysins (leucotoxins) are bi-component toxins that form lytic transmembrane pores. Their cytotoxic activities require the synergistic association of a class S component and a class F component, produced as water-soluble monomers that form hetero-oligomeric membrane-associated complexes. Strains that produce the Panton-Valentine leucocidin are clinically associated with cutaneous lesions and community-acquired pneumonia. In a previous study, we determined the crystal structure of the F monomer from the Panton-Valentine leucocidin. To derive information on the second component of the leucotoxins, the x-ray structure of the S protein from the Panton-Valentine leucocidin was solved to 2.0 angstrom resolution using a tetragonal crystal form that contains eight molecules in the asymmetric unit. The structure demonstrates the different conformation of the domain involved in membrane contacts and illustrates sequence and tertiary structure variabilities of the pore-forming leucotoxins. Mutagenesis studies at a key surface residue (Thr-28) further support the important role played by these microheterogeneities for the assembly of the bipartite leucotoxins.

Single-molecule imaging of cooperative assembly of gamma-hemolysin on erythrocyte membranes

Single-molecule fluorescence imaging was used to investigate assembly of Staphylococcus aureus LukF and HS monomers into pore-forming oligomers (gamma-hemolysin) on erythrocyte membranes. We distinguished the hetero-oligomers from the monomers, as indicated by fluorescence resonance energy transfer between different dyes attached to monomeric subunits. The stoichiometry of LukF (donor) and HS (acceptor) subunits in oligomers was deduced from the acceptor emission intensities during energy transfer and by direct acceptor excitation, respectively. Based on populations of monomeric and oligomeric intermediates, we estimated 11 sequential equilibrium constants for the assembly pathway, beginning with membrane binding of monomers, proceeding through single pore oligomerization, and culminating in the formation of clusters of pores. Several stages are highly cooperative, critically enhancing the efficiency of assembly.

Molecular features of the cytolytic pore-forming bacterial protein toxins

The repertoire of the cytolytic pore-forming protein toxins (PFT) comprises 81 identified members. The essential feature of these cytolysins is their capacity to provoke the formation of hydrophilic pores in the cytoplasmic membranes of target eukaryotic cells. This process results from the binding of the proteins on the cell surface, followed by their oligomerization which leads to the insertion of the oligomers into the membrane and formation of protein-lined channels. It impairs the osmotic balance of the cell and causes cytolysis. In this review the molecular aspects of a number of important PFT and their respective encoding structural genes will be briefly described.

Purification, cloning and characterization of variant LukE-LukD with strong leukocidal activity of staphylococcal bi-component leukotoxin family

Staphylococcus aureus produces bi-component leukotoxins composed of non-associated soluble proteins, S and F. Neither S nor F component alone is cytotoxic, but components together are active. These include Panton-Valentine leukocidin (PVL), gamma-hemolysin, LukE-LukD and others. Purification of leukotoxin from Staphylococcus aureus V8 strain (ATCC 27733) which does not have PVL genes, identified an F component with 100% identical to that of PVL in the first twenty-five N-terminal amino acids. Molecular cloning of this toxin obtained 2,595 nucleotides sequences containing two novel open reading frames for S and F. Deduced amino acid sequences of the S and F were respectively 91 and 94% identical to those of LukE and LukD. These were named variant of LukE-LukD (LukEv-LukDv). The activity of the recombinant LukEv-LukDv to rabbit leukocytes was similar to that of recombinant PVL. LukEv-LukDv was hemolytic to rabbit red blood cells although the activity was only 8% of gamma-hemolysin, but PVL was not. These activities were quite different from the LukE-LukD which was reported no hemolytic and poorly cytotoxic to leukocytes compared to PVL. The lukEv-IukDv was found in 87% of clinical isolates of Staphylococcus aureus but lukE-lukD was not detected. These data demonstrate the existence of variant LukE-LukD in V8 strain (ATCC 27733).

