Delta-endotoxins induce cation channels in Spodoptera frugiperda brush border membranes in suspension and in planar lipid bilayers

Influence of Ephestia kuehniella stage larvae on the potency of Bacillus thuringiensis Cry1Aa delta-endotoxin

The economically important crop pest Ephestia kuehniella was tested at two stages of larval development for susceptibility to Bacillus thuringiensis Cry1Aa toxin. Bioassays showed that toxicity decreased during the development of larvae stage. In fact, Cry1Aa toxins from BNS3-Cry^- (pHT-cry1Aa) showed low toxicity against the first-instar larvae (L1) with a LC₅₀ value of about 421.02μg/g of diet and was not toxic against the fifth-instar (L5), comparing to the BLB1 toxins used as positive control which represent a LC₅₀ value of about 56.96 and 84.21μg/g of diet against L1 and L5 instars larvae, respectively. Effects of Cry1Aa toxins were reflected in histopathological observations by the weak destruction of midgut epithelium, slight hypertrophy of epithelial cells, and minor alteration of brush border membrane (BBM) detected mainly in L1 larvae stage comparing to the more extensive damage caused by BLB1 toxins. Interestingly, in vitro proteolysis of Cry1Aa toxins was found to correlate with the difference of toxicity during larval stage development. In fact, the weak proteinase activity detected inside the L1 midgut has led to the persistence of the Cry1Aa active forms (65 and 58kDa) during prolonged incubations, causing the alterations described previously. Three subfamilies of aminopeptidase (APN) receptors were detected in both larvae instars with different intensities and molecular weights (150kDa and 55kDa for APN1, and 90kDa for APN2 and APN4). Remarkably, binding assay using Cry1Aa toxin seems to have no direct correlation with larval stages toxicity differences, since same putative receptors were detected. Understanding the reasons for the clear differences in the effectiveness of Cry1Aa toxins during larval development stages of E. kuehniella is very important for the design of future improvement insecticidal approaches and for the accomplishment of resistance prevention strategies.

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.

A conserved tetrameric interaction of cry toxin helix α3 suggests a functional role for toxin oligomerization

Crystal (Cry) toxins are widely used for insect control, but their mechanism of toxicity is still uncertain. These toxins can form lytic pores in vitro, and water soluble tetrameric pre-pore intermediates have been reported. Even the precise oligomeric state of the toxin in membranes, trimeric or tetrameric, is still a debated issue. Based on previous reports, we have assumed that interactions between toxin monomers in solution are at least partly mediated by domain I, and we have analyzed in silico the homo-oligomerization tendencies of the domain I α-helices individually. Using many homologous sequences for each α-helix, our strategy allows selection of evolutionarily conserved interactions. These interactions appeared only in helices α3 and α5, but only α3 produced a suitably oriented or α-helical sample in lipid bilayers, forming homotetramers in C14-betaine, and allowing determination of its rotational orientation in lipid bilayers using site-specific infrared dichroism (SSID). The determined orientation in the tetrameric model is in agreement with only one of the evolutionarily conserved models. In addition mutation R99E, which was found to inhibit oligomerization experimentally, greatly destabilized the tetramer in molecular dynamic simulations. In this model, helix 3 is able to form inter-monomer interactions without significant rearrangements of domain I, which is compatible with the available crystal structure of Cry toxins in solution. The model presented here at least partially explains the reported tetrameric oligomerization of Cry toxins in solution and the inhibition of this oligomerization by a synthetic α3 peptide.

Combinatorial effect of Bacillus thuringiensis kurstaki and Photorhabdus luminescens against Spodoptera littoralis (Lepidoptera: Noctuidae)

Spodoptera littoralis, one of the major pests of many important crop plants, is more susceptible to Bacillus thuringiensis aizawai delta-endotoxins than to those of Bacillus thuringiensis kurstaki. Within the framework of the development of efficient bioinsecticides and the prevention against insect resistance, we tested the effect of mixing B. thuringiensis kurstaki delta-endotoxins and Photorhabdus luminescens cells on S. littoralis growth. The obtained results showed that the growth inhibition of this insect was more effective when B. thuringiensis kurstaki spore-crystal mixture and Photorhabdus luminescens cells were used in combination. Furthermore, this synergism is mainly due to the presence of Cry1Ac, which is one of the three delta-endotoxins that form the crystal of B. thuringiensis kurstaki strain BNS3 in addition to Cry1Aa and Cry2Aa. This work shows a possibility to use B. thuringiensis as a delivery means for Photorhabdus bacteria in order to infect the insect hemocoel and to reduce the risk of developing resistance in the target organism.

