RESUMO
Background: Major histocompatibility class I (MHC-I, human leukocyte antigen [HLA]-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-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-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-I. The role of the TAPBPR editing loop in opening the HLA-I peptide binding groove was assessed using a molecular dynamics simulation. 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-I to adopt an open state. L30 in the TAPBPR editing loop was typically sufficient to mediate peptide repertoire restriction on the three HLA-I allotypes expressed by HeLa cells. TAPBPR was also able to load peptides onto HLA-I in a loop-dependent manner. Conclusions: These results unify the previously hypothesised scoop loop and peptide trap mechanisms of TAPBPR-mediated peptide exchange, with the former involved in peptide filtering and the latter in peptide loading.
Major histocompatibility complex (MHC) class I molecules play an essential role in alerting the immune system to infection and cellular changes. They do this by displaying small fragments of proteins (peptides) from pathogen-infected cells and tumours on the cell surface to immune cells. When activated, immune cells can then destroy the target cell. In 2015, we discovered that a novel accessory protein, called TAPBPR, assists in the selection of peptides displayed on MHC class I molecules for immune surveillance. A specific region in the TAPBPR protein the editing loop is known to be involved in removing peptides from MHC class I. However, our understanding of the process of peptide selection on MHC class I molecules remains incomplete. Here, we show that TAPBPR is not only involved in removing peptides from MHC class I molecules but also assists in peptide loading. Additionally, we demonstrate that the TAPBPR editing loop is involved in both removing and loading of peptides. Our results suggest that TAPBPR fine-tunes the peptide repertoire displayed on three different types of MHC class I molecules. Developing our understanding of the mechanisms of peptide selection on MHC class I molecules has important implications in disease and the development of new therapies.
RESUMO
Since the discovery of Transporter associated with antigen processing-binding protein-related (TAPBPR) over two decades ago, extensive studies have explored its function in the context of the major histocompatibility complex class-I (MHC-I) antigen processing and presentation pathway. As a chaperone and peptide editor, TAPBPR was recently revealed to have overlapping structural features when resolved with peptide-receptive MHC-I molecules compared with the two newly solved tapasin:MHC-I structures. Despite this, the two chaperones seem to have a unique criteria for loading high-affinity peptides on MHC-I molecules. Yet, the mechanism of action of how TAPBPR creates its distinct filter in cargo selection for peptide-receptive MHC-I molecules continues to be a subject of debate.
