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 CD3ζ, CD3γδ and CD3ε in Atlantic salmon, and real-time RT-PCR analysis to reveal their tissue-specific expression. Salmon CD3ζ 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 CD3ζ in salmon has 7 exons. Salmon CD3γδ (a forerunner of CD3γ and CD3δ in mammals) and CD3ε are related molecules each having an Ig-like EX domain, a TM peptide and a CY tail with one ITAM. Two distinct CD3γδ genes were found, each having 6 exons. The gene encoding CD3ε in salmon has 5 exons. RT-PCR also revealed a transcript from a degenerated CD3ε gene in salmon (Salmo salar) and brown trout (Salmo trutta). This pseudogene is located tail to tail to a CD3γδ gene in salmon and has a typical CD3ε 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 CD3ε pseudogene was only a fraction of that from the intact CD3ε gene.