Differential protection of Cry1Fa toxin against Spodoptera frugiperda larval gut proteases by cadherin orthologs correlates with increased synergism

The Cry proteins produced by Bacillus thuringiensis (Bt) are the most widely used biopesticides effective against a range of crop pests and disease vectors. Like chemical pesticides, development of resistance is the primary threat to the long-term efficacy of Bt toxins. Recently discovered cadherin-based Bt Cry synergists showed the potential to augment resistance management by improving efficacy of Cry toxins. However, the mode of action of Bt Cry synergists is thus far unclear. Here we elucidate the mechanism of cadherin-based Cry toxin synergism utilizing two cadherin peptides, Spodoptera frugiperda Cad (SfCad) and Manduca sexta Cad (MsCad), which differentially enhance Cry1Fa toxicity to Spodoptera frugiperda neonates. We show that differential SfCad- and MsCad-mediated protection of Cry1Fa toxin in the Spodoptera frugiperda midgut correlates with differential Cry1Fa toxicity enhancement. Both peptides exhibited high affinity for Cry1Fa toxin and an increased rate of Cry1Fa-induced pore formation in S. frugiperda. However, only SfCad bound the S. frugiperda brush border membrane vesicle and more effectively prolonged the stability of Cry1Fa toxin in the gut, explaining higher Cry1Fa enhancement by this peptide. This study shows that cadherin fragments may enhance B. thuringiensis toxicity by at least two different mechanisms or a combination thereof: (i) protection of Cry toxin from protease degradation in the insect midgut and (ii) enhancement of pore-forming ability of Cry toxin.

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.

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.

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.

The pre-pore from Bacillus thuringiensis Cry1Ab toxin is necessary to induce insect death in Manduca sexta

The insecticidal Cry toxins from Bacillus thuringiensis bacteria are pore-forming toxins that lyse midgut epithelial cells in insects. We have previously proposed that they form pre-pore oligomeric intermediates before membrane insertion. For formation of these oligomers coiled-coil structures are important, and helix alpha-3 from Cry toxins could form coiled-coils. Our data shows that different mutations in helix alpha-3 are affected in pore formation and toxicity. Mutants affected in toxicity bind Bt-R(1) receptor with a similar K(D) as the wild type toxin but do not form oligomers nor induce pore formation in planar lipid bilayers, indicating that the pre-pore oligomer is an obligate intermediate in the intoxication of Cry1Ab toxin and that interaction of monomeric Cry1Ab with Bt-R(1) is not enough to kill susceptible larvae.

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.

Bacillus thuringiensis ssp. israelensis Cyt1Aa enhances activity of Cry11Aa toxin by facilitating the formation of a pre-pore oligomeric structure

Bacillus thuringiensis ssp. israelensis (Bti) has been used worldwide for the control of dipteran insect pests. This bacterium produces several Cry and Cyt toxins that individually show activity against mosquitoes but together show synergistic effect. Previous work demonstrated that Cyt1Aa synergizes the toxic activity of Cry11Aa by functioning as a membrane-bound receptor. In the case of Cry toxins active against lepidopteran insects, receptor interaction triggers the formation of a pre-pore oligomer that is responsible for pore formation and toxicity. In this work we report that binding of Cry11Aa to Cyt1Aa facilitates the formation of a Cry11Aa pre-pore oligomeric structure that is capable of forming pores in membrane vesicles. Cry11Aa and Cyt1A point mutants affected in binding and in synergism had a correlative effect on the formation of Cry11Aa pre-pore oligomer and on pore-formation activity of Cry11Aa. These data further support that Cyt1Aa interacts with Cry11Aa and demonstrate the molecular mechanism by which Cyt1Aa synergizes or suppresses resistance to Cry11Aa, by providing a binding site for Cry11Aa that will result in an efficient formation of Cry11Aa pre-pore that inserts into membranes and forms ionic pores.

