Figure 2.

Na⁺/H⁺ antiporter activity of rice SOS1. A, ATP-dependent pH gradient formation in membrane vesicles isolated from yeast cells expressing rice SOS1. An inside-acid ΔpH was formed after the addition of ATP to vesicles (arrow 1). Once fluorescence was stabilized, sodium salts were added to the cuvette (arrow 2) and fluorescence recovery, indicating proton exchange, was monitored for 2 min, after which ΔpH was disrupted by the addition of 25 mm (NH₄)₂SO₄ (arrow 3). Fluorescence is expressed as arbitrary units. B, Na⁺/H⁺ exchange as a function of Na₂SO₄ concentration and the presence of rice OsSOS1, with and without coexpression of the Arabidopsis SOS2/SOS3 kinase complex. Circles, AXT3K cells transformed with empty vector pDR195; squares, AXT3K cells expressing rice OsSOS1 alone; diamonds, AXT3K cells transformed to produce proteins OsSOS1, AtSOS2, and AtSOS3. Na⁺/H⁺ exchange activity is given as the proportion of dissipation of the preformed pH gradient per minute and milligrams of membrane protein. C, Specificity of sodium-induced proton exchange. Sodium was added to a final concentration of 75 mm as sulfate (gray bars) or gluconate (black bars) salt. Values are the mean and se of percent fluorescence dissipation of triplicate samples.