Ion Binding Affinity in the Cavity of the KcsA Potassium Channel

Abstract

The hydrophobic cell membrane interior presents a large energy barrier for ions to permeate. Potassium channels reduce this barrier by creating a water-filled cavity at the middle of their ion conduction pore to allow ion hydration and by directing the C-terminal “end charge” of four α-helices toward the water-filled cavity. Here we have studied the interaction of monovalent cations with the cavity of the KcsA K⁺ channel using X-ray crystallography. In these studies, Tl⁺ was used as an analogue for K⁺ and the total ion-stabilization energy for Tl⁺ in the cavity was estimated by measuring its binding affinity. Binding affinity for the Na⁺ ion was also measured, revealing a weak selectivity (∼7-fold) favoring Tl⁺ over Na⁺. The structures of the cavity containing Na⁺, K⁺, Tl⁺, Rb⁺, and Cs⁺ are compared. These results are consistent with a fairly large (more negative than −100 mV) electrostatic potential inside the cavity, and they also imply the presence of a weak nonelectrostatic component to a cation's interaction with the cavity.