Reconstitution of glycan-driven MHC I recycling reveals calreticulin as mediator between TAPBPR and tapasin

Protein folding in the endoplasmic reticulum (ER) is essential for about one-third of the mammalian proteome. N-linked glycosylation and subsequent glycan remodeling barcodes glycoproteins during their maturation in the ER. Major histocompatibility complex class I (MHC I) molecules, key for adaptive immunity, rely on a dedicated quality control cycle that involves specialized chaperones and glycan-modifying enzymes for their maturation and loading of immunogenic peptides. However, the functional interplay of the MHC I editors tapasin as part of the peptide-loading complex (PLC), TAP-binding protein-related (TAPBPR), the UDP-glucose:glycoprotein glucosyltransferase 1 (UGGT1), and calreticulin in glycan-dependent transfer of MHC I clients has not been determined in a reconstituted system. With isolated components, we show that transfer of peptide-receptive MHC I from the downstream quality control factor TAPBPR back to tapasin depends on the recognition of the monoglucosylated glycan of MHC I by calreticulin. While calreticulins C-terminal acidic helix is dispensable for disengaging reglucosylated MHC I from TAPBPR, it is essential for docking MHC I onto tapasin. Our data provide a mechanistic basis for glycan-surveillance by calreticulin necessary for retrograde trafficking of misfolded or suboptimally loaded MHC I that escaped the first quality control at the PLC and were trapped by TAPBPR. Such finetuned dynamic network of glycan-dependent and MHC I-specific chaperones guarantees maturation of MHC I molecules and highlight the fundamental processes driving ER protein quality control. TeaserOur study dissects the mechanistic network of dedicated chaperones and glycan modifiers that quality-control MHC I. Significance StatementThe immune system relies on major histocompatibility complex (MHC) class I molecules to present protein fragments from within cells, enabling detection of infection or disease. This study uncovers how a dynamic, glycan-dependent chaperone network in the endoplasmic reticulum (ER) orchestrates the recycling of misfolded or suboptimally loaded MHC I. Using isolated components, the work shows how monoglucosylation of MHC I glycans by UDP-glucose:glycoprotein glucosyltransferase 1 (UGGT1) allows calreticulin to mediate transfer from the post-ER chaperone TAPBPR back to the ER-resident chaperone tapasin as part of the peptide-loading complex. These findings illuminate the coordinated action of TAPBPR, UGGT1, calreticulin, and tapasin-ERp57 in MHC I quality control, offering new insights in immune surveillance and how its disruption may contribute to disease.