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.

Identification of the catalytic triad of the lipase/acyltransferase from Aeromonas hydrophila

Aeromonas hydrophila secretes a lipolytic enzyme that has several properties in common with the mammalian enzyme lecithin-cholesterol acyltransferase. We have recently shown that it is a member of a newly described group of proteins that contain five similar blocks of sequence arranged in the same order in their primary structures (C. Upton and J. T. Buckley, Trends Biochem. Sci. 233:178-179, 1995). Assuming that, like other lipases, these enzymes have a Ser-Asp-His catalytic triad, we used these blocks to predict which aspartic acid and histidine would be at the active site of the Aeromonas enzyme. Targeted residues were replaced with other amino acids by site-directed mutagenesis, and the effects on secretion and activity were assessed. Changing His-291 to asparagine completely abolished enzyme activity, although secretion by the bacteria was not affected. Only very small amounts of the D116N mutant appeared in the culture supernatant, likely because it is sensitive to periplasmic proteases it encounters en route. Assays of crude preparations containing this variant showed no detectable enzyme activity. We conclude that, together with Ser-16, which we have identified previously, Asp-116 and His-291 compose the catalytic triad of the enzyme.

Optical single-channel analysis of the aerolysin pore in erythrocyte membranes

Scanning microphotolysis (Scamp), a recently developed photobleaching technique, was used to analyze the transport of two small organic anions and one inorganic cation through single pores formed in human erythrocyte membranes by the channel-forming toxin aerolysin secreted by Aeromonas species. The transport rate constants of erythrocyte ghosts carrying a single aerolysin pore were determined to be (1.83 +/- 0.43) x 10(-3) s-1 for Lucifer yellow, (0.33 +/- 0.10) x 10(-3) s-1 for carboxyfluorescein, and (8.20 +/- 2.30) x 10(-3) s-1 for Ca2+. The radius of the aerolysin pore was derived from the rate constants to be 19-23 A, taking steric hindrance and viscous drag into account. The size of the Ca2+ rate constant implies that at physiological extracellular Ca2+ concentrations (> 1 mM) the intracellular Ca2+ concentration would be elevated to the critical level of > 1 microM in much less than a second after formation of a single aerolysin pore in the plasma membrane. Thus changes in the levels of Ca2+ or other critical intracellular components may be more likely to cause cell death than osmotic imbalance.

Aerolysin--the ins and outs of a model channel-forming toxin

Aerolysin is one of a large group of bacterial proteins that can kill target cells by forming discrete channels in their plasma membranes. The toxin has many properties in common with the porins of the Gram-negative bacterial outer membrane, including an extensive amount of beta-structure, a high proportion of hydrophilic amino acid side-chains and no hydrophobic stretches in the primary structure. It also oligomerizes to produce an insertion-competent state. Aerolysin is secreted as a dimer by members of the Aeromonas family. It binds to a high-affinity receptor on the target cell that has recently been shown to be a glycosylphosphatidylinositol-anchored glycoprotein. Binding is followed by heptamerization to form a structure that we propose contains a beta-barrel which can insert into the membrane and produce a channel.

A new family of lipolytic plant enzymes with members in rice, arabidopsis and maize

We have noted a striking similarity between the sequences of proteins in a novel family of lipases we recently reported [Upton, C. and Buckley, J. T. (1995) Trends Biol. Sci. 20, 178-9] and more than 120 sequences from the database of Expressed Sequence Tags (dbEST) which correspond to at least 30 unique genes from arabidopsis, rice and maize. A cDNA (Arab-1) corresponding to one of these sequences was isolated, sequenced and translated. There was significant similarity to sequences in the new lipase family over the entire open reading frame of Arab-1 and when expressed in E. coli, the gene product was lipolytic. Arab-1 and genes for some of the other plant proteins appear to be differentially expressed. They may play a role in the regulation of lipid metabolism during plant development.

