Cytolytic toxin Cyt1A and its mechanism of membrane damage: data and hypotheses

Binding of Bacillus thuringiensis subsp. israelensis Cry4Ba to Cyt1Aa has an important role in synergism

Bacillus thuringiensis subsp. israelensis (Bti) produces at least four different crystal proteins that are specifically toxic to different mosquito species and that belong to two non-related family of toxins, Cry and Cyt named Cry4Aa, Cry4Ba, Cry11Aa and Cyt1Aa. Cyt1Aa enhances the activity of Cry4Aa, Cry4Ba or Cry11Aa and overcomes resistance of Culex quinquefasciatus populations resistant to Cry11Aa, Cry4Aa or Cry4Ba. Cyt1Aa synergized Cry11Aa by their specific interaction since single point mutants on both Cyt1Aa and Cry11Aa that affected their binding interaction affected their synergistic insecticidal activity. In this work we show that Cyt1Aa loop β6-αE K198A, E204A and β7 K225A mutants affected binding and synergism with Cry4Ba. In addition, site directed mutagenesis showed that Cry4Ba domain II loop α-8 is involved in binding and in synergism with Cyt1Aa since Cry4Ba SI303-304AA double mutant showed decreased binding and synergism with Cyt1Aa. These data suggest that similarly to the synergism between Cry11Aa and Cyt1Aa toxins, the Cyt1Aa also functions as a receptor for Cry4Ba explaining the mechanism of synergism between these two Bti toxins.

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.

Persistence of Bacillus thuringiensis israelensis (Bti) in the environment induces resistance to multiple Bti toxins in mosquitoes

BACKGROUND

The simultaneous production of six different toxins by Bacillus thuringiensis israelensis (Bti) is thought to delay the evolution of resistance in treated mosquito populations. Recent studies have shown that Bti can persist and proliferate in the environment, thereby imposing continuous selective pressure on mosquito populations, raising concerns about the long-term effectiveness of this bioinsecticide. In order to evaluate the effect of Bti persistence on the evolution of resistance, the authors selected a laboratory Aedes aegypti L. strain with field-collected leaf litter containing Bti toxins.

RESULTS

It is shown that resistance to each individual Bti toxin (up to 30-fold) can be obtained after only a few generations of selection. However, the resistance to commercial Bti and to environmental Bti remains low (twofold and 3.4-fold respectively) in the selected strain. Furthermore, some selected individuals exhibited resistance to Cry4B but not to Cry4A, suggesting that two distinct resistance mechanisms are involved in the resistance to these two toxins.

CONCLUSION

Considering that resistance to Cry toxins might act as a first step to resistance to a complete Bti toxin mixture, the present results highlight the importance of testing each toxin individually in order accurately to monitor Bti toxin resistance evolution in field populations.

Amino acid substitution on Beta1 and alphaF of Cyt2Aa2 affects molecular interaction of protoxin

Cyt2Aa2 is a mosquito-larvicidal protein produced as a 29 kDa crystalline protoxin from Bacillus thuringiensis subsp. darmstadiensis. To become an active toxin, proteolytic processing is required to remove amino acids from its N- and C-termini. This study aims to investigate the functional role of amino acid residues on the N-terminal Beta1 and C-terminal alphaF of Cyt2Aa2 protoxin. Mutant protoxins were constructed, characterized and compared to the wild type Cyt2Aa2. Protein expression data and SDS-PAGE analysis revealed that substitution at leucine- 33 (L33) of Beta1 has a critical effect on dimer formation and structural stability against proteases. In addition, amino acids N230 and I233-F237 around the C-terminus alphaF demonstrated a crucial role in protecting the protoxin from proteolytic digestion. These results suggested that Beta1 and alphaF on the Nand C-terminal ends of Cyt2Aa2 protoxin play an important role in the molecular interaction and in maintaining the structural stability of the protoxin.

