a, b, Cavity gate in S6 (viewed from the extracellular side). c, Inner gate in S6 (viewed from the extracellular side). d, Amino acid sequence alignment of the gate-forming region of S6 of B3 from different species. Sequences and species abbreviations were obtained from the KEGG database (http://www.genome.ad.jp/kegg/). e-h, Electrostatic potential (calculated with APBS plug-in in PyMol) of the interior surface of the ion conduction pathway in apo A3/B3 (e, f), CNGA1 (g, PDB ID: 7LFT), and TAX-4 (h). Front halves of the channels were cut away and rear halves were overlayed with the SF and S6.