Lepidopteran-specific crystal toxins from Bacillus thuringiensis form cation- and anion-selective channels in planar lipid bilayers

Engineering of Bacillus thuringiensis Cry2Ab toxin for improved insecticidal activity

Bacillus thuringiensis Cry2Ab toxin was a widely used bioinsecticide to control lepidopteran pests all over the world. In the present study, engineering of Bacillus thuringiensis Cry2Ab toxin was performed for improved insecticidal activity using site-specific saturation mutation. Variants L183I were screened with lower LC50 (0.129 µg/cm2) against P. xylostella when compared to wild-type Cry2Ab (0.267 µg/cm2). To investigate the molecular mechanism behind the enhanced activity of variant L183I, the activation, oligomerization and pore-formation activities of L183I were evaluated, using wild-type Cry2Ab as a control. The results demonstrated that the proteolytic activation of L183I was the same as that of wild-type Cry2Ab. However, variant L183I displayed higher oligomerization and pore-formation activities, which was consistence with its increased insecticidal activity. The current study demonstrated that the insecticidal activity of Cry2Ab toxin could be assessed using oligomerization and pore-formation activities, and the screened variant L183I with improved activity might contribute to Cry2Ab toxin's future application.

Channel Formation in Cry Toxins: An Alphafold-2 Perspective

Bacillus thuringiensis (Bt) strains produce pore-forming toxins (PFTs) that attack insect pests. Information for pre-pore and pore structures of some of these Bt toxins is available. However, for the three-domain (I-III) crystal (Cry) toxins, the most used Bt toxins in pest control, this crucial information is still missing. In these Cry toxins, biochemical data have shown that 7-helix domain I is involved in insertion in membranes, oligomerization and formation of a channel lined mainly by helix α4, whereas helices α1 to α3 seem to have a dynamic role during insertion. In the case of Cry1Aa, toxic against Manduca sexta larvae, a tetrameric oligomer seems to precede membrane insertion. Given the experimental difficulty in the elucidation of the membrane insertion steps, we used Alphafold-2 (AF2) to shed light on possible oligomeric structural intermediates in the membrane insertion of this toxin. AF2 very accurately (<1 Å RMSD) predicted the crystal monomeric and trimeric structures of Cry1Aa and Cry4Ba. The prediction of a tetramer of Cry1Aa, but not Cry4Ba, produced an 'extended model' where domain I helices α3 and α2b form a continuous helix and where hydrophobic helices α1 and α2 cluster at the tip of the bundle. We hypothesize that this represents an intermediate that binds the membrane and precedes α4/α5 hairpin insertion, together with helices α6 and α7. Another Cry1Aa tetrameric model was predicted after deleting helices α1 to α3, where domain I produced a central cavity consistent with an ion channel, lined by polar and charged residues in helix α4. We propose that this second model corresponds to the 'membrane-inserted' structure. AF2 also predicted larger α4/α5 hairpin n-mers (14 ≤n ≤ 17) with high confidence, which formed even larger (~5 nm) pores. The plausibility of these models is discussed in the context of available experimental data and current paradigms.

Molecular and Kinetic Models for Pore Formation of Bacillus thuringiensis Cry Toxin

Cry proteins from Bacillus thuringiensis (Bt) and other bacteria are pesticidal pore-forming toxins. Since 2010, when the ABC transporter C2 (ABCC2) was identified as a Cry1Ac protein resistant gene, our understanding of the mode of action of Cry protein has progressed substantially. ABCC2 mediates high Cry1A toxicity because of its high activity for helping pore formation. With the discovery of ABCC2, the classical killing model based on pore formation and osmotic lysis became nearly conclusive. Nevertheless, we are still far from a complete understanding of how Cry proteins form pores in the cell membrane through interactions with their host gut membrane proteins, known as receptors. Why does ABCC2 mediate pore formation with high efficiency unlike other Cry1A-binding proteins? Is the “prepore” formation indispensable for pore formation? What is the mechanism underlying the synergism between ABCC2 and the 12-cadherin domain protein? We examine potential mechanisms of pore formation via receptor interactions in this paper by merging findings from prior studies on the Cry mode of action before and after the discovery of ABC transporters as Cry protein receptors. We also attempt to explain Cry toxicity using Cry–receptor binding affinities, which successfully predicts actual Cry toxicity toward cultured cells coexpressing ABC transporters and cadherin.

Cry6Aa1, a Bacillus thuringiensis nematocidal and insecticidal toxin, forms pores in planar lipid bilayers at extremely low concentrations and without the need of proteolytic processing

Cry6Aa1 is a Bacillus thuringiensis (Bt) toxin active against nematodes and corn rootworm insects. Its 3D molecular structure, which has been recently elucidated, is unique among those known for other Bt toxins. Typical three-domain Bt toxins permeabilize receptor-free planar lipid bilayers (PLBs) by forming pores at doses in the 1-50 μg/ml range. Solubilization and proteolytic activation are necessary steps for PLB permeabilization. In contrast to other Bt toxins, Cry6Aa1 formed pores in receptor-free bilayers at doses as low as 200 pg/ml in a wide range of pH (5.5-9.5) and without the need of protease treatment. When Cry6Aa1 was preincubated with Western corn rootworm (WCRW) midgut juice or trypsin, 100 fg/ml of the toxin was sufficient to form pores in PLBs. The overall biophysical properties of the pores were similar for all three forms of the toxin (native, midgut juice- and trypsin-treated), with conductances ranging from 28 to 689 pS, except for their ionic selectivity, which was slightly cationic for the native and midgut juice-treated Cry6Aa1, whereas dual selectivity (to cations or anions) was observed for the pores formed by the trypsin-treated toxin. Enrichment of PLBs with WCRW midgut brush-border membrane material resulted in a 2000-fold reduction of the amount of native Cry6Aa1 required to form pores and affected the biophysical properties of both the native and trypsin-treated forms of the toxin. These results indicate that, although Cry6Aa1 forms pores, the molecular determinants of its mode of action are significantly different from those reported for other Bt toxins.