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.

Permeability changes of Manduca sexta midgut brush border membranes induced by oligomeric structures of different cry toxins

The pore-formation activity of monomeric and oligomeric forms of different Cry1 toxins (from Cry1A to Cry1G) was analyzed by monitoring ionic permeability across Manduca sexta brush border membrane vesicles. The membrane vesicles were isolated from microvilli structures, showing a high enrichment of apical membrane markers and low intrinsic K(+) permeability. A fluorometric assay performed with 3,3'-dipropylthiodicarbocyanine fluorescent probe, sensitive to changes in membrane potential, was used. Previously, it was suggested that fluorescence determinations with this dye could be strongly influenced by the pH, osmolarity and ionic strength of the medium. Therefore, we evaluated these parameters in control experiments using the K(+)-selective ionophore valinomycin. We show here that under specific ionic conditions changes in fluorescence can be correlated with ionic permeability without effects on osmolarity or ionic strength of the medium. It is extremely important to attenuate the background response due to surface membrane potential and the participation of the endogenous permeability of the membrane vesicles. Under these conditions, we analyzed the pore-formation activity induced by monomeric and oligomeric structures of different Cry1 toxins. The Cry1 toxin samples containing oligomeric structures correlated with high pore activity, in contrast to monomeric samples that showed marginal pore-formation activity, supporting the hypothesis that oligomer formation is a necessary step in the mechanism of action of Cry toxins.

Laboratory and field tests of spray-dried and granular formulations of a Bacillus thuringiensis strain with insecticidal activity against the sugarcane borer

Formulations of Bacillus thuringiensis Berliner (Bt) with insecticidal activity against the sugarcane borer, Diatraea saccharalis Fabricius (Lepidoptera: Pyralidae), were developed and tested under laboratory and field conditions. The formulations were prepared using biodegradable polymers such as modified corn starch as an encapsulating agent, gelatin as an adherent, powdered sugarcane as a feeding stimulant and a Bt var. kurstaki GM-34 strain from a non-sugarcane region as the active ingredient. The spore-crystal complex of this strain was mixed at three different concentrations (30, 70 and 100 g kg(-1)) with the other ingredients. The blends were prepared as spray-dried and granular formulations, and then submitted to laboratory tests with two day old larvae of D. saccharalis and field tests in sugarcane crops with natural sugarcane borer infestation. Spray-dried formulations in laboratory bioassays caused mortality near 100% with all three concentrations, and granular formulations caused mortality around 84%. The field tests showed that spray-dried formulations at 70 and 100 g kg(-1) concentrations were as effective as a commercial bioinsectide (Lepinox), while granular formulations were ineffective.

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.

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.

Role of tryptophan residues in toxicity of Cry1Ab toxin from Bacillus thuringiensis

Bacillus thuringiensis produces insecticidal proteins (Cry protoxins) during the sporulation phase as parasporal crystals. During intoxication, the Cry protoxins must change from insoluble crystals into membrane-inserted toxins which form ionic pores. The structural changes of Cry toxins during oligomerization and insertion into the membrane are still unknown. The Cry1Ab toxin has nine tryptophan residues; seven are located in domain I, the pore-forming domain, and two are located in domain II, which is involved in receptor recognition. Eight Trp residues are highly conserved within the whole family of three-domain Cry proteins, suggesting an essential role for these residues in the structural folding and function of the toxin. In this work, we analyzed the role of Trp residues in the structure and function of Cry1Ab toxin. We replaced the Trp residues with phenylalanine or cysteine using site-directed mutagenesis. Our results show that W65 and W316 are important for insecticidal activity of the toxin since their replacement by Phe reduced the toxicity against Manduca sexta. The presence of hydrophobic residue is important at positions 117, 219, 226, and 455 since replacement by Cys affected either the crystal formation or the insecticidal activity of the toxin in contrast to replacement by Phe in these positions. Additionally, some mutants in positions 219, 316, and 455 were also affected in binding to brush border membrane vesicles (BBMV). This is the first report that studies the role of Trp residues in the activity of Cry toxins.

