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Dr. Stefan Haberland

Dr. Stefan Haberland
Dr. Stefan Haberland


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Signalling Function of the Cysteine Synthase Complex

 

Sulfur is an essential component maintaining cell viability. It is present in the proteinogenic amino acid cysteine, which is the major source for reduced sulfur in the metabolism of all living organisms. Heterotrophic organisms like humans are not able to produce cysteine and have therefore to take it up via nutrition. In contrast Arabidopsis thaliana and other higher plants are able to synthesise cysteine. Sulfate is assimilated from the soil by specific membrane-localized sulphate transporters (Sultr) and then reduced to sulfide in the plastids. The biosynthesis of cysteine in Arabidopsis thaliana takes place in three different compartments of the cell, namely the cytosol, the plastids and the mitochondria.

Serine acetyltransferase (SAT) and O-acetylserine(thiol)lyase (OAS-TL) form together the cysteine synthase complex (CSC). In this decameric complex, consisting of two SAT trimers and two OAS-TL dimers, SAT is highly active and produces OAS from serine and acetyl-CoA. Due to the inactivity of OAS-TL the CSC the produced OAS is not channelled but released into the surrounding solution. In an additional step OAS and sulfide are then metabolised by free OAS-TL dimers to cysteine. Dissociation of the CSC is mediated by OAS, whereas sulfide stabilises the complex. The regulation of the whole cysteine biosynthesis pathway and how communication between the three compartments takes place have not been understood in detail, yet. A possible role of the CSC in terms of communication and transcriptional regulation of Sultr genes could be clarified experimentally with labelled SATs and OAS-TLs and genetic screens.

 


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