Stable integration and expression of a cry1Ia gene conferring resistance to fall armyworm and boll weevil in cotton plants

BACKGROUND

Boll weevil is a serious pest of cotton crop. Effective control involves applications of chemical insecticides, increasing the cost of production and environmental pollution. The current genetically modified Bt crops have allowed great benefits to farmers but show activity limited to lepidopteran pests. This work reports on procedures adopted for integration and expression of a cry transgene conferring resistance to boll weevil and fall armyworm by using molecular tools.

RESULTS

Four Brazilian cotton cultivars were microinjected with a minimal linear cassette generating 1248 putative lines. Complete gene integration was found in only one line (T0-34) containing one copy of cry1Ia detected by Southern blot. Protein was expressed in high concentration at 45 days after emergence (dae), decreasing by approximately 50% at 90 dae. Toxicity of the cry protein was demonstrated in feeding bioassays revealing 56.7% mortality to boll weevil fed buds and 88.1% mortality to fall armyworm fed leaves. A binding of cry1Ia antibody was found in the midgut of boll weevils fed on T0-34 buds in an immunodetection assay.

CONCLUSION

The gene introduced into plants confers resistance to boll weevil and fall armyworm. Transmission of the transgene occurred normally to T1 progeny. All plants showed phenotypically normal growth, with fertile flowers and abundant seeds. © 2015 Society of Chemical Industry.

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.

Canavalia ensiformis urease, Jaburetox and derived peptides form ion channels in planar lipid bilayers

Ureases catalyze the hydrolysis of urea into NH3 and CO2. They are synthesized by plants, fungi and bacteria but not by animals. Ureases display biological activities unrelated to their enzymatic activity, i.e., platelet and neutrophil activation, fungus inhibition and insecticidal effect. Urease from Canavalia ensiformis (jack bean) is toxic to several hemipteran and coleopteran insects. Jaburetox is an insecticidal fragment derived from jack bean urease. Among other effects, Jaburetox has been shown to interact with lipid vesicles. In this work, the ion channel activity of C. ensiformis urease, Jaburetox and three deletion mutants of Jaburetox (one lacking the N-terminal region, one lacking the C-terminal region and one missing the central β-hairpin) were tested on planar lipid bilayers. All proteins formed well resolved, highly cation-selective channels exhibiting two conducting states whose conductance ranges were 7-18pS and 32-79pS, respectively. Urease and the N-terminal mutant of Jaburetox were more active at negative potentials, while the channels of the other peptides did not display voltage-dependence. This is the first direct demonstration of the capacity of C. ensiformis urease and Jaburetox to permeabilize membranes through an ion channel-based mechanism, which may be a crucial step of their diverse biological activities, including host defense.

New insight to structure-function relationship of GalNAc mediated primary interaction between insecticidal Cry1Ac toxin and HaALP receptor of Helicoverpa armigera

Over the last few decades Cry1Ac toxin has been widely used in controlling the insect attack due to its high specificity towards target insects. The pore-forming toxin undergoes a complex mechanism in the insect midgut involving sequential interaction with specific glycosylated receptors in which terminal GalNAc molecule plays a vital role. Recent studies on Cry toxins interactions with specific receptors revealed the importance of several amino acid residues in domain III of Cry1Ac, namely Q509, N510, R511, Y513 and W545, serve as potential binding sites that surround the putative GalNAc binding pocket and mediate the toxin-receptor interaction. In the present study, alanine substitution mutations were generated in the Cry1Ac domain III region and functional significance of those key residues was monitored by insect bioassay on Helicoverpa armigera larvae. In addition, ligand blot analysis and SPR binding assay was performed to monitor the binding characteristics of Cry1Ac wild type and mutant toxins towards HaALP receptor isolated from Helicoverpa armigera. Mutagenesis data revealed that, alanine substitutions in R511, Y513 and W545 substantially impacted the relative affinity towards HaALP receptor and toxicity toward target insect. Furthermore, in silico study of GalNAc-mediated interaction also confirmed the important roles of these residues. This structural analysis will provide a detail insight for evaluating and engineering new generation Cry toxins to address the problem of change in insect behavioral patterns.

Single molecule fluorescence study of the Bacillus thuringiensis toxin Cry1Aa reveals tetramerization

Pore-forming toxins constitute a class of potent virulence factors that attack their host membrane in a two- or three-step mechanism. After binding to the membrane, often aided by specific receptors, they form pores in the membrane. Pore formation either unfolds a cytolytic activity in itself or provides a pathway to introduce enzymes into the cells that act upon intracellular proteins. The elucidation of the pore-forming mechanism of many of these toxins represents a major research challenge. As the toxins often refold after entering the membrane, their structure in the membrane is unknown, and key questions such as the stoichiometry of individual pores and their mechanism of oligomerization remain unanswered. In this study, we used single subunit counting based on fluorescence spectroscopy to explore the oligomerization process of the Cry1Aa toxin of Bacillus thuringiensis. Purified Cry1Aa toxin molecules labeled at different positions in the pore-forming domain were inserted into supported lipid bilayers, and the photobleaching steps of single fluorophores in the fluorescence time traces were counted to determine the number of subunits of each oligomer. We found that toxin oligomerization is a highly dynamic process that occurs in the membrane and that tetramers represent the final form of the toxins in a lipid bilayer environment.

