{"gene":"LRPAP1","run_date":"2026-04-28T18:30:27","timeline":{"discoveries":[{"year":1991,"finding":"The 39-kDa receptor-associated protein (RAP/LRPAP1) binds directly to the 515-kDa subunit of LRP/alpha2-macroglobulin receptor and inhibits binding and uptake of both beta-migrating VLDL (apoE-enriched) and activated alpha2-macroglobulin, demonstrating that RAP modulates receptor ligand-binding activity.","method":"Recombinant fusion protein binding assay, ligand uptake inhibition assay in cells","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1–2 — direct binding with recombinant protein plus functional ligand inhibition, foundational paper >500 citations","pmids":["1718973"],"is_preprint":false},{"year":1991,"finding":"LRPAP1 (RAP) primary structure was determined by cDNA cloning, revealing a 323-residue mature protein with 73% identity to rat Heymann nephritis antigen and 77% identity to mouse HBP-44; pulse-chase experiments showed newly formed RAP remains cell-associated, and surface labeling showed RAP forms a complex with the alpha2M receptor heavy and light chains on the cell surface.","method":"cDNA cloning, pulse-chase radiolabeling, cell surface labeling and immunoprecipitation","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1–2 — multiple orthogonal methods in foundational paper; >200 citations","pmids":["1712782"],"is_preprint":false},{"year":1992,"finding":"RAP binds to two equivalent sites on LRP/alpha2MR with a KD of 14 nM; as RAP-binding sites are saturated, LRP completely loses the ability to bind ligands (Ki ~23–26 nM for inhibition of alpha2M binding), indicating that the number of RAP molecules bound dictates receptor activity. A model was proposed in which LRP contains multiple ligand-binding sites, each regulated by a separate RAP-binding site.","method":"Titration binding studies, heterologous ligand displacement assays with purified LRP and on human fibroblasts","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — quantitative binding with purified components, reconstitution-level rigor; >300 citations","pmids":["1374383"],"is_preprint":false},{"year":1994,"finding":"RAP contains two distinct gp330/megalin-binding sites (amino acids 85–148 and 178–248) and a separate heparin-binding site (amino acids 261–323), demonstrating that the gp330-binding, heparin-binding, and Heymann nephritis pathogenic epitope (aa 1–86) domains are functionally distinct.","method":"Direct binding studies using GST fusion proteins representing overlapping RAP domains, ligand blotting, affinity chromatography, density-gradient sedimentation, radiolabeled heparin binding","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 — domain mapping with multiple orthogonal in vitro binding assays; >60 citations","pmids":["7512726"],"is_preprint":false},{"year":1994,"finding":"Immunohistochemical mapping in rat organs showed RAP is found in intracellular vesicles in all cells expressing gp330 or LRP, and is not definitively detected on the cell surface, indicating its major physiological function is intracellular (assembly/trafficking of receptors). RAP binding sites were found on cell surfaces where surface gp330 or LRP is expressed.","method":"Immunohistochemistry with organ distribution mapping; exogenous RAP binding experiments in vivo","journal":"The journal of histochemistry and cytochemistry","confidence":"Medium","confidence_rationale":"Tier 2–3 — direct localization by immunohistochemistry across multiple tissues; >290 citations but localization-focused","pmids":["7510321"],"is_preprint":false},{"year":1995,"finding":"RAP is an endoplasmic reticulum (ER) resident protein; the C-terminal tetrapeptide HNEL is both necessary and sufficient for ER retention. RAP interacts with LRP in vivo (demonstrated by cross-linking), the association is transient and occurs early in the secretory pathway, and dissociation at lower pH later in the pathway correlates with increased LRP ligand-binding activity. RAP functions as a molecular chaperone for LRP, preventing ligand-induced aggregation and maintaining LRP in a ligand-binding competent state.","method":"Metabolic labeling, cross-linking, pulse-chase analysis, C-terminal deletion/mutation analysis of HNEL signal, transfection of HNEL-tagged reporter proteins","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1 — multiple orthogonal methods including mutagenesis of retention signal, cross-linking, pulse-chase; >280 citations","pmids":["7774585"],"is_preprint":false},{"year":1995,"finding":"Gene targeting (RAP-knockout mice) demonstrated that RAP is required for functional expression of LRP in vivo: RAP-deficient mice show reduced mature LRP in liver and brain, impaired hepatic clearance of alpha2-macroglobulin, and accumulation of remnant lipoproteins in plasma (in LDL receptor-null background), confirming RAP stabilizes LRP within the secretory pathway.","method":"Gene targeting/knockout mouse model, LRP immunoblotting, alpha2M clearance assay, plasma lipoprotein analysis","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 — clean KO with defined in vivo functional phenotype; >240 citations","pmids":["7538675"],"is_preprint":false},{"year":1995,"finding":"RAP was identified as a component of the Heymann nephritis antigenic complex; it associates with megalin (gp330) and functions as a chaperone assisting in megalin folding in the ER and its transport to the cell surface. Binding to megalin is Ca2+-dependent (also shown in Orlando & Farquhar 1994).","method":"Biochemical characterization, ligand blotting, immunocytochemistry, affinity chromatography","journal":"Journal of the American Society of Nephrology","confidence":"High","confidence_rationale":"Tier 2 — multiple biochemical methods, consistent with parallel literature; >180 citations","pmids":["7579068"],"is_preprint":false},{"year":1996,"finding":"RAP-deficient mice generated by gene targeting showed that RAP prevents premature binding of co-expressed ligands (e.g., apoE) to newly synthesized LRP and other LDL receptor family members in the ER, preventing ligand-induced aggregation and degradation. Overexpression of apoE dramatically reduced cellular LRP expression, an effect prevented by RAP co-expression, establishing RAP as a specialized chaperone class that selectively protects endocytic receptors.","method":"Gene targeting (RAP-null mice), apoE overexpression rescue experiments, LRP expression analysis, embryonic fibroblast studies","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1–2 — genetic rescue + overexpression experiments with molecular readouts; >245 citations","pmids":["8654360"],"is_preprint":false},{"year":1997,"finding":"ERD2 proteins (ERD2.1 and ERD2.2, the mammalian KDEL receptors) mediate ER retention of RAP via its novel C-terminal HNEL signal. Overexpression of GH tagged with HNEL or KDEL both saturated ER retention receptors and caused secretion of endogenous RAP. Overexpression of either ERD2 isoform significantly increased the retention capacity for both KDEL- and HNEL-containing proteins, and functional LRP was reduced ~60% when intracellular RAP was decreased.","method":"Immunoelectron microscopy, stable transfection of GH-HNEL and GH-KDEL constructs, pulse-chase labeling, ERD2 overexpression, immunolocalization","journal":"Journal of cell science","confidence":"High","confidence_rationale":"Tier 1–2 — multiple orthogonal methods including electron microscopy, mutagenesis, and overexpression with quantitative functional readout; >35 citations","pmids":["9010785"],"is_preprint":false},{"year":1997,"finding":"RAP affinity chromatography from human brain extracts purified a novel ~95-kDa membrane glycoprotein (gp95/sortilin), demonstrating that LRPAP1 can be used as an affinity ligand to identify novel receptors beyond the canonical LDL receptor family, and that sortilin is a RAP-binding protein.","method":"RAP affinity chromatography, protein purification from human brain, cDNA cloning","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 — affinity purification with functional validation; RAP used as biochemical tool to discover new binding partners","pmids":["9013611"],"is_preprint":false},{"year":1996,"finding":"RAP affinity chromatography purified a novel ~250-kDa brain protein (sorLA-1/SORL1), a hybrid receptor containing both Vps10p-homologous domains and LDL receptor family repeats, demonstrating LRPAP1 binds this receptor and validating RAP as a tool to identify LDL receptor family-related proteins.","method":"RAP affinity chromatography from human brain, cDNA cloning, Northern blotting","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 — affinity purification; RAP binding to SorLA established biochemically","pmids":["8940146"],"is_preprint":false},{"year":1998,"finding":"RAP is defined as a novel class of molecular chaperone specifically designed to assist biosynthesis of endocytic receptors of the LDL receptor family; it acts as a receptor antagonist in the secretory pathway, preventing premature receptor-ligand association in cell types that co-express both receptor and ligand, thereby ensuring proper receptor export to the cell surface.","method":"Review synthesizing experimental evidence from multiple studies including RAP-null mice, ER localization, and ligand binding inhibition experiments","journal":"Biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — mechanistic synthesis of multiple experimental lines; consistent across multiple independent labs","pmids":["9792434"],"is_preprint":false},{"year":2000,"finding":"RAP's role in biogenesis of lipoprotein receptors was established: RAP serves as a molecular chaperone assisting folding of lipoprotein receptors and their safe passage through the secretory pathway. Multiple non-exclusive models were proposed: (1) RAP competitively inhibits ligand binding by occupying ligand-binding sites, and (2) RAP may directly assist folding of the cysteine-rich ligand-binding repeats.","method":"Review of experimental evidence; biochemical studies of RAP-receptor interactions in secretory pathway","journal":"Trends in cardiovascular medicine","confidence":"Medium","confidence_rationale":"Tier 3 — review synthesizing established experimental data, no new primary data","pmids":["11239794"],"is_preprint":false},{"year":2004,"finding":"Blood-borne RAP crosses the blood-brain barrier (BBB) by a saturable transport process; ~70% of transported RAP localizes in brain parenchyma rather than vasculature. Transport across polarized epithelial cell monolayers (MDCK) is consistent with megalin (LRP2) mediating most apical-to-basolateral transcytosis. Inhibition by excess RAP and involvement of megalin indicate a specific saturable transport system at the BBB.","method":"[125I]-RAP pharmacokinetics in intact mice, in situ brain perfusion, capillary depletion of brain homogenates, transport assays in stably transfected polarized MDCK cell monolayers","journal":"Journal of cell science","confidence":"High","confidence_rationale":"Tier 2 — multiple in vivo and in vitro methods with saturation and competition controls; >100 citations","pmids":["15383619"],"is_preprint":false},{"year":2007,"finding":"Recombinant RAP was successfully expressed in E. coli using pT7-PL vector in BL21(DE3,plysS) strain with optimal conditions (chloramphenicol selection), and purified RAP retained functional binding to RAW264.7 macrophage cells (which are rich in LDL receptor family members), confirming that recombinant RAP binds LDL receptor family receptors.","method":"Prokaryotic expression in E. coli, Ni-NTA affinity purification, cell binding assay with RAW264.7 macrophages","journal":"Journal of Huazhong University of Science and Technology. Medical sciences","confidence":"Medium","confidence_rationale":"Tier 2 — direct binding assay with purified recombinant protein; single lab, functional validation","pmids":["17393096"],"is_preprint":false},{"year":2018,"finding":"LRPAP1-reactive