Molecular Basis for Bacillus thuringiensis Cry1Ab Toxin Specificity: Two Structural Determinants in the Manduca sexta Bt-R1 Receptor Interact with Loops α-8 and 2 in Domain II of Cy1Ab Toxin

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Abstract
The identification of epitopes involved in Cry toxin−receptor interaction could provide insights into the molecular basis of insect specificity and for designing new toxins to overcome the potential problem of insect resistance. In previous works, we determined that the Manduca sexta Cry1A cadherin-like receptor (Bt-R1) interacts with Cry1A toxins through epitope 865NITIHITDTNN875 and by loop 2 of domain II in the toxin (Gomez, I., Miranda-Rios, J., Rudiño-Piñera, E., Oltean, D. I., Gill, S. S., Bravo, A., and Soberón, M. (2002) J. Biol. Chem. 277, 30137−30143.). In this work, we narrowed to 12 amino acids a previously identified Bt-R1 66 amino acids epitope (Dorsch, J. A., Candas, M., Griko, N. B., Maaty, W. S. A., Midbo, E. G., Vadlamudi, R. K., and Bulla, L. A., Jr. (2002) Insect Biochem. Mol. Biol. 32, 1025−1036) and identified loop α-8 of Cry1Ab domain II as its cognate binding epitope. Two amino acid Bt-R1 toxin binding regions of 70 residues, one comprised of residues 831−900 containing the 865NITIHITDTNN875 epitope (TBR1) and the other comprised of residues 1291−1360 (TBR2) were cloned by RT-PCR and produced in Escherichia coli. Cry1A toxins bind with the two TBR regions in contrast with the nontoxic Cry3A toxin. The loop 2 synthetic peptide competed with the binding of Cry1Ab toxin to both TBR regions in contrast to the α-8 synthetic peptide that only competed with Cry1Ab binding to TBR2. Western blots and competition ELISA analysis showed that the Cry1Ab loop 2 RR368-9EE mutant did not show observable binding to TBR1 but still bound the TBR2 peptide. This result suggests that loop α-8 interacts with the TBR2 region. Competition ELISA analysis of Cry1Ab binding to the two TBR peptides revealed that the toxin binds the TBR1 region with 6-fold higher affinity than the TBR2 region. The amino acid sequence of TBR2 involved on Cry1Ab interaction was narrowed to 12 amino acids, 1331IPLPASILTVTV1342, by using synthetic peptides as competitors for Cry1Ab binding to Bt-R1. Our results show that the specificity of Cry1A involves at least two structural determinants on both molecules.

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