Functional divergence in the glutathione transferase super-family in plants : identification of two classes with putative functions in redox homeostasis in Arabidopsis thaliana.

Abstract

Searches with the human omega glutathione transferase (GST), identified two outlying groups of the GST super-family in Arabidopsis thaliana which differed from all other plant GSTs in containing a cysteine in place of a serine at the active site. One group comprised four genes, three of which encoded active glutathione-dependent dehydroascorbate reductases (DHARs). Two DHARs were predicted to be cytosolic while the other contained a chloroplast targeting peptide. The DHARs were also active as thiol transferases but had no glutathione conjugating activity. Unlike most other GSTs, DHARs were monomeric. The other class of GST comprised two genes termed the lambda GSTs (GSTLs). The recombinant GSTLs were also monomeric and had glutathione-dependent thiol transferase activity. One GSTL was cytosolic, while the other was chloroplast targeted. When incubated with oxidised glutathione, the putative active site cysteine of the GSTLs and cytosolic DHARs formed mixed disulphides with glutathione, whereas the plastidic DHAR formed an intramolecular disulphide. DHAR S-glutathionylation was consistent with a proposed catalytic mechanism for dehydroascorbate reduction. Roles for the cytosolic DHARs and GSTLs as antioxidant enzymes were also inferred from the induction of the respective genes following exposure to chemicals and oxidative stress.