{"gene":"TAP2","run_date":"2026-04-28T21:42:58","timeline":{"discoveries":[{"year":1994,"finding":"TAP1 and TAP2 both contribute to the peptide-recognition site; photoaffinity labeling showed both subunits are photolabeled by peptide analogues, and efficient peptide-binding site formation requires coexpression of both subunits. MHC class I/β2m dimers associate with TAP1 but not detectably with individual TAP2 chains, indicating the MHC class I interaction site resides on TAP1.","method":"Photoaffinity labeling of TAP subunits with photopeptide analogues; co-immunoprecipitation of MHC class I with individual TAP subunits in transfectant cell lines","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1/2 — direct biochemical reconstitution with photoaffinity labeling plus Co-IP, orthogonal methods in one study","pmids":["7809108"],"is_preprint":false},{"year":1996,"finding":"Transport specificity of the TAP complex for peptide C-terminal residues is primarily determined by the TAP2 subunit; a single point mutation in human TAP2 (374A→D) drastically alters peptide transport specificity, and the N-terminal region (residues 1–361) of TAP2 critically controls selective transport of peptides with C-terminal positively charged residues.","method":"Interspecies TAP hybrid construction and site-directed mutagenesis of hTAP2 expressed in Sf9 insect cells and TAP-deficient T2 cells; peptide transport assays with 20 C-terminal peptide variants","journal":"European journal of immunology","confidence":"High","confidence_rationale":"Tier 1 — in vitro transport assay with mutagenesis, multiple variants tested","pmids":["8765016"],"is_preprint":false},{"year":2000,"finding":"Walker A lysine mutations in TAP2 (K509M) and TAP1 (K544M) selectively impair peptide translocation but not peptide binding. TAP2 K509M does not significantly impair nucleotide binding, whereas TAP1 K544M substantially reduces nucleotide binding, indicating distinct roles for the two NBDs; TAP1(K544M)·TAP2 retains low-level translocation but TAP1·TAP2(K509M) loses all translocation, suggesting both intact NBDs are required for efficient peptide transport.","method":"Site-directed mutagenesis of Walker A lysine in TAP1 and TAP2 NBDs; nucleotide binding assays, fluorescence quenching peptide binding assays, and peptide translocation assays in insect cells","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — reconstituted in vitro with mutagenesis and multiple functional readouts","pmids":["11099504"],"is_preprint":false},{"year":2002,"finding":"Tapasin binds to the membrane-spanning domains of both TAP1 and TAP2 (not the isolated NBDs), and tapasin enhances structural stability of TAP1·TAP2 complexes at near-physiological temperatures; tapasin is not required for high-affinity peptide binding to TAP and slightly reduces peptide affinity.","method":"Co-immunoprecipitation of tapasin with TAP truncation constructs (membrane-spanning domain only vs. NBD only) in insect cells; thermostability assays of peptide-binding site with and without tapasin; peptide binding affinity measurements","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1/2 — multiple orthogonal methods (Co-IP with domain mapping, thermostability, binding assays) in one study","pmids":["12213826"],"is_preprint":false},{"year":2002,"finding":"Individual TAP2 polypeptide (but not TAP1) can form homodimers in whole cells and detergent lysates, as shown by chemical cross-linking; individual TAP1 and TAP2 subunits each associate with HLA class I molecules and form peptide-loading complexes, with their NBDs retaining ATP-binding capacity.","method":"Chemical cross-linking of individual TAP subunits; immunoprecipitation of HLA class I with individually expressed rat TAP1 or TAP2 in T2 cells; vaccinia virus recombinants for multiple HLA alleles","journal":"Immunology","confidence":"Medium","confidence_rationale":"Tier 2/3 — multiple Co-IP and cross-linking experiments, single lab","pmids":["12047747"],"is_preprint":false},{"year":2005,"finding":"The N-terminal domain (N domain) of TAP2 (but not TAP1) is critical for functional integrity of the MHC class I peptide-loading complex; TAP variants lacking the N domain of TAP2 build PLCs that fail to generate stable MHC I-peptide complexes and show substantially reduced recruitment of accessory chaperones.","method":"Expression of N-terminally truncated TAP1 and TAP2 variants (individually, in combination with wild-type chains, or as fusion proteins) in cell lines; co-immunoprecipitation of PLC components; MHC class I surface expression assays","journal":"Journal of immunology (Baltimore, Md. : 1950)","confidence":"High","confidence_rationale":"Tier 2 — domain truncation experiments with multiple orthogonal readouts (Co-IP, MHC surface expression), single lab","pmids":["16210614"],"is_preprint":false},{"year":2005,"finding":"N-terminal domains of TAP1 and TAP2 (cleaved at residues 131 and 88, respectively) are important for tapasin binding and tapasin-mediated increase in MHC class I peptide loading; truncated TAP variants retain peptide binding and nucleotide binding but show reduced tapasin binding and consequently reduced tapasin-dependent enhancement of HLA-B*2705 and HLA-B*4402 processing.","method":"Expression and purification of human TAP1/TAP2 complexes from insect cells; proteolytic mapping; peptide translocation assays in vitro; insect cell-based MHC class I loading reconstitution assay; tapasin binding assays","journal":"Immunology and cell biology","confidence":"High","confidence_rationale":"Tier 1 — in vitro reconstitution with domain mapping and multiple functional readouts","pmids":["16174096"],"is_preprint":false},{"year":2006,"finding":"Catalytic activity at the TAP2 nucleotide-binding site (NBS2) is the main driver of peptide translocation; mutations at TAP2 Glu632 and His661 (consensus Walker B and switch region residues at NBS2) significantly reduce peptide translocation and MHC class I surface expression, while analogous non-consensus residues Asp668 and Gln701 at the TAP1 site (NBS1) have only minor effects, consistent with NBS1 being the attenuated site.","method":"Site-directed mutagenesis of catalytic site residues in TAP1 and TAP2 NBDs; peptide translocation assays; MHC class I surface expression assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — mutagenesis at defined catalytic residues with functional transport readouts","pmids":["17068338"],"is_preprint":false},{"year":2006,"finding":"Biogenesis of functional TAP requires assembly of pre-existing TAP1 with newly synthesized TAP2 (not vice versa); TAP2 is highly unstable when expressed alone and requires heterodimerization with TAP1 for stability; the core transmembrane domain (core TMD) of TAP2 is necessary and sufficient for functional complex formation with pre-existing TAP1.","method":"In vitro expression system with pulse-chase and assembly assays; domain truncation mapping of TAP2 TMD; stability and functional transport assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — in vitro reconstitution with domain mutagenesis and pulse-chase stability assays","pmids":["16624807"],"is_preprint":false},{"year":2007,"finding":"The transmembrane domain (TMD) and adjacent ER-luminal connecting peptide (CP) of tapasin stabilize the TAP2 subunit; a spatially arranged motif in the tapasin TMD (including a conserved Lys plus four neighboring residues in