Cyt1Aa protein from Bacillus thuringiensis (Berliner) serovar israelensis is active against the Mediterranean fruit fly, Ceratitis capitata (Wiedemann)

BACKGROUND

Ceratitis capitata (Wiedemann) is one of the world's most destructive fruit pests. The aim of this study was to ascertain insecticidal activity of Bacillus thuringiensis (Berliner) delta-endotoxins to C. capitata.

RESULTS

Among 42 selected Bacillus strains, only B. thuringiensis serovar israelensis (Bti) solubilised protoxins showed biological activity against C. capitata neonate larvae, whereas Bti spore and crystal mixture was inactive. Insecticidal activity of Bti protoxins was significantly enhanced by incubation with Culex pipiens L. gut extracts. Overdigestion of Bti protoxins with Sesamia nonagrioides (Lefebvre) gut extracts suppressed biological activity against C. capitata, and this correlated with degradation of Cyt toxins. Cyt1Aa solubilised protoxin showed the highest toxicity, LC(50) after 7 days of 4.93 microg cm(-2), while proteolytical processing of Cyt1Aa protoxins by larval gut extracts did not enhance insecticidal activity.

CONCLUSION

The present study provides evidence for the first time of the insecticidal activity of a B. thuringiensis strain against C. capitata and identifies a single delta-endotoxin with potential for controlling this pest.

Influence of selected antibiotics on the response of black fly (Simulium vittatum) larvae to insecticidal proteins produced by Bacillus thuringiensis subsp. israelensis

A controlled current toxicity test (CCTT) was developed to evaluate the response of black fly (Simulium vittatum) larvae to insecticidal proteins following exposure to various antibiotics. The bacterium, Bacillus thuringiensis subsp. israelensis (Bti), produces proteins that are toxic to Nemotoceran Diptera, such as black flies and mosquitoes, when ingested. These insecticidal crystalline proteins (ICPs) are highly efficacious in controlling black flies; however, speculation has arisen regarding the potential for antibiotic contamination of waterways to mitigate the toxicity of these proteins. A series of experiments was conducted with the CCTT in which black fly larvae were exposed to enrofloxacin, tylosin, sulfamethoxazole, and trimethoprim followed by exposure to Bti ICPs. These antibiotics were selected based on their use in agricultural and documented anthropogenic contamination of rivers. Anthropogenic concentrations of a mixture of these four antibiotics did not affect the response of the larvae to Bti ICPs. Subsequent experiments were conducted with antibiotic concentrations 10,000 to 80,000 times higher than those found in contaminated rivers. Exposure of black fly larvae to high levels of enrofloxacin (0.5 mg/L) had no effect upon the susceptibility to Bti ICPs; however, exposure to high levels of tylosin (8 mg/L) resulted in a significant increase in the susceptibility of the larvae to Bti ICPs at 72 h of exposure, but not at 48 h. Exposure of black fly larvae to high concentrations of a mixture of sulfamethoxazole and trimethoprim resulted in a significant increase in the efficacy of the larvicide after 48 and 72 h of exposure. These results suggest that impairment of the efficacy of Bti ICPs to black fly larvae is not due to antibiotic contamination of the larval environment.

Isolation, characterization and biological role of camelysin from Bacillus thuringiensis subsp. israelensis

The present study reports a simple rapid method for isolating the zinc-containing metalloprotease camelysin from Bacillus thuringiensis subsp. israelensis (Bti) by extraction from intact bacterial cells with egg L-alpha-phosphatidylcholine containing monolamellar liposomes, followed by separation on a sucrose gradient. Characterization of the isolated camelysin revealed a molecular weight of 23 kDa and a pI of 6.2. The camelysin exhibited maximal activity against the substrate azocasein at a temperature of 37 degrees C and pH 7.5. However, the enzyme's activity remained high also at basic pH values (8-10). In a rich growth medium (LB), camelysin appeared at the late logarithmic phase of Bti growth and reached its maximum in the stationary phase. Camelysin was shown to activate the protoxins Cyt1Aa and Cyt2Ba produced by Bti. The hemolytic activity of Cyt1Aa increased from 40 to 70% and that of Cyt2Ba from 6 to 50% in the presence of 50% (w/w) camelysin. It is concluded that these protoxins can be activated not only by insect gut proteases, but also by the endogeneous metalloprotease camelysin of the Bti bacterium.

