The Bacillus thuringiensis Cry1Ac δ-endotoxin was shown to bind in a biphasic manner to Manduca sexta aminopeptidase N (APN) present in a novel model membrane. Surface plasmon resonance analysis allowed the quantification of toxin binding to M. sexta APN in a supported lipid monolayer. The initial binding was rapid and reversible, with an affinity constant of 110 nM. The second phase was slower and resulted in an overall affinity constant of 3.0 nM. Reagents used to disrupt protein-protein interactions did not dissociate the toxin after highaffinity binding was attained. The initial association between Cry1Ac and APN was inhibited by the sugar GalNAc, but the