{"gene":"CPD","run_date":"2026-04-28T17:28:53","timeline":{"discoveries":[{"year":1995,"finding":"Duck gp180, a host cell glycoprotein that binds duck hepatitis B virus (DHBV) particles via the pre-S region of the large envelope protein, was purified and its cDNA cloned, revealing it is a novel member of the basic carboxypeptidase gene family with three carboxypeptidase-like domains.","method":"Protein purification from duck liver, cDNA cloning, DNA sequence analysis, virus-binding assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — direct biochemical purification and molecular cloning with functional binding characterization","pmids":["7797483"],"is_preprint":false},{"year":1997,"finding":"A membrane-bound carboxypeptidase in human skin fibroblasts and mouse macrophages was identified as the mammalian homolog of duck gp180/carboxypeptidase D, exhibiting low-pH (regulatory B-type metallocarboxypeptidase) enzymatic activity distinct from carboxypeptidases B, E, M, N, and U.","method":"Partial cDNA cloning, recombinant protein expression in bacteria, enzyme activity assays, immunoprecipitation, Western blot","journal":"Life sciences","confidence":"High","confidence_rationale":"Tier 1-2 — enzymatic characterization combined with immunoprecipitation and molecular cloning","pmids":["9064476"],"is_preprint":false},{"year":1997,"finding":"Intestinal epithelial cell gp180 (carboxypeptidase D) is a 180-kDa glycoprotein that exists in two forms — an apically sorted GPI-anchored form and a basolateral transmembrane form — is heavily N-glycosylated, binds to peripheral blood T cells, activates p56lck, and selectively activates p56lck through CD8α but not CD4, identifying it as a novel CD8 ligand.","method":"SDS-PAGE, Western blot, N-glycanase treatment, flow cytometry, T cell binding assay, p56lck phosphorylation assay using CD8/CD4 transfectants","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods establishing binding and functional kinase activation in a single study","pmids":["9139738"],"is_preprint":false},{"year":1998,"finding":"Soluble duck gp180 (a 170-kDa form lacking the C-terminal transmembrane domain) expressed in a baculovirus system displayed metallocarboxypeptidase D-like enzymatic activity, cleaving substrates with a pH optimum of 5.5–6.5 and Km values of 12–21 µM. Deletion analysis showed that the third carboxypeptidase domain is responsible for pre-S (DHBV envelope protein) binding, while carboxypeptidase activity resides in the first and second domains and is not required for viral pre-S binding.","method":"Baculovirus expression, enzyme kinetics with fluorescent substrates, inhibitor profiling, deletion mutagenesis, expression in 293T cells","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — in vitro enzymatic reconstitution with mutagenesis mapping of functional domains","pmids":["9525948"],"is_preprint":false},{"year":1998,"finding":"Carboxypeptidase D (gp180) was demonstrated to be the cellular receptor for avian hepatitis B viruses (DHBV): it was the only host protein binding with high affinity to the pre-S ectodomain of the large DHBV envelope protein; a pre-S subdomain matching the receptor-binding domain competed with viral infection; soluble gp180 inhibited DHBV infection; and expression of gp180 in a heterologous hepatoma cell line mediated viral particle attachment and internalization into vesicular structures. However, gp180 expression alone did not render cells permissive, indicating a species-specific coreceptor is required for full viral entry.","method":"Competitive infection inhibition assay, soluble receptor inhibition, fluorescent viral particle internalization imaging in transfected cells, co-isolation/binding assays","journal":"Journal of virology","confidence":"High","confidence_rationale":"Tier 2 — multiple complementary functional assays (competition, inhibition, transfection-based internalization) in a single study","pmids":["9733850"],"is_preprint":false},{"year":1998,"finding":"Human and mouse carboxypeptidase D (gp180 homologs) were cloned from HepG2 cells and mouse liver; they encode 1380 and 1377 amino acid proteins with three carboxypeptidase homologous domains (A, B, C). Domains A and B retain conserved catalytic residues; domain C does not. Expression of human CPD in 293T cells conferred carboxypeptidase activity. Fluorescence in situ hybridization mapped the human CPD gene to chromosome region 17q11.2.","method":"cDNA cloning, sequence analysis, heterologous expression in 293T cells, radiometric carboxypeptidase activity assay, FISH chromosomal mapping, Northern blot","journal":"Gene","confidence":"High","confidence_rationale":"Tier 1-2 — molecular cloning with direct enzymatic verification in mammalian expression system","pmids":["9714835"],"is_preprint":false},{"year":1998,"finding":"CPD (gp180) is concentrated in the trans-Golgi network (TGN) in AtT-20 mouse pituitary corticotroph cells, co-localizing with furin and wheat germ agglutinin markers, and cycles to the cell surface and back. CPD is present in the TGN even under brefeldin A treatment, and antibodies to the full-length protein (but not to the C-terminal cytosolic tail) are internalized within 15–30 minutes to a furin-positive compartment, distinguishable from transferrin recycling endosomes.","method":"Immunofluorescence microscopy, co-localization with TGN markers, antibody internalization assay, brefeldin A treatment, subcellular fractionation","journal":"Journal of cell science","confidence":"High","confidence_rationale":"Tier 2 — direct subcellular localization with functional internalization assay and pharmacological validation, replicated with multiple antibody epitopes","pmids":["9490632"],"is_preprint":false},{"year":1999,"finding":"Multiple sequence elements in the 58-residue cytoplasmic C-terminal tail of gp180/CPD mediate TGN localization and intracellular trafficking: truncation of the C-terminal 56 residues abolished TGN enrichment and surface retrieval; a FxxL-like motif (mutated to AxxL) slowed internalization from the cell surface; and casein kinase II phosphorylation sites within an acidic cluster regulated trafficking. Truncation of 12–43 residues reduced TGN retention and accelerated protein turnover, while deletion of C-terminal 45 residues caused cell surface accumulation.","method":"Expression of full-length and deletion/point mutants in AtT-20 cells, immunofluorescence microscopy, protein turnover assays, site-directed mutagenesis","journal":"Molecular biology of the cell","confidence":"High","confidence_rationale":"Tier 1-2 — systematic mutagenesis of functional elements with multiple cellular readouts in a single study","pmids":["9880325"],"is_preprint":false},{"year":1997,"finding":"Intestinal epithelial cell gp180 (CPD) expression is defective in inflammatory bowel disease (IBD): normal bowel shows bright gp180 staining on all intestinal epithelial cells, while both ulcerative colitis and Crohn's disease specimens show patchy or absent staining. Loss of gp180 expression correlates with altered CD8-associated p56lck activation — IBD intestinal epithelial cells activated CD4- as well as CD8-associated lck, unlike normal cells which selectively activate CD8-lck.","method":"Immunohistochemistry on frozen sections, flow cytometry, p56lck phosphorylation assay","journal":"The Journal of clinical investigation","confidence":"Medium","confidence_rationale":"Tier 2-3 — correlative but links CPD expression loss to a defined signaling functional defect","pmids":["9329971"],"is_preprint":false},{"year":1999,"finding":"On intestinal epithelial cells, gp180 (CPD) forms a complex with the non-classical MHC class I molecule CD1d on the cell surface, as demonstrated by co-immunoprecipitation and ELISA. Functional dissection showed that gp180/CD8 interaction activates p56lck, while CD1d/TCR interaction activates p59fyn, and the CD1d-gp180 complex coordinates both signals for full CD8+ T cell activation by intestinal epithelial cells.","method":"Co-immunoprecipitation, ELISA, blocking monoclonal antibodies (anti-gp180 mAb B9, anti-CD1d mAb D5), CD1d transfectant functional assays measuring p56lck and p59fyn phosphorylation","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — reciprocal co-immunoprecipitation combined with functional dissection using specific blocking antibodies and transfectants","pmids":["10473580"],"is_preprint":false},{"year":2002,"finding":"Soluble gp180 (CPD) binds directly to CD8-Fc fusion proteins and is absorbed by human CD8α (but not CD4) transfected murine T cells; this binding is dependent on carbohydrate moieties on gp180 and uses sites distinct from those used by the conventional CD8 ligand MHC class I, establishing CPD as a non-classical CD8 ligand on intestinal epithelial cells.","method":"Soluble gp180 binding to CD8-Fc fusion proteins, absorption assay with CD8α/CD4 transfected cells, carbohydrate-dependency testing, competitive binding with MHC class I","journal":"Clinical immunology (Orlando, Fla.)","confidence":"High","confidence_rationale":"Tier 2 — direct binding assay with multiple controls establishing specificity and molecular requirements","pmids":["11890713"],"is_preprint":false}],"current_model":"Carboxypeptidase D (CPD/gp180) is a type I transmembrane metallocarboxypeptidase concentrated in the trans-Golgi network that cycles to the cell surface; its cytoplasmic tail contains a FxxL motif and casein kinase II phosphorylation sites that mediate TGN retention and internalization; it harbors enzymatic activity in its first two carboxypeptidase domains (pH optimum 5.5–6.5) while the third domain mediates binding to the pre-S protein of avian hepatitis B virus, functioning as the primary receptor for duck HBV infection; on intestinal epithelial cells CPD acts as a non-classical, carbohydrate-dependent CD8 ligand that forms a complex with CD1d to coordinate CD8+ T cell activation via p56lck and p59fyn signaling, and its expression is defective in inflammatory bowel disease."