Gating Phi Values in the Muscle Acetylcholine Receptor αM2-αM3 Linker with vs. without Agonists

Abstract

Acetylcholine receptors are ligand gated ion channels that transduce a chemical signal into an electrical signal at the neuromuscular junction (α2βδe subunits arranged around a central pore). A global, reversible allosteric transition between stable closed- and open-channel states ('gating') takes place either in the presence or absence of agonists. Phi (the slope a log-log plot of channel opening rate constant vs. equilibrium constant for a series of mutation of one residue) gives the extent of the perturbed residue's progress in the transition state of the gating reaction, on a scale 1 to 0. Previously-estimated diliganded gating phi values in the α subunit decrease approximately linearly between the transmitter binding site (∼0.95, same as for the agonist), through the extracellular domain (0.8), through M2 (0.6) to the gate region (∼0.3). Some residues away from the binding site have the same phi value without or without agonists (Purohit and Auerbach, PNAS, 2007). However, unliganded phi values for eight binding site residues (αY93, αW149, αY190, αY198, αG147, αG153, eP121, δP123; 0.77±0.023) are lower than the corresponding diliganded values (0.94±0.017). We estimated di- and un-liganded phi gating values for four positions in the αM2-αM3 linker (αI260, αE262, αP265, αS268), another hi-phi domain that is apparently not connected by a high-phi pathway to the binding sites. The unliganded phi values for these four positions (0.88±0.022) were about the same as in diliganded gating (0.90±0.031). So far the mutability in phi appears to pertain only to the binding site, perhaps because the affinity change for the agonist contributes to the gating energy only when ligands are present.