Replacement of loop2 and 3 of Cry1Ai in domain II affects specificity to the economically important insect Bombyx mori

Bacillus thuringiensis Cry1Ai belongs to three-domain Cry toxins and only shows growth inhibition effects against the agricultural pest Helicoverpa armigera, although it exhibits high toxicity against the non-target insect Bombyx mori. In previous studies, loop2 and loop3 on domain II from Cry1Ah were found to be related to binding and high toxicity against H. armigera. However, toxicity for B. mori of Cry1Ai-h-loop2, obtained by replacing loop 2 from Cry1Ah into Cry1Ai, was not modified. In this study, to further characterize the role of loop2 and loop3 in Cry1Ai, all of the amino acids in these two loops were substituted with the same amount of alanine residues. The Cry1Ai-loop3 mutant exhibited significantly lower toxicity against B. mori, but the toxicity of the loop2 mutant was not significantly changed. Furthermore, the double-exchange mutant Cry1Ai-h-loop2&3, replacing loop2 and loop3 from Cry1Ah into Cry1Ai, showed decreased toxicity against B. mori related to Cry1Ai. In addition, we found that the binding affinity of Cry1Ai-h-loop2&3 with brush border membrane vesicles (BBMVs) from the midgut of B. mori was lower than that of Cry1Ai, which correlates with the reduced toxicity.