Bt Research 2024, Vol.15, No.1, 42-52 http://microbescipublisher.com/index.php/bt 43 highlight the critical aspects of Bt toxin-receptor interactions and their implications for sustainable pest management in agriculture. 2 Overview of Bt Toxins Bacillus thuringiensis (Bt) is a bacterium known for producing crystal proteins, commonly referred to as Cry toxins, which are highly specific insecticidal agents. These toxins are widely used in biological insecticides and genetically modified crops to control pest populations. The specificity and safety of Bt toxins make them valuable alternatives to chemical pesticides (Likitvivatanavong et al., 2011; Vachon et al., 2012; Mendoza-Almanza et al., 2020). 2.1 Types of Bt toxins Bt produces several types of toxins, each targeting different biological organisms. The most well-known are the Cry proteins, which are toxic to various insect larvae, including those affecting important crops. Other types include Cyt toxins, which primarily target mosquito larvae, and parasporins, which can kill mammalian cancer cells (Mendoza-Almanza et al., 2020). Additionally, Bt strains produce Vip (Vegetative insecticidal proteins) and Sip (Secreted insecticidal proteins) during their vegetative growth phase, which also exhibit insecticidal activity (Mendoza-Almanza et al., 2020). 2.2 Structure and function of Bt toxins Cry toxins are composed of three distinct domains that play crucial roles in their insecticidal activity. Domain I is involved in pore formation in the insect midgut cells, while Domains II and III are responsible for binding to specific receptors on the insect midgut epithelium (Gómez et al., 2003). The binding of Cry toxins to these receptors is highly specific, involving interactions with proteins such as cadherin, alkaline phosphatase, and aminopeptidase-N (Bretschneider et al., 2016; Yuan et al., 2017). The structural determinants of these interactions have been studied extensively, revealing that specific loops and epitopes in the toxin domains are critical for binding to the receptors (Liu et al., 2018). 2.3 Mechanism of action in target insects The mechanism of action of Cry toxins involves several steps. Initially, the protoxin is ingested by the insect and activated by proteolytic cleavage in the midgut, converting it into an active toxin (Liu et al., 2018). The active toxin then binds to specific receptors on the midgut epithelial cells, such as cadherin, ABCC2, and alkaline phosphatase (Bretschneider et al., 2016; Yuan et al., 2017). This binding triggers a series of events, including toxin oligomerization and insertion into the cell membrane, leading to pore formation (Vachon et al., 2012). The formation of pores disrupts the osmotic balance of the cells, causing cell swelling, lysis, and ultimately, insect death (Figure 1). Recent studies have also suggested that Cry toxins may activate intracellular signaling pathways, such as the adenylyl cyclase/PKA pathway, leading to cell death without the need for pore formation (Zhang et al., 2006). This dual mechanism of action highlights the complexity of Bt toxin interactions with insect receptors and underscores the importance of understanding these interactions to develop more effective and sustainable pest control strategies. Figure 1 shows the mechanism of action of Cyt protein. After interacting with phosphatidylcholine, phosphatidylethanolamine and sphingomyelin, Cyt toxin undergoes conformational changes, prompting six Cyt monomers to assemble into an “open umbrella” structure to form pores, resulting in increased membrane permeability and ultimately death of the larvae. At high concentrations, Cyt toxin binds to the lipid bilayer on the surface of the cell membrane through a detergent-like effect and destroys the membrane structure. These two models are not mutually exclusive. The detergent-like effect works at high concentrations of Cyt toxin, while the pore formation model works at low concentrations of Cyt toxin. In general, Cyt protein effectively destroys insect cell membranes through these two mechanisms to exert its insecticidal effect.
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