Ion Selectivity and Gating in the NAK Channel
Ion selectivity and gating are two fundamental properties central to proper physiological functioning of ion channels. In this work, a thorough structural characterization of ion binding profiles is presented for the NaK channel, a non selective cation channel from Bacillus cereus, along with an analysis of channel opening and closing mechanisms. An introduction to the overall structure of the full length channel is presented along with initial characterization of its ion selectivity properties. The body of the thesis encompasses a detailed analysis of Ca2+ binding within NaK, followed by additional gating and selectivity studies using high resolution structures of a truncated form of the channel. Two Ca2+ binding sites are observed, both utilizing a unique Ca2+ chelation chemistry involving only backbone carbonyl groups as ligands, with Ca2+ selectivity at the extracellular site mediated by a through space interaction with a conserved acidic residue, Asp66, seen in other Ca2+ conducting channels. In the high resolution structure of the truncated NaK channel, we observe the intracellular gate in an open conformation much like that seen in MthK, making NaK the first channel for which both open and closed state structures are known. This is combined with a structural analysis of ion binding within the NaK selectivity filter, which reveals several interesting features that shed light on the possible underlying mechanisms of ion non-selectivity.