Chemistry of ion coordination and hydration revealed by a K channel±Fab complex at 2.0 AÊ resolution

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 AÊ 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 ®lter form a very similar square antiprism around each K binding site, as if to mimic the waters of hydration. The selectivity ®lter changes its ion coordination structure in low K solutions. This structural change is crucial to the operation of the selectivity ®lter in the cellular context, where the K ion concentration near the selectivity ®lter varies in response to channel gating.