STRUCTURE AND DIVERSITY OF THE T-CELL RECEPTOR-ALPHA CHAIN IN THE MEXICAN AXOLOTL

Polymerase chain reaction was used to isolate cDNA clones encoding put ative T-cell receptor (TCR) alpha chains in an amphibian, the Mexican axolotl (Ambystoma mexicanum). Five TCR alpha-V chain-encoding segment s were identified, each belonging to a separate family. The best ident ity scores for these axolotl TCR alpha-V segments were all provided by sequences belonging to the human TCR alpha-VI family and the mouse TC R alpha-V3 and TCR alpha-V8 families. A total of 14 different TCRA-J s egments were identified from 44 TCRA-V/TCRA-J regions sequenced, sugge sting that a large repertoire of TCRA-J segments is a characteristic o f most vertebrates. The structure of the axolotl CDR3 alpha chain loop is in good agreement with that of mammals, including a majority of sm all hydrophobic residues at position 92 and of charged, hydrophilic, o r polar residues at positions 93 and 94, which are highly variable and correspond to the TCRA-V/J junction. This suggests that some position s of the axolotl CDR3 alpha chain loop are positively selected during T-cell differentiation, particularly around residue 93 that could be s elected for its ability to makes contacts with major histocompatibilit y complex-associated antigenic peptides, as in mammals. The axolotl C alpha domain had the typical structure of mammalian and avian C alpha domains, including the charged residues in the TM segment that are tho ught to interact with other proteins in the membrane, as well as most of the residues forming the conserved antigen receptor transmembrane m otif.