Determination of key structural requirements of a K⁺ channel pore

Among the highly conserved sites in K⁺ channel pores, the tyrosine- glycine sequence is believed to play an important role in selectivity. Here we describe a novel approach in which comprehensive mutagenesis of the YG sites of the voltage-gated K⁺ channel, Kat1, is combined with phenotypic screening in Saccharomyces cerevisiae and electrophysiological analysis in Xenopus oocytes to determine the roles of these sites in K⁺ selectivity. We show that structural constraints necessitate a tyrosine or phenylalanine at the first position to confer full K⁺ selectivity. Substitution to arginine creates a channel titratable by external pH, suggesting that the side group at this position may line the channel pore. Permeation is abolished by any increase in bulk at the adjacent glycine position unless accompanied by a compensatory mutation at the tyrosine site. These results suggest a model in which the selectivity filter of the K⁺ channel requires an aromatic residue paired with glycine within the pore loop in order to maintain maximal K⁺ selectivity.