Solubilization, Activation, and Insecticidal Activity of Bacillus thuringiensis Serovar thompsoni HD542 Crystal Proteins

Cry15Aa protein, produced by Bacillus thuringiensis serovar thompsoni HD542 in a crystal together with a 40-kDa accompanying protein, is one of a small group of nontypical, less well-studied members of the Cry family of insecticidal proteins and may provide an alternative for the more commonly used Cry proteins in insect pest management. In this paper, we describe the characterization of the Cry15Aa and 40-kDa protein's biochemical and insecticidal properties and the mode of action. Both proteins were solubilized above pH 10 in vitro. Incubation of solubilized crystal proteins with trypsin or insect midgut extracts rapidly processed the 40-kDa protein to fragments too small to be detected by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, whereas the Cry15 protein yielded a stable product of approximately 30 kDa. Protein N-terminal sequencing showed that Cry15 processing occurs exclusively at the C-terminal end. Cry15 protein showed in vitro hemolytic activity, which was greatly enhanced by preincubation with trypsin or insect gut extract. Larvae of the lepidopteran insects Manduca sexta, Cydia pomonella, and Pieris rapae were susceptible to crystals, and presolubilization of the crystals enhanced activity to P. rapae. Activity for all three species was enhanced by preincubation with trypsin. Larvae of Helicoverpa armigera and Spodoptera exigua were relatively insensitive to crystals, and activity against these insects was not enhanced by prior solubilization or trypsin treatment. The 40-kDa crystal protein showed no activity in the insects tested, nor did its addition or coexpression in Escherichia coli increase the activity of Cry15 in insecticidal and hemolytic assays.     

Single cysteine substitution in Bacillus thuringiensis Cry7Ba1 improves the crystal solubility and produces toxicity to Plutella xylostella larvae

Many Bacillus thuringiensis isolates have no demonstrated toxicity against insects. In this study, a novel holotype crystal protein gene cry7Ba1 was isolated from a 'non-insecticidal'B. thuringiensis strain YBT-978. The Cry7Ba1 protein showed high toxicity against Plutella xylostella larvae after the crystals were dissolved at pH 12.5, suggesting that the 'non-insecticidal' properties of this protein were due to insolubility in the normal insect midgut pH environment. After the C-terminal half of Cry7Ba1 was replaced by that of Cry1Ac or Cry1C proteins, the recombinant protein inclusions could be dissolved at pH 9.5, and exhibited high toxicity against P. xylostella larvae. This result proved the insolubility of Cry7Ba1 crystal was determined by the structure of its C-terminal half. Further, six mutations were constructed by substituting cysteine residues with serine. Solubility studies showed that the crystals from mutants C697S, C834S and C854S could be dissolved at lower pH (10.5, 9.5 and 11.5 respectively). Bioassays showed that crystals from mutant C834S were toxic to P. xylostella larvae. Our discoveries suggest that a single cysteine residue located in the C-terminal half of the protein determines the solubility and toxicity of some nontoxic crystal proteins. This study provides a strategy to isolate novel insecticidal crystal protein genes from 'non-insecticidal'B. thuringiensis strains.     

Binding of Cyt1Aa and Cry11Aa toxins of Bacillus thuringiensis serovar israelensis to brush border membrane vesicles of Tipula paludosa (Diptera: Nematocera) and subsequent pore formation