Coronatin-1 isolated from entomopathogenic fungus Conidiobolus coronatus kills Galleria mellonella hemocytes in vitro and forms potassium channels in planar lipid membrane

Entomopathogenic fungi are important natural regulatory factors of insect populations and have potential as biological control agents of insect pests. The cosmopolitan soil fungus Conidiobolus coronatus (Entomopthorales) easily attacks Galleria mellonella (Lepidoptera) larvae. Prompt death of invaded insects is attributed to the action of toxic metabolites released by the invader. Effect of fungal metabolites on hemocytes, insect blood cells involved in innate defense response, remains underexplored to date. C. coronatus isolate 3491 inducing 100% mortality of G. mellonella last instar larvae exposed to sporulating colonies, was cultivated at 20 °C in minimal medium. Post-incubation filtrates were used as a source of fungal metabolites. A two-step HPLC (1 step: Shodex KW-803 column eluted with 50 mM KH(2)PO(4) supplemented with 0.1 M KCl, pH 6.5; 2 step: ProteinPak™ CM 8HR column equilibrated with 5 mM KH(2)PO(4), pH 6.5, proteins eluted with a linear gradient of 0.5 M KCl) allowed the isolation of coronatin-1, an insecticidal 36 kDa protein showing both elastolytic and chitinolytic activities. Addition of coronatin-1 into primary in vitro cultures of G. mellonella hemocytes resulted in rapid disintegration of spherulocytes freely floating in culture medium and shrinkage of plasmatocytes adhering to the bottom of culture well. Coronatin-1 stimulated pseudopodia atrophy and, in consequence, disintegration of nets formed by cultured hemocytes. After incorporation of coronatin-1 into planar lipid membrane (PLM) ion channels selective for K(+) ions in 50/450 mM KCl solutions were observed. Potassium current flows were recorded in nearly 70% of experiments with conductance from 300 pS up to 1 nS. All observed channels were active at both positive and negative membrane potentials. Under experimental conditions incorporated coronatin-1 exhibited a zero current potential (E(rev)) of 47.7 mV, which indicates K(+)-selectivity of this protein. The success of the purification of coronatin-1 will allow further characterization of the mode of action of this molecule, including ability of coronatin-1 to form potassium channels in immunocompetent hemocytes.

Global functional analyses of cellular responses to pore-forming toxins

Here we present the first global functional analysis of cellular responses to pore-forming toxins (PFTs). PFTs are uniquely important bacterial virulence factors, comprising the single largest class of bacterial protein toxins and being important for the pathogenesis in humans of many Gram positive and Gram negative bacteria. Their mode of action is deceptively simple, poking holes in the plasma membrane of cells. The scattered studies to date of PFT-host cell interactions indicate a handful of genes are involved in cellular defenses to PFTs. How many genes are involved in cellular defenses against PFTs and how cellular defenses are coordinated are unknown. To address these questions, we performed the first genome-wide RNA interference (RNAi) screen for genes that, when knocked down, result in hypersensitivity to a PFT. This screen identifies 106 genes (∼0.5% of genome) in seven functional groups that protect Caenorhabditis elegans from PFT attack. Interactome analyses of these 106 genes suggest that two previously identified mitogen-activated protein kinase (MAPK) pathways, one (p38) studied in detail and the other (JNK) not, form a core PFT defense network. Additional microarray, real-time PCR, and functional studies reveal that the JNK MAPK pathway, but not the p38 MAPK pathway, is a key central regulator of PFT-induced transcriptional and functional responses. We find C. elegans activator protein 1 (AP-1; c-jun, c-fos) is a downstream target of the JNK-mediated PFT protection pathway, protects C. elegans against both small-pore and large-pore PFTs and protects human cells against a large-pore PFT. This in vivo RNAi genomic study of PFT responses proves that cellular commitment to PFT defenses is enormous, demonstrates the JNK MAPK pathway as a key regulator of transcriptionally-induced PFT defenses, and identifies AP-1 as the first cellular component broadly important for defense against large- and small-pore PFTs.

The plasma membrane surrounds cells and protects their interior from the environment and from attack by disease-causing agents like bacteria and viruses. Bacteria that cause disease have discovered that an effective way to attack cells is to secrete proteins (pore-forming toxins) that breach, i.e., form holes in, the plasma membrane. How cells deal with and survive this kind of attack is poorly understood. Here, we report on the first large-scale study of the genes and mRNA transcripts that respond to pore-forming toxin attack in cells. We find that a remarkable portion, >0.5%, of the cell's genome protects it against pore-forming toxins. These data led us to look more closely at mitogen-activated protein kinase pathways as regulators of pore-forming toxin defenses. We find that half of the PFT-induced protective response is controlled by a single, conserved signaling pathway in cells, which controls a complex array of downstream targets and which protects against both large pore and small pore toxins. Our results indicate that defense against pore-forming toxins is a very ancient defense that utilizes a much more complex and extensive response in cells than previously demonstrated.

An ABC transporter mutation is correlated with insect resistance to Bacillus thuringiensis Cry1Ac toxin

Transgenic crops producing insecticidal toxins from Bacillus thuringiensis (Bt) are commercially successful in reducing pest damage, yet knowledge of resistance mechanisms that threaten their sustainability is incomplete. Insect resistance to the pore-forming Cry1Ac toxin is correlated with the loss of high-affinity, irreversible binding to the mid-gut membrane, but the genetic factors responsible for this change have been elusive. Mutations in a 12-cadherin-domain protein confer some Cry1Ac resistance but do not block this toxin binding in in vitro assays. We sought to identify mutations in other genes that might be responsible for the loss of binding. We employed a map-based cloning approach using a series of backcrosses with 1,060 progeny to identify a resistance gene in the cotton pest Heliothis virescens that segregated independently from the cadherin mutation. We found an inactivating mutation of the ABC transporter ABCC2 that is genetically linked to Cry1Ac resistance and is correlated with loss of Cry1Ac binding to membrane vesicles. ABC proteins are integral membrane proteins with many functions, including export of toxic molecules from the cell, but have not been implicated in the mode of action of Bt toxins before. The reduction in toxin binding due to the inactivating mutation suggests that ABCC2 is involved in membrane integration of the toxin pore. Our findings suggest that ABC proteins may play a key role in the mode of action of Bt toxins and that ABC protein mutations can confer high levels of resistance that could threaten the continued utilization of Bt-expressing crops. However, such mutations may impose a physiological cost on resistant insects, by reducing export of other toxins such as plant secondary compounds from the cell. This weakness could be exploited to manage this mechanism of Bt resistance in the field.

