The connecting peptide domain of pT alpha dictates weak association of the pre-T cell receptor with the TCR zeta subunit.

Signals delivered through the pre-TCR, a heterodimer of pT alpha and TCR beta chains, are crucial for the maturation and proliferation of immature alphabeta 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 alpha chains were generated by replacing domains of the alpha chain cytoplasmic, transmembrane and constant regions with homologous domains from the pT alpha 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 alpha chain was substituted by the one from pT alpha, were weakly associated with the TCR zeta chain, showing that functional but not physical interactions were preserved in such chimeras. In contrast, introduction of the connecting peptide of TCR alpha in the pT alpha chain was insufficient to confer stable association with the TCR zeta chain. These results demonstrate that the inability of the pre-TCR to interact strongly with TCR zeta 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 alphand the zeta2 homodimer prevents the activation of specific TCR zeta-dependent signaling pathways, and thus confers unique signaling properties upon the pre-TCR. In addition, this structural difference between the pT alpha/beta and alphabeta TCR might constitute a means to regulate the expression of these receptors at the surface of thymocytes, at different stages of their maturation.