Catalyzes the reduction of free and protein-bound methionine sulfoxide to methionine. [thioredoxin]-disulfide + H2O + L-methionyl-[protein] = [thioredoxin]-dithiol + L-methionyl-(R)-S-oxide-[protein] [thioredoxin]-disulfide + H2O + L-methionine = [thioredoxin]-dithiol + L-methionine (R)-S-oxide Binds 1 zinc ion per subunit. Monomer. Not detected in the mitochondrion. Widely expressed. Detected in the sensory epithelia of the organ of Corti and vestibular end organs as early as P2 up to adulthood (at protein level). In the organ of Corti, present in inner and outer hair cells and, to a lesser extent, in supporting cells (at protein level). In hair cells, distributed throughout the cell body. Barely detectable level in stereocilia. Also observed in spiral ganglion neurons, but not in the stria vascularis. In the vestibular end organs, found throughout the sensory epithelium, but more intense expression in hair cells than in supporting cells (at protein level). In vestibular hair cells, present within cell bodies and to a lesser extent in kinocilia. Barely detectable in stereocilia. Expressed in temporal bones at 15 dpc through postnatal 30, levels decrease thereafter, but is still present in the inner ear at P180. Contains a signal peptide followed by a mitochondrial transit peptide within a single transcript. However, the protein is detected exclusively within the endoplasmic reticulum. This contasts with other species, including Homo sapiens, where the signal and transit peptides are encoded on 2 mutually exclusive exons (PubMed:15249228), and where alternative splicing leads to the synthesis of either the endoplasmic reticulum resident form or the mitochondrial form. Belongs to the MsrB Met sulfoxide reductase family. Truncated N-terminus. Truncated N-terminus. Truncated N-terminus.