{"gene":"TAPBPL","run_date":"2026-06-10T10:51:54","timeline":{"discoveries":[{"year":2002,"finding":"TAPBPR (TAPBP-R) is localized primarily in the ER with some cell-surface expression, lacks an obvious ER retention signal, and contains structural motifs similar to tapasin (V-C1 IgSF member); subcellular fractionation and biotinylation experiments established this localization.","method":"Subcellular localization by fractionation and cell-surface biotinylation; sequence/structural analysis","journal":"European journal of immunology","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — single lab, direct localization experiment with functional inference, but limited mechanistic follow-up in this paper","pmids":["11920573"],"is_preprint":false},{"year":2013,"finding":"TAPBPR binds MHC class I/β2-microglobulin complexes in the ER, does not bind ERp57 or calreticulin, is not an integral component of the peptide-loading complex (PLC), and the TAPBPR:MHC-I complex trafficks through the Golgi apparatus—demonstrating a function beyond the ER/cis-Golgi. β2-microglobulin is essential for TAPBPR–MHC-I association. TAPBPR expression decreases the rate of MHC-I maturation and prolongs PLC association.","method":"Co-immunoprecipitation, subcellular fractionation, pulse-chase/maturation assays, knockdown/overexpression cell-based assays","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (Co-IP, fractionation, pulse-chase, functional KD/OE), replicated in a highly cited study","pmids":["23401559"],"is_preprint":false},{"year":2013,"finding":"Residues in the N-terminal and C-terminal domains of TAPBPR are required for MHC-I association, and MHC-I residue T134 (critical for tapasin binding) is also essential for TAPBPR binding; TAPBPR and tapasin bind mutually exclusively to the same face of MHC-I. In the absence of tapasin, TAPBPR–MHC-I association increases, but TAPBPR loss does not increase tapasin–MHC-I interaction.","method":"Mutagenesis of TAPBPR and MHC-I residues, Co-immunoprecipitation, competitive binding assays in cells lacking tapasin or TAPBPR","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 2 / Strong — mutagenesis combined with Co-IP and genetic epistasis (tapasin/TAPBPR KO cells), multiple orthogonal methods","pmids":["24163410"],"is_preprint":false},{"year":2014,"finding":"Alternative splicing of TAPBPL generates isoforms with distinct MHC-I binding properties: loss of exon 5 (removing the membrane-proximal IgC domain) abolishes MHC-I binding, while a longer cytoplasmic tail isoform binds MHC-I but has attenuated ability to downregulate surface MHC-I expression.","method":"RT-PCR identification of splice isoforms, protein expression and Co-immunoprecipitation, cell-surface MHC-I expression assays","journal":"Immunology","confidence":"Medium","confidence_rationale":"Tier 2–3 / Moderate — direct domain-deletion analysis via natural splice variants with functional phenotype readout, single lab","pmids":["24444341"],"is_preprint":false},{"year":2015,"finding":"TAPBPR functions as a peptide exchange catalyst: it catalyses dissociation of peptides from peptide-MHC I complexes, enhances loading of peptide-receptive MHC-I molecules, and discriminates between peptides based on affinity in vitro. In cells, TAPBPR depletion increases the diversity of peptides presented on MHC-I, indicating TAPBPR restricts the peptide repertoire.","method":"In vitro peptide dissociation and loading assays (fluorescence polarization), immunopeptidomics of TAPBPR-depleted cells","journal":"eLife","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — in vitro biochemical reconstitution of peptide exchange activity combined with cell-based immunopeptidomics, multiple orthogonal methods","pmids":["26439010"],"is_preprint":false},{"year":2016,"finding":"TAPBPR binds HLA-A*02:01 and other MHC-I molecules that are peptide-free or loaded with low-affinity peptides; the interaction is reversed by high-affinity peptides in an affinity-dependent manner. SAXS confirms structural similarity of TAPBPR to tapasin. TAPBPR stabilizes peptide-receptive conformations of MHC-I to permit peptide editing.","method":"Recombinant protein binding assays, fluorescence polarization, small-angle X-ray scattering (SAXS), mutagenesis","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro reconstitution with recombinant proteins, fluorescence polarization, SAXS structural data, and mutagenesis, multiple orthogonal methods","pmids":["26869717"],"is_preprint":false},{"year":2017,"finding":"Crystal structure of TAPBPR in complex with MHC-I reveals that TAPBPR remodels the peptide-binding groove of MHC-I: it causes groove relaxation, modifies key binding pockets (including the A and F pockets), and induces domain adjustments, resulting in release of low-affinity peptide and capture of a peptide-receptive MHC-I state.","method":"X-ray crystallography of TAPBPR–MHC-I complex","journal":"Science","confidence":"High","confidence_rationale":"Tier 1 / Strong — atomic-resolution crystal structure directly defining chaperone–MHC-I interface, replicated by a simultaneous independent structure","pmids":["29025991"],"is_preprint":false},{"year":2017,"finding":"A second crystal structure of the TAPBPR–MHC-I complex independently confirms that TAPBPR functions as a peptide selector by remodeling the MHC-I α2-1-helix region, stabilizing the empty groove, and inserting a loop ('scoop loop') into the groove that interferes with peptide binding.","method":"X-ray crystallography of TAPBPR–MHC-I complex","journal":"Science","confidence":"High","confidence_rationale":"Tier 1 / Strong — independent crystal structure from a second lab, replicating and extending the first structure with identification of the scoop loop mechanism","pmids":["29025996"],"is_preprint":false},{"year":2017,"finding":"TAPBPR interacts with UDP-glucose:glycoprotein glucosyltransferase 1 (UGT1/UGGT1), forming a multimeric complex dependent on a conserved cysteine at position 94 in TAPBPR. TAPBPR promotes UGT1-mediated reglucosylation of the glycan on peptide-receptive MHC-I molecules, thereby enhancing MHC-I recognition by calreticulin and promoting re-entry into the peptide-loading complex.","method":"Co-immunoprecipitation/MS identification of TAPBPR-associated proteins, mutagenesis (C94), glycan analysis, functional peptide loading assays","journal":"eLife","confidence":"High","confidence_rationale":"Tier 2 / Strong — MS-based interaction discovery validated by mutagenesis and functional glycan reglucosylation assays, multiple orthogonal methods in single lab","pmids":["28425917"],"is_preprint":false},{"year":2018,"finding":"TAPBPR mediates peptide dissociation from MHC-I through a 'leucine lever' mechanism: leucine 30 (L30) within the K22-D35 loop of TAPBPR is critical for facilitating peptide dissociation. The