Alteration of ionic selectivity of a K⁺ channel by mutation of the H5 region

Abstract

THE high ionic selectivity of K⁺ channels is a unifying feature of this diverse class of membrane proteins. Though K⁺ channels differ widely in regulation and kinetics, physiological studies have suggested a common structure: a single file pore containing multiple ion-binding sites and having broader vestibules at both ends^1–5. We have used site-directed mutagenesis and single-channel recordings to identify a molecular region that influences ionic selectivity in a cloned A-type K⁺ channel from Drosophila. Single ammo-acid substitutions in H5, the fifth hydrophobic region⁶, enhanced the passage of NH⁺₄ and Rb⁺, ions with diameters larger than K⁺, without compromising the ability of the channel to exclude the smaller cation, Na⁺. The mutations that substantially altered selectivity had little effect on the gating properties of the channel. We conclude that the H5 region is likely to line the pore of the K⁺ channel.