The cytolytic toxin aerolysin must aggregate to disrupt erythrocytes, and aggregation is stimulated by human glycophorin

Protein Aggregation as a Bacterial Strategy to Survive Antibiotic Treatment

While protein aggregation is predominantly associated with loss of function and toxicity, it is also known to increase survival of bacteria under stressful conditions. Indeed, protein aggregation not only helps bacteria to cope with proteotoxic stresses like heat shocks or oxidative stress, but a growing number of studies suggest that it also improves survival during antibiotic treatment by inducing dormancy. A well-known example of dormant cells are persisters, which are transiently refractory to the action of antibiotics. These persister cells can switch back to the susceptible state and resume growth in the absence of antibiotics, and are therefore considered an important cause of recurrence of infections. Mounting evidence now suggests that this antibiotic-tolerant persister state is tightly linked to-or perhaps even driven by-protein aggregation. Moreover, another dormant bacterial phenotype, the viable but non-culturable (VBNC) state, was also shown to be associated with aggregation. These results indicate that persisters and VBNC cells may constitute different stages of the same dormancy program induced by progressive protein aggregation. In this mini review, we discuss the relation between aggregation and bacterial dormancy, focusing on both persisters and VBNC cells. Understanding the link between protein aggregation and dormancy will not only provide insight into the fundamentals of bacterial survival, but could prove highly valuable in our future battle to fight them.

Binary Toxin Subunits of Lysinibacillus sphaericus Are Monomeric and Form Heterodimers after In Vitro Activation

The binary toxin from Lysinibacillus sphaericus has been successfully used for controlling mosquito-transmitted diseases. An activation step shortens both subunits BinA and BinB before their interaction with membranes and internalization in midgut cells, but the precise role of this activation step is unknown. Herein, we show conclusively using three orthogonal biophysical techniques that protoxin subunits form only monomers in aqueous solution. However, in vitro activated toxins readily form heterodimers. This oligomeric state did not change after incubation of these heterodimers with detergent. These results are consistent with the evidence that maximal toxicity in mosquito larvae is achieved when the two subunits, BinA and BinB, are in a 1:1 molar ratio, and directly link proteolytic activation to heterodimerization. Formation of a heterodimer must thus be necessary for subsequent steps, e.g., interaction with membranes, or with a suitable receptor in susceptible mosquito species. Lastly, despite existing similarities between BinB C-terminal domain with domains 3 and 4 of pore-forming aerolysin, no aerolysin-like SDS-resistant heptameric oligomers were observed when the activated Bin subunits were incubated in the presence of detergents or lipidic membranes.

The Unique Molecular Choreography of Giant Pore Formation by the Cholesterol-Dependent Cytolysins of Gram-Positive Bacteria

The mechanism by which the cholesterol-dependent cytolysins (CDCs) assemble their giant β-barrel pore in cholesterol-rich membranes has been the subject of intense study in the past two decades. A combination of structural, biophysical, and biochemical analyses has revealed deep insights into the series of complex and highly choreographed secondary and tertiary structural transitions that the CDCs undergo to assemble their β-barrel pore in eukaryotic membranes. Our knowledge of the molecular details of these dramatic structural changes in CDCs has transformed our understanding of how giant pore complexes are assembled and has been critical to our understanding of the mechanisms of other important classes of pore-forming toxins and proteins across the kingdoms of life. Finally, there are tantalizing hints that the CDC pore-forming mechanism is more sophisticated than previously imagined and that some CDCs are employed in pore-independent processes.

Protein aggregation in bacteria: the thin boundary between functionality and toxicity

Misfolding and aggregation of proteins have a negative impact on all living organisms. In recent years, aggregation has been studied in detail due to its involvement in neurodegenerative diseases, including Alzheimer's, Parkinson's and Huntington's diseases, and type II diabetes--all associated with accumulation of amyloid fibrils. This research highlighted the central importance of protein homeostasis, or proteostasis for short, defined as the cellular state in which the proteome is both stable and functional. It implicates an equilibrium between synthesis, folding, trafficking, aggregation, disaggregation and degradation. In accordance with the eukaryotic systems, it has been documented that protein aggregation also reduces fitness of bacterial cells, but although our understanding of the cellular protein quality control systems is perhaps most detailed in bacteria, the use of bacterial proteostasis as a drug target remains little explored. Here we describe protein aggregation as a normal physiological process and its role in bacterial virulence and we shed light on how bacteria defend themselves against the toxic threat of aggregates. We review the impact of aggregates on bacterial viability and look at the ways that bacteria use to maintain a balance between aggregation and functionality. The proteostasis in bacteria can be interrupted via overexpression of proteins, certain antibiotics such as aminoglycosides, as well as antimicrobial peptides--all leading to loss of cell viability. Therefore intracellular protein aggregation and disruption of proteostatic balance in bacteria open up another strategy that should be explored towards the discovery of new antimicrobials.

Biomphalysin, a new β pore-forming toxin involved in Biomphalaria glabrata immune defense against Schistosoma mansoni

Aerolysins are virulence factors belonging to the β pore-forming toxin (β-PFT) superfamily that are abundantly distributed in bacteria. More rarely, β-PFTs have been described in eukaryotic organisms. Recently, we identified a putative cytolytic protein in the snail, Biomphalaria glabrata, whose primary structural features suggest that it could belong to this β-PFT superfamily. In the present paper, we report the molecular cloning and functional characterization of this protein, which we call Biomphalysin, and demonstrate that it is indeed a new eukaryotic β-PFT. We show that, despite weak sequence similarities with aerolysins, Biomphalysin shares a common architecture with proteins belonging to this superfamily. A phylogenetic approach revealed that the gene encoding Biomphalysin could have resulted from horizontal transfer. Its expression is restricted to immune-competent cells and is not induced by parasite challenge. Recombinant Biomphalysin showed hemolytic activity that was greatly enhanced by the plasma compartment of B. glabrata. We further demonstrated that Biomphalysin with plasma is highly toxic toward Schistosoma mansoni sporocysts. Using in vitro binding assays in conjunction with Western blot and immunocytochemistry analyses, we also showed that Biomphalysin binds to parasite membranes. Finally, we showed that, in contrast to what has been reported for most other members of the family, lytic activity of Biomphalysin is not dependent on proteolytic processing. These results provide the first functional description of a mollusk immune effector protein involved in killing S. mansoni.

Schistosomiasis is the second most widespread tropical parasitic disease after malaria. It is caused by flatworms of the genus Schistosoma. Its life cycle is complex and requires certain freshwater snail species as intermediate host. Given the limited options for treating S. mansoni infections, much research has focused on a better understanding of the immunobiological interactions between the invertebrate host Biomphalaria glabrata and its parasite S. mansoni. A number of studies published over the last two decades have contributed greatly to our understanding of B. glabrata innate immune mechanisms involved in the defense against parasite. However, most studies have focused on the identification of recognition molecules or immune receptors involved in the host/parasite interplay. In the present study, we report the first functional description of a mollusk immune effector protein involved in killing S. mansoni, a protein related to the β pore forming toxin that we named Biomphalysin.

