Cryo-electron microscopy of Na ,K -ATPase reveals how the extracellular gate locks in the E2·2K state.

Na ,K -ATPase (NKA) is one of the most important members of the P-type ion-translocating ATPases and plays a pivotal role in establishing electrochemical gradients for Na and K across the cell membrane. Presented here is a 3.3 Å resolution structure of NKA in the E2·2K state solved by cryo-electron microscopy. It is a stable state with two occluded K after transferring three Na into the extracellular medium and releasing inorganic phosphate bound to the cytoplasmic P domain. We describe how the extracellular ion pathway wide open in the E2P state becomes closed and locked in E2·2K , linked to events at the phosphorylation site more than 50 Å away. We also show, although at low resolution, how ATP binding to NKA in E2·2K relaxes the gating machinery and thereby accelerates the transition into the next step, that is, the release of K into the cytoplasm, more than 100 times.