Arresting and releasing Staphylococcal alpha-hemolysin at intermediate stages of pore formation by engineered disulfide bonds

alpha-Hemolysin (alphaHL) is secreted by Staphylococcus aureus as a water-soluble monomer that assembles into a heptamer to form a transmembrane pore on a target membrane. The crystal structures of the LukF water-soluble monomer and the membrane-bound alpha-hemolysin heptamer show that large conformational changes occur during assembly. However, the mechanism of assembly and pore formation is still unclear, primarily because of the difficulty in obtaining structural information on assembly intermediates. Our goal is to use disulfide bonds to selectively arrest and release alphaHL from intermediate stages of the assembly process and to use these mutants to test mechanistic hypotheses. To accomplish this, we created four double cysteine mutants, D108C/K154C (alphaHL-A), M113C/K147C (alphaHL-B), H48C/ N121C (alphaHL-C), I5C/G130C (alphaHL-D), in which disulfide bonds may form between the pre-stem domain and the beta-sandwich domain to prevent pre-stem rearrangement and membrane insertion. Among the four mutants, alphaHL-A is remarkably stable, is produced at a level at least 10-fold greater than that of the wild-type protein, is monomeric in aqueous solution, and has hemolytic activity that can be regulated by the presence or absence of reducing agents. Cross-linking analysis showed that alphaHL-A assembles on a membrane into an oligomer, which is likely to be a heptamer, in the absence of a reducing agent, suggesting that oxidized alphaHL-A is halted at a heptameric prepore state. Therefore, conformational rearrangements at positions 108 and 154 are critical for the completion of alphaHL assembly but are not essential for membrane binding or for formation of an oligomeric prepore intermediate.

Vibrio cholerae cytolysin is composed of an alpha-hemolysin-like core

The enteric pathogen Vibrio cholerae secretes a water-soluble 80-kD cytolysin, Vibrio cholerae cytolysin (VCC) that assembles into pentameric channels following proteolytic activation by exogenous proteases. Until now, VCC has been placed in a unique class of pore-forming toxins, distinct from paradigms such as Staphyloccal alpha-hemolysin. However, as reported here, amino acid sequence analysis and three-dimensional structure modeling indicate that the core component of the VCC toxin is related in sequence and structure to a family of hemolysins from Staphylococcus aureus that include leukocidin F and alpha-hemolysin. Furthermore, our analysis has identified the channel-forming region of VCC and a potential lipid head-group binding site, and suggests a conserved mechanism of assembly and lysis. An additional domain in the VCC toxin is related to plant lectins, conferring additional target cell specificity to the toxin.

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.

Acute septic arthritis

Acute septic arthritis may develop as a result of hematogenous seeding, direct introduction, or extension from a contiguous focus of infection. The pathogenesis of acute septic arthritis is multifactorial and depends on the interaction of the host immune response and the adherence factors, toxins, and immunoavoidance strategies of the invading pathogen. Neisseria gonorrhoeae and Staphylococcus aureus are used in discussing the host-pathogen interaction in the pathogenesis of acute septic arthritis. While diagnosis rests on isolation of the bacterial species from synovial fluid samples, patient history, clinical presentation, laboratory findings, and imaging studies are also important. Acute nongonococcal septic arthritis is a medical emergency that can lead to significant morbidity and mortality. Therefore, prompt recognition, rapid and aggressive antimicrobial therapy, and surgical treatment are critical to ensuring a good prognosis. Even with prompt diagnosis and treatment, high mortality and morbidity rates still occur. In contrast, gonococcal arthritis is often successfully treated with antimicrobial therapy alone and demonstrates a very low rate of complications and an excellent prognosis for full return of normal joint function. In the case of prosthetic joint infections, the hardware must be eventually removed by a two-stage revision in order to cure the infection.

