Abstract
Cells employ membrane-embedded antiporter proteins to control their pH, salt concentration, and volume. The large family of cation/proton antiporters is dominated by Na+/H+ antiporters that exchange sodium ions against protons, but homologous K+/H+ exchangers have recently been characterized. We show experimentally that the electroneutral antiporter NhaP1 of Methanocaldococcus jannaschii (MjNhaP1) is highly selective for Na+ ions. We then characterize the ion selectivity in both the inward-open and outward-open states of MjNhaP1 using classical molecular dynamics simulations, free energy calculations, and hybrid quantum/classical (QM/MM) simulations. We show that MjNhaP1 is highly selective for binding of Na+ over K+ in the inward-open state, yet it is only weakly selective in the outward-open state. These findings are consistent with the function of MjNhaP1 as a sodium-driven deacidifier of the cytosol that maintains a high cytosolic K+ concentration in environments of high salinity. By combining experiment and computation, we gain mechanistic insight into the Na+/H+ transport mechanism and help elucidate the molecular basis for ion selectivity in cation/proton exchangers.
Highlights
Sodium/proton (Na+/H+) antiporters or exchangers (NHE)are membrane proteins involved in the control of cellular pH, salt concentration, and volume.[1]
The experiments demonstrate that K+ does not serve as a substrate for WT MjNhaP1
In the environment of submarine hydrothermal vents, the typical habitat of M. jannaschii, MjNhaP1 is believed to work as Na+-driven proton exporter, powered by a Na+ gradient between the saline environment and the inside of the cell
Summary
Sodium/proton (Na+/H+) antiporters or exchangers (NHE)are membrane proteins involved in the control of cellular pH, salt concentration, and volume.[1]. In a tightly coupled exchange process, they employ an electrochemical gradient of one ion species across a membrane to drive the thermodynamically unfavorable transport of another ion. To this end, the Na+/H+ antiporters employ conformational transitions between two alternate access states,[4,5] in which the ion binding sites face opposite sides of the membrane. If transitions between the inward-open and outward-open access states are feasible only with bound Na+ and/or H+, conformational switching between these states results in selective ion exchange.[4,5]
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