Contributed Lecture
Subunit Stoichiometry and Organization in a Heteromeric Glutamate-Gated Chloride Channel

The invertebrate chloride-selective glutamate receptors (GluClRs) are ion channels serving as targets for ivermectin, a broad-spectrum anthelmintic drug used to treat human parasitic diseases like river blindness and lymphatic filariasis. The native GluClR is a heteropentamer consisting of α and β subunit types, with as yet unknown stoichiometry and organization. Based on the recent crystal structure of a homomeric GluClαR (1), we introduced mutations at the intersubunit interfaces where glutamate (the neurotransmitter) binds. By electrophysiological characterization of these mutants, we found heteromeric assemblies with two equivalent glutamate-binding sites at β/α intersubunit interfaces, where the GluCl β and α subunits respectively contribute the "principal" and "complementary" components to the putative Glu-binding pocket. We identified a mutation in the ivermectin-binding site (far away from the glutamate-binding site) that significantly increased the sensitivity of the heteromeric mutant receptor to both glutamate and ivermectin. This mutation also improved the receptor subunits` cooperativity. We further characterized this heteromeric GluClR mutant as a receptor having a third glutamate-binding site at an α/α intersubunit interface. Altogether, our experimental data and its analysis using the Monod-Wyman-Changeux (MWC) theory disclose the subunit organization and the stoichiometry of the glutamate-binding sites in a native-like heteromeric GluClα/βR (2).

(1) Hibbs R. E. and Gouaux E. (2011) Principles of activation and permeation in an anion-selective Cys-loop receptor. Nature 474:54-60.
(2) Degani-Katzav N., Gortler R., Gorodetzki L. and Paas Y. (2015) Subunit stoichiometry and organization in a heteromeric glutamate-gated chloride channel. PNAS, under revision.