the predicted α-helical arrangement) and a conserved Glu in the CP are each required for TAP2 stabilization, and loss of TAP2 stabilization impairs MHC class I surface expression.","method":"Detailed mutational analysis of tapasin TMD and CP; transfection of tapasin-deficient cells; TAP2 expression level assays; MHC class I surface expression assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — systematic mutagenesis with defined functional and stability readouts","pmids":["17244610"],"is_preprint":false},{"year":1997,"finding":"MHC class I molecules interact with both TAP1 and TAP2 subunits; in TAP-deficient T2 cells, transfected rat TAP2 alone associates with endogenous HLA-A2 and HLA-B51 as well as with calreticulin and tapasin, forming a peptide-loading complex.","method":"Immunoprecipitation of MHC class I with individually transfected TAP2 polypeptide in TAP-deficient T2 cells; vaccinia virus-encoded HLA alleles","journal":"European journal of immunology","confidence":"Medium","confidence_rationale":"Tier 3 — single Co-IP/pulldown, single lab, but multiple HLA alleles tested","pmids":["9368636"],"is_preprint":false},{"year":2001,"finding":"EBV latent membrane protein 1 (LMP-1) induces TAP2 expression via IRF-7 as a secondary mediator; LMP-1 stimulates IRF-7 expression and facilitates its phosphorylation and nuclear translocation, and activated IRF-7 then binds the ISRE in the TAP2 promoter to activate transcription.","method":"Reporter gene assays with TAP2 promoter constructs; gel mobility shift assays (EMSA) with ISRE; formaldehyde cross-linking ChIP; nuclear translocation assays; knockdown/complementation in Burkitt lymphoma cell lines with and without LMP-1-inducible IRF-7","journal":"Journal of virology","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods (reporter assay, EMSA, ChIP, complementation) in one study","pmids":["11119603"],"is_preprint":false},{"year":1993,"finding":"TAP2 (Ham-2/HAM2 in mouse) encodes an ATP-binding transporter protein required for efficient MHC class I antigen presentation; transfection of the Tap-2 gene into the TAP2-defective RMA-S cell line restores antigen presentation to CTLs and cell surface MHC class I expression.","method":"Gene transfection of murine Tap-2 into RMA-S cells; CTL killing assays; antigen presentation kinetic studies with Sendai virus","journal":"European journal of immunology","confidence":"High","confidence_rationale":"Tier 2 — genetic complementation with defined functional phenotypic readout, consistent with multiple independent reports","pmids":["8393798"],"is_preprint":false},{"year":1994,"finding":"TAP2-deficient RMA-S cells fail to express stable surface QA-1b (and class Ia) MHC class I molecules; transfection with Tap-2 rescues QA-1b surface expression, demonstrating that TAP-2-dependent peptide delivery is required for stable surface expression of these class Ib molecules.","method":"TAP2 gene transfection of RMA-S cells; CTL recognition assays; temperature-shift experiments","journal":"Journal of immunology (Baltimore, Md. : 1950)","confidence":"Medium","confidence_rationale":"Tier 2 — genetic complementation with defined functional readout, single study","pmids":["8189046"],"is_preprint":false},{"year":1995,"finding":"Mouse thymus-leukemia antigen (TL), a class Ib MHC molecule, is expressed efficiently at the cell surface in the absence of functional TAP2, indicating that TL does not require TAP2-dependent peptides for export and stable surface expression, though TL heavy chains expressed without TAP2 show altered conformation (increased proteolytic susceptibility).","method":"Expression of TL in TAP2-deficient RMA-S cells; immunoprecipitation; SDS-PAGE; temperature-shift stability assays","journal":"European journal of immunology","confidence":"Medium","confidence_rationale":"Tier 2 — biochemical characterization with multiple readouts, single lab","pmids":["7737270"],"is_preprint":false},{"year":2003,"finding":"Cysteine-less human TAP1 and TAP2 subunits (with all 19 cysteines replaced by de novo gene synthesis) are functional for ATP-dependent peptide transport and are inhibited normally by ICP47 (HSV-1 immune evasion protein), demonstrating that no cysteine residue in TAP2 is essential for its core transport function.","method":"De novo gene synthesis of cysteine-less TAP subunits; expression in TAP-deficient human fibroblasts; ATP-dependent peptide transport assays; ICP47 inhibition assays; MHC class I surface expression","journal":"FEBS letters","confidence":"Medium","confidence_rationale":"Tier 1 — in vitro functional assay with engineered cysteine-less protein, single lab","pmids":["12505156"],"is_preprint":false},{"year":2007,"finding":"TAP2 coding SNPs allele-dependently control alternative splicing into two isoforms (NM_000544 and NM_018833) with known differential peptide selectivities; the G(Ala) allele at codon 665 is enriched >2-fold in isoform NM_000544, while isoform NM_018833 derives almost exclusively from the A(Thr) allele, providing a functional mechanism by which TAP2 polymorphisms modulate peptide selectivity.","method":"Allele-specific isoform quantification by RT-PCR from heterozygous lymphoblastoid cell lines; transmission disequilibrium test in type 1 diabetes families","journal":"Diabetes","confidence":"Medium","confidence_rationale":"Tier 2 — direct RNA quantification from heterozygous cells linking coding SNPs to splicing outcomes","pmids":["17192492"],"is_preprint":false},{"year":2014,"finding":"Natural polymorphisms in rat Tap2 influence the MHC class I peptide ligandome and thereby affect negative selection and CD4:CD8 lineage commitment; a recombination between RT1-A and Tap2 alleles in MHC-recombinant congenic rats revealed that Tap2 variants altering the peptide repertoire presented by MHC class I lead to reduced negative selection of CD8 single-positive thymocytes.","method":"Generation of MHC-recombinant congenic rat strains; QTL mapping in outbred Heterogeneous Stock rats; flow cytometry of thymocyte populations; genetic interaction analysis between Tap2 and RT1-A intervals","journal":"PLoS genetics","confidence":"Medium","confidence_rationale":"Tier 2 — genetic epistasis with multiple congenic strains and defined cellular phenotype","pmids":["24586191"],"is_preprint":false},{"year":2016,"finding":"A T/C SNP (rs2071473) in a progesterone-responsive cis-regulatory element controls endometrial TAP2 expression by switching the element's activity from a repressor (T allele) to an enhancer (C allele); TAP2 is expressed by decidual stromal cells at the maternal-fetal interface, and this regulatory variant is associated with fecundability.","method":"Reporter gene assays demonstrating allele-specific enhancer/repressor activity; GTEx eQTL replication; cell-type expression analysis in decidual stromal cells; evolutionary analysis of balancing selection signatures","journal":"American journal of human genetics","confidence":"Medium","confidence_rationale":"Tier 2 — functional reporter assay with allele-specific activity, replicated eQTL, defined cell-type expression","pmids":["27745831"],"is_preprint":false}],"current_model":"TAP2 is an ABC transporter half-subunit that heterodimerizes with TAP1 in the ER membrane to form the TAP complex, which transports antigenic peptides from the cytosol into the ER for loading onto MHC class I molecules; both subunits contribute to the peptide-binding site, with TAP2 primarily governing peptide C-terminal specificity and its second nucleotide-binding site (NBS2, containing consensus catalytic residues Glu632/His661) serving as the main ATP hydrolysis site driving translocation, while the TAP2 N-terminal domain and tapasin (which contacts TAP2 transmembrane helices) are required for assembly of a functional MHC class I peptide-loading complex; TAP2 stability depends on heterodimerization with pre-existing TAP1, and TAP2 transcription is upregulated by IFN-γ and by EBV LMP-1 via IRF-7 binding to an ISRE in the TAP2 promoter."