The regulatory function of the plant cysteine synthase protein complex for cellular cysteine homeostasis

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Cysteine synthesis provides the only entry of assimilated sulfur into plant metabolism and forms one essential step for human and animal nutrition. Biochemical evidence suggests that cellular homeostasis of cysteine in plants is controlled by a unique sensing mechanism and subcellular organisation. The hetero-oligomeric cysteine synthase protein complex is located in cytosol, plastids and mitochondria. Its reversible activation/inactivation depends on whether its subunits are inside or outside the complex and this equilibrium is determined by the inverse effects of the pathway intermediates sulfide and O-acetylserine. The central function of the cysteine synthase complex will be addressed in two independent approaches. One, the reversible association of the complex and its response to changes in homeostasis will be investigated in vivo using fluorescence resonance energy transfer (FRET). Two, artificial microRNA interference and T-DNA mutants will be used to evaluate the hypothesis that mitochondria, and not plastids, are most important for the rate of cysteine synthesis. The results will clarify the current hypothesis of dynamic flux control by the cysteine synthase complex and unravel the role of cellular compartments in cysteine homeostasis. They will be significant for understanding of regulatory protein interactions in metabolism in general.