Has an important function as a repair enzyme for proteins that have been inactivated by oxidation. Catalyzes the reduction of methionine sulfoxide in proteins to methionine (PubMed:12145281, PubMed:30529269). Does not catalyze the reverse reaction involving the oxidation of methionine residues (PubMed:30529269). [thioredoxin]-disulfide + H2O + L-methionyl-[protein] = [thioredoxin]-dithiol + L-methionyl-(S)-S-oxide-[protein] [thioredoxin]-disulfide + H2O + L-methionine = [thioredoxin]-dithiol + L-methionine (S)-S-oxide By ecdysone. Conjugated to URM1, a ubiquitin-like protein. Belongs to the MsrA Met sulfoxide reductase family. Unlike mammalian MSRA, lacks methionine oxidase activity because Cys-232 cannot function as a resolving cysteine to perform the disulfide exchange and instead remains as a free thiol (PubMed:30529269). As a result, the active site cysteine is trapped in disulfide linkage with Cys-246 and is therefore unable to react with a second molecule of methionine sulfoxide to complete methionine oxidation (PubMed:30529269).