title: Potassium-selective block of barium permeation through single KcsA channels.
abstract: Ba(2+), a doubly charged analogue of K(+), specifically blocks K(+) channels by virtue of electrostatic stabilization in the permeation pathway. Ba(2+) block is used here as a tool to determine the equilibrium binding affinity for various monovalent cations at specific sites in the selectivity filter of a noninactivating mutant of KcsA. At high concentrations of external K(+), the block-time distribution is double exponential, marking at least two Ba(2+) sites in the selectivity filter, in accord with a Ba(2+)-containing crystal structure of KcsA. By analyzing block as a function of extracellular K(+), we determined the equilibrium dissociation constant of K(+) and of other monovalent cations at an extracellular site, presumably S1, to arrive at a selectivity sequence for binding at this site: Rb(+) (3 µM) > Cs(+) (23 µM) > K(+) (29 µM) > NH(4)(+) (440 µM) >> Na(+) and Li(+) (>1 M). This represents an unusually high selectivity for K(+) over Na(+), with |ΔΔG(0)| of at least 7 kcal mol(-1). These results fit well with other kinetic measurements of selectivity as well as with the many crystal structures of KcsA in various ionic conditions.
authors.affiliation: Department of Biochemistry, Howard Hughes Medical Institute, Brandeis University, Waltham, MA 02454, USA.
authors.forename: Kene N
authors.initials: KN
authors.lastname: Piasta
authors.forename: Douglas L
authors.initials: DL
authors.lastname: Theobald
authors.forename: Christopher
authors.initials: C
authors.lastname: Miller
mesh_terms: D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D001464:Barium / Q000378:metabolism*; D001665:Binding Sites; D002462:Cell Membrane / Q000502:physiology*; D002586:Cesium; D004305:Dose-Response Relationship, Drug; D004926:Escherichia coli / Q000235:genetics / Q000378:metabolism*; D015964:Gene Expression Regulation, Bacterial; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D010539:Permeability; D011188:Potassium / Q000378:metabolism*; D024642:Potassium Channels, Voltage-Gated / Q000235:genetics / Q000378:metabolism*; D000644:Quaternary Ammonium Compounds; D012413:Rubidium
chemical_list: D001426:Bacterial Proteins; C504448:KcsA protein, Streptomyces coelicolor; D024642:Potassium Channels, Voltage-Gated; D000644:Quaternary Ammonium Compounds; D002586:Cesium; D001464:Barium; D012413:Rubidium; D011188:Potassium