A comparison of glycine- and ivermectin-mediated conformational changes in the glycine receptor ligand-binding domain

Glycine receptor chloride channels are Cys-loop receptor proteins that isomerize between a low affinity closed state and a high affinity ion-conducting state. There is currently much interest in understanding the mechanisms that link affinity changes with conductance changes. This essentially involves an agonist binding in the glycine receptor ligand-binding site initiating local conformational changes that propagate in a wave towards the channel gate. However, it has proved difficult to convincingly distinguish those agonist-induced domain movements that are critical for triggering activation from those that are simply local deformations to accommodate ligands in the site. We employed voltage-clamp fluorometry to compare conformational changes in the ligand-binding site in response to activation by glycine, which binds locally, and ivermectin, which binds in the transmembrane domain. We reasoned that ivermectin-mediated activation should initiate a conformational wave that propagates from the pore-lining domain towards the ligand-binding domain, eliciting conformational changes in those extracellular domains that are allosterically linked to the gate. We found that ivermectin indeed elicited conformational changes in ligand-binding domain loops C, D and F. This implies that conformational changes in these domains are important for activation. This result also provides a mechanism to explain how ivermectin potentiates glycine-induced channel activation.