Controlling pore assembly of staphylococcal gamma-haemolysin by low temperature and by disulphide bond formation in double-cysteine LukF mutants

Staphylococcal LukF and Hlg2 are water-soluble monomers of gamma-haemolysin that assemble into oligomeric pores on the erythrocyte membranes. Here, we have created double-cysteine LukF mutants, in which single disulphide bonds connect either the prestem domain and the cap domain (V12C-T136C, Cap-Stem), or two beta-strands within the prestem domain (T117C-T136C, Stem-Stem) to control pore assembly of gamma-haemolysin at intermediate stages. The disulphide-trapped mutants were inactive in erythrocyte lysis, but gained full haemolytic activity if the disulphide bonds were reduced. The disulphide bonds blocked neither the membrane binding ability nor the intermediate prepore oligomerization, but efficiently inhibited the transition from prepores to pores. The prepores of Cap-Stem were dissociated into monomers in 1% SDS. In contrast, the prepores of Stem-Stem were stable in SDS and had ring-shaped structures similar to those of wild-type LukF, as observed by transmission electron microscopy. The transition of both mutants from prepores to pores could even be achieved by reducing disulphide bonds at low temperature (2 degrees C), whereas prepore oligomerization was effectively inhibited by low temperature. Finally, real-time transition of Stem-Stem from prepores to pores on ghost cells, visualized using a Ca2+-sensitive fluorescent indicator (Rhod2), was shown by the sequential appearance of fluorescence spots, indicating pore-opening events. Taken together, these data indicate that the prepores are legitimate intermediates during gamma-haemolysin pore assembly, and that conformational changes around residues 117 and 136 of the prestem domain are essential for pore formation, but not for membrane binding or prepore oligomerization. We propose a mechanism for gamma-haemolysin pore assembly based on the demonstrated intermediates.

Properties of Bacillus cereus hemolysin II: a heptameric transmembrane pore

The gene encoding hemolysin II (HlyII) was amplified from Bacillus cereus genomic DNA and a truncated mutant, HlyII(DeltaCT), was constructed lacking the 94 amino acid extension at the C terminus. The proteins were produced in an E. coli cell-free in vitro transcription and translation system, and were shown to assemble into SDS-stable oligomers on rabbit erythrocyte membranes and liposomes. The hemolytic activity of HlyII was measured with rabbit erythrocytes yielding an HC(50) value of 1.64 ng mL(-1), which is over 15 times more potent than staphylococcal alpha-hemolysin. HlyII(DeltaCT) was about eight times less potent than HlyII in this assay. Limited proteolysis of the oligomers formed by HlyII and HlyII(DeltaCT) on red cell membranes showed that the C-terminal extension is sensitive to digestion, while HlyII(DeltaCT) is protease resistant and migrates with an electrophoretic mobility similar to that of digested HlyII. HlyII forms moderately anion selective, rectifying pores (I(+80)/I(-80) = 0.57, 1 M KCl, pH 7.4) in planar lipid bilayers of diphytanoylphosphatidylcholine with a unitary conductance of 637 pS (1 M KCl, 5 mM HEPES, pH 7.4) and exhibits no gating over a wide range of applied potentials (-160 to +160 mV). In addition, it was demonstrated that HlyII forms a homoheptameric pore by using gel shift electrophoresis aided by a genetically encoded oligoaspartate tag. Although they share limited primary sequence identity (30%), these data confirm that HlyII is a structural and functional homolog of staphylococcal alpha-hemolysin.

Subunit composition of a bicomponent toxin: staphylococcal leukocidin forms an octameric transmembrane pore

Staphylococcal leukocidin pores are formed by the obligatory interaction of two distinct polypeptides, one of class F and one of class S, making them unique in the family of beta-barrel pore-forming toxins (beta-PFTs). By contrast, other beta-PFTs form homo-oligomeric pores; for example, the staphylococcal alpha-hemolysin (alpha HL) pore is a homoheptamer. Here, we deduce the subunit composition of a leukocidin pore by two independent methods: gel shift electrophoresis and site-specific chemical modification during single-channel recording. Four LukF and four LukS subunits coassemble to form an octamer. This result in part explains properties of the leukocidin pore, such as its high conductance compared to the alpha HL pore. It is also pertinent to the mechanism of assembly of beta-PFT pores and suggests new possibilities for engineering these proteins.

