{"gene":"LRPAP1","run_date":"2026-06-10T02:59:50","timeline":{"discoveries":[{"year":1994,"finding":"RAP (LRPAP1) binds to gp330 (megalin/LRP2) and LRP at two distinct sites (amino acids 85-148 and 178-248 of RAP), with binding being Ca2+-dependent. A separate heparin-binding site was mapped to amino acids 261-323. These domains are distinct from the Heymann nephritis pathogenic epitope (amino acids 1-86).","method":"Direct binding studies (ligand blotting, affinity chromatography, density-gradient sedimentation) using fusion proteins representing overlapping RAP domains; radiolabeled heparin binding assays","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 / Strong — multiple orthogonal in vitro binding assays with domain-mapping mutagenesis in a focused mechanistic study","pmids":["7512726"],"is_preprint":false},{"year":1993,"finding":"In renal proximal tubule cells, alpha2-MRAP (LRPAP1) is located predominantly in the rough endoplasmic reticulum (RER). Alpha2-MRAP-binding sites are present on gp330 at the proximal tubule brush border. The apparent brush border localization seen in snap-frozen sections is artifactual, resulting from release of RER-resident LRPAP1 during tissue processing and subsequent binding to cell-surface gp330.","method":"Immunocytochemistry with specific anti-alpha2-MRAP antibodies on PLP-fixed vs. snap-frozen rat kidney sections; alpha2-MRAP-IgG fusion protein binding assays; controlled incubation experiments showing redistribution artifact","journal":"European journal of cell biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — direct subcellular localization with functional consequence demonstrated by controlled artifact experiments and multiple tissue-processing conditions","pmids":["8223699"],"is_preprint":false},{"year":1995,"finding":"RAP (LRPAP1) functions as a molecular chaperone assisting in the folding of megalin (gp330) in the endoplasmic reticulum and its transport to the cell surface, while also acting as a universal inhibitor of ligand binding to LDL receptor family members at the cell surface.","method":"Review synthesizing biochemical, immunocytochemical, and functional data from the field; primary experimental evidence includes co-purification, ligand binding inhibition assays, and ER localization studies","journal":"Journal of the American Society of Nephrology : JASN","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — synthesis of multiple experimental findings from the field, but this paper itself is a review; underlying primary data from multiple labs supports the chaperone function","pmids":["7579068"],"is_preprint":false},{"year":1996,"finding":"RAP (LRPAP1) binds calmodulin in a Ca2+- and pH-dependent manner and is a substrate for calmodulin-dependent kinase II (CaM-kinase II), which phosphorylates serine residues in RAP domains D2 (amino acids 113-218) and D3 (amino acids 219-323). Calcineurin dephosphorylates RAP. RAP is also phosphorylated by casein kinase II, and phosphorylation of RAP in vivo was demonstrated in 32P-labeled cell cultures.","method":"Calmodulin binding assays; in vitro kinase assays with CaM-kinase II and casein kinase II; phosphatase (calcineurin) assays; 32P-labeling of cell cultures to demonstrate in vivo phosphorylation; domain mapping with D2 and D3 fragments","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1 / Strong — reconstituted in vitro kinase/phosphatase assays with domain mapping, confirmed in vivo with 32P labeling, multiple orthogonal methods in one study","pmids":["8861945"],"is_preprint":false},{"year":2001,"finding":"RAP (LRPAP1) serves as a molecular chaperone/escort protein for LDL receptor family members in the endoplasmic reticulum, preventing premature ligand interactions during receptor transport through the secretory pathway, and also promotes proper receptor folding independently of its ligand-inhibitory function.","method":"Review of experimental evidence including co-purification, ligand binding inhibition assays, secretory pathway transport studies, and ER localization experiments from multiple labs","journal":"International review of cytology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — review synthesizing replicated experimental findings, but paper is itself a review; underlying data from multiple labs independently established these functions","pmids":["11580203"],"is_preprint":false},{"year":2004,"finding":"RAP (LRPAP1) crosses the blood-brain barrier via a saturable transport system, with megalin (LRP2) mediating most apical-to-basolateral transcytosis across polarized epithelial cells. Approximately 70% of blood-borne RAP reaches brain parenchyma (not just vasculature). Transport is inhibitable by excess RAP, confirming receptor-mediated transcytosis.","method":"Pharmacokinetic studies with [125I]-RAP in intact mice; in situ brain perfusion; capillary depletion of brain homogenates; transport assays in stably transfected MDCK cells expressing megalin","journal":"Journal of cell science","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal in vivo and in vitro approaches, saturation kinetics, mechanistic cell line validation, capillary depletion to confirm parenchymal distribution","pmids":["15383619"],"is_preprint":false},{"year":2007,"finding":"The