title: Agonist-induced conformational changes in the extracellular domain of alpha 7 nicotinic acetylcholine receptors.
abstract: The molecular mechanisms that couple agonist binding to the gating of Cys-loop ionotropic receptors are not well understood. The crystal structure of the acetylcholine (ACh) binding protein has provided insights into the structure of the extracellular domain of nicotinic receptors and a framework for testing mechanisms of activation. Key ligand binding residues are located at the C-terminal end of the beta9 strand. At the N-terminal end of this strand (loop 9) is a conserved glutamate [E172 in chick alpha7 nicotinic acetylcholine receptors (nAChRs)] that is important for modulating activation. We hypothesize that agonist binding induces the movement of loop 9. To test this, we used the substituted-cysteine accessibility method to examine agonist-dependent changes in the modification of cysteines introduced in loop 9 of L247T alpha7 nAChRs. In the absence of agonist, ACh-evoked responses of E172C/L247T alpha7 nAChRs were inhibited by 2-trimethylammonioethylmethane thiosulfonate (MTSET). Agonist coapplication with MTSET reduced the extent and rate of modification. The dose-dependence of ACh activation was nearly identical with that of ACh-dependent protection from modification. ACh increased the inhibition by methanethiosulfonate reagents of N170C and did not change inhibition of G171C receptors. The antagonist dihydro-beta-erythroidine did not mimic the effects of ACh. Combined with a structural model, the data suggest that receptor activation includes subunit rotation and/or intrasubunit conformational changes that move N170 to a more accessible position and E172 to a more protected position away from the vestibule. Thus, loop 9, located near the junction between the extracellular and transmembrane domains, participates in conformational changes triggered by ligand binding.
authors.affiliation: Department of Pharmacology, CB #7365, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-7365, USA.
authors.forename: Lisa K
authors.initials: LK
authors.lastname: Lyford
authors.forename: Adrian D
authors.initials: AD
authors.lastname: Sproul
authors.forename: Donnie
authors.initials: D
authors.lastname: Eddins
authors.forename: James T
authors.initials: JT
authors.lastname: McLaughlin
authors.forename: Robert L
authors.initials: RL
authors.lastname: Rosenberg
mesh_terms: D000109:Acetylcholine / Q000378:metabolism / Q000494:pharmacology*; D000818:Animals; D002645:Chickens; D004305:Dose-Response Relationship, Drug; D005110:Extracellular Space / Q000737:chemistry / Q000187:drug effects* / Q000378:metabolism; D005260:Female; D018722:Nicotinic Agonists / Q000378:metabolism / Q000494:pharmacology; D009865:Oocytes; D011485:Protein Binding / Q000187:drug effects / Q000502:physiology; D011978:Receptors, Nicotinic / Q000737:chemistry* / Q000378:metabolism*; D014982:Xenopus laevis; D064569:alpha7 Nicotinic Acetylcholine Receptor
chemical_list: D018722:Nicotinic Agonists; D011978:Receptors, Nicotinic; D064569:alpha7 Nicotinic Acetylcholine Receptor; D000109:Acetylcholine