Effects of Midgut-Protein-Preparative and Ligand Binding Procedures on the Toxin Binding Characteristics of BT-R1, a Common High-Affinity Receptor in Manduca sexta for Cry1A Bacillus thuringiensis Toxins

The identity of the physiologically important Cry1A receptor protein(s) in the lepidopteran Manduca sexta has been a matter of dispute due to the multiple proteins which bind the Cry1Ac toxin. Cry1Aa, Cry1Ab, and Cry1Ac exhibit essentially identical toxicities toward M. sexta larvae and show a high degree of sequence and presumed structural identities. These similarities make it likely that there is a common mechanism of toxicity in these lepidopteran-specific toxins in terms of both mode of action and the receptor proteins through which these toxins exert their lepidopteran-specific toxicity. Investigators in our laboratory previously demonstrated that the cloned 210-kDa glycoprotein BT-R₁ binds all three Cry1A toxins (T. P. Keeton and L. A. Bulla, Jr., Appl. Environ. Microbiol. 63:3419–3425, 1997). This protein remains a common binding protein even after being subjected to various midgut membrane preparation and processing protocols. The method used to isolate proteins from the M. sexta larval midgut in no significant way affects the results of ligand binding and vacuum blotting experiments, and we have been unable to detect specific, high-affinity binding of any Cry1A toxin to Cry1Ac binding proteins other than BT-R₁. Alterations in blot substrate and blocking, hybridization, and washing buffers support these conclusions. Collectively, these results indicate that in M. sexta the cadherin-like BT-R₁ protein is a common high-affinity receptor protein for the Cry1A family of toxins.