Probing the function of STAS domains of the Arabidopsis sulfate transporters

Sulfate transporters in plants and animals are structurally conserved and have an amino-terminal domain that functions in transport and a carboxyl-terminal region that has been designated the STAS domain. The STAS domain in sulfate transporters has significant similarity to bacterial anti-sigma factor antagonists. To determine if the STAS domain has a role in controlling the activity of sulfate transporters, their stability, or their localization to the plasma membrane, we examined the effect of deleting or modifying the STAS domain of dominant sulfate transporters in roots of Arabidopsis thaliana. The A. thaliana Sultr1;2 and Sultr1; 1 sulfate transporters rescue the methionine-dependent growth phenotype of the yeast sulfate transporter mutant strain CP154-7B. Constructs of Sultr1; 2 in which the STAS domain was deleted (DeltaSTAS) resulted in synthesis of a truncated polypeptide that was unable to rescue the CP154-7B phenotype. The inability of these constructs to rescue the mutant phenotype probably reflected both low level cellular accumulation of the transporter and the inability of the truncated protein to localize to the plasma membrane. Fusing the STAS domain from other sulfate transporters to Sultr1; 2 DeltaSTAS constructs restored elevated accumulation and plasma membrane localization, although the kinetics of sulfate uptake in the transformants were markedly altered with respect to transformants synthesizing wild-type Sultr1; 2 protein. These results suggest that the STAS domain is essential, either directly or indirectly, for facilitating localization of the transporters to the plasma membrane, but it also appears to influence the kinetic properties of the catalytic domain of transporters.