Proteolytic processing of a coleopteran-specific delta-endotoxin produced by Bacillus thuringiensis var. tenebrionis

Structural and functional insights into the first Bacillus thuringiensis vegetative insecticidal protein of the Vpb4 fold, active against western corn rootworm

The western corn rootworm (WCR), Diabrotica virgifera virgifera LeConte, is a major maize pest in the United States causing significant economic loss. The emergence of field-evolved resistant WCR to Bacillus thuringiensis (Bt) traits has prompted the need to discover and deploy new insecticidal proteins in transgenic maize. In the current study we determined the crystal structure and mode of action (MOA) of the Vpb4Da2 protein (formerly known as Vip4Da2) from Bt, the first identified insecticidal Vpb4 protein with commercial level control against WCR. The Vpb4Da2 structure exhibits a six-domain architecture mainly comprised of antiparallel β-sheets organized into β-sandwich layers. The amino-terminal domains 1-3 of the protein share structural homology with the protective antigen (PA) PA14 domain and encompass a long β-pore forming loop as in the clostridial binary-toxB module. Domains 5 and 6 at the carboxyl-terminal half of Vpb4Da2 are unique as this extension is not observed in PA or any other structurally-related protein other than Vpb4 homologs. These unique Vpb4 domains adopt the topologies of carbohydrate-binding modules known to participate in receptor-recognition. Functional assessment of Vpb4Da2 suggests that domains 4-6 comprise the WCR receptor binding region and are key in conferring the observed insecticidal activity against WCR. The current structural analysis was complemented by in vitro and in vivo characterizations, including immuno-histochemistry, demonstrating that Vpb4Da2 follows a MOA that is consistent with well-characterized 3-domain Bt insecticidal proteins despite significant structural differences.

Engineering of multiple trypsin/chymotrypsin sites in Cry3A to enhance its activity against Monochamus alternatus Hope larvae

BACKGROUND

Bacillus thuringiensis Cry3 toxins exhibit specific toxicity against several coleopteran larvae. However, owing to its low toxicity to Monochamus alternatus, Cry3A toxin is not useful for managing M. alternatus larvae. Here we assessed the proteolytic activation of Cry3Aa toxin in M. alternatus larval midgut and increased its toxicity by molecular modification.

RESULTS

Our results indicated that insufficient processing of Cry3Aa protoxin and non-specific enzymatic digestion of Cry3Aa toxin in the midgut of M. alternatus larvae led to low toxicity. The results of transcriptome analysis, enzymatic assay with fluorogenic substrates, and multiplex substrate profiling by mass spectrometry showed that the main digestive enzymes in M. alternatus larval midgut were trypsin-like proteases that preferentially cleaved peptides with arginine and lysine residues. Consequently, trypsin recognition sites were introduced into the Domain I of Cry3Aa protoxin in the loop regions between α-helix 3 and α-helix 4 to facilitate proteolytic activation. Multiple potential trypsin cleavage sites away from the helix sheet and functional regions in Cry3Aa proteins were also mutated to alanine to prevent non-specific enzymatic digestion. Bioassays indicated that a modified Cry3Aa-T toxin (K65A, K70A, K231A, K468A, and K596A) showed a 9.5-fold (LC₅₀ = 12.3 μg/mL) increase in toxicity to M. alternatus larvae when compared to native Cry3Aa toxin.

CONCLUSION

This study highlights an effective way to increase the toxicity of Cry3Aa toxin to M. alternatus, which may be suitable for managing the resistance of transgenic plants to other pests, including some of the most important pests in agriculture. © 2020 Society of Chemical Industry.

Insecticidal Activity of Bacillus thuringiensis Proteins Against Coleopteran Pests

Bacillus thuringiensis is the most successful microbial insecticide agent and its proteins have been studied for many years due to its toxicity against insects mainly belonging to the orders Lepidoptera, Diptera and Coleoptera, which are pests of agro-forestry and medical-veterinary interest. However, studies on the interactions between this bacterium and the insect species classified in the order Coleoptera are more limited when compared to other insect orders. To date, 45 Cry proteins, 2 Cyt proteins, 11 Vip proteins, and 2 Sip proteins have been reported with activity against coleopteran species. A number of these proteins have been successfully used in some insecticidal formulations and in the construction of transgenic crops to provide protection against main beetle pests. In this review, we provide an update on the activity of Bt toxins against coleopteran insects, as well as specific information about the structure and mode of action of coleopteran Bt proteins.

Meeting technical challenges for protein characterization and surrogate equivalence studies that resulted from insecticidal protein co-expression in maize event MZIR098

Safety assessment of genetically modified plants includes protein characterization to confirm the intended trait protein expression. In addition, to conduct safety tests, the large amount of purified protein needed is usually met through the use of a surrogate, microbially produced protein source. Characterization of the eCry3.1Ab and mCry3A proteins as derived from Event MZIR098 maize was challenging because of the difficulty in purifying/isolating these proteins that are of similar molecular weight and have considerable shared sequence and immunogenicity. This also applies to establishing the biochemical equivalence to the microbially produced surrogate proteins, as highly-purified plant protein is required. While use of crude plant extracts facilitated functional equivalence testing with the surrogate proteins, a separate technical challenge had to be met. The eCry3.1Ab and mCry3A proteins display differentiated modes of action toward CRW pests, however, with the same overall target pest spectrum, no differential test organism existed to allow equivalence testing for one insecticidal protein in the presence of the other. To establish that the microbially produced proteins are suitable surrogates for the plant-produced proteins, the challenges in the protein purification and bioactivity testing had to be addressed. This article describes technical solutions to assess and characterize the insecticidal proteins in this new event and thereby confirm equivalence/suitability of the microbially produced protein surrogates.

