Molecular mechanisms in the TCR (TCRαβ–CD3δϵ,γϵ) interaction with ζ2 homodimers: clues from a `phenotypic revertant' clone

The association between the TCRαβ–CD3γϵδϵ hexamers and ζ2 homodimers in the endoplasmic reticulum (ER) constitutes a key step in TCR assembly and export to the T cell surface. Incompletely assembled TCR–CD3 complexes are degraded in the ER or the lysosomes. A previously described Jurkat variant (J79) has a mutation at position 195 on the TCR Cα domain causing a phenylalanine to valine exchange. This results in a lack of association between TCRαβ–CD3γϵδϵ hexamers and ζ2 homodimers. Two main hypotheses could explain this phenomenon in J79 cells: TCR–CD3 hexamers may be incapable of interacting with ζ2 due to a structural change in the TCR Cα region; alternatively, TCR–CD3 hexamers may be incapable of interacting with ζ2 due to factors unrelated to either molecular complex. In order to assess these two possibilities, the TCR–CD3 membrane-negative J79 cells were treated with ethylmethylsulfonate and clones positive for TCR membrane expression were isolated. The characterization of the J79r58 phenotypic revertant cell line is the subject of this study. The main question was to assess the reason for the TCR re-expression. The TCR on J79r58 cells appears qualitatively and functionally equivalent to wild-type TCR complexes. Nucleotide sequence analysis confirmed the presence of the original mutation in the TCR Cα region but failed to detect compensatory mutations in α, β, γ, δ, ϵ or ζ chains. Thus, mutated J79-TCR–CD3 complexes can interact with ζ2 homodimers. Possible mechanisms for the unsuccessful TCR–CD3 interaction with ζ2 homodimers are presented and discussed.