Domain III exchanges of Bacillus thuringiensis CryIA toxins affect binding to different gypsy moth midgut receptors

Aminopeptidase-N, purified from gypsy moth (Lymantria dispar L.) brush border membrane vesicles, exhibited specific binding to CryIAc toxin but not to CryIAa toxin. CryIAa-CryIAc hybrid toxins were used to localize the aminopeptidase-N binding region on CryIAc. Slot blot assays and ligand blot experiments demonstrated that the hybrid toxins which have the residues 451 to 623, comprising essentially domain III, from CryIAc toxin exhibited strong binding to purified aminopeptidase-N and 120 kDa brush border membrane protein. In contrast, the hybrid toxins which have the residues 451 to 623 from CryIAa toxin failed to bind to aminopeptidase-N, but did bind to another receptor, a 210 kDa protein. This is the first direct evidence that domain III is involved in receptor binding and the first to demonstrate that domain III substitutions direct the binding of these toxins to different gypsy moth midgut receptors.