Effective bacterial insecticidal proteins against coleopteran pests: A review

Coleoptera, the order of insects commonly referred to as beetles, are able to survive in various environments, and thus, comprise the largest order in the animal kingdom. Coleopterans mainly include coprophagous and phytophagous lineages, and many species of the latter lineage are serious pests. In addition to traditional chemical methods, biocontrol measures using various bacterial insecticidal proteins have also gradually been developed to control these insect pests. In this review, we summarized the possible coleopteran-pest-specific bacteria and insecticidal proteins that have been reported in the literature thus far and have provided a comprehensive overview and long-term guidance for the control of coleopteran pests in the future.

Unshared binding sites for Bacillus thuringiensis Cry3Aa and Cry3Ca proteins in the weevil Cylas puncticollis (Brentidae)

Bacillus thuringiensis Cry3Aa and Cry3Ca proteins have been reported to be toxic against the African sweetpotato pest Cylas puncticollis. In the present work, the binding sites of these proteins in C. puncticollis brush border vesicles suggest the occurrence of different binding sites, but only one of them is shared. Our results suggest that pest resistance mediated by alteration of the shared Cry-receptor binding site might not render both Cry proteins ineffective.

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N-terminal sequence of Cry3C activated by trypsin or chymotrypsin was identified at 159 and 153 positions, respectively.

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Cry3Aa and Cry3Ca proteins bound specifically to C. puncticollis BBMV.

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Cry3Aa and Cry3Ca proteins do not completely compete for the same binding sites.

Bt toxin modification for enhanced efficacy

Insect-specific toxins derived from Bacillus thuringiensis (Bt) provide a valuable resource for pest suppression. Here we review the different strategies that have been employed to enhance toxicity against specific target species including those that have evolved resistance to Bt, or to modify the host range of Bt crystal (Cry) and cytolytic (Cyt) toxins. These strategies include toxin truncation, modification of protease cleavage sites, domain swapping, site-directed mutagenesis, peptide addition, and phage display screens for mutated toxins with enhanced activity. Toxin optimization provides a useful approach to extend the utility of these proteins for suppression of pests that exhibit low susceptibility to native Bt toxins, and to overcome field resistance.

Protein crystal structure obtained at 2.9 Å resolution from injecting bacterial cells into an X-ray free-electron laser beam

It has long been known that toxins produced by Bacillus thuringiensis (Bt) are stored in the bacterial cells in crystalline form. Here we describe the structure determination of the Cry3A toxin found naturally crystallized within Bt cells. When whole Bt cells were streamed into an X-ray free-electron laser beam we found that scattering from other cell components did not obscure diffraction from the crystals. The resolution limits of the best diffraction images collected from cells were the same as from isolated crystals. The integrity of the cells at the moment of diffraction is unclear; however, given the short time (∼ 5 µs) between exiting the injector to intersecting with the X-ray beam, our result is a 2.9-Å-resolution structure of a crystalline protein as it exists in a living cell. The study suggests that authentic in vivo diffraction studies can produce atomic-level structural information.

Bacillus thuringiensis Cry34Ab1/Cry35Ab1 interactions with western corn rootworm midgut membrane binding sites

Background

Bacillus thuringiensis (Bt) Cry34Ab1/Cry35Ab1 are binary insecticidal proteins that are co-expressed in transgenic corn hybrids for control of western corn rootworm, Diabrotica virgifera virgifera LeConte. Bt crystal (Cry) proteins with limited potential for field-relevant cross-resistance are used in combination, along with non-transgenic corn refuges, as a strategy to delay development of resistant rootworm populations. Differences in insect midgut membrane binding site interactions are one line of evidence that Bt protein mechanisms of action differ and that the probability of receptor-mediated cross-resistance is low.

Methodology/Principal Findings

Binding site interactions were investigated between Cry34Ab1/Cry35Ab1 and coleopteran active insecticidal proteins Cry3Aa, Cry6Aa, and Cry8Ba on western corn rootworm midgut brush border membrane vesicles (BBMV). Competitive binding of radio-labeled proteins to western corn rootworm BBMV was used as a measure of shared binding sites. Our work shows that ¹²⁵I-Cry35Ab1 binds to rootworm BBMV, Cry34Ab1 enhances ¹²⁵I-Cry35Ab1 specific binding, and that ¹²⁵I-Cry35Ab1 with or without unlabeled Cry34Ab1 does not share binding sites with Cry3Aa, Cry6Aa, or Cry8Ba. Two primary lines of evidence presented here support the lack of shared binding sites between Cry34Ab1/Cry35Ab1 and the aforementioned proteins: 1) No competitive binding to rootworm BBMV was observed for competitor proteins when used in excess with ¹²⁵I-Cry35Ab1 alone or combined with unlabeled Cry34Ab1, and 2) No competitive binding to rootworm BBMV was observed for unlabeled Cry34Ab1 and Cry35Ab1, or a combination of the two, when used in excess with ¹²⁵I-Cry3Aa, or ¹²⁵I-Cry8Ba.

Conclusions/Significance

Combining two or more insecticidal proteins active against the same target pest is one tactic to delay the onset of resistance to either protein. We conclude that Cry34Ab1/Cry35Ab1 are compatible with Cry3Aa, Cry6Aa, or Cry8Ba for deployment as insect resistance management pyramids for in-plant control of western corn rootworm.

Transgenic approaches to western corn rootworm control

The western corn rootworm, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae) is a significant corn pest throughout the United States corn belt. Rootworm larvae feed on corn roots causing yield losses and control expenditures that are estimated to exceed US$1 billion annually. Traditional management practices to control rootworms such as chemical insecticides or crop rotation have suffered reduced effectiveness due to the development of physiological and behavioral resistance. Transgenic maize expressing insecticidal proteins are very successful in protecting against rootworm damage and preserving corn yield potential. However, the high rate of grower adoption and early reliance on hybrids expressing a single mode of action and low-dose traits threatens the durability of commercialized transgenic rootworm technology for rootworm control. A summary of current transgenic approaches for rootworm control and the corresponding insect resistance management practices is included. An overview of potential new modes of action based on insecticidal proteins, and especially RNAi targeting mRNA coding for essential insect proteins is provided.

