The receptor of Bacillus sphaericus binary toxin in Culex pipiens (Diptera: Culicidae) midgut: Molecular cloning and expression

Abstract

Culex pipiens larval midgut is the primary target of the binary toxin (Bin) present in parasporal inclusions of Bacillus sphaericus. Cpml, a 60-kDa protein purified from brush border membranes, has been proposed as the receptor of the Bin toxin in the midgut epithelial cells of mosquitoes. We have cloned and characterized the corresponding cDNA from midgut of Culex pipiens larvae. The open reading frame predicted a 580 amino-acid protein with a putative signal peptide at the N-terminus and a putative GPI-anchoring signal at the C-terminus. The amino acid sequence of the cloned Cpml exhibited 39-43% identities with insect maltases (α-glucosidases and α-amylases). Recombinant Cpml expressed in E. coli specifically bound to the Bin toxin and had a significant α-glucosidase activity but no α-amylase activity. These results support the view that Cpml is an α-glucosidase expressed in Culex midgut where it constitutes the receptor for the Bin toxin. To date, this is the first component involved in the mosquitocidal activity of the Bacillus sphaericus Bin toxin to be characterized. Its identification provides a key step to elucidate the mode of action of the Bin toxin and the mechanisms of resistance developed against it by some mosquito strains. © 2001 Elsevier Science Ltd.