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 receptor (TCR) molecules or TCR genes have been identified in ectothermic vertebrate species. Here we describe cDNA clones from an amphibian species, Ambystoma mexicanum (the Mexican axoloti), that have sequences highly homologous to the avian and mammalian TCRbeta chains. The cloned amphibian beta chain variable region (Vbeta) shares most of the structural characteristics with the more evolved vertebrate Vbeta and presents almost-equal-to 56% amino acid identities with the murine Vbeta14 and 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 conserved glycine at the Vbeta-Jbeta junction suggests the existence of diversity elements. The extracellular domains of the two axolotl beta chain constant region isotypes Cbeta1 and Cbeta2 show an impressively high degree of identity, thus suggesting that a very efficient mechanism of gene correction has been in operation to preserve this structure at least 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 involved in the charged interaction between the TCRalphabeta heterodimer and the CD3 complex.