Characterization of Critical Amino Acids in the Transport and Selectivity of the Plant Na+/H+ Exchanger Plasma Membrane SOS1.

SOS1 is a Na+/H+ antiporter found in the plant membrane of Arabidopsis thaliana and serves as a major transporter that extrudes Na+ across the plasma membrane of cells in exchange for intracellular H+. The first 450 amino acids comprise the membrane transport domain. Using a yeast heterologous expression system, we examined nine different mutations that may either change specificity or improve salt tolerance. E261K had minor negative effects on the ability to confer tolerance to LiCl and NaCl. Mutation A399V had minor effects, lowering LiCl tolerance and slightly improving NaCl tolerance, as did the double mutant E261KA399V. Four different mutations of amino acid Y346 had varying effects. The Y346R mutation resulted in a major improvement in LiCl tolerance but did not affect NaCl tolerance. The L375I mutant showed impaired NaCl tolerance, whereas the Q362L mutant exhibited minor effects on salt tolerance. Our results demonstrate that amino acid Y346 is critical in ion selectivity and its mutation can dramatically improve LiCl salt tolerance. Other mutations showed minor improvements in the ability to confer NaCl tolerance (Y346F, A399V, and Y346A), leaving open the possibility that such mutations might improve salt tolerance in intact plant species.