Bacillus thuringiensis serovar israelensis (B. thuringiensis subsp. israelensis) produces four insecticidal crystal proteins (ICPs) (Cry4A, Cry4B, Cry11A, and Cyt1A). Toxicity of recombinant B. thuringiensis subsp. israelensis strains expressing only one of the toxins was determined with first instars of Tipula paludosa (Diptera: Nematocera). Cyt1A was the most toxic protein, whereas Cry4A, Cry4B, and Cry11A were virtually nontoxic. Synergistic effects were recorded when Cry4A and/or Cry4B was combined with Cyt1A but not with Cry11A. The binding and pore formation are key steps in the mode of action of B. thuringiensis subsp. israelensis ICPs. Binding and pore-forming activity of Cry11Aa, which is the most toxic protein against mosquitoes, and Cyt1Aa to brush border membrane vesicles (BBMVs) of T. paludosa were analyzed. Solubilization of Cry11Aa resulted in two fragments, with apparent molecular masses of 32 and 36 kDa. No binding of the 36-kDa fragment to T. paludosa BBMVs was detected, whereas the 32-kDa fragment bound to T. paludosa BBMVs. Only a partial reduction of binding of this fragment was observed in competition experiments, indicating a low specificity of the binding. In contrast to results for mosquitoes, the Cyt1Aa protein bound specifically to the BBMVs of T. paludosa, suggesting an insecticidal mechanism based on a receptor-mediated action, as described for Cry proteins. Cry11Aa and Cyt1Aa toxins were both able to produce pores in T. paludosa BBMVs. Protease treatment with trypsin and proteinase K, previously reported to activate Cry11Aa and Cyt1Aa toxins, respectively, had the opposite effect. A higher efficiency in pore formation was observed when Cyt1A was proteinase K treated, while the activity of trypsin-treated Cry11Aa was reduced. Results on binding and pore formation are consistent with results on ICP toxicity and synergistic effect with Cyt1Aa in T. paludosa.     

Bacillus thuringiensis insecticidal Cry1Aa toxin binds to a highly conserved region of aminopeptidase N in the host insect leading to its evolutionary success.

Bacillus thuringiensis insecticidal protein, Cry1Aa toxin, binds to a specific receptor in insect midguts and has insecticidal activity. Therefore, the structure of the receptor molecule is probably a key factor in determining the binding affinity of the toxin and insect susceptibility. The cDNA fragment (PX frg1) encoding the Cry1Aa toxin-binding region of an aminopeptidase N (APN) or an APN family protein from diamondback moth, Plutella xylostella midgut was cloned and sequenced. A comparison between the deduced amino acid sequence of PX frg1 and other insect APN sequences shows that Cry1Aa toxin binds to a highly conserved region of APN family protein. In this paper, we propose a model to explain the mechanism that causes B. thuringiensis evolutionary success and differing insect susceptibility to Cry1Aa toxin.     

Binding of Cyt1Aa and Cry11Aa Toxins of Bacillus thuringiensis Serovar israelensis to Brush Border Membrane Vesicles of Tipula paludosa (Diptera: Nematocera) and Subsequent Pore Formation

Bacillus thuringiensis serovar israelensis (B. thuringiensis subsp. israelensis) produces four insecticidal crystal proteins (ICPs) (Cry4A, Cry4B, Cry11A, and Cyt1A). Toxicity of recombinant B. thuringiensis subsp. israelensis strains expressing only one of the toxins was determined with first instars of Tipula paludosa (Diptera: Nematocera). Cyt1A was the most toxic protein, whereas Cry4A, Cry4B, and Cry11A were virtually nontoxic. Synergistic effects were recorded when Cry4A and/or Cry4B was combined with Cyt1A but not with Cry11A. The binding and pore formation are key steps in the mode of action of B. thuringiensis subsp. israelensis ICPs. Binding and pore-forming activity of Cry11Aa, which is the most toxic protein against mosquitoes, and Cyt1Aa to brush border membrane vesicles (BBMVs) of T. paludosa were analyzed. Solubilization of Cry11Aa resulted in two fragments, with apparent molecular masses of 32 and 36 kDa. No binding of the 36-kDa fragment to T. paludosa BBMVs was detected, whereas the 32-kDa fragment bound to T. paludosa BBMVs. Only a partial reduction of binding of this fragment was observed in competition experiments, indicating a low specificity of the binding. In contrast to results for mosquitoes, the Cyt1Aa protein bound specifically to the BBMVs of T. paludosa, suggesting an insecticidal mechanism based on a receptor-mediated action, as described for Cry proteins. Cry11Aa and Cyt1Aa toxins were both able to produce pores in T. paludosa BBMVs. Protease treatment with trypsin and proteinase K, previously reported to activate Cry11Aa and Cyt1Aa toxins, respectively, had the opposite effect. A higher efficiency in pore formation was observed when Cyt1A was proteinase K treated, while the activity of trypsin-treated Cry11Aa was reduced. Results on binding and pore formation are consistent with results on ICP toxicity and synergistic effect with Cyt1Aa in T. paludosa.     

