The aromatic ring of phenylalanine 334 is essential for oligomerization of Vibrio vulnificus hemolysin

Vibrio vulnificus hemolysin (VVH) is thought to be a member of the cholesterol-dependent cytolysin (CDC) family of pore-forming toxins. To date, the structure-function relationships of CDCs produced by Gram-negative bacteria remain largely unknown. We show here that the aromatic ring of phenylalanine residue conserved in Vibrionaceae hemolysins is essential for oligomerization of VVH. We generated the VVH mutants; substituted Phe 334 for Ile (F334I), Ala (F334A), Tyr (F334Y), or Trp (F334W); and tested their binding and oligomerizing activity on Chinese hamster ovary cells. Binding in all mutants fell by approximately 50% compared with that in the wild type. Oligomerizing activities were completely eliminated in F334I and F334A mutants, whereas this ability was partially retained in F334Y and F334W mutants. These findings indicate that both hydrophobicity and an aromatic ring residue at the 334th position were needed for full binding activity and that the oligomerizing activity of this toxin was dependent on the existence of an aromatic ring residue at the 334th position. Our findings might help further understanding of the structure-and-function relationships in Vibrionaceae hemolysins.

Cholesterol exposure at the membrane surface is necessary and sufficient to trigger perfringolysin O binding

Perfringolysin O (PFO) is the prototype for the cholesterol-dependent cytolysins, a family of bacterial pore-forming toxins that act on eukaryotic membranes. The pore-forming mechanism of PFO exhibits an absolute requirement for membrane cholesterol, but the complex interplay between the structural arrangement of the PFO C-terminal domain and the distribution of cholesterol in the target membrane is poorly understood. Herein we show that PFO binding to the bilayer and the initiation of the sequence of events that culminate in the formation of a transmembrane pore depend on the availability of free cholesterol at the membrane surface, while changes in the acyl chain packing of the phospholipids and cholesterol in the membrane core, or the presence or absence of detergent-resistant domains do not correlate with PFO binding. Moreover, PFO association with the membrane was inhibited by the addition of sphingomyelin, a typical component of membrane rafts in cell membranes. Finally, addition of molecules that do not interact with PFO, but intercalate into the membrane and displace cholesterol from its association with phospholipids (e.g., epicholesterol), reduced the amount of cholesterol required to trigger PFO binding. Taken together, our studies reveal that PFO binding to membranes is triggered when the concentration of cholesterol exceeds the association capacity of the phospholipids, and this cholesterol excess is then free to associate with the toxin.

Cytolysins augment superantigen penetration of stratified mucosa

Staphylococcus aureus and Streptococcus pyogenes colonize mucosal surfaces of the human body to cause disease. A group of virulence factors known as superantigens are produced by both of these organisms that allows them to cause serious diseases from the vaginal (staphylococci) or oral mucosa (streptococci) of the body. Superantigens interact with T cells and APCs to cause massive cytokine release to mediate the symptoms collectively known as toxic shock syndrome. In this study we demonstrate that another group of virulence factors, cytolysins, aid in the penetration of superantigens across vaginal mucosa as a representative nonkeratinized stratified squamous epithelial surface. The staphylococcal cytolysin alpha-toxin and the streptococcal cytolysin streptolysin O enhanced penetration of toxic shock syndrome toxin-1 and streptococcal pyrogenic exotoxin A, respectively, across porcine vaginal mucosa in an ex vivo model of superantigen penetration. Upon histological examination, both cytolysins caused damage to the uppermost layers of the vaginal tissue. In vitro evidence using immortalized human vaginal epithelial cells demonstrated that although both superantigens were proinflammatory, only the staphylococcal cytolysin alpha-toxin induced a strong immune response from the cells. Streptolysin O damaged and killed the cells quickly, allowing only a small release of IL-1beta. Two separate models of superantigen penetration are proposed: staphylococcal alpha-toxin induces a strong proinflammatory response from epithelial cells to disrupt the mucosa enough to allow for enhanced penetration of toxic shock syndrome toxin-1, whereas streptolysin O directly damages the mucosa to allow for penetration of streptococcal pyrogenic exotoxin A and possibly viable streptococci.

