Kcsa: It's a Potassium Channel

Ion conduction and selectivity properties of KcsA, a bacterial ion channel of known structure, were studied in a planar lipid bilayer system at the single-channel level. Selectivity sequences for permeant ions were determined by symmetrical solution conductance (K⁺ > Rb⁺, NH₄⁺, Tl⁺ ≫ Cs⁺, Na⁺, Li⁺) and by reversal potentials under bi-ionic or mixed-ion conditions (Tl⁺ > K⁺ > Rb⁺ > NH₄⁺ ≫ Na⁺, Li⁺). Determination of reversal potentials with submillivolt accuracy shows that K⁺ is over 150-fold more permeant than Na⁺. Variation of conductance with concentration under symmetrical salt conditions is complex, with at least two ion-binding processes revealing themselves: a high affinity process below 20 mM and a low affinity process over the range 100–1,000 mM. These properties are analogous to those seen in many eukaryotic K⁺ channels, and they establish KcsA as a faithful structural model for ion permeation in eukaryotic K⁺ channels.