Cry51Aa Proteins Are Active against Apolygus lucorum and Show a Mechanism Similar to Pore Formation Model

Reduced insecticide spray in crop fields due to the widespread adoption of Bacillus thuringiensis (Bt) crops has favored the population increases of mirid bugs. Cry51Aa proteins are new types of Bt proteins that belong to aerolysin-like β pore-forming proteins with insecticidal activity against hemipteran and coleopteran pests. Here, we studied the activity of Bt Cry51Aa1 and Cry51Aa2 against Apolygus lucorum, an emerging pest in cotton, and their mechanism of action. Cry51Aa1 exhibited almost 5-fold higher toxicity than Cry51Aa2 with LC50 of 11.87 and 61.34 μg/mL, respectively. Protoxins could be activated both in vitro, by trypsin and midgut contents, and in vivo, by A. lucorum midgut. Both Cry51Aa protoxins were processed in two steps, producing pre-activated (∼30 kDa) and final activated (∼25–28 kDa) proteins. Cry51Aa proteins bound to a 25 kDa midgut protein, and Cry51Aa2 showed 2 times higher binding affinity than Cry51Aa1. Incubating Cry51Aa proteins with midgut homogenate resulted in toxin oligomers of 150–200 kDa. Our findings provide a theoretical basis for using Cry51Aa proteins to control A. lucorum and a better understanding of their mode of action.