Methionine sulphoxide reductases protect iron-sulphur clusters from oxidative inactivation in yeast

Methionine residues and iron-sulphur (FeS) clusters are primary targets of reactive oxygen species in the proteins of micro-organisms. Here, we show that methionine redox modifications help to preserve essential FeS cluster activities in yeast. Mutants defective for the highly conserved methionine sulphoxide reductases (MSRs; which re-reduce oxidized methionines) are sensitive to many pro-oxidants, but here exhibited an unexpected copper resistance. This phenotype was mimicked by methionine sulphoxide supplementation. Microarray analyses highlighted several Cu and Fe homeostasis genes that were upregulated in the mxrDelta double mutant, which lacks both of the yeast MSRs. Of the upregulated genes, the Cu-binding Fe transporter Fet3p proved to be required for the Cu-resistance phenotype. FET3 is known to be regulated by the Aft1 transcription factor, which responds to low mitochondrial FeS-cluster status. Here, constitutive Aft1p expression in the wild-type reproduced the Cu-resistance phenotype, and FeS-cluster functions were found to be defective in the mxrDelta mutant. Genetic perturbation of FeS activity also mimicked FET3-dependent Cu resistance. 55Fe-labelling studies showed that FeS clusters are turned over more rapidly in the mxrDelta mutant than the wild-type, consistent with elevated oxidative targeting of the clusters in MSR-deficient cells. The potential underlying molecular mechanisms of this targeting are discussed. Moreover, the results indicate an important new role for cellular MSR enzymes in helping to protect the essential function of FeS clusters in aerobic settings.