Abstract
The glycine receptor (GlyR), a member of the pentameric ligand-gated ion channel family (pLGIC), displays remarkable variations in the affinity and efficacy of the full agonist glycine and the partial agonist taurine depending on the cell system used. Despite detailed insights in the GlyR three-dimensional structure and activation mechanism, little is known about conformational rearrangements induced by these agonists. Here, we characterized the conformational states of the α1 GlyR upon binding of glycine and taurine by microscale thermophoresis expressed in HEK293 cells and Xenopus oocytes after solubilization in amphipathic styrene-maleic acid copolymer nanodiscs. Our results show that glycine and taurine induce different conformational transitions of the GlyR upon ligand binding. In contrast, the variability of agonist affinity is not mediated by an altered conformational change. Thus, our data shed light on specific agonist induced conformational features and mechanisms of pLGIC upon ligand binding determining receptor activation in native environments.
Highlights
The glycine receptor (GlyR), a member of the pentameric ligand-gated ion channel family, displays remarkable variations in the affinity and efficacy of the full agonist glycine and the partial agonist taurine depending on the cell system used
The efficacy to open the ion channel depends on the agonist used, i.e. glycine acts as a full agonist inducing maximal current responses with opening times of 95–98% seen in single-channel recordings[9], whereas taurine acts as a partial agonist producing a decreased maximal response relative to the effect produced by glycine that is reflected by lower open-channel probabilities[10]
Despite several available crystal structures of glycine receptors[8,19] and extended kinetic models based on single-channel r ecordings[10,20], so far little is known about the specific conformational rearrangements of the GlyR during receptor activation induced by full and partial agonists and even less which conformations can be adopted during activation in different cell systems
Summary
The glycine receptor (GlyR), a member of the pentameric ligand-gated ion channel family (pLGIC), displays remarkable variations in the affinity and efficacy of the full agonist glycine and the partial agonist taurine depending on the cell system used. Despite several available crystal structures of glycine receptors[8,19] and extended kinetic models based on single-channel r ecordings[10,20], so far little is known about the specific conformational rearrangements of the GlyR during receptor activation induced by full and partial agonists and even less which conformations can be adopted during activation in different cell systems. Upon successful solubilization of α1 GlyR from HEK293 cells and X. laevis oocytes in nanodiscs we can show by MST that both the affinity and efficacy of the full agonist glycine to induce a conformational change was identical in both expression systems the respective electrophysiological E C50 values differed by an order of magnitude. We provide experimental evidence for the underlying mechanism of partial agonism at the GlyR and that variations in E C50 values observed in different expression systems are likely mediated by an impaired ability of the receptor to open the channel once the agonist has bound
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