},"narrative":{"teleology":[{"year":1993,"claim":"Establishing that TAP2 is required for MHC class I antigen presentation answered the fundamental question of whether this ABC transporter gene is functionally necessary for peptide supply to MHC class I molecules.","evidence":"Transfection of murine Tap-2 into TAP2-defective RMA-S cells restored CTL-mediated antigen presentation and surface MHC class I expression","pmids":["8393798"],"confidence":"High","gaps":["Biochemical mechanism of peptide transport not yet defined","Relative contributions of TAP1 vs TAP2 unknown"]},{"year":1994,"claim":"Photoaffinity labeling demonstrated that both TAP1 and TAP2 subunits directly contact transported peptides, establishing that the peptide-binding site spans the heterodimer rather than residing on a single subunit.","evidence":"Photoaffinity labeling with peptide analogues in transfectant cell lines; co-immunoprecipitation of MHC class I with individual TAP subunits","pmids":["7809108"],"confidence":"High","gaps":["Which subunit determines peptide selectivity was unresolved","Stoichiometry and topology of the binding site unknown"]},{"year":1996,"claim":"Demonstrating that TAP2 governs C-terminal peptide selectivity — with a single residue (374) critical for specificity — resolved which subunit controls the immunologically important selection of peptide cargo.","evidence":"Interspecies TAP hybrids and site-directed mutagenesis of human TAP2 expressed in insect cells; peptide transport assays with 20 C-terminal variants","pmids":["8765016"],"confidence":"High","gaps":["Structural basis of residue 374 selectivity unknown","In vivo impact on peptide repertoire not tested"]},{"year":2000,"claim":"Walker A mutations revealed asymmetric roles for the two NBDs: TAP2's NBS2 is indispensable for translocation while TAP1's NBS1 contributes to nucleotide binding, establishing the functional asymmetry typical of heterodimeric ABC transporters.","evidence":"Site-directed mutagenesis of Walker A lysine in TAP1 (K544M) and TAP2 (K509M); nucleotide binding, peptide binding, and translocation assays in insect cells","pmids":["11099504"],"confidence":"High","gaps":["Catalytic residues at each NBS not yet identified","Mechanism coupling ATP hydrolysis to peptide translocation unknown"]},{"year":2001,"claim":"Discovery that EBV LMP-1 upregulates TAP2 via IRF-7 binding to a promoter ISRE established the first virus-mediated transcriptional activation pathway for TAP2, linking viral immune modulation to antigen processing.","evidence":"Reporter assays, EMSA, ChIP, and complementation in Burkitt lymphoma cell lines with inducible LMP-1/IRF-7","pmids":["11119603"],"confidence":"High","gaps":["Whether this pathway operates in non-EBV contexts unknown","Relationship to IFN-γ-mediated TAP2 induction not clarified"]},{"year":2002,"claim":"Mapping the tapasin interaction to the transmembrane domains (not NBDs) of both TAP1 and TAP2, and showing tapasin stabilizes the TAP complex at physiological temperature, established tapasin as a structural chaperone of the heterodimer.","evidence":"Co-immunoprecipitation of tapasin with TAP domain truncations in insect cells; thermostability and peptide binding assays","pmids":["12213826"],"confidence":"High","gaps":["Specific tapasin-contacting residues on TAP2 TMD not identified","Mechanism of thermostabilization unclear"]},{"year":2005,"claim":"Truncation of the TAP2 N-terminal domain (but not TAP1's) disrupted peptide-loading complex assembly and chaperone recruitment, identifying the TAP2 N-domain as uniquely critical for PLC integrity.","evidence":"N-terminal truncations of TAP1 and TAP2 expressed in cell lines; co-immunoprecipitation of PLC components; MHC class I surface expression assays","pmids":["16210614","16174096"],"confidence":"High","gaps":["Structural basis for N-domain–tapasin interaction unknown","Whether N-domain directly contacts MHC class I not tested"]},{"year":2006,"claim":"Mutagenesis of consensus catalytic residues (Glu632, His661) at NBS2 confirmed TAP2's NBS2 as the primary hydrolytic engine, while TAP1's degenerate NBS1 plays a minor catalytic role — completing the functional asymmetry model of the TAP heterodimer.","evidence":"Site-directed mutagenesis of Walker B and switch residues at NBS1 and NBS2; peptide translocation and MHC class I surface expression assays","pmids":["17068338"],"confidence":"High","gaps":["No structural data for the catalytic cycle","How ATP hydrolysis at NBS2 couples to conformational change in the TMDs remains unknown"]},{"year":2006,"claim":"Pulse-chase and domain-mapping experiments showed TAP2 is intrinsically unstable and must assemble with pre-existing TAP1 via its core TMD, establishing the ordered biogenesis pathway of the TAP complex.","evidence":"In vitro expression with pulse-chase; TAP2 domain truncation; stability and transport assays","pmids":["16624807"],"confidence":"High","gaps":["Degradation pathway of unassembled TAP2 not characterized","Whether ER quality-control factors participate is unknown"]},{"year":2007,"claim":"Identification of a specific TMD motif (conserved Lys and neighboring residues) plus an ER-luminal Glu in tapasin required for TAP2 stabilization provided the first residue-level map of the tapasin–TAP2 stabilization interface.","evidence":"Systematic mutagenesis of tapasin TMD and connecting peptide in tapasin-deficient cells; TAP2 protein levels and MHC class I surface expression","pmids":["17244610"],"confidence":"High","gaps":["Reciprocal residues on TAP2 TMD not mapped","No high-resolution structural data for the interface"]},{"year":2014,"claim":"Genetic epistasis between Tap2 alleles and MHC class I haplotype in congenic rats demonstrated that TAP2 polymorphisms shape the peptide ligandome in vivo, with consequences for thymic negative selection and CD4:CD8 lineage commitment.","evidence":"MHC-recombinant congenic rat strains; QTL mapping in outbred rats; thymocyte flow cytometry","pmids":["24586191"],"confidence":"Medium","gaps":["Specific peptides differentially transported by Tap2 alleles not identified","Mechanistic link to lineage commitment not defined at molecular level"]},{"year":null,"claim":"No high-resolution structure of the human TAP1–TAP2 heterodimer has been determined, and the conformational coupling between NBS2 ATP hydrolysis and peptide translocation through the TMD channel remains structurally undefined.","evidence":"","pmids":[],"confidence":"High","gaps":["No cryo-EM or crystallographic structure of human TAP heterodimer available in the timeline","Mechanism of conformational coupling between NBS2 hydrolysis and TMD gating unknown","Role of TAP2 in ER-associated degradation quality control pathway not characterized"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140657","term_label":"ATP-dependent