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 A. The crystal belongs to space group P212121, with unit-cell parameters a = 38.72, b = 40.65, c = 71.12 A. The Matthews coefficient and the solvent content were calculated to be 2.56 A 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.