The Related Molecular Chaperones Calnexin and Calreticulin Differentially Associate with Nascent T Cell Antigen Receptor Proteins within the Endoplasmic Reticulum*

Abstract

Assembly of the multisubunit T cell antigen receptor (TCR) complex is an intricate process requiring coordinated regulation of at least six different gene products (α, β, γ, δ, ϵ, and ζ) and the ordered pairing of partner chains within the endoplasmic reticulum (ER). To date, two proteins have been implicated as functioning as molecular chaperones in the assembly of nascent TCR proteins: calnexin, a resident ER transmembrane protein, which associates with all TCR components except ζ, and T cell receptor-associated protein, which selectively associates with CD3γϵ pairs. In this study, we examined the association of calreticulin, a soluble protein with significant sequence homology to calnexin, with newly synthesized TCR proteins. Analogous to calnexin, processing of glycan chains by glucosidase enzymes was required for initial association of TCRα and -β proteins with calreticulin; however, several major differences were noted regarding interaction of calnexin and calreticulin chaperones with TCR proteins. First, TCRα and -β proteins showed prolonged association with calnexin molecules compared with calreticulin; interaction of TCRα proteins with calreticulin was particularly transient, with most calreticulin-TCRα protein complexes dissociating within 15 min of their initial assembly. Second, we found that, unlike calnexin, which associated with clonotypic TCRα and -β proteins and invariant CD3δ and -ϵ polypeptides, calreticulin associated specifically with clonotypic TCRα and -β proteins. These studies identify calreticulin as a molecular chaperone for nascent clonotypic TCRα and -β proteins and demonstrate that calreticulin and calnexin differentially associate with newly synthesized TCR proteins within the ER.