Fig. 1.

The network of hydrogen bonds that tether the selectivity filter to the pore helix and involve residues E71, D80, Y78, G77, T75, and W67 in KcsA are shown. (A) Shows the high K⁺, conductive state (PDB 1K4C). (B) Shows the collapsed, low K⁺ state (PDB 1K4D). Notice that although the E71–D80–W76 hydrogen bond network is generally maintained between the two states, the geometry of the network between E71 and D80 becomes more compact and the hydrogen bond Y78 N–E71 CO2 becomes much weaker at low potassium. A water molecule between E71 and D80 is included in the majority (approximately 80%) of all crystal structures of WT-KcsA with a resolution lower than 2.5 Å. Heavy-atom distances corresponding to the hydrogen bonds are listed in Table S1.