B-cell receptors (BCRs) in mantle cell lymphoma (MCL) recognize LRPAP1 as a proliferation-inducing autoantigen; LRPAP1 stimulation activates the BCR signaling pathway in MCL cells. An LRPAP1-ETA' immunotoxin conjugate specifically killed MCL cells bearing LRPAP1-reactive BCRs, demonstrating that LRPAP1 can trigger BCR pathway signaling and be used for targeted therapy.","method":"Recombinant BCR expression, antigen screening, LRPAP1-induced proliferation assay, BCR pathway activation assay, immunotoxin killing assay","journal":"Leukemia","confidence":"Medium","confidence_rationale":"Tier 2 — functional BCR activation and killing assays; novel mechanism but single lab","pmids":["29955130"],"is_preprint":false},{"year":2023,"finding":"Secreted LRPAP1 (upregulated by viral proteases SARS-CoV-2 3CLpro and EV71 2Apro) binds via its N-terminus to the extracellular domain of IFNAR1 (interferon-alpha/beta receptor 1), triggering IFNAR1 ubiquitination and degradation, thereby suppressing innate immune signaling and facilitating viral infection. A small N-terminal LRPAP1 peptide recapitulated IFNAR1 degradation and enhanced infection by multiple viruses (HSV-1, HBV, EV71, HCoV-OC43); alpha2M (a LRPAP1 inhibitor) stabilized IFNAR1 and arrested viral infections.","method":"Viral protease overexpression, ELISA/Western blot for LRPAP1 secretion, co-IP of LRPAP1 with IFNAR1 extracellular domain, IFNAR1 ubiquitination assay, viral infection assays in vitro/ex vivo (mouse brain)/in vivo (newborn mice), peptide competition, alpha2M inhibition","journal":"Signal transduction and targeted therapy","confidence":"High","confidence_rationale":"Tier 1–2 — multiple orthogonal methods including co-IP, ubiquitination assay, in vivo validation, peptide/inhibitor rescue across multiple viral systems","pmids":["37743411"],"is_preprint":false},{"year":2001,"finding":"Genetic variation at the LRPAP1 locus (intron 5 insertion/deletion polymorphism) is associated with late-onset Alzheimer disease risk; homozygosity for the rare insertion allele was significantly underrepresented in patients vs. controls (OR=0.29), suggesting LRPAP1 variation modulates disease risk, consistent with its role in LRP-mediated amyloid-beta clearance pathway.","method":"Genotyping of LRPAP1 I/D polymorphism by PCR in 373 patients and 400 controls, logistic regression analysis","journal":"American journal of medical genetics","confidence":"Low","confidence_rationale":"Tier 4 — genetic association study, no direct mechanistic experiment on LRPAP1 protein function","pmids":["11425005"],"is_preprint":false}],"current_model":"LRPAP1 (RAP) is a specialized ER-resident molecular chaperone (retained via its C-terminal HNEL signal through ERD2/KDEL receptors) that transiently associates with newly synthesized LRP and other LDL receptor family members (megalin/gp330, VLDLR, SorLA) in the early secretory pathway, preventing premature ligand-induced aggregation and degradation by competitively occupying ligand-binding sites; RAP-null mice show reduced mature LRP expression and impaired lipoprotein clearance; additionally, secreted LRPAP1 has a newly discovered extracellular function whereby its N-terminus binds IFNAR1 to trigger ubiquitination and degradation of the interferon receptor, facilitating viral immune evasion."},"narrative":{"teleology":[{"year":1991,"claim":"Identification of RAP as a direct LRP-binding protein that modulates receptor ligand-binding activity established the first functional connection between this 39-kDa protein and lipoprotein receptor biology.","evidence":"Recombinant fusion protein binding assays and ligand uptake inhibition in cultured cells; concurrent cDNA cloning with pulse-chase and cell surface labeling","pmids":["1718973","1712782"],"confidence":"High","gaps":["Binding stoichiometry and affinity not yet determined","Physiological relevance (intracellular vs. surface function) unresolved","No data on other LDL receptor family members"]},{"year":1992,"claim":"Quantitative binding analysis revealed RAP occupies two equivalent high-affinity sites on LRP (KD ~14 nM), with progressive saturation of these sites completely ablating ligand binding, resolving how RAP stoichiometrically controls receptor activity.","evidence":"Titration binding and heterologous displacement assays with purified LRP on fibroblasts","pmids":["1374383"],"confidence":"High","gaps":["Structural basis of two-site binding unknown","Whether RAP acts inside the cell or at the surface still debated"]},{"year":1994,"claim":"Domain mapping identified two distinct megalin/gp330-binding domains and a separate heparin-binding domain in RAP, demonstrating modular architecture that enables interaction with multiple receptors.","evidence":"GST fusion protein binding studies, ligand blotting, and heparin-binding assays","pmids":["7512726"],"confidence":"High","gaps":["Whether domain modularity dictates receptor selectivity in vivo unresolved","Structural basis of domain-receptor interactions not determined"]},{"year":1995,"claim":"Three converging lines of evidence—ER localization via the HNEL retention signal, pH-dependent dissociation from LRP, and RAP-knockout mouse phenotype—established RAP as an ER-resident molecular chaperone essential for functional maturation and surface delivery of LDL receptor family members.","evidence":"C-terminal mutagenesis of HNEL, cross-linking and pulse-chase in the secretory pathway, gene-targeted RAP-null mice with reduced LRP and impaired alpha-2-macroglobulin clearance; parallel studies on megalin chaperoning","pmids":["7774585","7538675","7579068"],"confidence":"High","gaps":["Whether RAP directly assists receptor folding (vs. only blocking ligands) not distinguished","Structural basis of pH-dependent dissociation unknown"]},{"year":1996,"claim":"Rescue experiments in RAP-null fibroblasts demonstrated that RAP prevents co-expressed ligands (apoE) from binding newly synthesized LRP in the ER, causing aggregation and degradation—defining RAP as a competitive antagonist-type chaperone specific to endocytic receptors. RAP affinity chromatography also identified SorLA (SORL1) as a new binding partner.","evidence":"ApoE overexpression with/without RAP co-expression in RAP-null embryonic fibroblasts; RAP affinity purification from human brain identifying SorLA","pmids":["8654360","8940146"],"confidence":"High","gaps":["Full repertoire of RAP client receptors not mapped","Physiological consequences of SorLA–RAP interaction not tested in vivo"]},{"year":1997,"claim":"The ER retention mechanism was resolved: ERD2 (KDEL receptor) isoforms recognize the non-canonical HNEL signal of RAP, and saturation of ERD2 capacity causes RAP secretion and consequent ~60% loss of functional LRP, quantifying the dependence of receptor biogenesis on the retrieval cycle.","evidence":"ERD2 overexpression, immunoelectron microscopy, pulse-chase with GH-HNEL/KDEL reporter constructs","pmids":["9010785"],"confidence":"High","gaps":["Whether ERD2 isoforms show preferential specificity for HNEL vs. KDEL in vivo not determined","Consequences of physiological RAP secretion not explored"]},{"year":2004,"claim":"Circulating RAP was shown to cross the blood-brain barrier via a saturable, megalin-dependent transcytosis process, with the majority reaching brain parenchyma—revealing a potential extracellular trafficking function beyond ER chaperoning.","evidence":"Radiolabeled RAP pharmacokinetics in mice, in situ brain perfusion, capillary depletion, polarized MDCK transcytosis assays","pmids":["15383619"],"confidence":"High","gaps":["Physiological significance of blood-borne RAP in the brain not established","Whether BBB-transported RAP modulates neuronal LRP function in vivo unknown"]},{"year":2023,"claim":"A previously unrecognized extracellular function was discovered: secreted LRPAP1, upregulated by viral proteases, binds IFNAR1 via its N-terminus, triggers ubiquitination and degradation of the interferon receptor, and suppresses innate immunity—establishing LRPAP1 as a host factor exploited for viral immune evasion.","evidence":"Co-IP of LRPAP1 with IFNAR1, ubiquitination assays, peptide competition, alpha-2-macroglobulin rescue, multi-virus infection assays in vitro and in vivo (newborn mice)","pmids":["37743411"],"confidence":"High","gaps":["Whether IFNAR1 degradation by LRPAP1 occurs during non-infectious physiology unknown","Structural basis of LRPAP1 N-terminus–IFNAR1 interaction not determined","Whether other interferon receptor subunits are affected not tested"]},{"year":null,"claim":"Key unresolved questions include whether RAP directly assists folding of LDL receptor cysteine-rich repeats (beyond competitive ligand blocking), the structural basis of pH-dependent receptor dissociation, and the physiological scope of the newly discovered extracellular IFNAR1-degradation function outside viral infection contexts.","evidence":"","pmids":[],"confidence":"Low","gaps":["No high-resolution structure of a full RAP–receptor complex exists","Direct folding chaperone activity vs. competitive shielding not experimentally separated","IFNAR1 degradation pathway not yet confirmed by independent laboratories"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0044183","term_label":"protein folding chaperone","supporting_discovery_ids":[5,6,7,8,12]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[0,2,17]}],"localization":[{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[4,5,9]},{"term_id":"GO:0005576","term_label":"extracellular region","supporting_discovery_ids":[14,17]}],"pathway":[{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[5,6,8,9]},{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[17]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[17]}],"complexes":[],"partners":["LRP1","LRP2","SORL1","IFNAR1","SORT1","KDELR1","KDELR2"],"other_free_text":[]},"mechanistic_narrative":"LRPAP1 (RAP) is a specialized endoplasmic reticulum-resident molecular chaperone that escorts newly synthesized members of the LDL receptor family—including LRP, megalin/gp330, SorLA, and VLDLR—through the secretory pathway by competitively occupying their ligand-binding sites, thereby preventing premature ligand-induced aggregation and degradation [PMID:7774585, PMID:8654360]. RAP binds LRP at two equivalent high-affinity sites (KD ~14 nM) and is retained in the ER via its C-terminal HNEL tetrapeptide recognized by ERD2/KDEL receptors; pH-dependent dissociation later in the secretory pathway restores receptor ligand-binding competence [PMID:1374383, PMID:9010785]. RAP-knockout mice exhibit reduced mature LRP in liver and brain, impaired alpha-2-macroglobulin clearance, and remnant lipoprotein accumulation, confirming its essential in vivo chaperone function [PMID:7538675, PMID:8654360]. Secreted LRPAP1 also has an extracellular role: its N-terminus binds IFNAR1, triggering ubiquitination-dependent receptor degradation that suppresses type I interferon signaling and facilitates viral immune evasion [PMID:37743411]."