Identification of Henosepilachna vigintioctomaculata (Coleoptera: Coccinellidae) midgut putative receptor for Bacillus thuringiensis insecticidal Cry7Ab3 toxin

Bt WZ-9 strain, containing a single Cry7Ab3 toxin, had effective insecticidal activity against larvae of Henosepilachna vigintioctomaculata. By incubation with larvae midgut homogenate and trypsin in vitro, 130 kDa Cry7Ab3 protoxin was degraded into the ～75 kDa proteinase-resistant fragments. In vivo analysis, 130 kDa Cry7Ab3 protoxin was also processed into ～75 kDa fragment. Histopathological observations indicated that Cry7Ab3 ingestion by H. vigintioctomaculata larvae causes acceleration in the blebbing of the midgut epithelium cells into the gut lumen and eventual lysis of the epithelium cells resulting in larval death. A ligand blotting experiment demonstrated that Cry7Ab3 toxin bound a 220 kDa BBMV protein. This receptor protein was identified as cadherin by matrix assisted laser desorption-time of flight-mass spectrometry (MALDI-TOF-MS). The cadherin protein may be the receptor of Cry7Ab3. The data obtained may contribute to a better understanding of the mechanism of Cry7Ab3 toxin against H. vigintioctomaculata larvae.     

Molecular and insecticidal characterization of a Cry1I protein toxic to insects of the families Noctuidae, Tortricidae, Plutellidae, and Chrysomelidae

The most notable characteristic of Bacillus thuringiensis is its ability to produce insecticidal proteins. More than 300 different proteins have been described with specific activity against insect species. We report the molecular and insecticidal characterization of a novel cry gene encoding a protein of the Cry1I group with toxic activity towards insects of the families Noctuidae, Tortricidae, Plutellidae, and Chrysomelidae. PCR analysis detected a DNA sequence with an open reading frame of 2.2 kb which encodes a protein with a molecular mass of 80.9 kDa. Trypsin digestion of this protein resulted in a fragment of ca. 60 kDa, typical of activated Cry1 proteins. The deduced sequence of the protein has homologies of 96.1% with Cry1Ia1, 92.8% with Cry1Ib1, and 89.6% with Cry1Ic1. According to the Cry protein classification criteria, this protein was named Cry1Ia7. The expression of the gene in Escherichia coli resulted in a protein that was water soluble and toxic to several insect species. The 50% lethal concentrations for larvae of Earias insulana, Lobesia botrana, Plutella xylostella, and Leptinotarsa decemlineata were 21.1, 8.6, 12.3, and 10.0 microg/ml, respectively. Binding assays with biotinylated toxins to E. insulana and L. botrana midgut membrane vesicles revealed that Cry1Ia7 does not share binding sites with Cry1Ab or Cry1Ac proteins, which are commonly present in B. thuringiensis-treated crops and commercial B. thuringiensis-based bioinsecticides. We discuss the potential of Cry1Ia7 as an active ingredient which can be used in combination with Cry1Ab or Cry1Ac in pest control and the management of resistance to B. thuringiensis toxins.     

Identification, cloning and expression of a Cry1Ab cadherin receptor from European corn borer, Ostrinia nubilalis (Hubner) (Lepidoptera: Crambidae)

Transgenic corn expressing the Cry1Ab toxin from Bacillus thuringiensis is highly toxic to European corn borer, Ostrinia nubilalis, larvae. A putative Cry1Ab receptor (OnBt-R(1)) molecule was cloned and sequenced from a cDNA library prepared from midgut tissue of O. nubilalis larvae. The 5.6 Kb gene is homologous with a number of cadherin genes identified as Cry1 binding proteins in other lepidopterans. Brush border membrane vesicles were prepared using dissected midguts from late instars. A 220-kDa protein was identified as a cadherin-like molecule, which bound to Cry1Ab toxin and cross-reacted with an anti-cadherin serum developed from recombinant expression of a partial O. nubilalis cadherin peptide. Two additional proteins of smaller size cross-reacted with the anti-cadherin serum indicating that Cry1Ab binds to multiple receptors or to different forms of the same protein. Spodoptera frugiperda (SF9) cells transfected with the OnBt-R(1) gene were shown to express the receptor molecule which caused functional susceptibility to Cry1Ab at concentrations as low as 0.1 microg/ml. These results in combination suggest strongly that a cadherin-like protein acts as receptor and is involved with Cry1Ab toxicity in O. nubilalis.     

