Cry41-Related Mutants against Myzus persicae Based on Its Interaction with Cathepsin B

Cry toxins produced by Bacillus thuringiensis (Bt) are toxic to Lepidoptera, Coleoptera, and Diptera, but display very low activity against aphids. Recently, Cry41-related toxin was found to show moderate toxicity against Myzus persicae with the 50% lethal concentration (LC50) of 32.7 μg/mL, and its mode of action was proposed to be the acceleration of the apoptosis of aphid cells by enhancing Cathepsin B activity. This study focused on constructing Cry41-related mutants against M. persicae based on its interaction with Cathepsin B. First, eight key interacting residues in Cry41-related toxin were identified using alanine scanning and site-directed saturation mutagenesis. Subsequently, the positive mutant Cry41-7M protein (mutations of Gly48, Ile59, Lys364, Gln367, Gln377, Tyr378, and Ser400 to Tyr, Ala, Arg, Lys, Lys, Lys, and Ala in Cry41-related toxin, respectively) and the negative mutant Cry41-6A protein (mutations of Gly48, Lys364, Gln367, Gln377, and Tyr378 to Ala and mutation of Pro453 to Glu in Cry41-related toxin) were constructed, expressed, and purified. We then found that Cry41-7M protein performed slightly stronger than Cry41-related toxin in enhancing the enzymatic activity of Cathepsin B, whereas Cry41-6A protein did not affect Cathepsin B activity. A further bioassay showed that the mortality caused by Cry41-7M protein (LC50 = 19.144 μg/mL) was marginally higher than that of Cry41-related toxin, while Cry41-6A protein caused a decreased mortality (LC50 = 42.478 μg/mL). These results are expected to open new avenues for improving Cry aphidicidal activity.