Enhanced toxicity of Bacillus thuringiensis Cry3A delta-endotoxin in coleopterans by mutagenesis in a receptor binding loop

Making 3D-Cry Toxin Mutants: Much More Than a Tool of Understanding Toxins Mechanism of Action

3D-Cry toxins, produced by the entomopathogenic bacterium Bacillus thuringiensis, have been extensively mutated in order to elucidate their elegant and complex mechanism of action necessary to kill susceptible insects. Together with the study of the resistant insects, 3D-Cry toxin mutants represent one of the pillars to understanding how these toxins exert their activity on their host. The principle is simple, if an amino acid is involved and essential in the mechanism of action, when substituted, the activity of the toxin will be diminished. However, some of the constructed 3D-Cry toxin mutants have shown an enhanced activity against their target insects compared to the parental toxins, suggesting that it is possible to produce novel versions of the natural toxins with an improved performance in the laboratory. In this report, all mutants with an enhanced activity obtained by accident in mutagenesis studies, together with all the variants obtained by rational design or by directed mutagenesis, were compiled. A description of the improved mutants was made considering their historical context and the parallel development of the protein engineering techniques that have been used to obtain them. This report demonstrates that artificial 3D-Cry toxins made in laboratories are a real alternative to natural toxins.

Replacement of loop2 and 3 of Cry1Ai in domain II affects specificity to the economically important insect Bombyx mori

Bacillus thuringiensis Cry1Ai belongs to three-domain Cry toxins and only shows growth inhibition effects against the agricultural pest Helicoverpa armigera, although it exhibits high toxicity against the non-target insect Bombyx mori. In previous studies, loop2 and loop3 on domain II from Cry1Ah were found to be related to binding and high toxicity against H. armigera. However, toxicity for B. mori of Cry1Ai-h-loop2, obtained by replacing loop 2 from Cry1Ah into Cry1Ai, was not modified. In this study, to further characterize the role of loop2 and loop3 in Cry1Ai, all of the amino acids in these two loops were substituted with the same amount of alanine residues. The Cry1Ai-loop3 mutant exhibited significantly lower toxicity against B. mori, but the toxicity of the loop2 mutant was not significantly changed. Furthermore, the double-exchange mutant Cry1Ai-h-loop2&3, replacing loop2 and loop3 from Cry1Ah into Cry1Ai, showed decreased toxicity against B. mori related to Cry1Ai. In addition, we found that the binding affinity of Cry1Ai-h-loop2&3 with brush border membrane vesicles (BBMVs) from the midgut of B. mori was lower than that of Cry1Ai, which correlates with the reduced toxicity.

Enhancement of insect susceptibility and larvicidal efficacy of Cry4Ba toxin by calcofluor

Background

Mosquitoes transmit many vector-borne infectious diseases including malaria, dengue, chikungunya, yellow fever, filariasis, and Japanese encephalitis. The insecticidal δ-endotoxins Cry4, Cry11, and Cyt produced from Bacillus thuringiensis have been used for bio-control of mosquito larvae. Cry δ-endotoxins are synthesised as inactive protoxins in the form of crystalline inclusions in which they are processed to active toxins in larval midgut lumen. Previously, we demonstrated that the activated Cry4Ba toxin has to alter the permeability of the peritrophic membrane (PM), allowing toxin passage across PM to reach specific receptors on microvilli of larval midgut epithelial cells, where the toxin undergoes conformational changes, followed by membrane insertion and pore formation, resulting in larval death. A peritrophic membrane (PM)-binding calcofluor has been proposed to inhibit chitin formation and enhance baculovirus infection of lepidopteran Trichoplusia ni.

Methods

In this study, Aedes aegypti larvae were fed with the calcofluor and Cry4Ba toxin to investigate the effect of this agent on the toxicity of the Cry4Ba toxin.

