Surfing the Conformational Wave of Acetylcholine Receptor Channel Gating

Abstract

The muscle nicotinic acetylcholine receptor channel (AChR) is a cylindrical allosteric membrane protein (∼120 x 60 Å Fig. 1) that adopts alternative quaternary conformations (“open” and “closed”) with different functional properties (ion-conducting and ion-impermeable, respectively). We have characterized, residue-by-residue, the dynamics of the conformational change associated with gating using the framework of linear free energy relationships (LFER). The sequence of molecular events that underlies the closed-to-open gating transition was inferred from kinetic measurements of the receptor at the single molecule level.

Specific regions of the AChR were perturbed using site-directed mutagenesis, changes in the membrane potential, or different agonists. Single-channel currents were recorded from cell-attached patches (Fig. 2). For the gain-of-function mutations, choline was used as the agonist because of its low efficacy. The opening rate constant was determined at a saturating concentration of agonist (for choline, 20 mM) in order to isolate gating from binding steps. to avoid bias introduced by fast channel blockade, the closing rate constant was measured at a low concentration (for choline, 200 μM). The diliganded channel opening (β) and closing (α) rate constants were estimated using the QuB suite of kinetic analysis programs. in general, a plot of the log rate constant vs. log equilibrium constant was linear.