TY - JOUR T1 - Activation of Guanosine 5′-[γ-<sup>35</sup>S]thio-triphosphate Binding through M<sub>1</sub> Muscarinic Receptors in Transfected Chinese Hamster Ovary Cell Membranes: 2. Testing the “Two-States” Model of Receptor Activation JF - Molecular Pharmacology JO - Mol Pharmacol SP - 886 LP - 893 DO - 10.1124/mol.59.4.886 VL - 59 IS - 4 AU - Magali Waelbroeck Y1 - 2001/04/01 UR - http://molpharm.aspetjournals.org/content/59/4/886.abstract N2 - I suggested in the accompanying article [Mol Pharmacol2001;59:875–885] that muscarinic receptors catalyzed G protein activation. Acetylcholine or carbamylcholine recognition facilitated not only the GDP release from receptor-coupled inactive G proteins but also the release ofG GTPγS* from the (unstable) HRG GTPγS* complex. The two effects facilitated [35S]GTPγS binding in the presence of GDP, but could be studied separately by comparing [35S]GTPγS binding in the absence and presence of GTP. Guanyl nucleotides affected the efficiency of receptor-G protein coupling. The relative efficacies of partial agonists in the absence and presence of GTP should remain nonlinearly correlated if all agonists stabilize (to different extents) the same active receptor conformation. The correlation between M1 muscarinic agonists' efficacy in accelerating [35S]GTPγS binding in the absence of other nucleotides and their in vivo efficacy (inositol phosphate accumulation) was in fact very poor. This probably reflected the presence of GTP in intact cells: pertussis toxin pretreatment (which inactivates the Gi/o proteins) did not affect the agonists' efficacy profile (evaluated in the absence of spare receptors), but the addition of GTP to the [35S]GTPγS binding medium did. These results did not support the allosteric “two states” model of receptor activation, but suggested that different agonists induced different receptor conformations (“induced fit”). ER -