Preliminary crystallographic analysis of two oligomerization-deficient mutants of the aerolysin toxin, H132D and H132N, in their proteolyzed forms

Aerolysin is a major virulence factor produced by the Gram-negative bacterium Aeromonas hydrophila and is a member of the β-pore-forming toxin family. Two oligomerization-deficient aerolysin mutants, H132D and H132N, have been overproduced, proteolyzed by trypsin digestion and purified. Crystals were grown from the proteolyzed forms and diffraction data were collected for the two mutants to 2.1 and 2.3 Å resolution, respectively. The prism-shaped crystals belonged to space group C2. The crystal structure of the mutants in the mature, but not heptameric, aerolysin form will provide insight into the intermediate states in the oligomerization process of a pore-forming toxin.

Live recombinant Lactococcus lactis vaccine expressing aerolysin genes D1 and D4 for protection against Aeromonas hydrophila in tilapia (Oreochromis niloticus)

AIMS

To evaluate a live recombinant Lactococcus lactis vaccine expressing aerolysin genes D1 (Lac-D1ae) and/or D4 (Lac-D4ae) in protection against Aeromonas hydrophila in tilapia (Oreochromis niloticus).

METHODS AND RESULTS

The polymerase chain reaction (PCR)-amplified 250- and 750-bp sequences coding for domains D1 and D4 of aerolysin were individually cloned into pNZ8048 and electrotransformed into L. lactis. The recombinant vaccine candidates were then either orally fed or injected intraperitoneally into tilapia. The development of antibodies in sampled fish compared to control groups implied that the recombinant epitopes expressed in L. lactis were able to elicit an immunogenic response in tilapia. Interestingly, the lower doses of both Lac-D1ae and Lac-D4ae gave higher antibody levels over the study period. Fish immunized with Lac-D1ae and Lac-D4ae together showed the highest level of protection, and the mortality was reduced significantly compared to control strains in both modes of vaccination.

CONCLUSIONS

The recombinant L. lactis strain expressing D1 and D4 produced aerolysin-specific serum IgM in tilapia. Both D1 and D4 promoted 55-82% relative per cent survival (RPS) against Aeromonas infection through intraperitoneal injection, whereas the RPS following oral feeding of the vaccine was 70-100%.

SIGNIFICANCE AND IMPACT OF THE STUDY

The D1 and D4 regions of the aerolysin protein have been successfully identified as immunogenic regions that can elicit antibody production in tilapia and protect against challenge with Aer. hydrophila. A promising oral vaccine using L. lactis harbouring the D1 and D4 regions has been developed to control Aer. hydrophila.

Roles of integral protein in membrane permeabilization by amphidinols

Amphidinols (AMs) are a group of dinoflagellate metabolites with potent antifungal activity. As is the case with polyene macrolide antibiotics, the mode of action of AMs is accounted for by direct interaction with lipid bilayers, which leads to formation of pores or lesions in biomembranes. However, it was revealed that AMs induce hemolysis with significantly lower concentrations than those necessary to permeabilize artificial liposomes, suggesting that a certain factor(s) in erythrocyte membrane potentiates AM activity. Glycophorin A (GpA), a major erythrocyte protein, was chosen as a model protein to investigate interaction between peptides and AMs such as AM2, AM3 and AM6 by using SDS-PAGE, surface plasmon resonance, and fluorescent-dye leakages from GpA-reconstituted liposomes. The results unambiguously demonstrated that AMs have an affinity to the transmembrane domain of GpA, and their membrane-permeabilizing activity is significantly potentiated by GpA. Surface plasmon resonance experiments revealed that their interaction has a dissociation constant of the order of 10 microM, which is significantly larger than efficacious concentrations of hemolysis by AMs. These results imply that the potentiation action by GpA or membrane integral peptides may be due to a higher affinity of AMs to protein-containing membranes than that to pure lipid bilayers.

Asn183 in α5 is essential for oligomerisation and toxicity of the Bacillus thuringiensis Cry4Ba toxin

The proposed toxicity mechanism of the Bacillus thuringiensis Cry insecticidal proteins involves membrane penetration and lytic pore formation of the alpha4-alpha5 hairpins in the target larval midgut cell membranes. In this study, alanine substitutions of selected polar residues (Tyr(178), Gln(180), Asn(183), Asn(185), and Asn(195)) in the hydrophobic helix-alpha5 of the Cry4Ba mosquito-larvicidal protein were initially conducted via PCR-based directed mutagenesis. Upon IPTG induction, all the 130-kDa mutant protoxins were highly expressed in Escherichia coli as cytoplasmic inclusions, with yields similar to the wild-type protoxin. When E. coli cells expressing each mutant toxin were tested against Stegomyia aegypti mosquito larvae, the larvicidal activity of the N183A mutant was almost completely abolished whereas the four other mutants showed only a small reduction in toxicity. Additionally, replacements of this critical residue with various amino acids revealed that the uncharged polar residue at position 183 in alpha5 is crucial for larvicidal activity. Further characterisation of the N183K bio-inactive mutant revealed that the 65-kDa activated toxin was unable to form oligomers in lipid vesicles and its ability to induce the release of entrapped calcein from liposomes was much weaker than that of the wild-type toxin. These results suggest that the highly conserved Asn(183) located in the middle of the transmembrane alpha5 of Cry4Ba plays a crucial role in toxicity and toxin oligomerisation in the lipid membranes.

Clinical evaluation of an aerolysin-based screening test for paroxysmal nocturnal haemoglobinuria

BACKGROUND

Recently, a toxin produced by Aeromonas hydrophila was demonstrated to bind directly to the glycosyl-phosphatidyl-inositol (GPI) anchor. After coupling it to a fluorescent dye and applying it in fluorescence-activated cell scanning (FACS), this property was exploited to detect GPI-negative cells in the diagnosis of paroxysmal nocturnal haemoglobinuria (PNH).

METHODS

We used this reagent according to a very simple staining protocol followed by single-colour FACS and compared the results in patients with PNH and normal controls with those obtained with antibody-mediated detection of cells lacking GPI-anchored proteins.

RESULTS

We observed very good concordance between the two methods, with correlation coefficients (R2) of quantified GPI-deficient cell populations ranging from 0.952 to 0.969. The lower limit of detection was determined at 0.50% GPI-negative cells, which was in the range obtained with double-colour staining with antibodies (0.20-1.00%, depending on the antibody). A significant correlation was observed between the fraction of GPI-negative granulocytes and laboratory parameters of haemolysis, with the erythrocyte creatine having the best correlation (R2 = 0.671, P < 0.0001).

CONCLUSIONS

Using this protocol, we were able to reliably diagnose PNH with a high sensitivity. The test allows the identification of GPI-negative granulocyte populations as small as 0.5%.

