Identification of a Bacillus thuringiensis Cry8Da toxin-binding glucosidase from the adult Japanese beetle, Popillia japonica

Cry8Da from Bacillus thuringiensis galleriae SDS-502 has insecticidal activity against both the larvae and adult Japanese beetle (Popillia japonica Newman). The receptor determines the specificity of the insecticidal activity of Cry proteins and hence, in order to reveal the mode of action of Cry toxin, receptor identification is a necessary step. However, a receptor for Cry8-type toxin has not been identified in the Scarabaeidae family of insects. Therefore, we aimed to identify the receptor of Cry8Da toxin in adult P. japonica BBMV. A ligand blot showed the Cry8Da toxin only bound to a 150kDa protein in the BBMV of adult P. japonica. In order to identify the Cry8Da toxin binding protein, it was purified by column chromatography and three internal amino acid sequences were determined. Two of the three internal amino acid sequences shared homology with Coleopteran β-glucosidases. In addition, the fraction containing the Cry8Da toxin binding protein had β-glucosidase activity but no aminopeptidase N and alkaline phosphatase activity, both of which are commonly reported as receptors for Cry toxins in Lepidopteran and Dipteran insects. The β-glucosidase homologous genes could be amplified by PCR using degenerate oligonucleotide primers designed from a conserved sequence of Coleopteran β-glucosidases and an internal amino acid sequence of the Cry8Da toxin binding protein. Taken together, the β-glucosidase in adult P. japonica BBMV is the receptor for B. thuringiensis Cry8Da toxin.