title: Evolving potassium channels by means of yeast selection reveals structural elements important for selectivity.
abstract: 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.
authors.affiliation: Departments of Physiology and Biochemistry, Howard Hughes Medical Institute, University of California, San Francisco, CA 94143-0725, USA.
authors.forename: Delphine
authors.initials: D
authors.lastname: Bichet
authors.forename: Yu-Fung
authors.initials: YF
authors.lastname: Lin
authors.forename: Christian A
authors.initials: CA
authors.lastname: Ibarra
authors.forename: Cindy Shen
authors.initials: CS
authors.lastname: Huang
authors.forename: B Alexander
authors.initials: BA
authors.lastname: Yi
authors.forename: Yuh Nung
authors.initials: YN
authors.lastname: Jan
authors.forename: Lily Yeh
authors.initials: LY
authors.lastname: Jan
mesh_terms: D019943:Amino Acid Substitution; D000818:Animals; D051676:G Protein-Coupled Inwardly-Rectifying Potassium Channels; D008958:Models, Molecular; D016297:Mutagenesis, Site-Directed; D009865:Oocytes / Q000502:physiology; D018408:Patch-Clamp Techniques; D015221:Potassium Channels / Q000737:chemistry / Q000502:physiology; D024661:Potassium Channels, Inwardly Rectifying; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D015183:Restriction Mapping; D012441:Saccharomyces cerevisiae / Q000235:genetics / Q000502:physiology; D029701:Saccharomyces cerevisiae Proteins / Q000737:chemistry / Q000235:genetics; D014981:Xenopus
chemical_list: D051676:G Protein-Coupled Inwardly-Rectifying Potassium Channels; D015221:Potassium Channels; D024661:Potassium Channels, Inwardly Rectifying; D029701:Saccharomyces cerevisiae Proteins