In mammals CD4 is a membrane glycoprotein on Th cells with four extracellular immunoglobulin-like (Ig-like) domains (D1-D4). It functions as a co-receptor during immune recognition between the TCR and the MHC II/peptide complex. The cytoplasmic domain binds p56lck, a protein kinase responsible for phosphorylating CD3 which is the first interaction in a cascade leading to T cell activation. We have previously reported a CD4-2 gene in rainbow trout (Oncorhynchus mykiss) which was found adjacent to the CD4-1 gene by synteny analysis. There are two subtypes (a and b) of CD4-2 in rainbow trout, with two Ig-like extracellular domains. Here we present the homologues of mammalian CD4 in Atlantic salmon (Salmo salar): CD4-1 with four extracellular domains and CD4-2a and CD4-2b with two extracellular domains. A Southern blot analysis shows two copies of the CD4-1 gene in the genomic DNA of the closely related rainbow trout. The genes for CD4-1 and CD4-2 have been sequenced and show typical traits for CD4 genes, such as the code for the first domain (D1) being divided between two exons and the other domains being largely coded for by single exons. The corresponding translated cDNAs show little (13-17%) identity to higher vertebrates and are approximately 37% similar to other translated, teleost sequences but are 89% identical to the closely related rainbow trout. However they exhibit conserved features such as the Lck binding motif in their cytoplasmic domains and the order of variable and constant type Ig-like domains. qRT-PCR data are presented describing the differential tissue expression of these genes together with other T cell markers (TCR and CD3) in several individuals.
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