Access of extracellular cations to their binding sites in Na,K-ATPase: role of the second extracellular loop of the alpha subunit.

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serval:BIB_36BA7217A68C
Type
Article: article from journal or magazin.
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Publications
Institution
Title
Access of extracellular cations to their binding sites in Na,K-ATPase: role of the second extracellular loop of the alpha subunit.
Journal
The Journal of general physiology
Author(s)
Capendeguy O., Chodanowski P., Michielin O., Horisberger J.D.
ISSN
0022-1295
Publication state
Published
Issued date
2006
Peer-reviewed
Oui
Volume
127
Number
3
Pages
341-s52
Language
english
Notes
Publication types: Journal Article ; Research Support, Non-U.S. Gov't
Publication Status: ppublish
Abstract
Na,K-ATPase, the main active transport system for monovalent cations in animal cells, is responsible for maintaining Na(+) and K(+) gradients across the plasma membrane. During its transport cycle it binds three cytoplasmic Na(+) ions and releases them on the extracellular side of the membrane, and then binds two extracellular K(+) ions and releases them into the cytoplasm. The fourth, fifth, and sixth transmembrane helices of the alpha subunit of Na,K-ATPase are known to be involved in Na(+) and K(+) binding sites, but the gating mechanisms that control the access of these ions to their binding sites are not yet fully understood. We have focused on the second extracellular loop linking transmembrane segments 3 and 4 and attempted to determine its role in gating. We replaced 13 residues of this loop in the rat alpha1 subunit, from E314 to G326, by cysteine, and then studied the function of these mutants using electrophysiological techniques. We analyzed the results using a structural model obtained by homology with SERCA, and ab initio calculations for the second extracellular loop. Four mutants were markedly modified by the sulfhydryl reagent MTSET, and we investigated them in detail. The substituted cysteines were more readily accessible to MTSET in the E1 conformation for the Y315C, W317C, and I322C mutants. Mutations or derivatization of the substituted cysteines in the second extracellular loop resulted in major increases in the apparent affinity for extracellular K(+), and this was associated with a reduction in the maximum activity. The changes produced by the E314C mutation were reversed by MTSET treatment. In the W317C and I322C mutants, MTSET also induced a moderate shift of the E1/E2 equilibrium towards the E1(Na) conformation under Na/Na exchange conditions. These findings indicate that the second extracellular loop must be functionally linked to the gating mechanism that controls the access of K(+) to its binding site.
Keywords
Amino Acid Substitution, Animals, Binding Sites, Cells, Cultured, Computer Simulation, Extracellular Fluid/chemistry, Extracellular Fluid/metabolism, Models, Chemical, Models, Molecular, Mutagenesis, Site-Directed, Oocytes/chemistry, Oocytes/physiology, Protein Binding, Protein Conformation, Protein Subunits, Rats, Sodium-Potassium-Exchanging ATPase/chemistry, Sodium-Potassium-Exchanging ATPase/metabolism, Structure-Activity Relationship, Xenopus laevis
Pubmed
Web of science
Open Access
Yes
Create date
28/01/2008 11:22
Last modification date
20/08/2019 13:24
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