Single-site mutations in the conserved alternating-arginine region affect ionic channels formed by CryIAa, a Bacillus thuringiensis toxin

The base and root of domain II loops of Cry toxins contribute to binding to Bombyx mori ABC transporter C2

Little information is available regarding the region of Cry toxins involved in binding to their major receptors, the ATP-binding cassette (ABC) transporters. We analyzed which Cry1Aa amino acid residues contribute to binding to Bombyx mori ABC transporter C2 (BmABCC2). Several two oxidized double-cysteine substitution mutant toxins were made. In these, two amino acids at distant positions on toxin loop α8 and loop 2 or loop 2 and loop 3 were substituted with cysteine residues and crosslinked. These mutants exhibited a marked reduction in binding affinity to BmABCC2, suggesting that the binding site comprises complex cavities formed by loops α8, 2, and 3. Loop swapping between Cry1Aa and other BmABCC2-incompatible toxins indicated that loop 2 acts as a binding affinity-generating part of Cry1Aa toxin. Using single amino acid substitution mutants, the results of surface plasmon resonance (SPR) analysis and response assays with BmABCC2-expressing Sf9 cells indicated that Y366, R367, R368, and L447 in the Cry1Aa root and base region of loops 2 and 3 play important roles in binding. Furthermore, SPR analyses of these mutants suggested that a two-state binding model fits best the data obtained. Moreover, complex cavities and the above-mentioned amino acid residues contribute to the generation of multiple binding points and high-affinity binding. Finally, we found that the binding site of B. mori cadherin-like protein consists of complex cavities comprising loops 1, 2, and 3, partially overlapping that of BmABCC2, suggesting that the loop region of Cry1Aa toxin acts as a promiscuous binding site.

A novel Vip3Aa16-Cry1Ac chimera toxin: Enhancement of toxicity against Ephestia kuehniella, structural study and molecular docking

Bacillus thuringiensis Vip3A protein has been widely used for crop protection and for delay resistance to existing insecticidal Cry toxins. During current study, a fusion between vip3Aa16 and the toxic core sequence of cry1Ac was constructed in pHT Blue plasmid. Vip3Aa16-Cry1Ac protein was expressed in the supernatant of B. thuringiensis with a size of about 150 kDa. Bioassays tested on Ephestia kuehniella showed that the use of the chimera toxin as biopesticide improved the toxicity to reach 90% ± 2 with an enhancement of 20% compared to the single Vip3Aa16 protein. The findings indicated that the fusion protein design opens new ways to enhance Vip3A toxicity against lepidopteran species and could avoiding insect tolerance of B. thuringiensis delta-endotoxins. Through computational study, we have predicted for the first time the whole 3D structure of a Vip3A toxin. We showed that Vip3Aa16 structure is composed by three domains like Cry toxins: an N-terminal domain containing hemolysin like fold as well as two others Carbohydrate Binding Module (CBM)-like domains. Molecular docking analysis of the chimera toxin and the single Vip3Aa16 protein against specific insect receptors revealed that residues of CBM like domains are clearly involved in the binding of the toxin to receptors.

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.

Impact of Q139R substitution of MEB4-Cry2Aa toxin on its stability, accessibility and toxicity against Ephestia kuehniella

The Bacillus thuringiensis subsp. kurstaki strain MEB4 was previously found to be highly toxic to Ephestia kuehniella. SDS-PAGE analysis of the recombinant strain DH5α (pBS-cry2Aa-MEB4) showed that Cry2Aa-MEB4 delta-endotoxins were forming inclusion bodies, and were 2.75 fold more toxic towards E. kuehniella than those of Cry2Aa-BNS3. Besides to the 65kDa active toxin, proteolysis activation of Cry2Aa-BNS3 protein with E. kuehniella midgut juice generated an extra proteolysis form of 49kDa, which was the result of another chymotrypsin cleavage located in Leu144. The amino acid sequences alignment of Cry2Aa-MEB4 and Cry2Aa-BNS3 showed that among the different 15 amino acids, the Q139R substitution was found to be interesting. In fact, due to its presence within the loop α3-α4, the chymotrypsin-like protease was unable to access to its site in Cry2Aa-MEB4, resulting to the production of only the 65kDa form. The accessible surface and the stability studies of the structure model of the Cry2Aa-BNS3-49 form showed a lower hydrophobicity surface due to the omission of 144 amino acids from the N-terminal comparing with the active Cry2Aa-MEB4 protein. All these features caused the diminishing of Cry2Aa-BNS3 toxicity towards E. kuehniella.

Bacillus thuringiensis Cry1A toxins are versatile proteins with multiple modes of action: two distinct pre-pores are involved in toxicity

Cry proteins from Bacillus thuringiensis are insecticidal PFTs (pore-forming toxins). In the present study, we show that two distinct functional pre-pores of Cry1Ab are formed after binding of the protoxin or the protease-activated toxin to the cadherin receptor, but before membrane insertion. Both pre-pores actively induce pore formation, although with different characteristics, and contribute to the insecticidal activity. We also analysed the oligomerization of the mutant Cry1AbMod protein. This mutant kills different insect populations that are resistant to Cry toxins, but lost potency against susceptible insects. We found that the Cry1AbMod-protoxin efficiently induces oligomerization, but not the activated Cry1AbMod-toxin, explaining the loss of potency of Cry1AbMod against susceptible insects. These data are relevant for the future control of insects resistant to Cry proteins. Our data support the pore-formation model involving sequential interaction with different midgut proteins, leading to pore formation in the target membrane. We propose that not only different insect targets could have different receptors, but also different midgut proteases that would influence the rate of protoxin/toxin activation. It is possible that the two pre-pore structures could have been selected for in evolution, since they have differential roles in toxicity against selected targets, increasing their range of action. These data assign a functional role for the protoxin fragment of Cry PFTs that was not understood previously. Most PFTs produced by other bacteria are secreted as protoxins that require activation before oligomerization, to finally form a pore. Thus different pre-pores could be also part of the general mechanism of action of other PFTs.