Protease inhibitors fail to prevent pore formation by the activated Bacillus thuringiensis toxin Cry1Aa in insect brush border membrane vesicles

To investigate whether membrane proteases are involved in the activity of Bacillus thuringiensis insecticidal toxins, the rate of pore formation by trypsin-activated Cry1Aa was monitored in the presence of a variety of protease inhibitors with Manduca sexta midgut brush border membrane vesicles and by a light-scattering assay. Most of the inhibitors tested had no effect on the pore-forming ability of the toxin. However, phenylmethylsulfonyl fluoride, a serine protease inhibitor, promoted pore formation, although this stimulation only occurred at higher inhibitor concentrations than those commonly used to inhibit proteases. Among the metalloprotease inhibitors, o-phenanthroline had no significant effect; EDTA and EGTA reduced the rate of pore formation at pH 10.5, but only EDTA was inhibitory at pH 7.5. Neither chelator affected the properties of the pores already formed after incubation of the vesicles with the toxin. Taken together, these results indicate that, once activated, Cry1Aa is completely functional and does not require further proteolysis. The effect of EDTA and EGTA is probably better explained by their ability to chelate divalent cations that could be necessary for the stability of the toxin's receptors or involved elsewhere in the mechanism of pore formation.

Differential Effects of Ionic Strength, Divalent Cations and pH on the Pore-forming Activity of Bacillus thuringiensis Insecticidal Toxins

The combined effects of ionic strength, divalent cations, pH and toxin concentration on the pore-forming activity of Cry1Ac and Cry1Ca were studied using membrane potential measurements in isolated midguts of Manduca sexta and a brush border membrane vesicle osmotic swelling assay. The effects of ionic strength and divalent cations were more pronounced at pH 10.5 than at pH 7.5. At the higher pH, lowering ionic strength in isolated midguts enhanced Cry1Ac activity but decreased considerably that of Cry1Ca. In vesicles, Cry1Ac had a stronger pore-forming ability than Cry1Ca at a relatively low ionic strength. Increasing ionic strength, however, decreased the rate of pore formation of Cry1Ac relative to that of Cry1Ca. The activity of Cry1Ca, which was small at the higher pH, was greatly increased by adding calcium or by increasing ionic strength. EDTA inhibited Cry1Ac activity at pH 10.5, but not at pH 7.5, indicating that trace amounts of divalent cations are necessary for Cry1Ac activity at the higher pH. These results, which clearly demonstrate a strong effect of ionic strength, divalent cations and pH on the pore-forming activity of Cry1Ac and Cry1Ca, stress the importance of electrostatic interactions in the mechanism of pore formation by B. thuringiensis toxins.

Oligomerization triggers binding of a Bacillus thuringiensis Cry1Ab pore-forming toxin to aminopeptidase N receptor leading to insertion into membrane microdomains

Bacillus thuringiensis Cry1A toxins, in contrast to other pore-forming toxins, bind two putative receptor molecules, aminopeptidase N (APN) and cadherin-like proteins. Here we show that Cry1Ab toxin binding to these two receptors depends on the toxins' oligomeric structure. Toxin monomeric structure binds to Bt-R1, a cadherin-like protein, that induces proteolytic processing and oligomerization of the toxin (Gomez, I., Sanchez, J., Miranda, R., Bravo A., Soberon, M., FEBS Lett. (2002) 513, 242-246), while the oligomeric structure binds APN, which drives the toxin into the detergent-resistant membrane (DRM) microdomains causing pore formation. Cleavage of APN by phospholipase C prevented the location of Cry1Ab oligomer and Bt-R1 in the DRM microdomains and also attenuates toxin insertion into membranes despite the presence of Bt-R1. Immunoprecipitation experiments demonstrated that initial Cry1Ab toxin binding to Bt-R1 is followed by binding to APN. Also, immunoprecipitation of Cry1Ab toxin-binding proteins using pure oligomeric or monomeric structures showed that APN was more efficiently detected in samples immunoprecipitated with the oligomeric structure, while Bt-R1 was preferentially detected in samples immunoprecipitated with the monomeric Cry1Ab. These data agrees with the 200-fold higher apparent affinity of the oligomer than that of the monomer to an APN enriched protein extract. Our data suggest that the two receptors interact sequentially with different structural species of the toxin leading to its efficient membrane insertion.

