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Sulfur represents a vital macronutrient for all living organisms since it is a constituent of the two proteinogenic amino acids cysteine and methionine. Primary metabolism of sulfur in plants centers on the assimilation of reduced sulfate (sulfide) that is ultimately incorporated into cysteine. Regulated uptake of sulfate is implemented by high and low affinity sulfate transporters (Sultr) that are located in the plasmalemma of root cells. Reduction of sulfate is restricted to the plastids of root and leaf cells whereas cysteine biosynthesis occurs in the plastids, cytosol and mitochondria of plant cells. Generation of cysteine is catalyzed by the hetero-oligomeric cystein synthase complex (CSC) that is formed reversibly by serine-acteyltransferase (SAT) hexamers and O-acetylserine (thiol)-lyase (OAS-TL) dimers. Remarkably, cellular sulfur homeostasis is regulated by the CSC itself: In the case of sufficient sulfur availability, high sulfide levels stabilize the CSC thereby retaining SAT hexamers in their active and OAS-TL dimers in their inactive state. Active SATs catalyze the formation of OAS which is the precursor molecule of cysteine. Free, active OAS-TLs act as catalysts for cysteine formation from OAS and hydrogen sulfide. Under sulfur deficiency, OAS accumulates and the CSC dissociates, resulting in an inactive SAT. Subsequently, transcription of genes that are involved in sulfate transport and reduction are upregulated thereby enhancing sulfide availability.
In my PhD project I want to further elucidate the role of the CSC as a key regulator in the context of sulfur metabolism and signaling. Thus, it is assumed that this protein complex possesses a regulatory function beyond modulation of SAT activity by interacting with proteins that are involved in sulfur sensing and signal transduction. In this context, identification of interaction partners and downstream components of the CSC signaling pathway can be insightful to better understand this complex regulatory network. Besides, global changes in the nuclear proteome of leaf cells compared to root cells in response to malnutrition such as sulfur deficiency will be addressed in this project.