Molecular pathogenesis of Shigella spp.: controlling host cell signaling, invasion, and death by type III secretion

Shigella spp. are gram-negative pathogenic bacteria that evolved from harmless enterobacterial relatives and may cause devastating diarrhea upon ingestion. Research performed over the last 25 years revealed that a type III secretion system (T3SS) encoded on a large plasmid is a key virulence factor of Shigella flexneri. The T3SS determines the interactions of S. flexneri with intestinal cells by consecutively translocating two sets of effector proteins into the target cells. Thus, S. flexneri controls invasion into EC, intra- and intercellular spread, macrophage cell death, as well as host inflammatory responses. Some of the translocated effector proteins show novel biochemical activities by which they intercept host cell signal transduction pathways. An understanding of the molecular mechanisms underlying Shigella pathogenesis will foster the development of a safe and efficient vaccine, which, in parallel with improved hygiene, should curb infections by this widespread pathogen.

Hydrophobic substitutions in the first residue of the CRAC segment of the gp41 protein of HIV

We investigated the peptides N-acetyl-AWYIK-amide and N-acetyl-VWYIK-amide corresponding to single amino acid substitutions in LWYIK, a segment found in the gp41 protein of HIV and believed to play a role in sequestering this protein to a cholesterol-rich domain in the membrane. The effects of these peptides on the thermotropic phase transitions of 1-stearoyl-2-oleoylphosphatidylcholine (SOPC) and mixtures of SOPC and cholesterol were intermediate between that having the wild-type sequence (LWYIK) and another (IWYIK), the least active peptide previously studied. This correlated with results from studies of single mutations in the gp41 protein of HIV-1, in which L679 of the LWYIK segment is replaced with either A or V, measuring the capability of TZM-BL HeLa-based HIV-1 indicator cells to form syncytia. The peptides were also comparatively analyzed in silico. All together, the results suggest that the mode of interaction of this region of gp41 with the polar heads of membrane lipids contributes to its cholesterol selectivity and that this is somehow related to the biological activity of the viral glycoprotein.

Mannheimia haemolytica leukotoxin binds to lipid rafts in bovine lymphoblastoid cells and is internalized in a dynamin-2- and clathrin-dependent manner

Mannheimia haemolytica is the principal bacterial pathogen of the bovine respiratory disease complex. Its most important virulence factor is a leukotoxin (LKT), which is a member of the RTX family of exotoxins produced by many gram-negative bacteria. Previous studies demonstrated that LKT binds to the beta(2)-integrin LFA-1 (CD11a/CD18) on bovine leukocytes, resulting in cell death. In this study, we demonstrated that depletion of lipid rafts significantly decreases LKT-induced bovine lymphoblastoid cell (BL-3) death. After binding to BL-3 cells, some of the LKT relocated to lipid rafts in an LFA-1-independent manner. We hypothesized that after binding to LFA-1 on BL-3 cells, LKT moves to lipid rafts and clathrin-coated pits via a dynamic process that results in LKT internalization and cytotoxicity. Knocking down dynamin-2 by small interfering RNA reduced both LKT internalization and cytotoxicity. Similarly, expression of dominant negative Eps15 protein expression, which is required for clathrin coat formation, reduced LKT internalization and LKT-mediated cytotoxicity to BL-3 cells. Finally, we demonstrated that inhibiting actin polymerization reduced both LKT internalization and LKT-mediated cytotoxicity. These results suggest that both lipid rafts and clathrin-mediated mechanisms are important for LKT internalization and cytotoxicity in BL-3 cells and illustrate the complex nature of LKT internalization by the cytoskeletal network.

Conformational changes that effect oligomerization and initiate pore formation are triggered throughout perfringolysin O upon binding to cholesterol