activity","supporting_discovery_ids":[2,7,12]},{"term_id":"GO:0005215","term_label":"transporter activity","supporting_discovery_ids":[1,2,7,12,15]}],"localization":[{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[3,5,8,9]}],"pathway":[{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[0,1,5,12,13]},{"term_id":"R-HSA-382551","term_label":"Transport of small molecules","supporting_discovery_ids":[2,7,15]}],"complexes":["TAP complex (TAP1–TAP2 heterodimer)","MHC class I peptide-loading complex (PLC)"],"partners":["TAP1","TAPBP","CALR","HLA-A","HLA-B","IRF7"],"other_free_text":[]},"mechanistic_narrative":"TAP2 is an ABC transporter half-subunit that heterodimerizes with TAP1 in the endoplasmic reticulum membrane to form the transporter associated with antigen processing (TAP), which translocates cytosolic peptides into the ER lumen for loading onto MHC class I molecules, a process essential for cell-surface MHC class I expression and antigen presentation to cytotoxic T lymphocytes [PMID:8393798, PMID:7809108]. Both TAP1 and TAP2 contribute to the peptide-binding site, but TAP2 primarily governs C-terminal peptide selectivity through its N-terminal transmembrane region and a critical residue at position 374, and its nucleotide-binding site (NBS2, containing consensus catalytic residues Glu632 and His661) serves as the principal ATP hydrolysis engine driving translocation [PMID:8765016, PMID:17068338, PMID:11099504]. TAP2 is intrinsically unstable when expressed alone and requires assembly with pre-existing TAP1 via its core transmembrane domain; the TAP2 N-terminal domain is additionally essential for recruitment of tapasin and accessory chaperones into a functional peptide-loading complex, and tapasin in turn stabilizes TAP2 through a defined transmembrane motif [PMID:16624807, PMID:16210614, PMID:17244610]. TAP2 transcription is induced by IFN-γ and by EBV LMP-1 via IRF-7 binding to an ISRE in the TAP2 promoter, and natural polymorphisms in TAP2 modulate peptide repertoire, influencing thymic selection and immune recognition [PMID:11119603, PMID:24586191]."},"prefetch_data":{"uniprot":{"accession":"Q03519","full_name":"Antigen peptide transporter 2","aliases":["ATP-binding cassette sub-family B member 3","Peptide supply factor 2","Peptide transporter PSF2","PSF-2","Peptide transporter TAP2","Peptide transporter involved in antigen processing 2","Really interesting new gene 11 protein","RING11"],"length_aa":686,"mass_kda":75.7,"function":"ABC transporter associated with antigen processing. In complex with TAP1 mediates unidirectional translocation of peptide antigens from cytosol to endoplasmic reticulum (ER) for loading onto MHC class I (MHCI) molecules (PubMed:25377891, PubMed:25656091). Uses the chemical energy of ATP to export peptides against the concentration gradient (PubMed:25377891). During the transport cycle alternates between 'inward-facing' state with peptide binding site facing the cytosol to 'outward-facing' state with peptide binding site facing the ER lumen. Peptide antigen binding to ATP-loaded TAP1-TAP2 induces a switch to hydrolysis-competent 'outward-facing' conformation ready for peptide loading onto nascent MHCI molecules. Subsequently ATP hydrolysis resets the transporter to the 'inward facing' state for a new cycle (PubMed:11274390, PubMed:25377891, PubMed:25656091). Typically transports intracellular peptide antigens of 8 to 13 amino acids that arise from cytosolic proteolysis via IFNG-induced immunoproteasome. Binds peptides with free N- and C-termini, the first three and the C-terminal residues being critical. Preferentially selects peptides having a highly hydrophobic residue at position 3 and hydrophobic or charged residues at the C-terminal anchor. Proline at position 2 has the most destabilizing effect (PubMed:11274390, PubMed:7500034, PubMed:9256420). As a component of the peptide loading complex (PLC), acts as a molecular scaffold essential for peptide-MHCI assembly and antigen presentation (PubMed:1538751, PubMed:25377891, PubMed:26611325)","subcellular_location":"Endoplasmic reticulum membrane","url":"https://www.uniprot.org/uniprotkb/Q03519/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/TAP2","classification":"Not Classified","n_dependent_lines":2,"n_total_lines":1208,"dependency_fraction":0.0016556291390728477},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[{"gene":"CANX","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/search/TAP2","total_profiled":1310},"omim":[{"mim_id":"620813","title":"MHC CLASS I DEFICIENCY 2; MHC1D2","url":"https://www.omim.org/entry/620813"},{"mim_id":"612825","title":"SEC14-LIKE LIPID-BINDING PROTEIN 4; SEC14L4","url":"https://www.omim.org/entry/612825"},{"mim_id":"612824","title":"SEC14-LIKE LIPID-BINDING PROTEIN 3; SEC14L3","url":"https://www.omim.org/entry/612824"},{"mim_id":"605464","title":"ATP-BINDING CASSETTE, SUBFAMILY B, MEMBER 8; ABCB8","url":"https://www.omim.org/entry/605464"},{"mim_id":"605453","title":"ATP-BINDING CASSETTE, SUBFAMILY B, MEMBER 9; ABCB9","url":"https://www.omim.org/entry/605453"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Endoplasmic reticulum","reliability":"Supported"},{"location":"Nuclear speckles","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in many","driving_tissues":[],"url":"https://www.proteinatlas.org/search/TAP2"},"hgnc":{"alias_symbol":["PSF2","RING11","D6S217E"],"prev_symbol":["ABCB3"]},"alphafold":{"accession":"Q03519","domains":[{"cath_id":"1.20.1560.10","chopping":"127-289_432-449","consensus_level":"medium","plddt":85.5256,"start":127,"end":449},{"cath_id":"3.40.50.300","chopping":"467-680","consensus_level":"high","plddt":87.7366,"start":467,"end":680},{"cath_id":"1.20.120","chopping":"5-118","consensus_level":"high","plddt":64.968,"start":5,"end":118},{"cath_id":"1.10.287","chopping":"290-403","consensus_level":"medium","plddt":90.8358,"start":290,"end":403}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q03519","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q03519-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q03519-F1-predicted_aligned_error_v6.png","plddt_mean":82.69},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=TAP2","jax_strain_url":"https://www.jax.org/strain/search?query=TAP2"},"sequence":{"accession":"Q03519","fasta_url":"https://rest.uniprot.org/uniprotkb/Q03519.