Evolving Potassium Channels by Means of Yeast Selection Reveals Structural Elements Important for Selectivity

Potassium channels are widely distributed. To serve their physiological functions, such as neuronal signaling, control of insulin release, and regulation of heart rate and blood flow, it is essential that K+ channels allow K+ but not the smaller and more abundant Na+ ions to go through. The narrowest part of the channel pore, the selectivity filter formed by backbone carbonyls of the GYG-containing K+ channel signature sequence, approximates the hydration shell of K+ ions. However, the K+ channel signature sequence is not sufficient for K+ selectivity. To identify structural elements important for K+ selectivity, we randomly mutagenized the G protein-coupled inwardly rectifying potassium channel 3.2 (GIRK2) bearing the S177W mutation on the second transmembrane segment. This mutation confers constitutive channel activity but abolishes K+ selectivity and hence the channel's ability to complement the K+ transport deficiency of Deltatrk1Deltatrk2 mutant yeast. S177W-containing GIRK2 mutants that support yeast growth in low-K+ medium contain multiple suppressors, each partially restoring K+ selectivity to S177W-containing double mutants. These suppressors include mutations in the first transmembrane segment and the pore helix, likely exerting long-range actions to restore K+ selectivity, as well as a mutation of a second transmembrane segment residue facing the cytoplasmic half of the pore, below the selectivity filter. Some of these suppressors also affected channel gating (channel open time and opening frequency determined in single-channel analyses), revealing intriguing interplay between ion permeation and channel gating.