Extended Data Figure 4: The constriction of the extracellular vestibule.

a, The structures of the extracellular vestibule of GLIC (pale green), GluCl (dark green) and Torpedo nAChR (pink) are superimposed to that of the 5-HT₃ receptor (blue). For clarity, only a small stretch of the β4–β5 loop is represented as ribbon for homologous structures. The side chains of D105 and K108 of the 5-HT₃ receptor, as well as the side chains of charged residues of the loop in superimposed structures, are represented as sticks. D97 of the Torpedo nAChR, which was shown to be an important determinant of conductance²⁴, is labelled. b, View parallel to the membrane plane of the same region. c, Sequence alignment for the superimposed structures, around D105 and K108. d, Ion-density distribution, for cations (left) and anions (right), integrated radially around the pore axis. The protein density and membrane are in grey.