ALTERATION OF IONIC SELECTIVITY OF A K+ CHANNEL BY MUTATION OF THE H5 REGION

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 mulitiple 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 amino-acid substitutions in H5, the fifth hydrophobic region 6, enhanced the passage of NH4+ 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.