Cytotoxicity of Clostridium septicum alpha-toxin: its oligomerization in detergent resistant membranes of mammalian cells

Alpha-toxin is an important agent of the virulence of Clostridium septicum. We examined cytotoxicity for alpha-toxin to various mammalian cells with recombinant toxin fused with a histidine-tag at the amino-terminal. The recombinant toxin retained the activity indistinguishable from the native form. Mammalian nucleated cells examined in this study are more sensitive to the protoxin than to the trypsinized toxin, except RAW 264.7 and P3U1 cells of myeloid lineage. Cellular proteins of various molecular sizes interacted with the toxin. The size and SDS-PAGE pattern of the proteins were different among cell lines but they were liberated from the cells by the treatment with phosphatidylinositol-specific phospholipase C. The toxin appeared to target and utilize detergent resistant membranes (DRMs) for binding and subsequent oligomerization. In discontinuous sucrose density gradient, we demonstrated by immunoblotting that the toxin bound to DRMs contained in L929 cells and caused the oligomer formation. Furthermore, cholesterol depletion with cholesterol-interacting agents reduced toxin oligomerization and lowered cytotoxicity of the toxin towards cells. These results suggest that alpha-toxin preferentially exploits DRMs for oligomerization.

Solution Structure of the Pore-forming Protein of Entamoeba histolytica

Amoebapore A is a 77-residue protein from the protozoan parasite and human pathogen Entamoeba histolytica. Amoebapores lyse both bacteria and eukaryotic cells by pore formation and play a pivotal role in the destruction of host tissues during amoebiasis, one of the most life-threatening parasitic diseases. Amoebapore A belongs to the superfamily of saposin-like proteins that are characterized by a conserved disulfide bond pattern and a fold consisting of five helices. Membrane-permeabilizing effector molecules of mammalian lymphocytes such as porcine NK-lysin and the human granulysin share these structural attributes. Several mechanisms have been proposed to explain how saposin-like proteins form membrane pores. All mechanisms indicate that the surface charge distribution of these proteins is the basis of their membrane binding capacity and pore formation. Here, we have solved the structure of amoebapore A by NMR spectroscopy. We demonstrate that the specific activation step of amoebapore A depends on a pH-dependent dimerization event and is modulated by a surface-exposed histidine residue. Thus, histidine-mediated dimerization is the molecular switch for pore formation and reveals a novel activation mechanism of pore-forming toxins.

The Identification and Structure of the Membrane-spanning Domain of the Clostridium septicum Alpha Toxin

Alpha toxin (AT) is a pore-forming toxin produced by Clostridium septicum that belongs to the unique aerolysin-like family of pore-forming toxins. The location and structure of the transmembrane domains of these toxins have remained elusive. Using deletion mutagenesis, cysteine-scanning mutagenesis and multiple spectrofluorimetric methods a membrane-spanning amphipathic beta-hairpin of AT has been identified. Spectrofluorimetric analysis of cysteine-substituted residues modified with an environmentally sensitive fluorescent probe via the cysteine sulfydryl showed that the side chains of residues 203-232 alternated between the aqueous milieu and the membrane core when the AT oligomer was inserted into membranes, consistent with the formation of an amphipathic transmembrane beta-hairpin. AT derivatives that contained deletions that removed up to 90% of the beta-hairpin did not form a pore but were similar to native toxin in all other aspects of the mechanism. Furthermore, a mutant of AT that contained an engineered disulfide, predicted to restrict the movement of the beta-hairpin, functioned similarly to native toxin except that it did not form a pore unless the disulfide bond was reduced. Together these studies revealed the location and structure of the membrane-spanning domain of AT.

Requirement of N-glycan on GPI-anchored proteins for efficient binding of aerolysin but not Clostridium septicum alpha-toxin

Aerolysin of the Gram-negative bacterium Aeromonas hydrophila consists of small (SL) and large (LL) lobes. The alpha-toxin of Gram-positive Clostridium septicum has a single lobe homologous to LL. These toxins bind to glycosylphosphatidylinositol (GPI)-anchored proteins and generate pores in the cell's plasma membrane. We isolated CHO cells resistant to aerolysin, with the aim of obtaining GPI biosynthesis mutants. One mutant unexpectedly expressed GPI-anchored proteins, but nevertheless bound aerolysin poorly and was 10-fold less sensitive than wild-type cells. A cDNA of N-acetylglucosamine transferase I (GnTI) restored the binding of aerolysin to this mutant. Therefore, N-glycan is involved in the binding. Removal of mannoses by alpha-mannosidase II was important for the binding of aerolysin. In contrast, the alpha-toxin killed GnTI-deficient and wild-type CHO cells equally, indicating that its binding to GPI-anchored proteins is independent of N-glycan. Because SL bound to wild-type but not to GnTI-deficient cells, and because a hybrid toxin consisting of SL and the alpha-toxin killed wild-type cells 10-fold more efficiently than GnTI- deficient cells, SL with its binding site for N-glycan contributes to the high binding affinity of aerolysin.

Clostridium septicum alpha-toxin is active against the parasitic protozoan Toxoplasma gondii and targets members of the SAG family of glycosylphosphatidylinositol-anchored surface proteins

As is the case with many other protozoan parasites, glycosylphosphatidylinositol (GPI)-anchored proteins dominate the surface of Toxoplasma gondii tachyzoites. The mechanisms by which T. gondii GPI-anchored proteins are synthesized and transported through the unusual triple-membrane structure of the parasite pellicle to the plasma membrane remain largely unknown. As a first step in developing tools to study these processes, we show here that Clostridium septicum alpha-toxin, a pore-forming toxin that targets GPI-anchored protein receptors on the surface of mammalian cells, is active against T. gondii tachyzoites (50% effective concentration, 0.2 nM). Ultrastructural studies reveal that a tight physical connection between the plasma membrane and the underlying membranes of the inner membrane complex is locally disrupted by toxin treatment, resulting in a massive outward extension of the plasma membrane and ultimately lysis of the parasite. Toxin treatment also causes swelling of the parasite endoplasmic reticulum, providing the first direct evidence that alpha-toxin is a vacuolating toxin. Alpha-toxin binds to several parasite GPI-anchored proteins, including surface antigen 3 (SAG3) and SAG1. Interestingly, differences in the toxin-binding profiles between the virulent RH and avirulent P strain were observed. Alpha-toxin may prove to be a powerful experimental tool for molecular genetic analysis of GPI anchor biosynthesis and GPI-anchored protein trafficking in T. gondii and other susceptible protozoa.

Multilineage glycosylphosphatidylinositol anchor-deficient haematopoiesis in untreated aplastic anaemia

Aplastic anaemia and paroxysmal nocturnal haemoglobinuria (PNH) are closely related disorders. In PNH, haematopoietic stem cells that harbour PIGA mutations give rise to blood elements that are unable to synthesize glycosylphosphatidylinositol (GPI) anchors. Because the GPI anchor is the receptor for the channel-forming protein aerolysin, PNH cells do not bind the toxin and are unaffected by concentrations that lyse normal cells. Exploiting these biological differences, we have developed two novel aerolysin-based assays to detect small populations of PNH cells. CD59 populations as small as 0.004% of total red cells could be detected when cells were pretreated with aerolysin to enrich the PNH population. All PNH patients displayed CD59-deficient erythrocytes, but no myelodysplastic syndrome (MDS) patient or control had detectable PNH cells before or after enrichment in aerolysin. Only one aplastic anaemia patient had detectable PNH red cells before exposure to aerolysin. However, 14 (61%) had detectable PNH cells after enrichment in aerolysin. The inactive fluorescent proaerolysin variant (FLAER) that binds the GPI anchors of a number of proteins on normal cells was used to detect a global GPI anchor deficit on granulocytes. Flow cytometry with FLAER showed that 12 out of 18 (67%) aplastic anaemia patients had FLAER-negative granulocytes, but none of the MDS patients or normal control subjects had GPI anchor-deficient cells. These studies demonstrate that aerolysin-based assays can reveal previously undetectable multilineage PNH cells in patients with untreated aplastic anaemia. Thus, clonality appears to be an early feature of aplastic anaemia.

