Analysis of the region for receptor binding and triggering of oligomerization on Bacillus thuringiensis Cry1Aa toxin

Utilization of Diverse Molecules as Receptors by Cry Toxin and the Promiscuous Nature of Receptor-Binding Sites Which Accounts for the Diversity

By 2013, it had been shown that the genes cadherin-like receptor (Cad) and ATP-binding cassette transporter subfamily C2 (ABCC2) were responsible for insect resistance to several Cry1A toxins, acting as susceptibility-determining receptors, and many review articles have been published. Therefore, this review focuses on information about receptors and receptor-binding sites that have been revealed since 2014. Since 2014, studies have revealed that the receptors involved in determining susceptibility vary depending on the Cry toxin subfamily, and that binding affinity between Cry toxins and receptors plays a crucial role. Consequently, models have demonstrated that ABCC2, ABCC3, and Cad interact with Cry1Aa; ABCC2 and Cad with Cry1Ab and Cry1Ac; ABCC2 and ABCC3 with Cry1Fa; ABCB1 with Cry1Ba, Cry1Ia, Cry9Da, and Cry3Aa; and ABCA2 with Cry2Aa and Cry2Ba, primarily in the silkworm, Bombyx mori. Furthermore, since 2017, it has been suggested that the binding sites of BmCad and BmABCC2 on Cry1Aa toxin overlap in the loop region of domain II, indicating that Cry toxins use various molecules as receptors due to their ability to bind promiscuously in this region. Additionally, since 2017, several ABC transporters have been identified as low-efficiency receptors that poorly induce cell swelling in heterologously expressing cultured cells. In 2024, research suggested that multiple molecules from the ABC transporter subfamily, including ABCC1, ABCC2, ABCC3, ABCC4, ABCC10, and ABCC11, act as low-efficiency receptors for a single Cry toxin in the midgut of silkworm larvae. This observation led to the hypothesis that the presence of such low-efficiency receptors contributes to the evolution of Cry toxins towards the generation of highly functional receptors that determine the susceptibility of individual insects. Moreover, this evolutionary process is considered to offer valuable insights for the engineering of Cry toxins to overcome resistance and develop countermeasures against resistance.

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.

Structural changes upon membrane insertion of the insecticidal pore-forming toxins produced by Bacillus thuringiensis

Different Bacillus thuringiensis (Bt) strains produce a broad variety of pore-forming toxins (PFTs) that show toxicity against insects and other invertebrates. Some of these insecticidal PFT proteins have been used successfully worldwide to control diverse insect crop pests. There are several studies focused on describing the mechanism of action of these toxins that have helped to improve their performance and to cope with the resistance evolved by different insects against some of these proteins. However, crucial information that is still missing is the structure of pores formed by some of these PFTs, such as the three-domain crystal (Cry) proteins, which are the most commercially used Bt toxins in the biological control of insect pests. In recent years, progress has been made on the identification of the structural changes that certain Bt insecticidal PFT proteins undergo upon membrane insertion. In this review, we describe the models that have been proposed for the membrane insertion of Cry toxins. We also review the recently published structures of the vegetative insecticidal proteins (Vips; e.g. Vip3) and the insecticidal toxin complex (Tc) in the membrane-inserted state. Although different Bt PFTs show different primary sequences, there are some similarities in the three-dimensional structures of Vips and Cry proteins. In addition, all PFTs described here must undergo major structural rearrangements to pass from a soluble form to a membrane-inserted state. It is proposed that, despite their structural differences, all PFTs undergo major structural rearrangements producing an extended α-helix, which plays a fundamental role in perforating their target membrane, resulting in the formation of the membrane pore required for their insecticidal activity.

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.

Abiotic Synthetic Antibodies to Target a Specific Protein Domain and Inhibit Its Function

