Biocidal activity of the Bacillus thuringiensis 3D Cry toxins: Molecular crosstalk at the insect midgut with implication in insect resistance development

Bacillus thuringiensis (Bt) is a Gram-positive bacteria characterized by the production of parasporal crystalline proteins toxic to a wide range of insect orders. Cry toxins targeted pests of crops of economic importance. Nowadays, it has been described around 600 genes encode crystalline proteins of molecular weight from a molecular weight of 50 to 130 kDa. These proteins are formed by three domains which X-rays have elucidated tridimensional structures. Their mode of action remains to be defined and understood. However, most of these proteins follow a basic program for their biocidal activity. A critical step in the mode of action of the 3D Cry toxins is the specific binding to the receptors present in the midgut epithelial. These receptors are determinants of the specificity and susceptibility of targeted insects. Among them is the classical set of glycosyl phosphate inositol (GPI)-anchored membrane receptors, such as N-aminopeptidase, Alkaline Phosphatase, and classical epithelial cadherins DE-Cadherins. The second group of binding proteins includes ABC transporters, V-ATPase, and other lipid rafts-associated proteins. The hallmark of this molecular crosstalk at the insect midgut epithelium is the conservation in the mode of action of the 3D-Cy toxins, but on the other side, a lock of the receptor expression, and the activation of the insect response which ultimately leads to insect resistance development and for instance a fight for ecological survival.