Ion binding affinity in the cavity of the KcsA potassium channel

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 a-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 (similar to7-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.