Protonation of histidine-132 promotes oligomerization of the channel-forming toxin aerolysin

Aerolysin is a bacterial toxin that binds to a receptor on eukaryotic cells and oligomerizes to form stable, SDS-resistant, noncovalent oligomers that insert into the plasma membrane and produce well-defined channels. Little is known about the mechanisms controlling this process. Here we show that the protonation of a single histidine is required for oligomerization of aerolysin in solution. First we have investigated the effect of pH on the activity of aerolysin. The toxin's ability to disrupt human erythrocytes declined as the pH increased above 7.4. Experiments with receptor-free planar lipid bilayers demonstrated that the rate at which aerolysin formed channels also decreased with increasing pH, although the conductance of preexisting channels was not affected. The reduction in the rate of channel formation was shown to be due to a decrease in the toxin's ability to oligomerize. Our data indicate that the pH effect on activity is due to the deprotonation of a single residue rather than a global effect of pH on the protein. In agreement with our previous site-directed mutagenesis studies, His-132 is most likely to be the target of this pH effect. This conclusion was reinforced by the fact that we could shift the pH dependence of the activity to lower pH values by mutating Asp-139, a residue less than 3 A away from His-132 and likely to contribute to the usually high pKa of this histidine.

Vibrio spp. secrete proaerolysin as a folded dimer without the need for disulphide bond formation

Proaerolysin is an extracellular dimeric protein that is secreted across the inner and outer membranes of Aeromonas spp. in separate steps. To investigate the role of protein folding in the second step, one or more cysteine residues were introduced and the mutant proaerolysins were expressed in Aeromonas hydrophila and Aeromonas salmonicida, as well as Vibrio cholerae. Replacing Met-41 with Cys resulted in expression of a protein that could form a dimer in which the monomers were linked together by a disulphide bridge. A double mutant was also made, in which Gly-202 and Ile-445 were replaced with cysteine in order to allow the formation of an intrachain disulphide bridge when the molecule was correctly folded. The M41C covalent dimer and G202C/I445C proaerolysin with the new intrachain bridge were both easily detected inside the bacteria, and they later appeared in the culture supernatants. Small amounts of incorrectly folded proaerolysin were also observed in the cells, but they were not secreted. We observed in the cells, but they were not secreted. We conclude that proaerolysin folds and dimerizes before being released from the cell, and that correct folding is a requirement for secretion to occur. The proton ionophore CCCP reduced release of the folded proteins. Unoxidized protein was secreted by cells grown in beta-mercaptoethanol and by a dsbA mutant of V. cholerae, indicating that disulphide bond formation may not be essential for release.

The C-terminal peptide produced upon proteolytic activation of the cytolytic toxin aerolysin is not involved in channel formation

The channel-forming toxin aerolysin is secreted by Aeromonas hydrophila as a protoxin that can be activated by nicking with endoproteinase Lys-C after Lys-427 near the C terminus of the protein. The fate of the 43-amino acid peptide distal to the activation site was investigated. A cysteine was introduced into the C-terminal region by replacing Ile-445, and another replaced Gly-202, which is on the proximal side of the activation site. In a double mutant, the two new cysteines were close enough in the folded molecule to form an intrachain 202-445 disulfide bond. Tryptophan fluorescence measurements on wild type and the 2 single cysteine mutants indicated that activation results in exposure of at least 1 tryptophan residue, leading to the conclusion that the peptide moves with respect to the protein when it is produced. This was supported by the observation that upon activation there was a decrease in energy transfer between a tryptophan in the bulk of the protein and a probe attached to Cys-445. The peptide could be separated from active toxin by several methods, indicating that it leaves the protein when it is produced, and that it plays no further role in the process of channel formation.

