title: Structural analysis of ion selectivity in the NaK channel.
abstract: Here we present a detailed characterization of ion binding in the NaK pore using the high-resolution structures of NaK in complex with various cations. These structures reveal four ion binding sites with similar chemical environments but vastly different ion preference. The most nonselective of all is site 3, which is formed exclusively by backbone carbonyl oxygen atoms and resides deep within the selectivity filter. Additionally, four water molecules in combination with four backbone carbonyl oxygen atoms are seen to participate in K(+) and Rb(+) ion chelation, at both the external entrance and the vestibule of the NaK filter, confirming the channel's preference for an octahedral ligand configuration for K(+) and Rb(+) binding. In contrast, Na(+) binding in the NaK filter, particularly at site 4, utilizes a pyramidal ligand configuration that requires the participation of a water molecule in the cavity. Therefore, the ability of the NaK filter to bind both Na(+) and K(+) ions seemingly arises from the ions' ability to use the existing environment in unique ways, rather than from any structural rearrangements of the filter itself.
authors.affiliation: Department of Physiology, University of Texas Southwestern Medical Center, 5323 Harry Hines Blvd, Dallas, Texas 75390-9040, USA.
authors.forename: Amer
authors.initials: A
authors.lastname: Alam
authors.forename: Youxing
authors.initials: Y
authors.lastname: Jiang
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D001426:Bacterial Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D001665:Binding Sites; D002412:Cations; D002586:Cesium / Q000378:metabolism; D007700:Kinetics; D008958:Models, Molecular; D011188:Potassium / Q000378:metabolism; D015221:Potassium Channels / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D011485:Protein Binding; D011487:Protein Conformation; D012413:Rubidium / Q000378:metabolism; D017384:Sequence Deletion; D013379:Substrate Specificity; D014867:Water / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; D002412:Cations; D015221:Potassium Channels; C107790:prokaryotic potassium channel; D014867:Water; D002586:Cesium; D000255:Adenosine Triphosphate; D012413:Rubidium; D011188:Potassium