},"narrative":{"teleology":[{"year":1995,"claim":"Identification of duck gp180 as a novel carboxypeptidase-family glycoprotein that binds DHBV particles resolved the molecular identity of the avian hepadnavirus receptor.","evidence":"Protein purification from duck liver, cDNA cloning, and virus-binding assay","pmids":["7797483"],"confidence":"High","gaps":["Whether gp180 alone is sufficient for viral entry was untested","Enzymatic activity of individual domains not yet dissected","Mammalian homolog not yet identified"]},{"year":1997,"claim":"Cloning of the mammalian CPD homolog and demonstration of its low-pH metallocarboxypeptidase activity established that the enzyme is conserved across vertebrates with a catalytic profile distinct from all known regulatory B-type carboxypeptidases.","evidence":"Partial cDNA cloning, recombinant expression in bacteria, enzyme activity assays, immunoprecipitation, and Western blot in human fibroblasts and mouse macrophages","pmids":["9064476"],"confidence":"High","gaps":["Full-length mammalian sequence and chromosomal location unknown","Physiological substrates not identified","Subcellular localization in mammalian cells not determined"]},{"year":1997,"claim":"Discovery that intestinal epithelial gp180/CPD is a CD8α ligand that selectively activates p56lck revealed an unexpected immunological function for a carboxypeptidase, linking it to mucosal T cell regulation.","evidence":"Flow cytometry, T cell binding assay, and p56lck phosphorylation assay using CD8/CD4 transfectants on intestinal epithelial cells","pmids":["9139738"],"confidence":"High","gaps":["Molecular basis of CD8 binding (carbohydrate vs. protein epitope) not resolved","Relationship to MHC class I–CD8 interaction unknown","Whether enzymatic and immune functions are independent not tested"]},{"year":1997,"claim":"Loss of CPD expression on intestinal epithelial cells in inflammatory bowel disease correlated with aberrant lck signaling, implicating CPD in mucosal immune homeostasis and IBD pathogenesis.","evidence":"Immunohistochemistry on frozen sections from ulcerative colitis and Crohn's disease patients, flow cytometry, and p56lck phosphorylation assay","pmids":["9329971"],"confidence":"Medium","gaps":["Correlative association — causal role of CPD loss in IBD not established","Sample size and patient heterogeneity not fully addressed","Mechanism of CPD downregulation in IBD unknown"]},{"year":1998,"claim":"Mapping enzymatic activity to domains A and B and pre-S binding to domain C demonstrated that CPD's carboxypeptidase and viral receptor functions are structurally and functionally separable.","evidence":"Baculovirus expression of soluble gp180, enzyme kinetics, inhibitor profiling, and deletion mutagenesis in 293T cells","pmids":["9525948"],"confidence":"High","gaps":["Structural basis for domain C–pre-S interaction not resolved","Whether domain C has any residual catalytic or regulatory function unknown","No crystal structure available"]},{"year":1998,"claim":"Functional receptor studies confirmed CPD/gp180 is the primary attachment and internalization receptor for DHBV but showed that a species-specific coreceptor is required for full viral entry, defining the limits of gp180-mediated infection.","evidence":"Competitive infection inhibition, soluble receptor blocking, and fluorescent viral particle internalization imaging in transfected heterologous hepatoma cells","pmids":["9733850"],"confidence":"High","gaps":["Identity of the species-specific coreceptor unknown","Post-internalization trafficking pathway for virus not mapped","Relevance to mammalian hepadnavirus entry not established"]},{"year":1998,"claim":"Localization of CPD to the TGN with cycling to the plasma surface established a trafficking itinerary analogous to furin, suggesting CPD processes substrates in the late secretory pathway.","evidence":"Immunofluorescence co-localization with TGN markers, antibody internalization assays, and brefeldin A treatment in AtT-20 pituitary cells","pmids":["9490632"],"confidence":"High","gaps":["Specific TGN sorting signals not yet mapped","Whether CPD processes neuropeptide precursors in this compartment not tested","Adaptor proteins mediating TGN retrieval not identified"]},{"year":1999,"claim":"Systematic mutagenesis of the CPD cytoplasmic tail identified a FxxL internalization motif and casein kinase II–phosphorylated acidic cluster as discrete elements governing TGN retention and surface retrieval, explaining the molecular basis of its intracellular cycling.","evidence":"Expression of truncation and point mutants in AtT-20 cells with immunofluorescence and protein turnover assays","pmids":["9880325"],"confidence":"High","gaps":["Direct demonstration of CKII phosphorylation in vivo not shown","Adaptor proteins (e.g., PACS-1) that recognize these signals not identified in this study","Whether tail signals regulate enzymatic access to substrates is unknown"]},{"year":1999,"claim":"Demonstration that CPD forms a surface complex with CD1d and that the complex coordinates dual kinase activation (p56lck via CPD–CD8 and p59fyn via CD1d–TCR) provided a mechanistic model for how intestinal epithelial cells activate CD8+ T cells.","evidence":"Co-immunoprecipitation, ELISA, blocking monoclonal antibodies, and CD1d transfectant functional assays measuring p56lck and p59fyn phosphorylation","pmids":["10473580"],"confidence":"High","gaps":["Structural basis of CPD–CD1d interaction not resolved","Whether the complex presents lipid antigens via CD1d concurrently with CPD–CD8 engagement is untested","In vivo relevance of CPD–CD1d complex for mucosal immunity not demonstrated"]},{"year":2002,"claim":"Establishing that CPD–CD8α binding is carbohydrate-dependent and uses sites distinct from classical MHC class I defined CPD as a structurally and mechanistically non-classical CD8 ligand.","evidence":"Soluble gp180 binding to CD8-Fc fusion proteins, absorption with CD8α/CD4 transfected cells, carbohydrate-dependency testing, and competitive binding with MHC class I","pmids":["11890713"],"confidence":"High","gaps":["Specific glycan structures required for CD8 binding not identified","Whether CPD enzymatic activity influences its CD8 ligand function not tested","No in vivo genetic model confirming the immune function of CPD"]},{"year":null,"claim":"The physiological substrates of CPD's carboxypeptidase activity, the structural basis of its multi-domain architecture, and whether its enzymatic and immune functions intersect in vivo remain unresolved.","evidence":"","pmids":[],"confidence":"Low","gaps":["No endogenous substrates definitively identified","No crystal or cryo-EM structure of any CPD domain","No genetic knockout or knockdown phenotype reported in mammals","Functional link between enzymatic activity and CD8 ligand or viral receptor roles untested"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[1,3,5]},{"term_id":"GO:0016787","term_label":"hydrolase activity","supporting_discovery_ids":[1,3,5]},{"term_id":"GO:0001618","term_label":"virus receptor activity","supporting_discovery_ids":[0,4]}],"localization":[{"term_id":"GO:0005794","term_label":"Golgi apparatus","supporting_discovery_ids":[6,7]},{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[2,6,7,9,10]}],"pathway":[{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[2,8,9,10]},{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[1,3,5]}],"complexes":["CPD–CD1d surface complex"],"partners":["CD1D","CD8A"],"other_free_text":[]},"mechanistic_narrative":"Carboxypeptidase D (CPD/gp180) is a type I transmembrane metallocarboxypeptidase with three carboxypeptidase-like domains, of which the first two harbor enzymatic activity (pH optimum 5.5–6.5) while the third mediates binding to the pre-S protein of duck hepatitis B virus, functioning as the primary receptor for avian hepadnavirus attachment and internalization [PMID:9525948, PMID:9733850]. CPD is concentrated in the trans-Golgi network where it cycles to the cell surface via trafficking signals in its cytoplasmic tail, including a FxxL internalization motif and casein kinase II phosphorylation sites within an acidic cluster [PMID:9490632, PMID:9880325]. On intestinal epithelial cells, CPD functions as a non-classical, carbohydrate-dependent CD8α ligand that forms a surface complex with CD1d, coordinating p56lck and p59fyn signaling for CD8+ T cell activation; its expression is defective in inflammatory bowel disease [PMID:9139738, PMID:10473580, PMID:9329971]."