Interaction between Calcium Ions and Bacillus thuringiensis Toxin Activity against Sf9 Cells (Spodoptera frugiperda, Lepidoptera)

The effects of calcium ions and modulators of calcium movement on Bacillus thuringiensis insecticidal protein toxicity were investigated with Sf9 cells (Spodoptera frugiperda, fall armyworm) by a new B. thuringiensis toxicity assay based on measurement of fluorescence of ethidium homodimer, a high-affinity DNA stain. CryIC toxicity was substantially stimulated by extracellular calcium in a dose-dependent way (in the millimolar range), while toxicity enhancement could not be replicated when calcium was replaced by barium. This incremental toxicity was reduced by cobalt and lanthanum ions, two inorganic-calcium transport inhibitors. Methoxyverapamil, a voltage-dependent calcium channel blocker, and nifedipine, an inhibitor of dihydropyridine-sensitive L-type calcium channels, had no effect on CryIC toxin activity, but BAY K 8644, an L-type calcium channel activator, increased CryIC activity at high concentrations of extracellular calcium. While A23187, a calcium ionophore, and TMB-8, an inhibitor of intracellular-calcium mobilization, did not change CryIC-induced mortality, thapsigargin, an inhibitor of calcium uptake in intracellular stores, and more particularly trifluoperazine, which inhibits calcium-calmodulin-dependent processes, increased CryIC-mediated toxicity. The incremental effect of extracellular calcium on CryIC-induced toxicity was consistent with an increased concentration of intracellular calcium.

Effect of entomocidal proteins from Bacillus thuringiensis on ion permeability of apical membranes of Tenebrio molitor larvae gut epithelium

Effects of entomocidal Cry-type proteins, delta-endotoxins Cry3A and Cry11A produced by Bacillus thuringiensis, on ion permeability of the apical membranes of intestinal epithelium from Tenebrio molitor larvae midgut were studied. Using potential-sensitive dyes safranine O and oxonol VI and DeltapH indicator acridine orange, it was shown that placing brush border membrane vesicles (BBMV) (loaded with Mg2+ during their preparation) into a salt-free buffer medium resulted in spontaneous generation of transmembrane electric potential on the vesicular membrane (negative inside the vesicles) accompanied by acidification of the aqueous phase inside the vesicles. The generation of transmembrane ion gradients on the vesicular membrane was a result of an electrogenic efflux of Mg2+ from the vesicles as shown by abolishing of the membrane potential by such agents as MgSO4 or CaCl2 in centimolar concentrations, a highly lipophilic cation tetraphenylphosphonium, and some blockers of cell membrane Ca2+-channels in submillimolar concentrations. A passive generation of membrane potential on the vesicular membrane (but positive inside the vesicles) was also observed upon addition of centimolar concentrations of K2SO4. Addition of delta-endotoxins Cry3A and Cry11A to the vesicle suspension in a salt-free buffer medium or in the same medium supplemented with centimolar concentrations of K2SO4 exerted a pronounced hyperpolarization of the vesicular membrane. This hyperpolarization was sensitive to the same agents, which abolished the membrane potential generation in the absence of delta-endotoxin. It is concluded that Cry proteins induced in BBMV from T. molitor opening pores or ion channels, which were considerably more permeable for alkaline- and alkaline-earth metal cations than for the accompanying anions.

Bacterial Insecticidal Toxins

Over the years it has been important for humans to control the populations of harmful insects and insecticides have been used for this purpose in agricultural and horticultural sectors. Synthetic insecticides, owing to their various side effects, have been widely replaced by biological insecticides. In this review we attempt to describe three bacterial species that are known to produce insecticidal toxins of tremendous biotechnological, agricultural, and economic importance. Bacillus thuringiensis (BT) accounts for 90% of the bioinsecticide market and it produces insecticidal toxins referred to as delta endotoxins. The other two bacteria belong to the genera Photorhabdus and Xenorhabdus, which are symbiotically associated with entomopathogenic nematodes of the families Heterorhabditidae and Steinernematidae respectively. Whereas, Xenorhabdus and Photorhabdus exist in a mutualistic association with the entomopathogenic nematodes, BT act alone. BT formulations are widely used in the field against insects; however, over the years there has been a gradual development of insect resistance against BT toxins. No resistance against Xenorhabdus or Photorhabdus has been reported to date. More recently BT transgenic crops have been prepared; however, there are growing concerns about the safety of these genetically modified crops. Nematodal formulations are also used in the field to curb harmful insect populations. Resistance development to entomopathogenic nematodes is unlikely due to the physical macroscopic nature of infection. Xenorhabdus and Photorhabdus transgenes have not yet been prepared; but are predicted to be available in the near future. In this review we start with an overview of the synthetic insecticides and then discuss Bacillus thuringiensis, Xenorhabdus nematophilus, and Photorhabdus luminescens in greater detail.

All domains of Cry1A toxins insert into insect brush border membranes

A critical step in understanding the mode of action of insecticidal crystal toxins from Bacillus thuringiensis is their partitioning into membranes and, in particular, the insertion of the toxin into insect brush border membranes. The Umbrella and Penknife models predict that only alpha-helix 5 of domain I along with adjacent helices alpha-4 or alpha-6 insert into the brush border membranes because of their hydrophobic nature. By employing fluorescent-labeled cysteine mutations, we observe that all three domains of the toxin insert into the insect membrane. Using proteinase K protection assays, steady state fluorescence quenching measurements, and blue shift analysis of acrylodan-labeled cysteine mutants, we show that regions beyond those proposed by the two models insert into the membrane. Based on our studies, the only extended region that does not partition into the membrane is that of alpha-helix 1. Bioassays and voltage clamping studies show that all mutations examined, except certain domain II mutations in loop 2 (e.g. F371C and G374C), which disrupt membrane partitioning, retain their ability to form ion channels and toxicity in Manduca sexta larvae. This study confirms our earlier hypothesis that insertion of crystal toxin does not occur as separate helices alone, but virtually the entire molecule inserts as one or more units of the whole molecule.