Identification of the minimal active fragment of the Cry1Ah toxin

cry1Ah1, a novel holo-type gene cloned from Bacillus thuringiensis strain BT-8, encoded a protein exhibiting strong insecticidal activity against lepidopteran insects. To identify the minimal active fragment of the Cry1Ah toxin, 9 pairs of primers were designed to generate different PCR products. Seven PCR products were amplified by different primers using the cry1Ah1 gene as a template and cloned into a pET-21b vector. These positive clones were separately transformed into Escherichia coli. Insecticidal activity against 2nd-instar larvae of Plutella xylostella was performed using the leaf-dip bioassay: the minimal active fragment of the Cry1Ah toxin was located between amino acid residues 50I and 639E.     

Helicoverpa armigera cadherin fragment enhances Cry1Ac insecticidal activity by facilitating toxin-oligomer formation

The interaction between Bacillus thuringiensis insecticidal crystal protein Cry1A and cadherin receptors in lepidopteran insects induces toxin oligomerization, which is essential for membrane insertion and mediates Cry1A toxicity. It has been reported that Manduca sexta cadherin fragment CR12-MPED and Anopheles gambiae cadherin fragment CR11-MPED enhance the insecticidal activity of Cry1Ab and Cry4Ba to certain lepidopteran and dipteran larvae species, respectively. This study reports that a Helicoverpa armigera cadherin fragment (HaCad1) containing its toxin binding region, expressed in Escherichia coli, enhanced Cry1Ac activity against H. armigera larvae. A binding assay showed that HaCad1 was able to bind to Cry1Ac in vitro and that this event did not block toxin binding to the brush border membrane microvilli prepared from H. armigera. When the residues ¹⁴²³GVLSLNFQ¹⁴³⁰ were deleted from the fragment, the subsequent mutation peptide lost its ability to bind Cry1Ac and the toxicity enhancement was also significantly reduced. Oligomerization tests showed that HaCad1 facilitates the formation of a 250-kDa oligomer of Cry1Ac-activated toxin in the midgut fluid environment. Oligomer formation was dependent upon the toxin binding to HaCad1, which was also necessary for the HaCad1-mediated enhancement effect. Our discovery reveals a novel strategy to enhance insecticidal activity or to overcome the resistance of insects to B. thuringiensis toxin-based biopesticides and transgenic crops.     

Novel Bacillus thuringiensis binary insecticidal crystal proteins active on western corn rootworm, Diabrotica virgifera virgifera LeConte

A new family of insecticidal crystal proteins was discovered by screening sporulated Bacillus thuringiensis cultures for oral activity against western corn rootworm (WCR) larvae. B. thuringiensis isolates PS80JJ1, PS149B1, and PS167H2 have WCR insecticidal activity attributable to parasporal inclusion bodies containing proteins with molecular masses of ca. 14 and 44 kDa. The genes encoding these polypeptides reside in apparent operons, and the 14-kDa protein open reading frame (ORF) precedes the 44-kDa protein ORF. Mutagenesis of either gene in the apparent operons dramatically reduced insecticidal activity of the corresponding recombinant B. thuringiensis strain. Bioassays performed with separately expressed, biochemically purified 14- and 44-kDa polypeptides also demonstrated that both proteins are required for WCR mortality. Sequence comparisons with other known B. thuringiensis insecticidal proteins failed to reveal homology with previously described Cry, Cyt, or Vip proteins. However, there is evidence that the 44-kDa polypeptide and the 41.9- and 51.4-kDa binary dipteran insecticidal proteins from Bacillus sphaericus are evolutionarily related. The 14- and 44-kDa polypeptides from isolates PS80JJ1, PS149B1, and PS167H2 have been designated Cry34Aa1, Cry34Ab1, and Cry34Ac1, respectively, and the 44-kDa polypeptides from these isolates have been designated Cry35Aa1, Cry35Ab1, and Cry35Ac1, respectively.     

