Scavenger Receptor C1 Mediates Toxicity of Binary Toxin from Lysinibacillus sphaericus to Ag55 Cells.

Lysinibacillus sphaericus harboring Binary (BinA and BinB) toxins is highly toxic against Anopheles and Culex mosquito larvae. The Anopheles Ag55 cell line is a suitable model for investigating the mode of Bin toxin action. Based on the low-levels of α-glycosidase Agm3 mRNA in Ag55 cells and the absence of detectable Agm3 proteins, we hypothesized that a scavenger receptor could be mediating Bin cytotoxicity. Preliminary RNA interference knockdown of the expressed scavenger receptors, combined with Bin cytotoxicity assays, was conducted. The scavenger Receptor C1 (SCRC1) became the focus of this study, as a putative receptor for Bin toxins in Ag55 cells, and SCRBQ2 was selected as a negative control. Open reading frames encoding SCRC1 and SCRBQ2 were cloned and expressed in vitro, and polyclonal antibodies were prepared for immunological analyses. The RNAi silencing of SCRC1 and SCRBQ2 resulted in the successful knockdown of both SCRC1 and SCRBQ2 transcripts and protein levels. The cytolytic toxicity of Bin against Ag55 cells was severely reduced after the SCRC1-RNAi treatment. The phagocytic receptor protein SCRC1 mediates endocytosis of the Bin toxin into Ag55 cells, thereby facilitating its internal cytological activity. The results support a mechanism of the Bin toxin entering Ag55 cells, possibly via SCRC1-mediated endocytosis, and encourage investigations into how Bin is transferred from its bound form on the midgut epithelial cells into the epithelial endocytic system.

A novel spider venom peptide from the predatory mite Neoseiulus barkeri shows lethal effect on phytophagous pests.

The long-term use of pesticides in the field, and the high fertility and adaptability of phytophagous mites have led to resistance problems; consequently, novel safe and efficient active substances are necessary to broaden the tools of pest mite control. Natural enemies of arthropods typically secrete substances with paralytic or lethal effects on their prey, and those substances are a resource for future biopesticides. In this study, two putative venom peptide genes were identified in a parasitic mite Neoseiulus barkeri transcriptome. Recombinant venom NbSP2 peptide injected into Tetranychus cinnabarinus mites was significantly more lethal than recombinant NBSP1. NbSP2 was also lethal to Spodoptera litura when injected but not when fed to third instar larvae. The interaction proteins of NbSP2 in T. cinnabarinus and S. litura were identified by affinity chromatography. Among these proteins, ATP synthase subunit ß (ATP SSß) was deduced as a potential target. Four binding sites were predicted between NBSP2 and ATP SSß of T. cinnabarinus and S. litura. In conclusion, we identified a venom peptide with activity against T. cinnabarinus and S. litura. This study provides a novel component for development of a new biological pesticide.

Anopheles gambiae strain (Ag55) cultured cells originated from Anopheles coluzzii and are phagocytic with hemocyte-like gene expression.

Anopheles gambiae and Anopheles coluzzii are closely related species that are predominant vectors of malaria in Africa. Recently, A. gambiae form M was renamed A. coluzzii and we now conclude on the basis of a diagnostic PCR-restriction fragment length polymorphism assay that Ag55 cells were derived from A. coluzzii. We established an Ag55 cell transcriptome, and KEGG pathway analysis showed that Ag55 cells are enriched in phagosome pathway transcripts. The Ag55 transcriptome has an abundance of specific transcripts characteristic of mosquito hemocytes. Functional E. coli bioparticle uptake experiments visualized by fluorescence microscopy and confocal microscopy and quantified by flow cytometry establish the phagocytic competence of Ag55 cells. Results from this investigation of Ag55 cell properties will guide researchers in the use and engineering of the Ag55 cell line to better enable investigations of Plasmodium, other microbes, and insecticidal toxins.

Transcription factors CncC and Maf connect the molecular network between pesticide resistance and resurgence of pest mites.

