Abstract
Multiple Resistance and pH (Mrp) antiporters are seven-subunit complexes that couple transport of ions across the membrane in response to a proton motive force (PMF) and have various physiological roles, including sodium ion sensing and pH homeostasis. The hyperthermophilic archaeon Pyrococcus furiosus contains three copies of Mrp encoding genes in its genome. Two are found as integral components of two respiratory complexes, membrane bound hydrogenase (MBH) and the membrane bound sulfane sulfur reductase (MBS) that couple redox activity to sodium translocation, while the third copy is a stand-alone Mrp. Sequence alignments show that this Mrp does not contain an energy-input (PMF) module but contains all other predicted functional Mrp domains. The P. furiosus Mrp deletion strain exhibits no significant changes in optimal pH or sodium ion concentration for growth but is more sensitive to medium acidification during growth. Cell suspension hydrogen gas production assays using the deletion strain show that this Mrp uses sodium as the coupling ion. Mrp likely maintains cytoplasmic pH by exchanging protons inside the cell for extracellular sodium ions. Deletion of the MBH sodium-translocating module demonstrates that hydrogen gas production is uncoupled from ion pumping and provides insights into the evolution of this Mrp-containing respiratory complex.
Highlights
Multiple resistance and pH adaptation (Mrp) antiporters are multisubunit complexes that couple transport of Na+ ions across the membrane to the proton motive force (PMF; Ito et al, 2017)
The two MrpB domains are homologous to the MbhD and E subunits, which form the secondary proton pathway at the junction between the proton pumping module and the sodium translocation module
P. furiosus MrpB is homologous to a fusion of MbhD and MbhE, while MrpB’ is homologous to MbhF
Summary
Multiple resistance and pH adaptation (Mrp) antiporters are multisubunit complexes that couple transport of Na+ (or K+) ions across the membrane to the proton motive force (PMF; Ito et al, 2017). They are part of a larger group of Na+/H+ antiporters including eukaryotic NHE families (Brett et al, 2005) and bacterial NhaA families (Lentes et al, 2014) and are the only subgroup that is encoded by a multi-gene operon. Group 1 mrp operons encode separate mrpA and mrpB genes, with both mrpB and mrpA containing an mrpB domain. Group 2 mrp operons lack the mrpB gene, but mrpA contains
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