title: Location of ligand-binding sites on the nicotinic acetylcholine receptor alpha-subunit.
abstract: The portions of the Torpedo californica nicotinic acetylcholine receptor (AChR) alpha-subunit that contribute to the allosteric antagonist-binding site and to the agonist-binding site have been localized by affinity labeling and proteolytic mapping. [3H]Meproadifen mustard was employed as an affinity label for the allosteric antagonist-binding site and [3H]tubocurare as a photoaffinity label for the agonist-binding site. Both labels were found in a 20-kDa proteolytic fragment generated from the AChR alpha-subunit by Staphylococcus aureus V8 protease. This 20-kDa peptide also contains the 3H-labeled 4-(N-maleimido)-alpha-benzyltrimethylammonium iodide-reactive site and binds 125I-alpha-bungarotoxin. N-terminal sequencing established that the 20-kDa fragment began at Ser-173 of the alpha-subunit. Fluorescein isothiocyanate-conjugated concanavalin A could be bound to the second of the two major V8 cleavage products, an 18-kDa peptide. This peptide was also sensitive to treatment with endo-beta-N-acetyl-glucosaminidase H, consistent with the presence of N-linked carbohydrate on this fragment. The N terminus of this peptide was found to be Val-46 of the alpha-subunit sequence. Experiments designed to map disulfide bonds within the AChR alpha-subunit indicate that no bonds exist between the 18-kDa fragment (containing Cys-128 and Cys-142) and the 20-kDa fragment (containing Cys-192, Cys-193, and Cys-222). These results establish that the 20-kDa fragment contributes to both the acetylcholine and the allosteric antagonist-binding sites, whereas there is no evidence that the 18-kDa fragment is part of either site.
mesh_terms: D000118:Acetylglucosaminidase; D000818:Animals; D001665:Binding Sites; D003208:Concanavalin A / Q000031:analogs & derivatives; D004557:Electric Organ / Q000378:metabolism; D010450:Endopeptidases; D016650:Fluorescein-5-isothiocyanate / Q000031:analogs & derivatives*; D005452:Fluoresceins; D008024:Ligands; D046911:Macromolecular Substances; D017038:Mannosyl-Glycoprotein Endo-beta-N-Acetylglucosaminidase; D008666:Metalloendopeptidases; D010449:Peptide Mapping; D011978:Receptors, Nicotinic / Q000302:isolation & purification / Q000378:metabolism*; D012697:Serine Endopeptidases; D014101:Torpedo
chemical_list: D005452:Fluoresceins; D008024:Ligands; D046911:Macromolecular Substances; D011978:Receptors, Nicotinic; C042784:fluorescein isothiocyanate-concanavalin A; D003208:Concanavalin A; D000118:Acetylglucosaminidase; D017038:Mannosyl-Glycoprotein Endo-beta-N-Acetylglucosaminidase; D010450:Endopeptidases; D012697:Serine Endopeptidases; C028344:glutamyl endopeptidase; D008666:Metalloendopeptidases; C022653:auR protein, Staphylococcus aureus; D016650:Fluorescein-5-isothiocyanate