Bt Research 2025, Vol.16, No.4, 168-181 http://microbescipublisher.com/index.php/bt 169 formation, namely Bt insecticidal crystals. Bt strains have been found to produce hundreds of insecticidal proteins, which are classified into various categories according to sequence homology, such as Cry toxin, Vip toxin, etc. (Mendoza-Almanza et al., 2020)). Cry protein is an insoluble crystal produced when the spores of Bt bacteria are formed. It mainly targets pests such as Lepidoptera and Coleoptera. There are more than a hundred different types, corresponding to different target pest profiles. Vip (Vegetative insecticidal protein) protein is a toxin secreted during the vegetative growth period of Bt bacteria. For example, Vip3A protein has high activity against a variety of lepidopteran pests, and it is different from the Cry protein and can serve as an important supplement to Cry protein (Jiang et al., 2020)). Bt toxin is highly specific, safe for humans and animals and has a strong toxic effect on specific pests, making Bt toxin protein an ideal source of insect-resistant genes for implanted plants. Currently, the main toxic proteins used in commercial Bt crops include Cry1Ab, Cry1Ac, Cry2Ab, Cry1F, Cry3Bb1, Vip3A, etc. Each protein plays a role in specific pest groups. For example, Cry1 toxins mainly kill Lepidopteran larvae, Cry3 targets Coleopteran larvae (such as corn rootworm), and Vip3A fights certain Lepidopteran pests that are resistant to Cry (Figure 1). Figure 1 Domain III is a potential receptor binding domain (Adopted from Jiang et al., 2020) Image caption: (A) Overall structure of Vip3Aa11 domain III shown as a ribbon cartoon. Two views of three antiparallel β sheets from domain III are shown in three different colors, the black arrow indicates the angle of rotation around the central axis. (B) Two views of the surface model of domain III of Vip3Aa11. Inside the domain III, there is a conserved hydrophobic core, and the conserved hydrophobic amino acid residues from three antiparallel β sheets are shown as sticks, the black arrow indicates the angle of rotation around the central axis. (C) The schematics of C-terminal RFP (red fluorescent protein)-tagged Vip3Aa and its truncation derivatives. (D) Fluorescence microscope images of Sf9 cells treated with Vip3Aa-RFP or its truncations, which were labeled with C-terminal RFP tag, for 6 h. Nuclei are stained with DAPI (blue). (E) Quantification of the number of Sf9 cells that can be bound by RFP-tagged Vip3Aa and its truncations of Figure 3D in a blind fashion (n = 100 cells per sample). Data are expressed as the mean ± SD from three independent experiments. ns, nonsignificant; **, p < 0.01; ***, p < 0.001. Scale bar: 10 μm (Adopted from Jiang et al., 2020)
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