Endoplasmic reticulum retention and degradation of T cell antigen receptor beta chain

The T cell antigen receptor-CD3 (TCR/CD3) complex is assembled in the endoplasmic reticulum (ER) of T cells after synthesis of individual chains, and is transported to the cell surface for immune recognition and regulation. Partially assembled or unassembled TCR chains are retained and rapidly degraded in the ER. These processes are strictly regulated in the ER at post-translational level for the maintenance of cellular homeostasis. In order to identify the region responsible for the ER retention and rapid degradation of the TCR beta chain, number of mutants were engineered and their fates, after synthesis in the ER of the HeLa cells, were investigated. Extensive mutagenic analysis of TCR beta chain, including changing the charged amino acid residues and two tyrosine residues of the transmembrane region into hydrophobic amino acid residues, did not alter the ER retention and rapid degradation. Soluble TCR beta chain and cytoplasmic tail truncation mutant were also rapidly degraded in the ER. However, N-glycosylation rate of soluble TCR beta chain in the ER was significantly increased possibly due to the increased exposure of the N-glycosylation site. These results suggest that the ER retention of TCR beta chain is mediated through its extracellular and transmembrane-cytoplasmic regions and that the rapid ER degradation can be caused by an exposure of unassembled subregion of TCR beta chain, either extracellular domain or hydrophobic transmembrane region to the hydrophilic environment (lumen of the ER) rather than by presence of a specific degradation signal.