Patch-clamp study of the apical membrane of the midgut of Manduca sexta larvae: direct demonstration of endogenous channels and effect of a Bacillus thuringiensis toxin

The patch-clamp technique was applied to the apical membrane of epithelial midgut cells of a lepidoptera, Manduca sexta L. Access to the apical membrane, the main target site of Bacillus thuringiensis (Bt) toxins, was achieved by using freshly isolated larval midgut preparations mounted onto holding glass pipettes. The epithelial cells retained their functional integrity, as evidenced by the magnitude of intracellular potentials recorded with microelectrodes. With standard 32 mM K(+) solution in the bath and the patch-clamp pipette, endogenous channel activity was detected in about 50% of experiments, mainly in moulting larvae and larvae that had been kept at reduced temperature for at least two days prior to the experiments. In both cell-attached and inside-out patch-clamp configurations, different types of channel were observed, with conductances varying between about 5 and 50 pS and different conducting properties. Addition of trypsin-activated Cry1Ac Bt toxin in the patch-clamp pipette triggered, after a delay, large conductances of a few nanosiemens. This is the first study allowing exploration, in the intact midgut, of the properties of apical membrane channels and the direct interaction between the apical membrane of epithelial cells and pathogenic agents such as Bt toxins.

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

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

A constitutively expressed 36 kDa exochitinase from Bacillus thuringiensis HD-1

A 36 kDa chitinase was purified by ion exchange and gel filtration chromatography from the culture supernatant of Bacillus thuringiensis HD-1. The chitinase production was independent of the presence of chitin in the growth medium and was produced even in the presence of glucose. The purified chitinase was active at acidic pH, had an optimal activity at pH 6.5, and showed maximum activity at 65 degrees C. Of the various substrates, the enzyme catalyzed the hydrolysis of the disaccharide 4-MU(GlnAc)(2) most efficiently and was therefore classified as an exochitinase. The sequence of the tryptic peptides showed extensive homology with Bacillus cereus 36 kDa exochitinase. The 1083 bp open reading frame encoding 36 kDa chitinase was amplified with primers based on the gene sequence of B. cereus 36 kDa exochitinase. The deduced amino-acid sequence showed that the protein contained an N-terminal signal peptide and consisted of a single catalytic domain. The two conserved signature sequences characteristic of family 18 chitinases were mapped at positions 105-109 and 138-145 of Chi36. The recombinant chitinase was expressed in a catalytically active form in Escherichia coli in the vector pQE-32. The expressed 36 kDa chitinase potentiated the insecticidal effect of the vegetative insecticidal protein (Vip) when used against neonate larvae of Spodoptera litura.

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.

N-terminal activation is an essential early step in the mechanism of action of the Bacillus thuringiensis Cry1Ac insecticidal toxin

A variant form of the Bacillus thuringiensis Cry1Ac toxin that is not cleaved at the N terminus during proteolytic activation with trypsin was found to be incapable of forming pores in Manduca sexta brush border membrane vesicles in vitro and had reduced insecticidal activity in vivo. Binding studies indicated an altered binding pattern of the mutant toxin in that bound toxin could not be fully displaced by a high molar excess of fully trypsin-activated toxin. These results suggest that proteolytic removal of the N-terminal peptide of Cry1Ac is an important step in toxin activation.

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.