The Bacillus thuringiensis (Bt) Cry proteins are widely used in insect pest control. Despite their economic benefits, remaining concerns over potential ecological and health risks warrant their ongoing surveillance. Affinity reagents, most often antibodies, protein scaffolds, and aptamers, are the traditional tools used for protein binding and detection. We report a synthetic antibody (SA) alternative to traditional biological affinity reagents for binding Bt Cry proteins. Analysis of hotspots of the Bt Cry protein-insect midgut cadherin-like receptor complexes was used for the design of the SA. The SA was selected from a small focused library of hydrogel copolymers containing functional monomers complementary to key exposed hotspots of Bt Cry proteins. A directed chemical evolution identified a SA, APhe-NP23, with affinity and selectivity for Bt Cry1Ab/Ac proteins. The putative intermolecular polymer-protein interfaces were identified by the SA's uptake of Bt Cry1Ac pepsin hydrolysates, binding epitope mutation studies, and protein-protein inhibition studies of the toxin binding to its native insect receptor binding domains. The SA inhibitor binds to the same protein domains as the insect's cadherin-like receptors, Bt-R₁ and SeCad1b. The SA binds rapidly to Bt Cry1Ab/Ac with high capacity, is pH-responsive, and is synthesized reproducibly. We believe that a hotspot-directed approach is general for creation of abiotic protein affinity reagents that target functional protein domains. Affinity ligands are typically high-information content biologicals. Their structure and function are determined from their amino acid or oligo sequence. In contract, the SA described in this work is a statistical copolymer that lacks sequence specificity. These results are an important contribution to the concept that randomness and biospecificity are not mutually exclusive.

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

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

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

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

Helix α-3 inter-molecular salt bridges and conformational changes are essential for toxicity of Bacillus thuringiensis 3D-Cry toxin family

Bacillus thuringiensis insecticidal Cry toxins break down larval midgut-cells after forming pores. The 3D-structures of Cry4Ba and Cry5Ba revealed a trimeric-oligomer after cleavage of helices α-1 and α-2a, where helix α-3 is extended and made contacts with adjacent monomers. Molecular dynamic simulations of Cry1Ab-oligomer model based on Cry4Ba-coordinates showed that E101 forms a salt-bridge with R99 from neighbor monomer. An additional salt bridge was identified in the trimeric-Cry5Ba, located at the extended helix α-3 in the region corresponding to the α-2b and α-3 loop. Both salt-bridges were analyzed by site directed mutagenesis. Single-point mutations in the Lepidoptera-specific Cry1Ab and Cry1Fa toxins were affected in toxicity, while reversed double-point mutant partially recovered the phenotype, consistent with a critical role of these salt-bridges. The single-point mutations in the salt-bridge at the extended helix α-3 of the nematicidal Cry5Ba were also non-toxic. The incorporation of this additional salt bridge into the nontoxic Cry1Ab-R99E mutant partially restored oligomerization and toxicity, supporting that the loop between α-2b and α-3 forms part of an extended helix α-3 upon oligomerization of Cry1 toxins. Overall, these results highlight the role in toxicity of salt-bridge formation between helices α-3 of adjacent monomers supporting a conformational change in helix α-3.

Extracellular loop structures in silkworm ABCC transporters determine their specificities for Bacillus thuringiensis Cry toxins

Bacillus thuringiensis Cry toxins are insecticidal proteins used widely for pest control. They are lethal to a restricted range of insects via specific interactions with insect receptors such as the ABC transporter subfamily members C2 (ABCC2) and C3 (ABCC3). However, it is still unclear how these different receptors contribute to insect susceptibility to Cry1A toxins. Here, we investigated the differences between the silkworm (Bombyx mori) ABCC2 (BmABCC2_S) and ABCC3 (BmABCC3) receptors in mediating Cry toxicity. Compared with BmABCC2_S, BmABCC3 exhibited 80- and 267-fold lower binding affinities to Cry1Aa and Cry1Ab, respectively, and these decreased affinities correlated well with the lower receptor activities of BmABCC3 for these Cry1A toxins. To identify the amino acid residues responsible for these differences, we constructed BmABCC3 variants containing a partial amino acid replacement with extracellular loops (ECLs) from BmABCC2_S. Replacing three amino acids from ECL 1 or 3 increased BmABCC3 activity toward Cry1Aa and enabled its activity toward Cry1Ab. Meanwhile, BmABCC2_S and BmABCC3 exhibited no receptor activities for Cry1Ca, Cry1Da, and Cry3Bb, correlating with markedly lower binding affinities for these Cry toxins. ABCC2 from a Cry1Ab-resistant B. mori strain (BmABCC2_R), which has a tyrosine insertion in ECL 2, displayed 93-fold lower binding affinity to Cry1Ab compared with BmABCC2_S but maintained high binding affinity to Cry1Aa. These results indicate that the Cry toxin-binding affinities of ABCC transporters are largely linked to the level of Cry susceptibility of ABCC-expressing cells and that the ABCC ECL structures determine the specificities to Cry toxins.