Atlantic salmon (Salmo salar L.) parr fed genetically modified soybeans and maize: Histological, digestive, metabolic, and immunological investigations

Physiological and health related responses to dietary inclusion of genetically modified (GM) full-fat soybean meal (Roundup Ready; GM-soy) and maize (MON810 Bt-maize; GM-maize), as well as non-parental, untransformed lines (nGM-soy and nGM-maize D2), were evaluated in farmed Atlantic salmon (Salmo salar L.) parr during the first 8 months of feeding. Significant effects of dietary GM presence were only found in intestinal Na+-dependent d-glucose uptake and SGLT1 protein level in the region pyloric caeca in which the highest values were found in the GM-soy, intermediate in the nGM-soy, and lowest in the standard FM fed groups. Data from this study confirm that GM soybeans (RRS) and maize (MON810) at inclusion levels of about 6% appear to be as safe as commercially available nGM soy and maize in diets for Atlantic salmon parr. Results from studies with higher inclusion levels and with non-modified, isogenic or near-isogenic parental lines as control groups are pending.

Impact of Bacillus thuringiensis toxin Cry1Ab on rumen epithelial cells (REC) - a new in vitro model for safety assessment of recombinant food compounds

The growing use of genetically modified crops necessitates viable screening methods for safety evaluation of recombinant feed, particularly for ruminants. A new sheep rumen epithelial cell culture is introduced as an in vitro cell system for safety evaluation especially focussing on feed and food compounds. We used lactate dehydrogenase (LDH) release, WST-1 conversion, ATP content and caspase 3/7 activity to evaluate cytotoxicity of Cry1Ab, one of the newly expressed Bt-proteins in transgene maize. The results were compared to the effects of valinomycin, a potassium ionophore known to induce cytotoxic effects on a wide range of cells. Whereas no toxicity of Cry1Ab was observed in short as well as in long term experiments, even at non-physiological high concentrations, exposure to valinomycin induced apoptosis and a significant response of all viability parameters after a number of hours. The ATP content and the WST-1 conversion reflecting the energy metabolism of the cells appear to be more sensitive indicators of valinomycin toxicity than the LDH release, a parameter which reflects the membrane integrity. This study presents an in vitro model system, that may be useful as a supplementary tool in toxicity screening before testing substances on animals in vivo.

A trimeric building block model for Cry toxins in vitro ion channel formation

The crystal (Cry) insecticidal toxins, or delta-endotoxins, are lethal to a wide variety of insect larvae, and are therefore very important in insect control. Toxicity has been explained by formation of transmembrane oligomeric pores or ion channels and, more recently, by the ability of the monomeric toxin to subvert cellular signaling pathways. The structure, topology, and precise role of the putative pore in toxicity are not known. However, in vitro biophysical studies suggest that helices alpha4 and alpha5 in domain I insert into the lipid bilayer as an alpha-helical hairpin. Mutagenesis studies have assigned an important role to alpha5 in maintaining oligomerization, and to alpha4 in channel formation. To detect the possible homo-oligomerizing tendencies of these two helices, we have used the evolutionary conservation data contained in sixteen Cry homologs in order to filter non-native interactions found during a global conformational search. No conserved homo-oligomer was found for alpha4, but a right handed trimeric alpha5 model was present in the simulations of all Cry sequences. We propose a model for Cry toxin oligomerization based on sequence analysis and available mutagenesis data.

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.

Kinetics of pore formation by the Bacillus thuringiensis toxin Cry1Ac

After binding to specific receptors, Cry toxins form pores in the midgut apical membrane of susceptible insects. The receptors could form part of the pore structure or simply catalyze pore formation and consequently be recycled. To discriminate between these possibilities, the kinetics of pore formation in brush border membrane vesicles isolated from Manduca sexta was studied with an osmotic swelling assay. Pore formation, as deduced from changes in membrane permeability induced by Cry1Ac during a 60-min incubation period, was strongly dose-dependent, but rapidly reached a maximum as toxin concentration was increased. Following exposure of the vesicles to the toxin, the osmotic swelling rate reached a maximum shortly after a delay period. Under these conditions, at relatively high toxin concentrations, the maximal osmotic swelling rate increased linearly with toxin concentration. When vesicles were incubated for a short time with the toxin and then rapidly cooled to prevent the formation of new pores before and during the osmotic swelling experiment, a plateau in the rate of pore formation was observed as toxin concentration was increased. Taken together, these results suggest that the receptors do not act as simple catalysts of pore formation, but remain associated with the pores once they are formed.

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.

Expression of Cry1Ac cadherin receptors in insect midgut and cell lines

Cadherin-like proteins have been identified as putative receptors for the Bacillus thuringiensis Cry1A proteins in Heliothis virescens and Manduca sexta. Immunohistochemistry showed the cadherin-like proteins are present in the insect midgut apical membrane, which is the target site of Cry toxins. This subcellular localization is distinct from that of classical cadherins, which are usually present in cell-cell junctions. Immunoreactivity of the cadherin-like protein in the insect midgut was enhanced by Cry1Ac ingestion. We also generated a stable cell line Flp-InT-REX-293/Full-CAD (CAD/293) that expressed the H. virescens cadherin. As expected, the cadherin-like protein was mainly localized in the cell membrane. Interestingly, toxin treatment of CAD/293 cells caused this protein to relocalize to cell membrane subdomains. In addition, expression of H. virescens cadherin-like protein affects cell-cell contact and cell membrane integrity when the cells are exposed to activated Cry1Ab/Cry1Ac.

Photorhabdus luminescens toxin-induced permeability change in Manduca sexta and Tenebrio molitor midgut brush border membrane and in unilamellar phospholipid vesicle

Photorhabdus luminescens, a Gram-negative bacterium, secretes a protein toxin (PL toxin) that is toxic to insects. In this study, the effects of the PL toxin on large receptor-free unilamellar phospholipid vesicles (LUVs) of Manduca sexta and on brush border membrane vesicles (BBMVs) of M. sexta and Tenebrio molitor were examined. Cry1Ac served as a positive control in our experiments due to its known channel-forming activity on M. sexta. Voltage clamping assays with dissected midguts of M. sexta and T. molitor clearly showed that both Cry1Ac and PL toxin caused channel formation in the midguts, although channel formation was not detected for T. molitor midguts under Cry1Ac and it was less sensitive to PL toxin than to Cry1Ac for M. sexta midguts. Calcein release experiments showed that both toxins made LUVs (unilamellar lipid vesicles) permeable, and at some concentrations of the toxins such permeabilizing effects were pH-dependent. The lowest concentrations of PL toxin were more than 600-fold and 24-fold lower to induce BBMV permeability of T. molitor and M. sexta than those to induce calcein release from LUVs of M. sexta. These further support that PL toxin is responsible for channel formation in the larvae midguts. The lower concentration to induce permeability in BBMV than in LUV is, probably, attributable to that BBMV has PL toxin receptors that facilitate the toxin to induce permeabilization. Furthermore, our results indicate that the effects of PL toxin on BBMV permeability of M. sexta were not significantly influenced by Gal Nac, but those of Cry1Ac were. This implies that PL toxin and Cry1Ac might use different molecular binding sites in BBMV to cause channel formation.

