Quinacrine noncompetitive inhibitor binding site localized on the Torpedo acetylcholine receptor in the open state.
Open-channel blockers of the nicotinic acetylcholine receptor (nAcChR) are widely thought to act sterically by entering and "plugging" the open channel of the nAcChR. However, quinacrine, a fluorescent open-channel blocker, has been recently shown to bind to the nAcChR at a site near the lipid bilayer while the receptor is in a closed, desensitized state, suggesting that at least one open-channel blocker might act allosterically outside the channel [Valenzuela et al. (1992) J. Biol. Chem. 267, 8238]. To determine whether or not quinacrine also binds near the lipid bilayer when the receptor is in an open state, a short-range lipophilic quencher (5-doxylstearate, 5-SA) was used to assess the proximity of the nAcChR-bound quinacrine to the lipid bilayer while the receptor was transiently open by an agonist. Initial experiments using a stopped-flow instrument established the conditions required to monitor a portion of the changes in quinacrine fluorescence associated with its binding to the receptor in the open state. 5-SA (80 microM) reduced the amplitude of the rapid agonist-induced change in quinacrine emission to 44% +/- 12% of the control value, indicating that the quinacrine was binding to a site proximal to the membrane-partitioned 5-SA. Control experiments established that 5-SA had no effect on the ability of the receptor to undergo agonist-induced conformational changes, suggesting that little, if any, 5-SA distributed into the channel lumen and perturbed the functional activity of the receptor. Together, the results indicate that quinacrine binds to a site on the open receptor that is in contact with the lipid bilayer and not in the channel lumen.
Johnson
D A
DA
Division of Biomedical Sciences, University of California, Riverside 92521-0121, USA.
Ayres
S
S
D000818:Animals; D001665:Binding Sites; D002217:Carbachol / Q000494:pharmacology; D003497:Cyclic N-Oxides / Q000494:pharmacology; D003619:Dansyl Compounds / Q000378:metabolism; D004557:Electric Organ / Q000378:metabolism; D005456:Fluorescent Dyes / Q000378:metabolism; D066298:In Vitro Techniques; D007700:Kinetics; D008051:Lipid Bilayers / Q000378:metabolism; D018733:Nicotinic Antagonists / Q000378:metabolism / Q000494:pharmacology; D011487:Protein Conformation; D000644:Quaternary Ammonium Compounds / Q000378:metabolism; D011796:Quinacrine / Q000378:metabolism* / Q000494:pharmacology; D011978:Receptors, Nicotinic / Q000737:chemistry* / Q000378:metabolism*; D013113:Spin Labels; D014101:Torpedo
D003497:Cyclic N-Oxides; D003619:Dansyl Compounds; D005456:Fluorescent Dyes; D008051:Lipid Bilayers; D018733:Nicotinic Antagonists; D000644:Quaternary Ammonium Compounds; D011978:Receptors, Nicotinic; D013113:Spin Labels; C005072:5-doxylstearic acid; C027933:(1-(5-dimethylaminonaphthalene)sulfonamido)-n-hexanoic acid-beta-N-trimethylammonium ethyl ester; D002217:Carbachol; D011796:Quinacrine