STRUCTURE AND DIVERSITY OF THE TCR ALPHA-CHAIN IN A TELEOST FISH

T cell receptor beta-chain genes are well characterized in representat ives of most vertebrate phyla, from sharks to mammals, but the molecul ar 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) T CR alpha-chains. Eight V alpha segments were identified, belonging to six different families, and the best amino acid sequence identity scor es 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 segme nts belong to the V alpha 2 family, which has limited homology with ma mmalian 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 alph a regions sequenced, suggesting that a large repertoire of J alpha seg ments is a characteristic of most vertebrates. The structural properti es of the TCR alpha-chain complementarity-determining region 3 loop ar e well conserved between trout and mammals, suggesting that this regio n has been under continuous selective pressure in jawed vertebrate evo lution. The trout C alpha segment has conserved N-terminal and transme mbrane domains, but the C alpha intercysteine distance contains only 4 0 residues, significantly smaller as compared with mammals (49-56 resi dues). The conserved features of teleost fish TCR beta- and alpha-chai ns with their mammalian equivalents suggest that TCR-alpha beta recept ors were still present in the common devonian ancestors of modern tele ost fish and mammals, about 450 million years ago.