Partial purification of the rat erythrocyte receptor for the channel-forming toxin aerolysin and reconstitution into planar lipid bilayers

The cytolytic toxin aerolysin binds to a receptor on the surface of eukaryotic cells. Murine erythrocytes are among the most sensitive to the toxin. Here we describe the detergent solubilization and partial purification of the receptor from rat erythrocytes. We show that it can be successfully incorporated into planar lipid bilayers, greatly decreasing the concentration of aerolysin required to form channels. Exploiting the ability of the receptor to bind aerolysin after SDS electrophoresis and blotting, we obtain evidence that it is a 47 kDa glycoprotein that is sensitive to proteases and N-glycosidase. It may correspond to CHIP28, the water channel of the human erythrocyte.

Characterization of interfacial catalysis by Aeromonas hydrophila lipase/acyltransferase in the highly processive scooting mode

A glycerophospholipid:cholesterol acyltransferase (GCAT) that also has lipase activity is secreted by the bacterium Aeromonas hydrophila. Hydrolysis of the sn-2-ester bond of 1,2-dimyristoyl-sn-glycero-3-phosphomethanol (DMPM) vesicles by this enzyme is shown to occur in a highly processive scooting mode in which the enzyme, substrate, and the products of hydrolysis remain bound to the vesicle interface. This conclusion is based on the following observations. (a) When there is an excess of vesicles over enzyme, the hydrolysis of the sn-2-acyl group ceases after only a fraction of the total available substrate is hydrolyzed. Addition of more enzyme, but not of more substrate, leads to a new round of hydrolysis. (b) The extent of hydrolysis of vesicles per enzyme increases with the size of the vesicles, and it corresponds to the total hydrolysis of the outer monolayer of one vesicle by one enzyme. (c) The enzyme bound to vesicles composed of reaction products or of the non-hydrolyzable phospholipid 1,2-ditetradecyl-sn-glycero-3-phosphomethanol (DTPM) is not able to undergo intervesicle exchange. Instead, intervesicle transfer of the substrate or the bound enzyme due to vesicle fusion promotes hydrolysis of all of the vesicles present in the reaction mixture. (d) Addition of DTPM vesicles to a reaction mixture containing DMPM substrate vesicles and the enzyme has no noticeable effect on the course of hydrolysis. Substrate specificity studies in the scooting mode on DMPM vesicles reveal that GCAT displays essentially no selectivity in the hydrolysis of phospholipids with different polar head groups. Treatment of GCAT with trypsin, which removes a small peptide, results in an enzyme that displays comparable catalytic activity but increased affinity for the interface. Alkyltrifluoromethyl ketones are shown to be tight-binding competitive inhibitors of GCAT. The scooting mode analysis, which has previously been shown to provide a simplified approach for analyzing the steady-state kinetics of interfacial catalysis by secreted phospholipase A2, is also useful for analyzing the interfacial kinetic behavior of lipases.

The cytolytic toxin aerolysin: from the soluble form to the transmembrane channel

Aerolysin is a cytolytic toxin which forms channels in the plasma membranes of eucaryotic cells. The protein is secreted by Aeromonas hydrophila as an inactive protoxin. Its stability and water solubility are conferred by its ability to dimerize. Maturation of the protein occurs through proteolytic removal of a C-terminal peptide outside the secreting cell. Although the aerolysin which is so produced is still a dimer, it then has the ability to oligomerize. The oligomer is the active form of the toxin, capable of forming the transmembrane channels that disrupt cells. We review here the present knowledge about the structure of aerolysin in relation to the various steps in channel formation.

