Evaluation of a proposed mechanism of ligand-gated ion channel activation in the GABAA and glycine receptors.

Ligand-gated ion channels (LGICs) mediate rapid chemical neurotransmission in the mammalian brain. This gene superfamily includes the nicotinic acetylcholine (nAChR), GABA(A), 5-hydroxytryptamine type 3, and glycine receptors. Upon agonist binding these receptors undergo a rapid allosteric transition from the closed to open state. The molecular mechanism of coupling between agonist binding and channel gating remains poorly understood, in part due to the lack of a high-resolution structure of the entire receptor. Miyazawa, Fujiyoshi, and Unwin published a 4A resolution structure of the nAChR, and proposed that a single residue--valine 44 in Loop 2 of the extracellular domain--functions as a critical determinant of a "pin-into-socket" mechanism for receptor activation in nAChR. Here we examined whether this proposed "pin-into-socket" mechanism also contributes to channel activation in the GABA(A) and glycine receptors. We mutated residues corresponding to nAChR valine 44 in the GABA(A) (alpha(1) histidine 56 and beta(2) valine 53) and glycine (alpha(1) threonine 54) receptors. The results obtained in this study do not support a simple "pin-into-socket" mechanism of activation for the activation of GABA(A) and glycine receptors. This conclusion is consistent with other recent reports in which mutations of residues distributed throughout several loops of nAChR, GABA(A) and glycine receptors had large effects on gating behavior.