Identification of the Genes for Intracellular Glutathione Degradation in Arabidopsis thaliana

To understand the physiological role of glutathione (GSH) degradation and how it contributes to other sulfur metabolites, it is necessary to determine the GSH degradation pathway. In mammals it has long been believed that GSH is degraded outside of the cell by gamma-glutamyl transpeptidase (GGT). However most GSH exists inside the cell. In Arabidopsis it was suggested that GSH is catabolized by the GGT-independent pathway via 5-oxoproline, by gamma-glutamyl cyclotransferase (GGCT). This study aims to identify gene(s) that code the degradation of GSH in the cytosol. In Saccharomyces cerevisiae, the DUG2-DUG3 complex degrades GSH to Glu and Cys-Gly then DUG1 cleaves the peptide bond of Cys-Gly. None of the dug. strains are able to grow on the medium where GSH is the sole sulfur source. Transformants of dug. strains with an A. thaliana cDNA library were screened on a medium with GSH as the sole sulfur source. Sequences of inserts in positive clones were searched against A. thaliana cDNA database by BLAST. One A. thaliana gene complemented dug1 Delta and four genes complimented dug2 Delta and dug3 Delta strains. The same genes complemented both of dug2 Delta and dug3 Delta strains, indicating that in Arabidopsis GSH is degraded by single proteins, unlike in yeast in which complexed proteins are required. Two pathways were suggested for GSH degradation in Arabidopsis, GGCT pathway and AtDUG pathway.