Cadherin-like receptor binding facilitates proteolytic cleavage of helix alpha-1 in domain I and oligomer pre-pore formation of Bacillus thuringiensis Cry1Ab toxin

Cry toxins form lytic pores in the insect midgut cells. The role of receptor interaction in the process of protoxin activation was analyzed. Incubation of Cry1Ab protoxin with a single chain antibody that mimics the cadherin-like receptor and treatment with Manduca sexta midgut juice or trypsin, resulted in toxin preparations with high pore-forming activity in vitro. This activity correlates with the formation of a 250 kDa oligomer that lacks the helix alpha-1 of domain I. The oligomer, in contrast with the 60 kDa monomer, was capable of membrane insertion as judged by 8-anilino-1-naphthalenesulfonate binding. Cry1Ab protoxin was also activated to a 250 kDa oligomer by incubation with brush border membrane vesicles, presumably by the action of a membrane-associated protease. Finally, a model where receptor binding allows the efficient cleavage of alpha-1 and formation of a pre-pore oligomeric structure that is efficient in pore formation, is presented.

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.

Processing of Cry1Ab delta-endotoxin from Bacillus thuringiensis by Manduca sexta and Spodoptera frugiperda midgut proteases: role in protoxin activation and toxin inactivation

Activation of Cry protoxins is carried out by midgut proteases. This process is important for toxicity and in some cases for specificity. Commercial proteases have been used for in vitro protoxin activation. In the case of Cry1A protoxins, trypsin digestion generates a toxic fragment of 60-65 kDa. Here, we have analyzed the in vitro and in vivo activation of Cry1Ab. We found differences in the processing of Cry1Ab protoxin by Manduca sexta and Spodoptera frugiperda midgut proteases as compared to trypsin. Midgut juice proteases produced two additional nicks at the N-terminal end removing helices alpha1 and alpha2a to produce a 58 kDa protein. A further cleavage within domain II splits the toxin into two fragments of 30 kDa. The resulting fragments were not separated, but instead coeluted with the 58 kDa monomer, in size-exclusion chromatography. To examine if this processing was involved in the activation or degradation of Cry1Ab toxin, binding, pore formation, and toxicity assays were performed. Pore formation assays showed that midgut juice treatment produced a more active toxin than trypsin treatment. In addition, it was determined that the alpha1 helix is dispensable for Cry1Ab activity. In contrast, the appearance of the 30 kDa fragments correlates with a decrease in pore formation and insecticidal activities. Our results suggest that the cleavage in domain II may be involved in toxin inactivation, and that the 30 kDa fragments are stable intermediates in the degradation pathway.

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.

Structural and functional studies of alpha-helix 5 region from Bacillus thuringiensis Cry1Ab delta-endotoxin

The crystal insecticidal proteins from Bacillus thuringiensis are modular proteins comprised of three domains connected by single linkers. Domain I is a seven alpha-helix bundle, which has been involved in membrane insertion and pore formation activity. Site-directed mutagenesis has contributed to identify regions that might play an important role in the structure of the pore-forming domain within the membrane. There are several evidences that support that the hairpin alpha4-alpha5 inserts into the membrane in an antiparallel manner, while other helices lie on the membrane surface. We hypothesized that highly conserved residues of alpha5 could play an important role in toxin insertion, oligomerization and/or pore formation. A total of 15 Cry1Ab mutants located in six conserved residues of Cry1Ab, Y153, Y161, H168, R173, W182 and G183, were isolated. Eleven mutants were located within helix alpha5, one mutant was located in the loop alpha4-alpha5 and three mutants, W182P, W182I and G183C, were located in the loop alpha5-alpha6. Their effect on binding, K(+) permeability and toxicity against Manduca sexta larvae was analyzed and compared. The results provide direct evidence that some residues located within alpha5 have an important role in stability of the toxin within the insect gut, while some others also have an important role in pore formation. The results also provide evidence that conserved residues within helix alpha5 are not involved in oligomer formation since mutations in these residues are able to make pores in vitro.

Bacillus thuringiensis cry3Ca1 protein is toxic to the Colorado potato beetle, Leptinotarsa decemlineata (Say)

We expressed the wild-type cry3Aa3 and cry3Ca1 Bacillus thuringiensis genes, which code for insecticidal proteins, in an Escherichia coli expression system. Highly purified preparations of the soluble delta-endotoxins were used to perform comparative bioassays with third-instar larvae of the Colorado potato beetle (CPB). Acute mortality data showed that Cry3Ca1 (LD(50) = 320.1 ng) was 2-fold more toxic than Cry3Aa3 (LD(50) = 672.9 ng). We also compared the chronic effects of sublethal doses of these toxins by measuring the consumption of untreated foliage and monitoring survival and development for 6 days after intoxication. No significant additional mortality was recorded, but we found that surviving larvae fed Cry3Ca1 consumed foliage at a slower rate than the larvae fed Cry3Aa3, suggesting more damage to their digestive epithelium. This study, the first assessment of the toxicity of cry3Ca1 in third-instar CPB, suggests cry3Ca1 will prove useful for the control of this important insect pest.