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q03519/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q03519"}},"corpus_meta":[{"pmid":"8428770","id":"PMC_8428770","title":"Alleles and haplotypes of the MHC-encoded ABC transporters TAP1 and TAP2.","date":"1993","source":"Immunogenetics","url":"https://pubmed.ncbi.nlm.nih.gov/8428770","citation_count":227,"is_preprint":false},{"pmid":"10749979","id":"PMC_10749979","title":"High resolution analysis of haplotype diversity and meiotic crossover in the human TAP2 recombination hotspot.","date":"2000","source":"Human molecular genetics","url":"https://pubmed.ncbi.nlm.nih.gov/10749979","citation_count":169,"is_preprint":false},{"pmid":"7809108","id":"PMC_7809108","title":"Characteristics of peptide and major histocompatibility complex class I/beta 2-microglobulin binding to the transporters associated with antigen processing (TAP1 and TAP2).","date":"1994","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/7809108","citation_count":152,"is_preprint":false},{"pmid":"9485029","id":"PMC_9485029","title":"HLA class I antigen and transporter associated with antigen processing (TAP1 and TAP2) down-regulation in high-grade primary breast carcinoma lesions.","date":"1998","source":"Cancer research","url":"https://pubmed.ncbi.nlm.nih.gov/9485029","citation_count":144,"is_preprint":false},{"pmid":"12694570","id":"PMC_12694570","title":"Total loss of MHC class I in colorectal tumors can be explained by two molecular pathways: beta2-microglobulin inactivation in MSI-positive tumors and LMP7/TAP2 downregulation in MSI-negative tumors.","date":"2003","source":"Tissue antigens","url":"https://pubmed.ncbi.nlm.nih.gov/12694570","citation_count":127,"is_preprint":false},{"pmid":"8477801","id":"PMC_8477801","title":"Linkage disequilibrium between TAP2 variants and HLA class II alleles; no primary association between TAP2 variants and insulin-dependent diabetes mellitus.","date":"1993","source":"European journal of 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monoiodoacetate-induced arthritis rat model.","date":"2020","source":"Scientific reports","url":"https://pubmed.ncbi.nlm.nih.gov/33060735","citation_count":12,"is_preprint":false},{"pmid":"1672656","id":"PMC_1672656","title":"Molecular analysis of tap2, an anther-specific gene from Antirrhinum majus.","date":"1991","source":"FEBS letters","url":"https://pubmed.ncbi.nlm.nih.gov/1672656","citation_count":12,"is_preprint":false},{"pmid":"27745831","id":"PMC_27745831","title":"An Ancient Fecundability-Associated Polymorphism Switches a Repressor into an Enhancer of Endometrial TAP2 Expression.","date":"2016","source":"American journal of human genetics","url":"https://pubmed.ncbi.nlm.nih.gov/27745831","citation_count":12,"is_preprint":false},{"pmid":"8851724","id":"PMC_8851724","title":"The human leukocyte antigen TAP2 gene defines the centromeric limit of melanoma susceptibility on chromosome 6p.","date":"1996","source":"Tissue antigens","url":"https://pubmed.ncbi.nlm.nih.gov/8851724","citation_count":12,"is_preprint":false},{"pmid":"9672156","id":"PMC_9672156","title":"New TAP2 polymorphisms in Africans.","date":"1998","source":"Tissue antigens","url":"https://pubmed.ncbi.nlm.nih.gov/9672156","citation_count":12,"is_preprint":false},{"pmid":"1300236","id":"PMC_1300236","title":"TAP1 and TAP2 transporter genes and predisposition to insulin dependent diabetes mellitus.","date":"1992","source":"Comptes rendus de l'Academie des sciences. Serie III, Sciences de la vie","url":"https://pubmed.ncbi.nlm.nih.gov/1300236","citation_count":11,"is_preprint":false},{"pmid":"9510369","id":"PMC_9510369","title":"Development and characterization of mouse anti-human LMP2, LMP7, TAP1 and TAP2 monoclonal antibodies.","date":"1998","source":"Tissue antigens","url":"https://pubmed.ncbi.nlm.nih.gov/9510369","citation_count":11,"is_preprint":false},{"pmid":"8913653","id":"PMC_8913653","title":"Association of TAP1 and TAP2 with systemic sclerosis in Japanese.","date":"1996","source":"Clinical and experimental rheumatology","url":"https://pubmed.ncbi.nlm.nih.gov/8913653","citation_count":11,"is_preprint":false},{"pmid":"7797612","id":"PMC_7797612","title":"TAP2 polymorphisms in Australian multiple sclerosis patients.","date":"1995","source":"Journal of neuroimmunology","url":"https://pubmed.ncbi.nlm.nih.gov/7797612","citation_count":11,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":49436,"output_tokens":4945,"usd":0.111241},"stage2":{"model":"claude-opus-4-6","input_tokens":8441,"output_tokens":2918,"usd":0.172733},"total_usd":0.283974,"stage1_batch_id":"msgbatch_01RPbVc9PDH9zQmYK5Hs6srm","stage2_batch_id":"msgbatch_01JVbVLzeKN2yVrsGp3zC8aM","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1994,\n      \"finding\": \"TAP1 and TAP2 both contribute to the peptide-recognition site; photoaffinity labeling showed both subunits are photolabeled by peptide analogues, and efficient peptide-binding site formation requires coexpression of both subunits. MHC class I/β2m dimers associate with TAP1 but not detectably with individual TAP2 chains, indicating the MHC class I interaction site resides on TAP1.\",\n      \"method\": \"Photoaffinity labeling of TAP subunits with photopeptide analogues; co-immunoprecipitation of MHC class I with individual TAP subunits in transfectant cell lines\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1/2 — direct biochemical reconstitution with photoaffinity labeling plus Co-IP, orthogonal methods in one study\",\n      \"pmids\": [\"7809108\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"Transport specificity of the TAP complex for peptide C-terminal residues is primarily determined by the TAP2 subunit; a single point mutation in human TAP2 (374A→D) drastically alters peptide transport specificity, and the N-terminal region (residues 1–361) of TAP2 critically controls selective transport of peptides with C-terminal positively charged residues.\",\n      \"method\": \"Interspecies TAP hybrid construction and site-directed mutagenesis of hTAP2 expressed in Sf9 insect cells and TAP-deficient T2 cells; peptide transport assays with 20 C-terminal peptide variants\",\n      \"journal\": \"European journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro transport assay with mutagenesis, multiple variants tested\",\n      \"pmids\": [\"8765016\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"Walker A lysine mutations in TAP2 (K509M) and TAP1 (K544M) selectively impair peptide translocation but not peptide binding. TAP2 K509M does not significantly impair nucleotide binding, whereas TAP1 K544M substantially reduces nucleotide binding, indicating distinct roles for the two NBDs; TAP1(K544M)·TAP2 retains low-level translocation but TAP1·TAP2(K509M) loses all translocation, suggesting both intact NBDs are required for efficient peptide transport.\",\n      \"method\": \"Site-directed mutagenesis of Walker A lysine in TAP1 and TAP2 NBDs; nucleotide binding assays, fluorescence quenching peptide binding assays, and peptide translocation assays in insect cells\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstituted in vitro with mutagenesis and multiple functional readouts\",\n      \"pmids\": [\"11099504\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Tapasin binds to the membrane-spanning domains of both TAP1 and TAP2 (not the isolated NBDs), and tapasin enhances structural stability of TAP1·TAP2 complexes at near-physiological temperatures; tapasin is not required for high-affinity peptide binding to TAP and slightly reduces peptide affinity.\",\n      \"method\": \"Co-immunoprecipitation of tapasin with TAP truncation constructs (membrane-spanning domain only vs. NBD only) in insect cells; thermostability assays of peptide-binding site with and without tapasin; peptide binding affinity measurements\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1/2 — multiple orthogonal methods (Co-IP with domain mapping, thermostability, binding assays) in one study\",\n      \"pmids\": [\"12213826\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Individual TAP2 polypeptide (but not TAP1) can form homodimers in whole cells and detergent lysates, as shown by chemical cross-linking; individual TAP1 and TAP2 subunits each associate with HLA class I molecules and form peptide-loading complexes, with their NBDs retaining ATP-binding capacity.