Calnexin Associates Exclusively with Individual CD3δ and T Cell Antigen Receptor (TCR) α Proteins Containing Incompletely Trimmed Glycans That Are Not Assembled into Multisubunit TCR Complexes (∗)

Most T lymphocytes express on their surfaces an oligomeric protein complex consisting of clonotypic αβ polypeptides associated with invariant CD3-γδε and ζ chains, designated the T cell antigen receptor (TCR) complex. Assembly and intracellular transport of nascent TCR proteins is believed to be assisted by their interaction with the molecular chaperone calnexin, which for certain molecules functions as a lectin for monoglucosylated glycans. However, as most of our knowledge about calnexin-TCR protein associations has been obtained under conditions of limited TCR assembly, the role of calnexin in the formation of nascent TCR complexes is unclear. Here, we studied the role of glucose (Glc) trimming and calnexin association in the oligomerization of TCRα and CD3δ glycoproteins in murine splenic T lymphocytes, a model cell type for efficient assembly of complete TCR complexes. We show that removal of Glc residues from both CD3δ proteins and TCRα proteins occurred prior to their association with any other TCR components and that calnexin specifically interacted with unassembled TCRα and CD3δ proteins containing incompletely trimmed oligosaccharides. Interestingly, we found that removal of Glc residues from glycan chains was necessary for efficient association of calnexin with TCRα glycoproteins but not with CD3δ glycoproteins. These studies define Glc trimming and calnexin association as initial molecular events in the translation of CD3δ and TCRα proteins, occurring coincident with or immediately after their translocation into the endoplasmic reticulum and preceding the ordered pairing of TCR chains. In addition, these data document that calnexin assembly with CD3δ and TCRα glycoproteins involves both glycan-dependent and glycan-independent mechanisms.