Introduction and Overall Architecture

 

Introduction

GlyR, a postsynaptic ligand-gated channel receptor, is responsible for mediation of inhibitory synaptic transmission in the CNS. The importance of GlyR and its link to neurological disorders, such as hyperekplexia, led to the use of electron cryo-microscopy, demonstrating molecular mechanisms. GlyR was captured in complex with Glycine, Strychnine, and Glycine/Ivermectin. Glycine exerts an inhibitory effect on GlyR by permitting Cl- influx (leading to hyperpolarization), stabilizing the receptor in an agonist-bound open state. In contrast, strychnine is a competitive antagonist for GlyR that arrests the receptor in a closed state, precluding Cl- entry. Ivermectin, a macrocyclic lactone, in complex with glycine, adopts a partially open state and potentiates glycine induced currents via allosteric mechanisms. 

Structure determination and refinement

The glycine-, strychnine- and glycine/ivermectin- bound structures, captured with excellent stereochemistry, were of sufficient quality for whole receptor modelling (Figure 1). Strychnine binding was identified at the intersubunit neurotransmitter binding pocket, and the glycine/ivermectin form identified ivermectin to wedge between TMD subunit interfaces. However, density of glycine was not discernible in either structure, therefore the pore lining of the M2 helix is best resolved in the strychnine/ivermectin structures. 

Figure 1 | GlyR architecture. a–c, viewed parallel to the membrane plane (strychnine bound in blue, glycine bound in yellow, and glycine/ivermectin bound in pink). d-f, viewed from the extracellular side for visualisation of conformational state. Str is closed, Gly open, and Gly/Ivm intermediate. Residues 229Pro and 99Leu reside on the pore-lining M2 helix.

Overall architecture

GlyR adopts a pentametric assemblage of subunits around a central fivefold symmetric axis, each subunit resembling an upright left forearm. The pore in the strychnine-bound form is constricted with M2 helices perpendicular to the membrane. In the glycine-bound form, M2 helices rotate ACW, widening the extracellular opening. Ivermectin promotes M2 helices rotation CW in the intracellular half, constricting the opening, but the extracellular entry is left expanded.