Substitution of residues on the proximal side of Cry1A Bacillus thuringiensis delta-endotoxins affects irreversible binding to Manduca sexta midgut membrane

Substitution of a positively charged residue (R93F) or addition of a negatively charged residue (A92D) at the N-terminal of alpha 3 helix of domain I of the Cry1Ac delta-endotoxin resulted in a substantial reduction in toxicity against Manduca sexta. The N-terminal residues of helix 3 are considered to be on the same (proximal) surface of the toxin as the loops in domain II which are involved in the binding of the toxin to the receptor. The loss of toxicity was not caused by a decrease in the initial binding but rather by reduced irreversible binding. Only 65 and 75% of the A92D and R93F mutant toxin, respectively, bound to midgut vesicles irreversibly, compared to 94% of the wild type toxin. On the other hand, replacing A119 in a loop on the distal side of the helices with negatively charged residues (A119D or A119E) did nor affect toxicity or irreversible binding. (C) 1996 Academic Press, Inc.