Two distinct functional pre-pore oligomers of the Cry1Ab insecticidal toxin are formed before membrane insertion. These oligomers are formed after binding of either the protoxin or the protease-activated toxin to the cadherin receptor. Both pre-pores have different characteristics and contribute to insecticidal activity.

In silico modeling and functional interpretations of Cry1Ab15 toxin from Bacillus thuringiensis BtB-Hm-16

The theoretical homology based structural model of Cry1Ab15 δ-endotoxin produced by Bacillus thuringiensis BtB-Hm-16 was predicted using the Cry1Aa template (resolution 2.25 Å). The Cry1Ab15 resembles the template structure by sharing a common three-domain extending conformation structure responsible for pore-forming and specificity determination. The novel structural differences found are the presence of β0 and α3, and the absence of α7b, β1a, α10a, α10b, β12, and α11a while α9 is located spatially downstream. Validation by SUPERPOSE and with the use of PROCHECK program showed folding of 98% of modeled residues in a favourable and stable orientation with a total energy Z-score of −6.56; the constructed model has an RMSD of only 1.15 Å. These increments of 3D structure information will be helpful in the design of domain swapping experiments aimed at improving toxicity and will help in elucidating the common mechanism of toxin action.

Computational Modeling Deduced Three Dimensional Structure of Cry1Ab16 Toxin from Bacillus thuringiensis AC11

The first theoretical structural model of newly reported Cry1Ab16 δ-endotoxin produced by Bacillus thuringiensis AC11 was predicted using homology modeling technique. Cry1Ab16 resembles the Cry1Aa protein structure by sharing a common three domains structure responsible in pore forming and specificity determination along with few structural deviations. The main differences between the two is in the length of loops, absence of α7b, α9a, α10b, α11a and presence of additional β12b, α13 components while α10a is spatially located at downstream position in Cry1Ab16. A better understanding of the 3D structure shall be helpful in the design of domain swapping and mutagenesis experiments aimed at improving toxicity.

Expression of cry3A gene and its toxicity against Asian Gray Weevil Myllocerus undecimpustulatus undatus Marshall (Coleoptera: Curculionidae)

Coleopterans are the most damaging pests of many agricultural and forestry crops; there is an urgent need to develop effective biopesticides against these insects. Enhancers of Bt toxicity typify an opportunity to improve currently available commercial products into more effective control agents against diverse pests. A 1.9 kb DNA fragment, PCR amplified from native isolates of Bt using cry3A gene specific primers was cloned in expression vector pQE-80L and then used for transformation of Escherichia coli M15 cells. The sequence of the cloned crystal protein gene showed almost complete homology with a Coleopteran active Cry3A toxin gene with 117 mutations scattered in different domain regions encoding a protein of 645 amino acid residues in length, with a predicted molecular mass of 77.4 kDa. Phylogenetic analysis could be compulsive for new/novel Bacillus thuringiensis strains, allowing them to be grouped with related Cry proteins. The toxicity of Bt protein was determined against Myllocerus undecimpustulatus undatus Marshall (Coleoptera: Curculionidae) LC50 152 ng cm(-2). Genes coding for Coleopteran active Cry3A proteins have been isolated and their efficient expression will provide the tools necessary to increase the efficacy of Cry-based biopesticide against economically important beetles.

Non-silent story on synonymous sites in voltage-gated ion channel genes

Synonymous mutations are usually referred to as “silent”, but increasing evidence shows that they are not neutral in a wide range of organisms. We looked into the relationship between synonymous codon usage bias and residue importance of voltage-gated ion channel proteins in mice, rats, and humans. We tested whether translationally optimal codons are associated with transmembrane or channel-forming regions, i.e., the sites that are particularly likely to be involved in the closing and opening of an ion channel. Our hypothesis is that translationally optimal codons are preferred at the sites within transmembrane domains or channel-forming regions in voltage-gated ion channel genes to avoid mistranslation-induced protein misfolding or loss-of-function. Using the Mantel-Haenszel procedure, which applies to categorical data, we found that translationally optimal codons are more likely to be used at transmembrane residues and the residues involved in channel-forming. We also found that the conservation level at synonymous sites in the transmembrane region is significantly higher than that in the non-transmembrane region. This study provides evidence that synonymous sites in voltage-gated ion channel genes are not neutral. Silent mutations at channel-related sites may lead to dysfunction of the ion channel.