A molecular dynamics model of the Bt toxin Cyt1A and its validation by resonance energy transfer

Cyt1A is a cytolytic toxin from Bacillus thuringiensis var. israelensis. A computer model of the toxin in solution was generated and validated by resonance energy transfer (RET). The average distance between the two tryptophans (residues 158 and 161) and the fluorescently labeled cysteine 190 was 2.16 nm, which closely matched the distance predicted in computer simulations, 2.2 nm. The simulation results were able to explain two previous experimental observations: (i) amino-acid sequences of all Cyt toxins contain four blocks of highly conserved residues; and (ii) several single-point mutations drastically abrogated Cyt1A's toxicity. Selective randomization of atomic coordinates in the computer model revealed that the conserved blocks are important for proper folding and stability of the toxin molecule. Replacing lysine 225 with alanine, a mutation that renders the toxin inactive, was shown to result in breaking the hydrogen bonds between K225 and V126, L123, and Y189. Calculated Helmholtz free energy difference of the inactive mutation K225A was higher by 12 kcal/mol and 5 kcal/mol than the values for the benign mutations K118A and K198A, respectively, which indicates that the K225A mutant is significantly destabilized. The normal-mode and principal-component analyses revealed that in the wild-type Cyt1A the region around the residue K225 is quite stationary, due to the hydrogen-bond network around K225. In contrast, pronounced twisting and stretching were observed in the mutant K225A, and the region around the residue K225 becomes unstable. Our results indicate that conformational differences in this mutant spread far away from the site of the mutation, suggesting that the mutant is inactivated due to an overall change in conformation and diminished stability rather than due to a localized alteration of a "binding" or "active" site.

Does distant homology with Evf reveal a lipid binding site in Bacillus thuringiensis cytolytic toxins?

The Cry and Cyt classes of insecticidal toxins derived from the sporulating bacterium Bacillus thuringiensis are valuable substitutes for synthetic pesticides in agricultural contexts. Crystal structures and many biochemical data have provided insights into their molecular mechanisms, generally thought to involve oligomerization and pore formation, but have not localised the site on Cyt toxins responsible for selective binding of phospholipids containing unsaturated fatty acids. Here, distant homology between the structure of Cyt toxins and Erwinia virulence factor (Evf) is demonstrated which, along with sequence conservation analysis, allows a putative lipid binding site to be localised in the toxins.

Strategies to improve the insecticidal activity of Cry toxins from Bacillus thuringiensis

Bacillus thuringiensis Cry toxins have been widely used in the control of insect pests either as spray products or expressed in transgenic crops. These proteins are pore-forming toxins with a complex mechanism of action that involves the sequential interaction with several toxin-receptors. Cry toxins are specific against susceptible larvae and although they are often highly effective, some insect pests are not affected by them or show low susceptibility. In addition, the development of resistance threatens their effectiveness, so strategies to cope with all these problems are necessary. In this review we will discuss and compare the different strategies that have been used to improve insecticidal activity of Cry toxins. The activity of Cry toxins can be enhanced by using additional proteins in the bioassay like serine protease inhibitors, chitinases, Cyt toxins, or a fragment of cadherin receptor containing a toxin-binding site. On the other hand, different modifications performed in the toxin gene such as site-directed mutagenesis, introduction of cleavage sites in specific regions of the protein, and deletion of small fragments from the amino-terminal region lead to improved toxicity or overcome resistance, representing interesting alternatives for insect pest control.