\",\n      \"method\": \"Chemical cross-linking of individual TAP subunits; immunoprecipitation of HLA class I with individually expressed rat TAP1 or TAP2 in T2 cells; vaccinia virus recombinants for multiple HLA alleles\",\n      \"journal\": \"Immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2/3 — multiple Co-IP and cross-linking experiments, single lab\",\n      \"pmids\": [\"12047747\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"The N-terminal domain (N domain) of TAP2 (but not TAP1) is critical for functional integrity of the MHC class I peptide-loading complex; TAP variants lacking the N domain of TAP2 build PLCs that fail to generate stable MHC I-peptide complexes and show substantially reduced recruitment of accessory chaperones.\",\n      \"method\": \"Expression of N-terminally truncated TAP1 and TAP2 variants (individually, in combination with wild-type chains, or as fusion proteins) in cell lines; co-immunoprecipitation of PLC components; MHC class I surface expression assays\",\n      \"journal\": \"Journal of immunology (Baltimore, Md. : 1950)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — domain truncation experiments with multiple orthogonal readouts (Co-IP, MHC surface expression), single lab\",\n      \"pmids\": [\"16210614\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"N-terminal domains of TAP1 and TAP2 (cleaved at residues 131 and 88, respectively) are important for tapasin binding and tapasin-mediated increase in MHC class I peptide loading; truncated TAP variants retain peptide binding and nucleotide binding but show reduced tapasin binding and consequently reduced tapasin-dependent enhancement of HLA-B*2705 and HLA-B*4402 processing.\",\n      \"method\": \"Expression and purification of human TAP1/TAP2 complexes from insect cells; proteolytic mapping; peptide translocation assays in vitro; insect cell-based MHC class I loading reconstitution assay; tapasin binding assays\",\n      \"journal\": \"Immunology and cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution with domain mapping and multiple functional readouts\",\n      \"pmids\": [\"16174096\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Catalytic activity at the TAP2 nucleotide-binding site (NBS2) is the main driver of peptide translocation; mutations at TAP2 Glu632 and His661 (consensus Walker B and switch region residues at NBS2) significantly reduce peptide translocation and MHC class I surface expression, while analogous non-consensus residues Asp668 and Gln701 at the TAP1 site (NBS1) have only minor effects, consistent with NBS1 being the attenuated site.\",\n      \"method\": \"Site-directed mutagenesis of catalytic site residues in TAP1 and TAP2 NBDs; peptide translocation assays; MHC class I surface expression assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — mutagenesis at defined catalytic residues with functional transport readouts\",\n      \"pmids\": [\"17068338\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Biogenesis of functional TAP requires assembly of pre-existing TAP1 with newly synthesized TAP2 (not vice versa); TAP2 is highly unstable when expressed alone and requires heterodimerization with TAP1 for stability; the core transmembrane domain (core TMD) of TAP2 is necessary and sufficient for functional complex formation with pre-existing TAP1.\",\n      \"method\": \"In vitro expression system with pulse-chase and assembly assays; domain truncation mapping of TAP2 TMD; stability and functional transport assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution with domain mutagenesis and pulse-chase stability assays\",\n      \"pmids\": [\"16624807\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"The transmembrane domain (TMD) and adjacent ER-luminal connecting peptide (CP) of tapasin stabilize the TAP2 subunit; a spatially arranged motif in the tapasin TMD (including a conserved Lys plus four neighboring residues in the predicted α-helical arrangement) and a conserved Glu in the CP are each required for TAP2 stabilization, and loss of TAP2 stabilization impairs MHC class I surface expression.\",\n      \"method\": \"Detailed mutational analysis of tapasin TMD and CP; transfection of tapasin-deficient cells; TAP2 expression level assays; MHC class I surface expression assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — systematic mutagenesis with defined functional and stability readouts\",\n      \"pmids\": [\"17244610\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"MHC class I molecules interact with both TAP1 and TAP2 subunits; in TAP-deficient T2 cells, transfected rat TAP2 alone associates with endogenous HLA-A2 and HLA-B51 as well as with calreticulin and tapasin, forming a peptide-loading complex.\",\n      \"method\": \"Immunoprecipitation of MHC class I with individually transfected TAP2 polypeptide in TAP-deficient T2 cells; vaccinia virus-encoded HLA alleles\",\n      \"journal\": \"European journal of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — single Co-IP/pulldown, single lab, but multiple HLA alleles tested\",\n      \"pmids\": [\"9368636\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"EBV latent membrane protein 1 (LMP-1) induces TAP2 expression via IRF-7 as a secondary mediator; LMP-1 stimulates IRF-7 expression and facilitates its phosphorylation and nuclear translocation, and activated IRF-7 then binds the ISRE in the TAP2 promoter to activate transcription.\",\n      \"method\": \"Reporter gene assays with TAP2 promoter constructs; gel mobility shift assays (EMSA) with ISRE; formaldehyde cross-linking ChIP; nuclear translocation assays; knockdown/complementation in Burkitt lymphoma cell lines with and without LMP-1-inducible IRF-7\",\n      \"journal\": \"Journal of virology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (reporter assay, EMSA, ChIP, complementation) in one study\",\n      \"pmids\": [\"11119603\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1993,\n      \"finding\": \"TAP2 (Ham-2/HAM2 in mouse) encodes an ATP-binding transporter protein required for efficient MHC class I antigen presentation; transfection of the Tap-2 gene into the TAP2-defective RMA-S cell line restores antigen presentation to CTLs and cell surface MHC class I expression.\",\n      \"method\": \"Gene transfection of murine Tap-2 into RMA-S cells; CTL killing assays; antigen presentation kinetic studies with Sendai virus\",\n      \"journal\": \"European journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic complementation with defined functional phenotypic readout, consistent with multiple independent reports\",\n      \"pmids\": [\"8393798\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1994,\n      \"finding\": \"TAP2-deficient RMA-S cells fail to express stable surface QA-1b (and class Ia) MHC class I molecules; transfection with Tap-2 rescues QA-1b surface expression, demonstrating that TAP-2-dependent peptide delivery is required for stable surface expression of these class Ib molecules.\",\n      \"method\": \"TAP2 gene transfection of RMA-S cells; CTL recognition assays; temperature-shift experiments\",\n      \"journal\": \"Journal of immunology (Baltimore, Md. : 1950)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic complementation with defined functional readout, single study\",\n      \"pmids\": [\"8189046\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"Mouse thymus-leukemia antigen (TL), a class Ib MHC molecule, is expressed efficiently at the cell surface in the absence of functional TAP2, indicating that TL does not require TAP2-dependent peptides for export and stable surface expression, though TL heavy chains expressed without TAP2 show altered conformation (increased proteolytic susceptibility).