Novel Bacillus thuringiensis δ-endotoxin active against Locusta migratoria manilensis

A novel δ-endotoxin gene was cloned from a Bacillus thuringiensis strain with activity against Locusta migratoria manilensis by PCR-based genome walking. The sequence of the cry gene was 3,432 bp long, and it encoded a Cry protein of 1,144 amino acid residues with a molecular mass of 129,196.5 kDa, which exhibited 62% homology with Cry7Ba1 in the amino acid sequence. The δ-endotoxin with five conserved sequence blocks in the amino-terminal region was designated Cry7Ca1 (GenBank accession no. EF486523). Protein structure analysis suggested that the activated toxin of Cry7Ca1 has three domains: 227 residues forming 7 α-helices (domain I); 213 residues forming three antiparallel β-sheets (domain II); and 134 residues forming a β-sandwich (domain III). The three domains, respectively, exhibited 47, 44, and 34% sequence identity with corresponding domains of known Cry toxins. SDS-PAGE and Western blot analysis showed that Cry7Ca1, encoded by the full-length open reading frame of the cry gene, the activated toxin 1, which included three domains but without the N-terminal 54 amino acid residues and the C terminus, and the activated toxin 2, which included three domains and N-terminal 54 amino acid residues but without the C terminus, could be expressed in Escherichia coli. Bioassay results indicated that the expressed proteins of Cry7Ca1 and the activated toxins (toxins 1 and 2) showed significant activity against 2nd instar locusts, and after 7 days of infection, the estimated 50% lethal concentrations (LC₅₀s) were 8.98 μg/ml for the expressed Cry7Ca1, 0.87 μg/ml for the activated toxin 1, and 4.43 μg/ml for the activated toxin 2. The δ-endotoxin also induced histopathological changes in midgut epithelial cells of adult L. migratoria manilensis.

A Cry1Ac toxin variant generated by directed evolution has enhanced toxicity against Lepidopteran insects

Cry1Ac insecticidal crystal proteins produced by Bacillus thuringiensis (Bt) have become an important natural biological agent for the control of lepidopteran insects. In this study, a cry1Ac toxin gene from Bacillus thuringiensis 4.0718 was modified by using error-prone PCR, staggered extension process (StEP) shuffling combined with Red/ET homologous recombination to investigate the insecticidal activity of delta-endotoxin Cry1Ac. A Cry1Ac toxin variant (designated as T524N) screened by insect bioassay showed increased insecticidal activity against Spodoptera exigua larvae while its original insecticidal activity against Helicoverpa armigera larvae was still retained. The mutant toxin T524N had one amino acid substitution at position 524 relative to the original Cry1Ac toxin, and it can accumulate within the acrystalliferous strain Cry-B and form more but a little smaller bipyramidal crystals than the original Cry1Ac toxin. Analysis of theoretical molecular models of mutant and original Cry1Ac proteins indicated that the mutation T524N located in the loop linking β16-β17 of domain III in Cry1Ac toxin happens in the fourth conserved block which is an arginine-rich region to form a highly hydrophobic surface involving interaction with receptor molecules. This study showed for the first time that single mutation T524N played an essential role in the insecticidal activity. This finding provides the biological evidence of the structural function of domain III in insecticidal activity of the Cry1Ac toxin, which probably leads to a deep understanding between the interaction of toxic proteins and receptor macromolecules.

Lepidopteran-active variable-region sequence imparts coleopteran activity in eCry3.1Ab, an engineered Bacillus thuringiensis hybrid insecticidal protein

A unique, coleopteran-active protein, termed eCry3.1Ab, was generated following variable-region exchange of a Bacillus thuringiensis lepidopteran-active protein, Cry1Ab, with a Cry3A region. Our results support the hypothesis that this variable-region exchange is responsible for imparting strong bioactivity against the larvae of western corn rootworm (WCR) (Diabrotica virgifera virgifera LeConte), a pest species which is not susceptible to either parent protein sequence. This study demonstrates the potential of successfully engineering a portion(s) of a lepidopteran-active B. thuringiensis sequence so that it has activity against coleopterans. Further elucidation of the eCry3.1Ab activity indicated the importance of variable regions 4 to 6 that were derived from Cry1Ab instead of Cry1Ac. There was some flexibility in making domain III of engineered hybrid insecticidal proteins even more Cry1Ab-like and retaining activity, while there was less flexibility in making domain III more Cry3A-like and retaining activity. In vitro binding studies with brush border membrane vesicles demonstrated that there was specific binding of chymotrypsin-processed modified Cry3A (mCry3A), which was not diminished by addition of a 100-fold molar excess of chymotrypsin-processed eCry3.1Ab or unprocessed eCry3.1Ab. In addition, in the converse experiment, specific binding of chymotrypsin-processed eCry3.1Ab was not diminished by the presence of a 75-fold molar excess of chymotrypsin-processed mCry3A. These data support the hypothesis that eCry3.1Ab can interact with different binding sites than the activated form of mCry3A in the WCR brush border and may provide a different mode of action from the standpoint of resistance management.

Pore formation by Cry toxins

Bacillus thuringiensis (Bt) bacteria produce insecticidal Cry and Cyt proteins used in the biological control of different insect pests. In this review, we will focus on the 3d-Cry toxins that represent the biggest group of Cry proteins and also on Cyt toxins. The 3d-Cry toxins are pore-forming toxins that induce cell death by forming ionic pores into the membrane of the midgut epithelial cells in their target insect. The initial steps in the mode of action include ingestion of the protoxin, activation by midgut proteases to produce the toxin fragment and the interaction with the primary cadherin receptor. The interaction of the monomeric CrylA toxin with the cadherin receptor promotes an extra proteolytic cleavage, where helix alpha-1 of domain I is eliminated and the toxin oligomerization is induced, forming a structure of 250 kDa. The oligomeric structure binds to a secondary receptor, aminopeptidase N or alkaline phosphatase. The secondary receptor drives the toxin into detergent resistant membrane microdomains formingpores that cause osmotic shock, burst of the midgut cells and insect death. Regarding to Cyt toxins, these proteins have a synergistic effect on the toxicity of some Cry toxins. Cyt proteins are also proteolytic activated in the midgut lumen of their target, they bind to some phospholipids present in the mosquito midgut cells. The proposed mechanism of synergism between Cry and Cyt toxins is that Cyt1Aa function as a receptor for Cry toxins. The Cyt1A inserts into midgut epithelium membrane and exposes protein regions that are recognized by Cry11Aa. It was demonstrated that this interaction facilitates the oligomerization of Cry11Aa and also its pore formation activity.

