Antagonistic Effect of Truncated Fragments of Bacillus thuringiensis Vip3Aa on the Larvicidal Activity of its Full-length Protein

Background: Vip3Aa is a vegetative insecticidal protein produced by Bacillus thuringiensis. The protein is produced as an 88-kDa protoxin that could be processed by insect gut proteases into a 22-kDa N-terminal and a 66-kDa C-terminal fragments. The C-terminal part could bind to a specific receptor while the N-terminal part is required for toxicity and structural stability. Objective: To demonstrate the antagonistic effect of truncated fragments on the insecticidal activity of the full-length Vip3Aa. Methods: The full-length protein (Vip3Aa), a 66-kDa C-terminal fragment (Vip3Aa-D199) and a predicted carbohydrate binding module (CBM) were produced in Escherichia coli. Purified proteins were mixed at different ratios and fed to Spodoptera litura and Spodoptera exigua larvae. Mortality was recorded and compared between larvae fed with individual toxin and mixtures of the full-length and truncated toxins. Results: Production level of the Vip3Aa-D199 was significantly decreased comparing to that of the full-length protein. Vip3Aa-D199 and CBM fragment were not toxic to insect larvae whereas Vip3Aa showed high toxicity with LC₅₀ about 200 ng/cm². Feeding the larvae with mixtures of the Vip3Aa and Vip3Aa-D199 at different ratios revealed antagonistic effect of the Vip3Aa-D199 on the toxicity of Vip3Aa. Results showed that the lethal time (LT ₅₀ and LT ₉₅) of larvae fed the mixture toxins was longer than those fed the Vip3Aa alone. In addition, a CBM fragment could inhibit toxicity of the full-length Vip3Aa. Conclusion: Our results demonstrated that the Vip3Aa-D199 and a CBM fragment could complete for the membrane binding thus rendering activity of the full-length Vip3Aa.