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Mechanism of Ba2+ block of a mouse inwardly rectifying K+ channel: differential contribution by two discrete residues

1 The block of the IRK1/Kir2.1 inwardly rectifying K+ channel by a Ba2+ ion is highly voltage dependent, where the ion binds approximately half‐way within the membrane electrical field. The mechanism by which two distinct mutations, E125N and T141A, affect Ba2+ block of Kir2.1 was investigated using heterologous expression in Xenopus oocytes. 2 Analysis of the blocking kinetics showed that E125 and T141 affect the entry and binding of Ba2+ to the channel, respectively. Replacing the glutamate at position 125 with an asparagine greatly decreased the rate at which the Ba2+ ions enter and leave the pore. In contrast, replacing the polar threonine at position 141 with an alanine affected the entry rate of the Ba2+ ions while leaving the exit rate unchanged. 3 Acidification of the extracellular solution slowed the exit rate of the Ba2+ from the wild‐type channel, but had no such effect on the Kir2.1(E125N) mutant. 4 These results thus reveal two unique roles for the amino acids at positions 125 and 141 in aiding the interaction of Ba2+ with the channel. Their possible roles in K+ permeation are discussed.