Ion channels formed in planar lipid bilayers by the dipteran-specific Cry4BBacillus thuringiensistoxin and its α1–α5 fragment

Trypsin activation of Cry4B, a 130-kDa Bacillus thuringiensis (Bt) protein, produces a 65-kDa toxin active against mosquito larvae. The active toxin is made of two protease resistant-products of ca. 45 kDa and ca. 20 kDa. The cloned 21-kDa fragment consisting of the N-terminal region of the toxin was previously shown to be capable of permeabilizing liposomes. The present study was designed to test the following hypotheses: (1) Cry4B, like several other Bt toxins, is a channel-forming toxin in plannar lipid bilayers; and (2) the 21-kDa N-terminal region, which maps for the first five helices (alpha1-alpha5) of domain 1 in other Cry toxins, and which putatively shares a similar tri-dimensional structure, is sufficient to account for the ion channel activity of the whole toxin. Using circular dichroism spectroscopy and planar lipid bilayers, we showed that the 21-kDa polypeptide existed as an alpha-helical structure and that both Cry4B and its alpha1-alpha5 fragment formed ion channels of 248 +/- 44 pS and 207 +/- 23 pS, respectively. The channels were cation-selective with a potassium-to-chloride permeability ratio of 6.7 for Cry4B and 4.5 for its fragment. However, contrary to the full-length toxin, the alpha1-alpha5 region formed channels at low dose; they tended to remain locked in their open state and displayed flickering activity bouts. Thus, like the full-length toxin, the alpha1-alpha5 region is a functional channel former. A pH-dependent, yet undefined region of the toxin may be involved in regulating the channel properties.

The theoretical three-dimensional structure of Bacillus thuringiensis Cry5Aa and its biological implications

Cry5Aa is a crystal protein produced by Bacillus thuringiensis serovar. damstadiensis during its stationary phase, this delta-endotoxin is active against nematodes and has great potential for nematodes control. The theoretical model of the three-dimensional structure of Cry5Aa was predicted by homology modeling on the structures of the Cry1Aa which is specific to Lepidopteran insects. The structure of the Cry5Aa resembles previously reported Cry toxin structures but shows the following distinctions. Cry5Aa has a long insertion in alpha2 of domain I. Some loops in the domain II and III of Cry5Aa are exposed to the solvent. In this work we give a brief description of our model and hypothesize the residues of the Cry5Aa that could be important in receptor recognition and pore formation. This model will be helpful for the design of mutagenesis experiments aimed to the improvement of toxicity, and lead to a deep understanding of the mechanism of action of nematicidal toxins.

The theoretical 3D structure of Bacillus thuringiensis Cry5Ba

Cry5Ba is a delta-endotoxin produced by Bacillus thuringiensis PS86A1 NRRL B-18900. It is active against nematodes and has great potential for nematode control. Here, we predict the first theoretical model of the three-dimensional (3D) structure of a Cry5Ba toxin by homology modeling on the structure of the Cry1Aa toxin, which is specific to Lepidopteran insects. Cry5Ba resembles the previously reported Cry1Aa toxin structure in that they share a common 3D structure with three domains, but there are some distinctions, with the main differences being located in the loops of domain I. Cry5Ba exhibits a changeable extending conformation structure, and this special structure may also be involved in pore-forming and specificity determination. A fuller understanding of the 3D structure will be helpful in the design of mutagenesis experiments aimed at improving toxicity, and lead to a deep understanding of the mechanism of action of nematicidal toxins.

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.

Effects of the Bacillus thuringiensis Toxin Cry1Ab on Membrane Currents of Isolated Cells of the Ruminal Epithelium

A previous study has shown that Cry1Ab, a lepidopteran-specific toxin derived from Bacillus thuringiensis, does not affect the vitality of cultured cells of the ruminal epithelium of the sheep. While this may be due to lack of specific receptors for toxin action, other mechanisms of resistance should also be considered. In order to directly assess the pore-forming potential of Cry1Ab, we studied the interaction of this toxin with isolated, perfused cells of the ruminal epithelium using the whole-cell and single-channel configurations of the patch-clamp technique. At concentrations found in vivo in the rumen of cows (<10 ng/ml) and at a temperature of 37 degrees C, no significant effects of Cry1Ab could be observed. At 100 ng/ml, exposure of ruminal cells to Cry1Ab induced a significant rise in outward current in 16 of 34 cells, with a fourfold increase in the conductance for potassium. The cell membrane remained selective for potassium over sodium (p(K)/p(Na) = 1.8 + or - 0.3), with a considerable additional chloride conductance. In outside-out patches, exposure to high Cry1Ab concentrations induced channel-like events that reached levels of over 500 pS. We conclude that the unchanged vitality of intact ruminal epithelial cells exposed to Cry1Ab in vitro at high concentrations may be related to other factors besides the proposed absence of a specific receptor for the membrane insertion of this toxin.

