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Title: Mechanism of extracellular ion exchange and binding-site occlusion in a sodium/calcium exchanger

Na +/Ca 2+ exchangers utilize the Na + electrochemical gradient across the plasma membrane to extrude intracellular Ca 2+, and play a central role in Ca 2+ homeostasis. Here, we elucidate their mechanisms of extracellular ion recognition and exchange through a structural analysis of the exchanger from Methanococcus jannaschii (NCX_Mj) bound to Na +, Ca 2+ or Sr 2+ in various occupancies and in an apo state. This analysis defines the binding mode and relative affinity of these ions, establishes the structural basis for the anticipated 3:1Na +/Ca 2+ exchange stoichiometry, and reveals the conformational changes at the onset of the alternating-access transport mechanism. An independent analysis of the dynamics and conformational free-energy landscape of NCX_Mj in different ion-occupancy states, based on enhanced-sampling molecular-dynamics simulations, demonstrates that the crystal structures reflect mechanistically relevant, interconverting conformations. Lastly, these calculations also reveal the mechanism by which the outward-to-inward transition is controlled by the ion-occupancy state, thereby explaining the emergence of strictly-coupled Na +/Ca 2+ antiport.
Authors:
 [1] ;  [2] ;  [3] ;  [3] ;  [2] ;  [3]
  1. Shanghai Tech Univ., Shanghai (China)
  2. National Institutes of Health, Bethesda, MD (United States)
  3. Univ. of Texas Southwestern Medical Center, Dallas, TX (United States)
Publication Date:
Grant/Contract Number:
AC02-06CH11357; I-1578; R01GM079179
Type:
Accepted Manuscript
Journal Name:
Nature Structural & Molecular Biology
Additional Journal Information:
Journal Volume: 23; Journal Issue: 6; Journal ID: ISSN 1545-9993
Publisher:
Nature Publishing Group
Research Org:
UChicago, LLC., Argonne, IL (United States)
Sponsoring Org:
USDOE Office of Science (SC), Biological and Environmental Research (BER) (SC-23); National Institutes of Health (NIH); Howard Hughes Medical Institute; Welch Foundation; National Natural Science Foundation of China (NNSFC)
Country of Publication:
United States
Language:
English
Subject:
59 BASIC BIOLOGICAL SCIENCES; 37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; computational biology and bioinformatics; membrane proteins; X-ray crystallography
OSTI Identifier:
1347115