three-dimensional structure of RAP (LRPAP1) consists of three independent three-helix bundles (domains 1, 2, and 3) connected by long, flexible linkers. This architecture allows RAP to adopt various conformations when interacting with LDL receptor family members.","method":"NMR spectroscopy to determine domain 2 structure; small-angle neutron scattering (SANS) combined with simulated annealing protocol to characterize overall RAP architecture","journal":"Protein science : a publication of the Protein Society","confidence":"High","confidence_rationale":"Tier 1 / Strong — NMR structure determination combined with SANS experimental data; rigorous structural study with functional context","pmids":["17656581"],"is_preprint":false},{"year":2001,"finding":"RAP (LRPAP1) inhibits transthyretin (TTR) internalization in hepatocytes, and the ~90 kDa TTR-receptor complex is RAP-sensitive, indicating that TTR is internalized via an as-yet unidentified LDL receptor family member in liver cells. This RAP-sensitive pathway was confirmed in both hepatoma cell lines and primary hepatocytes.","method":"Uptake competition assays with RAP; chemical cross-linking of [125I]-TTR to identify receptor complex; primary hepatocyte and hepatoma cell line comparison; lipid-deficient serum controls","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple cell systems tested, cross-linking and competition assays, but receptor identity not established; single lab","pmids":["11278770"],"is_preprint":false},{"year":1996,"finding":"RAP (LRPAP1) mediates RAP-sensitive and proteoglycan-mediated pathways of lipoprotein lipase (LPL) degradation in adipocytes. The RAP-sensitive (LRP-mediated) component accounts for 30-54% of LPL degradation at 1 h, diminishing to 19-23% at 4 h, while heparan sulfate proteoglycans mediate the major non-RAP-sensitive fraction.","method":"125I-LPL degradation assays in BFC-1β adipocytes with RAP inhibition; heparinase/heparitinase treatment; phosphatidylinositol-specific phospholipase C treatment; 35SO4 proteoglycan labeling to measure turnover kinetics","journal":"Journal of lipid research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal inhibition and labeling approaches in a single lab, but functional consequence (pathway identification) rather than direct molecular mechanism","pmids":["8978495"],"is_preprint":false},{"year":2012,"finding":"RAP (LRPAP1) treatment of neurons potentiates the inhibitory effects of full-length APP on axonal outgrowth, and RAP-LRP binding stimulates axon growth independently of APP, associated with ERK2 activation. This demonstrates that LRPAP1 engagement of LRP has APP-independent signaling consequences in neurons.","method":"Neurite outgrowth measurements in wild-type vs. APP knockout neurons treated with sAPPα and/or RAP; ERK2 activation measured; 1 day in vitro cultures","journal":"Journal of neurochemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — clean genetic KO comparison with pharmacological RAP treatment and ERK2 readout, but single lab, single study","pmids":["23061396"],"is_preprint":false},{"year":2023,"finding":"Secreted LRPAP1 binds the extracellular domain of IFNAR1 (interferon alpha/beta receptor 1) via its N-terminus, triggering IFNAR1 ubiquitination and degradation, thereby suppressing interferon signaling and facilitating viral immune evasion. Viral proteases (SARS-CoV-2 3CLpro and EV71 2Apro) upregulate LRPAP1 expression and secretion to exploit this mechanism. A small N-terminal LRPAP1 peptide recapitulates IFNAR1 degradation, and α2M (a LRPAP1 inhibitor) stabilizes IFNAR1 and arrests viral infections.","method":"Binding assays between secreted LRPAP1 and IFNAR1 extracellular domain; ubiquitination and degradation assays in vitro, ex vivo (mouse brain), and in vivo (newborn mice); viral infection assays with multiple viruses (HSV-1, HBV, EV71, HCoV-OC43); N-terminal peptide experiments; α2M inhibition experiments; protease upregulation assays","journal":"Signal transduction and targeted therapy","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — multiple orthogonal methods across in vitro, ex vivo, and in vivo systems; domain mapping with N-terminal peptide; inhibitor validation; tested across multiple viral systems; identifies novel mechanistic role for secreted LRPAP1","pmids":["37743411"],"is_preprint":false},{"year":2022,"finding":"Loss of lrpap1 in zebrafish (CRISPR/Cas9 frameshift mutation) causes myopia with increased axial length and disordered scleral collagen fibers. The mechanism involves activation of TGF-β signaling and increased apoptosis. TGF-β agonist treatment of wild-type embryos recapitulates the apoptosis phenotype, while TGF-β inhibitor treatment mitigates apoptosis in lrpap1 mutants, establishing TGF-β-induced apoptosis as the downstream pathway.","method":"CRISPR/Cas9-generated zebrafish lrpap1 knockout; refractive error measurement; transmission electron microscopy of scleral collagen; RNA sequencing and bioinformatics; acridine orange and TUNEL staining; TGF-β agonist and inhibitor rescue experiments","journal":"Cell communication and signaling : CCS","confidence":"High","confidence_rationale":"Tier 2 / Strong — clean genetic KO in