A mechanical force contributes to the "osmotic swelling" of brush-border membrane vesicles

Brush-border membrane vesicles and an osmotic swelling assay have been used extensively to monitor the pore-forming activity of Bacillus thuringiensis toxins. After a hypertonic shock, Manduca sexta midgut brush-border membrane vesicles shrink rapidly and reswell partially to a volume that depends on membrane permeability and toxin concentration rather than regaining their original volume as expected from theoretical models. Because efflux of buffer from the vesicles, as they shrink, could contribute to this phenomenon, vesicles were mixed with a hypertonic solution of the buffer with which they were loaded. Under these conditions, they are not expected to reswell, since the same solute is present on both sides of the membrane. Nevertheless, with several buffers, vesicles reswelled readily, an observation that demonstrates the involvement of an additional restoration force. Reswelling also occurred when, in the absence of toxin, the buffers were replaced by glucose, a solute that diffuses readily across the membrane, but did not occur with rat liver microsomes, despite their permeability to glucose. Unexpected swelling was also observed with rabbit jejunum brush-border membrane vesicles, suggesting that the cytoskeleton, present in brush-border membrane vesicles but absent from microsomes, could be responsible for the restoration force.

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.

Location of the Bombyx mori aminopeptidase N type 1 binding site on Bacillus thuringiensis Cry1Aa toxin

We analyzed the binding site on Cry1Aa toxin for the Cry1Aa receptor in Bombyx mori, 115-kDa aminopeptidase N type 1 (BmAPN1) (K. Nakanishi, K. Yaoi, Y. Nagino, H. Hara, M. Kitami, S. Atsumi, N. Miura, and R. Sato, FEBS Lett. 519:215-220, 2002), by using monoclonal antibodies (MAbs) that block binding between the binding site and the receptor. First, we produced a series of MAbs against Cry1Aa and obtained two MAbs, MAbs 2C2 and 1B10, that were capable of blocking the binding between Cry1Aa and BmAPN1 (blocking MAbs). The epitope of the Fab fragments of MAb 2C2 overlapped the BmAPN1 binding site, whereas the epitope of the Fab fragments of MAb 1B10 did not overlap but was located close to the binding site. Using three approaches for epitope mapping, we identified two candidate epitopes for the blocking MAbs on Cry1Aa. We constructed two Cry1Aa toxin mutants by substituting a cysteine on the toxin surface at each of the two candidate epitopes, and the small blocking molecule N-(9-acridinyl)maleimide (NAM) was introduced at each cysteine substitution to determine the true epitope. The Cry1Aa mutant with NAM bound to Cys582 did not bind either of the two blocking MAbs, suggesting that the true epitope for each of the blocking MAbs was located at the site containing Val582, which also consisted of 508STLRVN513 and 582VFTLSAHV589. These results indicated that the BmAPN1 binding site overlapped part of the region blocked by MAb 2C2 that was close to but excluded the actual epitope of MAb 2C2 on domain III of Cry1Aa toxin. We also discuss another area on Cry1Aa toxin as a new candidate site for BmAPN1 binding.

Characterization of a new Bacillus thuringiensis endotoxin, Cry47Aa, from strains that are toxic to the Australian sheep blowfly, Lucilia cuprina

Sixteen isolates of Bacillus thuringiensis, derived from various soil samples collected in Australia, are highly toxic to larvae of the sheep blowfly (Lucilia cuprina). The toxin gene from one of the strains (CAA890) was cloned by genome walking, and sequencing of the cloned fragments revealed a new cry gene, encoding a protein of 1134 amino acid residues, with a theoretical molecular mass of 139,209Da. Based on the amino acid sequence comparison with known Cry delta-endotoxins, the gene was designated cry47Aa. Homology modelling based on known crystal structures of the Cry toxins reveals the differences to be located in the loops of domain II in the putative toxin-receptor binding surfaces between Cry47Aa and the dipteran active Cry2Aa. We also showed that the cry47Aa gene is present in the other isolates that are highly toxic to the sheep blowfly.

Estimation of the radius of the pores formed by the Bacillus thuringiensis Cry1C delta-endotoxin in planar lipid bilayers

Pore formation constitutes a key step in the mode of action of Bacillus thuringiensis delta-endotoxins and various activated Cry toxins have been shown to form ionic channels in receptor-free planar lipid bilayers at high concentrations. Multiple conductance levels have been observed with several toxins, suggesting that the channels result from the multimeric assembly of a variable number of toxin molecules. To test this possibility, the size of the channels formed by Cry1C was estimated with the non-electrolyte exclusion technique and polyethylene glycols of various molecular weights. In symmetrical 300 mM KCl solutions, Cry1C induced channel activity with 15 distinct conductance levels ranging from 21 to 246 pS and distributed in two main conductance populations. Both the smallest and largest conductance levels and the mean conductance values of both populations were systematically reduced in the presence of polyethylene glycols with hydrated radii of up to 1.05 nm, indicating that these solutes can penetrate the pores formed by the toxin. Larger polyethylene glycols had little effect on the conductance levels, indicating that they were excluded from the pores. Our results indicate that Cry1C forms clusters composed of a variable number of channels having a similar pore radius of between 1.0 and 1.3 nm and gating synchronously.