},"prefetch_data":{"uniprot":{"accession":"O75976","full_name":"Carboxypeptidase D","aliases":["Metallocarboxypeptidase D","gp180"],"length_aa":1380,"mass_kda":152.9,"function":"Functions in the processing of proteins and peptides in the secretory pathway. Mechanistically, cleaves exclusively C-terminal basic residues. By removing terminal residues, can modulate the activity, stability, and receptor-binding properties of bioactive peptides (Microbial infection) Acts as an alternative entry receptor for adeno-associated virus (AAV)","subcellular_location":"Cell membrane","url":"https://www.uniprot.org/uniprotkb/O75976/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/CPD","classification":"Not Classified","n_dependent_lines":116,"n_total_lines":1208,"dependency_fraction":0.09602649006622517},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[{"gene":"CANX","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/search/CPD","total_profiled":1310},"omim":[{"mim_id":"613675","title":"CHROMOSOME 17q11.2 DELETION SYNDROME, 1.4-MB","url":"https://www.omim.org/entry/613675"},{"mim_id":"613208","title":"XPC COMPLEX SUBUNIT, DNA DAMAGE RECOGNITION AND REPAIR FACTOR; XPC","url":"https://www.omim.org/entry/613208"},{"mim_id":"612052","title":"SMOKING AS A QUANTITATIVE TRAIT LOCUS 3; SQTL3","url":"https://www.omim.org/entry/612052"},{"mim_id":"607999","title":"ASH1-LIKE HISTONE LYSINE METHYLTRANSFERASE; ASH1L","url":"https://www.omim.org/entry/607999"},{"mim_id":"603103","title":"CARBOXYPEPTIDASE N, POLYPEPTIDE 1, 50-KD; CPN1","url":"https://www.omim.org/entry/603103"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Cytosol","reliability":"Approved"},{"location":"Nucleoli","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/CPD"},"hgnc":{"alias_symbol":["GP180"],"prev_symbol":[]},"alphafold":{"accession":"O75976","domains":[{"cath_id":"3.40.630.10","chopping":"80-105_126-198_207-382","consensus_level":"high","plddt":92.1095,"start":80,"end":382},{"cath_id":"2.60.40.1120","chopping":"385-460","consensus_level":"high","plddt":91.8513,"start":385,"end":460},{"cath_id":"3.40.630.10","chopping":"507-794","consensus_level":"high","plddt":95.7874,"start":507,"end":794},{"cath_id":"2.60.40.1120","chopping":"799-871","consensus_level":"high","plddt":92.74,"start":799,"end":871},{"cath_id":"3.40.630.10","chopping":"901-1211","consensus_level":"high","plddt":87.5753,"start":901,"end":1211},{"cath_id":"2.60.40.1120","chopping":"1218-1294","consensus_level":"high","plddt":81.1253,"start":1218,"end":1294}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/O75976","model_url":"https://alphafold.ebi.ac.uk/files/AF-O75976-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-O75976-F1-predicted_aligned_error_v6.png","plddt_mean":82.5},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=CPD","jax_strain_url":"https://www.jax.org/strain/search?query=CPD"},"sequence":{"accession":"O75976","fasta_url":"https://rest.uniprot.org/uniprotkb/O75976.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/O75976/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/O75976"}},"corpus_meta":[{"pmid":"15576622","id":"PMC_15576622","title":"Crystal structure of a photolyase bound to a CPD-like DNA lesion after in situ repair.","date":"2004","source":"Science (New York, N.Y.)","url":"https://pubmed.ncbi.nlm.nih.gov/15576622","citation_count":297,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"17290000","id":"PMC_17290000","title":"CPD damage recognition by transcribing RNA polymerase II.","date":"2007","source":"Science (New York, N.Y.)","url":"https://pubmed.ncbi.nlm.nih.gov/17290000","citation_count":201,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"15668165","id":"PMC_15668165","title":"Powerful skin cancer protection by a CPD-photolyase transgene.","date":"2005","source":"Current biology : CB","url":"https://pubmed.ncbi.nlm.nih.gov/15668165","citation_count":170,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"9675902","id":"PMC_9675902","title":"Transcription of the Arabidopsis CPD gene, encoding a steroidogenic cytochrome P450, is negatively controlled by brassinosteroids.","date":"1998","source":"The Plant journal : for cell and molecular biology","url":"https://pubmed.ncbi.nlm.nih.gov/9675902","citation_count":157,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"3874872","id":"PMC_3874872","title":"A T-lymphoma transmembrane glycoprotein (gp180) is linked to the cytoskeletal protein, fodrin.","date":"1985","source":"The Journal of cell biology","url":"https://pubmed.ncbi.nlm.nih.gov/3874872","citation_count":141,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"22465715","id":"PMC_22465715","title":"Alterations of red blood cell metabolome during cold liquid storage of erythrocyte concentrates in CPD-SAGM.","date":"2012","source":"Journal of proteomics","url":"https://pubmed.ncbi.nlm.nih.gov/22465715","citation_count":127,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"22822057","id":"PMC_22822057","title":"CYP90A1/CPD, a brassinosteroid biosynthetic cytochrome P450 of Arabidopsis, catalyzes C-3 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carboxypeptidase-like domains; the third domain mediates binding to duck hepatitis B virus pre-S envelope protein, while the first and second domains carry enzymatic carboxypeptidase activity.\",\n      \"method\": \"cDNA cloning, DNA sequence analysis, purification from duck liver\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — original cloning and biochemical characterization with domain mapping\",\n      \"pmids\": [\"7797483\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"Human CPD/gp180 on intestinal epithelial cells exists in two forms (an apically sorted GPI-anchored form and a basolateral transmembrane form), is heavily N-glycosylated, binds to peripheral blood T cells via CD8, and activates the CD8-associated kinase p56lck.\",\n      \"method\": \"Biochemical characterization, N-glycanase treatment, flow cytometry, functional kinase activation assays with CD8 transfectants\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods including CD8 transfectant specificity and kinase activation\",\n      \"pmids\": [\"9139738\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"Defective gp180/CPD expression on intestinal epithelial cells in inflammatory bowel disease correlates with impaired CD8+ T cell activation and altered p56lck phosphorylation patterns, establishing a functional link between CPD expression and mucosal immune regulation.\",\n      \"method\": \"Immunohistochemistry, flow cytometry, functional CD8-associated p56lck phosphorylation assays\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple methods but correlative rather than direct mechanistic intervention\",\n      \"pmids\": [\"9329971\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"CPD/gp180 functions as the cellular receptor for avian hepatitis B viruses (DHBV); soluble gp180 lacking the membrane anchor inhibits DHBV infection, and expression of gp180 in heterologous hepatoma cells mediates viral particle attachment and internalization into vesicular structures, though a species-specific co-receptor is required for productive infection.\",\n      \"method\": \"Infection competition assay, soluble receptor inhibition, heterologous cell transfection with fluorescently labeled viral particle tracking\",\n      \"journal\": \"Journal of virology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal approaches establishing receptor function; replicated across two companion papers\",\n      \"pmids\": [\"9733850\", \"9733849\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"Duck gp180/CPD exhibits metallocarboxypeptidase D-like enzymatic activity, cleaving substrates with a pH optimum of 5.5–6.5; the first and second carboxypeptidase domains are enzymatically active while the third domain mediates pre-S binding independently of carboxypeptidase activity.\",\n      \"method\": \"Baculovirus expression of soluble gp170 protein, fluorometric enzyme assay, deletion mutagenesis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro enzymatic assay with domain-specific mutagenesis and kinetic characterization\",\n      \"pmids\": [\"9525948\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"Human and mouse CPD (gp180 homolog) cDNAs encode proteins with three carboxypeptidase homologous domains; domains A and B retain conserved catalytic residues while domain C does not; human gp180 expressed in 293T cells exhibits carboxypeptidase activity; the gene maps to chromosomal region 17q11.2.\",\n      \"method\": \"cDNA cloning, Northern blotting, heterologous expression with radiometric carboxypeptidase assay, FISH\",\n      \"journal\": \"Gene\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — functional expression confirming enzymatic activity with chromosomal localization\",\n      \"pmids\": [\"9714835\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"The mammalian homolog of duck gp180 is carboxypeptidase D (CPD); antibodies to the recombinant human protein immunoprecipitate low-pH carboxypeptidase activity from human skin fibroblasts and macrophage cell lines, confirming CPD is a membrane-bound metallocarboxypeptidase distributed in multiple cell types.\",\n      \"method\": \"Partial cDNA cloning, recombinant protein expression, antibody immunoprecipitation of enzymatic activity, Western blotting\",\n      \"journal\": \"Life sciences\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — activity immunoprecipitation directly links the protein to enzymatic function\",\n      \"pmids\": [\"9064476\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"Sequences within the cytoplasmic C-terminal tail of CPD/gp180 mediate trans-Golgi network (TGN) localization; truncation of 12–43 residues reduces TGN retention and accelerates protein turnover; an FxxL motif near the tail's end is required for retrieval from the cell surface; casein kinase II phosphorylation sites in an acidic cluster also regulate intracellular trafficking.\",\n      \"method\": \"Deletion and point mutagenesis expressed in AtT-20 cells, immunofluorescence localization, turnover assays\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — systematic mutagenesis with direct subcellular localization readout identifying multiple trafficking determinants\",\n      \"pmids\": [\"9880325\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"On intestinal epithelial cells, gp180/CPD associates with the non-classical class I molecule CD1d; gp180 activates CD8-associated p56lck while CD1d activates TCR-associated p59fyn; the gp180–CD1d complex on the IEC surface is recognized by the TCR–CD8 co-receptor to activate CD8+ T cells.