Structural Analysis of a Non-selective Cation
Channel
Youxing Jiang
University of Texas Southwestern Medical Center
The recently discovered NaK (Na- and K-conducting) channel from B. cereus has provided a unique structural model for probing mechanisms of ion selectivity compared to K+ selective channels hitherto used as models for both selective and non-selective tetrameric cation channels. The channel shares overall structural similarities with KcsA K+ channel but has been shown to conduct most group 1A cations as well as Ca2+. This ion non-selectivity is thought to arise from structural differences within the selectivity filter, where only the two most intracellular K+ binding sites of KcsA (sites 3 and 4) are preserved while the upper two are replaced by a vestibular structure. Using high resolution structures of NaK in complex with various cations, we were able to perform a detailed characterization of ion binding in the NaK pore. These structures reveal four ion binding sites with similar chemical environments but vastly different ion preference. The most non selective of all is site 3, which is formed exclusively by backbone carbonyl oxygen atoms and resides deep within the selectivity filter. Additionally, four water molecules are seen to form half hydration shells around K+ and Rb+ ions both at the external entrance and vestibule of the NaK filter, confirming the preference for an octahedral ligand configuration for K+ and Rb+ chelation. In contrast, Na+ binding in the NaK filter, particularly at site 4, utilizes a pyramidal ligand configuration requiring the participation of a water molecule in the cavity. Therefore, the ability of the NaK filter to bind both Na+ and K+ ions seemingly arises from ion's ability to utilize the existing environment in unique ways rather than any structural rearrangements of the filter itself. In addition, we also determined the structures of NaK in both open and closed conformation, making NaK the only channel for which the three-dimensional structures of both conformations are known. Channel opening follows a conserved mechanism of inner helix bending using a flexible glycine residue, the gating hinge, seen in MthK and most other tetrameric cation channels.