Influence of active site and tyrosine modification on the secretion and activity of the Aeromonas hydrophila lipase/acyltransferase

Aeromonas sp. secrete a lipase/acyltransferase that shares several properties with the mammalian plasma enzyme lecithin:cholesterol acyltransferase. Reaction of the enzyme with tetranitromethane led to modification of 2 tyrosines and a nearly 80% decline in enzyme activity. Replacing Tyr230 with Phe altered the activity of the enzyme in the same way as did treatment with tetranitromethane. Unlike the wild type enzyme, which preferentially hydrolyzes the 2-position acyl chain of phosphatidylcholine, the Y230F mutant enzyme did not discriminate between the 1- and 2-positions of the phospholipid. Tyr230 may be necessary to correctly position phospholipid substrates at the active site. Several amino acids around the active site Ser16 of the lipase were also changed. Replacing Ser18 with Gly, bringing the enzyme's sequence into line with the "lipase consensus sequence," resulted in reduced secretion of the protein and complete loss of activity. Changing this serine to Val led to an inactive protein that was not secreted at all. Substituting Phe13 in the hydrophobic region of the consensus sequence with Ser also prevented secretion, although the mutant protein appeared to be active. The Aeromonas lipase may represent a distinct group of lipolytic enzymes which have a novel active site structure.

Structure of the Aeromonas toxin proaerolysin in its water-soluble and membrane-channel states

Aerolysin is chiefly responsible for the pathogenicity of Aeromonas hydrophila, a bacterium associated with diarrhoeal diseases and deep wound infections. Like many other microbial toxins, the protein changes in a multistep process from a completely water-soluble form to produce a transmembrane channel that destroys sensitive cells by breaking their permeability barriers. Here we describe the structure of proaerolysin determined by X-ray crystallography at 2.8 A resolution. The protoxin (M(r) 52,000) adopts a novel protein fold. Images of an aerolysin oligomer derived from electron microscopy have assisted in constructing a model of the membrane channel and have led to the proposal of a scheme to account for insertion of the protein into lipid bilayers to form ion channels.

Physical and chemical characterization of the oligomerization state of the Aeromonas hydrophila lipase/acyltransferase

Aeromonas glycerophospholipid:cholesterol acyl transferase undergoes a conformational transition upon activation by treatment with trypsin. Chemical cross-linking and sedimentation velocity analysis showed that the lipase dimerizes due to removal of a region near its C-terminus. The lipase monomer has a sedimentation coefficient s20.w = 2.83 S, whereas the dimer has s20.w = 3.65 +/- 0.22 S. Hydrodynamic analysis using these sedimentation values and the masses determined by mass spectrometry indicated that the monomers are aligned side-by-side in the dimer. An important change occurs in the apparent partial specific volume of the molecule upon activation.

Aeromonas spp. can secrete Escherichia coli alkaline phosphatase into the culture supernatant, and its release requires a functional general secretion pathway

Aerolysin is a channel-forming protein secreted by Aeromonas hydrophila. To determine if regions of aerolysin could direct the secretion of another protein, portions of aerA were fused to phoA, the Escherichia coli alkaline phosphatase gene and cloned into E. coli, Aeromonas salmonicida, and A. hydrophila. We were surprised to find that secretion of the enzyme by both Aeromonas spp. was independent of the aerolysin segments fused to it. The smallest fusion product contained only the signal sequence and two amino acids of aerolysin. The largest had more than 90% of the aerolysin molecule. The fusion proteins were found in the periplasms of E. coli and A. salmonicida grown in LB medium containing glucose, as well as in the shocked cells. Aerolysin itself was secreted by A. salmonicida under these conditions. In contrast, when A. salmonicida containing any of the fused genes was grown in LB medium without glucose, most of the alkaline phosphatase activity was extracellular, whereas beta-lactamase remained in its normal periplasmic location. Similar results were obtained with A. hydrophila. The change in location of the enzyme in A. salmonicida appeared to be related to the pH of the growth medium. A. salmonicida and A. hydrophila also secreted native E. coli alkaline phosphatase, but A. hydrophila strains with mutations in the general secretion pathway were unable to release the enzyme. We conclude that the Aeromonas secretion system can recognize the E. coli enzyme as an extracellular protein and direct it outside the cell.