\",\n      \"method\": \"Expression of TL in TAP2-deficient RMA-S cells; immunoprecipitation; SDS-PAGE; temperature-shift stability assays\",\n      \"journal\": \"European journal of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — biochemical characterization with multiple readouts, single lab\",\n      \"pmids\": [\"7737270\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Cysteine-less human TAP1 and TAP2 subunits (with all 19 cysteines replaced by de novo gene synthesis) are functional for ATP-dependent peptide transport and are inhibited normally by ICP47 (HSV-1 immune evasion protein), demonstrating that no cysteine residue in TAP2 is essential for its core transport function.\",\n      \"method\": \"De novo gene synthesis of cysteine-less TAP subunits; expression in TAP-deficient human fibroblasts; ATP-dependent peptide transport assays; ICP47 inhibition assays; MHC class I surface expression\",\n      \"journal\": \"FEBS letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 — in vitro functional assay with engineered cysteine-less protein, single lab\",\n      \"pmids\": [\"12505156\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"TAP2 coding SNPs allele-dependently control alternative splicing into two isoforms (NM_000544 and NM_018833) with known differential peptide selectivities; the G(Ala) allele at codon 665 is enriched >2-fold in isoform NM_000544, while isoform NM_018833 derives almost exclusively from the A(Thr) allele, providing a functional mechanism by which TAP2 polymorphisms modulate peptide selectivity.\",\n      \"method\": \"Allele-specific isoform quantification by RT-PCR from heterozygous lymphoblastoid cell lines; transmission disequilibrium test in type 1 diabetes families\",\n      \"journal\": \"Diabetes\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct RNA quantification from heterozygous cells linking coding SNPs to splicing outcomes\",\n      \"pmids\": [\"17192492\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Natural polymorphisms in rat Tap2 influence the MHC class I peptide ligandome and thereby affect negative selection and CD4:CD8 lineage commitment; a recombination between RT1-A and Tap2 alleles in MHC-recombinant congenic rats revealed that Tap2 variants altering the peptide repertoire presented by MHC class I lead to reduced negative selection of CD8 single-positive thymocytes.\",\n      \"method\": \"Generation of MHC-recombinant congenic rat strains; QTL mapping in outbred Heterogeneous Stock rats; flow cytometry of thymocyte populations; genetic interaction analysis between Tap2 and RT1-A intervals\",\n      \"journal\": \"PLoS genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis with multiple congenic strains and defined cellular phenotype\",\n      \"pmids\": [\"24586191\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"A T/C SNP (rs2071473) in a progesterone-responsive cis-regulatory element controls endometrial TAP2 expression by switching the element's activity from a repressor (T allele) to an enhancer (C allele); TAP2 is expressed by decidual stromal cells at the maternal-fetal interface, and this regulatory variant is associated with fecundability.\",\n      \"method\": \"Reporter gene assays demonstrating allele-specific enhancer/repressor activity; GTEx eQTL replication; cell-type expression analysis in decidual stromal cells; evolutionary analysis of balancing selection signatures\",\n      \"journal\": \"American journal of human genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — functional reporter assay with allele-specific activity, replicated eQTL, defined cell-type expression\",\n      \"pmids\": [\"27745831\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"TAP2 is an ABC transporter half-subunit that heterodimerizes with TAP1 in the ER membrane to form the TAP complex, which transports antigenic peptides from the cytosol into the ER for loading onto MHC class I molecules; both subunits contribute to the peptide-binding site, with TAP2 primarily governing peptide C-terminal specificity and its second nucleotide-binding site (NBS2, containing consensus catalytic residues Glu632/His661) serving as the main ATP hydrolysis site driving translocation, while the TAP2 N-terminal domain and tapasin (which contacts TAP2 transmembrane helices) are required for assembly of a functional MHC class I peptide-loading complex; TAP2 stability depends on heterodimerization with pre-existing TAP1, and TAP2 transcription is upregulated by IFN-γ and by EBV LMP-1 via IRF-7 binding to an ISRE in the TAP2 promoter.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"TAP2 is an ABC transporter half-subunit that heterodimerizes with TAP1 in the endoplasmic reticulum membrane to form the transporter associated with antigen processing (TAP), which translocates cytosolic peptides into the ER lumen for loading onto MHC class I molecules, a process essential for cell-surface MHC class I expression and antigen presentation to cytotoxic T lymphocytes [PMID:8393798, PMID:7809108]. Both TAP1 and TAP2 contribute to the peptide-binding site, but TAP2 primarily governs C-terminal peptide selectivity through its N-terminal transmembrane region and a critical residue at position 374, and its nucleotide-binding site (NBS2, containing consensus catalytic residues Glu632 and His661) serves as the principal ATP hydrolysis engine driving translocation [PMID:8765016, PMID:17068338, PMID:11099504]. TAP2 is intrinsically unstable when expressed alone and requires assembly with pre-existing TAP1 via its core transmembrane domain; the TAP2 N-terminal domain is additionally essential for recruitment of tapasin and accessory chaperones into a functional peptide-loading complex, and tapasin in turn stabilizes TAP2 through a defined transmembrane motif [PMID:16624807, PMID:16210614, PMID:17244610]. TAP2 transcription is induced by IFN-γ and by EBV LMP-1 via IRF-7 binding to an ISRE in the TAP2 promoter, and natural polymorphisms in TAP2 modulate peptide repertoire, influencing thymic selection and immune recognition [PMID:11119603, PMID:24586191].\",\n  \"teleology\": [\n    {\n      \"year\": 1993,\n      \"claim\": \"Establishing that TAP2 is required for MHC class I antigen presentation answered the fundamental question of whether this ABC transporter gene is functionally necessary for peptide supply to MHC class I molecules.\",\n      \"evidence\": \"Transfection of murine Tap-2 into TAP2-defective RMA-S cells restored CTL-mediated antigen presentation and surface MHC class I expression\",\n      \"pmids\": [\"8393798\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Biochemical mechanism of peptide transport not yet defined\", \"Relative contributions of TAP1 vs TAP2 unknown\"]\n    },\n    {\n      \"year\": 1994,\n      \"claim\": \"Photoaffinity labeling demonstrated that both TAP1 and TAP2 subunits directly contact transported peptides, establishing that the peptide-binding site spans the heterodimer rather than residing on a single subunit.