The cysteine synthase complex from plants - Mitochondrial serine acetyltransferase from Arabidopsis thaliana carries a bifunctional domain for catalysis and protein-protein interaction

Serine acetyltransferase (SAT) catalyzes the rate-limiting step of cysteine biosynthesis in bacteria and plants and functions in association with O-ac etylserine (thiol) lyase (OAS-TL) in the cysteine synthase complex. Very li ttle is known about the structure and catalysis of SATs except that they sh are a characteristic C-terminal hexapeptide-repeat domain with a number of enzymatically unrelated acyltransferases. Computational modeling of this do main was performed for the mitochondrial SAT isoform from Arabidopsis thali ana, based on crystal structures of bacterial acyltransferases. The results indicate a left-handed parallel beta -helix consisting of beta -sheets alt ernating with turns, resulting in a prism-like structure. This model was ch allenged by site-directed mutagenesis and tested for a suspected dual funct ion of this domain in catalysis and hetero-oligomerization. The bifunctiona lity of the SAT C-terminus in transferase activity and interaction with OAS -TL is demonstrated and discussed with respect to the putative role of the cysteine synthase complex in regulation of cysteine biosynthesis.