Ligand-induced motions in pentameric ligand-gated ion channels revealed by EPR spectroscopy

Abstract

Signaling in the brain depends on rapid opening and closing of pentameric ligand-gated ion channels (pLGICs). These proteins are the targets of various clinical drugs and, defects in their function is linked to a variety of diseases including myasthenia, epilepsy and sleep-disorders. While recent high-resolution structures of prokaryotic and eukaryotic pLGICs have shed light on the molecular architecture of these proteins, describing their conformational dynamics in physiological lipids is essential for understanding their function. Here, we used site-directed spin labeling electron paramagnetic resonance (SDSL EPR) spectroscopy and functional channels reconstituted in liposomes to reveal ligand-induced structural changes in the extracellular domain (ECD) of GLIC. Proton-activation caused an inward motion of labeled sites at the top of β-strands (β1, 2, 5, 6, 8) towards the channel lumen, consistent with an agonist-induced inward tilting motion of the ECD. Similar proton-dependent GLIC ECD motions were detected in the presence of a non-activating (gating deficient) mutation, suggesting that the inward tilting of the ECD does not accompany channel opening but is associated with an agonist-induced closed pre-activated channel state. These findings provide new insights into the protein dynamics underlying pLGIC gating transitions.

Mutational analysis to explore long-range allosteric couplings involved in a pentameric channel receptor pre-activation and activation

“…Remarkably, both trajectories show complex quaternary asymmetric reorganizations of ECD compaction. As stated above, ECD compaction is critically involved in pre-activation, a feature consistent with recent work by electron paramagnetic resonance (EPR) spectroscopy ( Tiwari et al, 2020 ) showing a proton-induced inward tilting motion of the ECDs, and recent cryoEM work showing marked structural flexibility of the ECD in the closed-channel state at pH 7 ( Rovsnik et al, 2021 ). Interestingly, fluorescence curves of pre-activation, especially the one of Bim136-Q101W, are endowed with markedly low cooperativities.…”

“…Remarkably, both trajectories show complex quaternary asymmetric reorganizations of ECD compaction. As stated above, ECD compaction is critically involved in pre-activation, a feature consistent with recent work by electron paramagnetic resonance (EPR) spectroscopy (Tiwari et al, 2020) showing a proton-induced inward tilting motion of the ECDs, and recent cryoEM work…”