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.

Ion channel activity from the midgut brush-border membrane of gypsy moth (Lymantria dispar) larvae

Ion channels from the midgut apical membrane of gypsy moth (Lymantria dispar) larvae were studied following mechanical fusion of brush-border membrane vesicles with planar phospholipid bilayer membranes. In symmetrical 300 mmol l(−)(1) KCl (pH 9.0), nine different channels with conductances ranging from 27 to 795 pS and linear current/voltage relationships were resolved. In the presence of a KCl gradient across the bilayer (450 mmol l(−)(1)cis/150 mmol l(−)(1)trans), 11 different conductance levels ranging from 16 to 850 pS were detected. The channels were slightly cationic: the zero-current reversal potential was shifted by −5 mV to −21 mV compared with symmetrical KCl conditions, corresponding to p(K)/p(Cl) permeability ratios of 1.5-8.0. Most channels were neither voltage-dependent nor Ca(2+)-sensitive and displayed complex gating kinetics. Addition of Ba(2+) or Cs(+) to both sides of the bilayer had little effect on channel activity, but fewer distinct channels were observed when KCl was replaced by potassium gluconate, suggesting an effect of Cl(−) on channel activity. A reduced number of channels was also detected when KCl was replaced by N-methyl-d-glucamine-HCl. Under asymmetrical N-methyl-d-glucamine-HCl conditions, only anionic channels were observed. They exhibited current rectification (35 pS at negative voltages and 81 pS at positive voltages) and were strongly voltage-dependent.

A binding site for Bacillus thuringiensis Cry1Ab toxin is lost during larval development in two forest pests

The insecticidal activity and receptor binding properties of Bacillus thuringiensis Cry1A toxins towards the forest pests Thaumetopoea pityocampa (processionary moth) and Lymantria monacha (nun moth) were investigated. Cry1Aa, Cry1Ab, and Cry1Ac were highly toxic (corresponding 50% lethal concentration values: 956, 895, and 379 pg/microl, respectively) to first-instar T. pityocampa larvae. During larval development, Cry1Ab and Cry1Ac toxicity decreased with increasing age, although the loss of activity was more pronounced for Cry1Ab. Binding assays with (125)I-labelled Cry1Ab and brush border membrane vesicles from T. pityocampa first- and last-instar larvae detected a remarkable decrease in the overall Cry1Ab binding affinity in last-instar larvae, although saturable Cry1Ab binding to both instars was observed. Homologous competition experiments demonstrated the loss of one of the two Cry1Ab high-affinity binding sites detected in first-instar larvae. Growth inhibition assays with sublethal doses of Cry1Aa, Cry1Ab, and Cry1Ac in L. monacha showed that all three toxins were able to delay molting from second instar to third instar. Specific saturable binding of Cry1Ab was detected only in first- and second-instar larvae. Cry1Ab binding was not detected in last-instar larvae, although specific binding of Cry1Aa and Cry1Ac was observed. These results demonstrate a loss of Cry1Ab binding sites during development on the midgut epithelium of T. pityocampa and L. monacha, correlating in T. pityocampa with a decrease in Cry1Ab toxicity with increasing age.

