Long-Term Results of Organ Preservation in Patients With Rectal Adenocarcinoma Treated With Total Neoadjuvant Therapy: The Randomized Phase II OPRA Trial

Clinical trials frequently include multiple end points that mature at different times. The initial report, typically based on the primary end point, may be published when key planned co-primary or secondary analyses are not yet available. Clinical Trial Updates provide an opportunity to disseminate additional results from studies, published in JCO or elsewhere, for which the primary end point has already been reported.To assess long-term risk of local tumor regrowth, we report updated organ preservation rate and oncologic outcomes of the OPRA trial (ClinicalTrials.gov identifier: NCT02008656). Patients with stage II/III rectal cancer were randomly assigned to receive induction chemotherapy followed by chemoradiation (INCT-CRT) or chemoradiation followed by consolidation chemotherapy (CRT-CNCT). Patients who achieved a complete or near-complete response after finishing treatment were offered watch-and-wait (WW). Total mesorectal excision (TME) was recommended for those who achieved an incomplete response. The primary end point was disease-free survival (DFS). The secondary end point was TME-free survival. In total, 324 patients were randomly assigned (INCT-CRT, n = 158; CRT-CNCT, n = 166). Median follow-up was 5.1 years. The 5-year DFS rates were 71% (95% CI, 64 to 79) and 69% (95% CI, 62 to 77) for INCT-CRT and CRT-CNCT, respectively (P = .68). TME-free survival was 39% (95% CI, 32 to 48) in the INCT-CRT group and 54% (95% CI, 46 to 62) in the CRT-CNCT group (P = .012). Of 81 patients with regrowth, 94% occurred within 2 years and 99% occurred within 3 years. DFS was similar for patients who underwent TME after restaging (64% [95% CI, 53 to 78]) and patients in WW who underwent TME after regrowth (64% [95% CI, 53 to 78]; P = .94). Updated analysis continues to show long-term organ preservation in half of the patients with rectal cancer treated with total neoadjuvant therapy. In patients who enter WW, most cases of tumor regrowth occur in the first 2 years.

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.

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.

Lipids favoring inverted phase enhance the ability of aerolysin to permeabilize liposome bilayers

Channel formation by the bacterial toxin aerolysin follows oligomerization of the protein to produce heptamers that are capable of inserting into lipid bilayers. How insertion occurs is not understood, not only for aerolysin but also for other proteins that can penetrate membranes. We have studied aerolysin channel formation by measuring dye leakage from large unilamellar egg phosphatidylcholine vesicles containing varying amounts of other lipids. The rate of leakage was enhanced in a dose-dependent manner by the presence of phosphatidylethanolamine, diacylglycerol, cholesterol, or hexadecane, all of which are known to favor a lamellar-to-inverted hexagonal (L-H) phase transition. Phosphatidylethanolamine molecular species with low L-H transition temperatures had the largest effects on aerolysin activity. In contrast, the presence in the egg phosphatidylcholine liposomes of lipids that are known to stabilize the lamellar phase, such as sphingomyelin and saturated phosphatidylcholines, reduced the rate of channel formation, as did the presence of lysophosphatidylcholine, which favors positive membrane curvature. When two different lipids that favor hexagonal phase were present with egg PC in the liposomes, their stimulatory effects were additive. Phosphatidylethanolamine and lysophosphatidylcholine canceled each other's effect on channel formation.

Improved detection and characterization of paroxysmal nocturnal hemoglobinuria using fluorescent aerolysin

Paroxysmal nocturnal hemoglobinuria (PNH) is caused by a somatic mutation in the gene PIGA, which encodes an enzyme essential for the synthesis of glycosylphosphatidylinositol (GPI) anchors. The PIGA mutation results in absence or marked deficiency of more than a dozen proteins on PNH blood cells. Current flow cytometric assays for PNH rely on the use of labeled antibodies to detect deficiencies of specific GPI anchor proteins, such as CD59. However, because no single GPI anchor protein is always expressed in all cell lineages, no one monoclonal antibody can be used with confidence to diagnose PNH. We describe a new diagnostic test for PNH, based on the ability of a fluorescently labeled inactive variant of the protein aerolysin (FLAER) to bind selectively to GPI anchors. We compared GPI anchor protein expression in 8 patients with PNH using FLAER and anti-CD59. In all cases, FLAER detected similar or higher proportions of PNH monocytes and granulocytes compared with anti-CD59. Because of the increased sensitivity of detection, FLAER could detect small abnormal granulocyte populations in patients to a level of about 0.5%; samples from healthy control subjects contained substantially fewer FLAER-negative cells. FLAER gives a more accurate assessment of the GPI anchor deficit in PNH.

