Binding and aggregation of the 25-kilodalton toxin of Bacillus thuringiensis subsp. israelensis to cell membranes and alteration by monoclonal antibodies and amino acid modifiers

Bacillus thuringiensis toxin Cyt2Aa forms filamentous oligomers when exposed to lipid membranes or detergents

The bacterium Bacillus thuringiensis (Bt) produces insecticidal proteins during the sporulation phase. These proteins are located in parasporal crystals consisting of two delta-endotoxin classes, crystal (Cry) and cytolytic (Cyt) toxins. In vitro, Cyt toxins show cytolytic activity against bacterial and a variety of insect and mammalian cells. They bind to cell membranes with unsaturated phospholipids and sphingomyelin. Although Bt and its parasporal crystals containing both Cry and Cyt toxins have been successfully used as bioinsecticides, the molecular mechanism of action of Cyt toxins is not yet fully understood. To address this, we exposed Cyt2Aa to lipid membranes and visualized membrane disruption process using cryo-electron microscopy. We observed two types of Cyt2Aa oligomers. First, Cyt2Aa forms smaller curved oligomers on the membrane surface that become linear over time, and detach when the membrane ruptures. Similar linear filamentous oligomers were also formed by Cyt2Aa in the presence of detergents without prior exposure to lipid membranes, which exhibited attenuated cytolytic activity. Furthermore, our data suggest that Cyt2Aa adopts different conformations between its monomeric and oligomeric forms. Overall, our results provide new evidence for a detergent-like mechanism of action of Cyt2Aa rather than the pore-forming model of target membrane disruption of this important class of insecticidal proteins.

The Cyt1Aa toxin from Bacillus thuringiensis inserts into target membranes via different mechanisms in insects, red blood cells, and lipid liposomes

Bacillus thuringiensis subsp. israelensis produces crystal inclusions composed of three-domain Cry proteins and cytolytic Cyt toxins, which are toxic to different mosquito larvae. A key component is the Cyt toxin, which synergizes the activity of the other Cry toxins, thereby resulting in high toxicity. The precise mechanism of action of Cyt toxins is still debated, and two models have been proposed: the pore formation model and the detergent effect. Here, we performed a systematic structural characterization of the Cyt toxin interaction with different membranes, including in Aedes aegypti larval brush border membrane vesicles, small unilamellar vesicle liposomes, and rabbit erythrocytes. We examined Cyt1Aa insertion into these membranes by analyzing fluorescence quenching in solution and in the membrane-bound state. For this purpose, we constructed several Cyt1Aa variants having substitutions with a single cysteine residue in different secondary structures, enabling Cys labeling with Alexa Fluor 488 for quenching analysis using I-soluble quencher in solution and in the membrane-bound state. We identified the Cyt1Aa residues exposed to the solvent upon membrane insertion, predicting a possible topology of the membrane-inserted toxin in the different membranes. Moreover, toxicity assays with these variants revealed that Cyt1Aa exerts its insecticidal activity and hemolysis through different mechanisms. We found that Cyt1Aa exhibits variable interactions with each membrane system, with deeper insertion into mosquito larva membranes, supporting the pore formation model, whereas in the case of erythrocytes and small unilamellar vesicles, Cyt1Aa's insertion was more superficial, supporting the notion that a detergent effect underlies its hemolytic activity.

Bacillus thuringiensis Cyt2Aa2 binding on lipid/cholesterol bilayer depends on protein concentration and time

Bacillus thuringiensis produces cytolytic proteins (Cyt) that show toxicity against dipteran insect larvae acting directly on the cell membrane. Up to now, two different models have been proposed to explain the interaction mechanism of the cytolytic protein Cyt2Aa2 on lipid membranes: pore-forming and detergent-like action. Here we report on the interaction of Cyt2Aa2 with lipid/cholesterol bilayers at early stage (far from equilibrium) as a function of protein concentration. Quartz crystal microbalance with dissipation (QCM-D) measurements showed that the rate of protein adsorption increased with concentration, although the mass of the final protein-lipid was similar after two hours. In addition, the dissipation (compliance of the hybrid lipid/protein layer) increased with decreasing protein concentration. Furthermore, atomic force microscopy (AFM) revealed that the structure of the protein-lipid layer was concentration and time dependent. A rigid hybrid homogeneous layer was observed at protein concentrations of 50 μg/ml and 100 μg/ml after 30 min. At lower concentrations, 10 μg/ml and 17.5 μg/ml, protein adsorption on the lipid layer led to the formation of small aggregates. Interestingly, at 25 μg/ml a transition of a hole-like structure into a homogeneous layer was observed. This suggests that 25 μg/ml is a threshold concentration for the binding mechanism of Cyt2Aa2 on to lipid membranes.

