Conformational Dynamics of the αM3 Transmembrane Helix during Acetylcholine Receptor Channel Gating

Muscle acetylcholine receptors are synaptic ion channels that “gate” between closed- and open-channel conformations. We used Φ-value analysis to probe the transition state of the diliganded gating reaction with regard to residues in the M3, membrane-spanning helix of the muscle acetylcholine receptor α-subunit. Φ (a fraction between 1 and 0) parameterizes the extent to which a mutation changes the opening versus the closing rate constant and, for a linear reaction mechanism, the higher the Φ-value, the “earlier” the gating motion. In the upper half of αM3 the gating motions of all five tested residues were temporally correlated (Φ ≈ 0.30) and serve to link structural changes occurring at the middle of the M2, pore-lining helix with those occurring at the interface of the extracellular and transmembrane domains. αM3 belongs to a complex and diverse set of synchronously moving parts that change structure relatively late in the channel-opening process. The propagation of the gating Brownian conformational cascade has a complex spatial distribution in the transmembrane domain.