Results

Calcofluor displayed an enhancing effect when co-fed with the Cry4Ba wild-type toxin. The agent could restore the killing activity of the partially active Cry4Ba mutant E417A/Y455A toward Ae. aegypti larvae. PM destruction was observed after larval challenge with calcofluor together with the toxin. Interestingly, calcofluor increased Cry4Ba toxin susceptibility toward semi-susceptible Culex quinquefasciatus larvae. However, calcofluor alone or in combination with the toxin showed no mortality effect on non-susceptible fresh-water fleas, Moina macrocopa.

Conclusions

Our results suggest that PM may contribute to the resistance of the mosquito larvae to Cry4Ba toxin. The PM-permeability alternating calcofluor might be a promising candidate for enhancing insect susceptibility, which will consequently improve Cry4Ba efficacy in field settings in the future.

Using phage display technology to obtain Crybodies active against non-target insects

The insecticidal Cry toxins produced by Bacillus thuringiensis (Bt) are increasingly important in the biological control of insect pests and vectors of human disease. Markets for Bt products and transgenic plants expressing their toxins are driven by their specificity, safety and the move away from chemical control agents. However, the high specificity of Cry toxins can also prove to be a limitation when there is no known Cry toxin active against a particular target. Novel activities can be discovered by screening natural Bt isolates or through modifications of the Cry proteins. Here we demonstrate the use of λ-phage displaying Cry1Aa13 toxin variants modified in domain II loop 2 (Crybodies) to select retargeted toxins. Through biopanning using gut tissue from larvae of the non-target insect Aedes aegypti, we isolated a number of phage for further testing. Two of the overexpressed Cry toxin variants showed significant activity against A. aegypti larvae while another induced mortality at the pupal stage. We present the first report of the use of phage display to identify novel activities toward insects from distant taxonomic Orders and establish this technology based on the use of Crybodies as a powerful tool for developing tailor-made insecticides against new target insects.

Engineered Cry1Ac-Cry9Aa hybrid Bacillus thuringiensis delta-endotoxin with improved insecticidal activity against Helicoverpa armigera

Recombinant Bt construct was prepared by exchange of pore forming domain I with cry1Ac to cry9Aa gene by overlap extension PCR (OE-PCR) technique. Construction of cry1Ac-cry9Aa was accomplished by six base pair homology at 3' ends of PCR products of domain I of cry1Ac and domain II and III of cry9Aa. The recombinant toxin was also modified by deletion of N-terminal alpha helix-1 of recombinant toxin. Both Cry toxins were expressed in E. coli BL21(DE3) plysS and purified by His-tag purification. Upon insect bioassay analysis against devastating crop pest Helicoverpa armigera, toxicity of recombinant toxin was found around fivefold higher than native Cry1Ac while alpha helix-1 deleted N-terminal modified toxin did not resulted in significant increase in toxicity. The recombinant Cry toxins such as Cry1Ac-Cry9Aa and Cry1Ac-Cry9AaMod may be used for insect pest control.

Loop replacements with gut-binding peptides in Cry1Ab domain II enhanced toxicity against the brown planthopper, Nilaparvata lugens (Stål)

Bacillus thuringiensis (Bt) Cry toxins have been used widely in pest managements. However, Cry toxins are not effective against sap-sucking insects (Hemiptera), which limits the application of Bt for pest management. In order to extend the insecticidal spectrum of Bt toxins to the rice brown planthopper (BPH), Nilaparvata lugens, we modified Cry1Ab putative receptor binding domains with selected BPH gut-binding peptides (GBPs). Three surface exposed loops in the domain II of Cry1Ab were replaced with two GBPs (P2S and P1Z) respectively. Bioassay results showed that toxicity of modified toxin L2-P2S increased significantly (~9 folds) against BPH nymphs. In addition, damage of midgut cells was observed from the nymphs fed with L2-P2S. Our results indicate that modifying Cry toxins based on the toxin-gut interactions can broaden the insecticidal spectrum of Bt toxin. This method provides another approach for the development of transgenic crops with novel insecticidal activity against hemipteran insects and insect populations resistant to current Bt transgenic crops.

