Potassium channels selectively conduct K +, primarily to the exclusion of Na +, despite the fact that both ions can bind within the selectivity filter. Here we perform crystallographic titration and single-channel electrophysiology to examine the competition of Na + and K + binding within the filter of two NaK channel mutants; one is the potassium-selective NaK2K mutant and the other is the non-selective NaK2CNG, a CNG channel pore mimic. With high-resolution structures of these engineered NaK channel constructs, we explicitly describe the changes in K + occupancy within the filter upon Na + competition by anomalous diffraction. Our results demonstrate that the non-selective NaK2CNG still retains a K + -selective site at equilibrium, whereas the NaK2K channel filter maintains two high-affinity K + sites. A double-barrier mechanism is proposed to explain K + channel selectivity at low K + concentrations.
ASJC Scopus subject areas
- Biochemistry, Genetics and Molecular Biology(all)
- Physics and Astronomy(all)