Probing a potassium channel pore with an engineered protonatable site.

Blockade by intracellular cations reduces outward conduction of K+ in inward rectifier K+ channels. Mutations of residue 171 in the second transmembrane (M2) segment of the ROMK1 channel have been found to affect the affinity for blockade by intracellular Mg2+ and polyamines. In the present study, we examined the mechanism by which this residue mediates blockade by placing a proton acceptor (histidine) at this position. The results allow us to draw two conclusions. First, the side chain of residue 171 is located in the ion conduction pore about halfway across the transmembrane voltage drop. Second, its side chain comes into close contact and interacts electrostatically with a blocking ion.