Tribolium castaneum immune defense genes are differentially expressed in response to Bacillus thuringiensis toxins sharing common receptor molecules and exhibiting disparate toxicity

In Tribolium castaneum larvae we have demonstrated by RNA interference knockdown that the Bacillus thuringiensis Cry3Ba toxin receptors Cadherin-like and Sodium solute symporter proteins are also functional receptors of the less active Cry3Aa toxin. Differences in susceptibility to B. thuringiensis infection might not only rely on toxin-receptor interaction but also on host defense mechanisms. We compared the expression of the immune related genes encoding Apolipophorin-III and two antimicrobial peptides, Defensin3 and Defensin2 after B. thuringiensis challenge. All three genes were up-regulated following Cry3Ba spore-crystal intoxication whereas only Defensins gene expression was induced upon Cry3Aa spore-crystal treatment, evidencing a possible association between host immune response and larval susceptibility to B. thuringiensis. We assessed the antimicrobial activity spectra of T. castaneum defensins peptide fragments and found that a peptide fragment of Defensin3 was effective against the human microbial pathogens, Escherichia coli, Staphylococcus aureus and Candida albicans, being S. aureus the most susceptible one.

Identification of Bacillus thuringiensis Cry3Aa toxin domain II loop 1 as the binding site of Tenebrio molitor cadherin repeat CR12

Bacillus thuringiensis Cry toxins exert their toxic effect by specific recognition of larval midgut proteins leading to oligomerization of the toxin, membrane insertion and pore formation. The exposed domain II loop regions of Cry toxins have been shown to be involved in receptor binding. Insect cadherins have shown to be functionally involved in toxin binding facilitating toxin oligomerization. Here, we isolated a VHH (VHHA5) antibody by phage display that binds Cry3Aa loop 1 and competed with the binding of Cry3Aa to Tenebrio molitor brush border membranes. VHHA5 also competed with the binding of Cry3Aa to a cadherin fragment (CR12) that was previously shown to be involved in binding and toxicity of Cry3Aa, indicating that Cry3Aa binds CR12 through domain II loop 1. Moreover, we show that a loop 1 mutant, previously characterized to have increased toxicity to T. molitor, displayed a correlative enhanced binding affinity to T. molitor CR12 and to VHHA5. These results show that Cry3Aa domain II loop 1 is a binding site of CR12 T. molitor cadherin.

Bt toxin modification for enhanced efficacy

Insect-specific toxins derived from Bacillus thuringiensis (Bt) provide a valuable resource for pest suppression. Here we review the different strategies that have been employed to enhance toxicity against specific target species including those that have evolved resistance to Bt, or to modify the host range of Bt crystal (Cry) and cytolytic (Cyt) toxins. These strategies include toxin truncation, modification of protease cleavage sites, domain swapping, site-directed mutagenesis, peptide addition, and phage display screens for mutated toxins with enhanced activity. Toxin optimization provides a useful approach to extend the utility of these proteins for suppression of pests that exhibit low susceptibility to native Bt toxins, and to overcome field resistance.

Evolution of Bacillus thuringiensis Cry toxins insecticidal activity

Insecticidal Cry proteins produced by Bacillus thuringiensis are use worldwide in transgenic crops for efficient pest control. Among the family of Cry toxins, the three domain Cry family is the better characterized regarding their natural evolution leading to a large number of Cry proteins with similar structure, mode of action but different insect specificity. Also, this group is the better characterized regarding the study of their mode of action and the molecular basis of insect specificity. In this review we discuss how Cry toxins have evolved insect specificity in nature and analyse several cases of improvement of Cry toxin action by genetic engineering, some of these examples are currently used in transgenic crops. We believe that the success in the improvement of insecticidal activity by genetic evolution of Cry toxins will depend on the knowledge of the rate-limiting steps of Cry toxicity in different insect pests, the mapping of the specificity binding regions in the Cry toxins, as well as the improvement of mutagenesis strategies and selection procedures.

