Compromised calnexin function in calreticulin-deficient cells.

Calnexin and calreticulin are molecular chaperones, which are involved in the protein folding, assembly, and retention/retrieval. We know that calreticulin-deficiency is lethal in utero, but do not understand the contribution of chaperone function to this phenotype. Here we studied protein folding and chaperone function of calnexin in the absence of calreticulin. We show that protein folding is accelerated and quality control is compromised in calreticulin-deficient cells. Calnexin-substrate association is severely reduced, leading to accumulation of unfolded proteins and a triggering of the unfolded protein response (UPR). PERK and Ire1alpha and eIF2alpha are also activated in calreticulin-deficient cells. We show that the absence of calreticulin can have devastating effects on the function of the others, compromising overall quality control of the secretory pathway and activating UPR-dependent pathways.

Assembly and antigen-presenting function of MHC class I molecules in cells lacking the ER chaperone calreticulin.

MHC class I molecules expressed in a calreticulin-deficient cell line (K42) assembled with beta 2-microglobulin (beta2-m) normally, but their subsequent loading with optimal peptides was defective. Suboptimally loaded class I molecules were released into the secretory pathway. This occurred despite the ability of newly synthesized class I to interact with the transporter associated with antigen processing (TAP) loading complex. The efficiency of peptide loading was reduced by 50%-80%, and impaired T cell recognition was observed for three out of four antigens tested. The peptide-loading function was specific to calreticulin, since the defect in K42 could be rectified by transfection with calreticulin but not a soluble form of calnexin, which shares its lectin-like activity.