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.

Clostridium septicum alpha toxin uses glycosylphosphatidylinositol-anchored protein receptors

The alpha toxin produced by Clostridium septicum is a channel-forming protein that is an important contributor to the virulence of the organism. Chinese hamster ovary (CHO) cells are sensitive to low concentrations of the toxin, indicating that they contain toxin receptors. Using retroviral mutagenesis, a mutant CHO line (BAG15) was generated that is resistant to alpha toxin. FACS analysis showed that the mutant cells have lost the ability to bind the toxin, indicating that they lack an alpha toxin receptor. The mutant cells are also resistant to aerolysin, a channel-forming protein secreted by Aeromonas spp., which is structurally and functionally related to alpha toxin and which is known to bind to glycosylphosphatidylinositol (GPI)-anchored proteins, such as Thy-1. We obtained evidence that the BAG15 cells lack N-acetylglucosaminyl-phosphatidylinositol deacetylase-L, needed for the second step in GPI anchor biosynthesis. Several lymphocyte cell lines lacking GPI-anchored proteins were also shown to be less sensitive to alpha toxin. On the other hand, the sensitivity of CHO cells to alpha toxin was increased when the cells were transfected with the GPI-anchored folate receptor. We conclude that alpha toxin, like aerolysin, binds to GPI-anchored protein receptors. Evidence is also presented that the two toxins bind to different subsets of GPI-anchored proteins.

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.

The channel-forming toxin aerolysin neutralizes human immunodeficiency virus type 1

Aerolysin is a channel-forming toxin secreted by Aeromonas spp. that binds to glycosyl phosphatidylinositol (GPI)-anchored proteins, such as Thy-1, on sensitive target cells. Receptor binding is followed first by oligomerization of the toxin and then by insertion of the oligomers into the membrane to form stable channels that disrupt the permeability barrier. Human immunodeficiency virus type 1 (HIV-1) produced from T cells is known to incorporate Thy-1 and other GPI-anchored proteins into its membrane. Here, we show that aerolysin is capable of neutralizing HIV-1 in a dose-dependent manner and that neutralization depends upon the presence of these proteins in the viral envelope. Pretreatment with phosphatidylinositol-specific phospholipase C to remove GPI-anchored proteins greatly reduced HIV-1 sensitivity to the toxin, and virus originating from a mutant cell line that lacks GPI-anchored proteins was not neutralized. Aerolysin variants with single amino acid changes that prevent oligomerization or insertion of the toxin were unable to inactivate the virus, implying that channel formation is necessary for neutralization to occur. These findings represent the first evidence that a pathogenic human virus can be neutralized by a bacterial toxin.

Channels formed by subnanomolar concentrations of the toxin aerolysin trigger apoptosis of T lymphomas

Aerolysin is a channel-forming toxin that binds to glycosylphosphatidylinositol (GPI)-anchored proteins, such as Thy-1, on target cells. Here, we show that subnanomolar concentrations of aerolysin trigger apoptosis of T lymphomas. Using inactive aerolysin variants, we determined that apoptosis was not directly triggered by binding to GPI-anchored receptors, nor was it caused by receptor clustering induced by toxin oligomerization. Apoptosis was caused by the production of a small number of channels in the cell membrane. Channel formation resulted in a rapid increase in intracellular calcium, which may have been the signal for apoptosis. Overexpression of the antiapoptotic protein bcl-2 blocked aerolysin-induced apoptosis, although this effect was overcome at higher toxin concentrations.

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--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.

