Extended Data Figure 6: Comparison of the MA and M4 helices of the 5-HT₃ and nACh receptors.

a, Superimposition of the 5-HT₃ receptor (grey) and Torpedo nAChR (orange, α-subunit, 2BG9 chain D, r.m.s.d. = 3.7 Å), the 5 MA–M4 helices are shown as ribbon (for the superimposed subunit) or cartoon. b, Enlarged view of the superimposed subunit, showing as sticks the arginine triplet, the conserved W426 and D434 of the 5-HT₃ receptor and equivalent residues of the α-nACh receptor. There is an offset of −1 residue at the level of W426 and +1 residue at the level of D434 (the change in offset is due to the break of the helical structure in Torpedo nAChR between MA and M4). c, Superimposition of all Torpedo nAChR subunits with the 5-HT₃ receptor. The conserved tryptophan and aspartate residues are at equivalent positions in all Torpedo nAChR subunits. d, Sequence alignment showing the conserved positions in purple. Bars indicate the offset in structure superpositions. e, f, Two orthogonal views of the superimposition with proteins depicted as cartoons, illustrating the global conformations of the intracellular domain.