Direct visualization of electric-field-stimulated ion conduction in a potassium channel.

Understanding protein function would be facilitated by direct, real-time observation of chemical kinetics in the atomic structure. The selectivity filter (SF) of the K channel provides an ideal model, catalyzing the dehydration and transport of K ions across the cell membrane through a narrow pore. We used a "pump-probe" method called electric-field-stimulated time-resolved X-ray crystallography (EFX) to initiate and observe K conduction in the NaK2K channel in both directions on the timescale of the transport process. We observe both known and potentially new features in the high-energy conformations visited along the conduction pathway, including the associated dynamics of protein residues that control selectivity and conduction rate. A single time series of one channel in action shows the orderly appearance of features observed in diverse homologs with diverse methods, arguing for deep conservation of the dynamics underlying the reaction coordinate in this protein family.