Binding of phylogenetically distant Bacillus thuringiensis Cry toxins to aBombyx mori aminopeptidase N suggests importance of Cry toxin's conserved structure in receptor binding

We investigated the binding proteins for three Cry toxins, Cry1Aa, Cry1Ac, and the phylogenetically distant Cry9Da, in the midgut cell membrane of the silkworm. In a ligand blot experiment, Cry1Ac and Cry9Da bound to the same 120-kDa aminopeptidase N (APN) as Cry1Aa. A competition experiment with th e ligand blot indicated that the three toxins share the same binding site o n several proteins. The values of the dissociation constants of the three C ry toxins and 120-kDa APN are as low as the case of other Cry toxins and re ceptors. These results suggest that distantly related Cry toxins bind to th e same site on the same proteins, especially with APN. We propose that the conserved structure in these three toxins includes the receptor-binding sit e.