Pesticide resistance and resurgence are serious problems often occurring simultaneously in the field. In our long-term study of a fenpropathrin-resistant strain of Tetranychus cinnabaribus, enhancement of detoxification and modified fecundity mechanisms were both observed. Here we investigate the network across these two mechanisms and find a key node between resistance and resurgence. We show that the ecdysone pathway is involved in regulating the fecundity of T. cinnabaribus. The concentration change of ecdysone is consistent with the fecundity curve; the concentration of ecdysone is higher in the fenpropathrin-resistant strain which has stronger fecundity. The enhancement of ecdysone is due to overexpression of two P450 genes (CYP314A1 and CYP315A1) in the ecdysone synthesis pathway. Silencing expression of these CYP genes resulted in lower concentration of ecdysone, reduced expression of vitellogenin, and reduced fecundity of T. cinnabaribus. The expression of CYP315A1 is regulated by transcription factors Cap-n-collar isoform C (CncC) and Musculoaponeurotic fibrosarcoma protein (Maf), which are involved in regulating other P450 genes functioning in detoxification of fenpropathrin in T. cinnabaribus. A similar regulation is established in citrus pest mite Panonychus citri showing that the CncC pathway regulates expression of PcCYP315A1, which affects mite fecundity. Transcription factors are activated to upregulate detoxification genes facilitating pesticide resistance, while the "one to multiple" regulation mode of transcription factors simultaneously increases expression of metabolic enzyme genes in hormone pathways and alters the physiology of pests. This is an important response of arthropods to pesticides which leads to resistance and population resurgence.

Novel strategies for the biocontrol of noctuid pests (Lepidoptera) based on improving ascovirus infectivity using Bacillus thuringiensis.

Identifying novel biocontrol agents and developing new strategies are urgent goals in insect pest biocontrol. Ascoviruses are potential competent insect viruses that may be developed into bioinsecticides, but this aim is impeded by their poor oral infectivity. To improve the per os infectivity of ascovirus, Bacillus thuringiensis kurstaki (Btk) was employed as a helper to damage the midgut of lepidopteran larvae (Helicoverpa armigera, Mythimna separata, Spodoptera frugiperda, and S. litura) in formulations with Heliothis virescens ascovirus isolates (HvAV-3h and HvAV-3j). Btk and ascovirus mixtures (Btk/HvAV-3h and Btk/HvAV-3j) were fed to insect larvae (3rd instar). With the exception of S. frugiperda larvae, which exhibited low mortality after ingesting Btk, the larvae of the other tested species showed three types of response to feeding on the formulas: type I, the tested larvae (H. armigera) were killed by Btk infection so quickly that insufficient time and resources remained for ascoviral invasion; type II, both Btk and the ascovirus were depleted by their competition, such that neither was successfully released or colonized the tissue; type III, Btk was eliminated by the ascovirus, and the ascovirus achieved systemic infection in the tested larvae. The feeding of Btk/ascovirus formulas led to a great reduction in larval diet consumption and resulted in a significant decrease in the emergence rate of H. armigera, M. separata, and S. litura larvae, which suggested that the formulas exerted marked oral control effects on both the contemporary individuals and the next generation of these tested pest species.

Identification of Lysinibacillus sphaericus Binary toxin binding proteins in a malarial mosquito cell line by proteomics: A novel approach towards improving mosquito control.

