1. Howard Hughes Medical Institute, Laboratory of Molecular Neurobiology and Biophysics, Rockefeller University, 1230 York Avenue, New York, New York 10021, USA
2. Present address: Department of Molecular Biophysics and Biochemistry, Yale University, 260 Whitney Avenue, New Haven, Connecticut 06520, USA.

Correspondence to: Correspondence and requests for materials should be addressed to R.M. (email: Email: mackinn@rockvax.rockefeller.edu). Coordinates have been deposited with the Protein Data Bank under accession codes 1K4C and 1K4D.

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 Å 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.

1. Howard Hughes Medical Institute, Laboratory of Molecular Neurobiology and Biophysics, Rockefeller University, 1230 York Avenue, New York, New York 10021, USA
2. Present address: Department of Molecular Biophysics and Biochemistry, Yale University, 260 Whitney Avenue, New Haven, Connecticut 06520, USA.

Correspondence to: Correspondence and requests for materials should be addressed to R.M. (email: Email: mackinn@rockvax.rockefeller.edu). Coordinates have been deposited with the Protein Data Bank under accession codes 1K4C and 1K4D.