Type II secretion by Aeromonas salmonicida: evidence for two periplasmic pools of proaerolysin

Aeromonas salmonicida containing the cloned gene for proaerolysin secretes the protein via the type II secretory pathway. Here we show that altering a region near the beginning of aerA led to a dramatic increase in the amount of proaerolysin that was produced and that a large amount of the protein was cell associated. All of the cell-associated protein had crossed the cytoplasmic membrane, because the signal sequence had been removed, and all of it was accessible to processing by trypsin during osmotic shock. Enlargement of the periplasm was observed by electron microscopy in overproducing cells, likely caused by the osmotic effect of the very large concentrations of accumulated proaerolysin. Immunogold electron microscopy localized nearly all of the proaerolysin in the enlarged periplasm; however, only half of the protoxin was released from the cells by osmotic shocking. Cross-linking studies showed that this fraction contained normal dimeric proaerolysin but that proaerolysin in the fraction that was not shockable had not dimerized, although it appeared to be correctly folded. Both periplasmic fractions were secreted by the cells; however, the nonshockable fraction was secreted much more slowly than the shockable fraction. We estimated a rate for maximal secretion of proaerolysin from the bacteria that was much lower than the rates that have been estimated for inner membrane transit, which suggests that transit across the outer membrane is rate limiting and may account for the periplasmic accumulation of the protein. Finally, we show that overproduction of proaerolysin inhibited the release of the protease that is secreted by A. salmonicida.

Efficient retrovirus-mediated PIG-A gene transfer and stable restoration of GPI-anchored protein expression in cells with the PNH phenotype

Paroxysmal nocturnal hemoglobinuria (PNH) is a clonal hematopoietic stem cell disorder characterized by complement-mediated hemolysis due to deficiencies of glycosylphosphatidylinositol-anchored proteins (GPI-APs) in subpopulations of blood cells. Acquired mutations in the X-linked phosphatidylinositol glycan-class A (PIG-A) gene appear to be the characteristic and pathogenetic cause of PNH. To develop a gene therapy approach for PNH, a retroviral vector construct, termed MPIN, was made containing the PIG-A complementary DNA along with an internal ribosome entry site and the nerve growth factor receptor (NGFR) as a selectable marker. MPIN transduction led to efficient and stable PIG-A and NGFR gene expression in a PIG-A-deficient B-cell line (JY5), a PIG-A-deficient K562 cell line, an Epstein-Barr virus-transformed B-cell line (TK-14(-)) established from a patient with PNH, as well as peripheral blood (PB) mononuclear cells from a patient with PNH. PIG-A expression in these cell lines stably restored GPI-AP expression. MPIN was transduced into bone marrow mononuclear cells from a patient with PNH, and myeloid/erythroid colonies and erythroid cells were derived. These transduced erythroid cells restored surface expression of GPI-APs and resistance to hemolysis. These results indicate that MPIN is capable of efficient and stable functional restoration of GPI-APs in a variety of PIG-A-deficient hematopoietic cell types. Furthermore, MPIN also transduced into PB CD34(+) cells from a normal donor, indicating that MPIN can transduce primitive human progenitors. These findings set the stage for determining whether MPIN can restore PIG-A function in multipotential stem cells, thereby providing a potential new therapeutic option in PNH.

Cleavage of a C-terminal peptide is essential for heptamerization of Clostridium perfringens epsilon-toxin in the synaptosomal membrane

Activation of Clostridium perfringens epsilon-protoxin by tryptic digestion is accompanied by removal of the 13 N-terminal and 22 C-terminal amino acid residues. In this study, we examined the toxicity of four constructs: an epsilon-protoxin derivative (PD), in which a factor Xa cleavage site was generated at the C-terminal trypsin-sensitive site; PD without the 13 N-terminal residues (DeltaN-PD); PD without the 23 C-terminal residues (DeltaC-PD); and PD without either the N- or C-terminal residues (DeltaNC-PD). A mouse lethality test showed that DeltaN-PD was inactive, as is PD, whereas DeltaC-PD and DeltaNC-PD were equally active. DeltaC-PD and DeltaNC-PD, but not the other constructs formed a large SDS-resistant complex in rat synaptosomal membranes as demonstrated by SDS-polyacrylamide gel electrophoresis. When DeltaNC-PD and DeltaC-PD, both labeled with (32)P and mixed in various ratios, were incubated with membranes, eight distinct high molecular weight bands corresponding to six heteropolymers and two homopolymers were detected on a SDS-polyacrylamide gel, indicating the active toxin forms a heptameric complex. These results indicate that C-terminal processing is responsible for activation of the toxin and that it is essential for its heptamerization within the membrane.

Glycophorin as a receptor for Escherichia coli alpha-hemolysin in erythrocytes

Escherichia coli alpha-hemolysin (HlyA) can lyse both red blood cells (RBC) and liposomes. However, the cells are lysed at HlyA concentrations 1-2 orders of magnitude lower than liposomes (large unilamellar vesicles). Treatment of RBC with trypsin, but not with chymotrypsin, reduces the sensitivity of RBC toward HlyA to the level of the liposomes. Since glycophorin, one of the main proteins in the RBC surface, can be hydrolyzed by trypsin much more readily than by chymotrypsin, the possibility was tested of a specific binding of HlyA to glycophorin. With this purpose, a number of experiments were performed. (a) HlyA was preincubated with purified glycophorin, after which it was found to be inactive against both RBC and liposomes. (b) Treatment of RBC with an anti-glycophorin antibody protected the cells against HlyA lysis. (c) Immobilized HlyA was able to bind glycophorin present in a detergent lysate of RBC ghosts. (d) Incorporation of glycophorin into pure phosphatidylcholine liposomes increased notoriously the sensitivity of the vesicles toward HlyA. (e) Treatment of the glycophorin-containing liposomes with trypsin reverted the vesicles to their original low sensitivity. The above results are interpreted in terms of glycophorin acting as a receptor for HlyA in RBC. The binding constant of HlyA for glycophorin was estimated, in RBC at sublytic HlyA concentrations, to be 1.5 x 10(-9) m.

