Moore L J, Somamoto T, Lie K K, Dijkstra J M, Hordvik I
Department of Biology, University of Bergen, Thormølhensgate 55, 5008 Bergen, Norway.
Mol Immunol. 2005 Jun;42(10):1225-34. doi: 10.1016/j.molimm.2004.11.017.
The genes and corresponding cDNAs of both alpha and beta chains of the Atlantic salmon (Salmo salar) CD8 molecule have been sequenced and characterized. In addition, the cDNAs for alpha and beta chains of brown trout (Salmo trutta) and for the beta 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 TCRalpha gene, which is known to be co-expressed with CD8 on mammalian T cells. This is the first report of a sequence for CD8beta in a teleost and together with the CD8alpha sequence, it encodes the ortholog of the CD8 co-receptor molecule on mammalian T cells.
大西洋鲑（Salmo salar）CD8分子的α链和β链的基因及相应的cDNA已被测序和表征。此外，褐鳟（Salmo trutta）α链和β链以及虹鳟（Oncorhynchus mykiss）β链的cDNA也已被测序。这些cDNA编码信号序列，其前面是短的5'非翻译区（UTR）。随后是典型的免疫球蛋白超家族可变序列，所有这些序列都包含两个保守的半胱氨酸用于链内二硫键。铰链区显示出用于二聚化的保守半胱氨酸以及每个预测蛋白的几个O-糖基化基序。与哺乳动物具有最高序列同一性的结构域是所有序列的单次跨膜结构域。在鲑鱼中，除了细胞质/3'UTR结构域外，每个结构域主要由单个外显子编码，α和β基因的细胞质/3'UTR结构域分别由3个和2个外显子编码。在α基因中，第二个细胞质外显子可能被剪接掉以形成另一种较短的转录本，如果表达，将表现出截短的细胞质尾巴。在鲑鱼β基因中发现的一种剪接变体仅在40个氨基酸后引入一个终止密码子。鲑科鱼类序列之间的总体氨基酸同一性高于90％，而它们与鸡和人等物种仅共享15-20％的同一性。使用定量RT-PCR分析这两个鲑鱼基因的表达模式显示在胸腺中表达非常高。这与TCRα基因的表达情况相似，已知TCRα基因在哺乳动物T细胞上与CD8共表达。这是硬骨鱼中CD8β序列的首次报道，与CD8α序列一起，它编码哺乳动物T细胞上CD8共受体分子的直系同源物。