Structure and distribution of the Bacillus thuringiensis Cry4Ba toxin in lipid membranes

Bacillus thuringiensis Cry delta-endotoxins cause death of susceptible insect larvae by forming lytic pores in the midgut epithelial cell membranes. The 65 kDa trypsin activated Cry4Ba toxin was previously shown to be capable of permeabilizing liposomes and forming ionic channels in receptor-free planar lipid bilayers. Here, magnetic ACmode (MACmode) atomic force microscopy (AFM) was used to characterize the lateral distribution and the native molecular structure of the Cry4Ba toxin in the membrane. Liposome fusion and the Langmuir-Blodgett technique were employed for supported lipid bilayer preparations. The toxin preferentially inserted in a self-assembled structure, rather than as a single monomeric molecule. In addition, the spontaneous insertion into receptor-free lipid bilayers lead to formation of characteristic pore-like structures with four-fold symmetry, suggesting that tetramers are the preferred oligomerization state of this toxin.

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.

Implications of high-molecular-weight oligomers of the binary toxin from Bacillus sphaericus

The mosquito-larvicidal binary toxin produced by Bacillus sphaericus is composed of BinB and BinA, which have calculated molecular weights of 51.4 and 41.9 kDa, respectively. NaOH extracts of B. sphaericus spores were analyzed using SDS-PAGE. Stained gels showed bands with molecular weights corresponding to those of BinB and BinA as well as two additional bands at 110 and 125 kDa. The matrix-assisted laser desorption/ionization mass spectrum of the purified 110 and 125 kDa bands showed two peaks at 104,160 and 87,358 Da that are assigned to dimers of BinB and BinA, respectively. Mass spectral analysis of trypsin-digested 110 and 125 kDa bands showed peaks at 51,328, 43,523, 43,130, and 40,832 Da that assigned to undigested BinB, two forms of digested BinB and digested BinA, respectively. Dynamic light scattering studies showed a solution of the purified 110 and 125 kDa bands was comprised almost entirely (99.6% of total mass) of a particle with a hydrodynamic radius of 5.6+/-1.2 nm and a calculated molecular weight of 186+/-38 kDa. These data demonstrate that the binary toxin extracted from B. sphaericus spores can exist in solution as an oligomer containing two copies each of BinB and BinA.

A novel Bacillus thuringiensis (PS149B1) containing a Cry34Ab1/Cry35Ab1 binary toxin specific for the western corn rootworm Diabrotica virgifera virgifera LeConte forms ion channels in lipid membranes

The binary Bacillus thuringiensis PS149B1 insecticidal crystal (Cry) protein is comprised of two components, Cry34Ab1, a 14-kDa protein, and Cry35Ab1, a 44-kDa protein, the combination of which forms a novel binary toxin active on western corn rootworm larvae. The permeabilizing behavior of the native binary toxin and its two individual components expressed as recombinant proteins was studied using calcein efflux determination in liposomes and by ion channel activity measurements in planar lipid bilayers (PLBs). Data obtained with solubilized native PS149B1 binary protein revealed it to be a pore-forming toxin that can permeabilize liposomes and form ion channels ( approximately 300-900 pS) in PLBs at pH 5.5 but not pH 9.0. The 14-kDa component of the toxin also formed ion channels ( approximately 15-300 pS) at pH 5.5 but did not insert easily in PLBs. While the 44-kDa moiety did seldomly form resolvable ion channels ( approximately 15-750 pS) in PLBs, it did destabilize the membranes. It showed pH-dependent truncation to a stable 40-kDa protein. The purified 40-kDa truncated product formed channels ( approximately 10-450 pS) in PLBs at pH 5.5. At that same pH, while a 3:1 molar mixture (14:44 kDa) of the individual components of the toxin induced channel activity that resembled that of the 14-kDa component alone, the 3:1 molar mixture of the 14-kDa component and 40-kDa truncated product induced channel activity ( approximately 20-800 pS) similar to that of PS149B1 in planar lipid bilayers. We conclude that the overall membrane permeabilization process of Cry34Ab1/Cry35Ab1 is a result of ion channel formation.

Tryptophan spectroscopy studies and black lipid bilayer analysis indicate that the oligomeric structure of Cry1Ab toxin from Bacillus thuringiensis is the membrane-insertion intermediate

During intoxication, the Cry protoxins must change from insoluble crystals into membrane-inserted toxins, which form ionic pores. Binding of Cry1A toxins to the cadherin receptor promotes the formation of a 250 kDa oligomer. In this work, we analyzed for the first time the structural changes presented by Cry1Ab toxin upon membrane insertion. Trp fluorescence of pure monomeric and oligomeric structures in solution and in a membrane-bound state was analyzed. Cry1Ab has nine Trp residues, seven of them in pore-forming domain I. Trp quenching analysis with iodide indicated that oligomerization caused a 27% reduction in the level of Trp exposed to the solvent. Most of the oligomeric structure (96%) inserts into the membrane as a function of the lipid:protein ratio, in contrast to the monomer (10%). Additionally, the membrane-associated oligomer presented a blue shift of 5 nm in lambda(max) of the emission spectrum, indicating a more hydrophobic environment for some Trp residues. In agreement with this, iodide was unable to quench the Trp of the membrane-bound oligomer, suggesting that a significant part of the protein may be buried in the membrane. Quenching analysis using brominated and spin-labeled phospholipids in the vesicles indicates that most of the Trp residues are located close to the membrane-water interface. Finally, ionic currents in black lipid bilayers revealed that the oligomeric structure has kinetics different from those of the monomer, producing stable channels with a high probability of being open in contrast to the monomer that exhibited unstable opening patterns. These data show that the oligomer, in contrast to the monomer, is able to interact efficiently with phospholipid membranes forming stable pores.