Transcriptome profiling of the intoxication response of Tenebrio molitor larvae to Bacillus thuringiensis Cry3Aa protoxin

Bacillus thuringiensis (Bt) crystal (Cry) proteins are effective against a select number of insect pests, but improvements are needed to increase efficacy and decrease time to mortality for coleopteran pests. To gain insight into the Bt intoxication process in Coleoptera, we performed RNA-Seq on cDNA generated from the guts of Tenebrio molitor larvae that consumed either a control diet or a diet containing Cry3Aa protoxin. Approximately 134,090 and 124,287 sequence reads from the control and Cry3Aa-treated groups were assembled into 1,318 and 1,140 contigs, respectively. Enrichment analyses indicated that functions associated with mitochondrial respiration, signalling, maintenance of cell structure, membrane integrity, protein recycling/synthesis, and glycosyl hydrolases were significantly increased in Cry3Aa-treated larvae, whereas functions associated with many metabolic processes were reduced, especially glycolysis, tricarboxylic acid cycle, and fatty acid synthesis. Microarray analysis was used to evaluate temporal changes in gene expression after 6, 12 or 24 h of Cry3Aa exposure. Overall, microarray analysis indicated that transcripts related to allergens, chitin-binding proteins, glycosyl hydrolases, and tubulins were induced, and those related to immunity and metabolism were repressed in Cry3Aa-intoxicated larvae. The 24 h microarray data validated most of the RNA-Seq data. Of the three intoxication intervals, larvae demonstrated more differential expression of transcripts after 12 h exposure to Cry3Aa. Gene expression examined by three different methods in control vs. Cry3Aa-treated larvae at the 24 h time point indicated that transcripts encoding proteins with chitin-binding domain 3 were the most differentially expressed in Cry3Aa-intoxicated larvae. Overall, the data suggest that T. molitor larvae mount a complex response to Cry3Aa during the initial 24 h of intoxication. Data from this study represent the largest genetic sequence dataset for T. molitor to date. Furthermore, the methods in this study are useful for comparative analyses in organisms lacking a sequenced genome.

Efficacy of Bacillus thuringiensis Cry3Aa protoxin and protease inhibitors against coleopteran storage pests

BACKGROUND

Environmental impacts and resistance to insecticides pose serious challenges to stored-product insect and other types of pest control. Insect-resistant transgenic grain is a potential alternative to fumigants, but candidate control proteins are needed, especially for coleopterans. Therefore, we evaluated the efficacy of a coleopteran-active toxin, Bacillus thuringiensis Cry3Aa, with or without protease inhibitors, in laboratory feeding assays against coleopteran storage pests.

RESULTS

In a comparison of the toxicity of Cry3Aa protoxin towards three species of coleopteran storage pests, Tenebrio molitor L. was found to be most sensitive, Tribolium castaneum (Herbst.) was most refractory and Rhyzopertha dominica F. displayed an intermediate response. For R. dominica, Cry3Aa combined with 3500 mg potato carboxypeptidase inhibitor or 5000 mg aprotinin kg(-1) diet resulted in both delayed development and increased mortality. Potato carboxypeptidase inhibitor and bovine aprotinin reduced the LC(50) of Cry3Aa for R. dominica two- and threefold respectively. Cry3Aa treatment resulted in fewer progeny from R. dominica, and progeny was further reduced when the protoxin was combined with potato carboxypeptidase inhibitor.

CONCLUSIONS

These data support the hypothesis that a combination of Cry3Aa protoxin and protease inhibitors, particularly a potato carboxypeptidase inhibitor, may have applications in control strategies for preventing damage to stored products and grains by coleopteran pests.

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.

A novel Tenebrio molitor cadherin is a functional receptor for Bacillus thuringiensis Cry3Aa toxin

Cry toxins produced by the bacterium Bacillus thuringiensis are effective biological insecticides. Cadherin-like proteins have been reported as functional Cry1A toxin receptors in Lepidoptera. Here we present data that demonstrate that a coleopteran cadherin is a functional Cry3Aa toxin receptor. The Cry3Aa receptor cadherin was cloned from Tenebrio molitor larval midgut mRNA, and the predicted protein, TmCad1, has domain structure and a putative toxin binding region similar to those in lepidopteran cadherin B. thuringiensis receptors. A peptide containing the putative toxin binding region from TmCad1 bound specifically to Cry3Aa and promoted the formation of Cry3Aa toxin oligomers, proposed to be mediators of toxicity in lepidopterans. Injection of TmCad1-specific double-stranded RNA into T. molitor larvae resulted in knockdown of the TmCad1 transcript and conferred resistance to Cry3Aa toxicity. These data demonstrate the functional role of TmCad1 as a Cry3Aa receptor in T. molitor and reveal similarities between the mode of action of Cry toxins in Lepidoptera and Coleoptera.