Bacterial insecticidal proteins, such as the Bin toxin from Lysinibacillus sphaericus, could be used more extensively to control insecticide resistant mosquitoes. This study was aimed at identification of mosquito cell proteins binding Bin toxin. Results showed that purified toxin was toxic to Anopheles gambiae larvae and Ag55 cultured cells. Clathrin heavy chain (an endocytosis protein) and glycolytic enzymes such as pyruvate kinase, enolase and dihydrolipoamide dehydrogenase were identified as binders of Bin toxin. The viability of Ag55 cells in the presence of endocytosis inhibitor, pitstop2, was significantly decreased upon Bin treatment, while the inhibitor chlorpromazine did not affect Bin toxicity. Bin toxin treatment decreased ATP production and mitochondrial respiration in Ag55 cells, whereas non-mitochondrial oxygen consumption significantly increased after Bin toxin treatment. These findings are steps towards understanding how Bin toxin kills mosquitoes. SIGNIFICANCE: Mosquitoes are vectors of pathogens causing human diseases such as dengue fever, yellow fever, zika virus and malaria. An insecticidal toxin from Lysinibacillus sphaericus called Binary, or Bin, toxin could be used more extensively to control insecticide resistant mosquitoes. Bin toxin enter cells in susceptible mosquitoes and induces apoptosis or autophagy. In the current research, we used the malaria mosquito Anopheles gambiae Ag55 cell line as a model. A proteomic-based approach identified proteins that interact with Bin toxin. Interacting proteins include clathrin heavy chain (endocytosis protein) and glycolysis enzymes such as pyruvate kinase, enolase and dihydrolipoamide dehydrogenase. In Ag55 cell toxicity assays, an endocytosis inhibitor, pitstop2, increased Bin toxicity. Real time assays with a Seahorse™ flux analyzer showed that Bin significantly affects mitochondrial respiration, a result consistent with cell death via apoptosis or autophagy. These research findings add insights into how an unusual binary protein exploits cellular machinery to kill mosquitoes.

Fluorescent analyses of Bacillus thuringiensis Cry1Fa and Cry1Ab toxin binding sites on brush border membrane vesicles of Ostrinia nubilalis (Hübner), Diatraea grandiosella (Dyar), and Helicoverpa zea (Boddie) larvae.

Bacillus thuringiensis (Bt) Cry1Fa and Cry1Ab proteins are important Cry toxins due to their high, selective toxicity against a number of lepidopteran species, including important pests of corn and cotton. Competition binding assays are a classical tool for investigating Cry toxin interactions with target pest insects. We developed a fluorescence-based binding assay and assessed Cry1Fa and Cry1Ab toxin binding to brush border membrane preparations from lepidopteran corn pests including Ostrinia nubilalis (European corn borer, ECB), Diatraea grandiosella (south western corn borer, SWCB), and Helicoverpa zea (corn earworm, CEW). Homologous and heterologous competition binding assays with fluorophore-(Alexa488)-labeled Cry1Fa toxin showed that Cry1Fa shares binding site(s) with Cry1Ab toxin in ECB, and SWCB for which Cry1Ab has higher affinity than Cry1Fa. Apart from the shared binding sites, Cry1Ab and Cry1Fa bind an additional site(s) in ECB and SWCB. In CEW, Cry1Fa and Cry1Ab each, has a high affinity binding site(s), which binds the heterologous toxin with low affinity. The Cry1Ab-Cry1Fa toxin binding models for ECB, SWCB and CEW based on our results are considered in the context of what is known about acquired cross-resistance against Cry1Ab and Cry1Fa toxins.

Binding and Synergizing Motif within Coleopteran Cadherin Enhances Cry3Bb Toxicity on the Colorado Potato Beetle and the Lesser Mealworm.

Cry3Bb toxin from Bacillus thuringiensis is an important insecticidal protein due to its potency against coleopteran pests, especially rootworms. Cadherin, a protein in the insect midgut epithelium, is a receptor of Cry toxins; in some insect species toxin-binding domains of cadherins-synergized Cry toxicity. Previously, we reported that the DvCad1-CR8-10 fragment of Diabrotica virgifera virgifera cadherin-like protein (GenBank Accession #EF531715) enhanced Cry3Bb toxicity to the Colorado Potato Beetle (CPB), Leptinotarsadecimlineata (L. decimlineata). We report that individual CR domains of the DvCad1-CR8-10 fragment were found to have strong binding affinities to α-chymotrypsin-treated Cry3Bb. The dissociation constant (Kd) of Cry3Bb binding to the CR8, CR9, and CR10 domain was 4.9 nM, 28.2 nM, and 4.6 nM, respectively. CR8 and CR10, but not CR9, enhanced Cry3Bb toxicity against L. decimlineata and the lesser mealworm Alphitobius diaperinus neonates. In-frame deletions of the DvCad1-CR10 open reading frame defined a high-affinity binding and synergistic site to a motif in residues I1226-D1278. A 26 amino acid peptide from the high affinity Cry3Bb-binding region of CR10 functioned as a Cry3Bb synergist against coleopteran larvae.

