SO42- transporters in plants and animals are structurally conserved and have an amino terminal domain that functions in transport and a carboxy-terminal region that has been designated the STAS domain. The STAS domain in SO42- transporters has significant similarity to bacterial anti-sigma-factor antagonists. To determine if the STAS domain has a role in controlling the activity of SO42- transporters, their stability or their localization to the plasma membrane, we examined the effect of deleting or modifying the STAS domain of dominant SO42-transporters in roots of Arabidopsis thaliana. The Arabidopsis SULTR1;2 and SULTR1;1 SO42- transporters rescue the methionine-dependent growth phenotype of the yeast sulfate transporter mutant strain CP154-7B. We have concentrated on analyzing the STAS domain of SULTR1;2 and have found that it is both critical for activity and the biogenesis of the transporter (Shibagaki and Grossman, 2004; Shibagaki and Grossman 2006). The β core of the STAS domain is required for transporter stability while a specific face of the transporter formed by α helical domains is critical for maintaining transporter activity. Some of the most interesting results deal with the finding that the STAS domain interacts with O-acetylserine(thiol)lyase, which is the enzyme that bonds sulfide to O-acetylserine to make cysteine. This interaction and how it affects the activity of the transporter is being explored now and appears to be dependent on the substrate in the medium (e.g. O-acetylserine). The implications however, are that the enzymes involved in the first step in the pathway of SO42- assimilation (transporters) interact with enzymes involved in the final steps (e.g. generation of cysteine) and that this interaction is important for maintaining the sulfur homeostasis of the cell; indeed this interaction is important for coordinating the uptake of sulfate with its assimilation and incorporation into cysteine. It may also suggest that macromolecular complexes are formed in the cells between transporters and assimilatory enzymes (Shibagaki and Grossman, 2010). This is a novel new area that requires considerable additional work.