Novel Bacillus thuringiensis Binary Insecticidal Crystal Proteins Active on Western Corn Rootworm, Diabrotica virgifera virgifera LeConte

A new family of insecticidal crystal proteins was discovered by screening sporulated Bacillus thuringiensis cultures for oral activity against western corn rootworm (WCR) larvae. B. thuringiensis isolates PS80JJ1, PS149B1, and PS167H2 have WCR insecticidal activity attributable to parasporal inclusion bodies containing proteins with molecular masses of ca. 14 and 44 kDa. The genes encoding these polypeptides reside in apparent operons, and the 14-kDa protein open reading frame (ORF) precedes the 44-kDa protein ORF. Mutagenesis of either gene in the apparent operons dramatically reduced insecticidal activity of the corresponding recombinant B. thuringiensis strain. Bioassays performed with separately expressed, biochemically purified 14- and 44-kDa polypeptides also demonstrated that both proteins are required for WCR mortality. Sequence comparisons with other known B. thuringiensis insecticidal proteins failed to reveal homology with previously described Cry, Cyt, or Vip proteins. However, there is evidence that the 44-kDa polypeptide and the 41.9- and 51.4-kDa binary dipteran insecticidal proteins from Bacillus sphaericus are evolutionarily related. The 14- and 44-kDa polypeptides from isolates PS80JJ1, PS149B1, and PS167H2 have been designated Cry34Aa1, Cry34Ab1, and Cry34Ac1, respectively, and the 44-kDa polypeptides from these isolates have been designated Cry35Aa1, Cry35Ab1, and Cry35Ac1, respectively.     

Functional analysis of two processed fragments of Bacillus thuringiensis Cry11A toxin

The 70-kDa protoxin of Cry11A, a dipteran-specific insecticidal protein, was processed by trypsin into 36- and 32-kDa fragments. To investigate the potent function of the two processed fragments, a GST (Glutathione-S-transferase) fusion protein of each polypeptide was constructed. While neither the 36- nor the 32-kDa fragment was toxic to Culex pipiens larvae, coexpression of the two fragments restored the insecticidal activity. Furthermore, the coprecipitation experiment demonstrated that the 36-kDa fragment was associated with the 32-kDa fragment. It was, therefore, shown that the coexistence of the two processed fragments of Cry11A was essential for the toxicity. The mutant of the 36-kDa fragment lacking the region from Gly(257) to Arg(360) bound to the 32-kDa fragment but the coexpression with the 32-kDa fragment resulted in no toxicity, suggesting that this region was involved in insecticidal activity.     

Insecticidal toxins from Bacillus thuringiensis subsp. kenyae: gene cloning and characterization and comparison with B. thuringiensis subsp. kurstaki CryIA(c) toxins

Genes encoding insecticidal crystal proteins were cloned from three strains of Bacillus thuringiensis subsp. kenyae and two strains of B. thuringiensis subsp. kurstaki. Characterization of the B. thuringiensis subsp. kenyae toxin genes showed that they are most closely related to cryIA(c) from B. thuringiensis subsp. kurstaki. The cloned genes were introduced into Bacillus host strains, and the spectra of insecticidal activities of each Cry protein were determined for six pest lepidopteran insects. CryIA(c) proteins from B. thuringiensis subsp. kenyae are as active as CryIA(c) proteins from B. thuringiensis subsp. kurstaki against Trichoplusia ni, Lymantria dispar, Heliothis zea, and H. virescens but are significantly less active against Plutella xylostella and, in some cases, Ostrinia nubilalis. The sequence of a cryIA(c) gene from B. thuringiensis subsp. kenyae was determined (GenBank M35524) and compared with that of cryIA(c) from B. thuringiensis subsp. kurstaki. The two genes are more than 99% identical and show seven amino acid differences among the predicted sequences of 1,177 amino acids.     