Amino acids at N- and C-termini are required for the efficient production and folding of a cytolytic delta-endotoxin from Bacillus thuringiensis

Bacillus thuringiensis Cyt2Aa toxin is a mosquito-larvicidal and cytolytic delta-endotoxin, which is synthesized as a protoxin and forms crystalline inclusions within the cell. These inclusions are solubilized under alkaline conditions and are activated by proteases within the larval gut. In order to assess the functions of the N-and C-terminal regions of the protoxin, several N- and C-terminal truncated forms of Cyt2Aa were constructed. It was determined that amino acid removal at the N-terminal, which disrupts the beta1 structure, might critically influence toxin production and inclusion formation. The deletion of 22 amino acids from the C-terminus reduced the production and solubility of the toxin. However, the removal of more than 22 amino acids from the C-terminus or the addition of a bulky group to this region could result in the inability of the protein to adopt the proper folding. These findings directly demonstrated the critical roles of N- and C- terminal amino acids on the production and folding of the B. thuringiensis cytolytic delta-endotoxin.

Variations in the mosquito larvicidal activities of toxins from Bacillus thuringiensis ssp. israelensis

Comparing activities of purified toxins from Bacillus thuringiensis ssp. israelensis against larvae of seven mosquito species (vectors of tropical diseases) that belong to three genera, gleaned from the literature, disclosed highly significant variations in the levels of LC(50) as well as in the hierarchy of susceptibilities. Similar toxicity comparisons were performed between nine transgenic Gram-negative species, four of which are cyanobacterial, expressing various combinations of cry genes, cyt1Aa and p20, against larvae of four mosquito species as potential agents for biological control. Reasons for inconsistencies are listed and discussed. Standard conditions for toxin isolation and presentation to larvae are sought. A set of lyophilized powders prepared identically from six Escherichia coli clones expressing combinations of four genes displayed toxicities against larvae of three mosquito species, with levels that differed between them but with identical hierarchy.

High-resolution crystal structure of activated Cyt2Ba monomer from Bacillus thuringiensis subsp. israelensis

The Cyt family of proteins consists of delta-endotoxins expressed during sporulation of several subspecies of Bacillus thuringiensis. Its members possess insecticidal, hemolytic, and cytolytic activities through pore formation and attract attention due to their potential use as vehicles for targeted membrane destruction. The delta-endotoxins of subsp. israelensis include three Cyt species: a major Cyt1Aa and two minor proteins, Cyt2Ba and Cyt1Ca. A cleaved Cyt protein that lacks the N- and C-terminal segments forms a toxic monomer. Here, we describe the crystal structure of Cyt2Ba, cleaved at its amino and carboxy termini by bacterial endogenous protease(s). Overall, its fold resembles that of the previously described volvatoxin A2 and the nontoxic form of Cyt2Aa. The structural similarity between these three proteins may provide information regarding the mechanism(s) of membrane-perforating toxins.

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.