Dimerization stabilizes the pore-forming toxin aerolysin in solution

Aerolysin is a channel-forming protein secreted as a protoxin by Aeromonas hydrophila. Analytical centrifugation measurements showed that proaerolysin is a dimer in solution, and this was confirmed by chemical cross-linking with dimethyl suberimidate. Dissociation of proaerolysin with low concentrations of SDS resulted in the loss of tertiary structure, assessed by near ultraviolet circular dichroism. This was accompanied by an increase in the protein's ability to bind the hydrophobic dye 1-anilino-8-naphthalene sulfonate, as well as by increased sensitivity to proteolytic degradation. However, the monomer was not fully unfolded by the detergent, as the tryptophans remained in a hydrophobic environment, and the secondary structure measured by far ultraviolet circular dichroism did not seem to be affected. Aerolysin, the active form of the protein, was also shown to be a dimer, and its stability was found to be no different from the stability of the protoxin dimer. Substituting tryptophan 371 or tryptophan 373 with leucine greatly reduced the stability of dimeric proaerolysin. These substitutions are known to increase the protein's ability to oligomerize, supporting the conclusion that dimer dissociation is necessary for oligomerization to occur.

Oligomerization of the channel-forming toxin aerolysin precedes insertion into lipid bilayers

Oligomerization is a necessary step in channel formation by the bacterial toxin aerolysin. We have identified a region of aerolysin containing two tryptophans which influence the ability of the protein to oligomerize. Changing the tryptophan at position 371 or 373 to leucine resulted in mutant proteins that oligomerized at much lower concentrations than the wild-type toxin. Near-ultraviolet circular dichroism measurements showed that the tertiary structures of the L-371 and L-373 mutant toxins may be slightly different from the structure of wild type. Other single amino acid replacements in the same region of the protein as the two tryptophans appeared to have little or no effect on any properties of the protein. None of the changes we made had any measured effect on secretion of the protein by the bacteria. The L-373 and L-371 proteins induced chloride release from liposomes at lower concentrations than native toxin. Wild-type aerolysin solutions were completely unable to cause release when oligomeric toxin was absent or when it was removed by centrifugation. Aerolysin changed at H-132, which cannot form oligomers, was also inactive against liposomes. We conclude that aerolysin channels are produced by direct insertion of oligomers formed in solution, or assembled on the surface of the cell after binding to the receptor, and not by lateral diffusion of the monomer after it enters the lipid bilayer.

Purification of Aeromonas hydrophila major outer-membrane proteins: N-terminal sequence analysis and channel-forming properties

Four outer-membrane proteins of Aeromonas hydrophila were purified and their N-terminal sequences and channel-forming properties were determined. Three could be matched with proteins from other species. One was a maltoporin, as its level increased when cells were grown in maltose-containing media, and the channel it formed was blocked by maltose. Another was like OmpF and OmpC of Escherichia coli, except that its channel fluctuated much more rapidly. The third protein, which was produced in low-phosphate medium, exhibited several properties of the general anion porin PhoE. The fourth showed no similarity to any known proteins. It had a unique N-terminus and it formed small sharply-defined cation-selective channels. Two other proteins which corresponded to OmpW of Vibrio cholerae and E. coli OmpA were partly characterized.

Spectroscopic study of the activation and oligomerization of the channel-forming toxin aerolysin: identification of the site of proteolytic activation

The channel-forming protein aerolysin is secreted as a protoxin which can be activated by proteolytic removal of a C-terminal peptide. The activation and subsequent oligomerization of aerolysin were studied using a variety of spectroscopic techniques. Mass spectrometric determination of the molecular weights of proaerolysin and aerolysin permitted identification of the sites at which the protoxin is processed by trypsin and chymotrypsin. The results of far- and near-UV circular dichroism measurements indicated that processing with trypsin does not lead to major changes in secondary or tertiary structure of the protein. An increase in tryptophan fluorescence intensity and a small red shift in the maximum emission wavelength of tryptophans could be observed, suggesting that there is a change in the environment of some of the tryptophans. There was also a dramatic increase in the binding of the hydrophobic fluorescent probe 1-anilino-8-naphthalenesulfonate during activation, leading us to conclude that a hydrophobic region in the protein is exposed by trypsin treatment. Using measurements of light scattering, various parameters influencing oligomerisation of trypsin-activated aerolysin were determined. Oligomerization rates were found to increase with the concentration of aerolysin, whereas they decreased with increasing ionic strength.