},"prefetch_data":{"uniprot":{"accession":"P30533","full_name":"Alpha-2-macroglobulin receptor-associated protein","aliases":["Low density lipoprotein receptor-related protein-associated protein 1","RAP"],"length_aa":357,"mass_kda":41.5,"function":"Molecular chaperone for LDL receptor-related proteins that may regulate their ligand binding activity along the secretory pathway","subcellular_location":"Rough endoplasmic reticulum lumen; Endoplasmic reticulum-Golgi intermediate compartment lumen; Golgi apparatus, cis-Golgi network; Golgi apparatus lumen; Endosome lumen; Cell surface","url":"https://www.uniprot.org/uniprotkb/P30533/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/LRPAP1","classification":"Not Classified","n_dependent_lines":7,"n_total_lines":1208,"dependency_fraction":0.005794701986754967},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[{"gene":"CANX","stoichiometry":0.2},{"gene":"CAPRIN1","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/search/LRPAP1","total_profiled":1310},"omim":[{"mim_id":"615431","title":"MYOPIA 23, AUTOSOMAL RECESSIVE; MYP23","url":"https://www.omim.org/entry/615431"},{"mim_id":"605552","title":"ABDOMINAL OBESITY-METABOLIC SYNDROME 1; AOMS1","url":"https://www.omim.org/entry/605552"},{"mim_id":"603159","title":"LOW DENSITY LIPOPROTEIN RECEPTOR-RELATED PROTEIN 3; LRP3","url":"https://www.omim.org/entry/603159"},{"mim_id":"600073","title":"LOW DENSITY LIPOPROTEIN RECEPTOR-RELATED PROTEIN 2; LRP2","url":"https://www.omim.org/entry/600073"},{"mim_id":"160700","title":"MYOPIA 2, AUTOSOMAL DOMINANT; MYP2","url":"https://www.omim.org/entry/160700"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Enhanced","locations":[{"location":"Endoplasmic reticulum","reliability":"Enhanced"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/LRPAP1"},"hgnc":{"alias_symbol":["HBP44","Alpha-2-MRAP"],"prev_symbol":["A2MRAP","RAP"]},"alphafold":{"accession":"P30533","domains":[{"cath_id":"1.20.81.10","chopping":"58-123","consensus_level":"high","plddt":84.3752,"start":58,"end":123},{"cath_id":"1.20.81.10","chopping":"150-202_213-245","consensus_level":"high","plddt":78.3876,"start":150,"end":245},{"cath_id":"1.20.81.10","chopping":"256-356","consensus_level":"high","plddt":88.1812,"start":256,"end":356}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P30533","model_url":"https://alphafold.ebi.ac.uk/files/AF-P30533-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P30533-F1-predicted_aligned_error_v6.png","plddt_mean":75.19},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=LRPAP1","jax_strain_url":"https://www.jax.org/strain/search?query=LRPAP1"},"sequence":{"accession":"P30533","fasta_url":"https://rest.uniprot.org/uniprotkb/P30533.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P30533/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P30533"}},"corpus_meta":[{"pmid":"12202034","id":"PMC_12202034","title":"Ras 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C3G alterations in human hepatocarcinoma.","date":"2018","source":"Hepatic oncology","url":"https://pubmed.ncbi.nlm.nih.gov/30302196","citation_count":23,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"29659486","id":"PMC_29659486","title":"Vascular Endothelial Growth Factor-A Exerts Diverse Cellular Effects via Small G Proteins, Rho and Rap.","date":"2018","source":"International journal of molecular sciences","url":"https://pubmed.ncbi.nlm.nih.gov/29659486","citation_count":23,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"2514150","id":"PMC_2514150","title":"Expression of the ras-related rap genes in human tumors.","date":"1989","source":"International journal of cancer","url":"https://pubmed.ncbi.nlm.nih.gov/2514150","citation_count":23,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"37743411","id":"PMC_37743411","title":"Secreted LRPAP1 binds and triggers IFNAR1 degradation to facilitate virus evasion from cellular innate immunity.","date":"2023","source":"Signal transduction and targeted therapy","url":"https://pubmed.ncbi.nlm.nih.gov/37743411","citation_count":22,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"8620032","id":"PMC_8620032","title":"RAP kinase, a new enzyme phosphorylating the acidic P proteins from Saccharomyces cerevisiae.","date":"1996","source":"Biochimica et biophysica acta","url":"https://pubmed.ncbi.nlm.nih.gov/8620032","citation_count":22,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"18453105","id":"PMC_18453105","title":"Rac and Rap GTPase activation assays.","date":"2007","source":"Methods in molecular biology (Clifton, N.J.)","url":"https://pubmed.ncbi.nlm.nih.gov/18453105","citation_count":22,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"11181995","id":"PMC_11181995","title":"The sequence of the human genome.","date":"2001","source":"Science (New York, N.Y.)","url":"https://pubmed.ncbi.nlm.nih.gov/11181995","citation_count":8428,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"24097068","id":"PMC_24097068","title":"Discovery and refinement of loci associated with lipid levels.","date":"2013","source":"Nature genetics","url":"https://pubmed.ncbi.nlm.nih.gov/24097068","citation_count":2409,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"16169070","id":"PMC_16169070","title":"A human protein-protein interaction network: a resource for annotating the proteome.","date":"2005","source":"Cell","url":"https://pubmed.ncbi.nlm.nih.gov/16169070","citation_count":1704,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"12477932","id":"PMC_12477932","title":"Generation and initial analysis of more than 15,000 full-length human and mouse cDNA sequences.","date":"2002","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/12477932","citation_count":1479,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"28514442","id":"PMC_28514442","title":"Architecture of the human interactome defines protein communities and disease networks.","date":"2017","source":"Nature","url":"https://pubmed.ncbi.nlm.nih.gov/28514442","citation_count":1085,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"3266596","id":"PMC_3266596","title":"Surface location and high affinity for calcium of a 500-kd liver membrane protein closely related to the LDL-receptor suggest a physiological role as lipoprotein receptor.","date":"1988","source":"The EMBO journal","url":"https://pubmed.ncbi.nlm.nih.gov/3266596","citation_count":918,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"32296183","id":"PMC_32296183","title":"A reference map of the human binary protein interactome.","date":"2020","source":"Nature","url":"https://pubmed.ncbi.nlm.nih.gov/32296183","citation_count":849,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"10571241","id":"PMC_10571241","title":"Direct binding of Reelin to VLDL receptor and ApoE receptor 2 induces tyrosine phosphorylation of disabled-1 and modulates tau phosphorylation.","date":"1999","source":"Neuron","url":"https://pubmed.ncbi.nlm.nih.gov/10571241","citation_count":774,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"33961781","id":"PMC_33961781","title":"Dual proteome-scale networks reveal cell-specific remodeling of the human interactome.","date":"2021","source":"Cell","url":"https://pubmed.ncbi.nlm.nih.gov/33961781","citation_count":705,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"21873635","id":"PMC_21873635","title":"Phylogenetic-based propagation of functional annotations within the Gene Ontology consortium.","date":"2011","source":"Briefings in bioinformatics","url":"https://pubmed.ncbi.nlm.nih.gov/21873635","citation_count":656,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"15342917","id":"PMC_15342917","title":"Tumor-promoting phorbol esters and activated Ras inactivate the tuberous sclerosis tumor suppressor complex via p90 ribosomal S6 kinase.","date":"2004","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/15342917","citation_count":618,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"1718973","id":"PMC_1718973","title":"39-kDa protein modulates binding of ligands to low density lipoprotein receptor-related protein/alpha 2-macroglobulin receptor.","date":"1991","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/1718973","citation_count":543,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"8125298","id":"PMC_8125298","title":"Oligo-capping: a simple method to replace the cap structure of eukaryotic mRNAs with oligoribonucleotides.","date":"1994","source":"Gene","url":"https://pubmed.ncbi.nlm.nih.gov/8125298","citation_count":492,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"15489334","id":"PMC_15489334","title":"The status, quality, and expansion of the NIH full-length cDNA project: the Mammalian Gene Collection (MGC).","date":"2004","source":"Genome research","url":"https://pubmed.ncbi.nlm.nih.gov/15489334","citation_count":438,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"26638075","id":"PMC_26638075","title":"A Dynamic Protein Interaction Landscape of the Human Centrosome-Cilium Interface.","date":"2015","source":"Cell","url":"https://pubmed.ncbi.nlm.nih.gov/26638075","citation_count":433,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"35271311","id":"PMC_35271311","title":"OpenCell: Endogenous tagging for the cartography of human cellular organization.","date":"2022","source":"Science (New York, N.Y.)","url":"https://pubmed.ncbi.nlm.nih.gov/35271311","citation_count":432,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"16344560","id":"PMC_16344560","title":"Diversification of transcriptional modulation: large-scale identification and characterization of putative alternative promoters of human genes.","date":"2005","source":"Genome research","url":"https://pubmed.ncbi.nlm.nih.gov/16344560","citation_count":409,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"9013611","id":"PMC_9013611","title":"Molecular identification of a novel candidate sorting receptor purified from human brain by receptor-associated protein affinity chromatography.","date":"1997","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/9013611","citation_count":359,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"26005850","id":"PMC_26005850","title":"Central role for PICALM in amyloid-β blood-brain barrier transcytosis and clearance.","date":"2015","source":"Nature neuroscience","url":"https://pubmed.ncbi.nlm.nih.gov/26005850","citation_count":352,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"34079125","id":"PMC_34079125","title":"A proximity-dependent biotinylation map of a human cell.","date":"2021","source":"Nature","url":"https://pubmed.ncbi.nlm.nih.gov/34079125","citation_count":339,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"1374383","id":"PMC_1374383","title":"A novel mechanism for controlling the activity of alpha 2-macroglobulin receptor/low density lipoprotein receptor-related protein. Multiple regulatory sites for 39-kDa receptor-associated protein.","date":"1992","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/1374383","citation_count":311,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"7510321","id":"PMC_7510321","title":"Organ distribution in rats of two members of the low-density lipoprotein receptor gene family, gp330 and LRP/alpha 2MR, and the receptor-associated protein (RAP).","date":"1994","source":"The journal of histochemistry and cytochemistry : official journal of the Histochemistry Society","url":"https://pubmed.ncbi.nlm.nih.gov/7510321","citation_count":299,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"7774585","id":"PMC_7774585","title":"39 kDa receptor-associated protein is an ER resident protein and molecular chaperone for LDL receptor-related protein.","date":"1995","source":"The EMBO journal","url":"https://pubmed.ncbi.nlm.nih.gov/7774585","citation_count":280,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"7538675","id":"PMC_7538675","title":"Functional expression of low density lipoprotein receptor-related protein is controlled by receptor-associated protein in vivo.","date":"1995","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/7538675","citation_count":247,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"8940146","id":"PMC_8940146","title":"Molecular characterization of a novel human hybrid-type receptor that binds the alpha2-macroglobulin receptor-associated protein.","date":"1996","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/8940146","citation_count":230,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"1712782","id":"PMC_1712782","title":"Primary structure of alpha 2-macroglobulin receptor-associated protein. Human homologue of a Heymann nephritis antigen.","date":"1991","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/1712782","citation_count":229,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"19098903","id":"PMC_19098903","title":"SRF and myocardin regulate LRP-mediated amyloid-beta clearance in brain