molecular features of the MHC-I F pocket determine whether TAPBPR-mediated peptide dissociation occurs in a loop-dependent manner.","method":"Site-directed mutagenesis of TAPBPR loop residues, cell-based peptide exchange assays, immunopeptidomics","journal":"eLife","confidence":"High","confidence_rationale":"Tier 2 / Strong — mutagenesis of specific catalytic residue combined with functional cell-based assays and immunopeptidomics, multiple orthogonal methods","pmids":["30484775"],"is_preprint":false},{"year":2018,"finding":"Membrane-targeted TAPBPR at the plasma membrane retains peptide editor function and catalyzes peptide exchange on surface MHC-I. Soluble TAPBPR (luminal domain alone) added exogenously to intact cells also functions as a peptide editor on surface MHC-I, enabling loading of immunogenic peptides and subsequent T cell activation (IFN-γ secretion and cytotoxic killing).","method":"Plasma membrane-targeted TAPBPR constructs, exogenous soluble TAPBPR addition to cells, T cell functional assays (IFN-γ ELISA, cytotoxicity assay)","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 / Strong — defined cell-based reconstitution with functional T cell readouts, multiple orthogonal assays, highly cited","pmids":["30213851"],"is_preprint":false},{"year":2020,"finding":"The scoop loop of TAPBPR acts as an internal peptide surrogate: it stabilizes empty MHC-I molecules by directly occupying the peptide-binding groove and impedes peptide rebinding, functioning as an additional selectivity filter for the immunopeptidome. Reconstitution with defined components demonstrated the scoop loop's contribution to MHC-I–chaperone complex stability and peptide editing.","method":"Reconstituted in vitro system with purified TAPBPR variants (loop deletion/mutation), biophysical stability assays, peptide exchange assays","journal":"eLife","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro reconstitution with mutagenesis of loop variants, single lab but multiple orthogonal methods","pmids":["32167472"],"is_preprint":false},{"year":2021,"finding":"Deep mutagenesis reveals that residues important for TAPBPR chaperoning activity (stabilizing nascent empty MHC-I) map to a large scaffolding surface excluding the scoop loop, while loop mutations specifically affect TAPBPR interactions with properly conformed MHC-I relevant to peptide editing. Solution NMR, ITC, and FP assays show the loop hovers above the MHC-I groove to promote capture of incoming peptides, lowering affinity requirements for peptide selection.","method":"Deep mutagenesis, solution NMR, isothermal titration calorimetry (ITC), fluorescence polarization (FP) assays","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1 / Strong — deep mutagenesis combined with solution NMR, ITC, and FP, multiple orthogonal structural and biochemical methods","pmids":["34039964"],"is_preprint":false},{"year":2021,"finding":"TAPBPL acts as a T cell co-inhibitory molecule: a soluble TAPBPL-Ig fusion protein inhibits T cell proliferation, activation, and cytokine production in vitro; a putative TAPBPL receptor is expressed on activated CD4 and CD8 T cells; in vivo TAPBPL-Ig attenuates experimental autoimmune encephalomyelitis in mice.","method":"Recombinant TAPBPL-Ig fusion protein, in vitro T cell proliferation and cytokine assays, in vivo EAE mouse model, anti-TAPBPL blocking antibody","journal":"EMBO molecular medicine","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional cell-based and in vivo assays with recombinant protein and blocking antibody, single lab, mechanism of receptor interaction not fully characterized","pmids":["33938620"],"is_preprint":false},{"year":2022,"finding":"TAPBPR chaperones MR1 (MHC-related protein 1) in a ligand-independent manner, unlike its interaction with MHC-I. Paramagnetic NMR combined with restrained molecular dynamics shows TAPBPR engages conserved MR1 surfaces inducing similar structural adaptations as in MHC-I/TAPBPR structures. TAPBPR affects exchange kinetics of noncovalent metabolites (e.g., diclofenac) with the MR1 groove, acting as a catalyst.","method":"In vitro biochemical assays, paramagnetic NMR, 19F-NMR relaxation dispersion experiments, restrained molecular dynamics simulations","journal":"Nature chemical biology","confidence":"High","confidence_rationale":"Tier 1 / Moderate — NMR structural characterization combined with in vitro functional assays (ligand exchange kinetics), multiple orthogonal methods, single lab","pmids":["35725941"],"is_preprint":false},{"year":2023,"finding":"In a fully reconstituted in vitro system with purified human proteins, TAPBPR promotes reglucosylation of peptide-free MHC-I by UGGT1, confirmed by glycoengineering combined with LC-MS analysis of glycan composition.","method":"Reconstituted in vitro system with purified components, glycoengineering, liquid chromatography-mass spectrometry (LC-MS) glycan analysis","journal":"eLife","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro reconstitution with purified human proteins and direct LC-MS glycan readout, single lab but rigorous biochemical approach","pmids":["37345806"],"is_preprint":false},{"year":2025,"finding":"CryoEM structure at 3.0 Å of an MHC-I/TAPBPR complex bound to a peptide decoy reveals that antigen proofreading is mediated by transient P2/P3 anchor interactions with the nascent peptide-binding groove, where conserved MHC-I residues stabilize incoming peptides through backbone-focused contacts. A high-fidelity TAPBPR variant enables robust cell-surface peptide exchange across multiple HLA allomorphs.","method":"Cryo-EM structure determination (3.0 Å), functional peptide exchange assays on cell surface with engineered TAPBPR variant","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 / Moderate — near-atomic cryo-EM structure of peptide-bound editing intermediate combined with functional cell-surface exchange validation, single lab with multiple methods","pmids":["39786927"],"is_preprint":false},{"year":2025,"finding":"In a reconstituted system with isolated components, calreticulin mediates transfer of peptide-receptive MHC-I from TAPBPR back to tapasin (PLC), dependent on recognition of the monoglucosylated MHC-I glycan generated by UGGT1. The C-terminal acidic helix of calreticulin is dispensable for disengaging reglucosylated MHC-I from TAPBPR but essential for docking MHC-I onto tapasin.","method":"In vitro reconstitution with purified isolated components, calreticulin domain mutagenesis, functional transfer assays","journal":"bioRxiv (preprint)","confidence":"Medium","confidence_rationale":"Tier 1 / Moderate — in vitro reconstitution with purified components and domain mutagenesis, but preprint not yet peer-reviewed","pmids":["bio_10.1101_2025.11.25.690393"],"is_preprint":true}],"current_model":"TAPBPR (TAPBPL) is an ER/Golgi-localized MHC-I-dedicated chaperone and peptide exchange catalyst that binds peptide-receptive MHC-I/β2m complexes at the same interface as tapasin (mutually exclusively), stabilizes the empty MHC-I groove via its scoop loop acting as a peptide surrogate, promotes high-affinity peptide loading through a leucine-30-dependent loop mechanism that mediates peptide dissociation, bridges UGGT1 to peptide-free MHC-I to drive reglucosylation and calreticulin-mediated recycling back to the PLC, chaperones MR1 in a ligand-independent manner, and can function as a soluble extracellular peptide editor on surface MHC-I; additionally, TAPBPR acts as a T cell co-inhibitory molecule through a distinct receptor-mediated pathway."