Structure and diversity of the TCR alpha-chain in a teleost fish

T cell receptor beta-chain genes are well characterized in representatives of most vertebrate phyla, from sharks to mammals, but the molecular structure of complete TCR alpha-chains has not yet been established in cold-blooded vertebrates. We used a PCR approach to isolate cDNAs encoding putative teleost fish (Oncorhynchus mykiss, rainbow trout) TCR alpha-chains. Eight V alpha segments were identified, belonging to six different families, and the best amino acid sequence identity scores for these trout V alpha were all provided by mammalian V alpha or V delta sequences. Twenty-four (60.1 %) of the 39 analyzed V alpha segments belong to the V alpha 2 family, which has limited homology with mammalian V alpha/delta sequences and with the human V pre-B sequence. A total of 32 different J alpha segments were identified from 40 J alpha regions sequenced, suggesting that a large repertoire of J alpha segments is a characteristic of most vertebrates. The structural properties of the TCR alpha-chain complementarity-determining region 3 loop are well conserved between trout and mammals, suggesting that this region has been under continuous selective pressure in jawed vertebrate evolution. The trout C alpha segment has conserved N-terminal and transmembrane domains, but the C alpha intercysteine distance contains only 40 residues, significantly smaller as compared with mammals (49-56 residues). The conserved features of teleost fish TCR beta- and alpha-chains with their mammalian equivalents suggest that TCR-alpha beta receptors were still present in the common Devonian ancestors of modern teleost fish and mammals, about 450 million years ago.