Yeast cells contain a family of three monothiol glutaredoxins: Grx3, 4, and 5. Absence of Grx5 leads to constitutive oxidative damage, exacerbating that caused by external oxidants. Phenotypic defects associated with the absence of Grx5 are suppressed by overexpression of SSQ1 and ISA2, two genes involved in the synthesis and assembly of iron/sulfur clusters into proteins. Grx5 localizes at the mitochondrial matrix, like other proteins involved in the synthesis of these clusters, and the mature form lacks the first 29 amino acids of the translation product. Absence of Grx5 causes: 1) iron accumulation in the cell, which in turn could promote oxidative damage, and 2) inactivation of enzymes requiring iron/sulfur clusters for their activity. Reduction of iron levels in grx5 null mutants does not restore the activity of iron/sulfur enzymes, and cell growth defects are not suppressed in anaerobiosis or in the presence of disulfide reductants. Hence, Grx5 forms part of the mitochondrial machinery involved in the synthesis and assembly of iron/sulfur centers.

Glutaredoxins are members of a superfamily of thiol disulfide oxidoreductases involved in maintaining the redox state of target proteins. In Saccharomyces cerevisiae, two glutaredoxins (Grx1 and Grx2) containing a cysteine pair at the active site had been characterized as protecting yeast cells against oxidative damage. In this work, another subfamily of yeast glutaredoxins (Grx3, Grx4, and Grx5) that differs from the first in containing a single cysteine residue at the putative active site is described. This trait is also characteristic for a number of glutaredoxins from bacteria to humans, with which the Grx3/4/5 group has extensive homology over two regions. Mutants lacking Grx5 are partially deficient in growth in rich and minimal media and also highly sensitive to oxidative damage caused by menadione and hydrogen peroxide. A significant increase in total protein carbonyl content is constitutively observed in grx5 cells, and a number of specific proteins, including transketolase, appear to be highly oxidized in this mutant. The synthetic lethality of the grx5 and grx2 mutations on one hand and of grx5 with the grx3 grx4 combination on the other points to a complex functional relationship among yeast glutaredoxins, with Grx5 playing a specially important role in protection against oxidative stress both during ordinary growth conditions and after externally induced damage. Grx5-deficient mutants are also sensitive to osmotic stress, which indicates a relationship between the two types of stress in yeast cells.

In order to clarify their physiological functions, we have undertaken a characterization of the three-membered gene families SNZ1-3 and SNO1-3. In media lacking vitamin B 6 , SNZ1 and SNO1 were both required for growth in certain conditions, but neither SNZ2, SNZ3, SNO2 nor SNO3 were required. Copies 2 and 3 of the gene products have, in spite of their extremely close sequence similarity, slightly different functions in the cell. We have also found that copies 2 and 3 are activated by the lack of thiamine and that the Snz proteins physically interact with the thiamine biosynthesis Thi5 protein family. Whereas copy 1 is required for conditions in which B 6 is essential for growth, copies 2 and 3 seem more related with B 1 biosynthesis during the exponential phase.

Glutaredoxins are thiol oxidoreductases that regulate protein redox state. In Saccharomyces cerevisiae, Grx1 and Grx2 are cytosolic dithiol glutaredoxins, whereas Grx3, Grx4, and Grx5 are monothiol glutaredoxins. Grx5 locates at the mitochondrial matrix and is needed for iron/sulfur cluster biogenesis. Its absence causes phenotypes such as inactivation of iron/sulfur enzymes and sensitivity to oxidative stress. Whereas Grx5 contains a single glutaredoxin domain, in Grx3 and Grx4 a thioredoxin-like domain is fused to the glutaredoxin domain. Here we have shown that Grx3 locates at the nucleus and that the thioredoxin-like domain is required for such location. We have addressed the functional divergence among glutaredoxins by targeting Grx2/3/4 molecules to the mitochondrial matrix using the Grx5 targeting sequence. The mitochondrial forms of Grx3 and Grx4 partially rescue the defects of a grx5 null mutant. On the contrary, mitochondrially targeted Grx2 does not suppress the mutant phenotype. Both the thioredoxin-like and glutaredoxin domains are needed for the mitochondrial activity of Grx3, although none of the cysteine residues at the thioredoxin-like domain is required for rescue of the grx5 phenotypes. We have concluded that dithiol glutaredoxins are functionally divergent from monothiol ones, but the latter can interchange their biological activities when compartment barriers are surpassed.

Grx5 defines a family of yeast monothiol glutaredoxins that also includes Grx3 and Grx4. All three proteins display significant sequence homology with proteins found from bacteria to humans. Grx5 is involved in iron/ sulfur cluster assembly at the mitochondria, but the function of Grx3 and Grx4 is unknown. Three-dimensional modeling based on known dithiol glutaredoxin structures predicted a thioredoxin fold structure for Grx5. Positionally conserved amino acids in this glutaredoxin family were replaced in Grx5, and the effect on the biological function of the protein has been tested. For all changes studied, there was a correlation between the effects on several different phenotypes: sensitivity to oxidants, constitutive protein oxidation, ability for respiratory growth, auxotrophy for a number of amino acids, and iron accumulation. Cys 60 and Gly 61 are essential for Grx5 function, whereas other single or double substitutions in the same region had no phenotypic effects. Gly 115 and Gly 116 could be important for the formation of a glutathione cleft on the Grx5 surface, in contrast to adjacent Cys 117 . Substitution of Phe 50 alters the ␤-sheet in the thioredoxin fold structure and inhibits Grx5 function. None of the substitutions tested affect the structure at a significant enough level to reduce protein stability.Glutaredoxins are thiol oxidireductases that catalyze redox reactions involving reduced glutathione as a hydrogen donor for the reduction of protein disulfides (dithiol mechanism of action) or glutathione-protein-mixed disulfides (monothiol mechanism of action) (see Refs. 1 and 2 for review). Previously described glutaredoxins are small proteins (about 10 kDa) with a conserved active site that includes two cysteine residues (Cys-Pro-Tyr-Cys). Site-directed mutagenesis (3-5) has demonstrated that both cysteine residues in the active site are required for the dithiol reaction. In contrast, the amino-terminal cysteine is sufficient to catalyze the deglutathionylation of the reduced glutathione-mixed disulfides that are formed under oxidative stress conditions (5).Three-dimensional structures of oxidized and reduced forms of viral, bacterial, and mammalian glutaredoxins and also of reduced glutathione-glutaredoxin complexes have been identified using x-ray crystallography (6, 7) or nuclear magnetic resonance spectroscopy (8 -14). These studies have revealed which residues, apart from those at the active site, are important for stable interactions between glutathione and the glutaredoxin molecule (10,13,14). Dithiol glutaredoxins are members of the thioredoxin superfamily (15, 16) along with at least five other classes of proteins that interact with cysteine-containing substrates (thioredoxins, DbsA, protein disulfide isomerases, glutathione S-transferases, and glutathione peroxidases). This superfamily shares a structural motif (called the thioredoxin fold or ␣␤␣ fold) formed by a four or fivestranded ␤-sheet (with parallel and antiparallel strands) surrounded by three or more ␣-helices distributed on ei...