Glycosylphosphatidylinositol anchors of membrane glycoproteins are binding determinants for the channel-forming toxin aerolysin

Cells that are sensitive to the channel-forming toxin aerolysin contain surface glycoproteins that bind the toxin with high affinity. Here we show that a common feature of aerolysin receptors is the presence of a glycosylphosphatidylinositol anchor, and we present evidence that the anchor itself is an essential part of the toxin binding determinant. The glycosylphosphatidylinositol (GPI)-anchored T-lymphocyte protein Thy-1 is an example of a protein that acts as an aerolysin receptor. This protein retained its ability to bind aerolysin when it was expressed in Chinese hamster ovary cells, but could not bind the toxin when expressed in Escherichia coli, where the GPI anchor is absent. An unrelated GPI-anchored protein, the variant surface glycoprotein of trypanosomes, was shown to bind aerolysin with similar affinity to Thy-1, and this binding ability was significantly reduced when the anchor was removed chemically. Cathepsin D, a protein with no affinity for aerolysin, was converted to an aerolysin binding form when it was expressed as a GPI-anchored hybrid in COS cells. Not all GPI-anchored proteins bind aerolysin. In some cases this may be due to differences in the structure of the anchor itself. Thus the GPI-anchored proteins procyclin of Trypanosoma congolense and gp63 of Leishmania major did not bind aerolysin, but when gp63 was expressed with a mammalian GPI anchor in Chinese hamster ovary cells, it bound the toxin.

Movement of a loop in domain 3 of aerolysin is required for channel formation

Aerolysin is a channel-forming toxin that must oligomerize in order to become insertion-competent. Modeling based on the crystal structure of the proaerolysin dimer and electron microscopic images of the oligomer indicated that a loop in domain 3 must move away from the beta-sheet that forms the main body of the protein before oligomerization can proceed. In order to determine if movement actually occurs, strategically located amino acids in the loop and in the sheet were replaced with cysteines by site-directed mutagenesis. A double mutant was produced in which the new cysteines, at position 253 on the loop and position 300 in the sheet, were close enough together to allow formation of a disulfide bridge. The double mutant was unable to oligomerize, and it was completely inactive, showing not only that the bridge had formed but also that movement of the loop was essential for formation of the oligomer. The existence of the bridge was confirmed by X-ray crystallography. The reduced form of the protein and the single mutants T253C and A300C were as active as wild type, indicating that the amino acid replacements themselves had no functional consequences. Labeling studies using an environment-sensitive fluorescent sulfhydryl-reactive probe confirmed that the structure of the protein changes in the loop region as a consequence of proteolytic activation of proaerolysin, a step which also must precede oligomerization.

Conformational changes in aerolysin during the transition from the water-soluble protoxin to the membrane channel

Proteolytic activation, oligomerization, and membrane insertion are three steps that precede channel formation by the bacterial toxin aerolysin. Using attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) and hydrogen-deuterium exchange, the structural changes associated with each step were analyzed. Our results show that activation induces a significant change in secondary structure, characterized by a decrease in random structure and an increase in beta-sheet content. We show that release of the propeptide is essential for this conformational change to occur and that changes are not restricted to the vicinity of the cleavage site but appear to propagate along the molecule. In contrast, subsequent oligomerization of the mature toxin does not involve any change in overall secondary structure but does involve a modification of the tertiary interactions. Finally, insertion of the heptameric complex into dimyristoylphosphatidylcholine vesicles also occurs without major modification of the secondary structure. Studies on the orientations of the secondary structures of the heptamer in the lipid bilayer have also been performed.

The erythrocyte receptor for the channel-forming toxin aerolysin is a novel glycosylphosphatidylinositol-anchored protein

The plasma membrane of rat erythrocytes contains a 47-kDa glycoprotein that binds the channel-forming toxin aerolysin with high affinity and accounts for the sensitivity of these cells to the toxin. The receptor was purified so that its N-terminal sequence could be determined after Western blotting. The sequence did not match any sequences in the databases, indicating that the receptor is a novel erythrocyte surface protein. However, it exhibited considerable homology to the N-termini of a group of membrane proteins that are thought to be involved in ADP-ribosyl transfer reactions. A common property of these proteins is that they are attached to plasma membranes by C-terminal glycosylphosphatidylinositol (GPI) anchors. The aerolysin receptor was shown to be anchored in the same way by treating rat erythrocytes with phosphatidylinositol-specific phospholipase C. This caused the selective release of the receptor and a reduction in the rodent cells' sensitivity to aerolysin. Human and bovine erythrocytes were shown to contain an aerolysin-binding protein with similar properties to the rat erythrocyte receptor. Proteins with GPI anchors are thought to have unusually high lateral mobility, and this may be an advantage for a toxin, such as aerolysin, which must oligomerize after binding to become insertion competent.

