The γ and ε Subunits of the CD3 Complex Inhibit Pre-Golgi Degradation of Newly Synthesized T Cell Antigen Receptors

The T cell receptor for antigen (TCR) is composed of six different transmembrane proteins. T cells carefully control the intracellular transport of the receptor and allow only complete receptors to reach the plasma membrane. In an attempt to understand how T cells regulate this process, we used c-DNA transfection and subunit-specific antibodies to follow the intracellular transport of five subunits (αβγδε) of the receptor. In particular, we assessed the intracellular stability of each chain. Our results showed that the chains were markedly different in their susceptibility to intracellular degradation. TCR α and β and CD3 δ were degraded rapidly, whereas CD3 γ and ε were stable. An analysis of the N-linked oligosaccharides of the glycoprotein subunits suggested that the chains were unable to reach the medial Golgi during the metabolic chase. This was supported by immunofluorescence micrographs that showed both the stable CD3 γ and unstable CD3 δ chain localized in the endoplasmic reticulum. To study the effects of subunit associations on intracellular transport we used cotransfection to reconstitute precise combinations of subunits. Associations between stable and unstable subunits expressed in the same cell led to the formation of stable complexes. These complexes were retained in or close to the endoplasmic reticulum. The results suggested that the intracellular transport of the T cell receptor could be regulated by two mechanisms. The TCR α and β and CD3 δ subunits were degraded rapidly and as a consequence failed to reach the plasma membrane. CD3 γ or ε were stable but were retained inside the cell. The results also demonstrated that there was an interplay between the two pathways such that the CD3 γ and ε subunits were able to protect labile chains from rapid intracellular degradation. In this way, they could seed subunit assembly in or close to the endoplasmic reticulum and allow a stable receptor to form before its transport to the plasma membrane.