Characterization of leukocytotoxic and superantigen-like factors produced by Staphylococcus aureus isolates from milk of cows with mastitis

Staphylococcus aureus is a major pathogen for cattle, causing various forms of subclinical and clinical mastitis. Two groups of virulence factors (leukotoxins and superantigens) are supposed to play an important role in the initiation and/or the exacerbation of this disease. In order to detect all known and putative members of leukotoxins and SAgs (superantigens), we tested secreted factors of different S. aureus isolates in flow cytometry-based assays. Isolates were sampled from 68 cows of different farms and cultured for 24h in vitro. Supernatants were then coincubated with purified polymorphonuclear granulocytes (PMN) or combinations of blood mononuclear cells (MNC) and PMN. Viable PMN and MNC were determined by quantitative flow cytometry. In addition, we recorded the proliferation-inducing potential of isolate supernatants for bovine MNC. Based on these criteria, the supernatants of S. aureus isolates fell in three groups. The first group (n=32), termed LT-SNs (leukotoxin-containing supernatants), killed purified granulocytes (neutrophils and eosinophils) in vitro. The second group of supernatants (n=20), termed SAg-SN (superantigen-containing supernatants), induced activation and proliferation of mononuclear cells (MNC) and, only in the presence of MNC, resulted in a selective depletion of neutrophils after 24h in vitro. The third group of supernatants (n=16) contained neither LTs or SAgs. Functionally, SAg-SNs behaved like purified staphylococcal enterotoxin A (SEA) or SEB tested in parallel. The absence of SAg-like activity in LT-SNs was confirmed by heat treatment of LT-SNs, which destroyed the leukocytotoxic activity, but did not reveal any MNC-activating potential. This study, therefore, suggests, that pathogenic S. aureus isolates either produce leukotoxins or superantigens and that both groups of virulence factors can easily be differentiated by the functional assays described. The prevalence of leukotoxin- or superantigen-producing isolates was comparable among cattle with subclinical (LT=41%; SAg=30.8%) mastitis. The higher frequency of LT-producing isolates in cases of clinical mastitis (LT=55.2%; SAg=27.6%) was not significant. At least, these findings argue against the dominant role of superantigens or leukotoxins in S. aureus-induced bovine mastitis.

Effects of amoxicillin, gentamicin, and moxifloxacin on the hemolytic activity of Staphylococcus aureus in vitro and in vivo

In Staphylococcus aureus infection hemolysis caused by the extracellular protein alpha-toxin encoded by hla is thought to contribute significantly to its multifactorial virulence. In vitro, subinhibitory concentrations of beta-lactam antibiotics and fluoroquinolones increase the levels of hla and alpha-toxin expression, whereas aminoglycosides decrease the levels of hla and alpha-toxin expression. In the present study we investigated the effects of subinhibitory concentrations of amoxicillin, gentamicin, and moxifloxacin on hla and alpha-toxin expression and total hemolysis of S. aureus strain 8325-4, a high-level alpha-toxin producer, and its alpha-toxin-negative mutant, DU 1090, in vitro and in a rat model of chronic S. aureus infection. The levels of expression of hla and alpha-toxin and total hemolysis did not differ significantly when amoxicillin, gentamicin, or moxifloxacin was added to cultures of S. aureus strain 8325-4. In vivo, strain 8325-4 induced a significantly increased level of hemolysis in infected pouches compared to that in uninfected control pouches, but the hemolysis was reduced to control levels by treatment with doses of amoxicillin, gentamicin, or moxifloxacin that reduced bacterial numbers by 2 orders of magnitude. Additionally, the effects of subinhibitory concentrations of the three antibiotics on total hemolysis of four methicillin-resistant S. aureus and three methicillin-sensitive S. aureus (MSSA) clinical isolates were assessed in vitro. A significant increase in total hemolysis was observed for only one MSSA strain when it was treated with amoxicillin but not when it was treated with moxifloxacin or gentamicin. When purified alpha-toxin was incubated with purified human neutrophil elastase, alpha-toxin was cleaved nearly completely. The results suggest that the penicillin-induced increases in S. aureus alpha-toxin expression are strain dependent, that reduction of bacterial numbers in vivo counteracts this phenomenon effectively, and finally, that in localized S. aureus infections alpha-toxin activity is controlled by neutrophil elastase.

