Methionine-sulfoxide reductase that specifically reduces methionine (R)-sulfoxide back to methionine. While in many cases, methionine oxidation is the result of random oxidation following oxidative stress, methionine oxidation is also a post-translational modification that takes place on specific residue. Acts as a regulator of actin assembly by reducing methionine (R)-sulfoxide mediated by MICALs (MICAL1, MICAL2 or MICAL3) on actin, thereby promoting filament repolymerization. Plays a role in innate immunity by reducing oxidized actin, leading to actin repolymerization in macrophages. [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. Truncated MSRB1/SEPX1 proteins produced by failed UGA/Sec decoding are ubiquitinated by some Cul2-RING E3 ubiquitin-protein ligase complexes (containing either PRAME, PRAMF6, PRAMF9 or FEM1C as substrate-recognition component). Belongs to the MsrB Met sulfoxide reductase family. Truncated C-terminus.