Investigating the properties of Bacillus thuringiensis Cry proteins with novel loop replacements created using combinatorial molecular biology

Cry proteins are a large family of crystalline toxins produced by Bacillus thuringiensis. Individually, the family members are highly specific, but collectively, they target a diverse range of insects and nematodes. Domain II of the toxins is important for target specificity, and three loops at its apex have been studied extensively. There is considerable interest in determining whether modifications in this region may lead to toxins with novel specificity or potency. In this work, we studied the effect of loop substitution on toxin stability and specificity. For this purpose, sequences derived from antibody complementarity-determining regions (CDR) were used to replace native domain II apical loops to create "Crybodies." Each apical loop was substituted either individually or in combination with a library of third heavy-chain CDR (CDR-H3) sequences to create seven distinct Crybody types. An analysis of variants from each library indicated that the Cry1Aa framework can tolerate considerable sequence diversity at all loop positions but that some sequence combinations negatively affect structural stability and protease sensitivity. CDR-H3 substitution showed that loop position was an important determinant of insect toxicity: loop 2 was essential for activity, whereas the effects of substitutions at loop 1 and loop 3 were sequence dependent. Unexpectedly, differences in toxicity did not correlate with binding to cadherins--a major class of toxin receptors--since all Crybodies retained binding specificity. Collectively, these results serve to better define the role of the domain II apical loops as determinants of specificity and establish guidelines for their modification.

An engineered chymotrypsin/cathepsin G site in domain I renders Bacillus thuringiensis Cry3A active against Western corn rootworm larvae

The western corn rootworm remains one of the most important pests of corn in the United States despite the use of many pest management tools. Cry3A, the first coleopteran-active Bacillus thuringiensis toxin isolated, has not been useful for control of the corn rootworm pest complex. Modification of Cry3A so that it contained a chymotrypsin/cathepsin G protease recognition site in the loop between alpha-helix 3 and alpha-helix 4 of domain I, however, resulted in consistent activity of the toxin ("mCry3A") against neonate western corn rootworm. In vitro chymotrypsin digests showed that there was a substantial difference between the enzyme sensitivity of mCry3A and the enzyme sensitivity of Cry3A, with mCry3A rapidly converted from a 67-kDa form to a approximately 55-kDa form. The introduced protease site was also recognized in vivo, where the approximately 55-kDa form of mCry3A toxin was rapidly generated and associated with the membrane fraction. After a point mutation in mcry3A that resulted in the elimination of the native domain I chymotrypsin site (C terminal to the introduced chymotrypsin/cathepsin G protease site of mCry3A), the in vitro and in vivo digestion patterns remained the same, demonstrating that the introduced site was required for the enhanced activity. Also, 55-kDa mCry3A generated by cleavage with chymotrypsin exhibited specific binding to western corn rootworm brush border membrane, whereas untreated 67-kDa mCry3A did not. These data indicate that the mCry3A toxicity for corn rootworm larvae was due to the introduction of a chymotrypsin/cathepsin G site, which enhanced cleavage and subsequent binding of the activated toxin to midgut cells.

A membrane associated metalloprotease cleaves Cry3Aa Bacillus thuringiensis toxin reducing pore formation in Colorado potato beetle brush border membrane vesicles

Insect proteases are implicated in Bacillus thuringiensis insecticidal proteins mode of action determining toxin specificity and sensitivity. Few data are available on the involvement of proteases in the later steps of toxicity such as protease interaction with toxin-receptor complexes and the pore formation process. In this study, a Colorado potato beetle (CPB) midgut membrane metalloprotease was found to be involved in the proteolytic processing of Cry3Aa. Interaction of Cry3Aa with BBMV membrane proteases resulted in a distinct pattern of proteolysis. Cleavage was demonstrated to occur in protease accessible regions of domain III and was specifically inhibited by the metalloprotease inhibitors 1,10-phenanthroline and acetohydroxamic acid. Proteolytic inhibition by a peptide representing a segment of proteolysis in domain III and the metalloprotease inhibitor acetohydroxamic acid correlated with increased pore formation, evidencing that Cry3Aa is a specific target of a CPB membrane metalloprotease that degrades potentially active toxin.

The mutation R423S in the Bacillus thuringiensis hybrid toxin CryAAC slightly increases toxicity for Mamestra brassicae L

Bacillus thuringiensis Cry1Ac toxin is 100 times less toxic than Cry1C to Mamestra brassicae. An R(423)S mutation abolishes Cry1Ac toxin proteolysis in M. brassicae gut juice but does not increase its toxicity to this insect. The CryAAC hybrid toxin (1Ac/1Ac/1Ca) is toxic to M. brassicae but is susceptible to gut protease digestion at the R(423) residue. Accordingly we have investigated the effect of the R(423)S mutation in CryAAC on its toxicity for M. brassicae and Pieris brassicae. Bioassays demonstrated that the R(423)S mutation slightly increased the toxicity of CryAAC for M. brassicae by having a significantly inhibitory effect on the growth of surviving larvae. The mutant hybrid was still highly toxic to P. brassicae. Features of CryAACR(423)S such as, (1) stability in M. brassicae gut juice and (2) crystal solubility were investigated. Computer simulations suggest that a possible major increase in flexibility in the CryAAC loop beta7/beta8 (G(391)-P(397)) caused by the R(423)S substitution could be a reason for the increase in M. brassicae toxicity.

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.

Crystal structure of the mosquito-larvicidal toxin Cry4Ba and its biological implications

