Characterization of the CD3ζ, CD3γδ and CD3ε subunits of the T cell receptor complex in Atlantic salmon

The CD3 subunits are essential components of the T cell receptor complex, transmitting signals to the inside of the cell. We report here cDNAs and corresponding genes encoding CD3 zeta, CD3 gamma delta and CD3 epsilon in Atlantic salmon, and real-time RT-PCR analysis to reveal their tissue-specific expression. Salmon CD3 zeta is the subunit that shows the highest sequence similarity to the mammalian counterparts, comprising of a short extracellular (EX) part, a transmembrane (TM) peptide and a long cytoplasmic (CY) tail with three immunoreceptor tyrosine-based activation motifs (ITAMs). The gene encoding CD3 zeta in salmon has 7 exons. Salmon CD3 gamma delta (a forerunner of CD3 gamma and CD3 delta in mammals) and CD3 epsilon are related molecules each having an Ig-like EX domain, a TM peptide and a CY tail with one ITAM. Two distinct CD3 gamma delta genes were found, each having 6 exons. The gene encoding CD3 epsilon in salmon has 5 exons. RT-PCR also revealed a transcript from a degenerated CD3 epsilon gene in salmon (Salmo solar) and brown trout (Salmo trutta). This pseudogene is located tail to tail to a CD3 gamma delta gene in salmon and has a typical CD3 epsilon gene structure with the exception of 1 extra exon. All the CD3 genes in salmon were most abundantly expressed in thymus but the expression of the CD3 epsilon pseudogene was only a fraction of that from the intact CD3 epsilon gene. (C) 2007 Elsevier Ltd. All rights reserved.