title: The conserved potassium channel filter can have distinct ion binding profiles: structural analysis of rubidium, cesium, and barium binding in NaK2K.
abstract: Potassium channels are highly selective for K(+) over the smaller Na(+). Intriguingly, they are permeable to larger monovalent cations such as Rb(+) and Cs(+) but are specifically blocked by the similarly sized Ba(2+). In this study, we used structural analysis to determine the binding profiles for these permeant and blocking ions in the selectivity filter of the potassium-selective NaK channel mutant NaK2K and also performed permeation experiments using single-channel recordings. Our data revealed that some ion binding properties of NaK2K are distinct from those of the canonical K(+) channels KcsA and MthK. Rb(+) bound at sites 1, 3, and 4 in NaK2K, as it does in KcsA. Cs(+), however, bound predominantly at sites 1 and 3 in NaK2K, whereas it binds at sites 1, 3, and 4 in KcsA. Moreover, Ba(2+) binding in NaK2K was distinct from that which has been observed in KcsA and MthK, even though all of these channels show similar Ba(2+) block. In the presence of K(+), Ba(2+) bound to the NaK2K channel at site 3 in conjunction with a K(+) at site 1; this led to a prolonged block of the channel (the external K(+)-dependent Ba(2+) lock-in state). In the absence of K(+), however, Ba(2+) acts as a permeating blocker. We found that, under these conditions, Ba(2+) bound at sites 1 or 0 as well as site 3, allowing it to enter the filter from the intracellular side and exit from the extracellular side. The difference in the Ba(2+) binding profile in the presence and absence of K(+) thus provides a structural explanation for the short and prolonged Ba(2+) block observed in NaK2K.
authors.affiliation: Department of Physiology and Howard Hughes Medical Institute, University of Texas Southwestern Medical Center, Dallas, TX 75390.
authors.forename: Yee Ling
authors.initials: YL
authors.lastname: Lam
authors.affiliation: Department of Physiology and Howard Hughes Medical Institute, University of Texas Southwestern Medical Center, Dallas, TX 75390Department of Physiology and Howard Hughes Medical Institute, University of Texas Southwestern Medical Center, Dallas, TX 75390.
authors.forename: Weizhong
authors.initials: W
authors.lastname: Zeng
authors.affiliation: Department of Physiology and Howard Hughes Medical Institute, University of Texas Southwestern Medical Center, Dallas, TX 75390.
authors.forename: David Bryant
authors.initials: DB
authors.lastname: Sauer
authors.affiliation: Department of Physiology and Howard Hughes Medical Institute, University of Texas Southwestern Medical Center, Dallas, TX 75390Department of Physiology and Howard Hughes Medical Institute, University of Texas Southwestern Medical Center, Dallas, TX 75390 youxing.jiang@utsouthwestern.edu.
authors.forename: Youxing
authors.initials: Y
authors.lastname: Jiang
mesh_terms: D001464:Barium / Q000378:metabolism* / Q000494:pharmacology; D002586:Cesium / Q000378:metabolism* / Q000494:pharmacology; D006801:Humans; D007477:Ions / Q000378:metabolism*; D008958:Models, Molecular; D018408:Patch-Clamp Techniques; D011188:Potassium / Q000378:metabolism; D026902:Potassium Channel Blockers / Q000494:pharmacology; D015221:Potassium Channels / Q000187:drug effects / Q000378:metabolism*; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D011994:Recombinant Proteins / Q000378:metabolism; D012413:Rubidium / Q000378:metabolism* / Q000494:pharmacology
chemical_list: D007477:Ions; D026902:Potassium Channel Blockers; D015221:Potassium Channels; D011994:Recombinant Proteins; D002586:Cesium; D001464:Barium; D012413:Rubidium; D011188:Potassium