The genes and corresponding cDNAs of both α and β chains of the Atlantic salmon (Salmo salar) CD8 molecule have been sequenced and characterized. In addition, the cDNAs for α and β chains of brown trout (Salmo trutta) and for the β chain in rainbow trout (Oncorhynchus mykiss) have been sequenced. The cDNAs code for signal sequences which are preceded by short 5′ UTRs. These are followed by typical immunoglobulin superfamily variable sequences all of which contain two conserved cysteines for the intra-chain disulphide bond. The hinge regions display conserved cysteines for dimerisation and several O-glycosylation motifs for each predicted protein. The domain sharing the highest sequence identity with mammals is the single pass transmembrane domain for all sequences. In salmon, each domain is predominantly coded for by a single exon except the cytoplasmic/3′ UTR domains, which are coded for by 3 and 2 exons for the alpha and beta genes, respectively. In the alpha gene, the second cytoplasmic exon may be spliced out to form an alternative shorter transcript which if expressed would exhibit a truncated cytoplasmic tail. A splice variant found for the salmon beta gene introduces a stop codon after only 40 amino acids. Overall amino acid identities between salmonid sequences were higher than 90%, whereas they shared only 15-20% identity with species such as, chicken and human. Analysis of the expression patterns of the two salmon genes using quantitative RT-PCR shows a very high expression in the thymus. This is mirrored by the expression of the TCRα gene, which is known to be co-expressed with CD8 on mammalian T cells. This is the first report of a sequence for CD8β in a teleost and together with the CD8α sequence, it encodes the ortholog of the CD8 co-receptor molecule on mammalian T cells.
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