Tyrosine72 residue at the bottom of rim domain in LukF crucial for the sequential binding of the staphylococcal gamma-hemolysin to human erythrocytes

Staphylococcal bi-component cytotoxins, leukocidin (Luk), Panton-Valentine leukocidin (PVL), and gamma-hemolysin (Hlg) consist of LukF and LukS, LukF-PV and LukS-PV, and LukF and Hlg2, respectively, and Luk and Hlg share LukF. LukF-PV can not substitute for LukF for Hlg, despite 73% identity in amino acid sequence and close similarity in the 3-dimensional structure between them. Here, we demonstrated that the absence of hemolytic activity of LukF-PV in cooperation with Hlg2 is due to the failure of the binding of LukF-PV to human erythrocytes. We identified Y72 residue at the bottom of rim domain in LukF as the crucial residue for its binding, which is a prerequisite to the subsequent binding of Hlg2 to human erythrocytes. The data obtained showed that a mutant of LukF-PV in which T71 residue was replaced by the corresponding residue of LukF, Y72, endowed LukF-PV with the binding capability to human erythrocytes which was accompanied by its hemolytic activity in the presence of Hlg2.

A new cytotoxin from Bacillus cereus that may cause necrotic enteritis

A cytotoxin (CytK) has been isolated from a Bacillus cereus strain that caused a severe food poisoning outbreak killing three people. A protein of 34 kDa was highly cytotoxic, and the addition of other secreted proteins gave no synergistic effect. CytK was also necrotic and haemolytic. No known B. cereus enterotoxins were produced by this strain. A DNA sequence from 1.8 kb upstream to 0.2 kb downstream of the toxin gene was sequenced. The deduced amino acid sequence of the toxin showed similarity to Staphylococcus aureus leucocidins, gamma-haemolysin and alpha-haemolysin, Clostridium perfringens beta-toxin and B. cereus haemolysin II, all belonging to a family of beta-barrel channel-forming toxins. There was no sequence similarity between CytK and enterotoxins of B. cereus. The upstream sequence contained a partial sequence of a putative histidine kinase gene. A recognition site for PlcR, which regulates the transcription of enterotoxins HBL and Nhe of B. cereus, was found in the promoter region of the toxin. This new cytotoxin may be responsible for a disease that is similar to, although not as severe as, the necrotic enteritis caused by the beta-toxin of C. perfringens type C.

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.

Construction of a LukS-PV mutant of a staphylococcal Panton-Valentine leukocidin component having a high LukS-like function

A 2-residue (D12I13) segment of LukS of a staphylococcal leukocidin component is an essential region for the hemolytic function of LukS towards rabbit erythrocytes in the presence of LukF. Here, we report that insertion of D, I, or AA residue(s) between A11 and E12 residues of LukS-PV, in which the 2-residue D12I13 segment in LukS was absent, confers the full LukS function on LukS-PV, which has only 4% hemolytic activity of that of LukS towards rabbit erythrocytes.

The interaction of Staphylococcus aureus bi-component gamma-hemolysins and leucocidins with cells and lipid membranes

Staphylococcus aureus gamma-hemolysins (HlgA, HlgB and HlgC) and Panton-Valentine leucocidins (LukS-PV and LukF-PV) are bi-component toxins forming a protein family with some relationship to alpha-toxin. Active toxins are couples formed by taking one protein from each of the two subfamilies of the S-components (LukS-PV, HlgA and HlgC) and the F-components (LukF-PV and HlgB). We compared the mode of action of the six possible couples on leukocytes, red blood cells and model lipid membranes. All couples were leucotoxic on human monocytes, whereas only four couples (HlgA+HlgB, HlgC+HlgB, LukS-PV+HlgB and HlgA+LukF-PV) were hemolytic. Toxins HlgA+HlgB and HlgC+HlgB were also able to induce permeabilisation of model membranes by forming pores via oligomerisation. The presence of membrane-bound aggregates, the smallest and most abundant of which had molecular weight and properties similar to that formed by alpha-toxin, was detected by SDS-PAGE. By infrared spectroscopy in the attenuated total reflection configuration (FTIR-ATR), the secondary structure of both components and of the aggregate were determined to be predominantly beta-sheet and turn with small variations among different toxins. Polarisation experiments indicated that the structure of the membrane complex was compatible with the formation of a beta-barrel oriented perpendicularly to the plane of the membrane, similar to that of porins. The couple LukS-PV+LukF-PV was leucotoxic, but not hemolytic. When challenged against model membranes it was able to bind to the lipid vesicles and to form the aggregate with the beta-barrel structure, but not to increase calcein permeability. Thus, the pore-forming effect correlated with the hemolytic, but not with the complete leucotoxic activity of these toxins, suggesting that other mechanisms, like the interaction with endogenous cell proteins, might also play a role in their pathogenic action.

