Studies Of Ionic Interactions Of Ba²⁺ Blockades In The MthK K⁺ Channel

Potassium channels control the electric potential across cell membranes. They catalyze the rapid, selective diffusion of K+ions down their electrochemical gradient. Under physiological conditions, the K+selectivity filter usually contains two resident K+ ions separated by a water molecule. The ion pair moves back and forth in a concerted manner between the 1,3 and 2,4 configuration.Ba2+is the only alkaline earth metal that blocks K+channels, presumably because it uniquely shares high similarity with the alkali metal K+in terms of the ionic radius (1.35 A of Ba2+vs.1.33 A of K+) while doubling the charge. The similar size allows Ba2+ions to fit into the K+channel selectivity filter. However, the charge apparently causes tight binding, preventing the rapid flow of K+. Doctor Christopher Miller from Brandeis University, pioneer in the ion channel field found Ba2+have 3 effects on the K+channel through electrophysiology studies, that is external lock-in effect, enhancement effect and internal lock-in effect. He also supposed the structure of K+channel which was solved by x-ray crystallography. However, how Ba2+and K+interactions lead to these effects remains to be unclear, so further studies are needed.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 and proposed a model. Without Ba2+, two K+ions bind equivalently in the 1,3 and 2,4 configurations. With the addition of Ba2+from the internal side, Ba2+ enters the filter and initiates block by occupying the S4 site. The electrostatic repulsion between Ba2+and K+allows the Ba2+ion to effectively drive out K+ions from the filter, leaving a single Ba2+ion alternating between S2, S3, and S4 sites and blocking the passage of K+ions. Ba2+ions may leave the filter either by dissociation back to the internal side or proceeding to the external side. The addition of external K+in low concentration causes K+ion binding at its high affinity site 1, locking Ba2+at site 4 and preventing Ba2+from exiting to the external side (external lock-in effect). Although external K+in high concentration allows the K+ion to enter site 2, it forces Ba2+ion dissociation to the internal side by the electrostatic repulsion (enhancement effect). The internal K+, or other permeant ions, can occupy the central cavity, locking the Ba2+ion mainly at site 2 and preventing the Ba2+ion from exiting to the internal side (internal lock-in effect).