TAPBPR bridges UDP-glucose:glycoprotein glucosyltransferase 1 onto MHC class I to provide quality control in the antigen presentation pathway

Recently we revealed that TAPBPR is a peptide exchange catalyst important for optimal peptide selection by MHC class I molecules. Here we asked if any other co-factors associate with TAPBPR which would explain its effect on peptide selection. We identify an interaction between TAPBPR and UDP-glucose:glycoprotein glucosyltransferase 1 (UGT1), a folding sensor in the calnexin/calreticulin quality control cycle known to regenerate the Glc1Man9GlcNAc2 moiety on glycoproteins. Our results suggest the formation of a multimeric complex, dependent on a conserved cysteine at position 94 in TAPBPR, in which TAPBPR promotes the association of UGT1 with peptide-receptive class I molecules. We reveal that the interaction between TAPBPR and UGT1 facilities the reglucosylation of the glycan on class I, promoting their recognition by calreticulin. Our results suggest that in addition to being a peptide-editor, TAPBPR improves peptide optimisation by promoting peptide-receptive MHC class I molecules to associate with the peptide-loading complex.