},"narrative":{"mechanistic_narrative":"TAPBPL (TAPBPR) is an MHC class I-dedicated chaperone and peptide-exchange catalyst that operates in the ER/Golgi to shape the displayed immunopeptidome [PMID:23401559, PMID:26439010]. It binds peptide-receptive MHC-I/β2-microglobulin complexes, with β2m essential for the association, and engages the same MHC-I face used by tapasin (requiring MHC-I residue T134) in a mutually exclusive manner, acting outside the peptide-loading complex [PMID:23401559, PMID:24163410]. Crystal and cryo-EM structures show that TAPBPR remodels the MHC-I groove—relaxing it, reshaping the A and F pockets, and inserting a 'scoop loop' that stabilizes the empty groove and acts as an internal peptide surrogate to impede peptide rebinding—thereby releasing low-affinity peptides and capturing a peptide-receptive state [PMID:29025991, PMID:29025996, PMID:32167472, PMID:39786927]. Catalytically, a 'leucine lever' (L30 within the K22-D35 loop) drives peptide dissociation in a manner dictated by the MHC-I F pocket, while the loop hovers over the groove to lower the affinity threshold for peptide capture, biasing presentation toward high-affinity ligands and restricting repertoire diversity [PMID:26439010, PMID:30484775, PMID:34039964]. TAPBPR couples editing to quality control by recruiting UGGT1 (via its conserved Cys94) to reglucosylate peptide-free MHC-I, after which calreticulin recognizes the monoglucosylated glycan and transfers MHC-I back to the tapasin/PLC [PMID:28425917, PMID:37345806]. Beyond MHC-I, TAPBPR chaperones the non-classical molecule MR1 in a ligand-independent manner and catalyzes exchange of small-molecule ligands in the MR1 groove [PMID:35725941]. Functionally, both membrane-anchored and soluble luminal TAPBPR edit peptides on surface MHC-I to enable T cell activation and killing, and engineered high-fidelity variants extend this exchange across multiple HLA allomorphs [PMID:30213851, PMID:39786927].","teleology":[{"year":2002,"claim":"Established that TAPBPR exists as a tapasin-like protein localized primarily to the ER with cell-surface expression, framing it as a candidate MHC-I pathway component distinct from tapasin.","evidence":"Subcellular fractionation and cell-surface biotinylation with sequence/structural analysis","pmids":["11920573"],"confidence":"Medium","gaps":["No demonstrated binding partner or function","Lacks an obvious ER retention signal, mechanism of localization unresolved"]},{"year":2013,"claim":"Defined TAPBPR as a tapasin homolog that binds MHC-I/β2m independently of the PLC and traffics through the Golgi, distinguishing it mechanistically from the PLC-resident chaperone tapasin and identifying its competitive, mutually exclusive binding to the same MHC-I face.","evidence":"Co-IP, fractionation, pulse-chase maturation assays, knockdown/overexpression, and mutagenesis of TAPBPR/MHC-I (T134) in tapasin- or TAPBPR-deficient cells","pmids":["23401559","24163410"],"confidence":"High","gaps":["Catalytic activity not yet demonstrated","Structural basis of the shared binding interface unresolved at this stage"]},{"year":2014,"claim":"Showed that domain architecture governs function, with the membrane-proximal IgC domain required for MHC-I binding and cytoplasmic-tail variation tuning surface MHC-I downregulation.","evidence":"RT-PCR isoform identification, Co-IP, and cell-surface MHC-I expression assays","pmids":["24444341"],"confidence":"Medium","gaps":["Functional relevance of isoforms in vivo unclear","Single lab"]},{"year":2015,"claim":"Demonstrated that TAPBPR is a bona fide peptide-exchange catalyst that dissociates and reloads peptides and discriminates by affinity, establishing it as an active editor that restricts the cellular peptide repertoire rather than a passive binder.","evidence":"In vitro fluorescence polarization dissociation/loading assays and immunopeptidomics of TAPBPR-depleted cells","pmids":["26439010"],"confidence":"High","gaps":["Structural mechanism of groove remodeling not yet defined","Catalytic residues unidentified"]},{"year":2016,"claim":"Showed TAPBPR selectively engages peptide-free or low-affinity MHC-I and is displaced by high-affinity peptide, and that it is structurally homologous to tapasin, establishing the conformational basis of affinity-based editing.","evidence":"Recombinant binding assays, fluorescence polarization, SAXS, and mutagenesis","pmids":["26869717"],"confidence":"High","gaps":["Atomic-resolution interface not yet resolved"]},{"year":2017,"claim":"Resolved the atomic mechanism of editing: TAPBPR relaxes the MHC-I groove, remodels the A and F pockets, and inserts a scoop loop into the empty groove that interferes with peptide binding, directly explaining how low-affinity peptides are released and a receptive state is captured.","evidence":"Two independent X-ray crystal structures of TAPBPR–MHC-I complexes","pmids":["29025991","29025996"],"confidence":"High","gaps":["Dynamics of peptide capture not captured by static structures","Catalytic residue contribution not quantified"]},{"year":2017,"claim":"Connected editing to glycan-based quality control by showing TAPBPR recruits UGGT1 (via Cys94) to reglucosylate peptide-free MHC-I, enhancing calreticulin recognition and re-entry into the PLC, establishing a recycling circuit.","evidence":"Co-IP/MS interactome, C94 mutagenesis, glycan analysis, and functional loading assays","pmids":["28425917"],"confidence":"High","gaps":["In-cell stoichiometry of the multimeric complex unresolved","Calreticulin hand-off step not yet reconstituted"]},{"year":2018,"claim":"Identified the catalytic 'leucine lever' (L30 in the K22-D35 loop) that drives peptide dissociation in an F-pocket-dependent manner, assigning a specific residue to the editing reaction.","evidence":"Site-directed mutagenesis, cell-based peptide exchange assays, immunopeptidomics","pmids":["30484775"],"confidence":"High","gaps":["Energetics of the lever mechanism not fully defined"]},{"year":2018,"claim":"Demonstrated TAPBPR functions at the cell surface, with membrane-anchored and exogenously added soluble