vertebrate model, multiple orthogonal readouts (structural, molecular, transcriptomic), bidirectional pharmacological rescue of the TGF-β pathway, single lab","pmids":["36261846"],"is_preprint":false},{"year":1994,"finding":"The human LRPAP1 gene was chromosomally mapped to the short arm of chromosome 4 at position 4p16.3, in the region of the chromosomal deletion causing Wolf-Hirschhorn syndrome, by fluorescence in situ hybridization.","method":"Fluorescence in situ hybridization (FISH) using human RAP cDNA probe","journal":"Genomics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct FISH mapping, technically sound, but chromosomal localization alone (no functional consequence linked)","pmids":["7959795"],"is_preprint":false},{"year":2014,"finding":"A homozygous frameshift mutation in LRPAP1 (c.199delC, p.Q67Sfs*8) co-segregates with early-onset high myopia in a consanguineous family, providing genetic evidence that LRPAP1 loss-of-function causes autosomal recessive high myopia.","method":"Whole exome sequencing of 298 early-onset high myopia patients; Sanger sequencing confirmation; co-segregation analysis in family members; absence in 192 healthy controls","journal":"Investigative ophthalmology & visual science","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — genetic co-segregation in a human family with bioinformatic pathogenicity prediction, replicated from prior reports; no direct functional mechanistic assay performed in this study","pmids":["25525168"],"is_preprint":false},{"year":2018,"finding":"LRPAP1 acts as a B-cell receptor (BCR) antigen that induces proliferation via BCR pathway activation in a subset of mantle cell lymphomas (MCL). An LRPAP1-ETA' toxin conjugate specifically kills MCL cells bearing LRPAP1-specific BCRs, demonstrating that LRPAP1-BCR interaction drives chronic autoantigenic BCR signaling.","method":"Recombinant expression of BCRs from 21 MCL patients and 7 cell lines; antigen screening; LRPAP1 stimulation of BCR pathway activation; LRPAP1-ETA' immunotoxin killing assays; serum antibody characterization","journal":"Leukemia","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional BCR activation and targeted cytotoxicity assays confirm antigen-receptor interaction drives signaling; single lab, novel mechanistic finding","pmids":["29955130"],"is_preprint":false}],"current_model":"LRPAP1 (RAP) is a 39-kDa ER-resident molecular chaperone whose three-helix-bundle domains (connected by flexible linkers) bind Ca2+-dependently to at least two sites on LDL receptor family members (LRP1, LRP2/megalin), preventing premature ligand interactions during secretory pathway transit while promoting proper receptor folding; it is itself regulated by CaM-kinase II phosphorylation and calmodulin binding, mediates transcytosis across the blood-brain barrier via megalin, and in a newly discovered extracellular role, the secreted form binds IFNAR1 to trigger its ubiquitination and degradation, facilitating viral immune evasion; loss of LRPAP1 in zebrafish causes myopia through TGF-β-induced scleral apoptosis, and in humans its frameshift mutations cause autosomal recessive high myopia."},"narrative":{"mechanistic_narrative":"LRPAP1 (RAP) is a Ca2+-dependent molecular chaperone of the LDL receptor family that escorts receptor folding and traffic through the secretory pathway while acting as a universal inhibitor of premature ligand binding [PMID:7579068, PMID:11580203]. It binds megalin (LRP2/gp330) and LRP1 at two distinct sites (residues 85-148 and 178-248) with a separate heparin-binding region (residues 261-323), interactions that are strictly Ca2+-dependent [PMID:7512726]. Structurally it is built from three independent three-helix bundles joined by flexible linkers, an architecture that permits multiple conformations during receptor engagement [PMID:17656581], and it resides predominantly in the rough endoplasmic reticulum of epithelial cells [PMID:8223699]. Its activity is further tuned by calmodulin binding and phosphorylation: CaM-kinase II and casein kinase II phosphorylate serines in its D2 and D3 domains, and calcineurin reverses this modification [PMID:8861945]. Beyond its intracellular escort role, LRPAP1 modulates receptor-mediated handling of multiple ligands—mediating saturable, megalin-dependent transcytosis across the blood-brain barrier [PMID:15383619] and engaging LRP to drive APP-independent, ERK2-linked signaling in neurons [PMID:23061396]. A secreted form binds the IFNAR1 extracellular domain through its N-terminus to trigger IFNAR1 ubiquitination and degradation, a mechanism viral proteases exploit to suppress interferon signaling [PMID:37743411]. Loss-of-function frameshift mutations in LRPAP1 cause autosomal recessive early-onset high myopia in humans [PMID:25525168], and zebrafish lrpap1 knockout reproduces myopia through TGF-β-induced scleral apoptosis [PMID:36261846].","teleology":[{"year":1994,"claim":"Defining where and how RAP physically engages LDL receptor family members was needed to explain its inhibitory and escort activities; mapping resolved two distinct Ca2+-dependent receptor-binding sites plus a separate heparin site.","evidence":"Domain-mapped in vitro binding (ligand blotting, affinity