Intramolecular proteolytic nicking and binding of Bacillus thuringiensis Cry8Da toxin in BBMVs of Japanese beetle

Bacillus thuringiensis (Bt) Cry8D insecticidal proteins are unique among Cry8 family proteins in terms of its insecticidal activity against adult Scarab beetles, such as Japanese beetle (Popillia japonica Newman). From the sequence homology with other Bt Cry proteins especially those active against beetles, such as Cry3Aa whose 3D structure is available, the structure of the Cry8D protein has been predicted to be a typical three-domain Cry protein type. In addition, the activation process of Cry8D in gut juice of susceptible insects is presumed to be similar to that of Cry3A (Yamaguchi et al., 2008). In this study, the activation process of Cry8Da in insect gut juice was closely examined. Japanese beetle gut juice proteases digested the 130 kDa Cry8Da protein to produce a 64 kDa protein. This 64 kDa protein was active against both adult and larval Japanese beetle and considered to be an activated toxin. N-terminal sequencing of this 64 kDa protein revealed that the Cry8Da leader sequence consisting of 63 amino acid residues from M¹ to F⁶³ was removed. As in the case of Cry3Aa, the proteases further digested the 64 kDa protein to two 8 kDa and 54 kDa fragments. N-terminal amino acid analysis of these smaller fragments indicated that the proteases digested the loop between Alpha Helix (Alpha for short) 3 and Alpha 4. This means that the 8 kDa fragment consists of Alpha 1-3 of Domain I and that the 54 kDa fragment contains the remaining Domain I and full Domain II and Domain III. Size exclusion chromatography and anion exchange chromatography could not separate these 64, 54 and 8 kDa proteins suggesting that the 54 kDa and 8 kDa fragments are still forming the toxin complex equivalent to the 64 kDa protein by size and ionic charge. The sequencing and chromatography results suggest that the gut juice proteases merely nicked the loop between Alpha 3 and Alpha 4. This nicking process appeared to be essential for receptor binding of the Cry8Da toxin. BBMV binding assay revealed that the Cry8Da toxin bound to BBMV preparations from both adult and larval Japanese beetle only after the loop was nicked. Only the 54 kDa fragment bound to the BBMV preparations but not the 64 kDa protein. Ligand blot showed that the protease activated Cry8Da toxin, presumably the 54 kDa fragment, bound to specific BBMV proteins, one or more of those would be receptor(s). The sizes and binding affinities of these Cry8Da-bound proteins of Japanese beetle BBMV differed between larvae and adults.     

Activation pattern and toxicity of the Cry11Bb1 toxin of Bacillus thuringiensis subsp. medellin

Bacillus thuringiensis protoxins undergo proteolytic processing in the midgut of susceptible insects to become active. The ability to process the Cry11Bb1 protoxin by trypsin and Culex quinquefasciatus larval gut extracts was tested. The protease activity indicated by the appearance of proteolytic products increased with an increment in pH, with the highest activity being observed at pH 10.6. A time course study showed the proteolysis of the 94-kDa Cry11Bb protein ending with the production of fragments of relative molecular mass of 30 and 35 kDa within 5 min. In vitro, gut proteases extract cleaved the solubilized toxin between Ser59 and Ile60 and between Ala395 and Asn396, generating a 30-kDa N-terminal and a 35-kDa C-terminal fragment, respectively. Similarly, mosquito larvae processed in vivo the parasporal inclusions, generating the same fragments as those observed in vitro. The Cry11Bb1 protoxin activated with trypsin or gut proteases showed larvicidal activity against C. quinquefasciatus first instar larvae. The data suggest that gut proteases participate in the activation of CryllBbl protoxin, generating at least two different fragments on which the activity could reside.     

Binding of Bacillus thuringiensis Cry1Ac Toxin to Aminopeptidase in Susceptible and Resistant Diamondback Moths (Plutella xylostella)

Bacillus thuringiensis Cry1Ac toxin bound to a 120-kDa protein isolated from the brush border membranes of both susceptible and resistant larvae of Plutella xylostella, the diamondback moth. The 120-kDa protein was purified by Cry1Ac toxin affinity chromatography. Like Cry1Ac-binding aminopeptidase N (EC 3.4.11.2) from other insects, this protein was eluted from the affinity column with 200 mM N-acetylgalactosamine. The purified protein had aminopeptidase activity and bound Cry1Ac toxin on ligand blots. Purified aminopeptidase was recognized by antibodies to the cross-reacting determinant found on phosphatidylinositol-specific phospholipase C-solubilized proteins. The results show that the presence of Cry1Ac-binding aminopeptidase in the brush border membrane is not sufficient to confer susceptibility to Cry1Ac. Furthermore, the results do not support the hypothesis that resistance to Cry1Ac was caused by lack of a Cry1Ac-binding aminopeptidase.     