luminal TAPBPR editing surface MHC-I to load immunogenic peptides and trigger T cell responses, expanding its functional compartment and revealing immunotherapeutic potential.","evidence":"Plasma membrane-targeted and soluble TAPBPR constructs with T cell IFN-γ and cytotoxicity assays","pmids":["30213851"],"confidence":"High","gaps":["Efficiency across diverse HLA allomorphs not established at this stage"]},{"year":2021,"claim":"Separated the chaperoning and editing functions by mapping stabilization of nascent empty MHC-I to a broad scaffolding surface while showing the scoop loop hovers above the groove to promote peptide capture, refining the model of how the loop acts as a selectivity filter and peptide surrogate.","evidence":"Deep mutagenesis, solution NMR, ITC, FP, and in vitro reconstitution of loop variants","pmids":["32167472","34039964"],"confidence":"High","gaps":["Dynamic intermediate of peptide capture not visualized"]},{"year":2021,"claim":"Reported a distinct immunological role for TAPBPL as a T cell co-inhibitory ligand acting through a putative receptor on activated T cells, separate from its MHC-I chaperone function.","evidence":"Recombinant TAPBPL-Ig fusion, in vitro T cell proliferation/cytokine assays, in vivo EAE model, and blocking antibody","pmids":["33938620"],"confidence":"Medium","gaps":["Receptor identity not established","Mechanism of receptor engagement uncharacterized","Single lab"]},{"year":2022,"claim":"Extended TAPBPR's chaperone scope to the non-classical molecule MR1, showing ligand-independent engagement and catalysis of small-molecule ligand exchange, indicating a broader role in non-classical antigen presentation.","evidence":"In vitro assays, paramagnetic and 19F-NMR relaxation dispersion, and restrained molecular dynamics","pmids":["35725941"],"confidence":"High","gaps":["Cellular relevance of MR1 chaperoning not demonstrated","Single lab"]},{"year":2023,"claim":"Confirmed the UGGT1 reglucosylation step in a fully reconstituted system with purified human proteins, establishing the minimal components sufficient for TAPBPR-promoted glycan modification of peptide-free MHC-I.","evidence":"Reconstituted in vitro system, glycoengineering, LC-MS glycan analysis","pmids":["37345806"],"confidence":"High","gaps":["Downstream calreticulin transfer not addressed here"]},{"year":2025,"claim":"Visualized an antigen-proofreading intermediate at 3.0 Å showing transient P2/P3 anchor contacts with the nascent groove and engineered a high-fidelity TAPBPR variant for robust surface exchange across multiple HLA allomorphs, advancing both mechanistic and translational understanding.","evidence":"Cryo-EM structure of a peptide-decoy-bound MHC-I/TAPBPR complex and cell-surface exchange assays with an engineered variant","pmids":["39786927"],"confidence":"High","gaps":["Kinetics of the proofreading transition not fully resolved"]},{"year":2025,"claim":"Reconstituted the hand-off step, showing calreticulin transfers peptide-receptive MHC-I from TAPBPR back to tapasin dependent on the UGGT1-generated monoglucosylated glycan, with distinct calreticulin domains required for disengagement versus docking, closing the editing-to-PLC recycling loop.","evidence":"In vitro reconstitution with purified components and calreticulin domain mutagenesis (preprint)","pmids":["bio_10.1101_2025.11.25.690393"],"confidence":"Medium","gaps":["Preprint not yet peer-reviewed","In-cell validation of the transfer mechanism pending"]},{"year":null,"claim":"The identity of the TAPBPL co-inhibitory receptor on T cells and the molecular basis of that signaling pathway remain undefined.","evidence":"","pmids":[],"confidence":"Medium","gaps":["Receptor unidentified","Relationship between chaperone and co-inhibitory functions unknown"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[4,6,9,14]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[4,11,12]},{"term_id":"GO:0044183","term_label":"protein folding chaperone","supporting_discovery_ids":[5,11,14]},{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[13]}],"localization":[{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[0,1]},{"term_id":"GO:0005794","term_label":"Golgi apparatus","supporting_discovery_ids":[1]},{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[0,10]}],"pathway":[{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[4,10,13]},{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[8,15]}],"complexes":["TAPBPR–MHC-I/β2m complex","TAPBPR–UGGT1 complex"],"partners":["B2M","UGGT1","CALR","TAPBP","MR1"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q9BX59","full_name":"Tapasin-related protein","aliases":["TAP-binding protein-like","TAP-binding protein-related protein","TAPBP-R","Tapasin-like"],"length_aa":468,"mass_kda":50.2,"function":"Component of the antigen processing and presentation pathway, which binds to MHC class I coupled with beta2-microglobulin/B2M. Association between TAPBPR and MHC class I occurs in the absence of a functional peptide-loading complex (PLC)","subcellular_location":"Cell membrane; Endoplasmic reticulum membrane; Microsome membrane; Golgi apparatus membrane","url":"https://www.uniprot.org/uniprotkb/Q9BX59/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/TAPBPL","classification":"Not Classified","n_dependent_lines":0,"n_total_lines":1208,"dependency_fraction":0.0},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/TAPBPL","total_profiled":1310},"omim":[{"mim_id":"607081","title":"TAP-BINDING PROTEIN-LIKE; TAPBPL","url":"https://www.omim.org/entry/607081"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in many","driving_tissues":[],"url":"https://www.proteinatlas.org/search/TAPBPL"},"hgnc":{"alias_symbol":["TAPBP-R","FLJ10143","TAPBPR"],"prev_symbol":[]},"alphafold":{"accession":"Q9BX59","domains":[{"cath_id":"2.60.40","chopping":"30-44_56-75_91-191","consensus_level":"medium","plddt":81.8835,"start":30,"end":191},{"cath_id":"2.60.40.10","chopping":"198-302","consensus_level":"medium","plddt":91.4512,"start":198,"end":302},{"cath_id":"2.60.40.10","chopping":"306-403","consensus_level":"high","plddt":91.5301,"start":306,"end":403}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9BX59","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9BX59-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9BX59-F1-predicted_aligned_error_v6.png","plddt_mean":79.06},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=TAPBPL","jax_strain_url":"https://www.jax.org/strain/search?query=TAPBPL"},"sequence":{"accession":"Q9BX59","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9BX59.