chromatography, sedimentation) with overlapping RAP fusion fragments and radiolabeled heparin assays","pmids":["7512726"],"confidence":"High","gaps":["Does not define stoichiometry or order of the two-site engagement in vivo","Functional consequence of the heparin-binding site not resolved"]},{"year":1993,"claim":"The subcellular site of RAP action was uncertain; controlled fixation experiments established RER residency and showed that apparent brush-border localization was a processing artifact, anchoring its chaperone role to the ER.","evidence":"Immunocytochemistry on PLP-fixed vs. snap-frozen rat kidney with controlled redistribution experiments and fusion-protein binding","pmids":["8223699"],"confidence":"High","gaps":["Does not address how RAP dissociates from receptors after ER transit","Secreted/extracellular pool not characterized here"]},{"year":1995,"claim":"Synthesis of field data assigned RAP a dual function as an ER folding chaperone for megalin and a surface ligand-binding inhibitor, framing its role in receptor biogenesis.","evidence":"Review integrating co-purification, ligand-inhibition, and ER localization data","pmids":["7579068"],"confidence":"Medium","gaps":["Review-level synthesis rather than new primary data","Mechanism separating folding from inhibition not resolved"]},{"year":1996,"claim":"Whether RAP itself is regulated was unknown; reconstituted assays showed it binds calmodulin Ca2+/pH-dependently and is phosphorylated on D2/D3 serines by CaM-kinase II and casein kinase II, reversed by calcineurin.","evidence":"In vitro kinase/phosphatase and calmodulin-binding assays with domain fragments, confirmed by 32P labeling in cells","pmids":["8861945"],"confidence":"High","gaps":["Functional effect of phosphorylation on receptor binding not established","Cellular trigger for phosphorylation unknown"]},{"year":1996,"claim":"RAP's role in lipoprotein metabolism was addressed by quantifying its contribution to LPL clearance, distinguishing an LRP-mediated RAP-sensitive route from a dominant proteoglycan route.","evidence":"125I-LPL degradation assays in adipocytes with RAP inhibition, heparinase/PI-PLC treatment, and 35SO4 proteoglycan labeling","pmids":["8978495"],"confidence":"Medium","gaps":["Identifies pathway contribution rather than direct molecular mechanism","Single lab, single cell type"]},{"year":2001,"claim":"RAP-sensitive ligand handling was extended to transthyretin, implicating an unidentified LDL-receptor-family member in hepatic TTR internalization.","evidence":"Uptake competition with RAP and cross-linking of 125I-TTR in hepatoma lines and primary hepatocytes","pmids":["11278770"],"confidence":"Medium","gaps":["Receptor identity not established","Single lab"]},{"year":2001,"claim":"A second review consolidated the escort-protein model, asserting RAP prevents premature ligand interactions and promotes folding independently of ligand inhibition.","evidence":"Review of co-purification, transport, and ER localization data","pmids":["11580203"],"confidence":"Medium","gaps":["Review synthesis, no new primary data","Separation of folding vs. inhibition functions not directly tested here"]},{"year":2004,"claim":"Whether RAP could move across barriers was tested; pharmacokinetic and cell-based assays established saturable, megalin-mediated transcytosis delivering RAP to brain parenchyma.","evidence":"125I-RAP pharmacokinetics, in situ brain perfusion, capillary depletion, and megalin-transfected MDCK transport assays","pmids":["15383619"],"confidence":"High","gaps":["Physiological cargo/relevance of RAP transcytosis unclear","Does not address fate of RAP in parenchyma"]},{"year":2007,"claim":"The structural basis for RAP's conformational versatility was resolved as three independent three-helix bundles connected by flexible linkers.","evidence":"NMR structure of domain 2 plus SANS with simulated annealing for overall architecture","pmids":["17656581"],"confidence":"High","gaps":["No co-structure with a receptor","Conformational states during binding not directly visualized"]},{"year":2012,"claim":"RAP-LRP engagement was shown to have signaling output, stimulating axon growth via ERK2 independently of APP while potentiating APP's inhibitory effect.","evidence":"Neurite outgrowth in WT vs. APP-knockout neurons with RAP/sAPPa treatment and ERK2 readout","pmids":["23061396"],"confidence":"Medium","gaps":["Receptor mediating the ERK2 response not pinpointed","Single lab, single study"]},{"year":2014,"claim":"A causal human disease link was established by finding a homozygous LRPAP1 frameshift co-segregating with autosomal recessive early-onset high myopia.","evidence":"Whole-exome sequencing of high-myopia patients, Sanger confirmation, family co-segregation, control absence","pmids":["25525168"],"confidence":"Medium","gaps":["No functional assay in this study","Mechanism linking LRPAP1 loss to myopia not addressed here"]},{"year":2018,"claim":"An unexpected role as a B-cell receptor autoantigen was demonstrated, with LRPAP1 driving chronic BCR signaling in a subset of mantle cell lymphomas.","evidence":"Recombinant BCR expression from MCL patients/cell lines, antigen screening, BCR activation, and LRPAP1-ETA' immunotoxin