Improving Cry8Ka toxin activity towards the cotton boll weevil (Anthonomus grandis)

Background

The cotton boll weevil (Anthonomus grandis) is a serious insect-pest in the Americas, particularly in Brazil. The use of chemical or biological insect control is not effective against the cotton boll weevil because of its endophytic life style. Therefore, the use of biotechnological tools to produce insect-resistant transgenic plants represents an important strategy to reduce the damage to cotton plants caused by the boll weevil. The present study focuses on the identification of novel molecules that show improved toxicity against the cotton boll weevil. In vitro directed molecular evolution through DNA shuffling and phage display screening was applied to enhance the insecticidal activity of variants of the Cry8Ka1 protein of Bacillus thuringiensis.

Results

Bioassays carried out with A. grandis larvae revealed that the LC₅₀ of the screened mutant Cry8Ka5 toxin was 3.15-fold higher than the wild-type Cry8Ka1 toxin. Homology modelling of Cry8Ka1 and the Cry8Ka5 mutant suggested that both proteins retained the typical three-domain Cry family structure. The mutated residues were located mostly in loops and appeared unlikely to interfere with molecular stability.

Conclusions

The improved toxicity of the Cry8Ka5 mutant obtained in this study will allow the generation of a transgenic cotton event with improved potential to control A. grandis.

Biological properties of loop-replaced mutants of Bacillus thuringiensis mosquitocidal Cry4Aa

Cry4Aa produced by Bacillus thuringiensis subsp. israelensis (Bti) exhibits a specific toxicity to Anopheles, Aedes, and Culex larvae, which are vectors of serious diseases, and formulations of Bti are used worldwide for mosquito control. In general, domain II of the Cry toxin is believed to be important for target specificity, and three loops (loops 1, 2, and 3) in domain II have been studied extensively. In this report, to analyze the biological functions of loops 1, 2, and 3 of Cry4Aa, mutants were constructed in which one of the loops was replaced with either of the other two loops. A bioassay using Culex pipiens larvae revealed that the mosquitocidal activity was virtually lost upon replacement of loop2. The mutants in which loops 1 and/or 3 were replaced also showed decreased activity, but they still maintained some activities. This suggested that loop2, but not loops 1 and 3, was essential for the mosquitocidal activity of Cry4Aa. Proteolytic digestion revealed the involvement of loops in the stability of the Cry4Aa structure. No significant differences were observed in the amount of wild-type and mutant Cry4Aa bound to the BBMVs prepared from the C. pipiens larvae.

Alanine scanning analyses of the three major loops in domain II of Bacillus thuringiensis mosquitocidal toxin Cry4Aa

Cry4Aa produced by Bacillus thuringiensis is a dipteran-specific toxin and is of great interest for developing a bioinsecticide to control mosquitoes. Therefore, it is very important to characterize the functional motif of Cry4Aa that is responsible for its mosquitocidal activity. In this study, to characterize a potential receptor binding site, namely, loops 1, 2, and 3 in domain II, we constructed a series of Cry4Aa mutants in which a residue in these three loops was replaced with alanine. A bioassay using Culex pipiens larvae revealed that replacement of some residues affected the mosquitocidal activity of Cry4Aa, but the effect was limited. This finding was partially inconsistent with previous results which suggested that replacement of the Cry4Aa loop 2 results in a significant loss of mosquitocidal activity. Therefore, we constructed additional mutants in which multiple (five or six) residues in loop 2 were replaced with alanine. Although the replacement of multiple residues also resulted in some decrease in mosquitocidal activity, the mutants still showed relatively high activity. Since the insecticidal spectrum of Cry4Aa is specific, Cry4Aa must have a specific receptor on the surface of the target tissue, and loss of binding to the receptor should result in a complete loss of mosquitocidal activity. Our results suggested that, unlike the receptor binding site of the well-characterized molecule Cry1, the receptor binding site of Cry4Aa is different from loops 1, 2, and 3 or that there are multiple binding sites that work cooperatively for receptor binding.

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.