Cloning of a Heliothis virescens 110 kDa aminopeptidase N and expression in Drosophila S2 cells

We previously identified a novel Heliothis virescens 110 kDa aminopeptidase N (APN) that binds Bacillus thuringiensis (Bt) Cry1Ac and Cry1Fa delta-endotoxins, and cloned an internal region of the 110 kDa APN gene (Banks et al., 2001). Here we describe the RACE-PCR cloning and sequence of a cDNA encoding 110 kDa APN. The 110 kDa APN gene was transiently co-expressed with green fluorescent protein (GFP) in Drosophila S2 cells using the pIZT expression vector. Enrichment of total membranes purified from S2 cells transfected with the 110 kDa APN gene had 3.3 fold increased APN enzymatic activity relative to enriched total membranes purified from S2 cells transfected with vector alone. Whereas the majority of S2 cells transfected with the 110 kDa APN gene bound rhodamine-labeled Cry1Ac toxin, no S2 cells transfected with vector alone bound rhodamine-labeled Cry1Ac toxin. This indicates that toxin binding to whole cells is APN mediated. However, flow cytometry and microscopy indicated that 110 kDa APN transfected S2 cells exposed to Cry1Ac or Cry1Fa toxin did not experience an increase in membrane permeability, indicating that APN transfected cells were resistant to toxin. This suggests while the H. virescens 110 kDa APN functions as a Bt toxin binding protein, it does not mediate cytotoxicity when expressed in S2 cells.

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.

The toxicity test and hypothetical model of Bacillus thuringiensis Cry1Aa helix4

Development of targeted biological agents against agricultural insect pests is of prime importance for the elaboration and implementation of integrated pest management strategies that are environment-friendly, respectful of bio-diversity and safer to human health through reduced use of chemical pesticides. A major goal to understand how Bt toxins work is to elucidate the functions of their three domains. Domains II and III are involved in binding specificity and structural integrity, but the function of Domain I remains poorly understood. Using a Manduca sexta BBMV (brush border membrane vesicles) system, we analyzed its responses to Cry1Aa 15 single-point mutations with altered Domain I helix 4 residues. Light scattering assay showed that toxicity was almost lost in 3 mutants, and we observed significantly reduced toxicity in other 7 mutants. However, 5 mutants retained wild-type toxicity. Using computer software, we simulated the three-dimensional structures of helix 4. Both experimental and bioinformatic analysis showed that residues in Cry1Aa Domain I helix 4 were involved in the formation of ion channels that is critical for its insect toxicity.

Interaction of gene-cloned and insect cell-expressed aminopeptidase N of Spodoptera litura with insecticidal crystal protein Cry1C

Insecticidal toxins produced by Bacillus thuringiensis interact with specific receptors located in the midguts of susceptible larvae, and the interaction is followed by a series of biochemical events that lead to the death of the insect. In order to elucidate the mechanism of action of B. thuringiensis toxins, receptor protein-encoding genes from many insect species have been cloned and characterized. In this paper we report the cloning, expression, and characterization of Cry toxin-interacting aminopeptidase N (APN) isolated from the midgut of a polyphagous pest, Spodoptera litura. The S. litura APN cDNA was expressed in the Sf21 insect cell line by using a baculovirus expression system. Immunofluorescence staining of the cells revealed that the expressed APN was located at the surface of Sf21 cells. Treatment of Sf21 cells expressing S. litura APN with phosphatidylinositol-specific phospholipase C demonstrated that the APN was anchored in the membrane by a glycosylphosphatidylinositol moiety. Interaction of the expressed receptor with different Cry toxins was examined by immunofluorescence toxin binding studies and ligand blot and immunoprecipitation analyses. By these experiments we showed that the bioactive toxin, Cry1C, binds to the recombinant APN, while the nonbioactive toxin, Cry1Ac, showed no interaction.

Pore formation activity of Cry1Ab toxin from Bacillus thuringiensis in an improved membrane vesicle preparation from Manduca sexta midgut cell microvilli

The pore formation activity of Cry1Ab toxin is analyzed in an improved membrane preparation from apical microvilli structures of Manduca sexta midgut epithelium cells (MEC). A novel methodology is described to isolate MEC and brush border membrane vesicles (BBMV) from purified microvilli structures. The specific enrichment of apical membrane enzyme markers aminopeptidase (APN) and alkaline phosphatase (APh) were 35- and 22-fold, respectively, as compared to the whole midgut cell homogenate. Ligand-blot and Western-blot experiments showed that Cry1A specific receptors were also enriched. The pore formation activity of Cry1Ab toxin was fourfold higher in the microvilli membrane fraction that showed low intrinsic K+ channels and higher APN and APh activities than in the basal-lateral membrane fraction harboring high intrinsic K+ channels. These data suggest that basal-lateral membrane was separated from apical membrane.This procedure should allow more precise studies of the interaction of Cry toxins with their target membranes, avoiding unspecific interaction with other cellular membranes, as well as the study of the pore formation activity induced by Cry toxins in the absence of endogenous channels from M. sexta midgut cells.

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.

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.