Dias JMC, Ribeiro BM. Characterization ofBacillus thuringiensissubsp.kurstakistrain S93 effective against the fall armywormSpodoptera frugiperda)

A Brazilian strain of Bacillus thuringiensis subsp. kurstaki, designated S93, was analyzed regarding its cry gene and protein contents and activity against the fall armyworm (Spodoptera frugiperda, Smith 1797). Bioassays using lyophilized powders of S93 or HD-1 and third instar larvae of S. frugiperda showed a 12.3-fold lower LC50for the S93 strain when compared with the standard HD-1 strain. The spore-crystal mixture, analyzed by SDS-PAGE, showed two major polypeptides of 130 and 65 kDa, corresponding to Cry1 and Cry2 toxins, respectively. Western blot analysis showed that these proteins were immunologically related to the Cry1A protein from B. thuringiensis subsp. kurstaki HD-73. The polymerase chain reaction technique (PCR) using total DNA from the S93 strain and specific primers showed the presence of cry1Aa, cry1Ab, and cry1Ac genes, and a cry1A-type gene was localized in a plasmid of about 44 MDa. A cry1Ab gene was isolated from a S93 plasmid DNA library and completely sequenced. Computer analysis showed that the gene sequence (GenBank acession number AF059670) is identical to cry1Ab1 and has 91.6 and 85.9% identity with cry1Aa1 and cry1Ac1 genes, respectively. The deduced amino-acid sequence showed a high degree of similarity with the amino-acid sequences of the Cry1Ab1 (100%), Cry1Aa1 (93.8%), and Cry1Ac1 (90.6%) proteins.Key words: Bacillus thuringiensis, Spodoptera frugiperda, biological control, crystal protein, cry genes.

Toxicity, binding, and permeability analyses of four Bacillus thuringiensis Cry1 delta-endotoxins using brush border membrane vesicles of Spodoptera exigua and Spodoptera frugiperda

The binding and pore formation properties of four Bacillus thuringiensis Cry1 toxins were analyzed by using brush border membrane vesicles from Spodoptera exigua and Spodoptera frugiperda, and the results were compared to the results of toxicity bioassays. Cry1Fa was highly toxic and Cry1Ac was nontoxic to S. exigua and S. frugiperda larvae, while Cry1Ca was highly toxic to S. exigua and weakly toxic to S. frugiperda. In contrast, Cry1Bb was active against S. frugiperda but only marginally active against S. exigua. Bioassays performed with iodinated Cry1Bb, Cry1Fa, and Cry1Ca showed that the effects of iodination on toxin activity were different. The toxicities of I-labeled Cry1Bb and Cry1Fa against Spodoptera species were significantly less than the toxicities of the unlabeled toxins, while Cry1Ca retained its insecticidal activity when it was labeled with 125I. Binding assays showed that iodination prevented Cry1Fa from binding to Spodoptera brush border membrane vesicles. 125I-labeled Cry1Ac, Cry1Bb, and Cry1Ca bound with high-affinities to brush border membrane vesicles from S. exigua and S. frugiperda. Competition binding experiments performed with heterologous toxins revealed two major binding sites. Cry1Ac and Cry1Fa have a common binding site, and Cry1Bb, Cry1C, and Cry1Fa have a second common binding site. No obvious relationship between dissociation of bound toxins from brush border membrane vesicles and toxicity was detected. Cry1 toxins were also tested for the ability to alter the permeability of membrane vesicles, as measured by a light scattering assay. Cry1 proteins toxic to Spodoptera larvae permeabilized brush border membrane vesicles, but the extent of permeabilization did not necessarily correlate with in vivo toxicity.

cDNA cloning and expression of Bacillus thuringiensis Cry1Aa toxin binding 120 kDa aminopeptidase N from Bombyx mori

Bacillus thuringiensis Cry1Aa toxin binds to a 120 kDa putative receptor protein in the Bombyx mori midgut. Recently, this protein was purified and identified as glycosyl-phosphatidylinositol (GPI) anchored aminopeptidase N (APN). In this study, a full-length cDNA thought to encode this 120 kDa APN was isolated and sequenced. It has a 2958 bp ORF encoding 986 amino acids. In the deduced amino acid sequence, we identified GPI-anchor and zinc-metallopeptidase signals, which are the same as those of APNs of other insects that are reported to be putative Cry1 toxin receptors. The B. mori APN amino acid sequence also has a high similarity with those of the other APNs. Subsequently, the recombinant APN was expressed by Escherichia coli and its Cry1Aa toxin binding ability was analyzed. Ligand blotting showed that Cry1Aa toxin bound to the recombinant APN.