Single amino acid mutations in the cadherin receptor from Heliothis virescens affect its toxin binding ability to Cry1A toxins

Bacillus thuringiensis Cry protein exerts its toxic effect through a receptor- mediated process. Both aminopeptidases and cadherin proteins were identified as putative Cry1A receptors from Heliothis virescens and Manduca sexta. The importance of cadherin was implied by its correlation with a Cry1Ac resistant H. virescens strain ( Gahan, L. J., Gould, F., and Heckel, D. G. ( 2001) Science 293, 857 - 860). In this study, the Cry1Ac toxinbinding region in H. virescens cadherin was mapped to a 40- amino- acid fragment, from amino acids 1422 to 1440. This site overlaps with a Cry1Ab toxin- binding site, amino acids 1363 - 1464 recently reported in M. sexta ( Hua, G., Jurat- Fuentes, J. L., and Adang, M. J. ( 2004) J. Biol. Chem. 279, 28051 - 28056). Further, feeding of the anti- H. virescens cadherin antiserum or the partial cadherins, which contain the toxin- binding region, in combination with Cry1Ab/ Cry1Ac reduced insect mortality by 25.5 - 55.6% to first instar H. virescens and M. sexta larvae, suggesting a critical function for this cadherin domain in insect toxicity. Mutations in this region, to which the Cry1Ac binds through its loop 3, resulted in the loss of toxin binding. For the first time, we show that the cadherin amino acids Leu(1425) and Phe(1429) are critical for Cry1Ac toxin interaction, and if substituted with charged amino acids, result in the loss of toxin binding, with a K-D of < 10(-5) M. Mutation of Gln(1430) to an alanine, however, increased the Cry1Ac affinity 10- fold primarily due to an increase on rate. The L1425R mutant can result from a single nucleotide mutation, CTG --> CGG, suggesting that these mutants, which have decreased toxin binding, may lead to Cry1A resistance in insects.