Expression of a glutamate decarboxylase homologue is required for normal oxidative stress tolerance in Saccharomyces cerevisiae

The action of gamma -aminobutyrate (GABA) as an intercellular signaling molecule has been intensively studied, but the role of this amino acid metabolite in intracellular metabolism is poorly understood. In this work, we identify a Saccharomyces cerevisiae homologue of the GABA-producing enzyme glutamate decarboxylase (GAD) that is required for normal oxidative stress tolerance. A high copy number plasmid bearing the glutamate decarboxylase gene (GAD1) increases resistance to two different oxidants, H2O2 and diamide, in cells that contain an intact glutamate catabolic pathway. Structural similarity of the S. cerevisiae GAD to previously studied plant enzymes was demonstrated by the crossreaction of the yeast enzyme to a antiserum directed against the plant GAD. The yeast GAD also bound to calmodulin as did the plant enzyme, suggesting a conservation of calcium regulation of this protein. Loss of either gene encoding the downstream steps in the conversion of glutamate to succinate reduced oxidative stress tolerance in normal cells and was epistatic to high copy number GAD1. The gene encoding succinate semialdehyde dehydrogenase (UGA5) was identified and found to be induced by H2O2 exposure. Together, these data strongly suggest that increases in activity of the glutamate catabolic pathway can act to buffer redox changes in the cell.