CONSERVED STRUCTURE OF AMPHIBIAN T-CELL ANTIGEN RECEPTOR BETA-CHAIN

All jawed vertebrates possess well-differentiated thymuses and elicit T-cell-like cell-mediated responses; however, no surface T-cell recept or (TCR) molecules or TCR genes have been identified in ectothermic ve rtebrate species. Here we describe cDNA clones from an amphibian speci es, Ambystoma mexicanum (the Mexican axoloti), that have sequences hig hly homologous to the avian and mammalian TCRbeta chains. The cloned a mphibian beta chain variable region (Vbeta) shares most of the structu ral characteristics with the more evolved vertebrate Vbeta and present s almost-equal-to 56% amino acid identities with the murine Vbeta14 an d human Vbeta18 families. The two different cloned axolotl beta chain joining regions (Jbeta) were found to have conserved all the invariant mammalian Jbeta residues, and in addition, the presence of a conserve d glycine at the Vbeta-Jbeta junction suggests the existence of divers ity elements. The extracellular domains of the two axolotl beta chain constant region isotypes Cbeta1 and Cbeta2 show an impressively high d egree of identity, thus suggesting that a very efficient mechanism of gene correction has been in operation to preserve this structure at le ast from the early tetrapod evolution. The transmembrane axolotl Cbeta domains have been less well conserved when compared to the mammalian Cbeta but they do maintain the lysine residue that is thought to be in volved in the charged interaction between the TCRalphabeta heterodimer and the CD3 complex.