Plant serine acetyltransferase: new insights for regulation of sulphur metabolism in plant cells

Plant serine acetyltransferase (SAT, E.C. 2.3.1.30) catalyses the first connecting reaction between nitrogen/carbon and sulphate metabolism. SAT is associated with the second committed enzyme, O-acetylserine(thiol)lyase (OASTL, E.C. 4.2.99.8), in a bi-enzyme complex called cysteine synthase (CS). Metabolic regulation of SAT-bound OASTL in the presence of cysteine (Cys) is analysed with the extracts from the leaf cell compartments of Pisum sativum. To this end, a high performance liquid chromatography (HPLC) technique is developed to measure the rate of O-acetylserine (OAS) formation by SAT. Under physiological experimental conditions, L-Cys specifically inhibits chloroplast-SAT activity, which is linked to the sulphate assimilation network. This metabolic feedback control does not apply to the SAT activity located in the cytosol. The non-physiological range of L-Cys inhibits the mitochondrial isoform. L-Cys in a non-competitive manner in presence of L-serine or acetyl-CoA (Ki of 12–20 μM) inhibits partially purified chloroplast SAT, free of bound OASTL. The Ki values are in the range of Cys concentrations estimated in this compartment. Furthermore, we report for the first time that the multi-enzyme complex, CS dissociates in the presence of Cys as previously described with OAS. From this study, and with the integration of data previously reported in the literature, we hypothesize a new model for the regulation of Cys synthesis in plant cells containing a chloroplastic Cys-sensitive SAT.