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Direct Spectroscopic Studies of Cation Translocation by Torpedo Acetylcholine Receptor on a Time Scale of Physiological Relevance
Hsiao-Ping H. Moore and Michael A. Raftery
Proceedings of the National Academy of Sciences of the United States of America
Vol. 77, No. 8, [Part 2: Biological Sciences] (Aug., 1980), pp. 4509-4513
Published by: National Academy of Sciences
Stable URL: http://www.jstor.org/stable/9133
Page Count: 5
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The kinetics of carbamoylcholine-mediated cation transport across the membrane of vesicles containing acetylcholine receptor have been measured on the physiologically relevant time scale of a few milliseconds. The stopped-flow spectroscopic approach utilizes thallium(I) as the cation transported into sealed vesicles containing a water-soluble fluorophore. Upon entry of thallium(I), fluorescence quenching occurs by a heavy atom effect. Rapid thallium translocation into the vesicles is mediated by cholinergic agonists and is blocked by antagonists and neurotoxins and by desensitization. The kinetics of thallium transport are used to demonstrate that the four polypeptides known to comprise the receptor are the only protein components necessary for cation translocation. The kinetics of thallium(I) transport at saturating agonist concentrations are also used to calculate the apparent ion transport rate for a single receptor. The minimal value obtained is close to that for a single activated channel determined in vivo. This demonstrates that the physiological receptor has been isolated in intact form.
Proceedings of the National Academy of Sciences of the United States of America © 1980 National Academy of Sciences