\",\n      \"method\": \"Co-immunoprecipitation, ELISA binding assays, antibody blocking experiments, CD1d transfectant functional assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal co-IP and transfectant experiments with defined kinase activation readouts\",\n      \"pmids\": [\"10473580\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"gp180/CPD binds to CD8α but not CD4, and this interaction depends on carbohydrate on the gp180 molecule; the gp180 binding sites on CD8 are distinct from those used by classical class I MHC, establishing CPD as a novel non-MHC CD8 ligand.\",\n      \"method\": \"Soluble gp180 binding to CD8-Fc fusion proteins, absorption by human CD8α or CD4 transfected murine T cells, carbohydrate-dependence assay\",\n      \"journal\": \"Clinical immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple binding assays with site discrimination from classical MHC ligand\",\n      \"pmids\": [\"11890713\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1985,\n      \"finding\": \"gp180/CPD (postsynaptic density glycoprotein) is a substrate for endogenous tyrosine kinase activity within the postsynaptic density, as demonstrated by incorporation of [32P]phosphotyrosine resistant to hot KOH digestion.\",\n      \"method\": \"In vitro phosphorylation of PSD with [γ-32P]ATP, isolation of gp180, phosphoamino acid analysis\",\n      \"journal\": \"Brain research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct biochemical demonstration of tyrosine phosphorylation on the isolated glycoprotein\",\n      \"pmids\": [\"4039619\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1985,\n      \"finding\": \"gp180/CPD (postsynaptic density glycoprotein) is phosphorylated by Ca2+/calmodulin-dependent protein kinase at three sites (phosphoserine and phosphothreonine predominantly, with some phosphotyrosine), and calmodulin increases phosphorylation 7.5-fold.\",\n      \"method\": \"In vitro phosphorylation of PSDs in presence of Ca2+/calmodulin, glycoprotein isolation, peptide mapping, phosphoamino acid analysis\",\n      \"journal\": \"Journal of neurochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 — in vitro kinase assay with peptide mapping; single lab\",\n      \"pmids\": [\"2993521\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1985,\n      \"finding\": \"gp180/CPD is linked to the cytoskeletal protein fodrin in T-lymphoma cells; ligand-induced receptor patching/capping increases gp180 and fodrin in the membrane-associated cytoskeleton fraction with a 1:1 molar ratio complex (sedimentation coefficient ~20S, pI 5.2–5.7); this complex is proposed to link surface receptors to the intracellular microfilament network.\",\n      \"method\": \"125I/[3H]glucosamine labeling, immunobinding co-isolation, Triton X-114 extraction, sucrose gradient centrifugation, isoelectric focusing, double-label immunofluorescence\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — biochemical co-isolation and stoichiometric characterization with functional context\",\n      \"pmids\": [\"3874872\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1992,\n      \"finding\": \"CD45/gp180 (a CD45 isoform in T-lymphoma cells) binds directly and specifically to fodrin (Kd ~1.1 nM) and spectrin (Kd ~3.2 nM) via a 48-kDa phosphopeptide domain; this binding significantly stimulates CD45 protein tyrosine phosphatase (PTPase) activity by increasing Vmax 7.5-fold (fodrin) and 3.2-fold (spectrin) without changing Km.\",\n      \"method\": \"In vitro binding assays, high-salt dissociation, enzyme kinetic analysis of PTPase activity with and without cytoskeletal proteins\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution of binding and enzymatic stimulation with kinetic parameters; note this may refer to CD45 isoform, not CPD\",\n      \"pmids\": [\"1400466\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"Tyrosine phosphorylation of the postsynaptic density glycoprotein PSD-GP180/CPD undergoes a sustained, Ca2+-dependent decrease after brief ischemia in hippocampal slices, distinguishing it from rapidly reversible tyrosine phosphorylation of other PSD proteins.\",\n      \"method\": \"Hippocampal slice ischemia model, isolation of glycoproteins, phosphotyrosine immunodetection, Ca2+ chelation experiments\",\n      \"journal\": \"Journal of neurochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — defined cellular model with Ca2+-dependence mechanistic follow-up; single lab\",\n      \"pmids\": [\"7561882\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"Carboxypeptidase D (CPD/gp180) is a type I transmembrane metallocarboxypeptidase concentrated in the trans-Golgi network that cycles to the plasma membrane via cytoplasmic tail signals (FxxL motif and casein kinase II sites); it possesses carboxypeptidase activity in its first two of three domains, binds the pre-S envelope protein of avian hepatitis B viruses through its third domain to mediate viral attachment and entry, acts as a novel CD8 ligand on intestinal epithelial cells where it associates with CD1d and activates CD8-associated p56lck to regulate mucosal immune responses, and in the postsynaptic density is phosphorylated on tyrosine and serine/threonine residues by endogenous kinases in a developmentally and Ca2+-regulated manner.\"\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\nI'll systematically classify each paper and extract discoveries only from KEEP papers.\n\n**Classification:**\n\nThe target gene is CPD = Carboxypeptidase D (human gene, HGNC). Key papers about this gene: gp180/carboxypeptidase D, a Golgi-resident metallocarboxypeptidase.\n\nPapers about CPD photolyase (DNA repair enzyme) → EXCLUDE (symbol collision - different gene)\nPapers about CPD = citrate-phosphate-dextrose (blood storage) → EXCLUDE (abbreviation collision)\nPapers about CPD = cyclobutane pyrimidine dimer (DNA lesion) → EXCLUDE (abbreviation collision)\nPapers about CPD = continuing professional development → EXCLUDE\nPapers about CPD gene in Arabidopsis (CYP90A1 brassinosteroid biosynthesis) → EXCLUDE (plant gene, symbol collision)\nPapers about Cpd-71, Cpd-42, Cpd-6c, Cpd-A1 (compound names) → EXCLUDE\nPapers about cell population data (CPD) → EXCLUDE\nPapers about codon pair deoptimization (CPD) → EXCLUDE\n\n**KEEP papers** (about carboxypeptidase D / gp180):\n- PMID:7797483 - gp180 encoded by carboxypeptidase gene family\n- PMID:9733850 - CPD (gp180) as receptor for avian HBV\n- PMID:9525948 - gp180 has metallocarboxypeptidase D-like enzymatic activity\n- PMID:9880325 - Sequences in cytoplasmic domain mediate TGN localization\n- PMID:9490632 - Intracellular trafficking of CPD in AtT-20 cells\n- PMID:9714835 - Cloning of human/mouse gp180-CPD\n- PMID:9064476 - Mammalian homolog of duck gp180 identified as CPD\n- PMID:9329971 - gp180 defective in IBD\n- PMID:9139738 - Characterization of gp180 on intestinal epithelial cells\n- PMID:10473580 - CD1d associates with gp180\n- PMID:11890713 - gp180 binds CD8\n- PMID:1400466 - CD45/GP180 PTPase regulated by cytoskeleton (NOTE: this is CD45/GP180 in T-lymphoma = CD45 isoform, NOT CPD)\n- PMID:3874872 - gp180 linked to fodrin (T-lymphoma) - this is CD45/T200, NOT CPD\n- PMID:4039619 - PSD gp180 phosphorylated by tyrosine kinase (postsynaptic density protein, NOT CPD)\n- PMID:2993521 - PSD gp180 phosphorylated by Ca2+/calmodulin kinase (postsynaptic density, NOT CPD)\n- PMID:2325159 - PSD-GP180 tyrosine phosphorylation (postsynaptic density, NOT CPD)\n- PMID:7561882 - PSD-GP180 in ischemia (postsynaptic density, NOT CPD)\n\nThe T-lymphoma gp180/CD45 papers (PMIDs 3874872, 1400466) and the postsynaptic density gp180 papers (PMIDs 4039619, 2993521, 2325159, 7561882) describe different proteins that happen to have the same gp180 designation — these are NOT carboxypeptidase D. EXCLUDE.\n\n- PMID:9733849 - avian HBV infection via pre-S/gp180 interaction → this is about the viral ligand, discusses gp180 as receptor → borderline KEEP (describes receptor binding domain)\n\nFrom additional curated papers:\n- PMID:9490632 - KEEP (TGN trafficking of CPD)\n- PMID:26186194, 28514442, 33961781 - BioPlex interactome screens - CPD appears as a hit but no specific mechanistic finding about CPD\n- Other curated papers don't specifically characterize CPD mechanistically\n\n```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1995,\n      \"finding\": \"Duck gp180, a host cell glycoprotein that binds duck hepatitis B virus (DHBV) particles via the pre-S region of the large envelope protein, was purified and its cDNA cloned, revealing it is a novel member of the basic carboxypeptidase gene family with three carboxypeptidase-like domains.\",\n      \"method\": \"Protein purification from duck liver, cDNA cloning, DNA sequence analysis, virus-binding assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct biochemical purification and molecular cloning with functional binding characterization\",\n      \"pmids\": [\"7797483\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"A membrane-bound carboxypeptidase in human skin fibroblasts and mouse macrophages was identified as the mammalian homolog of duck gp180/carboxypeptidase D, exhibiting low-pH (regulatory B-type metallocarboxypeptidase) enzymatic activity distinct from carboxypeptidases B, E, M, N, and U.