National Research Council of Canada. NRC Biotechnology Research Institute
The Gram-positive bacterium Bacillus thuringiensis is the most extensively used environment-friendly bioinsecticide worldwide. It produces a variety of crystalline proteins which are specifically lethal to different insect species. Following solubilization in the midgut lumen of susceptible insect larvae and binding to specific receptors located on the surface of the luminal membrane of midgut epithelial cells, the toxins insert into the membrane and form pores that abolish transmembrane ionic gradients which are vital for cellular functions. The three-dimensional structures of four B. thuringiensis Cry toxins have been elucidated. They share a similar three-domain structure in which domain I, composed of amphipathic α-helices, plays a crucial role in pore formation. The other two domains are composed of β-sheets and are responsible for receptor binding and toxin specificity. The mechanism by which these toxins insert into the cell membrane remains one of the most challenging biophysical questions regarding their mode of action. It involves extensive conformational changes in the toxin molecule and assembly of an oligomeric structure composed of a yet undetermined number of toxin subunits. A better understanding of these events is gradually emerging from the analysis of the functional properties of genetically engineered mutant toxins using a variety of biophysical techniques.