Cation selectivity of the NA+/H+ exchanger AtSOS1

Abstract

The plasma membrane Na+/H+ antiporter AtSOS1 is a key determinant for salt tolerance. This protein mediates Na+ extrusion from plant cells and shows a high specificity for Na+ in biochemical and in vivo assays. Interestingly, the Arabidopsis transporter AtNHX8, phylogenetically related to AtSOS1, was characterized as a Li+/H+ antiporter with little affinity for Na+. Although the substrate specifity is different in the two proteins, they show a high degree of similarity at the protein sequence level. Little is known about topological determinants involved in the cation specifity of antiporters. Critical residues of the pore domain and the regulatory cytosolic C-terminal domains are both thought to be important. We have studied the effect of the C-terminal part of the Arabidopsis proteins in the process of cation selectivity. The C-terminal regions of AtSOS1 and AtNHX8 were swapped and the transport activity of the chimerical proteins was analyzed in a Na+ and Li+ sensitive yeast strain. Results supporting a role for the C-terminal region in determining the substrate specificity of the transporter will be presented.