The transition between the open and closed states of the pentameric ligand–gated ion channels Gloeobacter ligand–gated ion channel (GLIC) and glutamate-gated chloride channel (GluCl) identifies a hotspot of channel movement in the transmembrane domain (TM) at the top of TM2 and TM3. (A) Extracellular view of the GLIC receptor with two chains highlighted. Chain A is in yellow and chain B is in green. (B) A side view of the two chains A and B, viewed from the pore, in the direction indicated in the illustration in (A). This view is adopted in subsequent representations. (C) The open state of the GLIC channel (3EAM)¹⁸  with the cartoon color-coded to represent the movement of the Cα carbon atoms on channel closing. (D) The closed state of the GLIC channel (4LMK)¹⁷  with the cartoon color-coded to represent the movement of the Cα carbon atoms on channel opening. (E) The open state of the GluCl channel (3RHW)²⁰  with the cartoon color-coded to represent the movement of the Cα carbon atoms on channel closing. (F) The closed state of the GluCl channel (4TNV)¹⁹  with the cartoon color-coded to represent the movement of the Cα carbon atoms on channel opening.