High throughput sequencing reveals Drosophila suzukii responses to insecticides.

Global climate change and acquired resistance to insecticides are threats to world food security. Drosophila suzukii, a devastating invasive pest in many parts of the world, causes substantial economic losses to fruit production industries, forcing farmers to apply broad-spectrum insecticides frequently. This could lead to the development of insecticide resistance. We determined the Lethal Concentration 50 (median lethal concentration, LC50 ) values of zeta-cypermethrin, spinosad, and malathion insecticides against D. suzukii colonies established from Clarke and Pierce county Georgia, United States. The LC50 values were 3 fold higher in the Pierce county population for all insecticide treatments. We then used RNA sequencing to analyze the responses of Pierce and Clarke population flies surviving a LC50 treatment of the 3 insecticides. We identified a high number of differentially expressed genes that are likely involved in detoxification and reduced cuticular penetration, especially in the Pierce population, with extensive overlap in differentially expressed genes between the 3 insecticide treatments. Finally, we predicted fewer nonsynonymous single nucleotide variants having deleterious effects on protein function among detoxification, insecticide target, and cuticular protein encoding genes in Pierce flies. Thus a combination of increased gene expression and fewer deleterious single nucleotide variants highlights molecular mechanisms underlying the higher LC50 values for Pierce population flies.

Identification of Cry48Aa/Cry49Aa toxin ligands in the midgut of Culex quinquefasciatus larvae.

A binary mosquitocidal toxin composed of a three-domain Cry-like toxin (Cry48Aa) and a binary-like toxin (Cry49Aa) was identified in Lysinibacillus sphaericus. Cry48Aa/Cry49Aa has action on Culex quinquefasciatus larvae, in particular, to those that are resistant to the Bin Binary toxin, which is the major insecticidal factor from L. sphaericus-based biolarvicides, indicating that Cry48Aa/Cry49Aa interacts with distinct target sites in the midgut and can overcome Bin toxin resistance. This study aimed to identify Cry48Aa/Cry49Aa ligands in C. quinquefasciatus midgut through binding assays and mass spectrometry. Several proteins, mostly from 50 to 120 kDa, bound to the Cry48Aa/Cry49Aa toxin were revealed by toxin overlay and pull-down assays. These proteins were identified against the C. quinquefasciatus genome and after analysis a set of 49 proteins were selected which includes midgut bound proteins such as aminopeptidases, amylases, alkaline phosphatases in addition to molecules from other classes that can be potentially involved in this toxin's mode of action. Among these, some proteins are orthologs of Cry receptors previously identified in mosquito larvae, as candidate receptors for Cry48Aa/Cry49Aa toxin. Further investigation is needed to evaluate the specificity of their interactions and their possible role as receptors.

Effects and mechanisms of Bacillus thuringiensis crystal toxins for mosquito larvae.

Bacillus thuringiensis is a Gram-positive aerobic bacterium that produces insecticidal crystalline inclusions during sporulation phases of the mother cell. The virulence factor, known as parasporal crystals, is composed of Cry and Cyt toxins. Most Cry toxins display a common 3-domain topology. Cry toxins exert intoxication through toxin activation, receptor binding and pore formation in a suitable larval gut environment. The mosquitocidal toxins of Bt subsp. israelensis (Bti) were found to be highly active against mosquito larvae and are widely used for vector control. Bt subsp. jegathesan is another strain which possesses high potency against broad range of mosquito larvae. The present review summarizes characterized receptors for Cry toxins in mosquito larvae, and will also discuss the diversity and effects of 3-D mosquitocidal Cry toxin and the ongoing research for Cry toxin mechanisms generated from investigations of lepidopteran and dipteran larvae.

