Bt Research 2025, Vol.16, No.2, 55-62 http://microbescipublisher.com/index.php/bt 56 can bind to the receptor, determining the specificity of the toxin to the host. The third region is also a β -folded structure, which can increase the stability of the toxin and participate in receptor recognition (Torres et al., 2023). Cyt toxin is quite different from Cry toxin. It mainly has a β -barrel structure and does not require a receptor. Instead, it directly interacts with membrane lipids to form pores (Pacheco et al., 2015; Mendoza-Almanza et al., 2020). 2.2 Mechanism of action: binding, pore formation, and insect midgut disruption The action process of Cry toxin is roughly as follows: After insects consume Bt crystalline protein, the crystals will dissolve in the high pH environment of the intestinal tract. Subsequently, the protoxin is cleaved by proteases in the intestine and transformed into the active toxin. The activated toxins will first bind to specific receptors on the mesenteric brush margin membrane, such as cadherin, aminopeptase N, alkaline phosphatase or ABC transporter. They first reversibly bind and then form irreversible binding mediated by cadherin, thereby promoting the formation of oligomers of toxins (Figure 1) (Pacheco et al., 2015; Mendoza-Almanza et al., 2020; Torres et al., 2023). After these oligomers insert into the cell membrane, the α helix in region I forms ion channels, causing an imbalance in cell osmotic pressure, leading to cell rupture and eventually insect death. The effect of Cyt toxin is more direct. It can form pores by binding to membrane lipids without the need for protein receptors. It can also work in synergy with Cry toxin to enhance the insecticidal effect (Pacheco et al., 2015; Mendoza-Almanza et al., 2020). Figure 1 Mechanism of action of Cry proteins (Adopted from Mendoza-Almanza et al., 2020) Image caption: (a) Pore-forming model, once larvae ingest crystals, these are solubilized and proteolyzed in larvae midgut. Cry toxins recognize APN, ALP, and EC12/BT-R1 membrane receptors. Cry toxins suffer a proteolytic cleavage on helix α1, resulting in formation of a pre-pore oligomer structure. Posteriorly, oligomer structure is inserted into the cell membrane and creates an ionic pore that leads to osmotic failure, followed by septicemia and insect death. (b) Signaling pathway model, once Cry toxins recognize and bind to a cadherin receptor, induces activation of adenylyl cyclase that triggers an increase in cAMP and activates protein kinase A (PKA). This activation will induce a cascade of events that results in an ion channel formation in the membrane, cytoskeleton destabilization, and programmed cell death. A, Solubilization. B, Activation by proteolysis. C, Recognition of membrane receptor (Adopted from Mendoza-Almanza et al., 2020) 2.3 Natural diversity and genetic variation in Bt toxin families The Bt toxin family is very large. Nearly 800 types of Cry toxins have been discovered so far. They have diverse structures and functions and can act on various types of pests such as Lepidoptera, Diptera and Coleoptera (Torres et al., 2023). The diversity of these toxin genes stems from molecular evolutionary mechanisms such as horizontal gene transfer, point mutation and gene recombination (Crickmore, 2017). In addition, some structures of the receptor binding region of Cry toxin, such as the exposure loop in the second region, vary greatly. This is also the reason why different toxins have different host profiles and insecticidal specificities (Torres et al., 2023). Cyt toxin is quite different from Cry toxin in structure and mechanism of action, which makes Bt toxin as a whole more diverse (Pacheco et al., 2015; Mendoza-Almanza et al., 2020).
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