The channel-forming protein proaerolysin remains a dimer at low concentrations in solution

Proaerolysin, the proform of the channel-forming protein aerolysin, is secreted as a dimer by Aeromonas sp. The protein also exists as a dimer in the crystal, as well as in solution, at least at concentrations in the region of 500 microg/ml. Recently it has been argued that proaerolysin becomes monomeric at concentrations below 100 microg/ml and that only the monomeric form of the protoxin can bind to cell surface receptors (Fivaz, M., Velluz, M.-C., and van der Goot, F. G. (1999) J. Biol. Chem. 274, 37705-37708). Here we show, using non-denaturing polyacrylamide electrophoresis, chemical cross-linking, and analytical ultracentrifugation, that proaerolysin remains dimeric at the lowest concentrations of the protein that we measured (less than 5 microg/ml) and that the dimeric protoxin is quite capable of receptor binding.

Channel formation by the glycosylphosphatidylinositol-anchored protein binding toxin aerolysin is not promoted by lipid rafts

Glycosylphosphatidylinositol-anchored proteins may be concentrated in membrane microdomains (lipid rafts) that are also enriched in cholesterol and sphingolipids. The glycosyl anchor of these proteins is a specific, high affinity receptor for the channel-forming protein aerolysin. We wished to determine if the presence of rafts promotes the activity of aerolysin. Treatment of T lymphocytes with methyl-beta-cyclodextrin, which destroys lipid rafts by sequestering cholesterol, had no measurable effect on the sensitivity of the cells to aerolysin; nor did similar treatment of erythrocytes decrease the rate at which they were lysed by the toxin. We also studied the rate of aerolysin-induced channel formation in liposomes containing glycosylphosphatidylinositol-anchored placental alkaline phosphatase, which we show is a receptor for aerolysin. In liposomes containing sphingolipids as well as glycerophospholipids and cholesterol, most of the enzyme was Triton X-100-insoluble, indicating that it was localized in rafts, whereas in liposomes prepared without sphingolipids, all of the enzyme was soluble. Aerolysin was no more active against liposomes containing rafts than against those that did not. We conclude that lipid rafts do not promote channel formation by aerolysin.

The cytotoxic enterotoxin of Aeromonas hydrophila induces proinflammatory cytokine production and activates arachidonic acid metabolism in macrophages

An aerolysin-related cytotoxic enterotoxin (Act) of Aeromonas hydrophila possesses multiple biological activities, which include its ability to lyse red blood cells, destroy tissue culture cell lines, evoke a fluid secretory response in ligated intestinal loop models, and induce lethality in mice. The role of Act in the virulence of the organism has been demonstrated. In this study, we evaluated the potential of Act to induce production of proinflammatory cytokines associated with Act-induced tissue injury and Act's capacity to activate in macrophages arachidonic acid (AA) metabolism that leads to production of eicosanoids (e.g., prostaglandin E(2) [PGE(2)]). Our data indicated that Act stimulated the production of tumor necrosis factor alpha and upregulated the expression of genes encoding interleukin-1beta (IL-1beta) and IL-6 in the murine macrophage cell line RAW264.7. Act also activated transcription of the gene encoding inducible nitric oxide synthase. Act evoked the production of PGE(2) coupled to the cyclooxygenase-2 (COX-2) pathway. AA is a substrate for PGE(2), and Act produced AA from phospholipids by inducing group V secretory phospholipase A(2). We also demonstrated that Act increased cyclic AMP (cAMP) production in macrophages. cAMP, along with PGE(2), could potentiate fluid secretion in animal models because of infiltration and activation of macrophages resulting from Act-induced tissue injury. After Act treatment of RAW cells, we detected an increased translocation of NF-kappaB and cAMP-responsive element binding protein (CREB) to the nucleus using gel shift assays. Act also upregulated production of antiapoptotic protein Bcl-2 in macrophages, suggesting a protective role for Bcl-2 against cell death induced by proinflammatory cytokines. The increased expression of genes encoding the proinflammatory cytokines, COX-2, and Bcl-2 appeared correlated with the activation of NF-kappaB and CREB. This is the first report of the detailed mechanisms of action of Act from A. hydrophila.

Identification and characterization of the Aeromonas sobria hemolysin glycoprotein receptor on intestine 407 cells

Aeromonas sobria hemolysin is important in the pathogenesis of diarrhoea caused by this enteropathogenic bacterium. By immunoprecipitation analysis using hemolysin and anti-hemolysin antibody, a 66 kDa protein (p66) was identified as a receptor for A. sobria hemolysin on Intestine 407 cells. Treatment of p66 with N-glycosidase F reduced the apparent sized of p66 to 60 kDa on SDS-polyacrylamide gels. p66, released from Intestine 407 cells following incubation with phosphatidylinositol-specific phospholipase C (PI-PLC) treatment, bound A. sobria hemolysin. Thus treatment of Intestine 407 cells with PI-PLC resulted in the remarkable decrease of the sensitivity to A. sobria hemolysin. These results are consistent with the hypothesis that p66, the binding protein for A. sobria hemolysin, is a glycosylphosphatidylinositol-anchored glycoprotein expressed on the surface of Intestine 407 cells and probably plays a role as a receptor for A. sobria hemolysin on the intestinal cells.

Analysis of receptor binding by the channel-forming toxin aerolysin using surface plasmon resonance

Aerolysin is a channel-forming bacterial toxin that binds to glycosylphosphatidylinositol (GPI) anchors on host cell-surface structures. The nature of the receptors and the location of the receptor-binding sites on the toxin molecule were investigated using surface plasmon resonance. Aerolysin bound to the GPI-anchored proteins Thy-1, variant surface glycoprotein, and contactin with similar rate constants and affinities. Enzymatic removal of N-linked sugars from Thy-1 did not affect toxin binding, indicating that these sugars are not involved in the high affinity interaction with aerolysin. Aerolysin is a bilobal protein, and both lobes were shown to be required for optimal binding. The large lobe by itself bound Thy-1 with an affinity that was at least 10-fold weaker than that of the whole toxin, whereas the small lobe bound the GPI-anchored protein at least 1000-fold more weakly than the intact toxin. Mutation analyses provided further evidence that both lobes were involved in GPI anchor binding, with certain single amino acid substitutions in either domain leading to reductions in affinity of as much as 100-fold. A variant with single amino acid substitutions in both lobes of the protein was completely unable to bind the receptor. The membrane protein glycophorin, which is heavily glycosylated but not GPI-anchored, bound weakly to immobilized proaerolysin, suggesting that interactions with cell-surface carbohydrate structures other than GPI anchors may partially mediate toxin binding to host cells.

Hemolysin of Aeromonas sobria stimulates production of cyclic AMP by cultured cells

Hemolysin of Aeromonas sobria possesses both cytotoxic activity against mammalian cells and enterotoxic activity. Histopathological examination revealed that hemolysin causes diarrhea without damaging the intestinal epithelial cells. And the fluid accumulated in the mouse intestinal loop by the action of the hemolysin is watery. These observations indicated that the enterotoxic activity of hemolysin is not dependent on its cytotoxic activity. To clarify the mechanism of the enterotoxic action of hemolysin, we examined cyclic nucleotide levels in cultured cells exposed to this toxin. These results showed that hemolysin stimulates the production of cyclic AMP in cultured cells and the cyclic AMPs thus produced emerge in the milieu of cells.

