Ionic interactions of Ba2+ blockades in the MthK K+ channel.

The movement and interaction of multiple ions passing through in single file underlie various fundamental K+ channel properties, from the effective conduction of K+ ions to channel blockade by Ba2+ ions. In this study, we used single-channel electrophysiology and x-ray crystallography to probe the interactions of Ba2+ with permeant ions within the ion conduction pathway of the MthK K+ channel. We found that, as typical of K+ channels, the MthK channel was blocked by Ba2+ at the internal side, and the Ba2+-blocking effect was enhanced by external K+. We also obtained crystal structures of the MthK K+ channel pore in both Ba2+-Na+ and Ba2+-K+ environments. In the Ba2+-Na+ environment, we found that a single Ba2+ ion remained bound in the selectivity filter, preferably at site 2, whereas in the Ba2+-K+ environment, Ba2+ ions were predominantly distributed between sites 3 and 4. These ionic configurations are remarkably consistent with the functional studies and identify a molecular basis for Ba2+ blockade of K+ channels.