Antimicrobial mechanisms of ortho-phthalaldehyde action

Biocides generally have multiple biochemical targets. Such a feature easily entangles the analysis of the mechanisms of antimicrobial action. In this study, the action of the dialdehyde biocide ortho-phtalaldehyde (OPA), on bacteria, was investigated using the Gram-negative Pseudomonas fluorescens. The targets of the biocide action were studied using different bacterial physiological indices. The respiratory activity, membrane permeabilization, physico-chemical characterization of the bacterial surfaces, outer membrane proteins (OMP) expression, concomitant influence of pH, contact time and presence of bovine serum albumin (BSA) on respiratory activity, morphological changes and OPA-DNA interactions were assessed for different OPA concentrations. With the process conditions used, the minimum inhibitory concentration was 1500 mg/l, the concentration to promote total loss of bacterial culturability was 65 mg/l and the concentration needed to inactivate respiratory activity was 80 mg/l. These data are evidence that culturability and respiratory activity were markedly affected by the biocide. OPA lead, moreover, to a significant change in cell surface hydrophobicity and induced propidium iodide uptake. Such results suggest cytoplasmic membrane damage, although no release of ATP was detected. At pH 5, the bactericidal action of OPA was stronger, though not influenced by BSA presence. Nevertheless, at pH 9, BSA noticeably (p < 0.05) impaired biocide action. A time-dependent effect in OPA action was evident when contemplating respiratory activity variation, mainly for the lower exposure times. Scanning electron microscopy allowed to detect bacterial morphological changes, translated on cellular elongation, for OPA concentrations higher than 100 mg/l. Interferences at DNA level were, however, restricted to extreme biocide concentrations. The overall bactericidal events occurred without detectable OMP expression changes. In conclusion, the results indicated a sequence of events responsible for the antimicrobial action of OPA: it binds to membrane receptors due to cross-linkage; impairs the membrane functions allowing the biocide to enter through the permeabilized membrane; it interacts with intracellular reactive molecules, such as RNA, compromising the growth cycle of the cells and, at last, with DNA.

Larvicidal activities against agricultural pests of transgenic Escherichia coli expressing combinations of four genes from Bacillus thuringiensis

The genes cry1Ac and cry1Ca from Bacillus thuringiensis subsps. kurstaki HD-73 and aizawai 4J4, respectively, encoding delta-endotoxins against lepidopteran larvae were isolated, cloned and expressed in Escherichia coli, with and without cyt1Aa (encoding cytolytic protein) and p20 (accessory protein) from subsp. israelensis. Nine combinations of the genes under control of an early T7, P A1 inducible promoter, produced the encoding proteins. Toxicities were examined against larvae of three major agricultural pests: Pectinophora gossypiella, Helicoverpa armigera and Spodoptera littoralis. The clones expressing cyt1Aa, with or without p20, were not toxic. The clone expressing cry1Ac (pBt-1A) was the most toxic to P. gossypiella (LC50 of 0.27 x 10(8) cells g(-1)). Clone pBt-1CA expressing cry1Ca and cry1Ac displayed the highest toxicity (LC50 of 0.12 x 10(8) cells ml(-1)) against S. littoralis. Clone pBt-1CARCy expressing all four genes (cry1Ca, cry1Ac, p20, cyt1Aa) in tandem exhibited the highest toxicity to H. armigera (LC50 of 0.16 x 10(8) cells ml(-1)). Cyt1Aa failed to raise the toxicity of these Cry toxins against P. gossypiella and S. littoralis but significantly enhanced toxicity against H. armigera. Two additional clones expressing either cry1Ac or cry1Ca under tandem promoters, P A1 and P psbA (constitutive), displayed significantly higher toxicities (7.5- to 140-fold) than their counterparts with P A1 alone, reducing the LC50 values to below 10(7) cells ml(-1).

Cross-resistance spectra of Culex quinquefasciatus resistant to mosquitocidal toxins of Bacillus thuringiensis towards recombinant Escherichia coli expressing genes from B. thuringiensis ssp. israelensis

Sixteen Escherichia coli clones were assayed against susceptible and Bacillus thuringiensis-resistant Culex quinquefasciatus larvae. The clones expressed different combinations of four genes from Bacillus thuringiensis ssp. israelensis; three genes encoded mosquitocidal toxins (Cry11Aa, Cry4Aa and Cyt1Aa) and the fourth encoded an accessory protein (P20). The cross-resistance spectra of the mosquitoes were similar to the profiles for recombinant B. thuringiensis strains expressing B. thuringiensis toxin genes, but with varied toxicity levels. The toxicity of the recombinants towards resistant mosquito larvae was improved when p20 and cyt1Aa were expressed in combination with cry4Aa and/or cry11Aa. Recombinant pVE4-ADRC, expressing cry4Aa, cry11Aa, p20 and cyt1Aa, was the most active against the resistant Culex, and resistance levels did not exceed fourfold. These results indicate that B. thuringiensis ssp. israelensis genes expressed in a heterologous host such as E. coli can be effective against susceptible and B. thuringiensis-resistant larvae and suppress resistance.

