Characterization of arrangement and expression of the beta-2 microglobulin locus in the sandbar and nurse shark

Beta 2 microglobulin (β2m) is an essential subunit of major histocompatibility complex (MHC) type I molecules. In this report, β2m cDNAs were identified and sequenced from sandbar shark spleen cDNA library. Sandbar shark β2m gene encodes one amino acid less than most teleost β2m genes, and 3 amino acids less than mammal β2m genes. Although sandbar shark β2m protein contains one β sheet less than that of human in the predicted protein structure, the overall structure of β2m proteins is conserved during evolution. Germline gene for the β2m in sandbar and nurse shark is present as a single locus. It contains three exons and two introns. CpG sites are evenly distributed in the shark β2m loci. Several DNA repeat elements were also identified in the shark β2m loci. Sequence analysis suggests that the β2m locus is not linked to the MHC I loci in the shark genome.