In Vitro Translation and Assembly of a Complete T Cell Receptor–CD3 Complex

The T cell receptor for antigen (TCR) is a multisubunit complex that consists of at least seven polypeptides: the clonotypic, disulfide-linked α/β heterodimer that is noncovalently associated with the invariant polypeptides of the CD3 complex (CD3-γ, -δ, -ε) and ζ, a disulfide-linked homodimer. We achieved the complete assembly of the human TCR in an in vitro transcription/translation system supplemented with dog pancreas microsomes by simultaneous translation of the messenger RNAs encoding the TCR-α, -β and CD3-γ, -δ, -ε, and -ζ subunits. CD3-ε, one of the subunits that initiates the assembly of the TCR in living cells, forms misfolded, disulfide-linked homooligomers when translated alone. However, co-translation of one of its first binding partners in the course of assembly, CD3-γ or -δ, led to the expression of mainly monomeric and correctly folded ε subunits, the only form we could detect as part of a properly assembled TCR complex. In the absence of these subunits, the ER-resident chaperone calnexin interacted with oligomeric, i.e. misfolded, structures of CD3-ε in a glycan-independent manner. A glycan-dependent interaction between CD3-ε and calnexin was mediated by CD3-γ and concerned only monomeric CD3-ε complexed with CD3-γ, but was dispensable for proper folding of CD3-ε. We suggest that in addition to its signaling function, CD3-ε serves as a monitor for proper subunit assembly of the TCR.