Cross-linking of glycine receptor transmembrane segments two and three alters coupling of ligand binding with channel opening

Contact points between transmembrane segments (TMs) two and three of the glycine receptor are undefined and may play an important role in channel gating. We tested whether two amino acids in TM2 (S267) and TM3 (A288), known to be critical for alcohol and volatile anesthetic action, could cross-link by mutating both to cysteines and expressing the receptors in Xenopus laevis oocytes. In contrast with the wild-type receptor and single cysteine mutants, the S267C/A288C double mutant displayed unusual responses, including a tonic leak activity that was closed by strychnine and a run-down of the response upon repeated applications of glycine. We hypothesized that these characteristics were due to cross-linking of the two cysteines on opposing faces of these adjacent, alpha helical TMs. This would alter the movement of these two regions required for normal gating. To test this hypothesis, we used dithiothreitol to reduce the putative S267C-A288C disulfide bond. Reduction abolished the leak current and provided normal responses to glycine. Subsequent application of the cross-linking agent mercuric chloride caused the initial characteristics to return. These data demonstrate that S267 and A288 are near-neighbors and provide insight towards the location and role of the TM2-TM3 interface in ligand-gated ion channels.