Pore formation by the cholesterol-dependent cytolysins (CDCs) requires the presence of cholesterol in the target membrane. Cholesterol was long thought to be the cellular receptor for these toxins, but not all CDCs require cholesterol for binding. Intermedilysin, secreted by Streptococcus intermedius, only binds to membranes containing the human protein CD59 but forms pores only if the membrane contains sufficient cholesterol. In contrast, perfringolysin O (PFO), secreted by Clostridium perfringens, only binds to membranes containing substantial amounts of cholesterol. Given that different steps in the assembly of various CDC pores require cholesterol, here we have analyzed to what extent cholesterol molecules, by themselves, can modulate the conformational changes associated with PFO oligomerization and pore formation. PFO binds to cholesterol when dispersed in aqueous solution, and this binding triggers the distant rearrangement of a beta-strand that exposes an oligomerization interface. Moreover, upon binding to cholesterol, PFO forms a prepore complex, unfolds two amphipathic transmembrane beta-hairpins, and positions their nonpolar surfaces so they associate with the hydrophobic cholesterol surface. The interaction of PFO with cholesterol is therefore sufficient to initiate an irreversible sequence of coupled conformational changes that extend throughout the toxin molecule.

Membrane cholesterol is required for activity of Vibrio vulnificus cytolysin

Vibrio vulnificus cytolysin (VVC) forms a pore in the plasma membrane and induces cytolysis of various cells including erythrocytes, neutrophil and endothelial cells. The cytolytic activity of VVC is inhibited by exogenously added cholesterol, suggesting that membrane cholesterol might be required for VVC cytolytic activity. However, there is no direct evidence that membrane cholesterol is involved in VVC-induced cytolysis. Herein we demonstrate that membrane cholesterol is required for binding of VVC to the plasma membrane. Membrane cholesterol depletion with methyl-beta-cyclodextrin inhibited VVC-induced K(+) release, 2-deoxy glucose release and Ca(2+) influx, which are indicators of VVC pore formation. The cholesterol depletion-induced blockage of VVC cytolysis was due to the inhibition of VVC binding to membrane. These findings suggest that interaction with cholesterol is required for activity of VVC.

Juxtamembrane protein segments that contribute to recruitment of cholesterol into domains

We investigated the properties of several peptides with sequences related to LWYIK, a segment found in the gp41 protein of HIV and believed to play a role in sequestering this protein to a cholesterol-rich domain in the membrane. This segment fulfills the requirements to be classified as a CRAC motif that has been suggested to predict those proteins that will partition into cholesterol-rich regions of the membrane. All of the peptides were studied with the terminal amino and carboxyl groups blocked, i.e., as N-acetyl-peptide-amides. Effects of cholesterol on the intensity of W emission generally parallel DSC evidence of sequestration of cholesterol. Modeling studies indicate that all of these peptides tend to partition with their mass center at the membrane interface at the level of the hydroxyl of cholesterol. Interaction with cholesterol is dual: van der Waals interactions between mainly hydrophobic surfaces and electrostatic stabilization of the cholesterol OH group. Thus, both experiments and modeling studies indicate that the preference of CRAC motifs for cholesterol-rich domains might be related to a membrane interfacial preference of the motif, to a capacity to wrap and block the cholesterol polar OH group by H-bond interactions, and to a capacity for peptide aromatic side chains to stack with cholesterol. These results were supported by studies of single mutations in the gp41 protein of HIV-1, in which L(679) is replaced with I. Despite the similarity of the properties of these amino acid residues, this single substitution resulted in a marked attenuation of the ability of JC53-BL HeLa-based HIV-1 indicator cells to form syncytia.

Steroid structural requirements for interaction of ostreolysin, a lipid-raft binding cytolysin, with lipid monolayers and bilayers

Ostreolysin, a cytolytic protein from the edible oyster mushroom (Pleurotus ostreatus), recognizes and binds specifically to membrane domains enriched in cholesterol and sphingomyelin (or saturated phosphatidylcholine). These events, leading to permeabilization of the membrane, suggest that a cholesterol-rich liquid-ordered membrane phase, which is characteristic of lipid rafts, could be its possible binding site. In this work, we present effects of ostreolysin on membranes containing various steroids. Binding and membrane permeabilizing activity of ostreolysin was studied using lipid mono- and bilayers composed of sphingomyelin combined, in a 1/1 molar ratio, with natural and synthetic steroids (cholesterol, ergosterol, beta-sitosterol, stigmasterol, lanosterol, 7-dehydrocholesterol, cholesteryl acetate, and 5-cholesten-3-one). Binding to membranes and lytic activity of the protein are both shown to be dependent on the intact sterol 3beta-OH group, and are decreased by introducing additional double bonds and methylation of the steroid skeleton or C17-isooctyl chain. The activity of ostreolysin mainly correlates with the ability of the steroids to promote formation of liquid-ordered membrane domains, and is the highest with cholesterol-containing membranes. Furthermore, increasing the cholesterol concentration enhanced ostreolysin binding in a highly cooperative manner, suggesting that the membrane lateral distribution and accessibility of the sterols are crucial for the activity of this new member of cholesterol-dependent cytolysins.