Resistance of Paroxysmal Nocturnal Hemoglobinuria Cells to the Glycosylphosphatidylinositol-Binding Toxin Aerolysin

Paroxysmal nocturnal hemoglobinuria (PNH) is a clonal stem cell disorder caused by a somatic mutation of the PIGA gene. The product of this gene is required for the biosynthesis of glycosylphosphatidylinositol (GPI) anchors; therefore, the phenotypic hallmark of PNH cells is an absence or marked deficiency of all GPI-anchored proteins. Aerolysin is a toxin secreted by the bacterial pathogen Aeromonas hydrophila and is capable of killing target cells by forming channels in their membranes after binding to GPI-anchored receptors. We found that PNH blood cells (erythrocytes, lymphocytes, and granulocytes), but not blood cells from normals or other hematologic disorders, are resistant to the cytotoxic effects of aerolysin. The percentage of lysis of PNH cells after aerolysin exposure paralleled the percentage of CD59+ cells in the samples measured by flow cytometry. The kinetics of red blood cell lysis correlated with the type of PNH erythrocytes. PNH type III cells were completely resistant to aerolysin, whereas PNH type II cells displayed intermediate sensitivity. Importantly, the use of aerolysin allowed us to detect PNH populations that could not be detected by standard flow cytometry. Resistance of PNH cells to aerolysin allows for a simple, inexpensive assay for PNH that is sensitive and specific. Aerolysin should also be useful in studying PNH biology.

Resistance of Paroxysmal Nocturnal Hemoglobinuria Cells to the Glycosylphosphatidylinositol-Binding Toxin Aerolysin

Paroxysmal nocturnal hemoglobinuria (PNH) is a clonal stem cell disorder caused by a somatic mutation of the PIGA gene. The product of this gene is required for the biosynthesis of glycosylphosphatidylinositol (GPI) anchors; therefore, the phenotypic hallmark of PNH cells is an absence or marked deficiency of all GPI-anchored proteins. Aerolysin is a toxin secreted by the bacterial pathogen Aeromonas hydrophila and is capable of killing target cells by forming channels in their membranes after binding to GPI-anchored receptors. We found that PNH blood cells (erythrocytes, lymphocytes, and granulocytes), but not blood cells from normals or other hematologic disorders, are resistant to the cytotoxic effects of aerolysin. The percentage of lysis of PNH cells after aerolysin exposure paralleled the percentage of CD59+ cells in the samples measured by flow cytometry. The kinetics of red blood cell lysis correlated with the type of PNH erythrocytes. PNH type III cells were completely resistant to aerolysin, whereas PNH type II cells displayed intermediate sensitivity. Importantly, the use of aerolysin allowed us to detect PNH populations that could not be detected by standard flow cytometry. Resistance of PNH cells to aerolysin allows for a simple, inexpensive assay for PNH that is sensitive and specific. Aerolysin should also be useful in studying PNH biology.

Expression and properties of an aerolysin--Clostridium septicum alpha toxin hybrid protein

Aerolysin is a bilobal channel-forming toxin secreted by Aeromonas hydrophila. The alpha toxin produced by Clostridium septicum is homologous to the large lobe of aerolysin. However, it does not contain a region corresponding to the small lobe of the Aeromonas toxin, leading us to ask what the function of the small lobe is. We fused the small lobe of aerolysin to alpha toxin, producing a hybrid protein that should structurally resemble aerolysin. Unlike aerolysin, the hybrid was not secreted when expressed in Aeromonas salmonicida. The purified hybrid was activated by proteolytic processing in the same way as both parent proteins and, after activation, it formed oligomers that corresponded to the aerolysin heptamer. Like aerolysin, the hybrid was far more active than alpha toxin against human erythrocytes and mouse T lymphocytes. Both aerolysin and the hybrid bound to human glycophorin, and both were inhibited by preincubation with this erythrocyte glycoprotein, whereas alpha toxin was unaffected. We conclude that aerolysin contains two receptor binding sites, one for glycosyl-phosphatidylinositol-anchored proteins that is located in the large lobe and is also found in alpha toxin, and a second site, located in the small lobe, that binds a surface carbohydrate determinant.

The pore-forming toxin proaerolysin is activated by furin

Aerolysin is secreted as an inactive dimeric precursor by the bacterium Aeromonas hydrophila. Proteolytic cleavage within a mobile loop near the C terminus of the protoxin is required for oligomerization and channel formation. This loop contains the sequence KVRRAR432, which should be recognized by mammalian proprotein convertases such as furin, PACE4, and PC5/6A. Here we show that these three proteases cleave proaerolysin after Arg-432 in vitro, yielding active toxin. We also investigated the potential role of these enzymes in the in vivo activation of the protoxin. We found that Chinese hamster ovary cells were able to convert the protoxin to aerolysin in the absence of exogenous proteases and that activation did not require internalization of the toxin. The furin inhibitor alpha1-antitrypsin Portland reduced the rate of proaerolysin activation in vivo, and proaerolysin processing was even further reduced in furin-deficient FD11 Chinese hamster ovary cells. The cells were also less sensitive to proaerolysin than wild type cells; however, transient transfection of FD11 cells with the cDNA encoding furin conferred normal sensitivity to the protoxin. Together these findings argue that furin catalyzes the cell-surface activation of proaerolysin in vivo.

Secretion and properties of the large and small lobes of the channel-forming toxin aerolysin

Aerolysin is a dimeric protein secreted by Aeromonas spp. that binds to glycosylphosphatidylinositol-anchored receptors on target cells and becomes insertion competent by oligomerizing. The protein comprises two lobes joined by a short arm. The large lobe is thought to be responsible for channel formation, whereas the small lobe is believed to stabilize the dimer, and it may also contain the receptor binding site. We cloned and expressed the DNA for both lobes of the toxin separately and together in A. salmonicida. The large lobe produced alone was secreted, although more poorly than native protein. The small lobe with the arm produced by itself was not secreted. When the large lobe without the arm was co-produced with the small lobe with the arm, both were secreted, and they co-purified as a stoichiometric complex. Analytical ultracentrifugation showed that they form a heterotetramer corresponding to the native dimer. The purified product was nearly as active as aerolysin, but lost activity and became trypsin sensitive above 25 degreesC. The large lobe with the arm was also purified. It was shown to be monomeric, confirming that the small lobe is responsible for dimer stabilization. The large lobe had very low channel-forming activity, although it was correctly processed by trypsin, and it could form stable oligomers. Surprisingly, the large lobe was found to bind to several glycosylphosphatidylinositol-anchored proteins, indicating that it contains at least part of the receptor-binding domain.

