Identification of two distinct types of beta-2 microglobulin in marine fish, Pagrus major and Seriola quinqueradiata

Two types of beta-2 microglobulin (β2m) were identified in red seabream and yellowtail from expressed sequence tag (EST) analyses of kidney and spleen cells. A cluster of 21 ESTs from red seabream showed high homology to β2m from other teleosts, while another cluster containing 7 ESTs was also homologous to β2m. Similarly, two clusters containing 21 and 2 ESTs were identified to be homologous to β2m in yellowtail. The genes in the large and small clusters of each fish were designated β2m-1 and β2m-2, respectively. Although the amino acid sequences of β2m-1s were highly homologous to those identified in the other teleosts, those of β2m-2s were less homologous to fish β2m. β2m-1 of red seabream lacks the first intron of two introns observed in vertebrtes, and β2m-2 lacks both introns. Phylogenetic analysis of vertebrate β2m proteins suggested that the β2m-2 in marine fish are distinct from β2ms in the other teleosts.