The intracellular region of silkworm cadherin-like protein is not necessary to mediate the toxicity of Bacillus thuringiensis Cry1Aa and Cry1Ab toxins

The cadherin-like protein in lepidopteran insects, known as a receptor for Bacillus thuringiensis Cry1A toxins, is a single-pass membrane protein that can be divided into extracellular and intracellular regions. The extracellular region is important for toxin binding and oligomerization, whereas the role of the intracellular region during Cry1A intoxication is unclear. In the present study, we generated a deletion mutant of Bombyx mori cadherin-like protein (BtR175) that lacked the intracellular region to investigate its role in mediating Cry1A toxicity. Like wild-type BtR175, the mutant protein conferred susceptibility to Cry1Aa and Cry1Ab toxins in Sf9 cells, suggesting that the intracellular region is not required to mediate intoxication. The deletion mutant maintained another role of cadherin-like proteins; that it, synergistic activity with B. mori ABC transporter C2 (ABCC2) when mediating Cry1Aa and Cry1Ab toxicity. In addition, we evaluated the effects of reagents that have been reported to inhibit Cry1A toxicity (e.g., protein kinase A inhibitors, EDTA, and sucrose) on Cry1A toxicity in BtR175-expressing cells. Our results suggest that Cry1Aa-induced cell death in BtR175-expressing cells was not caused by signal transduction but by osmotic lysis. Overall, our data indicate that BtR175 mediates the toxicity of Cry1Aa and Cry1Ab toxins entirely via its extracellular region. They also indicate that the synergism between cadherin-like protein and ABCC2 occurs outside of cells or in the cell membrane.

Cry toxin specificities of insect ABCC transporters closely related to lepidopteran ABCC2 transporters

In this study, we examined insect and human ABCC transporters closely related to the lepidopteran ABC transporter C2 (ABCC2), a powerful receptor for the Bacillus thuringiensis Cry toxin, for their responses to various Cry toxins. ABCC2 and the lepidopteran ABC transporter C3 (ABCC3) conferred cultured cells with susceptibility to a lepidopteran-specific Cry1Aa toxin but not to lepidopteran-specific Cry1Ca and Cry1Da. One coleopteran ABCC transporter specifically responded to a coleopteran-specific Cry8Ca toxin. ABCC transporters from a dipteran insect and humans did not respond to any of the tested Cry toxins that are active to lepidopteran and coleopteran insects. These results yield important information for our understanding of insect specificity of Cry toxins and provide the first demonstration of a coleopteran ABCC transporter that serves as a Cry toxin receptor.

Water influx via aquaporin directly determines necrotic cell death induced by the Bacillus thuringiensis Cry toxin

The Bacillus thuringiensis Cry toxin causes swelling and necrosis in insect cells, but the route(s) and significance of the water influx involved in its cytotoxicity are unclear. Here, we assessed the role of aquaporins (AQPs), known as water channels, in Cry toxin intoxication. An AQP inhibitor did not interfere with any known process to form the toxin pore, but it diminished the cell swelling and loss of membrane integrity induced by the Cry toxin. Overexpression of AQPs facilitated water influx and cytotoxicity. Our results demonstrate that water influx via aquaporin directly determines necrotic cell death induced by the Cry toxin.

Identification of an alkaline phosphatase as a putative Cry1Ac binding protein in Ostrinia furnacalis (Guenée)

Asian corn borer (ACB), Ostrinia furnacalis, is an important insect pest of maize susceptible to different Cry1A toxins. Based on amino acid sequence alignment of ALP sequences from lepidopteran larvae an alp gene was cloned from ACB, named ofalp. Pull dawn assays using biotinylated Cry1Ac and brush border membrane vesicles isolated from second instar ACB larvae showed that four proteins of 50, 65, 68 and 70kDa precipitated with the Cry1Ac. The 65kDa band cross-reacted with the anti-OfALP monoclonal antibody. GalNac was able to release the binding of Cry1Ac to the 65kDa OfALP in pull down assays. A 37kDa fragment from residues D173 to D473 of OfALP was cloned and expressed in Escherichia coli cells. We show that this ALP-fragment was able to bind Cry1Ac in ligand blot analysis. Our data also indicate that different ALP isoforms or variants may be also Cry1Ac binding proteins since more ALP enzymatic activity was pull down with Cry1Ac than with anti-OfALP antibody. We also analyzed the expression levels of ALP throughout the larval development by qPCR and ALP enzymatic activity. Our data indicated that ALP expression in ACB was observed preferentially in young instar larvae. Finally, we show that resistance in O. furnacalis ACB-AcR strain resistant to Cry1Ac did not correlate with changes in expression of this ALP protein since it shows similar gene expression of ofalp than the susceptible insect strain. Identification of Cry1Ac receptors will help to understand mechanism of action of Cry1Ac in O. furnacalis and to understand mechanism of Cry toxin resistance. Our data indicate that at least one ALP protein is involved in the binding interaction with Cry1Ac in O. furnacalis.

