Comparison of in vitro and in vivo binding site competition of Bacillus thuringiensis Cry1 proteins in two important maize pests

Receptor interactions of protoxin and activated Vip3Aa structural conformations in Spodoptera exigua

BACKGROUND

The Vip3Aa insecticidal protein, produced by Bacillus thuringiensis, has been effectively used in commercial Bt-crops to manage lepidopteran pests. Upon ingestion by larvae, the protoxin is processed by midgut proteases into the activated protein and binds specifically to its receptors in the midgut, leading to insect mortality. Cryo-EM resolution of the trypsin-processed Vip3Aa protein unveiled structural remodelling of the N-terminal region during the transition from protoxin to activated protein. This conformational change has been demonstrated to be crucial for toxicity against Spodoptera exigua larvae, a major global lepidopteran pest. In this study, we investigated the relevance of the structural remodelling for the specific binding to midgut receptors.

RESULTS

We conducted in vitro binding assays with radiolabelled proteins and brush border membrane vesicles (BBMV) from S. exigua, employing structural mutants that lock the protein in either its protoxin or its activated conformation. Our results indicate that both structural stages of the protein share binding sites in the midgut epithelium. Moreover, in vivo competition assays revealed that Vip3Aa is able to bind to functional receptors in S. exigua larvae both as protoxin and as activated protein.

CONCLUSION

Altogether, our findings point to both structural conformations contributing to receptor binding. In vivo, either spontaneous structural shift upon proteolytic cleavage or receptor-mediated remodelling could be occurring. However, the timing and context in which the conformational change occurs could influence membrane insertion and toxicity. Our results show the complex interplay between proteolytic processing, protein structure and receptor interactions in Vip3Aa's toxicity. © 2024 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

A Shared Receptor Suggests a Common Ancestry between an Insecticidal Bacillus thuringiensis Cry Protein and an Anti-Cancer Parasporin

Cry toxins, produced by the bacterium Bacillus thuringiensis, are of significant agronomic value worldwide due to their potent and highly specific activity against various insect orders. However, some of these pore-forming toxins display specific activity against a range of human cancer cells whilst possessing no known insecticidal activity; Cry41Aa is one such toxin. Cry41Aa has similarities to its insecticidal counterparts in both its 3-domain toxic core structure and pore-forming abilities, but how it has evolved to target human cells is a mystery. This work shows that some insecticidal Cry toxins can enhance the toxicity of Cry41Aa against hepatocellular carcinoma cells, despite possessing no intrinsic toxicity themselves. This interesting crossover is not limited to human cancer cells, as Cry41Aa was found to inhibit some Aedes-active Cry toxins in mosquito larval assays. Here, we present findings that suggest that Cry41Aa shares a receptor with several insecticidal toxins, indicating a stronger evolutionary relationship than their divergent activities might suggest.

Individual transmembrane domains of SfABCC2 from Spodoptera frugiperda do not serve as functional Cry1F receptors

The fall armyworm (Spodoptera frugiperda) is a major global pest causing severe damage to various crops, especially corn. Transgenic corn producing the Cry1F pesticidal protein from the bacterium Bacillus thuringiensis (Cry1F corn) showed effectiveness in controlling this pest until S. frugiperda populations at locations in North and South America evolved practical resistance. The mechanism for practical resistance involved disruptive mutations in an ATP binding cassette transporter subfamily C2 gene (SfABCC2), which serves as a functional Cry1F receptor in the midgut cells of susceptible S. frugiperda. The SfABCC2 protein contains two transmembrane domains (TMD1 and TMD2), each with a cytosolic nucleotide (ATP) binding domain (NBD1 and NBD2, respectively). Previous reports have demonstrated that disruptive mutations in TMD2 were linked with resistance to Cry1F, yet whether the complete SfABCC2 structure is needed for receptor functionality or if a single TMD-NBD protein can serve as functional Cry1F receptor remains unknown. In the present study, we separately expressed TMD1 and TMD2 with their corresponding NBDs in cultured insect cells and tested their Cry1F receptor functionality. Our results show that the complete SfABCC2 structure is required for Cry1F receptor functionality. Moreover, binding competition assays revealed that Cry1F specifically bound to SfABCC2, whereas neither SfTMD1-NBD1 nor SfTMD2-NBD2 exhibited any significant binding. These results provide insights into the molecular mechanism of Cry1F recognition by SfABCC2 in S. frugiperda, which could facilitate the development of more effective insecticidal proteins.