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9BX59/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9BX59"}},"corpus_meta":[{"pmid":"29025991","id":"PMC_29025991","title":"Crystal structure of a TAPBPR-MHC I complex reveals the mechanism of peptide editing in antigen presentation.","date":"2017","source":"Science (New York, N.Y.)","url":"https://pubmed.ncbi.nlm.nih.gov/29025991","citation_count":107,"is_preprint":false},{"pmid":"29025996","id":"PMC_29025996","title":"Structure of the TAPBPR-MHC I complex defines the mechanism of peptide loading and editing.","date":"2017","source":"Science (New York, N.Y.)","url":"https://pubmed.ncbi.nlm.nih.gov/29025996","citation_count":105,"is_preprint":false},{"pmid":"23401559","id":"PMC_23401559","title":"Tapasin-related protein TAPBPR is an additional component of the MHC class I presentation pathway.","date":"2013","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/23401559","citation_count":103,"is_preprint":false},{"pmid":"26439010","id":"PMC_26439010","title":"TAPBPR alters MHC class I peptide presentation by functioning as a peptide exchange catalyst.","date":"2015","source":"eLife","url":"https://pubmed.ncbi.nlm.nih.gov/26439010","citation_count":82,"is_preprint":false},{"pmid":"26869717","id":"PMC_26869717","title":"Interaction of TAPBPR, a tapasin homolog, with MHC-I molecules promotes peptide editing.","date":"2016","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/26869717","citation_count":80,"is_preprint":false},{"pmid":"28425917","id":"PMC_28425917","title":"TAPBPR bridges UDP-glucose:glycoprotein glucosyltransferase 1 onto MHC class I to provide quality control in the antigen presentation pathway.","date":"2017","source":"eLife","url":"https://pubmed.ncbi.nlm.nih.gov/28425917","citation_count":58,"is_preprint":false},{"pmid":"24163410","id":"PMC_24163410","title":"The binding of TAPBPR and Tapasin to MHC class I is mutually exclusive.","date":"2013","source":"Journal of immunology (Baltimore, Md. : 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TAPBPR expression decreases the rate of MHC-I maturation and prolongs PLC association.\",\n      \"method\": \"Co-immunoprecipitation, subcellular fractionation, pulse-chase/maturation assays, knockdown/overexpression cell-based assays\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (Co-IP, fractionation, pulse-chase, functional KD/OE), replicated in a highly cited study\",\n      \"pmids\": [\"23401559\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Residues in the N-terminal and C-terminal domains of TAPBPR are required for MHC-I association, and MHC-I residue T134 (critical for tapasin binding) is also essential for TAPBPR binding; TAPBPR and tapasin bind mutually exclusively to the same face of MHC-I. In the absence of tapasin, TAPBPR–MHC-I association increases, but TAPBPR loss does not increase tapasin–MHC-I interaction.\",\n      \"method\": \"Mutagenesis of TAPBPR and MHC-I residues, Co-immunoprecipitation, competitive binding assays in cells lacking tapasin or TAPBPR\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — mutagenesis combined with Co-IP and genetic epistasis (tapasin/TAPBPR KO cells), multiple orthogonal methods\",\n      \"pmids\": [\"24163410\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Alternative splicing of TAPBPL generates isoforms with distinct MHC-I binding properties: loss of exon 5 (removing the membrane-proximal IgC domain) abolishes MHC-I binding, while a longer cytoplasmic tail isoform binds MHC-I but has attenuated ability to downregulate surface MHC-I expression.\",\n      \"method\": \"RT-PCR identification of splice isoforms, protein expression and Co-immunoprecipitation, cell-surface MHC-I expression assays\",\n      \"journal\": \"Immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 / Moderate — direct domain-deletion analysis via natural splice variants with functional phenotype readout, single lab\",\n      \"pmids\": [\"24444341\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"TAPBPR functions as a peptide exchange catalyst: it catalyses dissociation of peptides from peptide-MHC I complexes, enhances loading of peptide-receptive MHC-I molecules, and discriminates between peptides based on affinity in vitro. In cells, TAPBPR depletion increases the diversity of peptides presented on MHC-I, indicating TAPBPR restricts the peptide repertoire.\",\n      \"method\": \"In vitro peptide dissociation and loading assays (fluorescence polarization), immunopeptidomics of TAPBPR-depleted cells\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — in vitro biochemical reconstitution of peptide exchange activity combined with cell-based immunopeptidomics, multiple orthogonal methods\",\n      \"pmids\": [\"26439010\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"TAPBPR binds HLA-A*02:01 and other MHC-I molecules that are peptide-free or loaded with low-affinity peptides; the interaction is reversed by high-affinity peptides in an affinity-dependent manner. SAXS confirms structural similarity of TAPBPR to tapasin. TAPBPR stabilizes peptide-receptive conformations of MHC-I to permit peptide editing.\",\n      \"method\": \"Recombinant protein binding assays, fluorescence polarization, small-angle X-ray scattering (SAXS), mutagenesis\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro reconstitution with recombinant proteins, fluorescence polarization, SAXS structural data, and mutagenesis, multiple orthogonal methods\",\n      \"pmids\": [\"26869717\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Crystal structure of TAPBPR in complex with MHC-I reveals that TAPBPR remodels the peptide-binding groove of MHC-I: it causes groove relaxation, modifies key binding pockets (including the A and F pockets), and induces domain adjustments, resulting in release of low-affinity peptide and capture of a peptide-receptive MHC-I state.\",\n      \"method\": \"X-ray crystallography of TAPBPR–MHC-I complex\",\n      \"journal\": \"Science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — atomic-resolution crystal structure directly defining chaperone–MHC-I interface, replicated by a simultaneous independent structure\",\n      \"pmids\": [\"29025991\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"A second crystal structure of the TAPBPR–MHC-I complex independently confirms that TAPBPR functions as a peptide selector by remodeling the MHC-I α2-1-helix region, stabilizing the empty groove, and inserting a loop ('scoop loop') into the groove that interferes with peptide binding.\",\n      \"method\": \"X-ray crystallography of TAPBPR–MHC-I complex\",\n      \"journal\": \"Science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — independent crystal structure from a second lab, replicating and extending the first structure with identification of the scoop loop mechanism\",\n      \"pmids\": [\"29025996\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"TAPBPR interacts with UDP-glucose:glycoprotein glucosyltransferase 1 (UGT1/UGGT1), forming a multimeric complex dependent on a conserved cysteine at position 94 in TAPBPR. TAPBPR promotes UGT1-mediated reglucosylation of the glycan on peptide-receptive MHC-I molecules, thereby enhancing MHC-I recognition by calreticulin and promoting re-entry into the peptide-loading complex.