Localization of gamma-glutamyl-transferase gene transcripts, protein and activity in plant tissues

One of the less well-known aspects of glutathione (GSH) metabolism is its degradation. Two pathways have been suggested: in one, the glycine residue is first removed by a carboxypeptidase, then cysteine is hydrolyzed by a -glutamyl cyclo-transferase; in another, glutathione degradation is initiated by gamma-glutamyl transferase/transpeptidase (GGT) activity. The latter is well described in mammals, where it is thought to have a key role in cell redox balance and glutathione salvage from the extracellular space, but it is little known in plants. Previous investigations on UV-B photo-oxidative stress conditions in maize leaves showed an increase in cys-gly content that reportedly results from glutathione degradation by means of GGT activity; this study presents the results of our investigations on this enzyme in plants. As predicted by sequence analysis on known plant GGTs, there is a substantial similarity with GGTs from other organisms as concerns the presence of the amino acid residues needed for substrate binding and catalytic activity, glycosylation sites, the cleavage site and the putative transmembrane domain at the N terminus. By means of an enzyme-histochemical procedure, we localized GGT activity in various tissues from different plant species. Our results show that GGT activity is mainly localized in the parenchymal cells of conductive tissues closely associated with vascular bundles, but also in the stomata and epidermal cells. In maize leaves, GGT transcript localization, as revealed by in situ hybridization, correlates with the distribution of GGT activity. Immunocytochemical analysis confirms the localization of GGT in the plasma membrane, but also reveals its presence within the cell walls. The role of GGT in plant metabolism remains largely unknown. The histochemically-detected localization of GGT activity suggests that this enzyme may be involved in GSH redistribution in plant organs and tissues. GGT promotes the removal of a gamma-glutamyl moiety and the resulting cys-gly may either be degraded to its constituent amino acids and transported across the plasma membrane, or it may take part in redox signaling by influencing the redox state of free thiols of membrane proteins. It is therefore feasible to suggest that GGT fulfills important functions in addition to GSH degradation.