vascular cells.","date":"2008","source":"Nature cell biology","url":"https://pubmed.ncbi.nlm.nih.gov/19098903","citation_count":226,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"21988832","id":"PMC_21988832","title":"Toward an understanding of the protein interaction network of the human liver.","date":"2011","source":"Molecular systems biology","url":"https://pubmed.ncbi.nlm.nih.gov/21988832","citation_count":207,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"32203420","id":"PMC_32203420","title":"Systems analysis of RhoGEF and RhoGAP regulatory proteins reveals spatially organized RAC1 signalling from integrin adhesions.","date":"2020","source":"Nature cell biology","url":"https://pubmed.ncbi.nlm.nih.gov/32203420","citation_count":194,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"10747918","id":"PMC_10747918","title":"The YXXL motif, but not the two NPXY motifs, serves as the dominant endocytosis signal for low density lipoprotein receptor-related protein.","date":"2000","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/10747918","citation_count":185,"is_preprint":false,"source_track":"gene2pubmed"}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":46031,"output_tokens":2627,"usd":0.088749},"stage2":{"model":"claude-opus-4-6","input_tokens":5945,"output_tokens":4127,"usd":0.19935},"total_usd":0.674086,"stage1_batch_id":"msgbatch_01Mgbec4vM6c9vXht7JVJzss","stage2_batch_id":"msgbatch_01JkuuH5RB2BdJNsgnhjbC6s","note":"batch pricing = 50% of standard","round2_stage1":{"model":"claude-sonnet-4-6","input_tokens":59155,"output_tokens":5543,"usd":0.130305},"round2_rules_fired":"R3","round2_stage2":{"model":"claude-opus-4-6","input_tokens":8581,"output_tokens":5102,"usd":0.255682}},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1994,\n      \"finding\": \"LRPAP1 (RAP) binds to gp330 (megalin) and LRP with high affinity in a Ca2+-dependent manner. Two separate gp330-binding sites were mapped on RAP between amino acids 85-148 and 178-248, and a heparin-binding site was mapped between amino acids 261-323, demonstrating that RAP's receptor-binding and heparin-binding domains are distinct.\",\n      \"method\": \"Ligand blotting, affinity chromatography, density-gradient sedimentation, direct binding studies on fusion proteins representing overlapping RAP domains\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — multiple orthogonal in vitro binding methods with domain-mapping mutagenesis in a single study\",\n      \"pmids\": [\"7512726\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"LRPAP1 (RAP) functions as a specialized chaperone for LDL receptor-related endocytic receptors (LRP and megalin). RAP-deficient mice generated by gene targeting show impaired folding and ER export of these receptors, and overexpression of the high-affinity ligand apoE dramatically reduces LRP surface expression in a RAP-preventable manner, demonstrating that RAP prevents premature ligand-induced aggregation and degradation of receptors in the ER.\",\n      \"method\": \"Gene targeting (RAP-knockout mice), LRP surface expression assay, apoE overexpression rescue experiment, embryonic fibroblast biochemical analysis\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — in vivo genetic knockout with mechanistic rescue experiment; replicated across multiple tissues and cell types in one study\",\n      \"pmids\": [\"8654360\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"LRPAP1 (RAP) is retained in the ER via its C-terminal HNEL sequence, which functions as an ER retention signal analogous to KDEL and interacts with ERD2 retention receptors. Overexpression of ERD2.1 or ERD2.2 significantly increases retention of HNEL-tagged proteins, and HNEL and KDEL signals compete for the same ERD2 receptors.\",\n      \"method\": \"Immunoelectron microscopy of stably transfected cells, pulse-chase labeling, immunolocalization of ERD2 proteins, saturation of ER retention by overexpression of HNEL/KDEL-tagged growth hormone\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — direct subcellular localization by immunoelectron microscopy combined with functional competition experiments and transient overexpression studies\",\n      \"pmids\": [\"9010785\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"LRPAP1 (RAP) efficiently crosses the blood-brain barrier (BBB) via saturable receptor-mediated transcytosis, primarily mediated by megalin (LRP2). [125I]-RAP transport into brain parenchyma was shown to be saturable and inhibited by excess RAP; 70% of transported RAP was found in brain parenchyma rather than vasculature. Megalin mediates most of the apical-to-basolateral transport in polarized epithelial cells.\",\n      \"method\": \"In vivo pharmacokinetics with [125I]-RAP in mice, in situ brain perfusion, capillary depletion of brain homogenates, transport assays in stably transfected MDCK cells\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal in vivo and in vitro methods demonstrating saturable megalin-mediated transcytosis\",\n      \"pmids\": [\"15383619\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"LRPAP1 (RAP) associates with megalin (gp330) and functions as a chaperone assisting in the folding of megalin in the endoplasmic reticulum and its transport to the cell surface. RAP is also an antigen in Heymann nephritis.\",\n      \"method\": \"Biochemical characterization, immunocytochemistry, cloning and sequencing\",\n      \"journal\": \"Journal of the American Society of Nephrology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — review/characterization paper summarizing experimental findings on RAP-megalin complex and ER function\",\n      \"pmids\": [\"7579068\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"LRPAP1 (RAP) acts as a molecular chaperone specifically designed to assist in biosynthesis and intracellular transport of endocytic receptors (LRP and megalin) by functioning as a receptor antagonist that prevents premature receptor-ligand association during ER-to-cell-surface transport.\",\n      \"method\": \"Review synthesizing biochemical and cell biological experimental evidence including ER localization studies and ligand binding inhibition assays\",\n      \"journal\": \"Biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — review consolidating experimental findings from multiple labs; underlying experiments described in PMID 8654360 support the mechanism\",\n      \"pmids\": [\"9792434\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Recombinant LRPAP1 (RAP) expressed in E. coli and purified by Ni-NTA affinity chromatography retains functional binding capacity to RAW264.7 cells expressing LDL receptor family members, confirming that RAP's receptor-binding activity is an intrinsic property of the protein.\",\n      \"method\": \"Prokaryotic expression, Ni-NTA affinity purification, cell binding assay with RAW264.7 macrophages\",\n      \"journal\": \"Journal of Huazhong University of Science and Technology\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — single lab, single binding assay with limited mechanistic detail\",\n      \"pmids\": [\"17393096\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Secreted LRPAP1 binds via its N-terminus to the extracellular domain of IFNAR1, triggering IFNAR1 ubiquitination and degradation. Viral proteases (SARS-CoV-2 3CLpro and EV71 2Apro) upregulate LRPAP1 expression and secretion, facilitating immune evasion. A small N-terminal LRPAP1 peptide is sufficient to cause IFNAR1 degradation and enhance viral infection, while α2M (an LRPAP1 inhibitor) stabilizes IFNAR1 and arrests viral infections.\",\n      \"method\": \"Co-immunoprecipitation, in vitro binding assays, viral infection models in vitro/ex vivo/in vivo (mouse brain), peptide competition assays, ubiquitination assays, LRPAP1 expression knockdown/overexpression\",\n      \"journal\": \"Signal transduction and targeted therapy\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (Co-IP, in vivo, ex vivo, peptide mapping, ubiquitination) demonstrating a novel extracellular mechanism\",\n      \"pmids\": [\"37743411\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"LRPAP1 is a frequent autoantigen recognized by B-cell receptors (BCRs) from mantle cell lymphomas (MCL). LRPAP1 induces proliferation via BCR pathway activation in MCL cells with LRPAP1-specific BCRs, and an LRPAP1-ETA' toxin conjugate specifically kills these cells.\",\n      \"method\": \"Recombinant BCR expression, antigen screening, proliferation assays, immunotoxin killing assay, serum antibody detection in MCL patients\",\n      \"journal\": \"Leukemia\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal approach (BCR expression + antigen identification + functional proliferation assay) in multiple patient samples and cell lines\",\n      \"pmids\": [\"29955130\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"RAP plays a role in the biogenesis of lipoprotein receptors by acting as a molecular chaperone within the early secretory pathway, preventing premature receptor-ligand interaction. Multiple non-exclusive models for RAP's mechanism as both a receptor antagonist and a chaperone were discussed based on experimental evidence.\",\n      \"method\": \"Review of biochemical evidence including chaperone assays and ligand binding inhibition studies\",\n      \"journal\": \"Trends in cardiovascular medicine\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — review paper; primary mechanistic evidence derived from other studies (PMID 8654360)\",\n      \"pmids\": [\"11239794\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"LRPAP1 (receptor-associated protein, RAP) is a specialized ER-resident chaperone that binds with high affinity (via two distinct sites on its surface) to nascent LDL receptor-related proteins (LRP1 and megalin/LRP2) in a Ca2+-dependent manner, preventing premature ligand-induced aggregation and ER retention of these receptors, thereby facilitating their proper folding and export to the cell surface; RAP is itself retained in the ER via its C-terminal HNEL signal acting through ERD2 receptors, can be transcytosed across the blood-brain barrier via megalin-dependent transport, and—when secreted extracellularly under viral protease-induced conditions—binds IFNAR1 to trigger its ubiquitination and degradation, thereby facilitating viral immune evasion.\"\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\nI'll systematically classify each paper before extracting discoveries.\n\n**Classification of retrieved papers:**\n\nPapers about LRPAP1 (the 39 kDa receptor-associated protein / RAP chaperone for LDL receptor family):\n- KEEP: [4], [5], [16], [37], [38], [45], [62], [71], [76], [94], [98], [21] (gene2pubmed [12]), [22] (gene2pubmed [23]), [24] (gene2pubmed [24]), [26] (gene2pubmed [26]), [27] (gene2pubmed [22])\n\nPapers about Rap GTPases (Ras superfamily small G proteins): EXCLUDE (alias collision — \"Rap\" as in Rap1/Rap2 GTPases is unrelated to LRPAP1/RAP chaperone protein)\n- Papers [1-3], [6-8], [10-15], [17-20], [23-26], [29-30], [33-36], [39-43], [49-50], [54], [59], [61], [63], [65-70], [80], [82-83], [85-86], [88-92], [95-97], [100]\n\nPapers about bacterial Rap phosphatases/proteins: EXCLUDE [9], [27], [28 (bacterial)], [46], [48], [81]\nPapers about Plasmodium/Babesia RAP-1 (rhoptry-associated protein): EXCLUDE [44], [47], [56], [57 wait — 57 is N-RAP], [58], [74], [75], [77]\nPapers about N-RAP (nebulin-related anchoring protein): EXCLUDE [31], [51], [53], [57], [79], [84], [92]\nPapers about MIA/CD-RAP (cartilage-derived): EXCLUDE [60], [87]\nPapers about RAP tag, RAP polysaccharide, RAP RNA method, RAP kinase, etc.: EXCLUDE [28], [32], [41], [52], [55], [72], [73], [78], [93], [99]\nPapers about KSHV RAP: EXCLUDE [20]\nPapers about bacterial rap/hor: EXCLUDE [22-retrieved]\n\n**Gene2pubmed papers:**\n- KEEP: [12] (39-kDa modulates LRP binding), [21] (primary structure), [22] (organ distribution), [23] (ER resident/chaperone), [24] (functional expression controlled by RAP), [25] (sortilin purified by RAP affinity), [26] (SorLA/SORL1 purified by RAP affinity), [27] (SRF/MYOCD regulate LRP - mentions RAP context but mainly about LRP), [30] (LRP endocytosis signal - RAP context)\n\nLet me now extract mechanistic discoveries from KEPT papers only:\n\n```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1991,\n      \"finding\": \"The 39-kDa receptor-associated protein (RAP/LRPAP1) binds directly to the 515-kDa subunit of LRP/alpha2-macroglobulin receptor and inhibits binding and uptake of both beta-migrating VLDL (apoE-enriched) and activated alpha2-macroglobulin, demonstrating that RAP modulates receptor ligand-binding activity.