Alabyev B Y, Guselnikov S V, Najakshin A M, Mechetina L V, Taranin A V
Institute of Cytology and Genetics, Novosibirsk, Russia.
Immunogenetics. 2000 Oct;51(12):1012-20. doi: 10.1007/s002510000236.
CD3epsilon is an essential component of the T-cell receptor (TCR) complex for antigen. We report here molecular cloning and characterization of cDNAs encoding the CD3epsilon homologues in sterlet (Acipenser ruthenus), a representative of primitive chondrostean fishes. Sequence analysis of the cDNA clones demonstrated unexpectedly high CD3epsilon gene heterogeneity in this species. While some cDNAs encoded proteins with the structure typical of mammalian CD3epsilon, others coded for proteins lacking the membrane-proximal half of the extracellular domain. Two cDNAs contained in-frame stop codons in the region encoding the cytoplasmic domain. Based on genomic blot analysis and RT-PCR typing of individual spleen RNAs, we suggest that sterlet may possess two highly polymorphic CD3epsilon loci, of which one can produce alternatively spliced transcripts. The structural elements shown to be functionally important in the mammalian CD3epsilon are strongly conserved in the sterlet CD3epsilon. The cytoplasmic region contains an immunoreceptor tyrosine-based activation motif (ITAM) with YEPI and YSGL tyrosine-containing sequences that are characteristic of only this TCR subunit. The pattern of sequence conservation indicates also that strong selection pressure was imposed on a motif VYYW at the C-end of the transmembrane domain and on a CD3epsilon-specific proline-rich motif RXPPVP juxtaposed to the N-terminus of the ITAM. Weak similarity of the sterlet CD3epsilon with the chicken and Xenopus CD3gamma/delta indicates that these two TCR subunits diverged before radiation of bony fishes and tetrapods. While the role of CD3epsilon heterogeneity in sterlet remains to be elucidated, the data obtained show that the basic mechanisms of TCR signaling have ancient evolutionary origin.
CD3ε是T细胞抗原受体（TCR）复合物的一个重要组成部分。我们在此报告了原始软骨硬鳞鱼类代表——小体鲟（Acipenser ruthenus）中编码CD3ε同源物的cDNA的分子克隆及特征分析。对cDNA克隆的序列分析显示，该物种中CD3ε基因存在意外的高度异质性。虽然一些cDNA编码的蛋白质具有哺乳动物CD3ε典型的结构，但其他一些编码的蛋白质缺少细胞外结构域靠近膜的一半。两个cDNA在编码细胞质结构域的区域含有框内终止密码子。基于基因组印迹分析和单个脾脏RNA的RT-PCR分型，我们认为小体鲟可能拥有两个高度多态的CD3ε基因座，其中一个可以产生可变剪接的转录本。在哺乳动物CD3ε中显示具有功能重要性的结构元件在小体鲟CD3ε中高度保守。细胞质区域包含一个基于免疫受体酪氨酸的激活基序（ITAM），带有YEPI和YSGL含酪氨酸序列，这是该TCR亚基所特有的。序列保守模式还表明，跨膜结构域C端的基序VYYW以及与ITAM N端相邻的CD3ε特异性富含脯氨酸的基序RXPPVP受到了强烈的选择压力。小体鲟CD3ε与鸡和非洲爪蟾的CD3γ/δ的弱相似性表明，这两个TCR亚基在硬骨鱼和四足动物辐射分化之前就已经分化。虽然小体鲟中CD3ε异质性的作用仍有待阐明，但所获得的数据表明，TCR信号传导的基本机制具有古老的进化起源。