Anopheles gambiae Ag55 cell line as a model for Lysinibacillus sphaericus Bin toxin action.

Binary toxin (Bin) produced by Lysinibacillus sphaericus is toxic to Culex and Anopheles mosquito larvae. It has been used world-wide for control of mosquitoes that vector disease. The Bin toxin interacts with the glucosidase receptor, Cpm1, in Culex and its orthologue, Agm3, in Anopheles mosquitoes. However, the exact mechanism of its mode of action is not clearly understood. It is essential to understand mode of action of Bin toxin to circumvent the resistance that develops over generations of exposure. A suitable model cell line will facilitate investigations of the molecular action of Bin toxin. Here we report Bin toxin activity on Ag55 cell line that has been derived from an actual target, Anopheles gambiae larvae. The Bin toxin, both in pro and active forms, kills the Ag55 cells within 24h. Bin toxin internalizes in Ag55 cells and also induces vacuolation as tracked by Lysotracker dye. The dose response studies showed that 1.5nM of Bin toxin is sufficient to induce vacuolation and Ag55 cell death. Presence of α-glucosidase gene (Agm3) expression in the Ag55 cells was also confirmed. Thus, Ag55 cells constitute an appropriate model system to decipher the mode of Bin action in mosquito larvae.

Generation of a Transcriptome in a Model Lepidopteran Pest, Heliothis virescens, Using Multiple Sequencing Strategies for Profiling Midgut Gene Expression.

Heliothine pests such as the tobacco budworm, Heliothis virescens (F.), pose a significant threat to production of a variety of crops and ornamental plants and are models for developmental and physiological studies. The efforts to develop new control measures for H. virescens, as well as its use as a relevant biological model, are hampered by a lack of molecular resources. The present work demonstrates the utility of next-generation sequencing technologies for rapid molecular resource generation from this species for which lacks a sequenced genome. In order to amass a de novo transcriptome for this moth, transcript sequences generated from Illumina, Roche 454, and Sanger sequencing platforms were merged into a single de novo transcriptome assembly. This pooling strategy allowed a thorough sampling of transcripts produced under diverse environmental conditions, developmental stages, tissues, and infections with entomopathogens used for biological control, to provide the most complete transcriptome to date for this species. Over 138 million reads from the three platforms were assembled into the final set of 63,648 contigs. Of these, 29,978 had significant BLAST scores indicating orthologous relationships to transcripts of other insect species, with the top-hit species being the monarch butterfly (Danaus plexippus) and silkworm (Bombyx mori). Among identified H. virescens orthologs were immune effectors, signal transduction pathways, olfactory receptors, hormone biosynthetic pathways, peptide hormones and their receptors, digestive enzymes, and insecticide resistance enzymes. As an example, we demonstrate the utility of this transcriptomic resource to study gene expression profiling of larval midguts and detect transcripts of putative Bacillus thuringiensis (Bt) Cry toxin receptors. The substantial molecular resources described in this study will facilitate development of H. virescens as a relevant biological model for functional genomics and for new biological experimentation needed to develop efficient control efforts for this and related Noctuid pest moths.

Crystal structure of Cry51Aa1: A potential novel insecticidal aerolysin-type ß-pore-forming toxin from Bacillus thuringiensis.

The structures of several Bacillus thuringiensis (Bt) insecticidal crystal proteins have been determined by crystallographic methods and a close relationship has been explicated between specific toxicities and conserved three-dimensional architectures. In this study, as a representative of the coleopteran- and hemipteran-specific Cry51A group, the complete structure of Cry51Aa1 protoxin has been determined by X-ray crystallography at 1.65 Å resolution. This is the first report of a coleopteran-active Bt insecticidal toxin with high structural similarity to the aerolysin-type ß-pore forming toxins (ß-PFTs). Moreover, study of featured residues and structural elements reveal their possible roles in receptor binding and pore formation events. This study provides new insights into the action of aerolysin-type ß-PFTs from a structural perspective, and could be useful for the control of coleopteran and hemipteran insect pests in agricultures.