\",\n      \"method\": \"Recombinant fusion protein binding assay, ligand uptake inhibition assay in cells\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — direct binding with recombinant protein plus functional ligand inhibition, foundational paper >500 citations\",\n      \"pmids\": [\"1718973\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1991,\n      \"finding\": \"LRPAP1 (RAP) primary structure was determined by cDNA cloning, revealing a 323-residue mature protein with 73% identity to rat Heymann nephritis antigen and 77% identity to mouse HBP-44; pulse-chase experiments showed newly formed RAP remains cell-associated, and surface labeling showed RAP forms a complex with the alpha2M receptor heavy and light chains on the cell surface.\",\n      \"method\": \"cDNA cloning, pulse-chase radiolabeling, cell surface labeling and immunoprecipitation\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — multiple orthogonal methods in foundational paper; >200 citations\",\n      \"pmids\": [\"1712782\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1992,\n      \"finding\": \"RAP binds to two equivalent sites on LRP/alpha2MR with a KD of 14 nM; as RAP-binding sites are saturated, LRP completely loses the ability to bind ligands (Ki ~23–26 nM for inhibition of alpha2M binding), indicating that the number of RAP molecules bound dictates receptor activity. A model was proposed in which LRP contains multiple ligand-binding sites, each regulated by a separate RAP-binding site.\",\n      \"method\": \"Titration binding studies, heterologous ligand displacement assays with purified LRP and on human fibroblasts\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — quantitative binding with purified components, reconstitution-level rigor; >300 citations\",\n      \"pmids\": [\"1374383\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1994,\n      \"finding\": \"RAP contains two distinct gp330/megalin-binding sites (amino acids 85–148 and 178–248) and a separate heparin-binding site (amino acids 261–323), demonstrating that the gp330-binding, heparin-binding, and Heymann nephritis pathogenic epitope (aa 1–86) domains are functionally distinct.\",\n      \"method\": \"Direct binding studies using GST fusion proteins representing overlapping RAP domains, ligand blotting, affinity chromatography, density-gradient sedimentation, radiolabeled heparin binding\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — domain mapping with multiple orthogonal in vitro binding assays; >60 citations\",\n      \"pmids\": [\"7512726\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1994,\n      \"finding\": \"Immunohistochemical mapping in rat organs showed RAP is found in intracellular vesicles in all cells expressing gp330 or LRP, and is not definitively detected on the cell surface, indicating its major physiological function is intracellular (assembly/trafficking of receptors). RAP binding sites were found on cell surfaces where surface gp330 or LRP is expressed.\",\n      \"method\": \"Immunohistochemistry with organ distribution mapping; exogenous RAP binding experiments in vivo\",\n      \"journal\": \"The journal of histochemistry and cytochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — direct localization by immunohistochemistry across multiple tissues; >290 citations but localization-focused\",\n      \"pmids\": [\"7510321\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"RAP is an endoplasmic reticulum (ER) resident protein; the C-terminal tetrapeptide HNEL is both necessary and sufficient for ER retention. RAP interacts with LRP in vivo (demonstrated by cross-linking), the association is transient and occurs early in the secretory pathway, and dissociation at lower pH later in the pathway correlates with increased LRP ligand-binding activity. RAP functions as a molecular chaperone for LRP, preventing ligand-induced aggregation and maintaining LRP in a ligand-binding competent state.\",\n      \"method\": \"Metabolic labeling, cross-linking, pulse-chase analysis, C-terminal deletion/mutation analysis of HNEL signal, transfection of HNEL-tagged reporter proteins\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — multiple orthogonal methods including mutagenesis of retention signal, cross-linking, pulse-chase; >280 citations\",\n      \"pmids\": [\"7774585\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"Gene targeting (RAP-knockout mice) demonstrated that RAP is required for functional expression of LRP in vivo: RAP-deficient mice show reduced mature LRP in liver and brain, impaired hepatic clearance of alpha2-macroglobulin, and accumulation of remnant lipoproteins in plasma (in LDL receptor-null background), confirming RAP stabilizes LRP within the secretory pathway.\",\n      \"method\": \"Gene targeting/knockout mouse model, LRP immunoblotting, alpha2M clearance assay, plasma lipoprotein analysis\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean KO with defined in vivo functional phenotype; >240 citations\",\n      \"pmids\": [\"7538675\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"RAP was identified as a component of the Heymann nephritis antigenic complex; it associates with megalin (gp330) and functions as a chaperone assisting in megalin folding in the ER and its transport to the cell surface. Binding to megalin is Ca2+-dependent (also shown in Orlando & Farquhar 1994).\",\n      \"method\": \"Biochemical characterization, ligand blotting, immunocytochemistry, affinity chromatography\",\n      \"journal\": \"Journal of the American Society of Nephrology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple biochemical methods, consistent with parallel literature; >180 citations\",\n      \"pmids\": [\"7579068\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"RAP-deficient mice generated by gene targeting showed that RAP prevents premature binding of co-expressed ligands (e.g., apoE) to newly synthesized LRP and other LDL receptor family members in the ER, preventing ligand-induced aggregation and degradation. Overexpression of apoE dramatically reduced cellular LRP expression, an effect prevented by RAP co-expression, establishing RAP as a specialized chaperone class that selectively protects endocytic receptors.\",\n      \"method\": \"Gene targeting (RAP-null mice), apoE overexpression rescue experiments, LRP expression analysis, embryonic fibroblast studies\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — genetic rescue + overexpression experiments with molecular readouts; >245 citations\",\n      \"pmids\": [\"8654360\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"ERD2 proteins (ERD2.1 and ERD2.2, the mammalian KDEL receptors) mediate ER retention of RAP via its novel C-terminal HNEL signal. Overexpression of GH tagged with HNEL or KDEL both saturated ER retention receptors and caused secretion of endogenous RAP. Overexpression of either ERD2 isoform significantly increased the retention capacity for both KDEL- and HNEL-containing proteins, and functional LRP was reduced ~60% when intracellular RAP was decreased.\",\n      \"method\": \"Immunoelectron microscopy, stable transfection of GH-HNEL and GH-KDEL constructs, pulse-chase labeling, ERD2 overexpression, immunolocalization\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — multiple orthogonal methods including electron microscopy, mutagenesis, and overexpression with quantitative functional readout; >35 citations\",\n      \"pmids\": [\"9010785\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"RAP affinity chromatography from human brain extracts purified a novel ~95-kDa membrane glycoprotein (gp95/sortilin), demonstrating that LRPAP1 can be used as an affinity ligand to identify novel receptors beyond the canonical LDL receptor family, and that sortilin is a RAP-binding protein.\",\n      \"method\": \"RAP affinity chromatography, protein purification from human brain, cDNA cloning\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — affinity purification with functional validation; RAP used as biochemical tool to discover new binding partners\",\n      \"pmids\": [\"9013611\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"RAP affinity chromatography purified a novel ~250-kDa brain protein (sorLA-1/SORL1), a hybrid receptor containing both Vps10p-homologous domains and LDL receptor family repeats, demonstrating LRPAP1 binds this receptor and validating RAP as a tool to identify LDL receptor family-related proteins.\",\n      \"method\": \"RAP affinity chromatography from human brain, cDNA cloning, Northern blotting\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — affinity purification; RAP binding to SorLA established biochemically\",\n      \"pmids\": [\"8940146\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"RAP is defined as a novel class of molecular chaperone specifically designed to assist biosynthesis of endocytic receptors of the LDL receptor family; it acts as a receptor antagonist in the secretory pathway, preventing premature receptor-ligand association in cell types that co-express both receptor and ligand, thereby ensuring proper receptor export to the cell surface.\",\n      \"method\": \"Review synthesizing experimental evidence from multiple studies including RAP-null mice, ER localization, and ligand binding inhibition experiments\",\n      \"journal\": \"Biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — mechanistic synthesis of multiple experimental lines; consistent across multiple independent labs\",\n      \"pmids\": [\"9792434\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"RAP's role in biogenesis of lipoprotein receptors was established: RAP serves as a molecular chaperone assisting folding of lipoprotein receptors and their safe passage through the secretory pathway. Multiple non-exclusive models were proposed: (1) RAP competitively inhibits ligand binding by occupying ligand-binding sites, and (2) RAP may directly assist folding of the cysteine-rich ligand-binding repeats.\",\n      \"method\": \"Review of experimental evidence; biochemical studies of RAP-receptor interactions in secretory pathway\",\n      \"journal\": \"Trends in cardiovascular medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — review synthesizing established experimental data, no new primary data\",\n      \"pmids\": [\"11239794\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Blood-borne RAP crosses the blood-brain barrier (BBB) by a saturable transport process; ~70% of transported RAP localizes in brain parenchyma rather than vasculature. Transport across polarized epithelial cell monolayers (MDCK) is consistent with megalin (LRP2) mediating most apical-to-basolateral transcytosis. Inhibition by excess RAP and involvement of megalin indicate a specific saturable transport system at the BBB.