TAPBPR Promotes Antigen Loading on MHC-I Molecules Using a Peptide Trap

Chaperones tapasin and TAP-binding protein related (TAPBPR) perform the important functions of stabilizing nascent MHC-I molecules (chaperoning) and selecting high affinity peptides in the MHC-I groove (editing). While X-ray and cryo-EM snapshots of MHC-I in complex with TAPBPR and tapasin, respectively, have provided important insights into the peptide-deficient MHC-I groove structure, the molecular mechanism through which these chaperones influence the selection of specific amino acid sequences remains incompletely characterized. Of particular importance is a 16 residue loop in TAPBPR (corresponding to 11 residues in tapasin), which has been proposed to actively compete with incoming peptides by forming direct contacts in the F-pocket of the empty MHC-I groove. Using a deep mutational scanning functional analysis of TAPBPR, we find that important residues for the chaperoning activity are located on the major interaction surfaces with nascent MHC-I molecules, excluding the loop. However, interactions with properly conformed molecules toward peptide editing are influenced by loop mutations, in an MHC-I allele- and peptide-dependent manner. Detailed biophysical characterization by ITC, FP and NMR reveals that the loop does not interact with the peptide-deficient MHC-I groove to compete with incoming peptides, but instead promotes peptide loading by acting as a kinetic trap. Our results suggest that the longer loop of TAPBPR lowers the affinity threshold for peptide selection, to promote loading within subcellular compartments of reduced peptide concentration and to prevent disassembly of high affinity peptide-MHC-I complexes that are transiently interrogated by TAPBPR during editing.