Structure and diversity of the T-cell receptor α chain in the Mexican axolotl

Abstract

 Polymerase chain reaction was used to isolate cDNA clones encoding putative T-cell receptor (TCR) α chains in an amphibian, the Mexican axolotl (Ambystoma mexicanum). Five TCRα-V chain-encoding segments were identified, each belonging to a separate family. The best identity scores for these axolotl TCRα-V segments were all provided by sequences belonging to the human TCRα-V1 family and the mouse TCRα-V3 and TCRα-V8 families. A total of 14 different TCRA-J segments were identified from 44 TCRA-V/TCRA-J regions sequenced, suggesting that a large repertoire of TCRA-J segments is a characteristic of most vertebrates. The structure of the axolotl CDR3 α chain loop is in good agreement with that of mammals, including a majority of small hydrophobic residues at position 92 and of charged, hydrophilic, or polar residues at positions 93 and 94, which are highly variable and correspond to the TCRA-V/J junction. This suggests that some positions of the axolotl CDR3 α chain loop are positively selected during T-cell differentiation, particularly around residue 93 that could be selected for its ability to makes contacts with major histocompatibility complex-associated antigenic peptides, as in mammals. The axolotl Cα domain had the typical structure of mammalian and avian Cα domains, including the charged residues in the TM segment that are thought to interact with other proteins in the membrane, as well as most of the residues forming the conserved antigen receptor transmembrane motif.