MicroED structure of the NaK ion channel reveals a Na⁺ partition process into the selectivity filter

Sodium (Na⁺) is a ubiquitous and important inorganic salt mediating many critical biological processes such as neuronal excitation, signaling, and facilitation of various transporters. The hydration states of Na⁺ are proposed to play critical roles in determining the conductance and the selectivity of Na⁺ channels, yet they are rarely captured by conventional structural biology means. Here we use the emerging cryo-electron microscopy (cryoEM) method micro-electron diffraction (MicroED) to study the structure of a prototypical tetrameric Na⁺-conducting channel, NaK, to 2.5 Å resolution from nano-crystals. Two new conformations at the external site of NaK are identified, allowing us to visualize a partially hydrated Na⁺ ion at the entrance of the channel pore. A process of dilation coupled with Na⁺ movement is identified leading to valuable insights into the mechanism of ion conduction and gating. This study lays the ground work for future studies using MicroED in membrane protein biophysics.