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

Abstract

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. © 2013 Macmillan Publishers Limited. All rights reserved.