Cry4Ba, isolated from Bacillus thuringiensis subsp. israelensis, is specifically toxic to the larvae of Aedes and Anopheles mosquitoes. The structure of activated Cry4Ba toxin has been determined by multiple isomorphous replacement with anomalous scattering and refined to R(cryst) = 20.5% and R(free)= 21.8% at 1.75 Angstroms resolution. It resembles previously reported Cry toxin structures but shows the following distinctions. In domain I the helix bundle contains only the long and amphipathic helices alpha3-alpha7. The N-terminal helices alpha1-alpha2b, absent due to proteolysis during crystallisation, appear inessential to toxicity. In domain II the beta-sheet prism presents short apical loops without the beta-ribbon extension of inner strands, thus placing the receptor combining sites close to the sheets. In domain III the beta-sandwich contains a helical extension from the C-terminal strand beta23, which interacts with a beta-hairpin excursion from the edge of the outer sheet. The structure provides a rational explanation of recent mutagenesis and biophysical data on this toxin. Furthermore, added to earlier structures from the Cry toxin family, Cry4Ba completes a minimal structural database covering the Coleoptera, Lepidoptera, Diptera and Lepidoptera/Diptera specificity classes. A multiple structure alignment found that the Diptera-specific Cry4Ba is structurally more closely similar to the Lepidoptera-specific Cry1Aa than the Coleoptera-specific Cry3Aa, but most distantly related to Lepidoptera/Diptera-specific Cry2Aa. The structures are most divergent in domain II, supporting the suggestion that this domain has a major role in specificity determination. They are most similar in the alpha3-alpha7 major fragment of domain I, which contains the alpha4-alpha5 hairpin crucial to pore formation. The collective knowledge of Cry toxin structure and mutagenesis data will lead to a more critical understanding of the structural basis for receptor binding and pore formation, as well as allowing the scope of diversity to be better appreciated.

Effect of specific mutations in helix alpha7 of domain I on the stability and crystallization of Cry3A in Bacillus thuringiensis

Insecticidal crystal (Cry) proteins of Bacillus thuringiensis crystallize after synthesis forming large inclusions that stabilize these toxins in the environment after cell lysis until eaten by an insect. Despite the biological importance of crystallization, little is known about the structural elements of Cry molecules that facilitate this process. We identified subdomains that affect Cry3A structure possibly through improper folding by chimeric-scanning mutagenesis, substituting short peptides of a truncated 70-kDa Cry1C molecule that does not crystallize into Cry3A, a wild-type 70-kDa molecule that crystallizes readily. Cry3A consists of three domains that contain five different blocks of conserved amino acids. Domain substitution and mutagenesis within these blocks suggested that the specific structure of block 2, which spans the junction between domains I and II, was important to the relative stability of Cry3A and subsequent crystallization. Amino acid sequences of particular importance to stability in Cry3A block 2 were identified using three substitution mutants, each spanning about a third of this block. One that consisted of Cry1C helix alpha7 yielded no detectable protein, whereas the other two produced characteristic Cry3A crystals. Specific mutations in this region showed tyrosine 268 was critical to normal stability of Cry3A and subsequent crystallization in that a mutant, Y268L, was less stable than wild-type Cry3A and failed to form a characteristic Cry3A crystal. Circular dichroism analysis showed a decrease in this mutant's alpha-helicity, indicating the importance of tyrosine 268 to the specific conformation of helix alpha7 that facilitates stability and normal crystallization.

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.

Structure of Cry3A delta-endotoxin within phospholipid membranes

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

Deletion of aprA and nprA genes for alkaline protease A and neutral protease A from bacillus thuringiensis: effect on insecticidal crystal proteins

The aprA gene encoding alkaline protease A (AprA) was cloned from Bacillus thuringiensis subsp. kurstaki, and the cloned gene was used to construct aprA-deleted (aprA1) strains of B. thuringiensis. An aprA1 strain of B. thuringiensis that contained the wild-type gene for neutral protease A (nprA(+)) displayed levels of extracellular proteolytic activity that were similar to those of an aprA(+)nprA(+) strain. However, when EDTA was included in the protease assay to inhibit NprA activity the aprA1nprA(+) strain displayed only 2% of the extracellular proteolytic activity of the aprA(+)nprA(+) strain. A strain that was deleted for both aprA and nprA (aprA1nprA3 strain) failed to produce detectable levels of proteolytic activity either in the presence or absence of EDTA in the assay. Compared with the aprA(+)nprA(+) strain the aprA1nprA(+) strain yielded 10% more full-length Cry1Bb crystal protein and the aprA1nprA3 strain yielded 25% more full-length Cry1Bb protein. No significant differences were seen in the 50% lethal dose of Cry1Bb protein from aprA(+)nprA(+) and aprA1nprA3 strains against three species of lepidopteran insects. These results suggest that enhanced yield of certain crystal proteins can be obtained by deletion of the genes aprA and nprA which are the major extracellular proteases of B. thuringiensis.

Transcription of the insecticidal crystal protein genes of Bacillus thuringiensis

Production of a large amount of insecticidal crystal proteins encoded on large plasmids is largely dependent upon the mother cell, Bacillus thuringiensis (B. thuringiensis, also Bt), specific transcription systems attributable to sporulation. In the middle stages of sporulation, cry4A is most actively transcribed from the promoter cry4A-P1. The proximal transcriptional start point of cry4A, which is under the control of the promoter P1, is used in Bacillus subtilis (B. subtilis) in the middle stage of sporulation. The nucleotide sequence that determines the cry4A-P1 promoter is homologous to the consensus sequence for the promoter of sigma E-specific genes in B. subtilis, and to those promoters of the insecticidal protein genes that are efficiently transcribed in vitro with the RNA polymerase E sigma 35 isolated from B. thuringiensis. The sigma factor sigma 35 of B. thuringiensis is highly homologous and functionally equivalent to sigma E of B. subtilis. These results suggest that the cry4A transcription from P1 is under the control of sigma E in B. subtilis, and under the control of sigma 35 in B. thuringiensis.

Domain I plays an important role in the crystallization of Cry3A in Bacillus thuringiensis

The insecticidal bacterium Bacillus thuringiensis synthesizes endotoxin Cry proteins of two size classes, 135 and 70 kDa, and both form crystalline inclusions in cells after synthesis. Crystallization of 135-kDa proteins is due to intermolecular attraction of regions in the C-terminal half of the molecule, and the N-terminal half fails to crystallize when synthesized in vivo. Alternatively, endotoxins of the 70-kDa class such as Cry2A and Cry3A, which correspond to the N-terminal half of 135-kDa molecules, crystallize readily after synthesis. Cry molecules of this size class consist of three principal domains, but the domains responsible for crystallization are not known. To identify these domains, chimeric proteins were constructed in which Cry3A Domains I or III, or I and III were substituted for the corresponding domains in truncated Cry1C molecules. Cry1C molecules with only Cry3A Domain III did not crystallize, whereas when Cry3A Domains I and III, or Domain I alone, were substituted, large inclusions were obtained. Except for the chimera consisting of Cry3A Domains I and III and Cry1C Domain II, most chimeras were not as stable as wild-type Cry3A or truncated Cry1C. These results show that Cry3A Domain I plays an important role in its crystallization in vivo.