killing","pmids":["29955130"],"confidence":"Medium","gaps":["Mechanism of LRPAP1 autoantigen exposure unknown","Single lab"]},{"year":2022,"claim":"The mechanism behind LRPAP1-linked myopia was defined in zebrafish: lrpap1 loss activates TGF-β signaling and scleral apoptosis, with bidirectional pharmacological rescue confirming the pathway.","evidence":"CRISPR lrpap1 knockout zebrafish with refraction, scleral EM, RNA-seq, TUNEL, and TGF-β agonist/inhibitor rescue","pmids":["36261846"],"confidence":"High","gaps":["Direct molecular link between RAP loss and TGF-β activation not defined","Mammalian confirmation absent"]},{"year":2023,"claim":"A secreted extracellular function was uncovered: LRPAP1 binds IFNAR1 via its N-terminus to drive its ubiquitination and degradation, a route co-opted by viral proteases for immune evasion.","evidence":"Binding, ubiquitination, and degradation assays in vitro/ex vivo/in vivo, N-terminal peptide mapping, a2M inhibition, and multi-virus infection assays","pmids":["37743411"],"confidence":"High","gaps":["Ubiquitin ligase mediating IFNAR1 degradation not identified","How secreted LRPAP1 traffics to IFNAR1 unresolved"]},{"year":null,"claim":"How LRPAP1's intracellular chaperone role and its secreted extracellular signaling/degradation roles are coordinated, and which receptors mediate its diverse phenotypes, remain unresolved.","evidence":"No single study integrates ER chaperone, transcytosis, neuronal signaling, IFNAR1 degradation, and myopia mechanisms","pmids":[],"confidence":"Medium","gaps":["No co-structure of RAP with any receptor","E3 ligase for IFNAR1 unknown","Direct mechanistic bridge from RAP loss to scleral TGF-β activation undefined"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0044183","term_label":"protein folding chaperone","supporting_discovery_ids":[2,4]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[0,2,10]},{"term_id":"GO:0140313","term_label":"molecular sequestering activity","supporting_discovery_ids":[0,2]}],"localization":[{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[1,2,4]},{"term_id":"GO:0005576","term_label":"extracellular region","supporting_discovery_ids":[10]}],"pathway":[{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[2,4]},{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[10]},{"term_id":"R-HSA-9609507","term_label":"Protein localization","supporting_discovery_ids":[5]}],"complexes":[],"partners":["LRP2","LRP1","IFNAR1","CALM1","CAMK2","TTR"],"other_free_text":[]}},"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 and Rap control AMPA receptor trafficking during synaptic plasticity.","date":"2002","source":"Cell","url":"https://pubmed.ncbi.nlm.nih.gov/12202034","citation_count":648,"is_preprint":false},{"pmid":"15780587","id":"PMC_15780587","title":"Linking Rap to cell adhesion.","date":"2005","source":"Current opinion in cell biology","url":"https://pubmed.ncbi.nlm.nih.gov/15780587","citation_count":389,"is_preprint":false},{"pmid":"9789079","id":"PMC_9789079","title":"A Rap guanine nucleotide exchange factor enriched highly in the basal ganglia.","date":"1998","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/9789079","citation_count":294,"is_preprint":false},{"pmid":"11715024","id":"PMC_11715024","title":"A new phospholipase-C-calcium signalling pathway mediated by cyclic AMP and a Rap GTPase.","date":"2001","source":"Nature cell 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neurochemistry","url":"https://pubmed.ncbi.nlm.nih.gov/23061396","citation_count":21,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":46620,"output_tokens":4304,"usd":0.10221,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":12287,"output_tokens":4279,"usd":0.084205,"stage2_stop_reason":"end_turn"},"total_usd":0.186415,"stage1_batch_id":"msgbatch_01BbTCMyrRPyhtEq6b1MdwCN","stage2_batch_id":"msgbatch_011dEed5bbxYvTAaQWKJevxu","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1994,\n      \"finding\": \"RAP (LRPAP1) binds to gp330 (megalin/LRP2) and LRP at two distinct sites (amino acids 85-148 and 178-248 of RAP), with binding being Ca2+-dependent. A separate heparin-binding site was mapped to amino acids 261-323. These domains are distinct from the Heymann nephritis pathogenic epitope (amino acids 1-86).\",\n      \"method\": \"Direct binding studies (ligand blotting, affinity chromatography, density-gradient sedimentation) using fusion proteins representing overlapping RAP domains; radiolabeled heparin binding assays\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — multiple orthogonal in vitro binding assays with domain-mapping mutagenesis in a focused mechanistic study\",\n      \"pmids\": [\"7512726\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1993,\n      \"finding\": \"In renal proximal tubule cells, alpha2-MRAP (LRPAP1) is located predominantly in the rough endoplasmic reticulum (RER). Alpha2-MRAP-binding sites are present on gp330 at the proximal tubule brush border. The apparent brush border localization seen in snap-frozen sections is artifactual, resulting from release of RER-resident LRPAP1 during tissue processing and subsequent binding to cell-surface gp330.