Genetic mutations linked to field-evolved Cry1Fa-resistance in the European corn borer, Ostrinia nubilalis

Transgenic corn, Zea mays (L.), expressing insecticidal toxins such as Cry1Fa, from Bacillus thuringiensis (Bt corn) targeting Ostrinia nubilalis (Hübner) (Lepidoptera: Crambidae) resulted in over 20 years of management success. The first case of practical field-evolved resistance by O. nubilalis to a Bt corn toxin, Cry1Fa, was discovered in Nova Scotia, Canada, in 2018. Laboratory-derived Cry1Fa-resistance by O. nubilalis was linked to a genome region encoding the ATP Binding Cassette subfamily C2 (ABCC2) gene; however, the involvement of ABCC2 and specific mutations in the gene leading to resistance remain unknown. Using a classical candidate gene approach, we report on O. nubilalis ABCC2 gene mutations linked to laboratory-derived and field-evolved Cry1Fa-resistance. Using these mutations, a DNA-based genotyping assay was developed to test for the presence of the Cry1Fa-resistance alleles in O. nubilalis strains collected in Canada. Screening data provide strong evidence that field-evolved Cry1Fa-resistance in O. nubilalis maps to the ABCC2 gene and demonstrates the utility of this assay for detecting the Cry1Fa resistance allele in O. nubilalis. This study is the first to describe mutations linked to Bt resistance in O. nubilalis and provides a DNA-based detection method that can be used for monitoring.

Resistance of Cabbage Loopers to Bacillus thuringiensis (Bt) Toxin Cry1F and to Dual-Bt Toxin WideStrike Cotton Plants

The Cry proteins from Bacillus thuringiensis (Bt) are major insecticidal toxins in formulated Bt sprays and are expressed in genetically engineered Bt crops for insect pest control. However, the widespread application of Bt toxins in the field imposes strong selection pressure on target insects, leading to the evolution of insect resistance to the Bt toxins. Identification and understanding of mechanisms of insect resistance to Bt toxins are an important approach for dissecting the modes of action of Bt toxins and providing knowledge necessary for the development of resistance management technologies. In this study, cabbage looper (Trichoplusia ni) strains resistant to the transgenic dual-Bt toxin WideStrike cotton plants, which express Bt toxins Cry1Ac and Cry1F, were selected from T. ni strains resistant to the Bt formulation Bt-DiPel. The WideStrike-resistant T. ni larvae were confirmed to be resistant to both Bt toxins Cry1Ac and Cry1F. From the WideStrike-resistant T. ni, the Cry1F resistance trait was further isolated to establish a T. ni strain resistant to Cry1F only. The levels of Cry1F resistance in the WideStrike-resistant and the Cry1F-resistant strains were determined, and the inheritance of the Cry1F-resistant trait in the two strains was characterized. Genetic association analysis of the Cry1F resistance trait indicated that the Cry1F resistance in T. ni isolated in this study is not shared with the Cry1Ac resistance mechanism nor is it associated with a mutation in the ABCC2 gene, as has so far been reported in Cry1F-resistant insects. IMPORTANCE Insecticidal toxins from Bacillus thuringiensis (Bt) are highly effective for insect control in agriculture. However, the widespread application of Bt toxins exerts strong selection for Bt resistance in insect populations. The continuing success of Bt biotechnology for pest control requires the identification of resistance and understanding of the mechanisms of resistance to Bt toxins. Cry1F is an important Bt toxin used in transgenic cotton, maize, and soybean varieties adopted widely for insect control. To understand the mode of action of Cry1F and mechanisms of Cry1F resistance in insects, it is important to identify Cry1F-specific resistance and the resistance mechanisms. In this study, Trichoplusia ni strains resistant to commercial "WideStrike" cotton plants that express Bt toxins Cry1Ac and Cry1F were selected, and a Cry1F-specific resistant strain was isolated. The isolation of the novel Cry1F-specific resistance in the T. ni provided an invaluable biological system to discover a Cry1F-specific novel resistance mechanism.

In vivo competition assays between Vip3 proteins confirm the occurrence of shared binding sites in Spodoptera littoralis

Due to their different specificity, the use of Vip3 proteins from Bacillus thuringiensis in combination with the conventionally used Cry proteins in crop protection is being essential to counteract the appearance of insect resistance. Therefore, understanding the mode of action of Vip3 proteins is crucial for their better application, with special interest on the binding to membrane receptors as the main step for specificity. Derived from in vitro heterologous competition binding assays using ¹²⁵I-Vip3A and other Vip3 proteins as competitors, it has been shown that Vip3 proteins share receptors in Spodoptera frugiperda and Spodoptera exigua brush border membrane vesicles (BBMV). In this study, using ¹²⁵I-Vip3Aa, we have first extended the in vitro competition binding site model of Vip3 proteins to Spodoptera littoralis. With the aim to understand the relevance (in terms of toxicity) of the binding to the midgut sites observed in vitro on the insecticidal activity of these proteins, we have performed in vivo competition assays with S. littoralis larvae, using disabled mutant (non-toxic) Vip3 proteins as competitors for blocking the toxicity of Vip3Aa and Vip3Af. The results of the in vivo competition assays confirm the occurrence of shared binding sites among Vip3 proteins and help understand the functional role of the shared binding sites as revealed in vitro.