Hydration valve controlled non-selective conduction of Na+ and K+ in the NaK channel

The Na+ and K+ channels are essential to neural signaling, but our current knowledge at the atomic level is mainly limited to the conducting mechanism of K+. Unlike a K+ channel having four equivalent K+-binding sites in its selectivity filter, a NaK channel has a vestibule in the middle part of its selectivity filter, and can conduct both Na+ and K+ ions. However, the underlying mechanism for non-selective ion conduction in NaK remains elusive. Here we find four small grottos connecting with the vestibule of the NaK selectivity filter, which form a vestibule-grotto complex perpendicular to the filter pore with a few water molecules within it. It is shown that two or more of the water molecules coming to the vestibule to coordinate the cation are necessary for conducting both Na+ and K+ ions, while only one water molecule in the vestibule will obstruct ion permeation. Thus, the complex with the aid of interior water movement forms a dynamic hydration valve which is flexible in conveying different cations through the vestibule. Similar exquisite hydration valve mechanisms are expected to be utilized by other non-selective cation channels, and the results should shed new light on the importance of water in neural signaling.