Identification of amino acid residues of Bacillus thuringiensis delta-endotoxin CryIAa associated with membrane binding and toxicity to Bombyx mori.

Alanine substitution (A3) or deletion (D3) of residues 365 to 371 of Bacillus thuringiensis CryIAa insect toxin removed nearly all toxicity for Bombyx mori (> 1,000-fold less active than the wild type). The loss of larvicidal activity in the mutants was not caused by increased sensitivity to larval gut enzymes but could be attributed to significantly reduced binding to B. mori brush border membrane vesicles. Some or all of the affected amino acid residues may interact directly or indirectly with the B. mori membrane receptor(s). Such receptor binding appears to be directly correlated with insect toxicity.