Mode of Action of Bacillus thuringiensis Cry1Ab Toxin: Role of Domain II Residue in Insertion into Insect Brush Border Membranes

Cry1Ab is a member of the family of crystal (Cry) toxins, produced by a soil bacterium Bacillus thuringiensis, and used globally as a commercial pesticide. A thorough knowledge of the mechanism by which the toxin acts is essential for preventing possible pest resistance to the toxin. While the upstream steps of the mechanism, including activation of the toxin in the alkaline midgut of the target lepidopteran insect followed by binding to its receptors are well understood, the final step of insertion of the toxin into insect brush border membranes remains controversial. The Umbrella and Penknife models hypothesize that the toxin partitions into the apical membrane of the insect midgut by insertion of only two –helices from domain I of the protein, -helices 4 and 5 in the case of the Umbrella Model and -helices 5 and 6 in the case of the Penknife Model. Neither model envisages membrane partitioning by domains II and III. In this study, we present data suggesting that mutations in domain II residue, F371 affect insertion of the whole toxin into Manduca sexta brush border membrane vesicles (BBMV)