Proteolytic processing of the Cyt1Ab1 toxin produced by Bacillus thuringiensis subsp. medellin

Bacillus thuringiensis produces d-endotoxins that require proteolytic processing to become active. The activation of the B. thuringiensis subsp. medellin 28 kDa (Cyt1Ab1) cytolytic toxin by trypsin, chymotrypsin and gut extract from Culex quinquefasciatus larvae was analyzed. The Cyt1Ab1 toxin of B. thuringiensis subsp. medellin was processed by all proteases tested to fragments between 23 and 25 kDa, while processing of the Cyt1Aa1 toxin produce fragments between 22.5 and 24.5 kDa. The Cyt1Ab1 toxin was preferentially processed at the alkaline pH of 12. The in vitro proteolytic processing of the Cyt1Ab1 toxin by C. quinquefasciatus larvae midgut extract showed a 25 kDa fragment; a similar result was observed when the activation was performed in the in vivo experiments. The solubilized Cyt1Ab1 toxin and the protease resistant cores generated by in vitro processing showed hemolytic activity but not mosquitocidal activity. Amino terminal sequence of the C. quinquefasciatus gut extract resistant fragment indicated that the cutting site was located between Lys31 and Asp32, with a sequence DDPNEKNNHNS; while for the trypsin-resistant fragment the cutting site was determined between Leu29 and Arg30, and for the chymotrypsin-resistant fragment between Arg30 and Lys31.

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.

Identification and purification of the 69-kDa intracellular protease involved in the proteolytic processing of the crystal delta-endotoxin of Bacillus thuringiensis subsp. tenebrionis

The dynamics of appearance of intracellular proteases in relation to the synthesis of crystal delta-endotoxin was studied to identify the native intracellular protease(s) involved in the proteolytic processing of the 73-kDa protoxin of Bacillus thuringiensis subsp. tenebrionis. In vitro proteolytic activation of the 73-kDa protoxin indicated the possible role of 69-kDa protease in the proteolytic processing of 73-kDa protoxin. The purified 69-kDa protease was able to cause the proteolytic activation of the 73-kDa protoxin to 68-kDa toxin and this conversion was inhibited by ethylenediamine tetraacetic acid and 1,10-phenanthroline.

Bacillus thuringiensis crystal delta-endotoxin: role of proteases in the conversion of protoxin to toxin

The conversion of delta-endoprotoxins of Bacillus thuringiensis to active toxins is mediated by trypsin, insect gut (exogenous) and bacterial (endogenous) proteases. The biochemical aspects of exogenous and endogenous proteases involved in the conversion of protoxin to toxin are reviewed. Perhaps, these proteases also play a role in influencing the host range of toxin and in the development of resistance to toxin.

Activation process of dipteran-specific insecticidal protein produced by Bacillus thuringiensis subsp. israelensis

Dipteran-specific insecticidal protein Cry4A is produced as a protoxin of 130 kDa in Bacillus thuringiensis subsp. israelensis. Here we performed the in vitro processing of Cry4A and showed that the 130-kDa protoxin of Cry4A was processed into the two protease-resistant fragments of 20 and 45 kDa through the intramolecular cleavage of a 60-kDa intermediate. The processing into these two fragments was also observed in vivo. To investigate functional properties of the two fragments, GST (glutathione S-transferase) fusion proteins of the 60-kDa intermediate and the 20- and 45-kDa fragments were constructed. Neither the GST-20-kDa fusion protein (GST-20) nor the GST-45-kDa fusion protein (GST-45) was actively toxic against mosquito larvae of Culex pipiens, whereas the GST-60-kDa intermediate fusion protein (GST-60) exhibited significant toxicity. However, when the two fusion proteins GST-20 and GST-45 coexisted, significant toxicity was observed. The coprecipitation experiment demonstrated that the two fragments associated with each other. Therefore, it is strongly suggested that the two fragments formed an active complex of apparently 60 kDa. A mutant of the 60-kDa protein which was apparently resistant to the intramolecular cleavage with the midgut extract of C. pipiens larvae had toxicity slightly lower than that of GST-60.

Transition state of the rate-limiting step of heat denaturation of Cry3A delta-endotoxin

Heat denaturation of Cry3A delta-endotoxin from Bacillus thuringiensis var. tenebrionis and its 55 kDa fragment was studied by differential scanning microcalorimetry at low pH. Analysis of the calorimetric data has shown that denaturation of Cry3A delta-endotoxin is a nonequilibrium process at heating rates from 0. 125 to 2 K/min. This means that the stability of delta-endotoxin (the apparent temperature of denaturation Tm) under these conditions is under kinetic control rather than under thermodynamic control. It has been shown that heat denaturation of this protein is a one-step kinetic process. The enthalpy of the process and its activation energy were measured as functions of temperature. The data obtained allow confirmation of the fact that the conformation of delta-endotoxin at the transition state only slightly differs from its native conformation with respect to compactness and extent of hydration. The comparison of the activation energy for intact delta-endotoxin and the 55 kDa fragment showed that the transition of the molecule to a transition state does not cause any changes in the conformation of three N-terminal alpha-helices. Complete removal of the N-terminal domain of delta-endotoxin and 40 amino acids from the C-terminus beta-sheet domain III causes an irreversible loss of the tertiary structure. Thus, during protein folding the nucleation core determining protein stability does not involve its three initial alpha-helices but may include the remaining alpha-helices of the N-terminal domain. The functional significance of peculiarities of structure arrangement of the delta-endotoxin molecule is discussed.

