Role of structural and functional elements of mouse methionine-S-sulfoxide reductase in its subcellular distribution.

Oxidized forms of methionine residues in proteins can be repaired by methionine-S-sulfoxide reductase (MsrA) and methionine-R-sulfoxide reductase (MsrB). In mammals, three MsrBs are present, which are targeted to various subcellular compartments. In contrast, only a single mammalian MsrA gene is known whose products have been detected in both cytosol and mitochondria. Factors that determine the location of the protein in these compartments are not known. Here, we found that MsrA was present in cytosol, nucleus, and mitochondria in mouse cells and tissues and that the major enzyme forms detected in various compartments were generated from a single-translation product rather than by alternative translation initiation. Both cytosolic and mitochondrial forms were processed with respect to the N-terminal signal peptide, and the distribution of the protein occurred post-translationally. Deletion of amino acids 69-108, 69-83, 84-108, or 217-233, which contained elements important for MsrA structure and function, led to exclusive mitochondrial location of MsrA, whereas a region that affected substrate binding but was not part of the overall fold had no influence on the subcellular distribution. The data suggested that proper structure-function organization of MsrA played a role in subcellular distribution of this protein in mouse cells. These findings were recapitulated by expressing various forms of mouse MsrA in Saccharomyces cerevisiae, suggesting conservation of the mechanisms responsible for distribution of the mammalian enzyme among different cellular compartments.