Acetylcholine receptor-alpha-bungarotoxin interactions: determination of the region-to-region contacts by peptide-peptide interactions and molecular modeling of the receptor cavity.

In previous studies from this laboratory, the binding regions of alpha-neurotoxins on human and Torpedo acetylcholine (AcCho) receptors (AcChoRs) and the binding regions for the receptor on the toxin were characterized with synthetic peptides of the respective molecules. In the present work, peptides representing the active regions of one molecule are each allowed to bind to each of the active-region peptides of the other molecule. Thus, the interaction of three alpha-bungarotoxin (alpha-BTX) synthetic loop peptides with four synthetic peptides representing the toxin-binding regions on human AcChoR permitted the determination of the region-region interactions between alpha-BTX and the human receptor. Based on the known three-dimensional structure of the toxin, the active peptides of the receptor were then assembled to their appropriate toxin-contact regions by computer model building and energy minimization. This allowed the three-dimensional construction of the toxin-binding cavity on human AcChoR. The cavity appears to be conical, 30.5 A in depth, involving several receptor regions that make contact with the alpha-BTX loop regions. One AcChoR region (within residues 125-136) involved in the binding to alpha-BTX also resides in a known AcCho-binding site, thus demonstrating in three dimensions a critical site involved in both AcCho activation and alpha-BTX blocking. The validity of this approach was first established for three of four peptides corresponding to regions on the beta chain of human hemoglobin involved in binding to the alpha chain. Thus, studying the interaction between peptides representing the binding regions of two protein molecules may provide an approach in molecular recognition by which the binding site on one protein can be described if the three-dimensional structure of the other protein is known.