Cholesterol and the interaction of proteins with membrane domains

Cholesterol is not uniformly distributed in biological membranes. One of the factors influencing the formation of cholesterol-rich domains in membranes is the unequal lateral distribution of proteins in membranes. Certain proteins are found in cholesterol-rich domains. In some of these cases, it is as a consequence of the proteins interacting directly with cholesterol. There are several structural features of a protein that result in the protein preferentially associating with cholesterol-rich domains. One of the best documented of these is certain types of lipidations. In addition, however, there are segments of a protein that can preferentially sequester cholesterol. We discuss two examples of these cholesterol-recognition elements: the cholesterol recognition/interaction amino acid consensus (CRAC) domain and the sterol-sensing domain (SSD). The requirements for a CRAC motif are quite flexible and predict that a large number of sequences could recognize cholesterol. There are, however, certain proteins that are known to interact with cholesterol-rich domains of cell membranes that have CRAC motifs, and synthetic peptides corresponding to these segments also promote the formation of cholesterol-rich domains. Modeling studies have provided a rationale for certain requirements of the CRAC motif. The SSD is a larger protein segment comprising five transmembrane domains. The amino acid sequence YIYF is found in several SSD and in certain other proteins for which there is evidence that they interact with cholesterol-rich domains. The CRAC sequences as well as YIYF are generally found adjacent to a transmembrane helical segment. These regions appear to have a strong influence of the localization of certain proteins into domains in biological membranes. In addition to the SSD, there is also a domain found in soluble proteins, the START domain, that binds lipids. Certain proteins with START domains specifically bind cholesterol and are believed to function in intracellular cholesterol transport. One of these proteins is StAR-D1, that also has a mitochondrial targeting sequence and plays an important role in delivering cholesterol to the mitochondria of steroidogenic cells.

Type III secretion: what's in a name?

The term 'type III secretion' has seen widespread use. However, problems persist in nomenclature. We propose that the standard abbreviation for this kind of secretion should be 'T3S' and that 'type III secretion system' should be abbreviated to 'T3SS'. There is also a need for a new terminology to distinguish flagellar and non-flagellar type III secretion systems that reflects their common evolutionary ancestry but does not obscure their distinctive features. Finally, the use of the term 'type III secretion' to cover cytolysin-mediated translocation is to be deprecated because an authentic type III secretion system has already been described in gram-positive bacteria, namely the flagellar protein export apparatus.

Cholesterol-dependent pore formation of Clostridium difficile toxin A

The large clostridial cytotoxins toxin A and toxin B from Clostridium difficile are major virulence factors known to cause antibiotic-associated diarrhea and pseudomembranous colitis. Both toxins mono-glucosylate and thereby inactivate small GTPases of the Rho family. Recently, it was reported that toxin B, but not toxin A, induces pore formation in membranes of target cells under acidic conditions. Here, we reassessed data on pore formation of toxin A in cells derived from human colon carcinoma. Treatment of 86Rb+-loaded cells with native or recombinant toxin A resulted in an increased efflux of radioactive cations induced by an acidic pulse. The efficacy of pore formation was dependent on membrane cholesterol, since cholesterol depletion of membranes with methyl-beta-cyclodextrin inhibited 86Rb+ efflux, and cholesterol repletion reconstituted pore-forming activity of toxin A. Similar results were obtained with toxin B. Consistently, methyl-beta-cyclodextrin treatment delayed intoxication of cells in a concentration-dependent manner. In black lipid membranes, toxin A induced ion-permeable pores only in cholesterol containing bilayers and at low pH. In contrast, release of glycosylphosphatidylinositol-anchored structures by phosphatidylinositol specific phospholipase C treatment did not reduce cell sensitivity toward toxins A and B. These data indicate that in colonic cells toxin A induces pore formation in an acidic environment (e.g. endosomes) similar to that reported for toxin B and suggest that pore formation by clostridial glucosylating toxins depends on the presence of cholesterol.

