Amino acids Y229 and F603 are involved in Bacillus thuringiensis Cry1Ac δ-endotoxin stability and toxicity

Bacillus thuringiensis Cry1Ac toxin shares structurally five conserved blocs with the other δ-endotoxins. To study the role of some amino acids belonging to these regions, two mutations, Y(229) P and F(603) S, were introduced respectively in blocs 2 and 5. The stability and crystallization of the resulting mutant proteins Cry1Ac'1 and Cry1Ac'3 were affected. Both of them lost their toxicity to the Lepidopteran larvae Ephestia kuehniella. Unlike Cry1Ac'1, Cry1Ac'3 became very sensitive to proteases. Accordingly, the three-dimensional structures of the two mutants were studied. The obtained models showed that both of the residues, Y229, located near the bottom of the α7 helix, and F603, located in the core of domain III, are involved in hydrophobic interactions essential for protein stability and toxicity. These results reveal that conserved amino acids blocs of Cry toxins have conformational and functional roles.