Conformational variability of the glycine receptor M2 domain in response to activation by different agonists

Models describing the structural changes mediating Cys loop receptor activation generally give little attention to the possibility that different agonists may promote activation via distinct M2 pore-lining domain structural rearrangements. We investigated this question by comparing the effects of different ligands on the conformation of the external portion of the homomeric alpha1 glycine receptor M2 domain. Conformational flexibility was assessed by tethering a rhodamine fluorophore to cysteines introduced at the 19' or 22' positions and monitoring fluorescence and current changes during channel activation. During glycine activation, fluorescence of the label attached to R19'C increased by approximately 20%, and the emission peak shifted to lower wavelengths, consistent with a more hydrophobic fluorophore environment. In contrast, ivermectin activated the receptors without producing a fluorescence change. Although taurine and beta-alanine were weak partial agonists at the alpha1R19'C glycine receptor, they induced large fluorescence changes. Propofol, which drastically enhanced these currents, did not induce a glycine-like blue shift in the spectral emission peak. The inhibitors strychnine and picrotoxin elicited fluorescence and current changes as expected for a competitive antagonist and an open channel blocker, respectively. Glycine and taurine (or beta-alanine) also produced an increase and a decrease, respectively, in the fluorescence of a label attached to the nearby L22'C residue. Thus, results from two separate labeled residues support the conclusion that the glycine receptor M2 domain responds with distinct conformational changes to activation by different agonists.