(a) Crystal structure of Ec2-S9 complex (PDB #5A43, re-refined) showing double-pore assembly, with F- ions in purple and subunits in cyan and yellow. Grey mesh marks surfaces of the bound S9 monobodies delineating the channel's aqueous vestibules. (b) Blow-up of an individual pore region, with coordinating Phe and polar track side chains indicated and colored according to the subunit from which they project. Crystallographic waters are shown as dotted spheres. (c) Space-fill representation of same region. (d) Bacterial Fluc sequence alignment spanning polar track residues (TM2-TM4), with color code indicated. Top two sequences (Ec2, Bpe) refer to structurally known homodimeric homologues. Known or surmised heterodimers are shown in six lower sequences (La1/2, Sa1/2, Mt1/2). Other sequences are taken arbitrarily from Swissprot for illustration.