Discovery of a novel Bacillus thuringiensis Cry8D protein and the unique toxicity of the Cry8D-class proteins against scarab beetles

A novel cry gene, cry8Db, highly toxic to scarab beetles such as the Japanese beetle, Popillia japonica Newman, was cloned from an isolate of Bacillus thuringiensis(Bt), BBT2-5. The cry8Db gene has 3525bp nucleotides and codes for a protein of 1174 amino acid residues. The protein, Cry8Db, has typical Bt characteristics such as the 8-block, conserved sequences and the three-domain 3D toxin structure as defined with Cry3Aa. When the amino acid sequence of Cry8Db was compared with that of Cry8Da whose gene was cloned and characterized in our laboratory earlier, substantial sequence diversities were found in their Domain III. The cry8Db gene was expressed in an acrystalliferous B. thuringiensis strain, BT51. BT51 expressing cry8Db formed a spherical crystal like the natural crystal of BBT2-5. The Cry8Db protein was assayed along with the other scarab active Cry8Da and Cry8Ca against the Japanese beetle. While Cry8Da and Cry8Db had toxicity against both adults and larvae of the Japanese beetle, Cry8Ca was toxic to only larvae. Cry8Ca showed no toxicity against the adult beetle up to 30 microg per 1 cm(2) of leaf discs on which the protein was applied. The activation process of Cry8Db by adult and larval gut juice was compared in vitro with the processes of Cry8Da and Cry8Ca. All three proteins, Cry8Db, Cry8Da and Cry8Ca, produced a toxic core of approximately 70kDa equally indicating that the activation process does not inactivate the adult activity of Cry8Ca. We concluded that the adult activity of Cry8D proteins is encoded in Domain II. Further tests against other beetle species showed a significant difference between Cry8D's and Cry8Ca but no difference between Cry8Da and Cry8Db. Comparison of 3D structural models of Cry8Ca, Cry8Da and Cry8Db, which were constructed by using Cry3Bb as the structural template, indicated significant structural differences, especially between Cry8Ca and Cry8D proteins, in three major surface-exposed loops of Domain II that may be involved in determining the adult beetle activity.

Membrane insertion of the Bacillus thuringiensis Cry1Ab toxin: single mutation in domain II block partitioning of the toxin into the brush border membrane

The umbrella and penknife models hypothesize that insecticidal Bacillus thuringiensis Cry toxins partition into the apical membrane of the insect midgut by insertion of only two alpha-helices from domain I of the protein, alpha-helices 4 and 5 in the case of the umbrella model and alpha-helices 5 and 6 in the case of the penknife model. Neither model envisages membrane partitioning by domains II and III. In this study, we present data suggesting that mutations in the domain II residue, F371, affect insertion of the whole toxin into Manduca sexta brush border membrane vesicles (BBMVs). Using steady state fluorescence measurements combined with a proteinase K protection assay, we show that mutants of F371 have lost their ability to insert into the BBMV, even though binding to cadherin is almost unaffected. The study also identifies a difference in partitioning of toxins into artificial lipid vesicles (SUVs) as opposed to native BBMVs. While the F371 mutations block insertion of domains I and II into BBMVs, they only block domain II insertion into SUVs. Bioassay and voltage clamping of midguts also confirm the fluorescence data that the noninserting mutants are nontoxic. Our study leads us to propose that, in contrast to previous models of individual free helices inserting into the membrane, the toxin enters into the membrane as a whole molecule or oligomers of the molecule, wherein the domain II residue F371 has a vital role to play in membrane insertion.

Investigating the properties of Bacillus thuringiensis Cry proteins with novel loop replacements created using combinatorial molecular biology

Cry proteins are a large family of crystalline toxins produced by Bacillus thuringiensis. Individually, the family members are highly specific, but collectively, they target a diverse range of insects and nematodes. Domain II of the toxins is important for target specificity, and three loops at its apex have been studied extensively. There is considerable interest in determining whether modifications in this region may lead to toxins with novel specificity or potency. In this work, we studied the effect of loop substitution on toxin stability and specificity. For this purpose, sequences derived from antibody complementarity-determining regions (CDR) were used to replace native domain II apical loops to create "Crybodies." Each apical loop was substituted either individually or in combination with a library of third heavy-chain CDR (CDR-H3) sequences to create seven distinct Crybody types. An analysis of variants from each library indicated that the Cry1Aa framework can tolerate considerable sequence diversity at all loop positions but that some sequence combinations negatively affect structural stability and protease sensitivity. CDR-H3 substitution showed that loop position was an important determinant of insect toxicity: loop 2 was essential for activity, whereas the effects of substitutions at loop 1 and loop 3 were sequence dependent. Unexpectedly, differences in toxicity did not correlate with binding to cadherins--a major class of toxin receptors--since all Crybodies retained binding specificity. Collectively, these results serve to better define the role of the domain II apical loops as determinants of specificity and establish guidelines for their modification.

