Putative iron-transport channels. (a) The fivefold axis channel consists of His187 and Tyr188. The left image shows the top view looking at the channel from the exterior into the interior of TmEnc and the right image is a cut-through side view. Density corresponding to two solvent molecules or ions is present in the channel. Above the two molecules are fivefold-symmetric elongated densities that could not be identified confidently. (b, c) Radius and hydrophobicity plots using data from CHAP of (b) the TmEnc fivefold axis channel and (c) the ferritin threefold axis channel as a function of probe position s. More negative s values correspond to positions closer to the exterior and vice versa. Hydrophobicity is measured with the Wimley–White scale (Wimley & White, 1996). Scales for each graph are identical for comparison. (d) The left image shows the threefold axis channel as viewed from the interior of TmEnc looking outwards. The right image is a cut-through side view. The threefold axis channel is formed by residues Asp236, Arg237, Pro10, Tyr234 and Phe8. Density for solvent (red spheres) and chloride ions (green spheres) is present in the channel (Skitchenko et al., 2020). (e, f) Radius and hydrophobicity plots of (e) the TmEnc threefold axis channel and (f) the ferritin fourfold axis channel as in (b) and (c). (g) Channels in the dimer interface are indicated in boxes, with water molecules shown as red spheres. Residues within 5 Å of the channel and their side-chain densities from the cryo-EM map are shown. Red spheres represent water molecules.