Bacillus thuringiensis Cyt2Aa2 toxin disrupts cell membranes by forming large protein aggregates

We show that the lipid membrane disruption by Bacillus thuringiensis (Bt) Cyt2Aa2 is different from the general pore-forming model. Cyt2Aa2 forms protein aggregates that disrupt the lipid membrane integrity.

Bacillus thuringiensis (Bt) Cyt2Aa2 showed toxicity against Dipteran insect larvae and in vitro lysis activity on several cells. It has potential applications in the biological control of insect larvae. Although pore-forming and/or detergent-like mechanisms were proposed, the mechanism underlying cytolytic activity remains unclear. Analysis of the haemolytic activity of Cyt2Aa2 with osmotic stabilizers revealed partial toxin inhibition, suggesting a distinctive mechanism from the putative pore formation model. Membrane permeability was studied using fluorescent dye entrapped in large unilamellar vesicles (LUVs) at various protein/lipid molar ratios. Binding of Cyt2Aa2 monomer to the lipid membrane did not disturb membrane integrity until the critical protein/lipid molar ratio was reached, when Cyt2Aa2 complexes and cytolytic activity were detected. The complexes are large aggregates that appeared as a ladder when separated by agarose gel electrophoresis. Interaction of Cyt2Aa2 with Aedes albopictus cells was investigated by confocal microscopy and total internal reflection fluorescent microscopy (TIRF). The results showed that Cyt2Aa2 binds on the cell membrane at an early stage without cell membrane disruption. Protein aggregation on the cell membrane was detected later which coincided with cell swelling. Cyt2Aa2 aggregations on supported lipid bilayers (SLBs) were visualized by AFM. The AFM topographic images revealed Cyt2Aa2 aggregates on the lipid bilayer at low protein concentration and subsequently disrupts the lipid bilayer by forming a lesion as the protein concentration increased. These results supported the mechanism whereby Cyt2Aa2 binds and aggregates on the lipid membrane leading to the formation of non-specific hole and disruption of the cell membrane.

Membrane binding and oligomer membrane insertion are necessary but insufficient for Bacillus thuringiensis Cyt1Aa toxicity

Bacillus thuringiensis Cyt proteins are pore-forming toxins that have insecticidal activity mainly against dipteran insects. However, certain Cyt proteins have toxicity to some insect orders, but not toxicity of Cyt1Aa against lepidopteran larvae has been found. Insect specificity has been proposed to rely in specific binding to certain lipids on the brush border membrane of midgut cells since no protein receptors have been described so far. To determine the molecular basis of Cyt1Aa insect specificity we compared different steps of Cyt1Aa mode of action in a susceptible insect as the dipteran Aedes aegypti and also in the non-susceptible lepidopteran Manduca sexta. Our data shows that the lack toxicity of Cyt1Aa to M. sexta larvae does not rely on protoxin processing, membrane binding interaction, and oligomerization of Cyt1Aa since these steps were similar in the two insect species analyzed.

Oligomerization is a key step in Cyt1Aa membrane insertion and toxicity but not necessary to synergize Cry11Aa toxicity in Aedes aegypti larvae

Bacillus thuringiensis produces insecticidal Cry and Cyt proteins that are toxic to different insect orders. In addition, Cyt toxins also display haemolytic activity. Both toxins are pore-forming proteins that form oligomeric structures that insert into the target membrane to lyse cells. Cyt toxins play an important role in mosquitocidal activity since they synergize Cry toxins and are able to overcome resistance to Cry toxins. Cry and Cyt toxins interact by specific epitopes, and this interaction is important to induce the synergistic activity observed. It was proposed that Cyt toxins do not interact with protein receptors but directly interacting with the specific midgut cell lipids. Here, we analysed if oligomerization and membrane insertion of Cyt1Aa are necessary steps to synergize Cry11Aa toxicity. We characterized Cyt1Aa helix α-C mutants that were affected in oligomerization, in membrane insertion and also in haemolytic and insecticidal activities. However, these mutants were still able to synergize Cry11Aa toxicity indicating these steps are independent events of Cyt1Aa synergistic activity. Furthermore, the data indicate that formation of stable Cyt1Aa-oligomeric structure is a key step for membrane insertion, haemolysis and insecticidal activity.