Chitosan/DsiRNA nanoparticle targeting identifies AgCad1 cadherin in Anopheles gambiae larvae as an in vivo receptor of Cry11Ba toxin of Bacillus thuringiensis subsp. jegathesan.

The Cry11Ba protein of Bacillus thuringiensis subsp. jegathesan crystals has uniquely high toxicity against a spectrum of mosquito species. The high potency of Cry11Ba against Anopheles gambiae is caused by recognition of multiple midgut proteins including glycosyl phosphatidylinositol-anchored alkaline phosphatase AgALP1, aminopeptidase AgAPN2, α-amylase AgAmy1 and α-glucosidase Agm3 that bind Cry11Ba with high affinity and function as putative receptors. The cadherin AgCad2 in An. gambiae larvae also binds Cry11Ba with high affinity (Kd = 12 nM) and is considered a putative receptor, while cadherin AgCad1 bound Cry11Ba with low affinity (Kd = 766 nM), a property not supportive for a Cry11Ba receptor role. Here, we show the in vivo involvement of AgCad1 in Cry11Ba toxicity in An. gambiae larvae using chitosan/DsiRNA nanoparticles to inhibit AgCad expression in larvae. Cry11Ba was significantly less toxic to AgCad1-silenced larvae than to control larvae. Because AgCad1 was co-suppressed by AgCad2 DsRNAi, the involvement of AgCad2 in Cry11Ba toxicity could not be ascertained. The ratio of AgCad1:AgCad2 transcript level is 36:1 for gut tissue in 4th instar larvae. Silencing AgCad expression had no effect on transcript levels of other binding receptors of Cry11Ba. We conclude that AgCad1 and possibly AgCad2 in An. gambiae larvae are functional receptors of Cry11Ba toxin in vivo.

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.

A coleopteran cadherin fragment synergizes toxicity of Bacillus thuringiensis toxins Cry3Aa, Cry3Bb, and Cry8Ca against lesser mealworm, Alphitobius diaperinus (Coleoptera: Tenebrionidae).

The lesser mealworm, Alphitobius diaperinus, is a serious cosmopolitan pest of commercial poultry facilities because of its involvement in structural damage to poultry houses, reduction in feed conversion efficiency, and transfer of avian and human pathogens. Cry3Aa, Cry3Bb, and Cry8Ca insecticidal proteins of Bacillus thuringiensis are used to control coleopteran larvae. Cadherins localized in the midgut epithelium function as receptors for Cry toxins in lepidopteran, coleopteran, and dipteran insects. Previously, we demonstrated that the truncated cadherin (DvCad1) from Diabrotica virgifera virgifera, which consists of the C-terminal cadherin repeats (CR) 8-10 and expressed in Escherichia coli, enhanced Cry3Aa and Cry3Bb toxicity against several coleopteran species. Here we report that the DvCad1-CR8-10 enhances Cry3Aa, Cry3Bb, and Cry8Ca toxicity to lesser mealworm. Previously, by an enzyme linked immunosorbent microplate assay, we demonstrated that the DvCad1-CR8-10 binds activated-Cry3Aa (11.8 nM), -Cry3Bb (1.4nM), and now report that CR8-10 binds activated-Cry8Ca (5.7 nM) toxin. The extent of Cry toxins enhancement by DvCad1-CR8-10, which ranged from 3.30- to 5.93-fold, may have practical application for lesser mealworm control in preventing avian and human pathogen transfer in poultry facilities.

Cadherin AdCad1 in Alphitobius diaperinus larvae is a receptor of Cry3Bb toxin from Bacillus thuringiensis.

