Effects of Site-Mutations Within the 22 kDa No-Core Fragment of the Vip3Aa11 Insecticidal Toxin of Bacillus thuringiensis.

Bacillus thuringiensis vegetative insecticidal proteins (VIPs) are not homologous to other known Cry proteins, and they act against lepidopteran larvae via a unique process. All reported studies on the mode of action of Vip3 proteins have been performed on the Vip3A family, mostly on the Vip3Aa subfamily. Vip3Aa proteins are activated by midgut proteases, and they cross the peritrophic membrane and bind specific proteins in apical membrane epithelial midgut cells, which results in pore formation and, eventually, death to the insects. Some studies of trypsin-activated protein (core fragment) and the full-length protein show differences in mortality on the same insect species. The N-terminus of Vip3A proteins is responsible for the translocation of the protein across the cell membrane. To determine whether the N-terminus of Vip3Aa11 proteins contribute to insecticidal activity, we exchanged Vip3Aa11 residues with Vip3Aa39 no-core fragment residues using site-directed mutagenesis. Bioassays showed that the toxicity of S9N, S193T, and S194L mutants displayed approximately one- and twofold increases in toxicity against Helicoverpa armigera. Mutant protein R115H demonstrated a threefold decrease in toxicity. This work serves as a guideline for the study of the Vip3Aa11 no-core fragment protein insecticidal mechanism.