A computational analysis of Salt Overly Sensitive 1 homologs in halophytes and glycophytes

Soil salinity is one of the most serious impediments to global agricultural productivity. Although most terrestrial plants are glycophytes which cannot tolerate high salt concentrations, a small fraction of species are halophytes. Exactly what allows these extremophile plants to survive in saline conditions is not yet well understood. Several studies have established the Salt Overly Sensitive (SOS) pathway as the canonical model for the mechanism responsible for salt tolerance. The SOS pathway involves interplay among Na⁺-H⁺ antiporters for transporting sodium, and the activation of the kinase that phosphorylates the transporter. Among them, SOS1, a plasma membrane Na⁺-H⁺ antiporter, has been shown to be a critical component for maintaining salt homeostasis by pumping sodium out of cells upon activation. Therefore, it is of great interest to evaluate any differences of SOS1 in halophytes as compared to glycophytes. Here we report a computational analysis of the primary and secondary structures of eight halophytes and seven glycophytes. ClustalW alignment of the protein sequences as a whole reveals no regions conserved specifically in only halophytes or in only glycophytes. In addition, the key regulatory residues at the C-terminus of SOS1, S1136 and S1138, which were shown to be the phosphorylation sites by the kinase SOS2, were completely conserved in all 15 halophytes and glycophytes. The four amino acids, G136, R365, G777, and G784, in which alterations affect the function of SOS1, are mostly conserved in the 15 species. The 14-3-3 binding site in the C-terminus which is important in the phosphorylation step of SOS1 in the SOS signal transduction cascade is also well conserved. Furthermore, the number of transmembrane helices for each species is between 9 and 12 and there is no significant difference between halophytes and glycophytes. If halophytes present any special feature of SOS1, it likely involves the presence (halophytes) or absence (glycophytes) of a SOS1-interacting component.