Amino acid substitution in alpha-helix 7 of Cry1Ac delta-endotoxin of Bacillus thuringiensis leads to enhanced toxicity to Helicoverpa armigera Hubner

Insecticidal proteins or delta-endotoxins of Bacillus thuringiensis are hig hly toxic to a wide range of agronomically important pests. The toxins are formed of three structural domains. The N-terminal domain is a bundle of ei ght alpha-heliccs and is implicated in pore formation in insect midgut epit helial membranes. All the delta-endotoxins share a common hydrophobic motif of eight amino acids in alpha-helix 7. A similar motif is also present in fragment B of diphtheria toxin (DT), Site-directed mutagenesis of Cry1Ac de lta-endotoxin of B, thuringiensis was carried out to substitute its hydroph obic motif with that of DT fragment B, The mutant toxin was shown to be mor e toxic to the larvae of Helicoverpa armigera (cotton bollworm) than the wi ldtype toxin, Voltage clamp analysis with planar lipid bilayers revealed th at the mutant toxin opens larger ion channels and induces higher levels of conductance than the wild-type toxin, (C) 1999 Federation of European Bioch emical Societies.