Specific targeting to murine myeloma cells of Cyt1Aa toxin from Bacillus thuringiensis subspecies israelensis

Multiple myeloma is currently an incurable cancer of plasma B cells often characterized by overproduction of abnormally high quantities of a patient-specific, clonotypic immunoglobulin "M-protein." The M-protein is expressed on the cell membrane and secreted into the blood. We previously showed that ligand-toxin conjugates (LTC) incorporating the ribosome-inactivating Ricin-A toxin were very effective in specific cytolysis of the anti-ligand antibody-bearing target cells used as models for multiple myeloma. Here, we report on the incorporation of the membrane-disruptive Cyt1Aa toxin from Bacillus thuringiensis subsp. israelensis into LTCs targeted to murine myeloma cells. Proteolytically activated Cyt1Aa was conjugated chemically or genetically through either its amino or carboxyl termini to the major peptidic epitope VHFFKNIVTPRTP (p87-99) of the myelin basic protein. The recombinant fusion-encoding genes were cloned and expressed in acrystalliferous B. thuringiensis subsp. israelensis through the shuttle vector pHT315. Both chemically conjugated and genetically fused LTCs were toxic to anti-myelin basic protein-expressing murine hybridoma cells, but the recombinant conjugates were more active. LTCs comprising the Cyt1Aa toxin might be useful anticancer agents. As a membrane-acting toxin, Cyt1Aa is not likely to induce development of resistant cell lines.

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.

Mode of action of Bacillus thuringiensis Cry and Cyt toxins and their potential for insect control

Bacillus thuringiensis Crystal (Cry) and Cytolitic (Cyt) protein families are a diverse group of proteins with activity against insects of different orders--Lepidoptera, Coleoptera, Diptera and also against other invertebrates such as nematodes. Their primary action is to lyse midgut epithelial cells by inserting into the target membrane and forming pores. Among this group of proteins, members of the 3-Domain Cry family are used worldwide for insect control, and their mode of action has been characterized in some detail. Phylogenetic analyses established that the diversity of the 3-Domain Cry family evolved by the independent evolution of the three domains and by swapping of domain III among toxins. Like other pore-forming toxins (PFT) that affect mammals, Cry toxins interact with specific receptors located on the host cell surface and are activated by host proteases following receptor binding resulting in the formation of a pre-pore oligomeric structure that is insertion competent. In contrast, Cyt toxins directly interact with membrane lipids and insert into the membrane. Recent evidence suggests that Cyt synergize or overcome resistance to mosquitocidal-Cry proteins by functioning as a Cry-membrane bound receptor. In this review we summarize recent findings on the mode of action of Cry and Cyt toxins, and compare them to the mode of action of other bacterial PFT. Also, we discuss their use in the control of agricultural insect pests and insect vectors of human diseases.

Bacillus thuringiensis serovariety israelensis and Bacillus sphaericus for mosquito control

