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  • Title: Structural reorganization of the acetylcholine binding site of the torpedo nicotinic receptor as revealed by dynamic photoaffinity labeling.
    Author: Grutter T, Bertrand S, Kotzyba-Hibert F, Bertrand D, Goeldner M.
    Journal: Chembiochem; 2002 Jul 02; 3(7):652-8. PubMed ID: 12324999.
    Abstract:
    We explored the structural changes that occur at the acetylcholine binding site of the Torpedo marmorata nicotinic receptor during activation by the tritiated photoactivatable agonist (diazocyclohexadienoylpropyl)trimethylammonium ([(3)H]DCTA). We quantified the incorporation of radioactivity into the receptor subunits as a function of the mixing time of [(3)H]DCTA with the receptor by using a rapid-mixing device adapted with a photochemical quenching system. A saturable increase of the specific photolabeling on the alpha and gamma subunits was observed with a half-time of about 2 minutes. We further analyzed this photoincorporation either after rapid mixing for 500 ms or after equilibration for 50 minutes. Under these conditions, [(3)H]DCTA explored transient state(s) and the stable desensitized state, respectively. Comparative analyses showed that at a probe concentration of 10 microM the relative variation of photoincorporation was more pronounced for the gamma subunit (three- to fourfold) than for the alpha subunit (about twofold). By contrast, the relative distribution of radioactivity among alpha-subunit labeled residues (alphaTyr190, alphaCys192, alphaCysC193, and alphaTyr198) did not change. Altogether, these results reveal that during the course of agonist-induced receptor desensitization, the site-lining peptide loops, which belong to adjacent alpha and gamma subunits, move closer to each other.
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