\",\n      \"method\": \"[125I]-RAP pharmacokinetics in intact mice, in situ brain perfusion, capillary depletion of brain homogenates, transport assays in stably transfected polarized MDCK cell monolayers\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple in vivo and in vitro methods with saturation and competition controls; >100 citations\",\n      \"pmids\": [\"15383619\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Recombinant RAP was successfully expressed in E. coli using pT7-PL vector in BL21(DE3,plysS) strain with optimal conditions (chloramphenicol selection), and purified RAP retained functional binding to RAW264.7 macrophage cells (which are rich in LDL receptor family members), confirming that recombinant RAP binds LDL receptor family receptors.\",\n      \"method\": \"Prokaryotic expression in E. coli, Ni-NTA affinity purification, cell binding assay with RAW264.7 macrophages\",\n      \"journal\": \"Journal of Huazhong University of Science and Technology. Medical sciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct binding assay with purified recombinant protein; single lab, functional validation\",\n      \"pmids\": [\"17393096\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"LRPAP1-reactive B-cell receptors (BCRs) in mantle cell lymphoma (MCL) recognize LRPAP1 as a proliferation-inducing autoantigen; LRPAP1 stimulation activates the BCR signaling pathway in MCL cells. An LRPAP1-ETA' immunotoxin conjugate specifically killed MCL cells bearing LRPAP1-reactive BCRs, demonstrating that LRPAP1 can trigger BCR pathway signaling and be used for targeted therapy.\",\n      \"method\": \"Recombinant BCR expression, antigen screening, LRPAP1-induced proliferation assay, BCR pathway activation assay, immunotoxin killing assay\",\n      \"journal\": \"Leukemia\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — functional BCR activation and killing assays; novel mechanism but single lab\",\n      \"pmids\": [\"29955130\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Secreted LRPAP1 (upregulated by viral proteases SARS-CoV-2 3CLpro and EV71 2Apro) binds via its N-terminus to the extracellular domain of IFNAR1 (interferon-alpha/beta receptor 1), triggering IFNAR1 ubiquitination and degradation, thereby suppressing innate immune signaling and facilitating viral infection. A small N-terminal LRPAP1 peptide recapitulated IFNAR1 degradation and enhanced infection by multiple viruses (HSV-1, HBV, EV71, HCoV-OC43); alpha2M (a LRPAP1 inhibitor) stabilized IFNAR1 and arrested viral infections.\",\n      \"method\": \"Viral protease overexpression, ELISA/Western blot for LRPAP1 secretion, co-IP of LRPAP1 with IFNAR1 extracellular domain, IFNAR1 ubiquitination assay, viral infection assays in vitro/ex vivo (mouse brain)/in vivo (newborn mice), peptide competition, alpha2M inhibition\",\n      \"journal\": \"Signal transduction and targeted therapy\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — multiple orthogonal methods including co-IP, ubiquitination assay, in vivo validation, peptide/inhibitor rescue across multiple viral systems\",\n      \"pmids\": [\"37743411\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Genetic variation at the LRPAP1 locus (intron 5 insertion/deletion polymorphism) is associated with late-onset Alzheimer disease risk; homozygosity for the rare insertion allele was significantly underrepresented in patients vs. controls (OR=0.29), suggesting LRPAP1 variation modulates disease risk, consistent with its role in LRP-mediated amyloid-beta clearance pathway.\",\n      \"method\": \"Genotyping of LRPAP1 I/D polymorphism by PCR in 373 patients and 400 controls, logistic regression analysis\",\n      \"journal\": \"American journal of medical genetics\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 4 — genetic association study, no direct mechanistic experiment on LRPAP1 protein function\",\n      \"pmids\": [\"11425005\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"LRPAP1 (RAP) is a specialized ER-resident molecular chaperone (retained via its C-terminal HNEL signal through ERD2/KDEL receptors) that transiently associates with newly synthesized LRP and other LDL receptor family members (megalin/gp330, VLDLR, SorLA) in the early secretory pathway, preventing premature ligand-induced aggregation and degradation by competitively occupying ligand-binding sites; RAP-null mice show reduced mature LRP expression and impaired lipoprotein clearance; additionally, secreted LRPAP1 has a newly discovered extracellular function whereby its N-terminus binds IFNAR1 to trigger ubiquitination and degradation of the interferon receptor, facilitating viral immune evasion.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"LRPAP1 (receptor-associated protein, RAP) is an ER-resident molecular chaperone that ensures proper folding, ER export, and surface delivery of LDL receptor family members, principally LRP1 and megalin (LRP2). RAP binds these receptors with high affinity in a Ca²⁺-dependent manner via two discrete receptor-binding sites (residues 85–148 and 178–248), acting as an intracellular receptor antagonist that prevents premature ligand engagement and aggregation during biosynthetic transit [PMID:7512726, PMID:8654360]. RAP is itself retained in the ER through its C-terminal HNEL sequence, which engages ERD2 retrieval receptors in a mechanism analogous to KDEL-mediated retention [PMID:9010785], and can undergo megalin-dependent transcytosis across the blood–brain barrier [PMID:15383619]. When secreted extracellularly—a process induced by certain viral proteases—LRPAP1 binds IFNAR1 via its N-terminus, triggering IFNAR1 ubiquitination and degradation, thereby attenuating type-I interferon signaling and facilitating viral immune evasion [PMID:37743411].\",\n  \"teleology\": [\n    {\n      \"year\": 1994,\n      \"claim\": \"Mapping RAP's binding architecture resolved the question of how a single chaperone engages both receptors and heparin: two independent receptor-binding sites and a separate heparin-binding domain were identified, establishing RAP as a multi-domain ligand of the LDL receptor family.\",\n      \"evidence\": \"Ligand blotting, affinity chromatography, and density-gradient sedimentation with overlapping RAP fusion proteins\",\n      \"pmids\": [\"7512726\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Structural basis of Ca²⁺-dependent binding not determined\",\n        \"Whether the two receptor-binding sites engage distinct or overlapping receptor surfaces was unresolved\",\n        \"No in vivo functional evidence at this stage\"\n      ]\n    },\n    {\n      \"year\": 1996,\n      \"claim\": \"RAP-knockout mice demonstrated that RAP is required in vivo for proper folding and surface delivery of LRP and megalin, directly establishing RAP as a specialized ER chaperone rather than merely an in vitro binding partner.\",\n      \"evidence\": \"Gene-targeted RAP-null mice; apoE overexpression rescue experiments in embryonic fibroblasts\",\n      \"pmids\": [\"8654360\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Molecular mechanism of chaperone action (conformational stabilization vs. ligand shielding) not distinguished\",\n        \"Phenotypic consequences beyond receptor surface expression incompletely characterized\",\n        \"Whether RAP assists other LDL receptor family members besides LRP and megalin was unclear\"\n      ]\n    },\n    {\n      \"year\": 1997,\n      \"claim\": \"Discovery that RAP's C-terminal HNEL sequence functions as an ER-retention signal via ERD2 receptors explained how RAP is normally confined to the secretory pathway despite lacking a canonical KDEL motif.\",\n      \"evidence\": \"Immunoelectron microscopy, pulse-chase labeling, and HNEL/KDEL competition assays in stably transfected cells\",\n      \"pmids\": [\"9010785\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Relative affinity of HNEL vs. KDEL for ERD2 isoforms not quantified\",\n        \"Whether physiological conditions exist that saturate HNEL retrieval and permit RAP secretion was unknown\"\n      ]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Demonstration that RAP crosses the blood–brain barrier via saturable, megalin-mediated transcytosis revealed a functional role for RAP beyond the ER and suggested it could serve as a CNS drug-delivery vehicle.\",\n      \"evidence\": \"In vivo [125I]-RAP pharmacokinetics in mice, in situ brain perfusion, capillary depletion, and polarized MDCK transport assays\",\n      \"pmids\": [\"15383619\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Physiological function of RAP transcytosis across the BBB unknown\",\n        \"Whether transcytosed RAP retains chaperone or receptor-antagonist activity in brain parenchyma not tested\"\n      ]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Identification of LRPAP1 as a frequent autoantigen for mantle cell lymphoma BCRs, capable of driving BCR-dependent proliferation, uncovered a disease-relevant extracellular role for the protein.\",\n      \"evidence\": \"Recombinant BCR expression, antigen screening, proliferation assays, and immunotoxin killing in MCL patient samples and cell lines\",\n      \"pmids\": [\"29955130\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Whether LRPAP1 autoantigenic recognition drives lymphomagenesis or is secondary remains unresolved\",\n        \"Source of extracellular LRPAP1 in MCL patients not identified\",\n        \"Limited to MCL; generalizability to other B-cell malignancies untested\"\n      ]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Discovery that viral proteases induce LRPAP1 secretion, and that secreted LRPAP1 binds IFNAR1 to trigger its ubiquitination and degradation, revealed an entirely new extracellular function for LRPAP1 in innate immune evasion.\",\n      \"evidence\": \"Co-IP, in vivo/ex vivo viral infection models (SARS-CoV-2, EV71), peptide competition, ubiquitination assays, knockdown/overexpression\",\n      \"pmids\": [\"37743411\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"IFNAR1-binding interface on LRPAP1 N-terminus not structurally resolved\",\n        \"Whether LRPAP1-IFNAR1 interaction occurs under non-viral physiological conditions is unknown\",\n        \"Mechanism by which viral proteases increase LRPAP1 expression and secretion not delineated\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"It remains unclear how LRPAP1's intracellular chaperone function and its newly discovered extracellular roles (IFNAR1 degradation, MCL antigenicity) are coordinately regulated, and whether conditions beyond viral infection lead to physiologically relevant LRPAP1 secretion.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\n        \"No structural model of full-length RAP in complex with any receptor\",\n        \"Mechanisms governing the switch from ER-retained chaperone to secreted ligand not elucidated\",\n        \"Potential roles in additional receptor family members (e.g., VLDLR, ApoER2) incompletely characterized\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0044183\", \"supporting_discovery_ids\": [1, 2, 4]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [1, 7]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [1, 2, 4]},\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [7, 8]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"GO:0009609507\", \"supporting_discovery_ids\": [1, 2]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [7]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"LRP1\",\n      \"LRP2\",\n      \"IFNAR1\",\n      \"KDELR1\",\n      \"KDELR2\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```\n\nWait, I need to fix the pathway term_ids. \"GO:0009609507\" is not valid. Let me correct:\n\n```json\n{\n  \"mechanistic_narrative\": \"LRPAP1 (receptor-associated protein, RAP) is an ER-resident molecular chaperone that ensures proper folding, ER export, and surface delivery of LDL receptor family members, principally LRP1 and megalin (LRP2). RAP binds these receptors with high affinity in a Ca²⁺-dependent manner via two discrete receptor-binding sites (residues 85–148 and 178–248), acting as an intracellular receptor antagonist that prevents premature ligand engagement and aggregation during biosynthetic transit [PMID:7512726, PMID:8654360]. RAP is itself retained in the ER through its C-terminal HNEL sequence, which engages ERD2 retrieval receptors in a mechanism analogous to KDEL-mediated retention [PMID:9010785], and can undergo megalin-dependent transcytosis across the blood–brain barrier [PMID:15383619]. When secreted extracellularly—a process induced by certain viral proteases—LRPAP1 binds IFNAR1 via its N-terminus, triggering IFNAR1 ubiquitination and degradation, thereby attenuating type-I interferon signaling and facilitating viral immune evasion [PMID:37743411].