Binding analyses of Bacillus thuringiensis Cry delta-endotoxins using brush border membrane vesicles of Ostrinia nubilalis

Transgenic corn expressing the Bacillus thuringiensis Cry1Ab gene is highly insecticidal to Ostrinia nubilalis (European corn borer) larvae. We ascertained whether Cry1F, Cry9C, or Cry9E recognizes the Cry1Ab binding site on the O. nubilalis brush border by three approaches. An optical biosensor technology based on surface plasmon resonance measured binding of brush border membrane vesicles (BBMV) injected over a surface of immobilized Cry toxin. Preincubation with Cry1Ab reduced BBMV binding to immobilized Cry1Ab, whereas preincubation with Cry1F, Cry9C, or Cry9E did not inhibit BBMV binding. BBMV binding to a Cry1F-coated surface was reduced when vesicles were preincubated in Cry1F or Cry1Ab but not Cry9C or Cry9E. A radioligand approach measured 125I-Cry1Ab toxin binding to BBMV in the presence of homologous (Cry1Ab) and heterologous (Cry1Ac, Cry1F, Cry9C, or Cry9E) toxins. Unlabeled Cry1Ac effectively competed for 125I-Cry1Ab binding in a manner comparable to Cry1Ab itself. Unlabeled Cry9C and Cry9E toxins did not inhibit (125)I-Cry1Ab binding to BBMV. Cry1F inhibited (125)I-Cry1Ab binding at concentrations greater than 500 nM. Cry1F had low-level affinity for the Cry1Ab binding site. Ligand blot analysis identified Cry1Ab, Cry1Ac, and Cry1F binding proteins in BBMV. The major Cry1Ab signals on ligand blots were at 145 kDa and 154 kDa, but a strong signal was present at 220 kDa and a weak signal was present at 167 kDa. Cry1Ac and Cry1F binding proteins were detected at 220 and 154 kDa. Anti-Manduca sexta aminopeptidase serum recognized proteins of 145, 154, and 167 kDa, and anti-cadherin serum recognized the 220 kDa protein. We speculate that isoforms of aminopeptidase and cadherin in the brush border membrane serve as Cry1Ab, Cry1Ac, and Cry1F binding proteins.     

Cry1A toxins of Bacillus thuringiensis bind specifically to a region adjacent to the membrane-proximal extracellular domain of BT-R(1) in Manduca sexta: involvement of a cadherin in the entomopathogenicity of Bacillus thuringiensis

Many subspecies of the soil bacterium Bacillus thuringiensis produce various parasporal crystal proteins, also known as Cry toxins, that exhibit insecticidal activity upon binding to specific receptors in the midgut of susceptible insects. One such receptor, BT-R(1) (210 kDa), is a cadherin located in the midgut epithelium of the tobacco hornworm, Manduca sexta. It has a high binding affinity (K(d) approximately 1nM) for the Cry1A toxins of B. thuringiensis. Truncation analysis of BT-R(1) revealed that the only fragment capable of binding the Cry1A toxins of B. thuringiensis was a contiguous 169-amino acid sequence adjacent to the membrane-proximal extracellular domain. The purified toxin-binding fragment acted as an antagonist to Cry1Ab toxin by blocking the binding of toxin to the tobacco hornworm midgut and inhibiting insecticidal action. Exogenous Cry1Ab toxin bound to intact COS-7 cells expressing BT-R(1) cDNA, subsequently killing the cells. Recruitment of BT-R(1) by B. thuringiensis indicates that the bacterium interacts with a specific cell adhesion molecule during its pathogenesis. Apparently, Cry toxins, like other bacterial toxins, attack epithelial barriers by targeting cell adhesion molecules within susceptible insect hosts.