J Biol Chem, Vol. 273, Issue 37, 23674-23680, September 11, 1998

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

Michael J. Bennett, Jeroen E. M. Van Leeuwen, and Kelly P. Kearse

From the Department of Microbiology & Immunology, East Carolina University, School of Medicine, Greenville, North Carolina 27858-4354 and  Experimental Immunology Branch, NCI, National Institutes of Health, Bethesda, Maryland 20892-1360

During T cell development, assembly of the mutisubunit T cell receptor (TCR) complex is regulated by the differential stability of newly synthesized TCR 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 TCR 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 TCR proteins in all other T cell types examined. Here, we compared the processing of TCR glycoproteins and their assembly with calnexin and calreticulin chaperones in CD4⁺CD8⁺ thymocytes and splenic T cells. These studies show that TCR glycoproteins synthesized in CD4⁺CD8⁺ thymocytes were processed in a similar manner as those made in splenic T cells and that TCR proteins stably associated with calnexin in both cell types. Interestingly, however, TCR association with the calnexin-related molecule calreticulin was decreased in CD4⁺CD8⁺ thymocytes compared with splenic T cells. Finally, TCR degradation in CD4⁺CD8⁺ thymocytes was impaired by inhibitors of proteasome activity, which was correlated with stabilization of calnexin·TCR complexes. These data demonstrate that calnexin association is not sufficient to protect TCR 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 TCR glycoproteins.