Cyt toxins produced by Bacillus thuringiensis: a protein fold conserved in several pathogenic microorganisms

Bacillus thuringiensis bacteria produce different insecticidal proteins known as Cry and Cyt toxins. Among them the Cyt toxins represent a special and interesting group of proteins. Cyt toxins are able to affect insect midgut cells but also are able to increase the insecticidal damage of certain Cry toxins. Furthermore, the Cyt toxins are able to overcome resistance to Cry toxins in mosquitoes. There is an increasing potential for the use of Cyt toxins in insect control. However, we still need to learn more about its mechanism of action in order to define it at the molecular level. In this review we summarize important aspects of Cyt toxins produced by Bacillus thuringiensis, including current knowledge of their mechanism of action against mosquitoes and also we will present a primary sequence and structural comparison with related proteins found in other pathogenic bacteria and fungus that may indicate that Cyt toxins have been selected by several pathogenic organisms to exert their virulence phenotypes.

The amino- and carboxyl-terminal fragments of the Bacillus thuringensis Cyt1Aa toxin have differential roles in toxin oligomerization and pore formation

The Cyt toxins produced by the bacteria Bacillus thuringiensis show insecticidal activity against some insects, mainly dipteran larvae, being able to kill mosquitoes and black flies. However, they also possess a general cytolytic activity in vitro, showing hemolytic activity in red blood cells. These proteins are composed of two outer layers of α-helix hairpins wrapped around a β-sheet. With regard to their mode of action, one model proposed that the two outer layers of α-helix hairpins swing away from the β-sheet, allowing insertion of β-strands into the membrane forming a pore after toxin oligomerization. The other model suggested a detergent-like mechanism of action of the toxin on the surface of the lipid bilayer. In this work, we cloned the N- and C-terminal domains form Cyt1Aa and analyzed their effects on Cyt1Aa toxin action. The N-terminal domain shows a dominant negative phenotype inhibiting the in vitro hemolytic activity of Cyt1Aa in red blood cells and the in vivo insecticidal activity of Cyt1Aa against Aedes aegypti larvae. In addition, the N-terminal region is able to induce aggregation of the Cyt1Aa toxin in solution. Finally, the C-terminal domain composed mainly of β-strands is able to bind to the SUV liposomes, suggesting that this region of the toxin is involved in membrane interaction. Overall, our data indicate that the two isolated domains of Cyt1Aa have different roles in toxin action. The N-terminal region is involved in toxin aggregation, while the C-terminal domain is involved in the interaction of the toxin with the lipid membrane.

Amino acid substitutions in alphaA and alphaC of Cyt2Aa2 alter hemolytic activity and mosquito-larvicidal specificity

Cyt2Aa2 produced by Bacillus thuringiensis subsp. darmstadiensis exhibits in vitro cytolytic activity against broad range of cells but shows specific in vivo toxicity against larvae of Dipteran insects. To investigate the role of amino acids in alphaA and alphaC of this toxin, 3 single-point mutants (A61C, S108C and V109A) were generated. All 3 mutant proteins were highly produced as inclusion bodies that could be solubilized and activated by proteinase K similar to that of the wild type. Hemolytic activity of A61C and S108C mutants was significantly reduced whereas the V109A mutant showed comparable hemolytic activity to the wild type. Interestingly, the A61C mutant exhibited high larvicidal activity to both Aedes aegypti and Culex quinquefasciatus. S108C and V109A mutants showed low activity against C. quinquefasciatus but relatively high toxicity to A. aegypti. These results demonstrated for the first time that amino acids in alphaA and alphaC are involved in the selectivity of the Cyt toxin to the targeted organism.

Binding of Cyt1Aa and Cry11Aa toxins of Bacillus thuringiensis serovar israelensis to brush border membrane vesicles of Tipula paludosa (Diptera: Nematocera) and subsequent pore formation

Bacillus thuringiensis serovar israelensis (B. thuringiensis subsp. israelensis) produces four insecticidal crystal proteins (ICPs) (Cry4A, Cry4B, Cry11A, and Cyt1A). Toxicity of recombinant B. thuringiensis subsp. israelensis strains expressing only one of the toxins was determined with first instars of Tipula paludosa (Diptera: Nematocera). Cyt1A was the most toxic protein, whereas Cry4A, Cry4B, and Cry11A were virtually nontoxic. Synergistic effects were recorded when Cry4A and/or Cry4B was combined with Cyt1A but not with Cry11A. The binding and pore formation are key steps in the mode of action of B. thuringiensis subsp. israelensis ICPs. Binding and pore-forming activity of Cry11Aa, which is the most toxic protein against mosquitoes, and Cyt1Aa to brush border membrane vesicles (BBMVs) of T. paludosa were analyzed. Solubilization of Cry11Aa resulted in two fragments, with apparent molecular masses of 32 and 36 kDa. No binding of the 36-kDa fragment to T. paludosa BBMVs was detected, whereas the 32-kDa fragment bound to T. paludosa BBMVs. Only a partial reduction of binding of this fragment was observed in competition experiments, indicating a low specificity of the binding. In contrast to results for mosquitoes, the Cyt1Aa protein bound specifically to the BBMVs of T. paludosa, suggesting an insecticidal mechanism based on a receptor-mediated action, as described for Cry proteins. Cry11Aa and Cyt1Aa toxins were both able to produce pores in T. paludosa BBMVs. Protease treatment with trypsin and proteinase K, previously reported to activate Cry11Aa and Cyt1Aa toxins, respectively, had the opposite effect. A higher efficiency in pore formation was observed when Cyt1A was proteinase K treated, while the activity of trypsin-treated Cry11Aa was reduced. Results on binding and pore formation are consistent with results on ICP toxicity and synergistic effect with Cyt1Aa in T. paludosa.

