Bt Research 2024, Vol.15, No.1, 42-52 http://microbescipublisher.com/index.php/bt 44 Figure 1 Mechanism of action of Cyt protein (Adopted from Mendoza-Almanza et al., 2020) Image captions: (a) pore-forming model; (b) detergent effect model (Adopted from Mendoza-Almanza et al., 2020) 3 Insect Receptors for Bt Toxins 3.1 Types of insect receptors Insect receptors for Bacillus thuringiensis (Bt) toxins are diverse and include cadherin-like proteins, aminopeptidase N (APN), and ATP-binding cassette (ABC) transporters. Cadherin-like proteins, such as BT-R1 in Manduca sexta, are primary receptors for Cry1A toxins and play a crucial role in the insecticidal process (Gómez et al., 2003; Adegawa et al., 2017; Liu et al., 2022). APNs, like the 106-kDa APN in Anopheles gambiae, also serve as important receptors for various Cry toxins, including Cry11Ba (Zhang et al., 2008). ABC transporters, such as ABCC2, have been identified as significant receptors for Cry1A toxins in species like Bombyx mori (Chen et al., 2015) 3.2 Receptor structure and function Cadherin-like receptors are single-membrane-spanning α-helical proteins that regulate intercellular adhesion and signaling activities. The BT-R1 receptor in M. sexta, for instance, binds Cry1A toxins tightly and triggers a signaling pathway leading to insect death (Liu et al., 2022). APNs are glycosylphosphatidylinositol (GPI)-anchored proteins with zinc-binding motifs and glycosylation sites, which facilitate their role in binding Cry toxins and initiating insecticidal activity. ABC transporters, such as ABCC2, are involved in the transport of various molecules across cellular membranes and have been shown to interact with Cry toxins, contributing to their cytotoxic effects (Adegawa et al., 2017). 3.3 Binding sites and specificity The binding sites on insect receptors for Bt toxins are highly specific and involve distinct epitopes. For example, the binding domain of BT-R1 for Cry1A toxins is localized in the 12th cadherin repeat (EC12), with a highly conserved 94-amino acid polypeptide designated as the toxin-binding site (TBS). In M. sexta, the Cry1Ab toxin interacts with specific epitopes in domain II loops and domain III of the toxin, which are crucial for binding to cadherin and APN receptors (Gómez et al., 2016). The specificity of Cry1A toxins also involves multiple structural determinants, such as the loop regions in domain II, which interact with both cadherin-like receptors and ABC transporters (Adegawa et al., 2017; Liu et al., 2022).
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