Glutathione peroxidase (Gpx) is one of the most important anti-oxidant enzymes in yeast. Gpx3 is a ubiquitously expressed isoform that modulates the activities of redox-sensitive thiol proteins, particularly those involved in signal transduction pathways and protein translocation. In order to search for the interaction partners of Gpx3, we carried out immunoprecipitation/2-dimensional gel electrophoresis (IP-2DE), MALDI-TOF mass spectrometry, and a pull down assay. We found that Mxr1, a peptide methionine sulfoxide reductase, interacts with Gpx3. By reducing methionine sulfoxide to methionine, Mxr1 reverses the inactivation of proteins caused by the oxidation of critical methionine residues. Gpx3 can interact with Mxr1 through the formation of an intermolecular disulfide bond. When oxidative stress is induced by H2O2, this interaction is compromised and the free Mxr1 then repairs the oxidized proteins. Our findings imply that this interaction links redox sensing machinery of Gpx3 to protein repair activity of Mxrl. Based on these results, we propose that Gpx3 functions as a redox-dependent exquisite regulator of the protein repair activity of Mxrl. (c) 2006 Elsevier Inc. All rights reserved.