Blocking binding of Bacillus thuringiensis Cry1Aa to Bombyx mori cadherin receptor results in only a minor reduction of toxicity

Background

Bacillus thuringiensis Cry1Aa insecticidal protein is the most active known B. thuringiensis toxin against the forest insect pest Lymantria dispar (gypsy moth), unfortunately it is also highly toxic against the non-target insect Bombyx mori (silk worm).

Results

Surface exposed hydrophobic residues over domains II and III were targeted for site-directed mutagenesis. Substitution of a phenylalanine residue (F328) by alanine reduced binding to the Bombyx mori cadherin by 23-fold, reduced biological activity against B. mori by 4-fold, while retaining activity against Lymantria dispar.

Conclusion

The results identify a novel receptor-binding epitope and demonstrate that virtual elimination of binding to cadherin BR-175 does not completely remove toxicity in the case of B. mori.

An ADAM metalloprotease is a Cry3Aa Bacillus thuringiensis toxin receptor

Bacillus thuringiensis insecticidal proteins toxic action relies on the interaction with receptor molecules on insect midgut target cells. Here, we describe an ADAM metalloprotease as a novel type of B. thuringiensis toxin receptor on the basis of the following data: (i) by ligand blot and N-terminal analysis, we detected a Colorado potato beetle Cry3Aa toxin binding molecule that shares homology with an ADAM10 metalloprotease; (ii) Colorado potato beetle brush border membrane vesicles display ADAM activity since it cleaves an ADAM fluorogenic substrate; (iii) Cry3Aa acts as a competitor of the cleavage of the ADAM fluorogenic substrate; (iv) Cry3Aa sequence contains the recognition motif R(345)FQPGYYGND(354) present in ADAM10 substrates. Accordingly, a peptide representative of the recognition motif localized within loop 1 of Cry3Aa domain II (Ac-F(341)HTRFQPGYYGNDSFN(358)-NH(2)) effectively prevented Cry3Aa proteolytic processing and nearly abolished pore formation, evidencing the functional significance of the Cry3Aa-ADAM interaction in relation to this toxin mode of action.

Structure of the functional form of the mosquito larvicidal Cry4Aa toxin from Bacillus thuringiensis at a 2.8-angstrom resolution

The Cry4Aa delta-endotoxin from Bacillus thuringiensis is toxic to larvae of Culex, Anopheles, and Aedes mosquitoes, which are vectors of important human tropical diseases. With the objective of designing modified toxins with improved potency that could be used as biopesticides, we determined the structure of this toxin in its functional form at a resolution of 2.8 angstroms. Like other Cry delta-endotoxins, the activated Cry4Aa toxin consists of three globular domains, a seven-alpha-helix bundle responsible for pore formation (domain I) and the following two other domains having structural similarities with carbohydrate binding proteins: a beta-prism (domain II) and a plant lectin-like beta-sandwich (domain III). We also studied the effect on toxicity of amino acid substitutions and deletions in three loops located at the surface of the putative receptor binding domain II of Cry4Aa. Our results indicate that one loop is an important determinant of toxicity, presumably through attachment of Cry4Aa to the surface of mosquito cells. The availability of the Cry4Aa structure should guide further investigations aimed at the molecular basis of the target specificity and membrane insertion of Cry endotoxins.