\",\n      \"method\": \"Partial cDNA cloning, recombinant protein expression in bacteria, enzyme activity assays, immunoprecipitation, Western blot\",\n      \"journal\": \"Life sciences\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — enzymatic characterization combined with immunoprecipitation and molecular cloning\",\n      \"pmids\": [\"9064476\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"Intestinal epithelial cell gp180 (carboxypeptidase D) is a 180-kDa glycoprotein that exists in two forms — an apically sorted GPI-anchored form and a basolateral transmembrane form — is heavily N-glycosylated, binds to peripheral blood T cells, activates p56lck, and selectively activates p56lck through CD8α but not CD4, identifying it as a novel CD8 ligand.\",\n      \"method\": \"SDS-PAGE, Western blot, N-glycanase treatment, flow cytometry, T cell binding assay, p56lck phosphorylation assay using CD8/CD4 transfectants\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods establishing binding and functional kinase activation in a single study\",\n      \"pmids\": [\"9139738\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"Soluble duck gp180 (a 170-kDa form lacking the C-terminal transmembrane domain) expressed in a baculovirus system displayed metallocarboxypeptidase D-like enzymatic activity, cleaving substrates with a pH optimum of 5.5–6.5 and Km values of 12–21 µM. Deletion analysis showed that the third carboxypeptidase domain is responsible for pre-S (DHBV envelope protein) binding, while carboxypeptidase activity resides in the first and second domains and is not required for viral pre-S binding.\",\n      \"method\": \"Baculovirus expression, enzyme kinetics with fluorescent substrates, inhibitor profiling, deletion mutagenesis, expression in 293T cells\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro enzymatic reconstitution with mutagenesis mapping of functional domains\",\n      \"pmids\": [\"9525948\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"Carboxypeptidase D (gp180) was demonstrated to be the cellular receptor for avian hepatitis B viruses (DHBV): it was the only host protein binding with high affinity to the pre-S ectodomain of the large DHBV envelope protein; a pre-S subdomain matching the receptor-binding domain competed with viral infection; soluble gp180 inhibited DHBV infection; and expression of gp180 in a heterologous hepatoma cell line mediated viral particle attachment and internalization into vesicular structures. However, gp180 expression alone did not render cells permissive, indicating a species-specific coreceptor is required for full viral entry.\",\n      \"method\": \"Competitive infection inhibition assay, soluble receptor inhibition, fluorescent viral particle internalization imaging in transfected cells, co-isolation/binding assays\",\n      \"journal\": \"Journal of virology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple complementary functional assays (competition, inhibition, transfection-based internalization) in a single study\",\n      \"pmids\": [\"9733850\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"Human and mouse carboxypeptidase D (gp180 homologs) were cloned from HepG2 cells and mouse liver; they encode 1380 and 1377 amino acid proteins with three carboxypeptidase homologous domains (A, B, C). Domains A and B retain conserved catalytic residues; domain C does not. Expression of human CPD in 293T cells conferred carboxypeptidase activity. Fluorescence in situ hybridization mapped the human CPD gene to chromosome region 17q11.2.\",\n      \"method\": \"cDNA cloning, sequence analysis, heterologous expression in 293T cells, radiometric carboxypeptidase activity assay, FISH chromosomal mapping, Northern blot\",\n      \"journal\": \"Gene\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — molecular cloning with direct enzymatic verification in mammalian expression system\",\n      \"pmids\": [\"9714835\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"CPD (gp180) is concentrated in the trans-Golgi network (TGN) in AtT-20 mouse pituitary corticotroph cells, co-localizing with furin and wheat germ agglutinin markers, and cycles to the cell surface and back. CPD is present in the TGN even under brefeldin A treatment, and antibodies to the full-length protein (but not to the C-terminal cytosolic tail) are internalized within 15–30 minutes to a furin-positive compartment, distinguishable from transferrin recycling endosomes.\",\n      \"method\": \"Immunofluorescence microscopy, co-localization with TGN markers, antibody internalization assay, brefeldin A treatment, subcellular fractionation\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — direct subcellular localization with functional internalization assay and pharmacological validation, replicated with multiple antibody epitopes\",\n      \"pmids\": [\"9490632\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"Multiple sequence elements in the 58-residue cytoplasmic C-terminal tail of gp180/CPD mediate TGN localization and intracellular trafficking: truncation of the C-terminal 56 residues abolished TGN enrichment and surface retrieval; a FxxL-like motif (mutated to AxxL) slowed internalization from the cell surface; and casein kinase II phosphorylation sites within an acidic cluster regulated trafficking. Truncation of 12–43 residues reduced TGN retention and accelerated protein turnover, while deletion of C-terminal 45 residues caused cell surface accumulation.\",\n      \"method\": \"Expression of full-length and deletion/point mutants in AtT-20 cells, immunofluorescence microscopy, protein turnover assays, site-directed mutagenesis\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — systematic mutagenesis of functional elements with multiple cellular readouts in a single study\",\n      \"pmids\": [\"9880325\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"Intestinal epithelial cell gp180 (CPD) expression is defective in inflammatory bowel disease (IBD): normal bowel shows bright gp180 staining on all intestinal epithelial cells, while both ulcerative colitis and Crohn's disease specimens show patchy or absent staining. Loss of gp180 expression correlates with altered CD8-associated p56lck activation — IBD intestinal epithelial cells activated CD4- as well as CD8-associated lck, unlike normal cells which selectively activate CD8-lck.\",\n      \"method\": \"Immunohistochemistry on frozen sections, flow cytometry, p56lck phosphorylation assay\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — correlative but links CPD expression loss to a defined signaling functional defect\",\n      \"pmids\": [\"9329971\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"On intestinal epithelial cells, gp180 (CPD) forms a complex with the non-classical MHC class I molecule CD1d on the cell surface, as demonstrated by co-immunoprecipitation and ELISA. Functional dissection showed that gp180/CD8 interaction activates p56lck, while CD1d/TCR interaction activates p59fyn, and the CD1d-gp180 complex coordinates both signals for full CD8+ T cell activation by intestinal epithelial cells.\",\n      \"method\": \"Co-immunoprecipitation, ELISA, blocking monoclonal antibodies (anti-gp180 mAb B9, anti-CD1d mAb D5), CD1d transfectant functional assays measuring p56lck and p59fyn phosphorylation\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal co-immunoprecipitation combined with functional dissection using specific blocking antibodies and transfectants\",\n      \"pmids\": [\"10473580\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Soluble gp180 (CPD) binds directly to CD8-Fc fusion proteins and is absorbed by human CD8α (but not CD4) transfected murine T cells; this binding is dependent on carbohydrate moieties on gp180 and uses sites distinct from those used by the conventional CD8 ligand MHC class I, establishing CPD as a non-classical CD8 ligand on intestinal epithelial cells.\",\n      \"method\": \"Soluble gp180 binding to CD8-Fc fusion proteins, absorption assay with CD8α/CD4 transfected cells, carbohydrate-dependency testing, competitive binding with MHC class I\",\n      \"journal\": \"Clinical immunology (Orlando, Fla.)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — direct binding assay with multiple controls establishing specificity and molecular requirements\",\n      \"pmids\": [\"11890713\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"Carboxypeptidase D (CPD/gp180) is a type I transmembrane metallocarboxypeptidase concentrated in the trans-Golgi network that cycles to the cell surface; its cytoplasmic tail contains a FxxL motif and casein kinase II phosphorylation sites that mediate TGN retention and internalization; it harbors enzymatic activity in its first two carboxypeptidase domains (pH optimum 5.5–6.5) while the third domain mediates binding to the pre-S protein of avian hepatitis B virus, functioning as the primary receptor for duck HBV infection; on intestinal epithelial cells CPD acts as a non-classical, carbohydrate-dependent CD8 ligand that forms a complex with CD1d to coordinate CD8+ T cell activation via p56lck and p59fyn signaling, and its expression is defective in inflammatory bowel disease.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"Carboxypeptidase D (CPD/gp180) is a type I transmembrane metallocarboxypeptidase that processes substrates at acidic pH via its first two of three carboxypeptidase-homologous domains, while its enzymatically inactive third domain serves as the attachment receptor for avian hepatitis B viruses [PMID:7797483, PMID:9525948, PMID:9733850]. CPD is concentrated in the trans-Golgi network through cytoplasmic tail signals including an FxxL retrieval motif and casein kinase II phosphorylation sites in an acidic cluster, and it cycles to the plasma membrane where it can be expressed as both a transmembrane and a GPI-anchored form on intestinal epithelial cells [PMID:9880325, PMID:9139738]. On intestinal epithelial cells, CPD functions as a novel non-MHC CD8α ligand in a carbohydrate-dependent manner, associates with CD1d, and activates CD8-associated p56lck to regulate mucosal CD8+ T cell responses; defective CPD expression correlates with impaired mucosal immunity in inflammatory bowel disease [PMID:9139738, PMID:10473580, PMID:11890713, PMID:9329971]. In the postsynaptic density, CPD is phosphorylated on tyrosine and serine/threonine residues by endogenous and Ca²⁺/calmodulin-dependent kinases, and this phosphorylation is developmentally regulated and sensitive to ischemic insult [PMID:4039619, PMID:2993521, PMID:7561882].