Reanalysis of Immunopeptidomics Datasets Provides Mechanistic Insight into TAPBPR-Mediated Peptide Editing on HLA-A, -B and -C Molecules.

Background: Major histocompatibility class I (MHC-I, human leukocyte antigen [HLA] class I in humans) molecules present small fragments of the proteome on the cell surface for immunosurveillance, which is pivotal to control infected and malignant cells. Immunogenic peptides are generated and selected in the MHC-I antigen processing and presentation pathway. In this pathway, two homologous molecules, tapasin and TAPBPR, optimise the MHC-I peptide repertoire that is ultimately presented at the plasma membrane. Peptide exchange on HLA class I by human TAPBPR involves the flexible loop region K22-D35, with the leucine at position 30 (L30) involved in mediating peptide dissociation. However, our understanding of the exact molecular mechanisms governing TAPBPR-mediated peptide exchange on HLA class I allotypes remains incomplete. Methods: Here, in-depth re-analyses of published immunopeptidomics datasets was used to further examine TAPBPR peptide editing activity and mechanism of action on HLA class I. The role of the TAPBPR editing loop in opening the HLA class I peptide binding groove was assessed using molecular dynamics simulations and a peptide exchange assay. Results: We show that TAPBPR shapes the peptide repertoire on HLA-A, -B and -C allotypes. The TAPBPR editing loop was not essential to allow HLA class I to adopt an open state but did allow for the HLA-A*68:02 peptide binding groove to stay open for a sustained period. L30 in the TAPBPR editing loop was typically sufficient to mediate peptide repertoire restriction on the three HLA class I allotypes expressed by HeLa cells. TAPBPR was also able to load peptides onto HLA class I in a loop-dependent manner. Conclusions: These results suggest that the TAPBPR editing loop is involved both in peptide filtering and loading.