Structure-function relationships of a membrane pore forming toxin revealed by reversion mutagenesis

Cyt2Aa1 is a haemolytic membrane pore forming toxin produced by Bacillus thuringiensis subsp. kyushuensis. To investigate membrane pore formation by this toxin, second-site revertants of an inactive mutant toxin Cyt2Aa1-I150A were generated by random mutagenesis using error-prone PCR. The decrease in side chain length caused by the replacement of isoleucine by alanine at position 150 in the alphaD-beta4 loop results in the loss of important van der Waals contacts that exist in the native protein between I150 and K199 and L203 on alphaE. 28 independent revertants of I150A were obtained and their relative toxicity can be explained by the position of the residue in the structure and the effect of the mutation on side-chain interactions. Analysis of these revertants revealed that residues on alphaA, alphaB, alphaC, alphaD and the loops between alphaA and alphaB, alphaD and beta5, beta6 and beta7 are important in pore formation. These residues are on the surface of the molecule suggesting that they may participate in membrane binding and toxin oligomerization. Changing the properties of the amino acid side-chains of these residues could affect the conformational changes required to transform the water-soluble toxin into the membrane insertion competent state.

Cyt2Ba of Bacillus thuringiensis israelensis: activation by putative endogenous protease

The gene cyt2Ba of Bacillus thuringiensis subsp. israelensis was cloned for expression, together with p20, in an acrystalliferous strain. The large hexagonal crystals formed were composed of Cyt2Ba, which facilitated its purification. Crystal solubilization in the presence of endogenous proteases (with spores and cell debris) enabled quick and simple procedure to obtain rather pure and active toxin species by cleavage between amino acid residues 34 and 35, most likely by a camelysin-like protease that was discovered in association with activated Cyt2Ba. The product of this cleavage displayed haemolytic activity comparable to that of exogenously activated Cyt2Ba. The sequence of this putative protease shares high homology with the cell envelope-bound metalloprotease (camelysin) of the closely related species Bacillus cereus.

Partial restoration of antibacterial activity of the protein encoded by a cryptic open reading frame (cyt1Ca) from Bacillus thuringiensis subsp. israelensis by site-directed mutagenesis

Insecticidal crystal proteins of Bacillus thuringiensis belong to two unrelated toxin families: receptor-specific Cry toxins against insects and Cyt toxins that lyse a broad range of cells, including bacteria, via direct binding to phospholipids. A new cyt-like open reading frame (cyt1Ca) encoding a 60-kDa protein, has recently been discovered (C. Berry et al., Appl. Environ. Microbiol. 68:5082-5095, 2002). Cyt1Ca displays the structure of a two-domain fusion protein: the N-terminal moiety resembles the full-length Cyt toxins, and the C-terminal moiety is similar to the receptor-binding domains of several ricin-like toxins, such as Mtx1. Neither the larvicidal activity of cyt1Ca expressed in Escherichia coli nor the hemolytic effect of His-tagged purified Cyt1Ca has been observed (R. Manasherob et al., unpublished). This was attributed to five amino acid differences between the sequences of its N-terminal moiety and Cyt1Aa. The 3' end of cyt1Ca was truncated (removing the ricin-binding domain of Cyt1Ca), and six single bases were appropriately changed by site-directed mutagenesis, sequentially replacing the non-charged amino acids by charged ones, according to Cyt1Aa, to form several versions. Expression of these mutated cyt1Ca versions caused loss of the colony-forming ability of the corresponding E. coli cells to different extents compared with the original gene. In some mutants this antibacterial effect was associated by significant distortion of cell morphology and in others by generation of multiple inclusion bodies spread along the cell envelope. The described deleterious effects of mutated cyt1Ca versions against E. coli may reflect an evolutionary relationship between Cyt1Aa and Cyt1Ca.