\",\n  \"teleology\": [\n    {\n      \"year\": 1985,\n      \"claim\": \"Before CPD was molecularly identified, biochemical studies established that the ~180 kDa postsynaptic density glycoprotein is a substrate for endogenous tyrosine kinases and Ca²⁺/calmodulin-dependent kinases, revealing it as a signaling target at the synapse.\",\n      \"evidence\": \"In vitro phosphorylation of isolated PSDs with [γ-32P]ATP, phosphoamino acid analysis, and calmodulin stimulation assays in rat brain preparations\",\n      \"pmids\": [\"4039619\", \"2993521\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Identity of gp180 as CPD was not yet established at this point\", \"Functional consequence of PSD phosphorylation on CPD unknown\", \"In vivo relevance of calmodulin-dependent phosphorylation not demonstrated\"]\n    },\n    {\n      \"year\": 1985,\n      \"claim\": \"Parallel work in T-lymphoma cells showed gp180 forms a stoichiometric complex with the cytoskeletal protein fodrin, linking CPD to the cortical cytoskeleton upon receptor activation.\",\n      \"evidence\": \"Co-isolation by Triton X-114 extraction, sucrose gradient sedimentation, and double-label immunofluorescence in BW5147 T-lymphoma cells\",\n      \"pmids\": [\"3874872\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether this gp180 is definitively the same gene product as CPD or a CD45 isoform is ambiguous in the literature\", \"Direct binding domain on CPD not mapped\", \"Functional significance of fodrin interaction for CPD trafficking or signaling unresolved\"]\n    },\n    {\n      \"year\": 1995,\n      \"claim\": \"Molecular cloning of duck CPD/gp180 revealed a novel three-domain metallocarboxypeptidase architecture, resolving the identity of the DHBV-binding protein and showing that enzymatic activity resides in domains A and B while domain C mediates pre-S viral envelope binding.\",\n      \"evidence\": \"cDNA cloning, sequence analysis, and purification from duck liver with domain-specific functional mapping\",\n      \"pmids\": [\"7797483\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Human CPD enzymatic characterization not yet performed\", \"Whether domain C has any catalytic or additional non-viral function unknown\"]\n    },\n    {\n      \"year\": 1995,\n      \"claim\": \"An ischemia model showed that tyrosine phosphorylation of PSD-GP180/CPD undergoes sustained Ca²⁺-dependent dephosphorylation after brief ischemia, distinguishing CPD from other PSD phosphoproteins and implicating it in ischemic signaling.\",\n      \"evidence\": \"Hippocampal slice oxygen-glucose deprivation, glycoprotein isolation, anti-phosphotyrosine immunoblotting with Ca²⁺ chelation\",\n      \"pmids\": [\"7561882\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab observation not independently replicated\", \"Identity of the phosphatase responsible not determined\", \"Downstream functional consequence of CPD dephosphorylation in ischemia unknown\"]\n    },\n    {\n      \"year\": 1997,\n      \"claim\": \"Identification of the mammalian CPD homolog and demonstration that it is a widely expressed membrane-bound metallocarboxypeptidase confirmed the conservation of enzymatic function from avian to human.\",\n      \"evidence\": \"Partial cDNA cloning, recombinant protein expression, immunoprecipitation of low-pH carboxypeptidase activity from human fibroblasts and macrophages\",\n      \"pmids\": [\"9064476\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Endogenous substrates of mammalian CPD not identified\", \"Relative contribution of domain A versus B to cellular peptide processing unknown\"]\n    },\n    {\n      \"year\": 1997,\n      \"claim\": \"Discovery that CPD/gp180 on intestinal epithelial cells binds CD8 and activates p56lck established an unexpected immune-regulatory function for this carboxypeptidase, independent of its enzymatic activity.\",\n      \"evidence\": \"Flow cytometry, N-glycanase treatment, CD8 transfectant binding specificity, and p56lck kinase activation assays on human IEC lines\",\n      \"pmids\": [\"9139738\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether carboxypeptidase activity contributes to or modifies the immune function not tested\", \"Structural basis for CD8 binding not resolved\"]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Functional receptor studies demonstrated that CPD/gp180 mediates DHBV attachment and internalization into vesicular structures, though productive infection requires an additional species-specific co-receptor.\",\n      \"evidence\": \"Soluble gp180 infection inhibition, heterologous transfection in non-duck hepatoma cells with fluorescent viral particle tracking\",\n      \"pmids\": [\"9733850\", \"9733849\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of the species-specific co-receptor unknown\", \"Whether CPD-mediated endocytic pathway is clathrin- or caveolae-dependent not determined\"]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Enzymatic characterization of recombinant duck CPD confirmed metallocarboxypeptidase activity with an acidic pH optimum (5.5–6.5), consistent with a function in post-Golgi compartments, and definitively separated enzymatic domains from the viral-binding domain.\",\n      \"evidence\": \"Baculovirus-expressed soluble gp170, fluorometric enzyme assays, deletion mutagenesis\",\n      \"pmids\": [\"9525948\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Endogenous peptide substrates in the TGN lumen not identified\", \"Whether both active domains process identical or distinct substrate preferences not fully resolved\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Mutagenesis of the cytoplasmic tail identified the trafficking code that concentrates CPD in the TGN: an FxxL motif for cell-surface retrieval and casein kinase II phosphorylation sites for steady-state localization, explaining how CPD cycles between the TGN and plasma membrane.\",\n      \"evidence\": \"Systematic deletion/point mutagenesis in AtT-20 neuroendocrine cells with immunofluorescence and turnover assays\",\n      \"pmids\": [\"9880325\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Adaptor proteins recognizing the FxxL motif not identified\", \"Whether phosphorylation-dependent trafficking is regulated by extracellular signals unknown\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"CPD/gp180 was shown to associate with CD1d on intestinal epithelial cells, forming a composite ligand where gp180 activates CD8/p56lck and CD1d activates TCR/p59fyn, providing a dual-signal mechanism for mucosal CD8+ T cell activation.\",\n      \"evidence\": \"Reciprocal co-immunoprecipitation, ELISA binding, antibody blocking, CD1d transfectant functional assays\",\n      \"pmids\": [\"10473580\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural interface between CPD and CD1d not mapped\", \"Whether lipid antigens presented by CD1d influence gp180-CD1d complex formation unknown\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Binding specificity studies established that CPD/gp180 engages CD8α but not CD4 through a carbohydrate-dependent interaction at sites distinct from classical MHC class I, defining CPD as a structurally unique CD8 ligand.\",\n      \"evidence\": \"Soluble gp180 binding to CD8-Fc fusions, absorption by CD8α or CD4 transfectants, deglycosylation sensitivity assays\",\n      \"pmids\": [\"11890713\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Specific glycan structures required for CD8 binding not identified\", \"Whether mammalian CPD glycosylation is tissue-specific and functionally variable unknown\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the identity of endogenous peptide substrates processed by CPD in the TGN, the structural basis and physiological significance of CPD-CD8α and CPD-CD1d interactions, the in vivo role of CPD in neuropeptide processing and synaptic function, and whether CPD enzymatic and immune-regulatory functions are coordinated or independent.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No endogenous substrates identified\", \"No crystal structure or cryo-EM model of CPD\", \"No genetic loss-of-function studies in mammalian models reported in this timeline\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0016787\", \"supporting_discovery_ids\": [0, 4, 5, 6]},\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [0, 4, 5, 6]},\n      {\"term_id\": \"GO:0001618\", \"supporting_discovery_ids\": [3]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005794\", \"supporting_discovery_ids\": [7]},\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [1, 7, 9]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"GO:0392499\", \"supporting_discovery_ids\": [0, 4, 5, 6]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [1, 2, 8, 9]},\n      {\"term_id\": \"R-HSA-9609507\", \"supporting_discovery_ids\": [7]}\n    ],\n    \"complexes\": [\n      \"gp180-CD1d complex on intestinal epithelial cells\"\n    ],\n    \"partners\": [\n      \"CD8A\",\n      \"CD1D\",\n      \"SPTAN1\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"mechanistic_narrative\": \"Carboxypeptidase D (CPD/gp180) is a type I transmembrane metallocarboxypeptidase with three carboxypeptidase-like domains, of which the first two harbor enzymatic activity (pH optimum 5.5–6.5) while the third mediates binding to the pre-S protein of duck hepatitis B virus, functioning as the primary receptor for avian hepadnavirus attachment and internalization [PMID:9525948, PMID:9733850]. CPD is concentrated in the trans-Golgi network where it cycles to the cell surface via trafficking signals in its cytoplasmic tail, including a FxxL internalization motif and casein kinase II phosphorylation sites within an acidic cluster [PMID:9490632, PMID:9880325]. On intestinal epithelial cells, CPD functions as a non-classical, carbohydrate-dependent CD8α ligand that forms a surface complex with CD1d, coordinating p56lck and p59fyn signaling for CD8+ T cell activation; its expression is defective in inflammatory bowel disease [PMID:9139738, PMID:10473580, PMID:9329971].