\",\n  \"teleology\": [\n    {\n      \"year\": 1994,\n      \"claim\": \"Mapping RAP's binding architecture resolved the question of how a single chaperone engages both receptors and heparin: two independent receptor-binding sites and a separate heparin-binding domain were identified, establishing RAP as a multi-domain ligand of the LDL receptor family.\",\n      \"evidence\": \"Ligand blotting, affinity chromatography, and density-gradient sedimentation with overlapping RAP fusion proteins\",\n      \"pmids\": [\"7512726\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Structural basis of Ca²⁺-dependent binding not determined\",\n        \"Whether the two receptor-binding sites engage distinct or overlapping receptor surfaces was unresolved\",\n        \"No in vivo functional evidence at this stage\"\n      ]\n    },\n    {\n      \"year\": 1996,\n      \"claim\": \"RAP-knockout mice demonstrated that RAP is required in vivo for proper folding and surface delivery of LRP and megalin, directly establishing RAP as a specialized ER chaperone rather than merely an in vitro binding partner.\",\n      \"evidence\": \"Gene-targeted RAP-null mice; apoE overexpression rescue experiments in embryonic fibroblasts\",\n      \"pmids\": [\"8654360\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Molecular mechanism of chaperone action (conformational stabilization vs. ligand shielding) not distinguished\",\n        \"Phenotypic consequences beyond receptor surface expression incompletely characterized\",\n        \"Whether RAP assists other LDL receptor family members besides LRP and megalin was unclear\"\n      ]\n    },\n    {\n      \"year\": 1997,\n      \"claim\": \"Discovery that RAP's C-terminal HNEL sequence functions as an ER-retention signal via ERD2 receptors explained how RAP is normally confined to the secretory pathway despite lacking a canonical KDEL motif.\",\n      \"evidence\": \"Immunoelectron microscopy, pulse-chase labeling, and HNEL/KDEL competition assays in stably transfected cells\",\n      \"pmids\": [\"9010785\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Relative affinity of HNEL vs. KDEL for ERD2 isoforms not quantified\",\n        \"Whether physiological conditions exist that saturate HNEL retrieval and permit RAP secretion was unknown\"\n      ]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Demonstration that RAP crosses the blood–brain barrier via saturable, megalin-mediated transcytosis revealed a functional role for RAP beyond the ER and suggested it could serve as a CNS drug-delivery vehicle.\",\n      \"evidence\": \"In vivo [125I]-RAP pharmacokinetics in mice, in situ brain perfusion, capillary depletion, and polarized MDCK transport assays\",\n      \"pmids\": [\"15383619\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Physiological function of RAP transcytosis across the BBB unknown\",\n        \"Whether transcytosed RAP retains chaperone or receptor-antagonist activity in brain parenchyma not tested\"\n      ]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Identification of LRPAP1 as a frequent autoantigen for mantle cell lymphoma BCRs, capable of driving BCR-dependent proliferation, uncovered a disease-relevant extracellular role for the protein.\",\n      \"evidence\": \"Recombinant BCR expression, antigen screening, proliferation assays, and immunotoxin killing in MCL patient samples and cell lines\",\n      \"pmids\": [\"29955130\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Whether LRPAP1 autoantigenic recognition drives lymphomagenesis or is secondary remains unresolved\",\n        \"Source of extracellular LRPAP1 in MCL patients not identified\",\n        \"Limited to MCL; generalizability to other B-cell malignancies untested\"\n      ]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Discovery that viral proteases induce LRPAP1 secretion, and that secreted LRPAP1 binds IFNAR1 to trigger its ubiquitination and degradation, revealed an entirely new extracellular function for LRPAP1 in innate immune evasion.\",\n      \"evidence\": \"Co-IP, in vivo/ex vivo viral infection models (SARS-CoV-2, EV71), peptide competition, ubiquitination assays, knockdown/overexpression\",\n      \"pmids\": [\"37743411\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"IFNAR1-binding interface on LRPAP1 N-terminus not structurally resolved\",\n        \"Whether LRPAP1-IFNAR1 interaction occurs under non-viral physiological conditions is unknown\",\n        \"Mechanism by which viral proteases increase LRPAP1 expression and secretion not delineated\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"It remains unclear how LRPAP1's intracellular chaperone function and its newly discovered extracellular roles (IFNAR1 degradation, MCL antigenicity) are coordinately regulated, and whether conditions beyond viral infection lead to physiologically relevant LRPAP1 secretion.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\n        \"No structural model of full-length RAP in complex with any receptor\",\n        \"Mechanisms governing the switch from ER-retained chaperone to secreted ligand not elucidated\",\n        \"Potential roles in additional receptor family members (e.g., VLDLR, ApoER2) incompletely characterized\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0044183\", \"supporting_discovery_ids\": [1, 2, 4]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [1, 7]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [1, 2, 4]},\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [7, 8]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-9609507\", \"supporting_discovery_ids\": [1, 2]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [7]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"LRP1\",\n      \"LRP2\",\n      \"IFNAR1\",\n      \"KDELR1\",\n      \"KDELR2\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"mechanistic_narrative\": \"LRPAP1 (RAP) is a specialized endoplasmic reticulum-resident molecular chaperone that escorts newly synthesized members of the LDL receptor family—including LRP, megalin/gp330, SorLA, and VLDLR—through the secretory pathway by competitively occupying their ligand-binding sites, thereby preventing premature ligand-induced aggregation and degradation [PMID:7774585, PMID:8654360]. RAP binds LRP at two equivalent high-affinity sites (KD ~14 nM) and is retained in the ER via its C-terminal HNEL tetrapeptide recognized by ERD2/KDEL receptors; pH-dependent dissociation later in the secretory pathway restores receptor ligand-binding competence [PMID:1374383, PMID:9010785]. RAP-knockout mice exhibit reduced mature LRP in liver and brain, impaired alpha-2-macroglobulin clearance, and remnant lipoprotein accumulation, confirming its essential in vivo chaperone function [PMID:7538675, PMID:8654360]. Secreted LRPAP1 also has an extracellular role: its N-terminus binds IFNAR1, triggering ubiquitination-dependent receptor degradation that suppresses type I interferon signaling and facilitates viral immune evasion [PMID:37743411].\",\n  \"teleology\": [\n    {\n      \"year\": 1991,\n      \"claim\": \"Identification of RAP as a direct LRP-binding protein that modulates receptor ligand-binding activity established the first functional connection between this 39-kDa protein and lipoprotein receptor biology.\",\n      \"evidence\": \"Recombinant fusion protein binding assays and ligand uptake inhibition in cultured cells; concurrent cDNA cloning with pulse-chase and cell surface labeling\",\n      \"pmids\": [\"1718973\", \"1712782\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Binding stoichiometry and affinity not yet determined\",\n        \"Physiological relevance (intracellular vs. surface function) unresolved\",\n        \"No data on other LDL receptor family members\"\n      ]\n    },\n    {\n      \"year\": 1992,\n      \"claim\": \"Quantitative binding analysis revealed RAP occupies two equivalent high-affinity sites on LRP (KD ~14 nM), with progressive saturation of these sites completely ablating ligand binding, resolving how RAP stoichiometrically controls receptor activity.\",\n      \"evidence\": \"Titration binding and heterologous displacement assays with purified LRP on fibroblasts\",\n      \"pmids\": [\"1374383\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Structural basis of two-site binding unknown\",\n        \"Whether RAP acts inside the cell or at the surface still debated\"\n      ]\n    },\n    {\n      \"year\": 1994,\n      \"claim\": \"Domain mapping identified two distinct megalin/gp330-binding domains and a separate heparin-binding domain in RAP, demonstrating modular architecture that enables interaction with multiple receptors.\",\n      \"evidence\": \"GST fusion protein binding studies, ligand blotting, and heparin-binding assays\",\n      \"pmids\": [\"7512726\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Whether domain modularity dictates receptor selectivity in vivo unresolved\",\n        \"Structural basis of domain-receptor interactions not determined\"\n      ]\n    },\n    {\n      \"year\": 1995,\n      \"claim\": \"Three converging lines of evidence—ER localization via the HNEL retention signal, pH-dependent dissociation from LRP, and RAP-knockout mouse phenotype—established RAP as an ER-resident molecular chaperone essential for functional maturation and surface delivery of LDL receptor family members.\",\n      \"evidence\": \"C-terminal mutagenesis of HNEL, cross-linking and pulse-chase in the secretory pathway, gene-targeted RAP-null mice with reduced LRP and impaired alpha-2-macroglobulin clearance; parallel studies on megalin chaperoning\",\n      \"pmids\": [\"7774585\", \"7538675\", \"7579068\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Whether RAP directly assists receptor folding (vs. only blocking ligands) not distinguished\",\n        \"Structural basis of pH-dependent dissociation unknown\"\n      ]\n    },\n    {\n      \"year\": 1996,\n      \"claim\": \"Rescue experiments in RAP-null fibroblasts demonstrated that RAP prevents co-expressed ligands (apoE) from binding newly synthesized LRP in the ER, causing aggregation and degradation—defining RAP as a competitive antagonist-type chaperone specific to endocytic receptors. RAP affinity chromatography also identified SorLA (SORL1) as a new binding partner.\",\n      \"evidence\": \"ApoE overexpression with/without RAP co-expression in RAP-null embryonic fibroblasts; RAP affinity purification from human brain identifying SorLA\",\n      \"pmids\": [\"8654360\", \"8940146\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Full repertoire of RAP client receptors not mapped\",\n        \"Physiological consequences of SorLA–RAP interaction not tested in vivo\"\n      ]\n    },\n    {\n      \"year\": 1997,\n      \"claim\": \"The ER retention mechanism was resolved: ERD2 (KDEL receptor) isoforms recognize the non-canonical HNEL signal of RAP, and saturation of ERD2 capacity causes RAP secretion and consequent ~60% loss of functional LRP, quantifying the dependence of receptor biogenesis on the retrieval cycle.