Academic journal article Genetics

The Alternative Pathway of Glutathione Degradation Is Mediated by a Novel Protein Complex Involving Three New Genes in Saccharomyces Cerevisiae

Academic journal article Genetics

The Alternative Pathway of Glutathione Degradation Is Mediated by a Novel Protein Complex Involving Three New Genes in Saccharomyces Cerevisiae

Article excerpt

ABSTRACT

Glutathione (GSH), L-γ-glutamyl-L-cysteinyl-glycine, is the major low-molecular-weight thiol compound present in almost all eukaryotic cells. GSH degradation proceeds through the γ-glutamyl cycle that is initiated, in all organisms, by the action of γ-glutamyl transpeptidase. A novel pathway for the degradation of GSH that requires the participation of three previously uncharacterized genes is described in the yeast Saccharomyces cerevisiae. These genes have been named DUG1 (YFR044c), DUG2 (YBR281c), and DUG3 (YNL191w) (defective in utilization of glutathione). Although dipeptides and tripeptides with a normal peptide bond such as cys-gly or glu-cys-gly required the presence of only a functional DUG1 gene that encoded a protein belonging to the M20A metallohydrolase family, the presence of an unusual peptide bond such as in the dipeptide, γ-glu-cys, or in GSH, required the participation of the DUG2 and DUG3 gene products as well. The DUG2 gene encodes a protein with a peptidase domain and a large WD40 repeat region, while the DUG3 gene encoded a protein with a glutamine amidotransferase domain. The Dug1p, Dug2p, and Dug3p proteins were found to form a degradosomal complex through Dug1p-Dug2p and Dug2p-Dug3p interactions. A model is proposed for the functioning of the Dug1p/Dug2p/Dug3p proteins as a specific GSH degradosomal complex.

GLUTATHIONE (GSH), L-γ-glutamyl-L-cysteinyl-glycine, is the major low-molecular-weight thiol compound present in almost all eukaryotic cells (MEISTER and ANDERSON 1983; FAHEY and SUNDQUIST 1991) at intracellular concentrations ranging from 0.1 to 10 mM (HWANG et al. 1992). This is in contrast to other redox couples that are in micromolar concentrations in the cell (HOLMGREN et al. 1978). GSH thus acts as the principal redox buffer, plays an important role in oxidative stress response and in the detoxification of metals and xenobiotics, and influences-through redox-several essential processes such as gene expression, cell proliferation, and apoptosis (PENNINCKX and ELSKENS 1993; ARRIGO 1999; FANG et al. 2002). The two important chemical properties from which glutathione derives its importance in the cell are its low redox potential (OSTERGAARD et al. 2004) and the stability of the tripeptide provided by the unusual γ-glutamyl bond, making it resistant to peptidases in the cell and allowing it to exist at high concentrations in the cell (GANGULY et al. 2003).

GSH levels (and the ratio of the oxidized and reduced forms of glutathione) need to be carefully maintained in the cell. In addition to its biosynthesis, degradation, and consumption in different processes, glutathione levels are altered by its compartmentalization and efflux from the cell (PERRONE et al. 2005). In addition to biosynthesis of GSH, which occurs in the cytoplasm through the sequential action of two cytosolic enzymes, γ-glutamyl cysteine synthase and glutathione synthase (MEISTER and ANDERSON 1983), GSH can also be transported from the extracellular medium through specific transporters (BOURBOULOUX et al. 2000). These multiple processes combine to maintain glutathione homeostasis in the cell. GSH deficiency in the cell has been associated with many disease states that include liver diseases, macular eye degeneration, Alzheimer's, aging, and HIV infections (WU et al. 2004). Higher levels of glutathione have also been shown to lead to glutathione toxicity at least in yeast (SRIKANTH et al. 2005).

In eukaryotes, glutathione is essential for growth. In the yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe, deletion of the first enzyme in GSH biosynthesis leads to growth stasis (unless supplied with exogenous glutathione) (WU and MOYE-ROWLEY 1994; GRANT et al. 1996; CHAUDHURI et al. 1997), while in mice deletion of the first enzyme leads to embryonic lethality (SHI et al. 2000). Overproduction of the rate-limiting enzyme in glutathione biosynthesis has recently been demonstrated as leading to increased longevity in Drosophila melanogaster (ORR et al. …

Search by... Author
Show... All Results Primary Sources Peer-reviewed

Oops!

An unknown error has occurred. Please click the button below to reload the page. If the problem persists, please try again in a little while.