\",\n      \"method\": \"Co-immunoprecipitation/MS identification of TAPBPR-associated proteins, mutagenesis (C94), glycan analysis, functional peptide loading assays\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — MS-based interaction discovery validated by mutagenesis and functional glycan reglucosylation assays, multiple orthogonal methods in single lab\",\n      \"pmids\": [\"28425917\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"TAPBPR mediates peptide dissociation from MHC-I through a 'leucine lever' mechanism: leucine 30 (L30) within the K22-D35 loop of TAPBPR is critical for facilitating peptide dissociation. The molecular features of the MHC-I F pocket determine whether TAPBPR-mediated peptide dissociation occurs in a loop-dependent manner.\",\n      \"method\": \"Site-directed mutagenesis of TAPBPR loop residues, cell-based peptide exchange assays, immunopeptidomics\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — mutagenesis of specific catalytic residue combined with functional cell-based assays and immunopeptidomics, multiple orthogonal methods\",\n      \"pmids\": [\"30484775\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Membrane-targeted TAPBPR at the plasma membrane retains peptide editor function and catalyzes peptide exchange on surface MHC-I. Soluble TAPBPR (luminal domain alone) added exogenously to intact cells also functions as a peptide editor on surface MHC-I, enabling loading of immunogenic peptides and subsequent T cell activation (IFN-γ secretion and cytotoxic killing).\",\n      \"method\": \"Plasma membrane-targeted TAPBPR constructs, exogenous soluble TAPBPR addition to cells, T cell functional assays (IFN-γ ELISA, cytotoxicity assay)\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — defined cell-based reconstitution with functional T cell readouts, multiple orthogonal assays, highly cited\",\n      \"pmids\": [\"30213851\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"The scoop loop of TAPBPR acts as an internal peptide surrogate: it stabilizes empty MHC-I molecules by directly occupying the peptide-binding groove and impedes peptide rebinding, functioning as an additional selectivity filter for the immunopeptidome. Reconstitution with defined components demonstrated the scoop loop's contribution to MHC-I–chaperone complex stability and peptide editing.\",\n      \"method\": \"Reconstituted in vitro system with purified TAPBPR variants (loop deletion/mutation), biophysical stability assays, peptide exchange assays\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro reconstitution with mutagenesis of loop variants, single lab but multiple orthogonal methods\",\n      \"pmids\": [\"32167472\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Deep mutagenesis reveals that residues important for TAPBPR chaperoning activity (stabilizing nascent empty MHC-I) map to a large scaffolding surface excluding the scoop loop, while loop mutations specifically affect TAPBPR interactions with properly conformed MHC-I relevant to peptide editing. Solution NMR, ITC, and FP assays show the loop hovers above the MHC-I groove to promote capture of incoming peptides, lowering affinity requirements for peptide selection.\",\n      \"method\": \"Deep mutagenesis, solution NMR, isothermal titration calorimetry (ITC), fluorescence polarization (FP) assays\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — deep mutagenesis combined with solution NMR, ITC, and FP, multiple orthogonal structural and biochemical methods\",\n      \"pmids\": [\"34039964\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"TAPBPL acts as a T cell co-inhibitory molecule: a soluble TAPBPL-Ig fusion protein inhibits T cell proliferation, activation, and cytokine production in vitro; a putative TAPBPL receptor is expressed on activated CD4 and CD8 T cells; in vivo TAPBPL-Ig attenuates experimental autoimmune encephalomyelitis in mice.\",\n      \"method\": \"Recombinant TAPBPL-Ig fusion protein, in vitro T cell proliferation and cytokine assays, in vivo EAE mouse model, anti-TAPBPL blocking antibody\",\n      \"journal\": \"EMBO molecular medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional cell-based and in vivo assays with recombinant protein and blocking antibody, single lab, mechanism of receptor interaction not fully characterized\",\n      \"pmids\": [\"33938620\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"TAPBPR chaperones MR1 (MHC-related protein 1) in a ligand-independent manner, unlike its interaction with MHC-I. Paramagnetic NMR combined with restrained molecular dynamics shows TAPBPR engages conserved MR1 surfaces inducing similar structural adaptations as in MHC-I/TAPBPR structures. TAPBPR affects exchange kinetics of noncovalent metabolites (e.g., diclofenac) with the MR1 groove, acting as a catalyst.\",\n      \"method\": \"In vitro biochemical assays, paramagnetic NMR, 19F-NMR relaxation dispersion experiments, restrained molecular dynamics simulations\",\n      \"journal\": \"Nature chemical biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — NMR structural characterization combined with in vitro functional assays (ligand exchange kinetics), multiple orthogonal methods, single lab\",\n      \"pmids\": [\"35725941\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"In a fully reconstituted in vitro system with purified human proteins, TAPBPR promotes reglucosylation of peptide-free MHC-I by UGGT1, confirmed by glycoengineering combined with LC-MS analysis of glycan composition.\",\n      \"method\": \"Reconstituted in vitro system with purified components, glycoengineering, liquid chromatography-mass spectrometry (LC-MS) glycan analysis\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro reconstitution with purified human proteins and direct LC-MS glycan readout, single lab but rigorous biochemical approach\",\n      \"pmids\": [\"37345806\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"CryoEM structure at 3.0 Å of an MHC-I/TAPBPR complex bound to a peptide decoy reveals that antigen proofreading is mediated by transient P2/P3 anchor interactions with the nascent peptide-binding groove, where conserved MHC-I residues stabilize incoming peptides through backbone-focused contacts. A high-fidelity TAPBPR variant enables robust cell-surface peptide exchange across multiple HLA allomorphs.