Characterization of a novel structural member, LukE-LukD, of the bi-component staphylococcal leucotoxins family

A new member of the staphylococcal bi-component leucotoxins family, LukE (32 kDa) and LukD (34.3 kDa) has been characterized from Staphylococcus aureus strain Newman. LukE was 58-68% identical with the class S proteins, whereas LukD was 71-77% identical with the class F proteins of the family. A partial immunoreactivity with the various affinity-purified antibodies specific for the other proteins was observed. Immunoprecipitation assay and gene probing confirmed a 30% frequency among human clinical isolates, differing from the distribution of the other known leucotoxins (P<0.005). LukE+LukD was as effective as the Panton-Valentine leucocidin for inducing dermonecrosis when injected in the rabbit skin, but not hemolytic and poorly leucotoxic compared to other leucotoxins expressed by Staphylococcus aureus.

Further study on the two pivotal parts of Hlg2 for the full hemolytic activity of staphylococcal gamma-hemolysin

Staphylococcal gamma-hemolysin consists of LukF of 34 kDa and Hlg2 (or H gamma II) of 32 kDa, which cooperatively lyse human and rabbit erythrocytes. Our previous data showed that the 5-residue segment K23R24L25A26I27 of Hlg2 is pivotal for the hemolytic activity [Nariya, H. and Kamio, Y., Biosci. Biotechnol. Biochem., 59, 1603-1604 (1997)]. Here, we identify an additional amino acid residue in Hlg2 necessary for the full gamma-hemolysin activity by measuring the toxin activity of Hlg2 mutants in the presence of LukF. The data obtained showed that Arg217 of Hlg2 is an additional pivotal amino acid residue besides the KRLAI segment for the full Hlg2-specific function in gamma-hemolysin. We also report evidence that the Hlg2 mutants showing a low or null hemolytic activity in the presence of LukF towards human erythrocytes had low or no binding activity to the cells, resulting in failure of formation of the ring-shaped pore-forming complex on the erythrocytes.

Phosphorylation of LukS by protein kinase A is crucial for the LukS-specific function of the staphylococcal leukocidin on human polymorphonuclear leukocytes

Staphylococcal leukocidin (Luk) consists of two protein components, LukF and LukS, which cooperatively lyse human and rabbit polymorphonuclear leukocytes. Here, we demonstrate that the phosphorylation of LukS by protein kinase A is crucial for the LukS-specific leukocytolytic function of Luk on HPMNLs by using N-[2(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide (H-89), which is a potent and selective inhibitor of protein kinase A. At 0.5 microM H-89 completely prevented the Luk-induced cell lysis accompanied by blocking of the incorporation of exogenous 32P-H3PO4 into LukS on HPMNLs. However, with LukS and LukF together, 0.5 microM H-89 did not inhibit the cell swelling which takes place before the cell lysis. HPMNLs also became swollen upon treating with both LukF and LukS mutants which could not be phosphorylated.

Voltage gating is a fundamental feature of porin and toxin beta-barrel membrane channels

Beta-barrel pores are found in outer membrane porins of gram-negative bacteria, bacterial toxins and mitochondrial channels. Apart from the beta-barrel the three groups show no close sequence or structural homology but these pores exhibit symmetrical voltage gating when reconstituted into planar lipid bilayers. The structures of several of these are known and many site-directed mutants have been examined. As a result it seems evident that the gating is a common characteristic of these unrelated large pores and is not generated by specialised structures in the pore lumen.

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.