\",\n      \"method\": \"Immunocytochemistry with specific anti-alpha2-MRAP antibodies on PLP-fixed vs. snap-frozen rat kidney sections; alpha2-MRAP-IgG fusion protein binding assays; controlled incubation experiments showing redistribution artifact\",\n      \"journal\": \"European journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — direct subcellular localization with functional consequence demonstrated by controlled artifact experiments and multiple tissue-processing conditions\",\n      \"pmids\": [\"8223699\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"RAP (LRPAP1) functions as a molecular chaperone assisting in the folding of megalin (gp330) in the endoplasmic reticulum and its transport to the cell surface, while also acting as a universal inhibitor of ligand binding to LDL receptor family members at the cell surface.\",\n      \"method\": \"Review synthesizing biochemical, immunocytochemical, and functional data from the field; primary experimental evidence includes co-purification, ligand binding inhibition assays, and ER localization studies\",\n      \"journal\": \"Journal of the American Society of Nephrology : JASN\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — synthesis of multiple experimental findings from the field, but this paper itself is a review; underlying primary data from multiple labs supports the chaperone function\",\n      \"pmids\": [\"7579068\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"RAP (LRPAP1) binds calmodulin in a Ca2+- and pH-dependent manner and is a substrate for calmodulin-dependent kinase II (CaM-kinase II), which phosphorylates serine residues in RAP domains D2 (amino acids 113-218) and D3 (amino acids 219-323). Calcineurin dephosphorylates RAP. RAP is also phosphorylated by casein kinase II, and phosphorylation of RAP in vivo was demonstrated in 32P-labeled cell cultures.\",\n      \"method\": \"Calmodulin binding assays; in vitro kinase assays with CaM-kinase II and casein kinase II; phosphatase (calcineurin) assays; 32P-labeling of cell cultures to demonstrate in vivo phosphorylation; domain mapping with D2 and D3 fragments\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — reconstituted in vitro kinase/phosphatase assays with domain mapping, confirmed in vivo with 32P labeling, multiple orthogonal methods in one study\",\n      \"pmids\": [\"8861945\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"RAP (LRPAP1) serves as a molecular chaperone/escort protein for LDL receptor family members in the endoplasmic reticulum, preventing premature ligand interactions during receptor transport through the secretory pathway, and also promotes proper receptor folding independently of its ligand-inhibitory function.\",\n      \"method\": \"Review of experimental evidence including co-purification, ligand binding inhibition assays, secretory pathway transport studies, and ER localization experiments from multiple labs\",\n      \"journal\": \"International review of cytology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — review synthesizing replicated experimental findings, but paper is itself a review; underlying data from multiple labs independently established these functions\",\n      \"pmids\": [\"11580203\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"RAP (LRPAP1) crosses the blood-brain barrier via a saturable transport system, with megalin (LRP2) mediating most apical-to-basolateral transcytosis across polarized epithelial cells. Approximately 70% of blood-borne RAP reaches brain parenchyma (not just vasculature). Transport is inhibitable by excess RAP, confirming receptor-mediated transcytosis.\",\n      \"method\": \"Pharmacokinetic studies with [125I]-RAP in intact mice; in situ brain perfusion; capillary depletion of brain homogenates; transport assays in stably transfected MDCK cells expressing megalin\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal in vivo and in vitro approaches, saturation kinetics, mechanistic cell line validation, capillary depletion to confirm parenchymal distribution\",\n      \"pmids\": [\"15383619\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"The three-dimensional structure of RAP (LRPAP1) consists of three independent three-helix bundles (domains 1, 2, and 3) connected by long, flexible linkers. This architecture allows RAP to adopt various conformations when interacting with LDL receptor family members.\",\n      \"method\": \"NMR spectroscopy to determine domain 2 structure; small-angle neutron scattering (SANS) combined with simulated annealing protocol to characterize overall RAP architecture\",\n      \"journal\": \"Protein science : a publication of the Protein Society\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — NMR structure determination combined with SANS experimental data; rigorous structural study with functional context\",\n      \"pmids\": [\"17656581\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"RAP (LRPAP1) inhibits transthyretin (TTR) internalization in hepatocytes, and the ~90 kDa TTR-receptor complex is RAP-sensitive, indicating that TTR is internalized via an as-yet unidentified LDL receptor family member in liver cells. This RAP-sensitive pathway was confirmed in both hepatoma cell lines and primary hepatocytes.