Aerolysin and pertussis toxin share a common receptor-binding domain

We have discovered that the bacterial toxins aerolysin and pertussis toxin share a common domain. This is surprising because the two toxins affect cells in very different ways. The common domain, which we call the APT domain, consists of two three-stranded antiparallel beta-sheets that come together and wrap around a central pair of helices. The APT domain shares a common fold with the C-type lectins and Link modules, and there appears to be a divergent relationship among the three families. One surface region of the APT domain is highly conserved, raising the possibility that the domains have a common function in both proteins. Mutation of one of the conserved surface residues in aerolysin, Tyr61, results in reduced receptor binding and activity, thus providing evidence that the APT domain may be involved in interaction with the toxin's receptor. Structural and biochemical evidence suggests that the APT domain contains a carbohydrate-binding site that can direct the toxins to their target cells.

The glycosylphosphatidylinositol-anchored surface glycoprotein Thy-1 is a receptor for the channel-forming toxin aerolysin

Aerolysin is a channel-forming protein secreted by virulent Aeromonas spp. Some eucaryotic cells, including T-lymphocytes, are sensitive to very low concentrations of the toxin (<10(-9) M). Here we show that aerolysin binds selectively and with high affinity to the glycosylphosphatidylinositol (GPI)-anchored surface protein Thy-1, which is found on T-lymphocyte populations as well as in brain. Less than 1 ng of purified Thy-1 could be detected by probing Western blots with the toxin. Mutant T-cell lines that lack the ability to add GPI anchors to Thy-1 and other surface proteins were much less sensitive to aerolysin, as were wild-type cells that were pretreated with phosphatidylinositol-specific phospholipase C to remove GPI-anchored proteins. Phosphatidylcholine/cholesterol liposomes containing purified Thy-1 in their membranes were much more sensitive to aerolysin than protein-free liposomes.

The disulfide bond in the Aeromonas hydrophila lipase/acyltransferase stabilizes the structure but is not required for secretion or activity

Vibrio and Aeromonas spp. secrete an unusual 35-kDa lipase that shares several properties with mammalian lecithin-cholesterol acyltransferase. The Aeromonas hydrophila lipase contains two cysteine residues that form an intramolecular disulfide bridge. Here we show that changing either of the cysteines to serine does not reduce enzymatic activity, indicating that the disulfide bond is not required for correct folding. However, when either of the cysteines is replaced, the enzyme is more readily denatured by urea and more sensitive to degradation by trypsin than is the wild-type enzyme, evidence that the bridge has an important role in stabilizing the protein's structure. The two mutant proteins with serine-for-cysteine replacements were secreted by Aeromonas salmonicida containing the cloned genes, although the levels of both in the culture supernatants were lower than the level of the wild-type enzyme. When the general secretory pathway was blocked with carbonyl cyanide chlorophenylhydrazone, the cell-associated pools of the mutant enzymes appeared to be degraded, whereas the wild-type pool remained stable. We conclude that reduced extracellular levels of the mutant proteins are the result of their increased sensitivities to proteases encountered inside the cell during export.

Studies on the energetics of proaerolysin secretion across the outer membrane of Aeromonas species. Evidence for a requirement for both the protonmotive force and ATP

Aeromonas spp. secrete the channel-forming protein proaerolysin across their inner and outer membranes in separate steps using the general secretion pathway. Here we show that treating A. hydrophila or A. salmonicida with the protonophore carbonyl cyanide m-chorophenyl hydrazone blocks the second step in transport, secretion across the outer membrane from the periplasm, under conditions where the ATP levels in the cell are no different than the levels in control, secreting cells. A threshold for DeltaPsi was observed in the region of 120 mV, below which secretion by both species was inhibited. Treatment of cells with arsenate, which lowered ATP levels but did not affect DeltaPsi, also reduced secretion from the periplasm, an indication that there is an ATP requirement for this step independent of the requirement for DeltaPsi. Secretion across the outer membrane was also arrested by increasing the osmotic pressure of the medium, even though cellular ATP levels and DeltaPsi were not affected. This may be due to disruption of some necessary association between the inner and outer membranes.

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.