Aerolysin induces G-protein activation and Ca2+ release from intracellular stores in human granulocytes

Aerolysin is a pore-forming toxin that plays a key role in the pathogenesis of Aeromonas hydrophila infections. In this study, we have analyzed the effect of aerolysin on human granulocytes (HL-60 cells). Proaerolysin could bind to these cells, was processed into active aerolysin, and led to membrane depolarization, indicating that granulocytes are potential targets for this toxin. Fura-2 measurements were used to analyze the effect of aerolysin on cytosolic [Ca2+] homeostasis. As expected for a pore-forming toxin, aerolysin addition led to Ca2+ influx across the plasma membrane. In addition, the toxin triggered Ca2+ release from agonist and thapsigargin-sensitive intracellular Ca2+ stores. This Ca2+ release was independent of the aerolysin-induced Ca2+ influx and occurred in two kinetically distinct phases: an initial rapid and transient phase and a second, more sustained, phase. The first, but not the second phase was sensitive to pertussis toxin. Activation of pertussis toxin-sensitive G-proteins appeared to be a consequence of pore formation, rather than receptor activation through aerolysin-binding, as it: (i) was not observed with a binding competent, insertion-incompetent aerolysin mutant, (ii) had a marked lag time, and (iii) was also observed in response to other bacterial pore-forming toxins (staphylococcal alpha-toxin, streptolysin O) which are thought to bind to different receptors. G-protein activation through pore-forming toxins stimulated cellular functions, as evidenced by pertussis toxin-sensitive chemotaxis. Our results demonstrate that granulocytes are potential target cells for aerolysin and that in these cells, Ca2+ signaling in response to a pore-forming toxin involves G-protein-dependent cell activation and Ca2+ release from intracellular stores.

Aerolysin--a paradigm for membrane insertion of beta-sheet protein toxins?

The determination of the crystal structure of the bacterial protein proaerolysin provided the first view of a pore-forming toxin constructed mainly from beta-sheet. The structure that was obtained and subsequent crystallographic and biochemical studies have together allowed us to explain how the toxin is transformed from a water-soluble dimer to a heptameric transmembrane pore. Recent discoveries of structural similarities between aerolysin and other toxins suggest that the structure/function studies we have made may prove useful in understanding the actions of a number of pore-forming proteins.

A pore-forming toxin interacts with a GPI-anchored protein and causes vacuolation of the endoplasmic reticulum

In this paper, we have investigated the effects of the pore-forming toxin aerolysin, produced by Aeromonas hydrophila, on mammalian cells. Our data indicate that the protoxin binds to an 80-kD glycosyl-phosphatidylinositol (GPI)-anchored protein on BHK cells, and that the bound toxin is associated with specialized plasma membrane domains, described as detergent-insoluble microdomains, or cholesterol-glycolipid "rafts." We show that the protoxin is then processed to its mature form by host cell proteases. We propose that the preferential association of the toxin with rafts, through binding to GPI-anchored proteins, is likely to increase the local toxin concentration and thereby promote oligomerization, a step that it is a prerequisite for channel formation. We show that channel formation does not lead to disruption of the plasma membrane but to the selective permeabilization to small ions such as potassium, which causes plasma membrane depolarization. Next we studied the consequences of channel formation on the organization and dynamics of intracellular membranes. Strikingly, we found that the toxin causes dramatic vacuolation of the ER, but does not affect other intracellular compartments. Concomitantly we find that the COPI coat is released from biosynthetic membranes and that biosynthetic transport of newly synthesized transmembrane G protein of vesicular stomatitis virus is inhibited. Our data indicate that binding of proaerolysin to GPI-anchored proteins and processing of the toxin lead to oligomerization and channel formation in the plasma membrane, which in turn causes selective disorganization of early biosynthetic membrane dynamics.

Hyperproduction, purification, and mechanism of action of the cytotoxic enterotoxin produced by Aeromonas hydrophila

A gene encoding the cytotoxic enterotoxin (Act) from Aeromonas hydrophila was hyperexpressed with the pET, pTRX, and pGEX vector systems. Maximum toxin yield was obtained with the pTRX vector. Approximately 40 to 60% of Act was in a soluble form with the pTRX and pET vector systems. The toxin protein was purified to homogeneity by a combination of ammonium sulfate precipitation and fast protein liquid chromatography-based column chromatographies, including hydrophobic, anion-exchange, sizing, and hydroxylapatite chromatographies. Purified mature toxin migrated as a 52-kDa polypeptide on a sodium dodecyl sulfate (SDS)polyacrylamide gel that reacted with Act-specific antibodies in immunoblots. The minimal amount of toxin needed to cause fluid secretion in rat ileal loops was 200 ng, and the 50% lethal dose for mice was 27.5 ng when injected intravenously. Binding of the toxin to erythrocytes was temperature dependent, with no binding occurring at 4 degrees C. However, at 37 degrees C the toxin bound to erythrocytes within 1 to 2 min. It was determined that the mechanism of action of the toxin involved the formation of pores in erythrocyte membranes, and the diameter of the pores was estimated to be 1.14 to 2.8 nm, as determined by the use of saccharides of different sizes and by electron microscopy. Calcium chloride prevented lysis of erythrocytes by the toxin; however, it did not affect the binding and pore-forming capabilities of the toxin. A dose-dependent reduction in hemoglobin release from erythrocytes was observed when Act was preincubated with cholesterol, but not with myristylated cholesterol. With 14C-labeled cholesterol and gel filtration, the binding of cholesterol to Act was demonstrated. None of the other phospholipids and glycolipids tested reduced the hemolytic activity of Act. The toxin also appeared to undergo aggregation when preincubated with cholesterol, as determined by SDS-polyacrylamide gel electorphoresis. As a result of this aggregation, Act's capacity to form pores in the erythrocyte membrane was inhibited.

Small-angle X-ray scattering study on CEL-III, a hemolytic lectin from Holothuroidea Cucumaria echinata, and its oligomer induced by the binding of specific carbohydrate

Hemolytic lectin CEL-III from a marine invertebrate Cucumaria echinata forms an oligomer upon binding of specific carbohydrate such as lactose at high pH values and in the presence of high concentrations of salt. In this study, using small-angle X-ray scattering, we characterized CEL-III and its oligomer induced by the binding of lactose. The molecular mass of the oligomer was determined as 1019 kDa from its forward scattering value, compared with 47,490 Da for the monomer. This oligomer size is much larger than that estimated using SDS-polyacrylamide gel electrophoresis (SDS-PAGE, 270 kDa). The monomer has a 24.6 A radius of gyration and can be approximated by a rod which has a 20 A radius and a height of 75 A, while the oligomer has a 101.4 A radius of gyration. Together with the comparison of the radii of gyration and the forward scattering of the cross-section of the monomer and oligomer, it is suggested that in aqueous solution the oligomer comprises three or four molecules of a smaller unit which was observed by SDS-PAGE (270 kDa), held by a relatively weak interaction. The scattering profile also suggests that the oligomer has a hole in its central axis which might be associated with the formation of ion-permeable pores in the erythrocyte membrane by CEL-III during the hemolytic process.