\",\n      \"evidence\": \"Photoaffinity labeling with peptide analogues in transfectant cell lines; co-immunoprecipitation of MHC class I with individual TAP subunits\",\n      \"pmids\": [\"7809108\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Which subunit determines peptide selectivity was unresolved\", \"Stoichiometry and topology of the binding site unknown\"]\n    },\n    {\n      \"year\": 1996,\n      \"claim\": \"Demonstrating that TAP2 governs C-terminal peptide selectivity — with a single residue (374) critical for specificity — resolved which subunit controls the immunologically important selection of peptide cargo.\",\n      \"evidence\": \"Interspecies TAP hybrids and site-directed mutagenesis of human TAP2 expressed in insect cells; peptide transport assays with 20 C-terminal variants\",\n      \"pmids\": [\"8765016\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of residue 374 selectivity unknown\", \"In vivo impact on peptide repertoire not tested\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Walker A mutations revealed asymmetric roles for the two NBDs: TAP2's NBS2 is indispensable for translocation while TAP1's NBS1 contributes to nucleotide binding, establishing the functional asymmetry typical of heterodimeric ABC transporters.\",\n      \"evidence\": \"Site-directed mutagenesis of Walker A lysine in TAP1 (K544M) and TAP2 (K509M); nucleotide binding, peptide binding, and translocation assays in insect cells\",\n      \"pmids\": [\"11099504\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Catalytic residues at each NBS not yet identified\", \"Mechanism coupling ATP hydrolysis to peptide translocation unknown\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Discovery that EBV LMP-1 upregulates TAP2 via IRF-7 binding to a promoter ISRE established the first virus-mediated transcriptional activation pathway for TAP2, linking viral immune modulation to antigen processing.\",\n      \"evidence\": \"Reporter assays, EMSA, ChIP, and complementation in Burkitt lymphoma cell lines with inducible LMP-1/IRF-7\",\n      \"pmids\": [\"11119603\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether this pathway operates in non-EBV contexts unknown\", \"Relationship to IFN-γ-mediated TAP2 induction not clarified\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Mapping the tapasin interaction to the transmembrane domains (not NBDs) of both TAP1 and TAP2, and showing tapasin stabilizes the TAP complex at physiological temperature, established tapasin as a structural chaperone of the heterodimer.\",\n      \"evidence\": \"Co-immunoprecipitation of tapasin with TAP domain truncations in insect cells; thermostability and peptide binding assays\",\n      \"pmids\": [\"12213826\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Specific tapasin-contacting residues on TAP2 TMD not identified\", \"Mechanism of thermostabilization unclear\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Truncation of the TAP2 N-terminal domain (but not TAP1's) disrupted peptide-loading complex assembly and chaperone recruitment, identifying the TAP2 N-domain as uniquely critical for PLC integrity.\",\n      \"evidence\": \"N-terminal truncations of TAP1 and TAP2 expressed in cell lines; co-immunoprecipitation of PLC components; MHC class I surface expression assays\",\n      \"pmids\": [\"16210614\", \"16174096\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis for N-domain–tapasin interaction unknown\", \"Whether N-domain directly contacts MHC class I not tested\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Mutagenesis of consensus catalytic residues (Glu632, His661) at NBS2 confirmed TAP2's NBS2 as the primary hydrolytic engine, while TAP1's degenerate NBS1 plays a minor catalytic role — completing the functional asymmetry model of the TAP heterodimer.\",\n      \"evidence\": \"Site-directed mutagenesis of Walker B and switch residues at NBS1 and NBS2; peptide translocation and MHC class I surface expression assays\",\n      \"pmids\": [\"17068338\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No structural data for the catalytic cycle\", \"How ATP hydrolysis at NBS2 couples to conformational change in the TMDs remains unknown\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Pulse-chase and domain-mapping experiments showed TAP2 is intrinsically unstable and must assemble with pre-existing TAP1 via its core TMD, establishing the ordered biogenesis pathway of the TAP complex.\",\n      \"evidence\": \"In vitro expression with pulse-chase; TAP2 domain truncation; stability and transport assays\",\n      \"pmids\": [\"16624807\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Degradation pathway of unassembled TAP2 not characterized\", \"Whether ER quality-control factors participate is unknown\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Identification of a specific TMD motif (conserved Lys and neighboring residues) plus an ER-luminal Glu in tapasin required for TAP2 stabilization provided the first residue-level map of the tapasin–TAP2 stabilization interface.\",\n      \"evidence\": \"Systematic mutagenesis of tapasin TMD and connecting peptide in tapasin-deficient cells; TAP2 protein levels and MHC class I surface expression\",\n      \"pmids\": [\"17244610\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Reciprocal residues on TAP2 TMD not mapped\", \"No high-resolution structural data for the interface\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Genetic epistasis between Tap2 alleles and MHC class I haplotype in congenic rats demonstrated that TAP2 polymorphisms shape the peptide ligandome in vivo, with consequences for thymic negative selection and CD4:CD8 lineage commitment.\",\n      \"evidence\": \"MHC-recombinant congenic rat strains; QTL mapping in outbred rats; thymocyte flow cytometry\",\n      \"pmids\": [\"24586191\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Specific peptides differentially transported by Tap2 alleles not identified\", \"Mechanistic link to lineage commitment not defined at molecular level\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"No high-resolution structure of the human TAP1–TAP2 heterodimer has been determined, and the conformational coupling between NBS2 ATP hydrolysis and peptide translocation through the TMD channel remains structurally undefined.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No cryo-EM or crystallographic structure of human TAP heterodimer available in the timeline\", \"Mechanism of conformational coupling between NBS2 hydrolysis and TMD gating unknown\", \"Role of TAP2 in ER-associated degradation quality control pathway not characterized\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140657\", \"supporting_discovery_ids\": [2, 7, 12]},\n      {\"term_id\": \"GO:0005215\", \"supporting_discovery_ids\": [1, 2, 7, 12, 15]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [3, 5, 8, 9]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"GO:0140657\", \"supporting_discovery_ids\": []},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [0, 1, 5, 12, 13]},\n      {\"term_id\": \"R-HSA-382551\", \"supporting_discovery_ids\": [2, 7, 15]}\n    ],\n    \"complexes\": [\n      \"TAP complex (TAP1–TAP2 heterodimer)\",\n      \"MHC class I peptide-loading complex (PLC)\"\n    ],\n    \"partners\": [\n      \"TAP1\",\n      \"TAPBP\",\n      \"CALR\",\n      \"HLA-A\",\n      \"HLA-B\",\n      \"IRF7\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}