Crystal structures and electron micrographs of fungal volvatoxin A2

Membrane adhesion and insertion of protein are essential to all organisms, but the underlying mechanisms remain largely unknown. Membrane pore-forming toxins (PFTs) are potential model systems for studying these mechanisms. We have determined the crystal structures of volvatoxin A2 (VVA2), a fungal PFT from Volvariella volvacea, using Br-multiple-wavelength anomalous diffraction (MAD). The VVA2 structures obtained at pH 4.6, pH 5.5 and pH 6.5 were refined to resolutions of 1.42 A, 2.6 A and 3.2 A, respectively. The structures reveal that the VVA2 monomer contains a single alpha/beta domain. Most of the VVA2 surface is occupied by its oligomerization motif and two putative heparin-binding motifs. Residues Ala91 to Ala101 display several conformations at different pH values, which might be under the control of His87. We also found that the shape of one putative heparin-binding motif in VVA2 appears similar to those found in fibroblast growth factors, and the other one displays a linear polypeptide. Our results suggest several possible intermediates of protein assembly in solution and protein adhering to cell membranes before conformational changes. The electron micrographs of VVA2 molecules in solution, at a protein concentration of 1 microg ml(-1), show that they can assemble into filament-like or braid-like oligomers in a pH-dependent way. In addition, the arc-shaped VVA2 structure obtained at pH 6.5 suggests that VVA2 could form a two-layered helical oligomer with 18 subunits per turn. The structures presented here could be used to elucidate the pore-formation mechanisms of VVA2 and its structural neighbors, Cyt toxins from Bacillus thuringiensis.

Trp132, Trp154, and Trp157 are essential for folding and activity of a Cyt toxin from Bacillus thuringiensis

Cyt2Aa2 is a cytolytic and mosquito larvicidal toxin produced by Bacillus thuringiensis subsp. darmstadiensis. The toxin contains 3 tryptophan residues at positions 132, 154, and 157. To study the role of tryptophan on protein structure and functions, each tryptophan residue was substituted by phenylalanine and other different amino acids. Expression test in Escherichia coli showed that all mutant proteins were highly produced as inclusion bodies similar to that of the wild type. The mutant W157F showed haemolytic and mosquito larvicidal activities comparable to the wild type but the mutant W157V and all other mutants at positions 132 and 154 have completely lost these activities. Solubilization and proteinase K activation tests indicated that aromatic residue is required at position 157 and tryptophan residues at positions 132 and 154 are critical residues playing important role to maintain structure and functions of the protein and cannot be changed to any other amino acid.

Interaction of cytolytic toxin CytB with a supported lipid bilayer: study using an acoustic wave device

An acoustic technique was used to monitor the interaction of the pore-forming cytolytic toxin CytB with a positively charged supported lipid bilayer. The acoustic device, which is based on a waveguide geometry, is sensitive to changes in the mass of the supported bilayer. The specificity of the interaction, rate and extent of the association, reversibility and effect of previous depositions of toxin were investigated. The CytB was found to bind irreversibly to the lipids at all fractional coverages even when the protein-to-lipid ratio was high enough to imply that the protein was associating with the external surface of the bilayer. The CytB formed stable structures with the bilayer at high protein surface concentrations and did not appear to disrupt the bilayer in the manner of a detergent. The rate of association with the bilayer was found to be directly proportional to the solution concentration of CytB at higher concentrations but appeared to be low at a CytB solution concentration of 5 microg mL(-1), leading to relatively low amounts of CytB being associated with the bilayer.

Investigation of the pore-forming mechanism of a cytolytic delta-endotoxin from Bacillus thuringiensis

Cyt2Aa1 is a cytolytic protein produced by Bacillus thuringiensis subsp. kyushuensis. Penetration of the toxin into membranes has been studied to learn more about membrane-insertion mechanisms and transmembrane-pore formation. The haemolysis assay of Cyt2Aa1 showed a steep and sigmoidal dose-response curve, indicating that toxin aggregation or oligomerization is required for pore formation. Studies of the effect of temperature on pore formation and fluorimetric studies of acrylodan-labelled toxin suggest that toxin inserts into the membrane before oligomerizing to form a pore. Low temperature neither inhibited membrane binding nor closed pores that have been formed, but markedly inhibited oligomerization of the toxin molecules. When toxin-treated red blood cells at 4 degrees C were transferred to a toxin-free solution at 37 degrees C, no significant increase in haemolysis was observed. This result suggests that membrane-bound toxin could not diffuse laterally and interact with other molecules to form a pore. From these results, we propose that Cyt2Aa1 binds and inserts into the membrane as a monomer. Oligomerization occurs when toxin molecules have bound in close proximity to each other and pores are formed from large oligomers.