Amino acid and divalent ion permeability of the pores formed by the Bacillus thuringiensis toxins Cry1Aa and Cry1Ac in insect midgut brush border membrane vesicles

The pores formed by Bacillus thuringiensis insecticidal toxins have been shown to allow the diffusion of a variety of monovalent cations and anions and neutral solutes. To further characterize their ion selectivity, membrane permeability induced by Cry1Aa and Cry1Ac to amino acids (Asp, Glu, Ser, Leu, His, Lys and Arg) and to divalent cations (Mg(2+), Ca(2+) and Ba(2+)) and anions (SO(4)(2-) and phosphate) was analyzed at pH 7.5 and 10.5 with midgut brush border membrane vesicles isolated from Manduca sexta and an osmotic swelling assay. Shifting pH from 7.5 to 10.5 increases the proportion of the more negatively charged species of amino acids and phosphate ions. All amino acids diffused well across the toxin-induced pores, but, except for aspartate and glutamate, amino acid permeability was lower at the higher pH. In the presence of either toxin, membrane permeability was higher for the chloride salts of divalent cations than for the potassium salts of divalent anions. These results clearly indicate that the pores are cation-selective.

Mutagenic analysis of a conserved region of domain III in the Cry1Ac toxin of Bacillus thuringiensis

We used site-directed mutagenesis to probe the function of four alternating arginines located at amino acid positions 525, 527, 529, and 531 in a highly conserved region of domain III in the Cry1Ac toxin of Bacillus thuringiensis. We created 10 mutants: eight single mutants, with each arginine replaced by either glycine (G) or aspartic acid (D), and two double mutants (R525G/R527G and R529G/R531G). In lawn assays of the 10 mutants with a cultured Choristoneura fumiferana insect cell line (Cf1), replacement of a single arginine by either glycine or aspartic acid at position 525 or 529 decreased toxicity 4- to 12-fold relative to native Cry1Ac toxin, whereas replacement at position 527 or 531 decreased toxicity only 3-fold. The reduction in toxicity seen with double mutants was 8-fold for R525G/R527G and 25-fold for R529G/R531G. Five of the mutants (R525G, R525D, R527G, R529D, and R525G/R527G) were tested in bioassays with Plutella xylostella larvae and ion channel formation in planar lipid bilayers. In the bioassays, R525D, R529D, and R525G/R527G showed reduced toxicity. In planar lipid bilayers, the conductance and the selectivity of the mutants were similar to those of native Cry1Ac. Toxins with alteration at position 527 or 529 tended to remain in their subconducting states rather than the maximally conducting state. Our results suggest that the primary role of this conserved region is to maintain both the structural integrity of the native toxin and the full functionality of the formed membrane pore.

Unfolding affects insect cell permeabilization by Bacillus thuringiensis Cry1C toxin

Bacillus thuringiensis Cry toxins are efficient, environment-friendly biological insecticides. Their molecular mode of action on target insect cells remains largely unknown. The aim of this study was to investigate the relation between the conformational state of the Cry1C toxin and its ionophoric activity on live Sf9 cells of Spodoptera frugiperda, a target insect for this protein. Potassium ion movement induced by Cry1C across the cell membrane was measured with a fluorescent assay developed previously and the conformation of the toxin was studied using tryptophan spectroscopy. Following treatment with 4 M guanidinium hydrochloride, which resulted in the unfolding of its N-terminal half, the toxin retained its full capacity to permeabilize the cells while the fully unfolded toxin did not induce potassium leakage. Therefore, permeabilization of Sf9 cells by Cry1C requires the integrity of the C-terminal half of the toxin and may depend on an initial unfolding step provided by the acidic environment of the cells.

The alpha-helix 4 residue, Asn135, is involved in the oligomerization of Cry1Ac1 and Cry1Ab5 Bacillus thuringiensis toxins

The insecticidal Cry toxins produced by the bacterium Bacillus thuringiensis are comprised of three structural domains. Domain I, a seven-helix bundle, is thought to penetrate the insect epithelial cell plasma membrane through a hairpin composed of alpha-helices 4 and 5, followed by the oligomerization of four hairpin monomers. The alpha-helix 4 has been proposed to line the lumen of the pore, whereas some residues in alpha-helix 5 have been shown to be responsible for oligomerization. Mutation of the Cry1Ac1 alpha-helix 4 amino acid Asn135 to Gln resulted in the loss of toxicity to Manduca sexta, yet binding was still observed. In this study, the equivalent mutation was made in the Cry1Ab5 toxin, and the properties of both wild-type and mutant toxin counterparts were analyzed. Both mutants appeared to bind to M. sexta membrane vesicles, but they were not able to form pores. The ability of both N135Q mutants to oligomerize was also disrupted, providing the first evidence that a residue in alpha-helix 4 can contribute to toxin oligomerization.

Structure of Cry3A delta-endotoxin within phospholipid membranes

Interaction of delta-endotoxin and its proteolytic fragments with phospholipid vesicles was studied using electron microscopy, scanning microcalorimetry, and limited proteolysis. It was shown that native protein destroys liposomes. The removal of 4 N-terminal alpha-helices and the extreme 56 C-terminal amino acid residues did not affect this ability. The results obtained by limited proteolysis of delta-endotoxin bound to lipid vesicles show essential conformational changes in three or four N-terminal helices and in the C-terminal region. The calorimetric method used in this study provides a unique possibility for the validation of existing models of protein binding and for a more accurate determination of the regions where conformational changes take place. It was found that the binding of the protein to model liposomes does not alter its structure in the regions starting with the fourth alpha-helix of domain I. This can be concluded from the fact that the activation energy of denaturation of the protein remains unchanged upon its binding to the phospholipid membranes. A new structural model has been proposed which agrees with the data obtained.