\",\n      \"evidence\": \"ERD2 overexpression, immunoelectron microscopy, pulse-chase with GH-HNEL/KDEL reporter constructs\",\n      \"pmids\": [\"9010785\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Whether ERD2 isoforms show preferential specificity for HNEL vs. KDEL in vivo not determined\",\n        \"Consequences of physiological RAP secretion not explored\"\n      ]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Circulating RAP was shown to cross the blood-brain barrier via a saturable, megalin-dependent transcytosis process, with the majority reaching brain parenchyma—revealing a potential extracellular trafficking function beyond ER chaperoning.\",\n      \"evidence\": \"Radiolabeled RAP pharmacokinetics in mice, in situ brain perfusion, capillary depletion, polarized MDCK transcytosis assays\",\n      \"pmids\": [\"15383619\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Physiological significance of blood-borne RAP in the brain not established\",\n        \"Whether BBB-transported RAP modulates neuronal LRP function in vivo unknown\"\n      ]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"A previously unrecognized extracellular function was discovered: secreted LRPAP1, upregulated by viral proteases, binds IFNAR1 via its N-terminus, triggers ubiquitination and degradation of the interferon receptor, and suppresses innate immunity—establishing LRPAP1 as a host factor exploited for viral immune evasion.\",\n      \"evidence\": \"Co-IP of LRPAP1 with IFNAR1, ubiquitination assays, peptide competition, alpha-2-macroglobulin rescue, multi-virus infection assays in vitro and in vivo (newborn mice)\",\n      \"pmids\": [\"37743411\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Whether IFNAR1 degradation by LRPAP1 occurs during non-infectious physiology unknown\",\n        \"Structural basis of LRPAP1 N-terminus–IFNAR1 interaction not determined\",\n        \"Whether other interferon receptor subunits are affected not tested\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include whether RAP directly assists folding of LDL receptor cysteine-rich repeats (beyond competitive ligand blocking), the structural basis of pH-dependent receptor dissociation, and the physiological scope of the newly discovered extracellular IFNAR1-degradation function outside viral infection contexts.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\n        \"No high-resolution structure of a full RAP–receptor complex exists\",\n        \"Direct folding chaperone activity vs. competitive shielding not experimentally separated\",\n        \"IFNAR1 degradation pathway not yet confirmed by independent laboratories\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0044183\", \"supporting_discovery_ids\": [5, 6, 7, 8, 12]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [0, 2, 17]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [4, 5, 9]},\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [14, 17]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"GO:0009609507\", \"supporting_discovery_ids\": [5, 6, 8, 9]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [5, 6, 8, 9]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [17]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [17]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"LRP1\", \"LRP2\", \"SORL1\", \"IFNAR1\", \"SORT1\", \"KDELR1\", \"KDELR2\"],\n    \"other_free_text\": []\n  }\n}\n```\n\nWait, I need to fix the pathway term_id. \"GO:0009609507\" is not valid — I should use \"R-HSA-9609507\" for Protein localization. Let me correct:\n\n```json\n{\n  \"mechanistic_narrative\": \"LRPAP1 (RAP) is a specialized endoplasmic reticulum-resident molecular chaperone that escorts newly synthesized members of the LDL receptor family—including LRP, megalin/gp330, SorLA, and VLDLR—through the secretory pathway by competitively occupying their ligand-binding sites, thereby preventing premature ligand-induced aggregation and degradation [PMID:7774585, PMID:8654360]. RAP binds LRP at two equivalent high-affinity sites (KD ~14 nM) and is retained in the ER via its C-terminal HNEL tetrapeptide recognized by ERD2/KDEL receptors; pH-dependent dissociation later in the secretory pathway restores receptor ligand-binding competence [PMID:1374383, PMID:9010785]. RAP-knockout mice exhibit reduced mature LRP in liver and brain, impaired alpha-2-macroglobulin clearance, and remnant lipoprotein accumulation, confirming its essential in vivo chaperone function [PMID:7538675, PMID:8654360]. Secreted LRPAP1 also has an extracellular role: its N-terminus binds IFNAR1, triggering ubiquitination-dependent receptor degradation that suppresses type I interferon signaling and facilitates viral immune evasion [PMID:37743411].\",\n  \"teleology\": [\n    {\n      \"year\": 1991,\n      \"claim\": \"Identification of RAP as a direct LRP-binding protein that modulates receptor ligand-binding activity established the first functional connection between this 39-kDa protein and lipoprotein receptor biology.\",\n      \"evidence\": \"Recombinant fusion protein binding assays and ligand uptake inhibition in cultured cells; concurrent cDNA cloning with pulse-chase and cell surface labeling\",\n      \"pmids\": [\"1718973\", \"1712782\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Binding stoichiometry and affinity not yet determined\",\n        \"Physiological relevance (intracellular vs. surface function) unresolved\",\n        \"No data on other LDL receptor family members\"\n      ]\n    },\n    {\n      \"year\": 1992,\n      \"claim\": \"Quantitative binding analysis revealed RAP occupies two equivalent high-affinity sites on LRP (KD ~14 nM), with progressive saturation completely ablating ligand binding, resolving how RAP stoichiometrically controls receptor activity.\",\n      \"evidence\": \"Titration binding and heterologous displacement assays with purified LRP on fibroblasts\",\n      \"pmids\": [\"1374383\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Structural basis of two-site binding unknown\",\n        \"Whether RAP acts inside the cell or at the surface still debated\"\n      ]\n    },\n    {\n      \"year\": 1994,\n      \"claim\": \"Domain mapping identified two distinct megalin-binding domains and a separate heparin-binding domain in RAP, demonstrating modular architecture enabling interaction with multiple receptor family members.\",\n      \"evidence\": \"GST fusion protein binding studies, ligand blotting, and heparin-binding assays\",\n      \"pmids\": [\"7512726\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Whether domain modularity dictates receptor selectivity in vivo unresolved\",\n        \"Structural basis of domain-receptor interactions not determined\"\n      ]\n    },\n    {\n      \"year\": 1995,\n      \"claim\": \"Three converging lines of evidence—ER localization via the HNEL retention signal, pH-dependent dissociation from LRP, and RAP-knockout mouse phenotype—established RAP as an ER-resident molecular chaperone essential for functional maturation and surface delivery of LDL receptor family members.\",\n      \"evidence\": \"C-terminal mutagenesis of HNEL, cross-linking and pulse-chase in the secretory pathway, gene-targeted RAP-null mice with reduced LRP and impaired alpha-2-macroglobulin clearance; parallel studies on megalin chaperoning\",\n      \"pmids\": [\"7774585\", \"7538675\", \"7579068\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Whether RAP directly assists receptor folding vs. only blocking ligands not distinguished\",\n        \"Structural basis of pH-dependent dissociation unknown\"\n      ]\n    },\n    {\n      \"year\": 1996,\n      \"claim\": \"Rescue experiments in RAP-null fibroblasts demonstrated that RAP prevents co-expressed ligands (apoE) from binding newly synthesized LRP in the ER, causing aggregation and degradation—defining RAP as a competitive antagonist-type chaperone. RAP affinity chromatography also identified SorLA/SORL1 as a new binding partner.\",\n      \"evidence\": \"ApoE overexpression with/without RAP co-expression in RAP-null embryonic fibroblasts; RAP affinity purification from human brain identifying SorLA\",\n      \"pmids\": [\"8654360\", \"8940146\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Full repertoire of RAP client receptors not mapped\",\n        \"Physiological consequences of SorLA-RAP interaction not tested in vivo\"\n      ]\n    },\n    {\n      \"year\": 1997,\n      \"claim\": \"The ER retention mechanism was resolved: ERD2/KDEL receptor isoforms recognize RAP's non-canonical HNEL signal, and saturation of ERD2 capacity causes RAP secretion and ~60% loss of functional LRP, quantifying the dependence of receptor biogenesis on the retrieval cycle. Sortilin was also identified as a RAP-binding partner.\",\n      \"evidence\": \"ERD2 overexpression, immunoelectron microscopy, pulse-chase with GH-HNEL/KDEL reporter constructs; RAP affinity chromatography from brain\",\n      \"pmids\": [\"9010785\", \"9013611\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Whether ERD2 isoforms show preferential specificity for HNEL vs. KDEL in vivo not determined\",\n        \"Physiological significance of RAP-sortilin interaction unknown\"\n      ]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Circulating RAP was shown to cross the blood-brain barrier via a saturable, megalin-dependent transcytosis process, revealing a potential extracellular trafficking function beyond ER chaperoning.\",\n      \"evidence\": \"Radiolabeled RAP pharmacokinetics in mice, in situ brain perfusion, capillary depletion, polarized MDCK transcytosis assays\",\n      \"pmids\": [\"15383619\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Physiological significance of blood-borne RAP in the brain not established\",\n        \"Whether BBB-transported RAP modulates neuronal LRP function in vivo unknown\"\n      ]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"A previously unrecognized extracellular function was discovered: secreted LRPAP1, upregulated by viral proteases, binds IFNAR1 via its N-terminus, triggers receptor ubiquitination and degradation, and suppresses innate immunity—establishing LRPAP1 as a host factor exploited for viral immune evasion.\",\n      \"evidence\": \"Co-IP of LRPAP1 with IFNAR1, ubiquitination assays, peptide competition, alpha-2-macroglobulin rescue, multi-virus infection assays in vitro and in vivo\",\n      \"pmids\": [\"37743411\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Whether IFNAR1 degradation by LRPAP1 occurs during non-infectious physiology unknown\",\n        \"Structural basis of LRPAP1 N-terminus-IFNAR1 interaction not determined\",\n        \"Whether other interferon receptor subunits are affected not tested\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include whether RAP directly assists folding of LDL receptor cysteine-rich repeats beyond competitive ligand blocking, the structural basis of pH-dependent receptor dissociation, and the physiological scope of the extracellular IFNAR1-degradation function outside viral infection contexts.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\n        \"No high-resolution structure of a full RAP-receptor complex exists\",\n        \"Direct folding chaperone activity vs. competitive shielding not experimentally separated\",\n        \"IFNAR1 degradation pathway not yet confirmed by independent laboratories\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0044183\", \"supporting_discovery_ids\": [5, 6, 7, 8, 12]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [0, 2, 17]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [4, 5, 9]},\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [14, 17]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-9609507\", \"supporting_discovery_ids\": [5, 6, 8, 9]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [5, 6, 8, 9]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [17]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [17]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"LRP1\", \"LRP2\", \"SORL1\", \"IFNAR1\", \"SORT1\", \"KDELR1\", \"KDELR2\"],\n    \"other_free_text\": []\n  }\n}\n```"}