title: Chemistry of ion coordination and hydration revealed by a K+ channel-Fab complex at 2.0 A resolution.
abstract: Ion transport proteins must remove an ion's hydration shell to coordinate the ion selectively on the basis of its size and charge. To discover how the K+ channel solves this fundamental aspect of ion conduction, we solved the structure of the KcsA K+ channel in complex with a monoclonal Fab antibody fragment at 2.0 A resolution. Here we show how the K+ channel displaces water molecules around an ion at its extracellular entryway, and how it holds a K+ ion in a square antiprism of water molecules in a cavity near its intracellular entryway. Carbonyl oxygen atoms within the selectivity filter form a very similar square antiprism around each K+ binding site, as if to mimic the waters of hydration. The selectivity filter changes its ion coordination structure in low K+ solutions. This structural change is crucial to the operation of the selectivity filter in the cellular context, where the K+ ion concentration near the selectivity filter varies in response to channel gating.
authors.affiliation: Department of Molecular Biophysics and Biochemistry, Yale University, 260 Whitney Avenue, New Haven, Connecticut 06520, USA.
authors.forename: Y
authors.initials: Y
authors.lastname: Zhou
authors.forename: J H
authors.initials: JH
authors.lastname: Morais-Cabral
authors.forename: A
authors.initials: A
authors.lastname: Kaufman
authors.forename: R
authors.initials: R
authors.lastname: MacKinnon
mesh_terms: D000818:Animals; D000911:Antibodies, Monoclonal / Q000737:chemistry / Q000276:immunology / Q000378:metabolism; D001426:Bacterial Proteins; D018360:Crystallography, X-Ray; D004563:Electrochemistry; D007140:Immunoglobulin Fab Fragments / Q000737:chemistry* / Q000276:immunology / Q000378:metabolism; D051379:Mice; D008954:Models, Biological; D008958:Models, Molecular; D010316:Particle Size; D011188:Potassium / Q000737:chemistry* / Q000378:metabolism; D015221:Potassium Channels / Q000737:chemistry* / Q000276:immunology / Q000378:metabolism; D011487:Protein Conformation; D012996:Solutions; D014867:Water / Q000737:chemistry*
chemical_list: D000911:Antibodies, Monoclonal; D001426:Bacterial Proteins; D007140:Immunoglobulin Fab Fragments; D015221:Potassium Channels; D012996:Solutions; C107790:prokaryotic potassium channel; D014867:Water; D011188:Potassium