Determination of Receptor Binding Properties of Bacillus thuringiensis δ-Endotoxins to Cotton Bollworm (Helicoverpa zea) and Pink Bollworm (Pectinophora gossypiella) Midgut Brush Border Membrane Vesicles

Abstract

Pesticidal activity and receptor binding properties of Bacillus thuringiensis toxins to cotton pink bollworm (Pectinophora gossypiella) and cotton bollworm (Helicoverpa zea) were investigated. P. gossypiella was susceptible to Cry1Aa, Cry1Ab, Cry1Ac, and Cry2Aa toxins. To H. zea, Cry1Ac and Cry1Ab were more potent than Cry1Aa and Cry2Aa. Cry1Ba, Cry1Ca, Cry1Da, Cry1Ea, Cry1Fa, Cry1Ga, Cry1Ha, and Cry2Ba were not potent against both pests. Binding assays were performed with 125I-labeled toxins (Cry1Aa, Cry1Ab, Cry1Ac, and Cry2Aa) and brush border membrane vesicles (BBMVs) prepared from H. zea and P. gossypiella midguts. Both Cry1Ab and Cry1Ac toxins showed saturable, high-affinity binding to P. gossypiella and H. zea BBMVs. Cry2Aa and Cry1Aa toxins bound to BBMVs with relatively low binding affinity but with high binding site concentration. Heterologous competition binding assays were performed to investigate the binding site cross reactivity. The results showed that Cry1Aa, Cry1Ab, and Cry1Ac recognize the same binding site, which is different from Cry2Aa. Ligand blot assay showed that Cry1Ac toxin binds to a 120-kDa BBMV protein in P. gossypiella and Cry1Ab binds to a major 210-kDa protein.