Since the discovery of Bacillus thuringiensis (Berliner) serovariety israelensis de Barjac (Bti) and efficacious isolates of Bacillus sphaericus Neide, formulations of these bacteria have become the predominant non-chemical means employed for control of mosquito larvae at several locations in the United States and other countries. An overview of developments in the past 20 years is presented in this chapter regarding the toxins of Bti and B. sphaericus, their modes of action, efficacy and factors that affect larvicidal activity, development of resistance, safety, and their roles in integrated mosquito control. The efficacy of Bti formulations has been demonstrated in a variety of habitats against a multitude of species of mosquitoes. B. sphaericus formulations have been utilized predominantly in organically enriched habitats against Culex species, but they are also active in a variety of habitats having low organic enrichment, against numerous species, and across several genera. Stegomyia spp. are not susceptible to practical doses of B. sphaericus formulations. B. sphaericus has been shown to persist longer than Bti in polluted habitats and, under certain circumstances, can recycle in larval cadavers. A disadvantage of B. sphaericus has been the development of resistance in certain populations of Cx. quinquefasciatus Say and Cx. pipiens Linnaeus. Biotic and abiotic factors that influence the larvicidal activity of Bti and B. sphaericus include species of mosquito and their respective feeding strategies, rate of ingestion, age and density of larvae, habitat factors (temperature, solar radiation, depth of water, turbidity, tannin and organic content, presence of vegetation, etc.), formulation factors (type of formulation, toxin content, how effectively the material reaches the target, and settling rate), storage conditions, production factors, means of application and frequency of treatments. Due to their efficacy and relative specificity, both Bti and B. sphaericus can be ideal control agents in integrated programs especially where other biological control agents, environmental management, personal protection and the judicious use of insecticides are combined.

Developing recombinant bacteria for control of mosquito larvae

Genetic engineering techniques have been used to significantly improve mosquito larvicides based on the bacteria Bacillus thuringiensis (Bt) subsp. israelensis (Bti) and Bacillus sphaericus (Bs). These new larvicides hold excellent promise for providing better and more cost-effective control of nuisance mosquitoes and vectors of important diseases, including the anopheline vectors of malaria and culicine vectors responsible for filariasis and viral encephalitides. The toxicity of Bti and Bs is due primarily to endotoxin proteins produced during sporulation. After ingestion by larvae, these are activated and destroy the larval stomach, quickly resulting in death. By cloning the genes encoding various endotoxins from Bt and Bs species, and engineering these for high levels of synthesis, we have been able to generate recombinant bacterial strains based on Bti that are more than 10 times as effective as the conventional strains of Bti or Bs that serve as the active ingredients of commercial bacterial larvicides currently used for mosquito control. The best of these recombinants contain all major Bti endotoxins, specifically, Cry4A, Cry4B, Cry11A, and Cyt1A, plus the binary (Bin) endotoxin of Bs, the principal mosquitocidal protein responsible for the activity of this species. The presence of Cyt1A in these recombinants, which synergizes Cry toxicity and delays resistance to these proteins and Bs Bin, should enable long term use of these recombinants with little if any development of resistance. In the field, these new recombinants should be particularly effective larvicides against most important vectors and nuisance species of the genus Culex, the malaria vectors Anopheles gambiae and An. arabiensis, and species of Aedes and Ochlerotatus sensitive to Bs.

Bacillus thuringiensis subsp. israelensis Cyt1Aa synergizes Cry11Aa toxin by functioning as a membrane-bound receptor

Bacillus thuringiensis subsp. israelensis produces crystal proteins, Cry (4Aa, 4Ba, 10Aa, and 11Aa) and Cyt (1Aa and 2Ba) proteins, toxic to mosquito vectors of human diseases. Cyt1Aa overcomes insect resistance to Cry11Aa and Cry4 toxins and synergizes the toxicity of these toxins. However, the molecular mechanism of synergism remains unsolved. Here, we provide evidence that Cyt1Aa functions as a receptor of Cry11Aa. Sequential-binding analysis of Cyt1Aa and Cry11Aa revealed that Cyt1Aa binding to Aedes aegypti brush border membrane vesicles enhanced the binding of biotinylated-Cry11Aa. The Cyt1Aa- and Cry11Aa-binding epitopes were mapped by means of the yeast two-hybrid system, peptide arrays, and heterologous competition assays with synthetic peptides. Two exposed regions in Cyt1Aa, loop beta6-alphaE and part of beta7, bind Cry11Aa. On the other side, Cry11Aa binds Cyt1Aa proteins by means of domain II-loop alpha8 and beta-4, which are also involved in midgut receptor interaction. Characterization of single-point mutations in Cry11Aa and Cyt1Aa revealed key Cry11Aa (S259 and E266) and Cyt1Aa (K198, E204 and K225) residues involved in the interaction of both proteins and in synergism. Additionally, a Cyt1Aa loop beta6-alphaE mutant (K198A) with enhanced synergism to Cry11Aa was isolated. Data provided here strongly indicates that Cyt1Aa synergizes or suppresses resistance to Cry11Aa toxin by functioning as a membrane-bound receptor. Bacillus thuringiensis subsp. israelensis is a highly effective pathogenic bacterium because it produces a toxin and also its functional receptor, promoting toxin binding to the target membrane and causing toxicity.