Structure of Cry2Aa suggests an unexpected receptor binding epitope

BACKGROUND

Genetically modified (GM) crops that express insecticidal protein toxins are an integral part of modern agriculture. Proteins produced by Bacillus thuringiensis (Bt) during sporulation mediate the pathogenicity of Bt toward a spectrum of insect larvae whose breadth depends upon the Bt strain. These transmembrane channel-forming toxins are stored in Bt as crystalline inclusions called Cry proteins. These proteins are the active agents used in the majority of biorational pesticides and insect-resistant transgenic crops. Though Bt toxins are promising as a crop protection alternative and are ecologically friendlier than synthetic organic pesticides, resistance to Bt toxins by insects is recognized as a potential limitation to their application.

RESULTS

We have determined the 2.2 A crystal structure of the Cry2Aa protoxin by multiple isomorphous replacement. This is the first crystal structure of a Cry toxin specific to Diptera (mosquitoes and flies) and the first structure of a Cry toxin with high activity against larvae from two insect orders, Lepidoptera (moths and butterflies) and Diptera. Cry2Aa also provides the first structure of the proregion of a Cry toxin that is cleaved to generate the membrane-active toxin in the larval gut.

CONCLUSIONS

The crystal structure of Cry2Aa reported here, together with chimeric-scanning and domain-swapping mutagenesis, defines the putative receptor binding epitope on the toxin and so may allow for alteration of specificity to combat resistance or to minimize collateral effects on nontarget species. The putative receptor binding epitope of Cry2Aa identified in this study differs from that inferred from previous structural studies of other Cry toxins.

A theoretical model of the tridimensional structure of Bacillus thuringiensis subsp. medellin Cry 11Bb toxin deduced by homology modelling

Cry11Bb is an insecticidal crystal protein produced by Bacillus thuringiensis subsp. medellin during its stationary phase; this partial differential-endotoxin is active against dipteran insects and has great potential for mosquito borne disease control. Here, we report the first theoretical model of the tridimensional structure of a Cry11 toxin. The tridimensional structure of the Cry11Bb toxin was obtained by homology modelling on the structures of the Cry1Aa and Cry3Aa toxins. In this work we give a brief description of our model and hypothesize the residues of the Cry11Bb toxin that could be important in receptor recognition and pore formation. This model will serve as a starting point for the design of mutagenesis experiments aimed to the improvement of toxicity, and to provide a new tool for the elucidation of the mechanism of action of these mosquitocidal proteins.

Role of two arginine residues in domain II, loop 2 of Cry1Ab and Cry1Ac Bacillus thuringiensis delta-endotoxin in toxicity and binding to Manduca sexta and Lymantria dispar aminopeptidase N

Two arginine residues (368-369) of Cry1Ab and Cry1Ac were mutated to alanine, glutamic acid and lysine by site-directed mutagenesis. Insecticidal activities of the mutant toxins on Manduca sexta and Lymantria dispar larvae were examined. Cry1Ac mutant toxins (c)RR-AA and (c)RR-EE and Cry1Ab mutant toxins (b)RR-AA and (b)RR-EE showed great reductions in toxicity against both insects. In contrast, conservatively changed (c)RR-KK and (b)RR-KK mutants did not alter toxicity to either insect. Binding assays with brush border membrane vesicles (BBMVs) prepared from L. dispar midguts demonstrated that (c)RR-AA, (c)RR-EE, (b)RR-AA and (b)RR-EE bound with lower affinities compared with their respective wild-type toxins. To M. sexta BBMVs, (c)RR-AA and (c)RR-EE showed great reductions in BBMV binding. However, (b)RR-AA and (b)RR-EE did not alter BBMV competition patterns, despite their reduced toxicity. Further binding assays were performed with aminopeptidase N (APN) purified from L. dispar and M. sexta BBMVs using surface plasmon resonance (BIAcore). Direct correlation between toxicity and APN binding was observed for the mutant toxins using this technique. The inconsistency between BBMV and APN binding data with Cry1Ab to M. sexta suggests the possibility of a different Cry1Ab toxin-binding mechanism or the importance of another receptor in M. sexta.

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

We used site-directed mutagenesis to modify the Bacillus thuringiensis cry3A gene in amino acid residues 350-354. Two mutant toxins, A1 (R(345)A,Y(350)F,Y(351)F) and A2 (R(345)A,DeltaY(350), DeltaY(351)), showed significantly improved toxicity against Tenebrio molitor (yellow mealworm). The mutant toxin A1 was also more potent against both Leptinotarsa decemlineata (Colorado potato beetle) and Chrysomela scripta (cottonwood leaf beetle), while A2 displayed enhanced toxicity only in L. decemlineata. Competitive binding assays of L. decemlineata brush border membrane vesicles (BBMV) revealed that binding affinities for the A1 and A2 mutant toxins were ca. 2.5-fold higher than for the wild-type Cry3 toxin. Similar binding assays with C. scripta BBMV revealed a ca. 5-fold lower dissociation rate for the A1 mutant as compared to that of Cry3A.