Biochemical and molecular characterization of the insecticidal fragment of CryV

Two C-terminal deletion constructs were made to study the effect of such deletions on the biological activity of the CryV protein of Bacillus thuringiensis subsp. kurstaki. The results of feeding on neonatal larvae of Ostrinia nubilalis (European corn borer [ECB]) indicated that the 50% lethal dose of the full-length CryV protein was 3.34 micrograms/g of diet (95% fiducial limits, 2.53 to 4.32 micrograms/g of diet). Removal of 71 amino acids (aa) from the C terminus had little effect on toxicity, whereas deletion of 184 aa abolished the insecticidal activity of the CryV protein completely. Truncations of the full-length CryV protein were also generated with trypsin and the midgut protease of ECB. The proteolytically treated products were characterized by determining their N-terminal amino acid sequences. The CryV protein was found to be cleaved by both proteases through a two-step process. Initially an intermediary form was generated which contained aa 45 of full-length CryV as its N-terminal end. The C-terminal end of this peptide was not experimentally determined. However, analysis of the deduced amino acid sequence of CryV indicated that the C-terminal end of the intermediary form is likely either aa 655 or 659. Further N-terminal processing of the intermediary form resulted in a protease-resistant core form. The core included aa 156 to aa 655 or 659. While the intermediary form retained 100% of the ECB larval toxicity, the core form exhibited only approximately 22% of the toxicity of the full-length protein.

Intramolecular proteolytic cleavage of Bacillus thuringiensis Cry3A delta-endotoxin may facilitate its coleopteran toxicity

The Cry3A delta-endotoxin protein inclusion synthesized by Bacillus thuringiensis subsp. tenebrionis is soluble in alkaline and acid buffer solutions but the toxin precipitates when returned to neutral pH conditions. The midgut pH of susceptible beetle larvae is neutral to slightly acidic, a pH environment in which the Cry3A toxin is insoluble. To investigate this paradox we studied the Cry3A toxin after various proteolytic treatments. In many cases the toxin was cleaved into polypeptides that remained associated under non-denaturing conditions. Interestingly a chymotrypsinized Cry3A product was soluble under neutral pH conditions, retained full activity against susceptible beetle larvae, and exhibited specific binding to Leptinotarsa decemlineata midgut membranes.

Cloning of the nprA gene for neutral protease A of Bacillus thuringiensis and effect of in vivo deletion of nprA on insecticidal crystal protein

The nprA gene, encoding Bacillus thuringiensis neutral protease A, was cloned by the use of gene-specific oligonucleotides. The size of neutral protease A deduced from the nprA sequence was 566 amino acids (60,982 Da). The cloned nprA gene was partially deleted in vitro, and the deleted allele, designated nprA3, was used to construct an nprA3 strain (neutral protease A-deficient strain) of B. thuringiensis. Growth and sporulation of the nprA3 strain were similar to those of an isogenic nprA+ strain, although the extracellular proteolytic activity of the nprA3 strain was significantly less than that of the nprA+ strain. The nprA3 strain produced insecticidal crystal proteins that were more stable than those of the isogenic nprA+ strain after solubilization in vitro, and sporulated cultures of the nprA3 strain contained higher concentrations of full-length insecticidal crystal proteins than did those of its isogenic counterpart. The absence of neutral protease A did not affect the insecticidal activity of a lepidopteran-specific crystal protein of B. thuringiensis. These results indicate that crystal protein stability and yield may be improved by deletion of specific proteases from B. thuringiensis.

Mapping and characterization of the entomocidal domain of the Bacillus thuringiensis CryIA(b) protoxin

The amino acid sequences necessary for entomocidal activity of the CryIA(b) protoxin of Bacillus thuringiensis were determined. Introduction of stop codons behind codons Arg601, Phe604 or Ala607 showed that amino acid residues C-terminal to Ala607 are not required for insecticidal activity and that activation by midgut proteases takes place distal to Ala607. The two shortest polypeptides, deleted for part of the highly conserved beta-strand, were prone to proteolytic degradation, explaining their lack of toxicity. Apparently, this beta-strand is essential for folding of the molecule into a stable conformation. Proteolytic activation at the N-terminus was investigated by removing the first 28 codons, resulting in a translation product extending from amino acid 29 to 607. This protein appeared to be toxic not only to susceptible insect larvae such as Manduca sexta and Heliothis virescens, but also to Escherichia coli cells. An additional mutant, encoding only amino acid residues 29-429, encompassing the complete putative pore forming domain, but lacking a large part of the receptor-binding domain, was similarly toxic to E. coli cells. This suggests a role for the N-terminal 28 amino acids in rendering the toxin inactive in Bacillus thuringiensis, and indicates that the cytolytic potential of the pore forming domain is only realized after proteolytic removal of these residues by proteases in the insect gut. In line with this hypothesis are results obtained with a mutant protein in which Arg28 at the cleavage site was replaced by Asp. This substitution prevented the protein from being cleaved by trypsin in vitro, and reduced its toxicity to M. sexta larvae.

Functional analysis of block 5, one of the highly conserved amino acid sequences in the 130-kDa CryIVA protein produced by Bacillus thuringiensis subsp. israelensis

There are five amino acid sequences highly conserved among Bacillus thuringiensis delta-endotoxins. We have changed the amino acid residues in block 5, one of the conserved sequences, of CryIVA. When the amino acid residues with charged side chains were replaced by others, the amount of production of the altered CryIVA protein was markedly decreased. It is suggested that the decrease is caused by the unstable conformation of the altered CryIVA protein molecule, as judged by digestion with trypsin and thermolysin. On the other hand, the substitution of amino acid residues in block 5 did not affect the insecticidal activity of CryIVA. These results strongly suggest that block 5 of CryIVA is one of the stability-determining elements of the protoxin molecule.