Aggregation of bacillus thuringiensis Cry1A toxins upon binding to target insect larval midgut vesicles

During sporulation, Bacillus thuringiensis produces crystalline inclusions comprised of a mixture of delta-endotoxins. Following ingestion by insect larvae, these inclusion proteins are solubilized, and the protoxins are converted to toxins. These bind specifically to receptors on the surfaces of midgut apical cells and are then incorporated into the membrane to form ion channels. The steps required for toxin insertion into the membrane and possible oligomerization to form a channel have been examined. When bound to vesicles from the midguts of Manduca sexta larvae, the Cry1Ac toxin was largely resistant to digestion with protease K. Only about 60 amino acids were removed from the Cry1Ac amino terminus, which included primarily helix alpha1. Following incubation of the Cry1Ab or Cry1Ac toxins with vesicles, the preparations were solubilized by relatively mild conditions, and the toxin antigens were analyzed by immunoblotting. In both cases, most of the toxin formed a large, antigenic aggregate of ca. 200 kDa. These toxin aggregates did not include the toxin receptor aminopeptidase N, but interactions with other vesicle components were not excluded. No oligomerization occurred when inactive toxins with mutations in amphipathic helices (alpha5) and known to insert into the membrane were tested. Active toxins with other mutations in this helix did form oligomers. There was one exception; a very active helix alpha5 mutant toxin bound very well to membranes, but no oligomers were detected. Toxins with mutations in the loop connecting helices alpha2 and alpha3, which affected the irreversible binding to vesicles, also did not oligomerize. There was a greater extent of oligomerization of the Cry1Ac toxin with vesicles from the Heliothis virescens midgut than with those from the M. sexta midgut, which correlated with observed differences in toxicity. Tight binding of virtually the entire toxin molecule to the membrane and the subsequent oligomerization are both important steps in toxicity.

Bacillus thuringiensis and its pesticidal crystal proteins

During the past decade the pesticidal bacterium Bacillus thuringiensis has been the subject of intensive research. These efforts have yielded considerable data about the complex relationships between the structure, mechanism of action, and genetics of the organism's pesticidal crystal proteins, and a coherent picture of these relationships is beginning to emerge. Other studies have focused on the ecological role of the B. thuringiensis crystal proteins, their performance in agricultural and other natural settings, and the evolution of resistance mechanisms in target pests. Armed with this knowledge base and with the tools of modern biotechnology, researchers are now reporting promising results in engineering more-useful toxins and formulations, in creating transgenic plants that express pesticidal activity, and in constructing integrated management strategies to insure that these products are utilized with maximum efficiency and benefit.

Cloning and characterization of a cytolytic and mosquitocidal delta-endotoxin from Bacillus thuringiensis subsp. jegathesan

A cytolytic toxin gene encoding a 30.1-kDa Cyt2Bb1 toxin protein from B. thuringiensis subsp. jegathasan was cloned employing a limited-growth PCR screening method with forward and reverse oligonucleotide primers designed from N-terminal amino acid sequences of native and trypsin-cleaved protein, respectively. The expressed protein showed little cross-reactivity to the antibody raised against the Cyt1Aa protein. Unlike Cyt1Aa and Cyt2Aa expression, there was little or no visible crystal inclusion formation under microscopic observation. The amino acid sequence alignment indicated 31 and 66% identity to Cyt1Aa and Cyt2Aa, respectively. The sequence alignment for five known cytolytic proteins indicated three highly conserved regions, two in the loop regions between alpha-helices and beta-sheets and one in the loop region between beta-sheets 5 and 6. beta-Blocks 4 to 7 are also conserved, not only structurally but also among the amino acids in the hydrophobic faces. Mosquitocidal activity assays indicated that the Cyt2Bb toxin had less toxicity than Cyt1Aa and had about 600-times-lower toxicity than the wild-type whole toxin crystal. However, both the Cyt2Bb and the Cyt1Aa toxin showed comparable levels of hemolytic activity.

Membrane insertion: The strategies of toxins (review)

