The connecting peptide domain of pTα dictates weak association of the pre-T cell receptor with the TCR ζ subunit

Signals delivered through the pre-TCR, a heterodimer of pTα and TCR β chains, are crucial for the maturation and proliferation of immature αβ lineage thymocytes from the CD4^- CD8^- to the CD4⁺ CD8⁺ stage. To gain insight into the structural and functional properties of the pre-TCR, chimeric TCR α chains were generated by replacing domains of the α chain cytoplasmic, transmembrane and constant regions with homologous domains from the pTα chain. All chimeric TCR could be expressed stably at the cell surface and induce Ca²⁺ mobilization as well as phosphorylation of several protein substrates on tyrosine residues. However, chimeras wherein the connecting peptide of TCR α chain was substituted by the one from pTα, were weakly associated with the TCR ζ chain, showing that functional but not physical interactions were preserved in such chimeras. In contrast, introduction of the connecting peptide of TCR α in the pTα chain was insufficient to confer stable association with the TCR ζ chain. These results demonstrate that the inability of the pre-TCR to interact strongly with TCR ζ is attributable to amino acid residues present throughout the region comprised between the intrachain Cys and the transmembrane domain. It remains to be determined whether the weak physical interaction between the pre-TCR and the ζ₂ homodimer prevents the activation of specific TCR ζ-dependent signaling pathways, and thus confers unique signaling properties upon the pre-TCR. In addition, this structural difference between the pTα/β and αβ TCR might constitute a means to regulate the expression of these receptors at the surface of thymocytes, at different stages of their maturation.