Aromatic Residues at the Dimer-Dimer Interface in the Peroxiredoxin Tsa1 Facilitate Decamer Formation and Biological Function.

To prevent the accumulation of reactive oxygen species and limit associated damage to biological macromolecules, cells express a variety of oxidant-detoxifying enzymes, including peroxiredoxins. In Saccharomyces cerevisiae, the peroxiredoxin Tsa1 plays a key role in peroxide clearance and maintenance of genome stability. Five homodimers of Tsa1 can assemble into a toroid-shaped decamer, with the active sites in the enzyme being shared between individual dimers in the decamer. Here, we have examined whether two conserved aromatic residues at the decamer-building interface promote Tsa1 oligomerization, enzymatic activity, and biological function. When substituting either or both of these aromatic residues at the decamer-building interface with either alanine or leucine, we found that the Tsa1 decamer is destabilized, favoring dimeric species instead. These proteins exhibit varying abilities to rescue the phenotypes of oxidant sensitivity and genomic instability in yeast lacking Tsa1 and Tsa2, with the individual leucine substitutions at this interface partially complementing the deletion phenotypes. The ability of Tsa1 decamer interface variants to partially rescue peroxidase function in deletion strains is temperature-dependent and correlates with their relative rate of reactivity with hydrogen peroxide and their ability to interact with thioredoxin. Based on the combined results of in vitro and in vivo assays, our findings indicate that multiple steps in the catalytic cycle of Tsa1 may be impaired by introducing substitutions at its decamer-building interface, suggesting a multifaceted biological basis for its assembly into decamers.

Piecing Together How Peroxiredoxins Maintain Genomic Stability.

Peroxiredoxins, a highly conserved family of thiol oxidoreductases, play a key role in oxidant detoxification by partnering with the thioredoxin system to protect against oxidative stress. In addition to their peroxidase activity, certain types of peroxiredoxins possess other biochemical activities, including assistance in preventing protein aggregation upon exposure to high levels of oxidants (molecular chaperone activity), and the transduction of redox signals to downstream proteins (redox switch activity). Mice lacking the peroxiredoxin Prdx1 exhibit an increased incidence of tumor formation, whereas baker's yeast (Saccharomyces cerevisiae) lacking the orthologous peroxiredoxin Tsa1 exhibit a mutator phenotype. Collectively, these findings suggest a potential link between peroxiredoxins, control of genomic stability, and cancer etiology. Here, we examine the potential mechanisms through which Tsa1 lowers mutation rates, taking into account its diverse biochemical roles in oxidant defense, protein homeostasis, and redox signaling as well as its interplay with thioredoxin and thioredoxin substrates, including ribonucleotide reductase. More work is needed to clarify the nuanced mechanism(s) through which this highly conserved peroxidase influences genome stability, and to determine if this mechanism is similar across a range of species.

Trapping redox partnerships in oxidant-sensitive proteins with a small, thiol-reactive cross-linker.

A broad range of redox-regulated proteins undergo reversible disulfide bond formation on oxidation-prone cysteine residues. Heightened reactivity of the thiol groups in these cysteines also increases susceptibility to modification by organic electrophiles, a property that can be exploited in the study of redox networks. Here, we explored whether divinyl sulfone (DVSF), a thiol-reactive bifunctional electrophile, cross-links oxidant-sensitive proteins to their putative redox partners in cells. To test this idea, previously identified oxidant targets involved in oxidant defense (namely, peroxiredoxins, methionine sulfoxide reductases, sulfiredoxin, and glutathione peroxidases), metabolism, and proteostasis were monitored for cross-link formation following treatment of Saccharomyces cerevisiae with DVSF. Several proteins screened, including multiple oxidant defense proteins, underwent intermolecular and/or intramolecular cross-linking in response to DVSF. Specific redox-active cysteines within a subset of DVSF targets were found to influence cross-linking; in addition, DVSF-mediated cross-linking of its targets was impaired in cells first exposed to oxidants. Since cross-linking appeared to involve redox-active cysteines in these proteins, we examined whether potential redox partners became cross-linked to them upon DVSF treatment. Specifically, we found that several substrates of thioredoxins were cross-linked to the cytosolic thioredoxin Trx2 in cells treated with DVSF. However, other DVSF targets, like the peroxiredoxin Ahp1, principally formed intra-protein cross-links upon DVSF treatment. Moreover, additional protein targets, including several known to undergo S-glutathionylation, were conjugated via DVSF to glutathione. Our results indicate that DVSF is of potential use as a chemical tool for irreversibly trapping and discovering thiol-based redox partnerships within cells.