Participation of Valine 171 in α-Helix 5 of Bacillus thuringiensis Cry1Ab δ-Endotoxin in Translocation of Toxin into Lymantria dispar Midgut Membranes

The Cry1Ab delta-endotoxin V171C mutant protein exhibits a 25-fold increase in toxicity against Lymantria dispar, which correlates with a faster rate of partitioning into the midgut membrane and slightly decreased protein stability. This is an insect-specific mechanism; similar results were not observed in Manduca sexta, another Cry1Ab delta-endotoxin-susceptible insect.