Glycine receptors (GlyR) are the primary inhibitory receptors in the spinal cord and belong to a superfamily of ligand-gated ion channels (LGICs) that are extremely sensitive to low-affinity neurological agents such as general anesthetics and alcohols. The high-resolution pore architecture and the gating mechanism of this superfamily, however, remain unclear. The pore-lining second transmembrane (TM2) segments of the GlyR α1 subunit are unique in that they form functional homopentameric channels with conductance characteristics nearly identical to those of an authentic receptor (Opella, S. J., J. Gesell, A. R. Valente, F. M. Marassi, M. Oblatt-Montal, W. Sun, A. F. Montiel, and M. Montal. 1997. Chemtracts Biochem. Mol. Biol. 10:153-174). Using NMR and circular dichroism (CD), we determined the high-resolution structures of the TM2 segment of human α1 GlyR and an anesthetic-insensitive mutant (S267Y) in dodecyl phosphocholine (DPC) and sodium dodecyl sulfate (SDS) micelles. The NMR structures showed right-handed α-helices without kinks. A well-defined hydrophilic path, composed of side chains of G2′, T6′, T10′, Q14′, and S18′, runs along the helical surfaces at an angle ∼10-20° relative to the long axis of the helices. The side-chain arrangement of the NMR-derived structures and the energy minimization of a homopentameric TM2 channel in a fully hydrated DMPC membrane using large-scale computation suggest a model of pore architecture in which simultaneous tilting movements of entire TM2 helices by a mere 10° may be sufficient to account for the channel gating. The model also suggests that additional residues accessible from within the pore include L3′, T7′, T13′, and G17′. A similar pore architecture and gating mechanism may apply to other channels in the same superfamily, including GABAA, nACh, and 5-HT3 receptors.
Tang, P., Mandal, P. K., & Xu, Y. (2002). NMR structures of the second transmembrane domain of the human glycine receptor α1 subunit: Model of pore architecture and channel gating. Biophysical Journal, 83(1), 252–262. https://doi.org/10.1016/S0006-3495(02)75166-7