title: Structural and thermodynamic properties of selective ion binding in a K+ channel.
abstract: Thermodynamic measurements of ion binding to the Streptomyces lividans K(+) channel were carried out using isothermal titration calorimetry, whereas atomic structures of ion-bound and ion-free conformations of the channel were characterized by x-ray crystallography. Here we use these assays to show that the ion radius dependence of selectivity stems from the channel's recognition of ion size (i.e., volume) rather than charge density. Ion size recognition is a function of the channel's ability to adopt a very specific conductive structure with larger ions (K(+), Rb(+), Cs(+), and Ba(2+)) bound and not with smaller ions (Na(+), Mg(2+), and Ca(2+)). The formation of the conductive structure involves selectivity filter atoms that are in direct contact with bound ions as well as protein atoms surrounding the selectivity filter up to a distance of 15 A from the ions. We conclude that ion selectivity in a K(+) channel is a property of size-matched ion binding sites created by the protein structure.
authors.affiliation: Laboratory of Molecular Neurobiology and Biophysics, The Rockefeller University, New York, New York, United States of America.
authors.forename: Steve W
authors.initials: SW
authors.lastname: Lockless
authors.forename: Ming
authors.initials: M
authors.lastname: Zhou
authors.forename: Roderick
authors.initials: R
authors.lastname: MacKinnon
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry / Q000378:metabolism*; D002151:Calorimetry; D002412:Cations / Q000378:metabolism*; D018360:Crystallography, X-Ray; D008958:Models, Molecular; D015221:Potassium Channels / Q000737:chemistry / Q000378:metabolism*; D011189:Potassium Chloride; D011487:Protein Conformation; D012965:Sodium Chloride; D048170:Streptomyces lividans / Q000378:metabolism*; D013816:Thermodynamics
chemical_list: D001426:Bacterial Proteins; D002412:Cations; D015221:Potassium Channels; C107790:prokaryotic potassium channel; D012965:Sodium Chloride; D011189:Potassium Chloride