A novel Aedes aegypti cadherin is a functional receptor of the Cry11Aa toxin from Bacillus thuringiensis subsp. israelensis

Bacillus thuringiensis subsp. israelensis (Bti) has been used to control Aedes aegypti for more than three decades. However, its mechanism of action is still unclear. Using a whole genome screen to identify genes that are altered during Cry11Aa intoxication we identified two cadherin genes that were significantly down-regulated; AAEL000597 and AAEL001196. To further characterize the role of the two cadherins in Cry11Aa toxicity, we cloned and separately, expressed and purified the two functional domains (cadherin repeat domains and EGF-LamG domains) from AAEL000597 and AAEL001196. The binding affinity of Cry11Aa to these fragments was analyzed by ELISA, and we showed the EGF-LamG fragment from AAEL000597 bound Cry11Aa with high affinity.  Further this domain competed with Cry11Aa binding to mosquito midgut membranes. To show if the cadherin is involved in modulating Cry11Aa toxicity in vivo, we used dsRNA to silence the expression of the AAEL000597 cadherin in larval midgut. At doses that killed 50% of control mosquitoes only 30% of the AAEL000597-silenced Aedes mosquitoes died. These data suggest that AAEL000597-silenced mosquitoes had a tolerance to Cry11Aa, implying that this cadherin is involved in mediating Cry11Aa toxicity. We further investigated the binding regions of EGF-LamG domain from AAEL000597 and loop regions from Cry11Aa toxin. We found two separated regions of EGF-LamG domain interacted with Cry11Aa toxin and, correspondingly, two loops of Cry11Aa toxin, are involved in toxin binding.