Single amino acid insertions in extracellular loop 2 of Bombyx mori ABCC2 disrupt its receptor function for Bacillus thuringiensis Cry1Ab and Cry1Ac but not Cry1Aa toxins

In a previous report, seven Cry1Ab-resistant strains were identified in the silkworm, Bombyx mori; these strains were shown to have a tyrosine insertion at position 234 in extracellular loop 2 of the ABC transporter C2 (BmABCC2). This insertion was confirmed to destroy the receptor function of BmABCC2 and confer the strains resistance against Cry1Ab and Cry1Ac. However, these strains were susceptible to Cry1Aa. In this report, we examined the mechanisms of the loss of receptor function of the transporter by expressing mutations in Sf9 cells. After replacement of one or two of the five amino acid residues in loop 2 of the susceptible BmABCC2 gene [BmABCC2_S] with alanine, cells still showed susceptibility, retaining the receptor function. Five mutants with single amino acid insertions at position 234 in BmABCC2 were also generated, resulting in loop 2 having six amino acids, which corresponds to replacing the tyrosine insertion in the resistant BmABCC2 gene [BmABCC2_R(+(234)Y)] with another amino acid. All five mutants exhibited loss of function against Cry1Ab and Cry1Ac. These results suggest that the amino acid sequence in loop 2 is less important than the loop size (five vs. six amino acids) or loop structure for Cry1Ab and Cry1Ac activity. Several domain-swapped mutant toxins were then generated among Cry1Aa, Cry1Ab, and Cry1Ac, which are composed of three domains. Swapped mutants containing domain II of Cry1Ab or Cry1Ac did not kill Sf9 cells expressing BmABCC2_R(+(234)Y), suggesting that domain II of the Cry toxin is related to the interaction with the receptor function of BmABCC2. This also suggests that different reactions against Bt-toxins in some B. mori strains, that is, Cry1Ab resistance or Cry1Aa susceptibility, are attributable to structural differences in domain II of Cry1A toxins.

Induction of rapid and selective cell necrosis in Drosophila using Bacillus thuringiensis Cry toxin and its silkworm receptor

Background

Genetic ablation of target cells is a powerful tool to study the origins and functions of cells, tissue regeneration, or pathophysiology in a human disease model in vivo. Several methods for selective cell ablation by inducing apoptosis have been established, using exogenous toxins or endogenous proapoptotic genes. However, their application is limited to cells with intact apoptotic machinery.

Results

Herein, we established a method for inducing rapid and selective cell necrosis by the pore-forming bacterial toxin Cry1Aa, which is specifically active in cells expressing the Cry1Aa receptor (CryR) derived from the silkworm Bombyx mori. We demonstrated that overexpressing CryR in Drosophila melanogaster tissues induced rapid cell death of CryR-expressing cells only, in the presence of Cry1Aa toxin. Cry/CryR system was effective against both proliferating cells in imaginal discs and polyploid postmitotic cells in the fat body. Live imaging analysis of cell ablation revealed swelling and subsequent osmotic lysis of CryR-positive cells after 30 min of incubation with Cry1Aa toxin. Osmotic cell lysis was still triggered when apoptosis, JNK activation, or autophagy was inhibited, suggesting that Cry1Aa-induced necrotic cell death occurred independently of these cellular signaling pathways. Injection of Cry1Aa into the body cavity resulted in specific ablation of CryR-expressing cells, indicating the usefulness of this method for in vivo cell ablation.

Conclusions

With Cry toxins from Bacillus thuringiensis, we developed a novel method for genetic induction of cell necrosis. Our system provides a “proteinous drill” for killing target cells through physical injury of the cell membrane, which can potentially be used to ablate any cell type in any organisms, even those that are resistant to apoptosis or JNK-dependent programmed cell death.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-015-0160-2) contains supplementary material, which is available to authorized users.

