Key residues of Bacillus thuringiensis Cry2Ab for oligomerization and pore-formation activity

N-Terminal α-Helices in Domain I of Bacillus thuringiensis Vip3Aa Play Crucial Roles in Disruption of Liposomal Membrane

Bacillus thuringiensis Vip3 toxins form a tetrameric structure crucial for their insecticidal activity. Each Vip3Aa monomer comprises five domains. Interaction of the first four α-helices in domain I with the target cellular membrane was proposed to be a key step before pore formation. In this study, four N-terminal α-helix-deleted truncations of Vip3Aa were produced and, it was found that they lost both liposome permeability and insecticidal activity against Spodoptera litura. To further probe the role of domain I in membrane permeation, the full-length domain I and the fragments of N-terminal α-helix-truncated domain I were fused to green fluorescent protein (GFP), respectively. Only the fusion carrying the full-length domain I exhibited permeability against artificial liposomes. In addition, seven Vip3Aa-Cry1Ac fusions were also constructed by combination of α-helices from Vip3Aa domains I and II with the domains II and III of Cry1Ac. Five of the seven combinations were determined to show membrane permeability in artificial liposomes. However, none of the Vip3Aa-Cry1Ac combinations exhibited insecticidal activity due to the significant reduction in proteolytic stability. These results indicated that the N-terminal helix α1 in the Vip3Aa domain I is essential for both insecticidal activity and liposome permeability and that domain I of Vip3Aa preserved a high liposome permeability independently from domains II-V.

Engineering of Bacillus thuringiensis Cry2Ab toxin for improved insecticidal activity

Bacillus thuringiensis Cry2Ab toxin was a widely used bioinsecticide to control lepidopteran pests all over the world. In the present study, engineering of Bacillus thuringiensis Cry2Ab toxin was performed for improved insecticidal activity using site-specific saturation mutation. Variants L183I were screened with lower LC50 (0.129 µg/cm2) against P. xylostella when compared to wild-type Cry2Ab (0.267 µg/cm2). To investigate the molecular mechanism behind the enhanced activity of variant L183I, the activation, oligomerization and pore-formation activities of L183I were evaluated, using wild-type Cry2Ab as a control. The results demonstrated that the proteolytic activation of L183I was the same as that of wild-type Cry2Ab. However, variant L183I displayed higher oligomerization and pore-formation activities, which was consistence with its increased insecticidal activity. The current study demonstrated that the insecticidal activity of Cry2Ab toxin could be assessed using oligomerization and pore-formation activities, and the screened variant L183I with improved activity might contribute to Cry2Ab toxin's future application.

Development of a soybean leaf disc assay for determining oral insecticidal activity in the lepidopteran agricultural pest Helicoverpa armigera

Pest insects pose a heavy burden on global agricultural industries with small molecule insecticides being predominantly used for their control. Unwanted side effects and resistance development plagues most small molecule insecticides such as the neonicotinoids, which have been reported to be harmful to honeybees. Bioinsecticides like Bacillus thuringiensis (Bt) toxins can be used as environmentally-friendly alternatives. Arachnid venoms comprise another promising source of bioinsecticides, containing a multitude of selective and potent insecticidal toxins. Unfortunately, no standardised insect models are currently available to assess the suitability of insecticidal agents under laboratory conditions. Thus, we aimed to develop a laboratory model that closely mimics field conditions by employing a leaf disk assay (LDA) for oral application of insecticidal agents in a bioassay tray format. Neonate larvae of the cotton bollworm (Helicoverpa armigera) were fed with soybean (Glycine max) leaves that were treated with different insecticidal agents. We observed dose-dependent insecticidal effects for Bt toxin and the neonicotinoid insecticide imidacloprid, with imidacloprid exhibiting a faster response. Furthermore, we identified several insecticidal arachnid venoms that were active when co-applied with sub-lethal doses of Bt toxin. We propose the H. armigera LDA as a suitable tool for assessing the insecticidal effects of insecticidal agents against lepidopterans.

Bacillus thuringiensis Cry9Aa Insecticidal Protein Domain I Helices α3 and α4 Are Two Core Regions Involved in Oligomerization and Toxicity

Bacillus thuringiensis Cry9 proteins show high insecticidal activity against different lepidopteran pests. Cry9 could be a valuable alternative to Cry1 proteins because it showed a synergistic effect with no cross-resistance. However, the pore-formation region of the Cry9 proteins is still unclear. In this study, nine mutations of certain Cry9Aa helices α3 and α4 residues resulted in a complete loss of insecticidal activity against the rice pest Chilo suppressalis; however, the protein stability and receptor binding ability of these mutants were not affected. Among these mutants, Cry9Aa-D121R, Cry9Aa-D125R, Cry9Aa-D163R, Cry9Aa-E165R, and Cry9Aa-D167R are unable to form oligomers in vitro, while the oligomers formed by Cry9Aa-R156D, Cry9Aa-R158D, and Cry9Aa-R160D are unstable and failed to insert into the membrane. These data confirmed that helices α3 and α4 of Cry9Aa are involved in oligomerization, membrane insertion, and toxicity. The knowledge of Cry9 pore-forming action may promote its application as an alternative to Cry1 insecticidal proteins.

V-ATPase E mediates Cry2Ab binding and toxicity in Helicoverpa armigera

Cry2Ab is one of the important alternative Bt proteins that can be used to manage insect pests resistant to Cry1A toxins and to expand the insecticidal spectrum of pyramided Bt crops. Previous studies have showed that vacuolar H+-ATPase subunits A and B (V-ATPase A and B) may be involved in Bt insecticidal activities. The present study investigated the role of V-ATPases subunit E in the toxicity of Cry2Ab in Helicoverpa amigera. RT-PCR analysis revealed that oral exposure of H. amigera larvae to Cry2Ab led to a significant reduction in the expression of H. armigera V-ATPase E (HaV-ATPase E). Ligand blot, homologous and heterologous competition experiments confirmed that HaV-ATPases E physically and specifically bound to activated Cry2Ab toxin. Heterologous expressing of HaV-ATPase E in Sf9 cells made the cell line more susceptible to Cry2Ab, whereas knockdown of the endogenous V-ATPase E in H. zea midgut cells decreased Cry2Ab's cytotoxicity against this cell line. Further in vivo bioassay showed that H. armigera larvae fed a diet overlaid with both Cry2Ab and E. coli-expressed HaV-ATPase E protein suffered significantly higher mortality than those fed Cry2Ab alone. These results support that V-ATPases E is a putative receptor of Cry2Ab and can be used to improve Cry2Ab toxicity and manage Cry2Ab resistance at least in H. armigera.