Bacillus thuringiensis (Bt) Cry proteins are used as components of biopesticides or expressed in transgenic crops to control diverse insect pests worldwide. These Cry toxins bind to receptors on the midgut brush border membrane and kill enterocytes culminating in larval mortality. Cadherin proteins have been identified as Cry toxin receptors in diverse lepidopteran, coleopteran, and dipteran species. In the present work we report a 185 kDa cadherin (AdCad1) from larvae of the lesser mealworm (Alphitobius diaperinus) larvae as the first identified receptor for Cry3Bb toxin. The AdCad1 protein contains typical structural components for Cry toxin receptor cadherins, including nine cadherin repeats (CR9), a membrane-proximal extracellular domain (MPED) and a cytosolic region. Peptides corresponding to the CR9 and MPED regions bound Cry3Bb toxin with high affinities (23 nM and 40 nM) and significantly synergized Cry3Bb toxicity against A. diperinus larvae. Silencing of AdCad1 expression through RNA interference resulted in highly reduced susceptibility to Cry3Bb in A. diperinus larvae. The CR9 peptide fed with toxin to RNAi-treated larvae restored Cry3Bb toxicity. These results are evidences that AdCad1 is a functional receptor of Cry3Bb toxin and that exogenously fed CR9 peptide can overcome the effect of reduced AdCad1expression on Cry3Bb toxicity to larvae.

Analyses of α-amylase and α-glucosidase in the malaria vector mosquito, Anopheles gambiae, as receptors of Cry11Ba toxin of Bacillus thuringiensis subsp. jegathesan.

Bacillus thuringiensis subsp. jegathesan produces Cry11Ba crystal protein with high toxicity to mosquito larvae. The Cry11Ba toxicity is dependent on its receptors on mosquito larval midgut epithelial cells. Previously, a cadherin-like protein (AgCad2), aminopeptidase (AgAPN2) and alkaline phosphatase (AgALP1) were reported to be involved in regulation of Cry11Ba toxicity on Anopheles gambiae larvae. Here, the cDNAs encoding α-amylase (AgAmy1) and α-glucosidase (Agm3) were cloned from A. gambiae larva midgut. Both are glycophosphatidylinositol (GPI) anchored proteins on brush border membranes (BBMV). Immunohistochemistry revealed their localization on different regions of the larval midgut. AgAmy1 and Agm3 bound Cry11Ba with high affinity, 37.6 nM and 21.1 nM respectively. Cry11Ba toxicity against A. gambiae larvae was neutralized by both AgAmy1 and Agm3. The results provide evidence that both AgAmy1 and Agm3 function as receptors of Cry11Ba in A. gambiae.

AgCad2 cadherin in Anopheles gambiae larvae is a putative receptor of Cry11Ba toxin of Bacillus thuringiensis subsp. jegathesan.

In an effort to study the mode of action of Cry11Ba, we identified toxin binding proteins in Anopheles gambiae larval midgut and investigated their receptor roles. Previously, an aminopeptidase (AgAPN2) and an alkaline phosphatase (AgALP1) were identified as receptors for Cry11Ba toxin in A. gambiae. However, an A. gambiae cadherin (AgCad1) that bound Cry11Ba with low affinity (K(d) = 766 nM) did not support a receptor role of AgCad1 for Cry11Ba. Here, we studied a second A. gambiae cadherin (AgCad2) that shares 14% identity to AgCad1. Immunohistochemical study showed that the protein is localized on A. gambiae larval midgut apical membranes. Its cDNA was cloned and the protein was analyzed as a transmembrane protein containing 14 cadherin repeats. An Escherichia coli expressed CR14MPED fragment of AgCad2 bound Cry11Ba with high affinity (K(d) = 11.8 nM), blocked Cry11Ba binding to A. gambiae brush border vesicles and reduced Cry11Ba toxicity in bioassays. Its binding to Cry11Ba could be completely competed off by AgCad1, but only partially competed by AgALP1. The results are evidence that AgCad2 may function as a receptor for Cry11Ba in A. gambiae larvae.