\",\n      \"method\": \"Uptake competition assays with RAP; chemical cross-linking of [125I]-TTR to identify receptor complex; primary hepatocyte and hepatoma cell line comparison; lipid-deficient serum controls\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple cell systems tested, cross-linking and competition assays, but receptor identity not established; single lab\",\n      \"pmids\": [\"11278770\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"RAP (LRPAP1) mediates RAP-sensitive and proteoglycan-mediated pathways of lipoprotein lipase (LPL) degradation in adipocytes. The RAP-sensitive (LRP-mediated) component accounts for 30-54% of LPL degradation at 1 h, diminishing to 19-23% at 4 h, while heparan sulfate proteoglycans mediate the major non-RAP-sensitive fraction.\",\n      \"method\": \"125I-LPL degradation assays in BFC-1β adipocytes with RAP inhibition; heparinase/heparitinase treatment; phosphatidylinositol-specific phospholipase C treatment; 35SO4 proteoglycan labeling to measure turnover kinetics\",\n      \"journal\": \"Journal of lipid research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal inhibition and labeling approaches in a single lab, but functional consequence (pathway identification) rather than direct molecular mechanism\",\n      \"pmids\": [\"8978495\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"RAP (LRPAP1) treatment of neurons potentiates the inhibitory effects of full-length APP on axonal outgrowth, and RAP-LRP binding stimulates axon growth independently of APP, associated with ERK2 activation. This demonstrates that LRPAP1 engagement of LRP has APP-independent signaling consequences in neurons.\",\n      \"method\": \"Neurite outgrowth measurements in wild-type vs. APP knockout neurons treated with sAPPα and/or RAP; ERK2 activation measured; 1 day in vitro cultures\",\n      \"journal\": \"Journal of neurochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — clean genetic KO comparison with pharmacological RAP treatment and ERK2 readout, but single lab, single study\",\n      \"pmids\": [\"23061396\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Secreted LRPAP1 binds the extracellular domain of IFNAR1 (interferon alpha/beta receptor 1) via its N-terminus, triggering IFNAR1 ubiquitination and degradation, thereby suppressing interferon signaling and facilitating viral immune evasion. Viral proteases (SARS-CoV-2 3CLpro and EV71 2Apro) upregulate LRPAP1 expression and secretion to exploit this mechanism. A small N-terminal LRPAP1 peptide recapitulates IFNAR1 degradation, and α2M (a LRPAP1 inhibitor) stabilizes IFNAR1 and arrests viral infections.\",\n      \"method\": \"Binding assays between secreted LRPAP1 and IFNAR1 extracellular domain; ubiquitination and degradation assays in vitro, ex vivo (mouse brain), and in vivo (newborn mice); viral infection assays with multiple viruses (HSV-1, HBV, EV71, HCoV-OC43); N-terminal peptide experiments; α2M inhibition experiments; protease upregulation assays\",\n      \"journal\": \"Signal transduction and targeted therapy\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — multiple orthogonal methods across in vitro, ex vivo, and in vivo systems; domain mapping with N-terminal peptide; inhibitor validation; tested across multiple viral systems; identifies novel mechanistic role for secreted LRPAP1\",\n      \"pmids\": [\"37743411\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Loss of lrpap1 in zebrafish (CRISPR/Cas9 frameshift mutation) causes myopia with increased axial length and disordered scleral collagen fibers. The mechanism involves activation of TGF-β signaling and increased apoptosis. TGF-β agonist treatment of wild-type embryos recapitulates the apoptosis phenotype, while TGF-β inhibitor treatment mitigates apoptosis in lrpap1 mutants, establishing TGF-β-induced apoptosis as the downstream pathway.\",\n      \"method\": \"CRISPR/Cas9-generated zebrafish lrpap1 knockout; refractive error measurement; transmission electron microscopy of scleral collagen; RNA sequencing and bioinformatics; acridine orange and TUNEL staining; TGF-β agonist and inhibitor rescue experiments\",\n      \"journal\": \"Cell communication and signaling : CCS\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — clean genetic KO in vertebrate model, multiple orthogonal readouts (structural, molecular, transcriptomic), bidirectional pharmacological rescue of the TGF-β pathway, single lab\",\n      \"pmids\": [\"36261846\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1994,\n      \"finding\": \"The human LRPAP1 gene was chromosomally mapped to the short arm of chromosome 4 at position 4p16.3, in the region of the chromosomal deletion causing Wolf-Hirschhorn syndrome, by fluorescence in situ hybridization.\",\n      \"method\": \"Fluorescence in situ hybridization (FISH) using human RAP cDNA probe\",\n      \"journal\": \"Genomics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct FISH mapping, technically sound, but chromosomal localization alone (no functional consequence linked)\",\n      \"pmids\": [\"7959795\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"A homozygous frameshift mutation in LRPAP1 (c.199delC, p.Q67Sfs*8) co-segregates with early-onset high myopia in a consanguineous family, providing genetic evidence that LRPAP1 loss-of-function causes autosomal recessive high myopia.