Biochemical analysis of the Xenopus laevis TCR/CD3 complex supports the "stepwise evolution" model

The TCR/CD3 complex of a cold‐blooded vertebrate, the amphibian Xenopus laevis, was biochemically characterized with a cross‐reactive polyclonal antiserum recognizing a conserved epitope in the cytoplasmic domain of CD3ϵ. The specificity and utility of this reagent was validated by Western blot analysis and immunoprecipitation of the well‐characterized chicken TCR/CD3 complex. Cross‐reactivity with the X. laevis CD3ϵ protein was demonstrated by specific staining of sorted CD8+ cells. Immunohistology on both tadpoles and adult tissues suggests this antiserum will be instrumental in the localization of Xenopus T cells and most likely NK cells. Double staining of tissue sections with an anti‐CD8 monoclonal antibody confirmed that this staining is specific. The antiserum was also used for the biochemical analyses of X. laevis TCR/CD3 complex. The 75‐kDa α β TCR heterodimer could be separated into a 40‐kDa acidic TCR α chain and a 35‐kDa basic TCR β chain. Two CD3 proteins, both comigrating at approximately 19 kDa, were associated with the TCR heterodimer. Removal of N‐linked carbohydrates yielded CD3 proteins of 19 kDa and 16.5 kDa, most likely representing the CD3ϵ and CD3γ/δ homologues, respectively. An additional band of 110 kDa represents a multimeric complex of the TCR heterodimer covalently linked to a CD3 dimer. These properties of the Xenopus TCR/CD3 complex substantiate a stepwise evolutionary model for the CD3 protein family.