Signaling Efficiency of the T Cell Receptor Controlled by a Single Amino Acid in the β Chain Constant Region

A single amino acid residue, Gln₁₃₆, located within the connecting peptide domain of Cβ controls the ability of the α/β TCR to transmit a full signal. TCRs in which this Cβ residue is mutated to Phe, the residue found in TCR-γ, are unresponsive to antigenic ligands. Interestingly, this Cβ residue is either polar or charged in every species studied thus far, including the trout and the skate. In contrast, the analogous residue in Cγ is always hydrophobic. In spite of their compromised antigen responsiveness, the mutant TCR complex contains the CD3-γ, -δ, -ε, and -ζ chains, and undergoes ζ chain phosphorylation and ZAP-70 recruitment. However, the biological response of the mutant TCR could be rescued with a calcium ionophore, implying that mutant TCRs are defective in generating a calcium-mediated signal. The implications of the differences between Cβ and Cγ are considered.