Figure 7. Structural model of the KcsA pore and sequential model of K_ir1.1 gating by pH_i and K⁺_o.

A, structural model of the KcsA pore as determined by X-ray crystallography (Doyle et al. 1998) viewed from the side (left panel, only two subunits shown for clarity) and the top (right panel); transmembrane segments in green (M1) and blue (M2) and the P-helices (Pα) in red; K⁺ binding sites are indicated by grey spheres. Side chains of residues 70 and 74 homologous to residues 136 and 140 in K_ir1.1 were replaced by I and V and highlighted. Note that the side chains are oriented towards the neighbouring subunits rather than to the ion conducting pathway. B, sequential model of K_ir1.1 gating by pH_i and K⁺_o as deduced from the experiments presented here. ‘open-K⁺’ represents active channels; ‘closed-K⁺’ is pH-inactivated closed state with K⁺ bound to the pore binding site(s); ‘closed-’ is K⁺-inactivated closed state; possible intermediate states are omitted for simplicity. These channel conformations are linked through gating transitions (pH gating and K⁺-dependent gating, symbolized by arrows) which can be blocked independently by point mutations (indicated in red).