Expression, purification, crystallization and preliminary X-ray diffraction analysis of grass carp β₂-microglobulin

β₂-Microglobulin (β₂m) is an essential subunit of MHC I molecules; it stabilizes the structure of MHC I and plays a pivotal role in coreceptor recognition. To date, structures of β₂m have been solved for three different mammals: human, mouse and cattle. In order to illuminate the molecular evolutionary origin of β2m, an understanding of its structure in lower vertebrates becomes important. Here, grass carp (Ctenopharyngodon idellus) β₂m (Ctid-β₂m) was expressed, purified and crystallized. Diffraction data were collected to a resolution of 2.5 Å. The crystal belongs to space group P2₁2₁2₁, with unit-cell parameters a = 38.72, b = 40.65, c = 71.12 Å. The Matthews coefficient and the solvent content were calculated to be 2.56 Å Da^-1 and 52.07%, respectively, for one molecule per asymmetric unit. The structure has been solved by molecular replacement using monomeric human β₂m as a model.