\",\n  \"teleology\": [\n    {\n      \"year\": 1995,\n      \"claim\": \"Identification of duck gp180 as a novel carboxypeptidase-family glycoprotein that binds DHBV particles resolved the molecular identity of the avian hepadnavirus receptor.\",\n      \"evidence\": \"Protein purification from duck liver, cDNA cloning, and virus-binding assay\",\n      \"pmids\": [\"7797483\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Whether gp180 alone is sufficient for viral entry was untested\",\n        \"Enzymatic activity of individual domains not yet dissected\",\n        \"Mammalian homolog not yet identified\"\n      ]\n    },\n    {\n      \"year\": 1997,\n      \"claim\": \"Cloning of the mammalian CPD homolog and demonstration of its low-pH metallocarboxypeptidase activity established that the enzyme is conserved across vertebrates with a catalytic profile distinct from all known regulatory B-type carboxypeptidases.\",\n      \"evidence\": \"Partial cDNA cloning, recombinant expression in bacteria, enzyme activity assays, immunoprecipitation, and Western blot in human fibroblasts and mouse macrophages\",\n      \"pmids\": [\"9064476\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Full-length mammalian sequence and chromosomal location unknown\",\n        \"Physiological substrates not identified\",\n        \"Subcellular localization in mammalian cells not determined\"\n      ]\n    },\n    {\n      \"year\": 1997,\n      \"claim\": \"Discovery that intestinal epithelial gp180/CPD is a CD8α ligand that selectively activates p56lck revealed an unexpected immunological function for a carboxypeptidase, linking it to mucosal T cell regulation.\",\n      \"evidence\": \"Flow cytometry, T cell binding assay, and p56lck phosphorylation assay using CD8/CD4 transfectants on intestinal epithelial cells\",\n      \"pmids\": [\"9139738\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Molecular basis of CD8 binding (carbohydrate vs. protein epitope) not resolved\",\n        \"Relationship to MHC class I–CD8 interaction unknown\",\n        \"Whether enzymatic and immune functions are independent not tested\"\n      ]\n    },\n    {\n      \"year\": 1997,\n      \"claim\": \"Loss of CPD expression on intestinal epithelial cells in inflammatory bowel disease correlated with aberrant lck signaling, implicating CPD in mucosal immune homeostasis and IBD pathogenesis.\",\n      \"evidence\": \"Immunohistochemistry on frozen sections from ulcerative colitis and Crohn's disease patients, flow cytometry, and p56lck phosphorylation assay\",\n      \"pmids\": [\"9329971\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Correlative association — causal role of CPD loss in IBD not established\",\n        \"Sample size and patient heterogeneity not fully addressed\",\n        \"Mechanism of CPD downregulation in IBD unknown\"\n      ]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Mapping enzymatic activity to domains A and B and pre-S binding to domain C demonstrated that CPD's carboxypeptidase and viral receptor functions are structurally and functionally separable.\",\n      \"evidence\": \"Baculovirus expression of soluble gp180, enzyme kinetics, inhibitor profiling, and deletion mutagenesis in 293T cells\",\n      \"pmids\": [\"9525948\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Structural basis for domain C–pre-S interaction not resolved\",\n        \"Whether domain C has any residual catalytic or regulatory function unknown\",\n        \"No crystal structure available\"\n      ]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Functional receptor studies confirmed CPD/gp180 is the primary attachment and internalization receptor for DHBV but showed that a species-specific coreceptor is required for full viral entry, defining the limits of gp180-mediated infection.\",\n      \"evidence\": \"Competitive infection inhibition, soluble receptor blocking, and fluorescent viral particle internalization imaging in transfected heterologous hepatoma cells\",\n      \"pmids\": [\"9733850\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Identity of the species-specific coreceptor unknown\",\n        \"Post-internalization trafficking pathway for virus not mapped\",\n        \"Relevance to mammalian hepadnavirus entry not established\"\n      ]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Localization of CPD to the TGN with cycling to the plasma surface established a trafficking itinerary analogous to furin, suggesting CPD processes substrates in the late secretory pathway.\",\n      \"evidence\": \"Immunofluorescence co-localization with TGN markers, antibody internalization assays, and brefeldin A treatment in AtT-20 pituitary cells\",\n      \"pmids\": [\"9490632\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Specific TGN sorting signals not yet mapped\",\n        \"Whether CPD processes neuropeptide precursors in this compartment not tested\",\n        \"Adaptor proteins mediating TGN retrieval not identified\"\n      ]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Systematic mutagenesis of the CPD cytoplasmic tail identified a FxxL internalization motif and casein kinase II–phosphorylated acidic cluster as discrete elements governing TGN retention and surface retrieval, explaining the molecular basis of its intracellular cycling.\",\n      \"evidence\": \"Expression of truncation and point mutants in AtT-20 cells with immunofluorescence and protein turnover assays\",\n      \"pmids\": [\"9880325\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Direct demonstration of CKII phosphorylation in vivo not shown\",\n        \"Adaptor proteins (e.g., PACS-1) that recognize these signals not identified in this study\",\n        \"Whether tail signals regulate enzymatic access to substrates is unknown\"\n      ]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Demonstration that CPD forms a surface complex with CD1d and that the complex coordinates dual kinase activation (p56lck via CPD–CD8 and p59fyn via CD1d–TCR) provided a mechanistic model for how intestinal epithelial cells activate CD8+ T cells.\",\n      \"evidence\": \"Co-immunoprecipitation, ELISA, blocking monoclonal antibodies, and CD1d transfectant functional assays measuring p56lck and p59fyn phosphorylation\",\n      \"pmids\": [\"10473580\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Structural basis of CPD–CD1d interaction not resolved\",\n        \"Whether the complex presents lipid antigens via CD1d concurrently with CPD–CD8 engagement is untested\",\n        \"In vivo relevance of CPD–CD1d complex for mucosal immunity not demonstrated\"\n      ]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Establishing that CPD–CD8α binding is carbohydrate-dependent and uses sites distinct from classical MHC class I defined CPD as a structurally and mechanistically non-classical CD8 ligand.\",\n      \"evidence\": \"Soluble gp180 binding to CD8-Fc fusion proteins, absorption with CD8α/CD4 transfected cells, carbohydrate-dependency testing, and competitive binding with MHC class I\",\n      \"pmids\": [\"11890713\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Specific glycan structures required for CD8 binding not identified\",\n        \"Whether CPD enzymatic activity influences its CD8 ligand function not tested\",\n        \"No in vivo genetic model confirming the immune function of CPD\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The physiological substrates of CPD's carboxypeptidase activity, the structural basis of its multi-domain architecture, and whether its enzymatic and immune functions intersect in vivo remain unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\n        \"No endogenous substrates definitively identified\",\n        \"No crystal or cryo-EM structure of any CPD domain\",\n        \"No genetic knockout or knockdown phenotype reported in mammals\",\n        \"Functional link between enzymatic activity and CD8 ligand or viral receptor roles untested\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [1, 3, 5]},\n      {\"term_id\": \"GO:0016787\", \"supporting_discovery_ids\": [1, 3, 5]},\n      {\"term_id\": \"GO:0001618\", \"supporting_discovery_ids\": [0, 4]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005794\", \"supporting_discovery_ids\": [6, 7]},\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [2, 6, 7, 9, 10]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"GO:0168256\", \"supporting_discovery_ids\": [2, 8, 9, 10]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [2, 8, 9, 10]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [1, 3, 5]}\n    ],\n    \"complexes\": [\n      \"CPD–CD1d surface complex\"\n    ],\n    \"partners\": [\n      \"CD1D\",\n      \"CD8A\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```\n\nWait, I need to fix a duplicate/incorrect pathway entry. Let me correct:\n\n```json\n{\n  \"mechanistic_narrative\": \"Carboxypeptidase D (CPD/gp180) is a type I transmembrane metallocarboxypeptidase with three carboxypeptidase-like domains, of which the first two harbor enzymatic activity (pH optimum 5.5–6.5) while the third mediates binding to the pre-S protein of duck hepatitis B virus, functioning as the primary receptor for avian hepadnavirus attachment and internalization [PMID:9525948, PMID:9733850]. CPD is concentrated in the trans-Golgi network where it cycles to the cell surface via trafficking signals in its cytoplasmic tail, including a FxxL internalization motif and casein kinase II phosphorylation sites within an acidic cluster [PMID:9490632, PMID:9880325]. On intestinal epithelial cells, CPD functions as a non-classical, carbohydrate-dependent CD8α ligand that forms a surface complex with CD1d, coordinating p56lck and p59fyn signaling for CD8+ T cell activation; its expression is defective in inflammatory bowel disease [PMID:9139738, PMID:10473580, PMID:9329971].