Kcsa : It's a Potassium Channel

Abstract

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 ⁺, NH ₄ ⁺, Tl ⁺ ≫ Cs ⁺, Na ⁺, Li ⁺) and by reversal potentials under bi-ionic or mixed-ion conditions (Tl ⁺ > K ⁺ > Rb ⁺ > NH ₄ ⁺ ≫ 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.