Redox regulation of CLIC1 by cysteine residues associated with the putative channel pore

Chloride intracellular channels (CLICs) are putative pore-forming glutathione-S-transferase homologs that are thought to insert into cell membranes directly from the cytosol. We incorporated soluble, recombinant human CLIC1 into planar lipid bilayers to investigate the associated ion channels, and noted that channel assembly (unlike membrane insertion) required a specific lipid mixture. The channels formed by reduced CLIC1 were similar to those previously recorded from cells and "tip-dip" bilayers, and specific anti-CLIC1 antibodies inhibited them. However, the amplitudes of the filtered single-channel currents were strictly regulated by the redox potential on the "extracellular" (or "luminal") side of the membrane, with minimal currents under strongly oxidizing conditions. We carried out covalent functional modification and site-directed mutagenesis of this controversial ion channel to test the idea that cysteine 24 is a critical redox-sensitive residue located on the extracellular (or luminal) side of membrane CLIC1 subunits, in a cysteine-proline motif close to the putative channel pore. Our findings support a simple structural hypothesis to explain how CLIC1 oligomers form pores in membranes, and suggest that native channels may be regulated by a novel mechanism involving the formation and reduction of intersubunit disulphide bonds.

Effect of pH on the pore forming activity and conformational stability of ostreolysin, a lipid raft-binding protein from the edible mushroom Pleurotus ostreatus

Ostreolysin, a pore-forming protein from the edible oyster mushroom (Pleurotus ostreatus), is a member of the aegerolysin protein family, a novel group of small acidic proteins found in bacteria, molds, mushrooms, and plants. It binds to lipid rafts and interacts specifically with cholesterol-rich lipid domains. In this study, ostreolysin was classified as a single-domain all-beta-structured protein on the basis of cDNA sequencing. pH-induced and thermally induced unfolding of ostreolysin was studied by means of CD, UV absorption, and intrinsic tryptophan fluorescence to characterize conformational transitions associated with its functional properties, i.e., binding to lipid membranes, pore forming activity on lipid vesicles, and hemolysis. At 25 degrees C and between pH 6 and 9, ostreolysin adopted a monomeric and thermodynamically stable nativelike conformation, characterized by rigid tertiary structure and predominantly beta-sheet secondary structure. Between pH 2 and 3, the protein underwent an irreversible transition to a partially unfolded, molten globule-like state which bound ANS, and exhibited disrupted tertiary structure and enhanced non-native alpha-helical structure. Functional studies showed that, unlike colicins and some other bacterial pore-forming toxins, the acid-induced molten globule-like state of ostreolysin is not relevant for lipid binding and pore formation. Instead, the compact native state was necessary for binding to cholesterol/sphingomyelin multilamellar vesicles, optimally in the pH range from 6 to 7, and for pore formation and hemolysis, maximally between pH 7 and 8.

Vesicular polydiacetylene sensor for colorimetric signaling of bacterial pore-forming toxin

A vesicle-based polydiacetylene biosensor for colorimetric detection of bacterial pore-forming toxin streptolysin O (SLO) is reported. The sensor was constructed with three lipid constituents: glycine-terminated diacetylene lipid Gly-PCDA, cell membrane-mimicking component PC-DIYNE, and cholesterol (CHO), which serves as the bait molecule. UV irradiation led to photopolymerization of the diacetylene lipids that gave the material a blue appearance. Incubation of the vesicles with SLO from Streptococcus pyrogenes turned the vesicle solution red, and the color change was found to be correlated to SLO concentration. The optimal sensing performance was found with vesicles consisting of 71% Gly-PCDA, 25% CHO, and 4% PC-DIYNE, and a correlation relationship was obtained for 20 HU to 500 HU/mL, or 100 pM to 6.3 nM of SLO toxin. Transmission electron microscopy and dynamic light scattering was used for further characterization of the vesicular assemblies. Transmembrane pores (holes) with diameter around 30 nm were observed on the vesicle membranes, in particular on the peripheral of the membrane structures, suggesting pore formation by SLO toxin provides the driving force for the color change of the functional vesicles.