Functional display of Bacillus thuringiensis Cry1Ac toxin on T7 phage

The Cry1Ac toxin from Bacillus thuringiensis was displayed on the surface of T7 phage. The cry1Ac gene was fused to the C-terminal end of T7-10B capsid protein and displayed on the surface of T7 phage as revealed by Western blot analysis of the purified phage particles. The T7-Cry1Ac phages retained toxicity against Manduca sexta larvae. We demonstrated that the T7-Cry1Ac phage interacts with Cry1Ac receptors present in M. sexta BBMV either in solution or in overlay binding assays.

Redesigning Bacillus thuringiensis Cry1Aa toxin into a mosquito toxin

The Bacillus thuringiensis crystal protein Cry1Aa is normally selectively active to caterpillar larvae. Through rational design, toxicity (microg/ml) to the mosquito Culex pipiens was introduced by selected deletions and substitutions of the loop residues of domain II. Toxicity to its natural target Manduca sexta was concomitantly abolished. The successful grafting of the alternate mosquito toxicity onto the original lepidopteran Cry1Aa toxin demonstrates the possibility of designing and engineering a desired toxicity into any toxin of a common scaffold by reshaping the receptor binding region with desired specificities.

Targeted mutagenesis of loop residues in the receptor-binding domain of theBacillus thuringiensisCry4Ba toxin affects larvicidal activity

Loop residues in domain II of Bacillus thuringiensis Cry delta-endotoxins have been demonstrated to be involved in insecticidal specificity. In this study, selected residues in loops beta6-beta7 (S(387)SPS(390)), beta8-beta9 (S(410), N(411), T(413), T(415), E(417) and G(418)) and beta10-beta11 (D(454)YNS(457)) in domain II of the Cry4Ba mosquito-larvicidal protein were changed individually to alanine by PCR-based directed mutagenesis. All mutant toxins were expressed in Escherichia coli JM109 cells as 130-kDa protoxins at levels comparable to the wild type. Only E. coli cells that express the P389A, S410A, E417A, Y455A or N456A mutants exhibited a loss in toxicity against Aedes aegypti mosquito larvae of approximately 30% when compared to the wild type. In addition, E. coli cells expressing double mutants, S410A/E417A or Y455A/N456A, at wild-type levels revealed a significantly higher loss in larvicidal activity of approximately 70%. Similar to the wild-type protoxin, both double mutant toxins were structurally stable upon solubilisation and trypsin activation in carbonate buffer, pH 9.0. These results indicate that S(410) and E(417) in the beta8-beta9 loop, and Y(455) and N(456) in the beta10-beta11 loop are involved in larvicidal activity of the Cry4Ba toxin.

Enhancement of Cry19Aa mosquitocidal activity against Aedes aegypti by mutations in the putative loop regions of domain II

Improvements in the mosquitocidal activity of Bacillus thuringiensis Cry19Aa were achieved by protein engineering of putative surface loop residues in domain II through rational design. The improvement of Aedes toxicity in Cry19Aa was 42,000-fold and did not affect its toxicity against Anopheles or Culex.

Role of toxin activation on binding and pore formation activity of the Bacillus thuringiensis Cry3 toxins in membranes of Leptinotarsa decemlineata (Say)

Binding and pore formation constitute key steps in the mode of action of Bacillus thuringiensis delta-endotoxins. In this work, we present a comparative analysis of toxin-binding capacities of proteolytically processed Cry3A, Cry3B and Cry3C toxins to brush border membranes (BBMV) of the Colorado potato beetle Leptinotarsa decemlineata (CPB), a major potato coleopteran-insect pest. Competition experiments showed that the three Cry3 proteolytically activated toxins share a common binding site. Also heterologous competition experiments showed that Cry3Aa and Cry3Ca toxins have an extra binding site that is not shared with Cry3Ba toxin. The pore formation activity of the three different Cry3 toxins is analysed. High pore-formation activities were observed in Cry3 toxins obtained by proteolytical activation with CPB BBMV in contrast to toxins activated with either trypsin or chymotrypsin proteases. The pore-formation activity correlated with the formation of soluble oligomeric structures. Our data support that, similarly to the Cry1A toxins, the Cry3 oligomer is formed after receptor binding and before membrane insertion, forming a pre-pore structure that is insertion-competent.