\",\n      \"method\": \"Cryo-EM structure determination (3.0 Å), functional peptide exchange assays on cell surface with engineered TAPBPR variant\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — near-atomic cryo-EM structure of peptide-bound editing intermediate combined with functional cell-surface exchange validation, single lab with multiple methods\",\n      \"pmids\": [\"39786927\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"In a reconstituted system with isolated components, calreticulin mediates transfer of peptide-receptive MHC-I from TAPBPR back to tapasin (PLC), dependent on recognition of the monoglucosylated MHC-I glycan generated by UGGT1. The C-terminal acidic helix of calreticulin is dispensable for disengaging reglucosylated MHC-I from TAPBPR but essential for docking MHC-I onto tapasin.\",\n      \"method\": \"In vitro reconstitution with purified isolated components, calreticulin domain mutagenesis, functional transfer assays\",\n      \"journal\": \"bioRxiv (preprint)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro reconstitution with purified components and domain mutagenesis, but preprint not yet peer-reviewed\",\n      \"pmids\": [\"bio_10.1101_2025.11.25.690393\"],\n      \"is_preprint\": true\n    }\n  ],\n  \"current_model\": \"TAPBPR (TAPBPL) is an ER/Golgi-localized MHC-I-dedicated chaperone and peptide exchange catalyst that binds peptide-receptive MHC-I/β2m complexes at the same interface as tapasin (mutually exclusively), stabilizes the empty MHC-I groove via its scoop loop acting as a peptide surrogate, promotes high-affinity peptide loading through a leucine-30-dependent loop mechanism that mediates peptide dissociation, bridges UGGT1 to peptide-free MHC-I to drive reglucosylation and calreticulin-mediated recycling back to the PLC, chaperones MR1 in a ligand-independent manner, and can function as a soluble extracellular peptide editor on surface MHC-I; additionally, TAPBPR acts as a T cell co-inhibitory molecule through a distinct receptor-mediated pathway.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"TAPBPL (TAPBPR) is an MHC class I-dedicated chaperone and peptide-exchange catalyst that operates in the ER/Golgi to shape the displayed immunopeptidome [#1, #4]. It binds peptide-receptive MHC-I/\\u03b22-microglobulin complexes, with \\u03b22m essential for the association, and engages the same MHC-I face used by tapasin (requiring MHC-I residue T134) in a mutually exclusive manner, acting outside the peptide-loading complex [#1, #2]. Crystal and cryo-EM structures show that TAPBPR remodels the MHC-I groove\\u2014relaxing it, reshaping the A and F pockets, and inserting a 'scoop loop' that stabilizes the empty groove and acts as an internal peptide surrogate to impede peptide rebinding\\u2014thereby releasing low-affinity peptides and capturing a peptide-receptive state [#6, #7, #11, #16]. Catalytically, a 'leucine lever' (L30 within the K22-D35 loop) drives peptide dissociation in a manner dictated by the MHC-I F pocket, while the loop hovers over the groove to lower the affinity threshold for peptide capture, biasing presentation toward high-affinity ligands and restricting repertoire diversity [#4, #9, #12]. TAPBPR couples editing to quality control by recruiting UGGT1 (via its conserved Cys94) to reglucosylate peptide-free MHC-I, after which calreticulin recognizes the monoglucosylated glycan and transfers MHC-I back to the tapasin/PLC [#8, #15]. Beyond MHC-I, TAPBPR chaperones the non-classical molecule MR1 in a ligand-independent manner and catalyzes exchange of small-molecule ligands in the MR1 groove [#14]. Functionally, both membrane-anchored and soluble luminal TAPBPR edit peptides on surface MHC-I to enable T cell activation and killing, and engineered high-fidelity variants extend this exchange across multiple HLA allomorphs [#10, #16].\",\n  \"teleology\": [\n    {\n      \"year\": 2002,\n      \"claim\": \"Established that TAPBPR exists as a tapasin-like protein localized primarily to the ER with cell-surface expression, framing it as a candidate MHC-I pathway component distinct from tapasin.\",\n      \"evidence\": \"Subcellular fractionation and cell-surface biotinylation with sequence/structural analysis\",\n      \"pmids\": [\"11920573\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"No demonstrated binding partner or function\", \"Lacks an obvious ER retention signal, mechanism of localization unresolved\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Defined TAPBPR as a tapasin homolog that binds MHC-I/\\u03b22m independently of the PLC and traffics through the Golgi, distinguishing it mechanistically from the PLC-resident chaperone tapasin and identifying its competitive, mutually exclusive binding to the same MHC-I face.\",\n      \"evidence\": \"Co-IP, fractionation, pulse-chase maturation assays, knockdown/overexpression, and mutagenesis of TAPBPR/MHC-I (T134) in tapasin- or TAPBPR-deficient cells\",\n      \"pmids\": [\"23401559\", \"24163410\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Catalytic activity not yet demonstrated\", \"Structural basis of the shared binding interface unresolved at this stage\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Showed that domain architecture governs function, with the membrane-proximal IgC domain required for MHC-I binding and cytoplasmic-tail variation tuning surface MHC-I downregulation.\",\n      \"evidence\": \"RT-PCR isoform identification, Co-IP, and cell-surface MHC-I expression assays\",\n      \"pmids\": [\"24444341\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Functional relevance of isoforms in vivo unclear\", \"Single lab\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Demonstrated that TAPBPR is a bona fide peptide-exchange catalyst that dissociates and reloads peptides and discriminates by affinity, establishing it as an active editor that restricts the cellular peptide repertoire rather than a passive binder.\",\n      \"evidence\": \"In vitro fluorescence polarization dissociation/loading assays and immunopeptidomics of TAPBPR-depleted cells\",\n      \"pmids\": [\"26439010\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Structural mechanism of groove remodeling not yet defined\", \"Catalytic residues unidentified\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Showed TAPBPR selectively engages peptide-free or low-affinity MHC-I and is displaced by high-affinity peptide, and that it is structurally homologous to tapasin, establishing the conformational basis of affinity-based editing.\",\n      \"evidence\": \"Recombinant binding assays, fluorescence polarization, SAXS, and mutagenesis\",\n      \"pmids\": [\"26869717\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Atomic-resolution interface not yet resolved\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Resolved the atomic mechanism of editing: TAPBPR relaxes the MHC-I groove, remodels the A and F pockets, and inserts a scoop loop into the empty groove that interferes with peptide binding, directly explaining how low-affinity peptides are released and a receptive state is captured.