Crystallization and preliminary X-ray diffraction analysis of the two distinct types of zebrafish β2-microglobulin.

β(2)-Microglobulin (β(2)m) noncovalently associates with the heavy chain of major histocompatibility complex class I (MHC I) molecules, which bind foreign antigen peptides to control the cytotoxic T lymphocyte (CTL) immune response. In contrast to mammals, there are distinct types of β(2)ms derived from two loci in a number of teleost species. In order to clarify the structures of the β(2)ms, the zebrafish (Danio rerio) β(2)ms Dare-β(2)m-I and Dare-β(2)m-II were expressed in Escherichia coli, purified and crystallized, and diffraction data were collected to 1.6 and 1.9 Å resolution, respectively. Both crystals belonged to space group P2(1)2(1)2(1). The unit-cell parameters were determined to be a = 38.2, b = 50.4, c = 50.9 Å for Dare-β(2)m-I and a = 38.9, b = 52.7, c = 65.8 Å for Dare-β(2)m-II. Each asymmetric unit was constituted of one molecule, with Matthews coefficients of 2.22 and 3.01 Å(3) Da(-1) and solvent contents of 45 and 59% for Dare-β(2)m-I and Dare-β(2)m-II, respectively. These two β(2)m structures will provide relevant information for further studies of the structures of the MHC I complex.