Volume 90 Issue 7 | p. 37 | Concentrates
Issue Date: February 13, 2012

Cellular Sodium-Calcium Exchange Illuminated

Probe of membrane protein structure deepens understanding of critical cellular maintenance functions
Department: Science & Technology
News Channels: Biological SCENE, Analytical SCENE
Keywords: calcium, sodium, transport, exchange, signaling, membrane, protein
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Sodium (green) and calcium (orange) ions compete for ligands in the binding pocket of NCX (blue).
Credit: Youxing Jiang
Sodium (green) and calcium (orange) ions compete for ligands in the binding pocket of NCX (blue).
 
Sodium (green) and calcium (orange) ions compete for ligands in the binding pocket of NCX (blue).
Credit: Youxing Jiang
[+]Enlarge
NCX exchanges sodium ions (green) from outside the cell (top) with calcium ions (red) from inside the cell (bottom).
Credit: Science
NCX exchanges sodium ions (green) from outside the cell (top) with calcium ions (red) from inside the cell (bottom).
 
NCX exchanges sodium ions (green) from outside the cell (top) with calcium ions (red) from inside the cell (bottom).
Credit: Science

A clearer picture of how cells traffic calcium comes from a new structure of a membrane ion-exchange protein (Science, DOI: 10.1126/science.1215759). Cellular calcium transport is essential for processes including fertilization, muscle contraction, and cell death. A group led by Youxing Jiang, a Howard Hughes Medical Institute investigator at the University of Texas Southwestern Medical Center, crystallized and solved the structure of an archaeal Na+/Ca2+ exchanger (NCX). The protein is homologous to eukaryotic transport proteins that exchange three Na+ ions for one Ca2+ ion across cell membranes. The group found that NCX is made up of a bundle of 10 helices that span a membrane and enclose a central cation-binding pocket. The researchers propose a “helix sliding” conformational mechanism for ion exchange that alternately opens ion channels to the interior and exterior of a cell, with Na+ and Ca2+ ions competing for ligands in the binding pocket. The structure generally confirms what scientists had learned about eukaryotic exchangers through other methods. The archaeal protein, however, does not include a regulatory domain found in eukaryotic exchangers, so interactions between the regulatory and transport domains remain to be clarified.

 
Chemical & Engineering News
ISSN 0009-2347
Copyright © American Chemical Society

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