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Exploration of receptor binding of Bacillus thuringiensis toxins
Abstract:
Wild type and mutant toxins of Bacillus thuringiensis delta-endotoxins were examined for their binding to midgut brush border membrane vesicles (BBMV). CryIAa, CryIAb, and CryIAc were examined for their binding to Gypsy moth (Lymantria dispar) BBMV. The binding of CryIAa and CryIAc was directly correlated with their toxicity, while CryIAb was observed to have lower binding than expected from its toxicity. The latter observation confirms the observation of Wolfersberger (1990). The "rule" of reciprocity of binding and toxicity is apparently obeyed by CryIAa and CryIAc, but broken by CryIAb on L. dispar. Alanine substitutions were made in several positions of the putative loops of CryIAa to test the hypothesis that the loops are intimately involved in binding to the receptor. The mutant toxins showed minor shifts in heterologous binding to Bombyx mori BBMV, but not enough to conclude that the residues chosen play critical roles in receptor binding.
Insights
Bacillus thuringiensis delta-endotoxins CryIAa and CryIAc toxicity correlates with binding to Gypsy moth midgut vesicles. CryIAb showed lower binding than expected, breaking the binding-toxicity rule.
Area of Science:
- Insect toxicology
- Molecular biology
- Biochemistry
Background:
- Bacillus thuringiensis (Bt) delta-endotoxins are crucial insecticidal proteins.
- Understanding toxin-receptor interactions is key to Bt efficacy.
- Midgut brush border membrane vesicles (BBMV) are primary targets for Bt toxins.
Purpose of the Study:
- To investigate the binding characteristics of wild-type and mutant Bacillus thuringiensis delta-endotoxins.
- To determine the correlation between toxin binding and toxicity in Gypsy moth (Lymantria dispar).
- To test the role of specific loops in CryIAa toxin binding to receptors.
Main Methods:
- Binding assays using wild-type CryIAa, CryIAb, and CryIAc toxins with Lymantria dispar BBMV.
- Analysis of toxin binding affinity and correlation with known toxicity data.
- Site-directed mutagenesis of CryIAa to create alanine substitutions in putative receptor-binding loops.
- Heterologous binding assays of mutant CryIAa toxins to Bombyx mori BBMV.
Main Results:
- CryIAa and CryIAc binding to L. dispar BBMV directly correlated with their toxicity.
- CryIAb exhibited lower binding affinity than predicted by its toxicity, deviating from the binding-toxicity reciprocity rule.
- Mutant CryIAa toxins with alanine substitutions showed minimal changes in heterologous binding to Bombyx mori BBMV.
Conclusions:
- The binding-toxicity reciprocity rule is upheld by CryIAa and CryIAc but not by CryIAb in L. dispar.
- The specific loop residues investigated in CryIAa do not appear to play critical roles in receptor binding.
- Further research is needed to elucidate the precise mechanisms of CryIAb binding and the role of other structural elements.