Haemolytic activity of stonustoxin from stonefish (Synanceja horrida) venom: pore formation and the role of cationic amino acid residues

Stonustoxin (SNTX) is a two-subunit protein toxin purified from the venom of the stonefish (Synanceja horrida), which induces potent haemolytic activity. We examined the pore-forming property of this non-enzymic protein by an osmotic protection assay. SNTX-induced haemolysis was completely prevented by osmotic protectants of adequate size [poly(ethylene) glycol 3000; molecular diameter approx. 3.2 nm]. Uncharged molecules of smaller size, such as raffinose and poly(ethylene) glycol 1000-2000, failed to protect against cell lysis. These findings indicate that SNTX induces the formation of hydrophilic pores in the cell membrane, which results in the lysis of erythrocytes. Since cationic residues contribute significantly to the cytolytic activity of several other pore-forming toxins, we examined the role of positively charged lysine and arginine residues in the haemolytic activity of SNTX. SNTX lost its haemolytic activity when the positively charged side chains of lysine residues were neutralized or converted into negatively charged side chains upon carbamylation or succinylation respectively. The haemolytic activity of SNTX was also inhibited by the modification of positively charged arginine residues using 2,3-butanedione. The loss of haemolysis showed strong correlation with the number of Lys or Arg residues modified. CD analyses, however, showed that the conformation of SNTX was not significantly affected by these chemical modifications. Further, the haemolytic activity of SNTX was competitively inhibited by various negatively charged lipids, such as phosphatidylserine, cardiolipin and monosialogangliosides. These results indicate that SNTX induces potent haemolytic activity through the formation of pores in the cell membrane, and that cationic residues play a crucial role in its cytolytic mechanism.

Mode of primary binding to target membranes and pore formation induced by Vibrio cholerae cytolysin (hemolysin)

Vibrio cholerae cytolysin (VCC) is produced by many non-choleratoxigenic strains of V. cholerae, and possibly represents a relevant pathogenicity determinant of these bacteria. The protein is secreted as a pro-toxin that is proteolytically cleaved to yield the active toxin with a molecular mass of approximately 63 kDa. We here describe a simple procedure for preparative isolation of mature VCC from bacterial culture supernatants, and present information on its mode of binding and pore formation in biological membranes. At low concentrations, toxin monomers interact with a high-affinity binding site on highly susceptible rabbit erythrocytes. This as yet unidentified binding site is absent on human erythrocytes, which are less susceptible to the toxin action. At higher concentrations, binding of the toxin occurs to both rabbit and human erythrocytes in a non-saturable manner. Cell-bound toxin monomers oligomerize to form supramolecular structures that are seen in the electron microscope as apparently hollow funnels, and oligomerization correlates functionally with the appearance of small transmembrane pores. Osmotic protection experiments indicate that the toxin channels are of finite size with a diameter of 1-2 nm. The mode of action of VCC closely resembles that of classical pore-forming toxins such as staphylococcal alpha-toxin and the aerolysin of Aeromonas hydrophila.

Potent membrane-permeabilizing and cytocidal action of Vibrio cholerae cytolysin on human intestinal cells

Many strains of Vibrio cholerae non-O1 and O1 El Tor that cause diarrhea do not harbor genes for a known secretogenic toxin. However, these strains usually elaborate a pore-forming toxin, hitherto characterized as a hemolysin and here designated V. cholerae cytolysin, whose action on intestinal cells has not yet been described. We report that V. cholerae cytolysin binds as a monomer to Intestine 407 cells and then assembles into detergent-stable oligomers that probably represent tetra- or pentamers. Oligomer formation is accompanied by generation of small transmembrane pores that allow rapid flux of K+ but not influx of Ca2+ or propidium iodide. Pore formation is followed by irreversible ATP depletion and cell death. Binding of fewer than 10(4) toxin molecules per cell in vitro is lethal. The possibility is raised that production of this toxin by bacteria that are in close contact with intestinal cells is rapidly cytocidal in vivo, and death of intestinal cells may be a cause of diarrhea.

Oligomerization of the hemolytic lectin CEL-III from the marine invertebrate Cucumaria echinata induced by the binding of carbohydrate ligands

The hemolytic lectin CEL-III is a Ca2+-dependent, galactose/GalNAc-specific lectin purified from the marine invertebrate Cucumaria echinata (Holothuroidea). We found that this lectin forms ion-permeable pores in erythrocyte and artificial lipid membranes that have specific carbohydrate ligands on the surface. The hemolytic activity of CEL-III exhibited characteristic pH dependence; activity increased remarkably with pH in the alkaline region, especially above pH 9. When rabbit erythrocyte membrane was examined by immunoblotting using anti-CEL-III antiserum after treatment with CEL-III, the irreversible binding of the CEL-III oligomer increased with pH, indicating that the increase in hemolytic activity at higher pH is associated closely with the amount of oligomer irreversibly bound to the membrane. Surface hydrophobicity of CEL-III, as measured by the fluorescent probe 8-anilino-1-naphthalenesulfonate, increased markedly with the binding of specific ligands such as lactose, lactulose, and N-acetyllactosamine at pH 9-10 in the presence of 1 M NaCl. The enhancement of surface hydrophobicity induced by the binding of carbohydrates was also accompanied by the formation of a CEL-III oligomer, which was found to be the same size on sodium dodecyl sulfate-polyacrylamide gel electrophoresis as the oligomer that formed in CEL-III-treated erythrocyte membranes. Far-UV circular dichroism spectra of CEL-III and the oligomer revealed a definite difference in secondary structure. These data suggest that the binding of CEL-III to specific carbohydrate ligands on the erythrocyte surface induces a conformational change in the protein, leading to the exposure of a hydrophobic region which triggers oligomerization and the irreversible binding of the protein to the membrane.

Characterisation of the heptameric pore-forming complex of the Aeromonas toxin aerolysin using MALDI-TOF mass spectrometry

Aerolysin, a virulence factor secreted by Aeromonas hydrophila, is representative of a group of beta-sheet toxins that must form stable homooligomers in order to be able to insert into biological membranes and generate channels. Electron microscopy and image analysis of two-dimensional membrane crystals had previously revealed a structure with 7-fold symmetry suggesting that aerolysin forms heptameric oligomers [Wilmsen et al. (1992) EMBO J. 11, 2457-2463]. However, this unusual molecularity of the channel remained to be confirmed by an independent method since low-resolution electron crystallography had led to artefactual data for other pore-forming toxins. In this study, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) was used to measure the mass of the aerolysin oligomer preparation. A mass of 333 850 Da was measured, fitting very well with a heptameric complex (expected mass: 332 300 Da). These results confirm the earlier evidence that the aerolysin oligomer is a heptamer and also show that MALDI-TOF mass spectrometry could be a valuable tool to study non-covalent association of proteins.

Characterization of Vibrio cholerae El Tor cytolysin as an oligomerizing pore-forming toxin

V. cholerae El Tor cytolysin is a secreted, water-soluble protein of M(r) 60,000 that may be relevant to the pathogenesis of acute diarrhea. In this communication, we demonstrate that the toxin binds to and oligomerizes in target membranes to form SDS-stable aggregates of M(r) 200,000-250,000 that generate small transmembrane pores. Pores formed in erythrocytes were approximately 0.7 nm in size, as demonstrated by osmotic protection experiments. Binding was shown to occur in a temperature-independent manner preceding the temperature-dependent oligomerization step. Pores were also shown to be formed in L929 and HEp-2 cells, human fibroblasts and keratinocytes, albeit with highly varying efficacy. At neutral pH and in the presence of serum, human fibroblasts were able to repair a limited number of lesions. The collective data identify V. cholerae El Tor cytolysin as an oligomerizing toxin that damages cells by creating small transmembrane pores.