Functional expression of the multi-subunit type calcium/proton antiporter from Thermomicrobium roseum.

Multiple resistance and pH adaptation (Mrp) antiporters are widely distributed in various prokaryotes and have been reported to function as a hetero-oligomeric monovalent cation/proton antiporter, which exchanges a cytoplasmic monovalent cation (Na(+), Li(+), and/or K(+)) with extracellular H(+). In many organisms, they are essential for survival in alkaline or saline environments. Here, we report that the Mrp antiporter from the thermophilic gram-negative bacterium, Thermomicrobium roseum, does not catalyze monovalent cation/proton antiport like the Mrp antiporters studied to date, but catalyzes Ca(2+)/H(+) antiport in Escherichia coli membrane vesicles.