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.