Two subsites in the binding domain of the acetylcholine receptor: an aromatic subsite and a proline subsite.
The ligand binding site of the nicotinic acetylcholine receptor (AcChoR) is localized in the alpha-subunit within a domain containing the tandem Cys-192 and -193. By analyzing the binding-site region of AcChoR from animal species that are resistant to alpha-neurotoxins, we have previously shown that four residues in this region, at positions 187, 189, 194, and 197, differ between animals sensitive (e.g., mouse) and resistant (e.g., mongoose and snake) to alpha-bungarotoxin (alpha-BTX). In the present study, we performed site-directed mutagenesis on a fragment of the mongoose AcChoR alpha-subunit (residues 122-205) and exchanged residues 187, 189, 194, and 197, either alone or in combination, with those present in the mouse alpha-subunit sequence. Only the mongoose fragment in which all four residues were mutated to the mouse ones exhibited alpha-BTX binding similar to that of the mouse fragment. The mongoose double mutation in which Leu-194 and His-197 were replaced with proline residues, which are present at these positions in the mouse AcChoR and in all other toxin binders, bound alpha-BTX to approximately 60% of the level of binding exhibited by the mouse fragment. In addition, replacement of either Pro-194 or -197 in the mouse fragment with serine and histidine, respectively, markedly decreased alpha-BTX binding. All other mutations resulted in no or just a small increase in alpha-BTX binding. These results have led us to propose two subsites in the binding domain for alpha-BTX: the proline subsite, which includes Pro-194 and -197 and is critical for alpha-BTX binding, and the aromatic subsite, which includes amino acid residues 187 and 189 and determines the extent of alpha-BTX binding.
D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D001665:Binding Sites / Q000235:genetics; D002038:Bungarotoxins / Q000378:metabolism*; D004252:DNA Mutational Analysis; D004351:Drug Resistance; D008988:Herpestidae; D051379:Mice; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D018733:Nicotinic Antagonists / Q000378:metabolism*; D010446:Peptide Fragments / Q000494:pharmacology; D011978:Receptors, Nicotinic / Q000235:genetics / Q000378:metabolism*; D017386:Sequence Homology, Amino Acid; D013045:Species Specificity; D013329:Structure-Activity Relationship
D002038:Bungarotoxins; D018733:Nicotinic Antagonists; D010446:Peptide Fragments; D011978:Receptors, Nicotinic