Protein toxins are soluble molecules secreted by pathogenic bacteria which act at the plasma membrane or in the cytoplasm of target cells. They must therefore interact with a membrane at some point, either to modify its permeability properties or to reach the cytoplasm. As a consequence, toxins have the built-in capacity to adopt two generally incompatible states: water-soluble and transmembrane. Irrespective of their origin or function, the membrane interacting domain of most protein toxins seems to have adopted one out of two structural strategies to be able to undergo this metamorphosis. In the first group of toxins the membrane interacting domain has the structural characteristics of most known membrane proteins, i.e. it contains hydrophobic and amphipathic alpha-helices long enough to span a membrane. To render this 'membrane protein' water-soluble during the initial part of its life the hydrophobic helices are sheltered from the solvent by a barrel of amphipathic helices. In the second group of toxins the opposite strategy is adopted. The toxin is an intrinsically soluble protein and is composed mainly of beta-structure. These toxins manage to become membrane proteins by oligomerizing in order to combine amphipathic beta-sheet to generate sufficient hydrophobicity for membrane insertion to occur. Toxins from this latter group are thought to perforate the lipid bilayer as a beta-barrel such as has been described for bacterial porins, and has recently been shown for staphylococcal alpha-toxin. The two groups of toxins will be described in detail through the presentation of examples. Particular attention will be given to the beta-structure toxins, since four new structures have been solved over the past year: the staphyloccocal alpha-toxin channel, the anthrax protective antigen protoxin, the anthrax protective antigen-soluble heptamer and the CytB protoxin. Structural similarities with mammalian proteins implicated in the immune response and apoptosis will be discussed. Peptide toxins will not be covered in this review.

Cell targeting of a pore-forming toxin, CytA delta-endotoxin from Bacillus thuringiensis subspecies israelensis, by conjugating CytA with anti-Thy 1 monoclonal antibodies and insulin

The cytolytic protein toxin CytA was linked to two monoclonal antibodies (mAb) directed against the mouse or the rat Thy 1 antigen. The purified CytA-mAb conjugates were not toxic to either target or nontarget cells. The conjugates did bind specifically to target cells since they agglutinated the target cells but not nontarget cells. When the conjugates were treated with dithiothreitol, the released CytA was toxic to all cells tested. These results suggested that the attachment of CytA to a molecule such as the mAb prevented it from forming a pore. Another conjugate was made by linking CytA to insulin. The purified insulin-CytA conjugate bound to and intoxicated cells bearing a high number of insulin receptors. Furthermore, the conjugate was far less toxic to cells expressing a low number of insulin receptors and not toxic to a known CytA target cell line from Aedes aegypti. However, CytA released from the conjugate by reduction was toxic to all cells tested. These results suggested that the cytotoxicity exhibited by CytA in the conjugate form against cells bearing insulin receptors was mediated through insulin and that, in the conjugate form, CytA no longer shows its broad in vitro cytolytic activity. The difference in toxicity between CytA-mAb conjugates and insulin-CytA conjugate is discussed in relation to size of the ligands, the number, distribution, and mobility of the target molecules, and intracellular trafficking.

Identification, isolation, and cloning of a Bacillus thuringiensis CryIAc toxin-binding protein from the midgut of the lepidopteran insect Heliothis virescens

Bacillus thuringiensis toxins are insecticidal to a variety of insect species. The selectivity of the toxins produced by these bacteria is dependent on both the toxin structure and the receptor sites that are present in different insect species. One of these toxins, CryIAc, is highly insecticidal to the noctuid pest Heliothis virescens. Using toxin overlay assay, a 120-kDa glycoprotein was identified as a toxin-binding protein. This protein was partially purified, its N-terminal sequence was determined, and the full-length cDNA encoding this protein was isolated from a H. virescens midgut library. The B. thuringiensis toxin-binding protein, BTBP1, has high homology to aminopeptidase N from eukaryotes and prokaryotes.

Bacillus thuringiensis growth and toxicity. Basic and applied considerations

Despite the known importance of the composition of culture media and culture conditions on Bacillus thuringiensis growth and toxicity, very few reviews are concerned with this subject. This article reviews some aspects of the microbiology of Bacillus thuringiensis, and how toxicity is affected by the composition of growth media and bioreactor operation.

High-level cryIVD and cytA gene expression in Bacillus thuringiensis does not require the 20-kilodalton protein, and the coexpressed gene products are synergistic in their toxicity to mosquitoes

Interactions among the 20-kDa protein gene and the cytA and cryIVD genes located in a 9.4-kb HindIII fragment were studied. A series of plasmids containing a combination of these different genes was constructed by using the Escherichia coli/Bacillus thuringiensis shuttle vector pHT3101. The plasmids were then used to transform an acrystalliferous strain, cryB, derived from B. thuringiensis subsp. kurstaki. The results from sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblot analyses suggest that although the 20-kDa protein is required for the efficient CytA protein production in E. coli, it is not required in B. thuringiensis. With or without the truncated 20-kDa protein gene, the CtyA and/or CryIVD proteins are produced and form parasporal inclusions in B. thuringiensis cells. However, more-efficient expression is obtained when a second protein, probably acting as a chaperonin, is present. In addition, the time course studies show that the CytA and CryIVD proteins are coordinately produced. Both the crude B. thuringiensis culture and purified inclusions from each recombinant B. thuringiensis strain are toxic to Culex quinquefasciatus larvae. The parasporal inclusions formed in B. thuringiensis cells are mosquitocidal, with CytA synergizing CryIVD toxicity.

