Résumé
The binary toxin (Bin) from
Bacillus sphaericus exhibits a highly insecticidal activity against
Culex and
Anopheles mosquitoes. The cytotoxicity of Bin requires an interaction with a specific receptor present on the membrane of midgut epithelial cells in larvae. A direct correlation exists between binding affinity and toxicity. The toxin binds with high affinity to its receptor in its primary target,
Culex pipiens, and displays a lower affinity to the receptor in
Anopheles gambiae, which is less sensitive to Bin. Although the Bin receptor has previously been identified and named Cpm1 in
C. pipiens, its structure in
Anopheles remains unknown. In this study, we hypothesize that the
Anopheles Bin receptor is an ortholog of Cpm1. By screening the
Anopheles genomic database, we identified a candidate gene (
Agm3) which is expressed primarily on the surface of midgut cells in larvae and which functions as a receptor for Bin. A
Cpm1-like gene is also present in the Bin-refractory species
Aedes aegypti. Overall, our results indicate that the three mosquito genes examined share a very similar organization and are strongly conserved at the amino acid level, in particular in the NH
2-terminus, a region believed to contain the ligand binding site, suggesting that relatively few amino acids residues are critical for high affinity binding of the toxin.