Rendering a membrane protein soluble in water: a common packing motif in bacterial protein toxins

The recently determined structures of three different protein toxins by X-ray crystallography has unexpectedly revealed a common membrane-insertion domain. This domain consists of an alpha-helical bundle of between seven and ten helices, some of which are hydrophobic. The three toxins, colicin, insecticidal delta-endotoxin and diphtheria toxin are directed towards different hosts, have different killing mechanisms and bear no sequence homology. The observation of a common membrane-insertion domain has implications for the design of therapeutic agents in combating disease.

Effects on toxicity of eliminating a cleavage site in a predicted interhelical loop in Bacillus thuringiensis CryIVB delta-endotoxin

When activated by treatment with mosquito (Aedes aegypti) gut extract, the Bacillus thuringiensis CryIVB delta-endotoxin lysed A. aegypti cells in vitro. SDS-PAGE and N-terminal sequence determination showed that in addition to removal of the C-terminal half of the molecule, the activated toxin had undergone proteolytic cleavage at two internal regions producing 47-48-kDa and 16-18-kDa polypeptides. Aligning the CryIVB protein sequence with the crystallographic structure of the CryIIIA toxin suggested that one set of cleavages occurred in a region before the start of the N-terminal helical bundle and the second cleavage site occurred in a predicted loop between helices 5 and 6 in the bundle at arginine-203. To investigate the suggestion by Li et al. that interhelical proteolysis is important in the cytolytic mechanism of these toxins, arginine-203 was substituted by alanine. The mutated toxin now resisted proteolysis at this position and showed a marked decrease in cytolysis in vitro but an increase in larvicidal activity.

In vitro and in vivo proteolysis of the Bacillus thuringiensis subsp. israelensis CryIVD protein by Culex quinquefasciatus larval midgut proteases

Proteases with trypsin-, chymotrypsin- and thermolysin-like specificity were detected in Culex quinquefasciatus larval midguts. Their activities were monitored by N-terminal amino acid sequence analysis of the Bacillus thuringiensis subsp. israelensis CryIVD toxin proteolytic fragments. These proteases are located in the larval midgut and in different fractions obtained during the preparation of brush border membrane vesicles. The activity of the midgut proteases increased with an increase in pH. Both the chymotrypsin- and thermolysin-like activities are involved in the processing of solubilized CryIVD toxin, whereas an additional trypsin-like protease is necessary for the CryIVD parasporal inclusion processing. The solubilized CryIVD toxin was first cleaved between Thr347 and Phe348 and between Phe348 and Tyr349, generating a 40-kDa N-terminal fragment and a 32.5-kDa C-terminal fragment. The C-terminal domain was resistant to further processing, with only a small amount of a 31-kDa product appearing due to the action of a thermolysin-like protease. However, the N-terminal domain was very unstable, and was further degraded to about 30 kDa. Unlike the solubilized CryIVD toxin, the processing of the CryIVD parasporal inclusion was very slow at neutral pH. Three protease-resistant products were detected at pHs higher than 9.5 with an overnight incubation at 37 degrees C. The 30- and 28.5-kDa C-terminal peptides are proteolytic products of trypsin- and chymotrypsin-like proteases, respectively; while the 28-kDa N-terminal peptide has 27 amino acids deleted from the N-terminal end by a thermolysin-like protease.

Processing of delta-endotoxin from Bacillus thuringiensis subsp. kurstaki HD-1 and HD-73 by gut juices of various insect larvae

Midgut juices were prepared from Adoxophyes sp., smaller tea tortrix (STT); Bombyx mori, silkworm (SW); Spodoptera litura, common cutworm (CCW); Plutella xylostella, diamondback moth (DBM); and Musca domestica, housefly (HF) and immobilized onto Sepharose 4B. delta-Endotoxins (ICPs) from Bacillus thuringiensis subsp. kurstaki HD-1 and HD-73 were digested by these immobilized gut juice proteases. All gut juices tested derived relatively proteolytic resistant cores from ICP. The molecular sizes of these cores, about 55 kDa in SDS-PAGE, were resulted. In the case of CCW, however, digestion was very strong and only 1/20 concentration of core protein remained relative to other digests. The N-terminal amino acid sequencing of the core proteins showed that they were truncated at the very end of the N-terminus of protoxin, CryIA, at different sites. Although housefly larvae were completely insensitive to active toxin, the gut juice produced the core, suggesting that the housefly may lack the binding sites for the core-active toxin.

Synthetic cryIIIA gene from Bacillus thuringiensis improved for high expression in plants

A 1974 bp synthetic gene was constructed from chemically synthesized oligonucleotides in order to improve transgenic protein expression of the cryIIIA gene from Bacillus thuringiensis var. tenebrionis in transgenic tobacco. The crystal toxin genes (cry) from B. thuringiensis are difficult to express in plants even when under the control of efficient plant regulatory sequences. We identified and eliminated five classes of sequence found throughout the cryIIIA gene that mimic eukaryotic processing signals and which may be responsible for the low levels of transcription and translation. Furthermore, the GC content of the gene was raised from 36% to 49% and the codon usage was changed to be more plant-like. When the synthetic gene was placed behind the cauliflower mosaic virus 35S promoter and the alfalfa mosaic virus translational enhancer, up to 0.6% of the total protein in transgenic tobacco plants was cryIIIA as measured from immunoblot analysis. Bioassay data using potato beetle larvae confirmed this estimate.