The cytosolic Arabidopsis thaliana cysteine desulfurase ABA3 delivers sulfur to the sulfurtransferase STR18
ABSTRACTThe biosynthesis of many sulfur-containing biomolecules depends on cysteine as a sulfur source. Cysteine desulfurase (CD) and rhodanese (Rhd) domain-containing protein families participate in the trafficking of sulfur for various metabolic pathways in bacteria and human. However, their connection is not yet described in plants even though the existence of natural chimeric proteins containing both CD and Rhd domains in specific bacterial genera suggests that the interaction between both proteins should be universal. We report here the biochemical relationships between two cytosolic proteins from Arabidopsis thaliana, a Rhd domain containing protein, the sulfurtransferase 18 (STR18), and a CD isoform, ABA3, and compare these biochemical features to those of a natural CD-Rhd fusion protein from the bacterium Pseudorhodoferax sp.. We found that the bacterial enzyme is bifunctional exhibiting both CD and STR activities using L-cysteine and thiosulfate as sulfur donors. In vitro activity assays and mass spectrometry analyses revealed that STR18 stimulates the CD activity of ABA3 by recovering the intermediate persulfide on its catalytic cysteine. The ability of STR18 to catalyze trans-persulfidation reactions from ABA3 to a reduced roGFP2 used as a model acceptor protein reveals that the ABA3-STR18 couple may represent an uncharacterized pathway of sulfur trafficking in the cytosol of plant cells, independent of ABA3 function in molybdenum cofactor maturation.