Equilibrium selectivity alone does not create K+-selective ion conduction in K+ channels

Potassium (K+) channels are selective for K+ over Na+ ions during their transport across membranes. We and others have previously shown that tetrameric K+ channels are primarily occupied by K+ ions in their selectivity filters under physiological conditions, demonstrating the channel's intrinsic equilibrium preference for K+ ions. Based on this observation, we hypothesize that the preference for K+ ions over Na+ ions in the filter determines its selectivity during ion conduction. Here, we ask whether non-selective cation channels, which share an overall structure and similar individual ion-binding sites with K+ channels, have an ion preference at equilibrium. The variants of the non-selective Bacillus cereus NaK cation channel we examine are all selective for K+ over Na+ ions at equilibrium. Thus, the detailed architecture of the K+ channel selectivity filter, and not only its equilibrium ion preference, is fundamental to the generation of selectivity during ion conduction.