Driving force-dependent block by internal Ba2+ on the Kir2.1 channel: Mechanistic insight into inward rectification

- The internal Ba2+ block in Kir2.1 channels showed inward rectification fashion.
- The inward rectification was due to flux-dependent block.
- The steep changes in kon near EK of Ba2+ block is driving force-dependent.
- The high affinity Ba2+ binding site is located near T141.
- The single-file long cytoplasmic pore provides structural basis for flux coupling.

The Kir2.1 channel is characterized by strong inward rectification; however, the mechanism of the steep voltage dependence near the equilibrium potential remains to be investigated. Here, we studied the internal Ba2+ block of the Kir2.1 channel expressed in Xenopus oocytes. We showed that the driving force and thus the K+ ion flux significantly influenced the apparent affinity of the block by internal Ba2+. Kinetic analysis revealed that the binding rate shifted with the driving force and changed steeply near the equilibrium point, either in the presence or absence of the transmembrane electrical field. The unbinding rate was determined by the intrinsic affinity of the site. Mutagenesis studies revealed that the high-affinity binding site for Ba2+ was located near T141 at the internal entrance of the selectivity filter. The steep change of the blocking affinity near the equilibrium potential may result from the flux-coupling effect in the single-file, multi-ion cytoplasmic pore.