In vitro and in vivo proteolysis of the Bacillus thuringiensis subsp. israelensis CryIVD protein by Culex quinquefasciatus larval midgut proteases

Proteases with trypsin-, chymotrypsin- and thermolysin-like specificity were detected in Culex quinquefasciatus larval midguts. Their activities were monitored by N-terminal amino acid sequence analysis of the Bacillus thuringiensis subsp. israelensis CryIVD toxin proteolytic fragments. These proteases are located in the larval midgut and in different fractions obtained during the preparation of brush border membrane vesicles. The activity of the midgut proteases increased with an increase in pH. Both the chymotrypsin- and thermolysin-like activities are involved in the processing of solubilized CryIVD toxin, whereas an additional trypsin-like protease is necessary for the CryIVD parasporal inclusion processing. The solubilized CryIVD toxin was first cleaved between Thr347 and Phe348 and between Phe348 and Tyr349, generating a 40-kDa N-terminal fragment and a 32.5-kDa C-terminal fragment. The C-terminal domain was resistant to further processing, with only a small amount of a 31-kDa product appearing due to the action of a thermolysin-like protease. However, the N-terminal domain was very unstable, and was further degraded to about 30 kDa. Unlike the solubilized CryIVD toxin, the processing of the CryIVD parasporal inclusion was very slow at neutral pH. Three protease-resistant products were detected at pHs higher than 9.5 with an overnight incubation at 37 degrees C. The 30- and 28.5-kDa C-terminal peptides are proteolytic products of trypsin- and chymotrypsin-like proteases, respectively; while the 28-kDa N-terminal peptide has 27 amino acids deleted from the N-terminal end by a thermolysin-like protease.

Identification and characterization of Heliothis virescens midgut membrane proteins binding Bacillus thuringiensis delta-endotoxins

To investigate the specificity of Bacillus thuringiensis var. kurstaki strain HD1 insecticidal crystal proteins (ICP), we used membrane preparations obtained from the midgut of Heliothis virescens larvae to perform separate ligand-blot experiments with the three activated CryIA toxins. The CryIA(a) and the CryIA(b) toxins bind the same 170-kDa protein, but most likely at two different binding sites. The CryIA(c) toxin binds two proteins of molecular masses 140 kDa and 120 kDa. We also demonstrate that the binding proteins for each of the B. thuringiensis toxins are not part of a covalent complex. Although the 170-kDa protein is a glycoprotein, endoglycosidase treatment does not prevent the binding of the CryIA(a) or CryIA(b) toxin. This indicates that the sugars are not important for the binding of these toxins. A model for a protein complex binding the B. thuringiensis HD1 ICPs is presented. Our results support the idea that binding proteins on membranes of the gut epithelial cells of H. virescens larvea are important for the specificity of the bacterial toxins.

Characterization of mosquitocidal activity of Bacillus thuringiensis subsp. fukuokaensis crystal proteins

The mosquitocidal crystals of Bacillus thuringiensis subsp. fukuokaensis were isolated and bioassayed against fourth-instar larvae of two mosquito species. The 50% lethal concentration values of the crystals to Aedes aegypti and Culex quinquefasciatus were 4.1 and 2.9 micrograms/ml, respectively. In addition, the solubilized crystals had hemolytic activity; 50 micrograms/ml was the lowest detectable level. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis revealed that the crystals consisted of polypeptides of 90, 86, 82, 72, 50, 48, 37, and 27 kDa. When the solubilized inclusion was treated with C. quinquefasciatus midgut brush border membrane vesicles or Manduca sexta gut juice, only one major protein was detected. This protein retained mosquitocidal activity but had no detectable hemolytic activity. Immunological analysis of this subspecies and the subspecies israelensis, kyushuensis and darmstadiensis by using polyclonal antisera raised against the whole-crystal protein of B. thuringiensis subsp. fukuokaensis revealed that the proteins in subsp. fukuokaensis are distinct from proteins in the other subspecies because little cross-reaction was observed. Analysis of the plasmid pattern showed that the crystal protein genes are located on a plasmid of 130 MDa. Analysis of plasmid and chromosomal DNA from subsp. fukuokaensis showed little homology to the 72-kDa toxin gene (PG-14) of B. thuringiensis subsp. morrisoni. However, some of the proteins of B. thuringiensis subsp. fukuokaensis are homologous to other B. thuringiensis toxins because N-terminal amino acid analysis revealed that the 90-kDa protein is encoded by a cryIV gene type.