Helix 4 of the Bacillus thuringiensis Cry1Aa toxin lines the lumen of the ion channel

The mode of action of Bacillus thuringiensis insecticidal proteins is not well understood. Based on analogies with other bacterial toxins and ion channels, we hypothesized that charged amino acids in helix 4 of the Cry1Aa toxin are critical for toxicity and ion channel function. Using Plutella xylostella as a model target, we analyzed responses to Cry1Aa and eight proteins with altered helix 4 residues. Toxicity was abolished in five charged residue mutants (E129K, R131Q, R131D, D136N, D136C), however, two charged (R127E and R127N) and one polar (N138C) residue mutant retained wild-type toxicity. Compared with Cry1Aa and toxic mutants, nontoxic mutants did not show greatly reduced binding to brush border membrane vesicles, but their ion channel conductance was greatly reduced in planar lipid bilayers. Substituted cysteine accessibility tests showed that in situ restoration of the negative charge of D136C restored conductance to wild-type levels. The results imply that charged amino acids on the Asp-136 side of helix 4 are essential for toxicity and passage of ions through the channel. These results also support a refined version of the umbrella model of membrane integration in which the side of helix 4 containing Asp-136 faces the aqueous lumen of the ion channel.

Identification of residues in domain III of Bacillus thuringiensis Cry1Ac toxin that affect binding and toxicity

Alanine substitution mutations in the Cry1Ac domain III region, from amino acid residues 503 to 525, were constructed to study the functional role of domain III in the toxicity and receptor binding of the protein to Lymantria dispar, Manduca sexta, and Heliothis virescens. Five sets of alanine block mutants were generated at the residues (503)SS(504), (506)NNI(508), (509)QNR(511), (522)ST(523), and (524)ST(525). Single alanine substitutions were made at the residues (509)Q, (510)N, (511)R, and (513)Y. All mutant proteins produced stable toxic fragments as judged by trypsin digestion, midgut enzyme digestion, and circular dichroism spectrum analysis. The mutations, (503)SS(504)-AA, (506)NNI(508)-AAA, (522)ST(523)-AA, (524)ST(525)-AA, and (510)N-A affected neither the protein's toxicity nor its binding to brush border membrane vesicles (BBMV) prepared from these insects. Toward L. dispar and M. sexta, the (509)QNR(511)-AAA, (509)Q-A, (511)R-A, and (513)Y-A mutant toxins showed 4- to 10-fold reductions in binding affinities to BBMV, with 2- to 3-fold reductions in toxicity. Toward H. virescens, the (509)QNR(511)-AAA, (509)Q-A, (511)R-A, and (513)Y-mutant toxins showed 8- to 22-fold reductions in binding affinities, but only (509)QNR(511)-AAA and (511)R-A mutant toxins reduced toxicity by approximately three to four times. In the present study, greater loss in binding affinity relative to toxicity has been observed. These data suggest that the residues (509)Q, (511)R, and (513)Y in domain III might be only involved in initial binding to the receptor and that the initial binding step becomes rate limiting only when it is reduced more than fivefold.

Interaction between functional domains of Bacillus thuringiensis insecticidal crystal proteins

Interactions among the three structural domains of Bacillus thuringiensis Cry1 toxins were investigated by functional analysis of chimeric proteins. Hybrid genes were prepared by exchanging the regions coding for either domain I or domain III among Cry1Ab, Cry1Ac, Cry1C, and Cry1E. The activity of the purified trypsin-activated chimeric toxins was evaluated by testing their effects on the viability and plasma membrane permeability of Sf9 cells. Among the parental toxins, only Cry1C was active against these cells and only chimeras possessing domain II from Cry1C were functional. Combination of domain I from Cry1E with domains II and III from Cry1C, however, resulted in an inactive toxin, indicating that domain II from an active toxin is necessary, but not sufficient, for activity. Pores formed by chimeric toxins in which domain I was from Cry1Ab or Cry1Ac were slightly smaller than those formed by toxins in which domain I was from Cry1C. The properties of the pores formed by the chimeras are therefore likely to result from an interaction between domain I and domain II or III. Domain III appears to modulate the activity of the chimeric toxins: combination of domain III from Cry1Ab with domains I and II of Cry1C gave a protein which was more strongly active than Cry1C.

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.

Effects of midgut-protein-preparative and ligand binding procedures on the toxin binding characteristics of BT-R1, a common high-affinity receptor in Manduca sexta for Cry1A Bacillus thuringiensis toxins

The identity of the physiologically important Cry1A receptor protein(s) in the lepidopteran Manduca sexta has been a matter of dispute due to the multiple proteins which bind the Cry1Ac toxin. Cry1Aa, Cry1Ab, and Cry1Ac exhibit essentially identical toxicities toward M. sexta larvae and show a high degree of sequence and presumed structural identities. These similarities make it likely that there is a common mechanism of toxicity in these lepidopteran-specific toxins in terms of both mode of action and the receptor proteins through which these toxins exert their lepidopteran-specific toxicity. Investigators in our laboratory previously demonstrated that the cloned 210-kDa glycoprotein BT-R1 binds all three Cry1A toxins (T. P. Keeton and L. A. Bulla, Jr., Appl. Environ. Microbiol. 63:3419-3425, 1997). This protein remains a common binding protein even after being subjected to various midgut membrane preparation and processing protocols. The method used to isolate proteins from the M. sexta larval midgut in no significant way affects the results of ligand binding and vacuum blotting experiments, and we have been unable to detect specific, high-affinity binding of any Cry1A toxin to Cry1Ac binding proteins other than BT-R1. Alterations in blot substrate and blocking, hybridization, and washing buffers support these conclusions. Collectively, these results indicate that in M. sexta the cadherin-like BT-R1 protein is a common high-affinity receptor protein for the Cry1A family of toxins.