Bt Cry endotoxins are generally composed of three domains with conserved regions, that are crucial for insect species specificity and toxicity. Domain I, composed of 7 α-helices is responsible for penetration in the midgut by creating a helical hairpin, formed by the α5 and α4 or α6 helices.
Mutation in this domain lead to toxins that can bind to receptors but fail to insert into the membrane. The domain II is composed of three anti-parallel β-sheets forming a loop and is believed to play a major role in receptor binding and thus in specificity determination. The domain III has a β-sandwich structure implicated also in the determination of insect specificity (Li et al. 1991; Knowles, 1994; Smedley, 1996).