Cysteine synthesis in plants: protein-protein interactions of serine acetyltransferase from Arabidopsis thaliana.

The biosynthesis of cysteine represents the final step of sulfate assimilation in bacteria and plants. It is catalyzed by the sequential action of serine acetyltransferase (SAT) and O-acetylserine (thiol) lyase (OAS-TL) which form a cysteine synthase (CS) complex in vitro. SAT and OAS-TL from Arabidopsis thaliana have previously been cloned, and now the first evidence is presented for the CS complex and SAT self-interaction in vivo employing the yeast two-hybrid system. Application of this method proved to be an efficient tool for the analysis of protein-protein interactions within a plant metabolic protein complex. Mapping of SAT domain structure revealed two new, independent domains with specific functions in protein-protein interaction. Analysis using truncated proteins proved the C-terminus of SAT to be sufficient for association with OAS-TL and to correlate with the putative transferase activity domain. SAT/SAT interaction was localized in the central region of the protein and occurred also between SAT isoforms. Both protein interaction domains coincided with distinct alpha-helical and beta-sheet clusters and together correlated with the minimal protein structure required for SAT catalysis as shown by functional complementation of an Escherichia coli mutant. The homo- and hetero-oligomerization properties are discussed with respect to the assumed function of the CS complex in metabolic channeling and activation of SAT by interaction with OAS-TL.