Differential effects of pH on the pore-forming properties of Bacillus thuringiensis insecticidal crystal toxins

The effect of pH on the pore-forming ability of two Bacillus thuringiensis toxins, Cry1Ac and Cry1C, was examined with midgut brush border membrane vesicles isolated from the tobacco hornworm, Manduca sexta, and a light-scattering assay. In the presence of Cry1Ac, membrane permeability remained high over the entire pH range tested (6.5 to 10.5) for KCl and tetramethylammonium chloride, but was much lower at pH 6.5 than at higher pHs for potassium gluconate, sucrose, and raffinose. On the other hand, the Cry1C-induced permeability to all substrates tested was much higher at pH 6.5, 7.5, and 8.5 than at pH 9.5 and 10.5. These results indicate that the pores formed by Cry1Ac are significantly smaller at pH 6.5 than under alkaline conditions, whereas the pore-forming ability of Cry1C decreases sharply above pH 8.5. The reduced activity of Cry1C at high pH correlates well with the fact that its toxicity for M. sexta is considerably weaker than that of Cry1Aa, Cry1Ab, and Cry1Ac. However, Cry1E, despite having a toxicity comparable to that of Cry1C, formed channels as efficiently as the Cry1A toxins at pH 10.5. These results strongly suggest that although pH can influence toxin activity, additional factors also modulate toxin potency in the insect midgut.

Mapping the epitope in cadherin-like receptors involved in Bacillus thuringiensis Cry1A toxin interaction using phage display

In susceptible lepidopteran insects, aminopeptidase N and cadherin-like proteins are the putative receptors for Bacillus thuringiensis (Bt) toxins. Using phage display, we identified a key epitope that is involved in toxin-receptor interaction. Three different scFv molecules that bind Cry1Ab toxin were obtained, and these scFv proteins have different amino acid sequences in the complementary determinant region 3 (CDR3). Binding analysis of these scFv molecules to different members of the Cry1A toxin family and to Escherichia coli clones expressing different Cry1A toxin domains showed that the three selected scFv molecules recognized only domain II. Heterologous binding competition of Cry1Ab toxin to midgut membrane vesicles from susceptible Manduca sexta larvae using the selected scFv molecules showed that scFv73 competed with Cry1Ab binding to the receptor. The calculated binding affinities (K(d)) of scFv73 to Cry1Aa, Cry1Ab, and Cry1Ac toxins are in the range of 20-51 nm. Sequence analysis showed this scFv73 molecule has a CDR3 significantly homologous to a region present in the cadherin-like protein from M. sexta (Bt-R(1)), Bombyx mori (Bt-R(175)), and Lymantria dispar. We demonstrated that peptides of 8 amino acids corresponding to the CDR3 from scFv73 or to the corresponding regions of Bt-R(1) or Bt-R(175) are also able to compete with the binding of Cry1Ab and Cry1Aa toxins to the Bt-R(1) or Bt-R(175) receptors. Finally, we showed that synthetic peptides homologous to Bt-R(1) and scFv73 CDR3 and the scFv73 antibody decreased the in vivo toxicity of Cry1Ab to M. sexta larvae. These results show that we have identified the amino acid region of Bt-R(1) and Bt-R(175) involved in Cry1A toxin interaction.

Mode of action of Bacillus thuringiensis PS86Q3 strain in hymenopteran forest pests

The mode of action of Cry toxins has been described principally in lepidopteran insects as a multistep process. In this work we describe the mode of action of a Cry toxin active in the common pine sawfly Diprion pini (Hymenoptera, Diprionidae), considered a major forest pest in Europe. Strain PS86Q3 contains a long bipyramidal crystal composed of five major proteins. The N-terminal sequence shows that the 155 kDa protein corresponds to Cry5B toxin and the other proteins belong to the Cry5A subgroup. PCR analysis indicates the presence of cry5Ac and cry5Ba genes, suggesting that Cry5A protein should be Cry5Ac. Activation of protoxins with trypsin or with midgut content from D. pini and Cephacia abietis (Hymenoptera, Pamphiliidae) (spruce webspinning sawfly), another important hymenopteran forest pest, produced a single 75 kDa toxin that corresponded to Cry5A by N-terminal sequence and is responsible for the insecticidal activity. Homologous competition experiments with D. pini and C. abietis brush border membrane vesicles (BBMV) showed that the binding interaction of Cry5A is specific. Membrane potential measurements using a fluorescent dye indicate that Cry5A toxin at nM concentration caused immediate permeability changes in the BBMV isolated from both hymenopteran larvae. The initial response and the sustained permeability change are cationic as previously shown for Cry1 toxins. These results indicate that the hymenopteran specific Cry5A toxin exerts toxicity by a similar mechanism as Cry1 toxins.

Evidence for intermolecular interaction as a necessary step for pore-formation activity and toxicity of Bacillus thuringiensis Cry1Ab toxin

Based on the observation of large conductance states formed by Bacillus thuringiensis Cry toxins in synthetic planar lipid bilayers and the estimation of a pore size of 10-20 A, it has been proposed that the pore could be formed by an oligomer containing four to six Cry toxin monomers. However, there is a lack of information regarding the insertion of Cry toxins into the membrane and oligomer formation. Here we provide direct evidence showing that the intermolecular interaction between Cry1Ab toxin monomers is a necessary step for pore formation and toxicity. Two Cry1Ab mutant proteins affected in different steps of their mode of action (F371A in receptor binding and H168F in pore formation) were affected in toxicity against Manduca sexta larvae. Binding analysis showed that F371A protein bound more efficiently to M. sexta brush border membrane vesicles when mixed with H168F in a one to one ratio. These mutant proteins also recovered pore-formation activity, measured with a fluorescent dye with isolated brush border membrane vesicles, and toxicity against M. sexta larvae when mixed, showing that monomers affected in different steps of their mode of action can form functional hetero-oligomers.

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.

The oligomeric state of Bacillus thuringiensis Cry toxins in solution

The molecular mass of different Cry toxins produced by Bacillus thuringiensis bacteria was estimated by size-exclusion chromatography and non-denaturing polyacrylamide gel electrophoresis at neutral and alkaline pH in order to assess the existence of oligomers in solution. We found that Cry1Aa, Cry1Ac, Cry1C, Cry1D and Cry3A toxins exist in solution as a mixture of monomer and high molecular mass aggregates with an apparent molecular mass greater than 600 kDa, that depend on the time elapsed between toxin activation and analysis. Aggregation of toxins by disulfide bonds is unlikely because aggregates are also observed in samples incubated with DTT. These data show that the Cry toxins studied do not form oligomers of less than ten subunits in solution and suggest that oligomer formation may occur after the toxin binds to the receptor and inserts into the membrane.