Ion Selectivity in a Semisynthetic K + Channel Locked in the Conductive Conformation

Abstract

Potassium channels are K + -selective protein pores in cell membrane. The selectivity filter is the functional unit that allows K + channels to distinguish potassium (K + ) and sodium (Na + ) ions. The filter's structure depends on whether K + or Na + ions are bound inside it. We synthesized a K + channel containing the d -enantiomer of alanine in place of a conserved glycine and found by x-ray crystallography that its filter maintains the K + (conductive) structure in the presence of Na + and very low concentrations of K + . This channel conducts Na + in the absence of K + but not in the presence of K + . These findings demonstrate that the ability of the channel to adapt its structure differently to K + and Na + is a fundamental aspect of ion selectivity, as is the ability of multiple K + ions to compete effectively with Na + for the conductive filter.