\",\n  \"teleology\": [\n    {\n      \"year\": 1995,\n      \"claim\": \"Identification of duck gp180 as a novel carboxypeptidase-family glycoprotein that binds DHBV particles resolved the molecular identity of the avian hepadnavirus receptor.\",\n      \"evidence\": \"Protein purification from duck liver, cDNA cloning, and virus-binding assay\",\n      \"pmids\": [\"7797483\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Whether gp180 alone is sufficient for viral entry was untested\",\n        \"Enzymatic activity of individual domains not yet dissected\",\n        \"Mammalian homolog not yet identified\"\n      ]\n    },\n    {\n      \"year\": 1997,\n      \"claim\": \"Cloning of the mammalian CPD homolog and demonstration of its low-pH metallocarboxypeptidase activity established that the enzyme is conserved across vertebrates with a catalytic profile distinct from all known regulatory B-type carboxypeptidases.\",\n      \"evidence\": \"Partial cDNA cloning, recombinant expression in bacteria, enzyme activity assays, immunoprecipitation, and Western blot in human fibroblasts and mouse macrophages\",\n      \"pmids\": [\"9064476\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Full-length mammalian sequence and chromosomal location unknown\",\n        \"Physiological substrates not identified\",\n        \"Subcellular localization in mammalian cells not determined\"\n      ]\n    },\n    {\n      \"year\": 1997,\n      \"claim\": \"Discovery that intestinal epithelial gp180/CPD is a CD8α ligand that selectively activates p56lck revealed an unexpected immunological function for a carboxypeptidase, linking it to mucosal T cell regulation.\",\n      \"evidence\": \"Flow cytometry, T cell binding assay, and p56lck phosphorylation assay using CD8/CD4 transfectants on intestinal epithelial cells\",\n      \"pmids\": [\"9139738\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Molecular basis of CD8 binding (carbohydrate vs. protein epitope) not resolved\",\n        \"Relationship to MHC class I–CD8 interaction unknown\",\n        \"Whether enzymatic and immune functions are independent not tested\"\n      ]\n    },\n    {\n      \"year\": 1997,\n      \"claim\": \"Loss of CPD expression on intestinal epithelial cells in inflammatory bowel disease correlated with aberrant lck signaling, implicating CPD in mucosal immune homeostasis and IBD pathogenesis.\",\n      \"evidence\": \"Immunohistochemistry on frozen sections from ulcerative colitis and Crohn's disease patients, flow cytometry, and p56lck phosphorylation assay\",\n      \"pmids\": [\"9329971\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Correlative association — causal role of CPD loss in IBD not established\",\n        \"Sample size and patient heterogeneity not fully addressed\",\n        \"Mechanism of CPD downregulation in IBD unknown\"\n      ]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Mapping enzymatic activity to domains A and B and pre-S binding to domain C demonstrated that CPD's carboxypeptidase and viral receptor functions are structurally and functionally separable.\",\n      \"evidence\": \"Baculovirus expression of soluble gp180, enzyme kinetics, inhibitor profiling, and deletion mutagenesis in 293T cells\",\n      \"pmids\": [\"9525948\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Structural basis for domain C–pre-S interaction not resolved\",\n        \"Whether domain C has any residual catalytic or regulatory function unknown\",\n        \"No crystal structure available\"\n      ]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Functional receptor studies confirmed CPD/gp180 is the primary attachment and internalization receptor for DHBV but showed that a species-specific coreceptor is required for full viral entry, defining the limits of gp180-mediated infection.\",\n      \"evidence\": \"Competitive infection inhibition, soluble receptor blocking, and fluorescent viral particle internalization imaging in transfected heterologous hepatoma cells\",\n      \"pmids\": [\"9733850\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Identity of the species-specific coreceptor unknown\",\n        \"Post-internalization trafficking pathway for virus not mapped\",\n        \"Relevance to mammalian hepadnavirus entry not established\"\n      ]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Localization of CPD to the TGN with cycling to the plasma surface established a trafficking itinerary analogous to furin, suggesting CPD processes substrates in the late secretory pathway.\",\n      \"evidence\": \"Immunofluorescence co-localization with TGN markers, antibody internalization assays, and brefeldin A treatment in AtT-20 pituitary cells\",\n      \"pmids\": [\"9490632\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Specific TGN sorting signals not yet mapped\",\n        \"Whether CPD processes neuropeptide precursors in this compartment not tested\",\n        \"Adaptor proteins mediating TGN retrieval not identified\"\n      ]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Systematic mutagenesis of the CPD cytoplasmic tail identified a FxxL internalization motif and casein kinase II–phosphorylated acidic cluster as discrete elements governing TGN retention and surface retrieval, explaining the molecular basis of its intracellular cycling.\",\n      \"evidence\": \"Expression of truncation and point mutants in AtT-20 cells with immunofluorescence and protein turnover assays\",\n      \"pmids\": [\"9880325\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Direct demonstration of CKII phosphorylation in vivo not shown\",\n        \"Adaptor proteins (e.g., PACS-1) that recognize these signals not identified in this study\",\n        \"Whether tail signals regulate enzymatic access to substrates is unknown\"\n      ]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Demonstration that CPD forms a surface complex with CD1d and that the complex coordinates dual kinase activation (p56lck via CPD–CD8 and p59fyn via CD1d–TCR) provided a mechanistic model for how intestinal epithelial cells activate CD8+ T cells.\",\n      \"evidence\": \"Co-immunoprecipitation, ELISA, blocking monoclonal antibodies, and CD1d transfectant functional assays measuring p56lck and p59fyn phosphorylation\",\n      \"pmids\": [\"10473580\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Structural basis of CPD–CD1d interaction not resolved\",\n        \"Whether the complex presents lipid antigens via CD1d concurrently with CPD–CD8 engagement is untested\",\n        \"In vivo relevance of CPD–CD1d complex for mucosal immunity not demonstrated\"\n      ]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Establishing that CPD–CD8α binding is carbohydrate-dependent and uses sites distinct from classical MHC class I defined CPD as a structurally and mechanistically non-classical CD8 ligand.\",\n      \"evidence\": \"Soluble gp180 binding to CD8-Fc fusion proteins, absorption with CD8α/CD4 transfected cells, carbohydrate-dependency testing, and competitive binding with MHC class I\",\n      \"pmids\": [\"11890713\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Specific glycan structures required for CD8 binding not identified\",\n        \"Whether CPD enzymatic activity influences its CD8 ligand function not tested\",\n        \"No in vivo genetic model confirming the immune function of CPD\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The physiological substrates of CPD's carboxypeptidase activity, the structural basis of its multi-domain architecture, and whether its enzymatic and immune functions intersect in vivo remain unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\n        \"No endogenous substrates definitively identified\",\n        \"No crystal or cryo-EM structure of any CPD domain\",\n        \"No genetic knockout or knockdown phenotype reported in mammals\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [1, 3, 5]},\n      {\"term_id\": \"GO:0016787\", \"supporting_discovery_ids\": [1, 3, 5]},\n      {\"term_id\": \"GO:0001618\", \"supporting_discovery_ids\": [0, 4]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005794\", \"supporting_discovery_ids\": [6, 7]},\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [2, 6, 7, 9, 10]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [2, 9, 10]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [1, 3, 5]}\n    ],\n    \"complexes\": [\n      \"CPD–CD1d surface complex\"\n    ],\n    \"partners\": [\n      \"CD1D\",\n      \"CD8A\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}