Figure 1. The X-ray crystal structure of the water-soluble hexameric WSHC6 and the ion conductivity of the 12-helix TMHC6 transmembrane channel.

(a) Superposition of backbones of the crystal structure (blue) and the design model (gray) of WSHC6. The C-ɑ RMSD between the crystal structure and the design model is 0.89 Å. (b) The cross section of the WSHC6 channel. A chain of water molecules (red) occupies the central pore (Extended Data Fig. 1e). (c) TMHC6 channel model. The permeation path, calculated by HOLE²⁰, is illustrated by the blue surface in the left panel. Constriction sites along the channel are the E-ring (E44), the K-rings (K65, K68), and two intervening L-rings (L51, L58). Right: the radius of the pore along the permeation path. (d) CD spectra and temperature melt (inset) of the TMHC6 channel. No apparent unfolding transition is observed up to 95°C. Re25°C spectrum is taken when the sample cools back to 25°C after the thermal melt scan. (e) AUC sedimentation-equilibrium curves at three different rotor speeds for TMHC6 yield a measured molecular weight of ~58 kDa consistent with the designed hexamer. “MW (D)” and “MW (E)” indicate the molecular weight of the oligomer design and that calculated from the experiment, respectively. (f) Conductivity in whole-cell patch-clamp experiment on insect cells expressing TMHC6. The channel blocker Gd³⁺ diminished ion conductance to that of untransfected cells. (g) TMHC6 has considerably higher conductance for K⁺ than Na⁺, Cs⁺, CH₃NH₃⁺ and Ba²⁺. 10 cells were measured for each permeant ion species and the mean (data points) and standard error of measurement (s.e.m., error bars) are plotted.