Abstract

Barium ion (Ba2+) has the same crystal radius as potassium ion but blocks rather than permeates the ion-conducting pore of K+ channels. Ba2+ ion may therefore be used as a probe of residues lining the pore of K+ channels, and we applied it to test the position and function of a residue crucial for K+/Rb+ selectivity and blockade by internal tetraethylammonium. We examined blockade by internal and external Ba2+ of the delayed rectifier K+ channel DRK1 (Kv2.1) and tested the effects of point mutations at pore residue 374. Internal Ba2+ blocked the wild-type open channel with high affinity (Kd = 13 microM). Blockade involved more than one site, was voltage dependent, and increased at more positive potentials. Mutation of V374 to threonine or serine produced a significant decrease in the rate of dissociation of internal Ba2+ from the pore, whereas mutation of V374 to isoleucine produced no change. For wild-type channels, external Ba2+ decreased the rate of activation of the K+ current, suggesting that Ba2+ can interact with closed DRK1 channels. This result was unaffected by the V374T substitution. Furthermore, external Ba2+ also caused a very low affinity (Kd approximately 30 mM) and voltage-independent block of the open DRK1 channel. Thus, Ba2+ blocked the pore at internal and external sites, which were clearly distinguishable. The effects of substitution at position 374 with residues having polar hydroxyls are consistent with position 374 being at a surface position critical for ion permeation, near the inner mouth of the pore.