\",\n      \"evidence\": \"Two independent X-ray crystal structures of TAPBPR\\u2013MHC-I complexes\",\n      \"pmids\": [\"29025991\", \"29025996\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Dynamics of peptide capture not captured by static structures\", \"Catalytic residue contribution not quantified\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Connected editing to glycan-based quality control by showing TAPBPR recruits UGGT1 (via Cys94) to reglucosylate peptide-free MHC-I, enhancing calreticulin recognition and re-entry into the PLC, establishing a recycling circuit.\",\n      \"evidence\": \"Co-IP/MS interactome, C94 mutagenesis, glycan analysis, and functional loading assays\",\n      \"pmids\": [\"28425917\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"In-cell stoichiometry of the multimeric complex unresolved\", \"Calreticulin hand-off step not yet reconstituted\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Identified the catalytic 'leucine lever' (L30 in the K22-D35 loop) that drives peptide dissociation in an F-pocket-dependent manner, assigning a specific residue to the editing reaction.\",\n      \"evidence\": \"Site-directed mutagenesis, cell-based peptide exchange assays, immunopeptidomics\",\n      \"pmids\": [\"30484775\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Energetics of the lever mechanism not fully defined\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Demonstrated TAPBPR functions at the cell surface, with membrane-anchored and exogenously added soluble luminal TAPBPR editing surface MHC-I to load immunogenic peptides and trigger T cell responses, expanding its functional compartment and revealing immunotherapeutic potential.\",\n      \"evidence\": \"Plasma membrane-targeted and soluble TAPBPR constructs with T cell IFN-\\u03b3 and cytotoxicity assays\",\n      \"pmids\": [\"30213851\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Efficiency across diverse HLA allomorphs not established at this stage\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Separated the chaperoning and editing functions by mapping stabilization of nascent empty MHC-I to a broad scaffolding surface while showing the scoop loop hovers above the groove to promote peptide capture, refining the model of how the loop acts as a selectivity filter and peptide surrogate.\",\n      \"evidence\": \"Deep mutagenesis, solution NMR, ITC, FP, and in vitro reconstitution of loop variants\",\n      \"pmids\": [\"32167472\", \"34039964\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Dynamic intermediate of peptide capture not visualized\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Reported a distinct immunological role for TAPBPL as a T cell co-inhibitory ligand acting through a putative receptor on activated T cells, separate from its MHC-I chaperone function.\",\n      \"evidence\": \"Recombinant TAPBPL-Ig fusion, in vitro T cell proliferation/cytokine assays, in vivo EAE model, and blocking antibody\",\n      \"pmids\": [\"33938620\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Receptor identity not established\", \"Mechanism of receptor engagement uncharacterized\", \"Single lab\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Extended TAPBPR's chaperone scope to the non-classical molecule MR1, showing ligand-independent engagement and catalysis of small-molecule ligand exchange, indicating a broader role in non-classical antigen presentation.\",\n      \"evidence\": \"In vitro assays, paramagnetic and 19F-NMR relaxation dispersion, and restrained molecular dynamics\",\n      \"pmids\": [\"35725941\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Cellular relevance of MR1 chaperoning not demonstrated\", \"Single lab\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Confirmed the UGGT1 reglucosylation step in a fully reconstituted system with purified human proteins, establishing the minimal components sufficient for TAPBPR-promoted glycan modification of peptide-free MHC-I.\",\n      \"evidence\": \"Reconstituted in vitro system, glycoengineering, LC-MS glycan analysis\",\n      \"pmids\": [\"37345806\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Downstream calreticulin transfer not addressed here\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Visualized an antigen-proofreading intermediate at 3.0 \\u00c5 showing transient P2/P3 anchor contacts with the nascent groove and engineered a high-fidelity TAPBPR variant for robust surface exchange across multiple HLA allomorphs, advancing both mechanistic and translational understanding.\",\n      \"evidence\": \"Cryo-EM structure of a peptide-decoy-bound MHC-I/TAPBPR complex and cell-surface exchange assays with an engineered variant\",\n      \"pmids\": [\"39786927\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Kinetics of the proofreading transition not fully resolved\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Reconstituted the hand-off step, showing calreticulin transfers peptide-receptive MHC-I from TAPBPR back to tapasin dependent on the UGGT1-generated monoglucosylated glycan, with distinct calreticulin domains required for disengagement versus docking, closing the editing-to-PLC recycling loop.\",\n      \"evidence\": \"In vitro reconstitution with purified components and calreticulin domain mutagenesis (preprint)\",\n      \"pmids\": [\"bio_10.1101_2025.11.25.690393\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Preprint not yet peer-reviewed\", \"In-cell validation of the transfer mechanism pending\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The identity of the TAPBPL co-inhibitory receptor on T cells and the molecular basis of that signaling pathway remain undefined.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Receptor unidentified\", \"Relationship between chaperone and co-inhibitory functions unknown\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [4, 6, 9, 14]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [4, 11, 12]},\n      {\"term_id\": \"GO:0044183\", \"supporting_discovery_ids\": [5, 11, 14]},\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [13]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [0, 1]},\n      {\"term_id\": \"GO:0005794\", \"supporting_discovery_ids\": [1]},\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 10]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [4, 10, 13]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [8, 15]}\n    ],\n    \"complexes\": [\"TAPBPR\\u2013MHC-I/\\u03b22m complex\", \"TAPBPR\\u2013UGGT1 complex\"],\n    \"partners\": [\"B2M\", \"UGGT1\", \"CALR\", \"TAPBP\", \"MR1\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":7,"faith_total":7,"faith_pct":100.0}}