Molecular Dynamics Simulations of the Cytolytic Toxin Cyt1A in Solution

Cytolytic toxin Cyt1A from Bacillus thuringiensis var. israelensis is used as an environmentally friendly insecticide, but its mode of action has not been clearly established. One main obstacle seems to be the lack of the experimentally determined structure of the toxin. As a first step in computer simulations of Cyt1A, in this paper, a three-dimensional molecular structure of Cyt1A in solution was generated by homology modeling, potential energy minimization and molecular dynamics. Regions of the toxin molecule that manifest increased conformational flexibility--and thus are likely to participate in the initial membrane binding and conformational changes--were then identified. Finally, the simulated structure was used to study the effect of a single amino-acid mutation that is known to abrogate the toxicity of Cyt1A in vivo.

Cyt1A of Bacillus thuringiensis delays evolution of resistance to Cry11A in the mosquito Culex quinquefasciatus

Insecticides based on Bacillus thuringiensis subsp. israelensis have been used for mosquito and blackfly control for more than 20 years, yet no resistance to this bacterium has been reported. Moreover, in contrast to B. thuringiensis subspecies toxic to coleopteran or lepidopteran larvae, only low levels of resistance to B. thuringiensis subsp. israelensis have been obtained in laboratory experiments where mosquito larvae were placed under heavy selection pressure for more than 30 generations. Selection of Culex quinquefasciatus with mutants of B. thuringiensis subsp. israelensis that contained different combinations of its Cry proteins and Cyt1Aa suggested that the latter protein delayed resistance. This hypothesis, however, has not been tested experimentally. Here we report experiments in which separate C. quinquefasciatus populations were selected for 20 generations to recombinant strains of B. thuringiensis that produced either Cyt1Aa, Cry11Aa, or a 1:3 mixture of these strains. At the end of selection, the resistance ratio was 1,237 in the Cry11Aa-selected population and 242 in the Cyt1Aa-selected population. The resistance ratio, however, was only 8 in the population selected with the 1:3 ratio of Cyt1Aa and Cry11Aa strains. When the resistant mosquito strain developed by selection to the Cyt1Aa-Cry11Aa combination was assayed against Cry11Aa after 48 generations, resistance to this protein was 9.3-fold. This indicates that in the presence of Cyt1Aa, resistance to Cry11Aa evolved, but at a much lower rate than when Cyt1Aa was absent. These results indicate that Cyt1Aa is the principal factor responsible for delaying the evolution and expression of resistance to mosquitocidal Cry proteins.

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.

Structure, diversity, and evolution of protein toxins from spore-forming entomopathogenic bacteria

Gram-positive spore-forming entomopathogenic bacteria can utilize a large variety of protein toxins to help them invade, infect, and finally kill their hosts, through their action on the insect midgut. These toxins belong to a number of homology groups containing a diversity of protein structures and modes of action. In many cases, the toxins consist of unique folds or novel combinations of domains having known protein folds. Some of the toxins display a similar structure and mode of action to certain toxins of mammalian pathogens, suggesting a common evolutionary origin. Most of these toxins are produced in large amounts during sporulation and have the remarkable feature that they are localized in parasporal crystals. Localization of multiple toxin-encoding genes on plasmids together with mobilizable elements enables bacteria to shuffle their armory of toxins. Recombination between toxin genes and sequence divergence has resulted in a wide range of host specificities.