The channel-forming toxin aerolysin

Aeromonas sp. secrete a precursor of the cytolytic protein aerolysin into the culture medium, where it is activated by proteolytic removal of a C-terminal fragment. Activation can be achieved by a variety of mammalian proteases as well as by proteases released by the bacteria itself. Activated toxin binds with high affinity to the transmembrane protein glycophorin on the surface of eucaryotic cells. Binding is followed by oligomerization and the formation of transmembrane channels, leading to cell death. Using chemical modification and site-directed mutagenesis, we have identified regions of the molecule which are important in transfer across the outer membrane of the bacteria, and in proteolytic activation, binding, and oligomerization. A preliminary electron density map of proaerolysin crystals indicates that the protein is organized into three domains. Analysis of two-dimensional crystals of aerolysin suggests that the oligomeric form of the protein is heptameric.

Crossing three membranes. Channel formation by aerolysin

Aerolysin is a channel-forming toxin responsible for the pathogenicity of Aeromonas hydrophila. It crosses the inner and outer membranes of the bacteria in separate steps and is released as a 52-kDa inactive protoxin which is activated by proteolytic removal of approximately 40 amino acids from the C terminus. The toxin binds to the erythrocyte transmembrane protein glycophorin and oligomerizes before inserting into the membrane, producing a voltage gated, anion selective channel about 1 nm in diameter. Remarkably, proaerolysin appears to be dimeric, whereas the oligomer is a heptamer. Using chemical modification and site-directed mutagenesis, we have identified some of the regions of the molecule which appear to be involved in secretion and in channel formation.

The aerolysin membrane channel is formed by heptamerization of the monomer

The cytolytic toxin aerolysin has been found to form heptameric oligomers by SDS-PAGE electrophoresis, STEM mass measurements of single oligomers and image analysis of two-dimensional membrane crystals. Two types of crystal, flat sheets and long regular tubes, have been obtained by reconstitution of purified protein and Escherichia coli phospholipids. A noise-filtered image of the best crystalline sheets reveals a structure with 7-fold symmetry containing a central strongly stain-excluding ring that encircles a dark stain-filled channel 17 A in diameter. The ring is surrounded by seven arms each made up of two unequal sized domains. By combining projected views and side-views, a simplified model of the aerolysin channel complex has been constructed. The relevance of this structure to the mode of action of aerolysin is discussed.

Stereochemical and positional specificity of the lipase/acyltransferase produced by Aeromonas hydrophila

Aeromonas species secrete a glycerophospholipid-cholesterol acyltransferase (GCAT) which shares many properties with mammalian plasma lecithin-cholesterol acetyltransferase (LCAT). We have studied the stereochemical and positional specificity of GCAT against a variety of lipid substrates using NMR spectroscopy as well as other assay methods. The results show that both the primary and secondary acyl ester bonds of L-phosphatidylcholine can be hydrolyzed but only the sn-2 fatty acid can be transferred to cholesterol. The enzyme has an absolute requirement for the L configuration at the sn-2 position of phosphatidylcholine. The secondary ester bond of D-phosphatidylcholine cannot be hydrolyzed, and this lipid is not a substrate for acyl transfer. In contrast to the phospholipases, but similar to LCAT, the enzyme does not interact stereochemically with the phosphorus of phosphatidylcholine. In fact, the phosphorus is not required for enzyme activity, as GCAT will also hydrolyze monolayers of diglyceride, although at much lower rates.