Calnexin Association Is Not Sufficient to Protect T Cell Receptor α Proteins from Rapid Degradation in CD4+CD8+Thymocytes*

During T cell development, assembly of the mutisubunit T cell receptor (TCR) complex is regulated by the differential stability of newly synthesized TCRα molecules, having a half-life of approximately 20 min in immature CD4+CD8+thymocytes compared with >75 min in mature T cells. The molecular basis for TCRα instability in CD4+CD8+thymocytes is unknown but has been postulated to involve abnormalities in N-glycan processing and calnexin assembly as perturbation of these pathways markedly destabilizes TCRα proteins in all other T cell types examined. Here, we compared the processing of TCRα glycoproteins and their assembly with calnexin and calreticulin chaperones in CD4+CD8+thymocytes and splenic T cells. These studies show that TCRα glycoproteins synthesized in CD4+CD8+thymocytes were processed in a similar manner as those made in splenic T cells and that TCRα proteins stably associated with calnexin in both cell types. Interestingly, however, TCRα association with the calnexin-related molecule calreticulin was decreased in CD4+CD8+thymocytes compared with splenic T cells. Finally, TCRα degradation in CD4+CD8+thymocytes was impaired by inhibitors of proteasome activity, which was correlated with stabilization of calnexin·TCRα complexes. These data demonstrate that calnexin association is not sufficient to protect TCRα proteins from rapid degradation in CD4+CD8+thymocytes, suggesting that additional components of the quality control system of the endoplasmic reticulum operate to ensure the proper folding of nascent TCRα glycoproteins.