Aedes cadherin mediates the in vivo toxicity of the Cry11Aa toxin to Aedes aegypti

Cadherin plays an important role in the toxicity of Bacillus thuringiensis Cry proteins. We previously cloned a full-length cadherin from Aedes aegypti larvae and reported this protein binds Cry11Aa toxin from B. thuringiensis subsp. israelensis with high affinity, ≈16.7nM. Based on these results, we investigated if Aedes cadherin is involved in the in vivo toxicity of Cry11Aa toxin to Ae. aegypti. We established a mosquito cell line stably expressing the full-length Aedes cadherin and transgenic mosquitoes with silenced Aedes cadherin expression. Cells expressing the Aedes cadherin showed increased sensitivity to Cry11Aa toxin. Cry11Aa toxin at 400nM killed approximately 37% of the cells in 3h. Otherwise, transgenic mosquitoes with silenced Aedes cadherin expression showed increased tolerance to Cry11Aa toxin. Furthermore, cells expressing Aedes cadherin triggered Cry11Aa oligomerization. These results show the Aedes cadherin plays a pivotal role in Cry11Aa toxicity to Ae. aegypti larvae by mediating Cry11Aa oligomerization. However, since high toxicity was not obtained in cadherin-expressing cells, an additional receptor may be needed for manifestation of full toxicity. Moreover, cells expressing Aedes cadherin were sensitive to Cry4Aa and Cry11Ba, but not Cry4Ba. However transgenic mosquitoes with silenced Aedes cadherin expression showed no tolerance to Cry4Aa, Cry4Ba, and Cry11Ba toxins. These results suggest that while Aedes cadherin may mediate Cry4Aa and Cry11Ba toxicity, this cadherin but is not the main receptor of Cry4Aa, Cry4Ba and Cry11Ba toxin in Ae. aegypti.

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.

Cry1Aa binding to the cadherin receptor does not require conserved amino acid sequences in the domain II loops

Characterizing the binding mechanism of Bt (Bacillus thuringiensis) Cry toxin to the cadherin receptor is indispensable to understanding the specific insecticidal activity of this toxin. To this end, we constructed 30 loop mutants by randomly inserting four serial amino acids covering all four receptor binding loops (loops α8, 1, 2 and 3) and analysed their binding affinities for Bombyx mori cadherin receptors via Biacore. High binding affinities were confirmed for all 30 mutants containing loop sequences that differed from those of wild-type. Insecticidal activities were confirmed in at least one mutant from loops 1, 2 and 3, suggesting that there is no critical amino acid sequence for the binding of the four loops to BtR175. When two mutations at different loops were integrated into one molecule, no reduction in binding affinity was observed compared with wild-type sequences. Based on these results, we discussed the binding mechanism of Cry toxin to cadherin protein.

Insect tolerance to the crystal toxins Cry1Ac and Cry2Ab is mediated by the binding of monomeric toxin to lipophorin glycolipids causing oligomerization and sequestration reactions

Endotoxins from the soil bacterium Bacillus thuringiensis are used worldwide to control insect pests and vectors of diseases. Despite extensive use of the toxins as sprays and in transgenic crops, their mode of action is still not completely known. Here we show that two crystal toxins binding to different glycoprotein receptors have similar glycolipid binding properties. The glycolipid binding domain was identified in a recombinant peptide representing the domain II of the crystal toxin Cry1Ac (M-peptide). The recombinant M-peptide was isolated from bacterial lysates as a mixture of monomers and dimers and formed tetramers upon binding to glycolipid microvesicles from gut tissues and lipid particles from hemolymph plasma. Likewise, when mature toxins and M-peptides where mixed with plasma, these peptides bind to lipid particles and can be separated with lipophorin particles on low-density gradients. When mature toxin and M-peptides are added to lipid particles in increasing amounts, the peptide-particle complexes form higher aggregates that are similar to aggregates formed in low-density gradients in the presence of the toxin. This could indicate that glycolipids on lipid particles are possible targets for toxin monomers in the gut lumen, which upon binding to the glycolipids form tetramers and aggregate particles and thereby sequester the toxin inside the gut lumen before it can interact with receptors on the brush border membrane. The implication is that domain II interacting with glycolipids mediate tolerance to the toxin that is separate from interaction of the toxin with glycoprotein receptors causing toxicity.

Survey of the 2009 commercial optical biosensor literature

We took a different approach to reviewing the commercial biosensor literature this year by inviting 22 biosensor users to serve as a review committee. They set the criteria for what to expect in a publication and ultimately decided to use a pass/fail system for selecting which papers to include in this year's reference list. Of the 1514 publications in 2009 that reported using commercially available optical biosensor technology, only 20% passed their cutoff. The most common criticism the reviewers had with the literature was that "the biosensor experiments could have been done better." They selected 10 papers to highlight good experimental technique, data presentation, and unique applications of the technology. This communal review process was educational for everyone involved and one we will not soon forget.