\",\n      \"method\": \"Whole exome sequencing of 298 early-onset high myopia patients; Sanger sequencing confirmation; co-segregation analysis in family members; absence in 192 healthy controls\",\n      \"journal\": \"Investigative ophthalmology & visual science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — genetic co-segregation in a human family with bioinformatic pathogenicity prediction, replicated from prior reports; no direct functional mechanistic assay performed in this study\",\n      \"pmids\": [\"25525168\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"LRPAP1 acts as a B-cell receptor (BCR) antigen that induces proliferation via BCR pathway activation in a subset of mantle cell lymphomas (MCL). An LRPAP1-ETA' toxin conjugate specifically kills MCL cells bearing LRPAP1-specific BCRs, demonstrating that LRPAP1-BCR interaction drives chronic autoantigenic BCR signaling.\",\n      \"method\": \"Recombinant expression of BCRs from 21 MCL patients and 7 cell lines; antigen screening; LRPAP1 stimulation of BCR pathway activation; LRPAP1-ETA' immunotoxin killing assays; serum antibody characterization\",\n      \"journal\": \"Leukemia\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional BCR activation and targeted cytotoxicity assays confirm antigen-receptor interaction drives signaling; single lab, novel mechanistic finding\",\n      \"pmids\": [\"29955130\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"LRPAP1 (RAP) is a 39-kDa ER-resident molecular chaperone whose three-helix-bundle domains (connected by flexible linkers) bind Ca2+-dependently to at least two sites on LDL receptor family members (LRP1, LRP2/megalin), preventing premature ligand interactions during secretory pathway transit while promoting proper receptor folding; it is itself regulated by CaM-kinase II phosphorylation and calmodulin binding, mediates transcytosis across the blood-brain barrier via megalin, and in a newly discovered extracellular role, the secreted form binds IFNAR1 to trigger its ubiquitination and degradation, facilitating viral immune evasion; loss of LRPAP1 in zebrafish causes myopia through TGF-β-induced scleral apoptosis, and in humans its frameshift mutations cause autosomal recessive high myopia.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"LRPAP1 (RAP) is a Ca2+-dependent molecular chaperone of the LDL receptor family that escorts receptor folding and traffic through the secretory pathway while acting as a universal inhibitor of premature ligand binding [#2, #4]. It binds megalin (LRP2/gp330) and LRP1 at two distinct sites (residues 85-148 and 178-248) with a separate heparin-binding region (residues 261-323), interactions that are strictly Ca2+-dependent [#0]. Structurally it is built from three independent three-helix bundles joined by flexible linkers, an architecture that permits multiple conformations during receptor engagement [#6], and it resides predominantly in the rough endoplasmic reticulum of epithelial cells [#1]. Its activity is further tuned by calmodulin binding and phosphorylation: CaM-kinase II and casein kinase II phosphorylate serines in its D2 and D3 domains, and calcineurin reverses this modification [#3]. Beyond its intracellular escort role, LRPAP1 modulates receptor-mediated handling of multiple ligands—mediating saturable, megalin-dependent transcytosis across the blood-brain barrier [#5] and engaging LRP to drive APP-independent, ERK2-linked signaling in neurons [#9]. A secreted form binds the IFNAR1 extracellular domain through its N-terminus to trigger IFNAR1 ubiquitination and degradation, a mechanism viral proteases exploit to suppress interferon signaling [#10]. Loss-of-function frameshift mutations in LRPAP1 cause autosomal recessive early-onset high myopia in humans [#13], and zebrafish lrpap1 knockout reproduces myopia through TGF-\\u03b2-induced scleral apoptosis [#11].\",\n  \"teleology\": [\n    {\n      \"year\": 1994,\n      \"claim\": \"Defining where and how RAP physically engages LDL receptor family members was needed to explain its inhibitory and escort activities; mapping resolved two distinct Ca2+-dependent receptor-binding sites plus a separate heparin site.\",\n      \"evidence\": \"Domain-mapped in vitro binding (ligand blotting, affinity chromatography, sedimentation) with overlapping RAP fusion fragments and radiolabeled heparin assays\",\n      \"pmids\": [\"7512726\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Does not define stoichiometry or order of the two-site engagement in vivo\", \"Functional consequence of the heparin-binding site not resolved\"]\n    },\n    {\n      \"year\": 1993,\n      \"claim\": \"The subcellular site of RAP action was uncertain; controlled fixation experiments established RER residency and showed that apparent brush-border localization was a processing artifact, anchoring its chaperone role to the ER.\",\n      \"evidence\": \"Immunocytochemistry on PLP-fixed vs. snap-frozen rat kidney with controlled redistribution experiments and fusion-protein binding\",\n      \"pmids\": [\"8223699\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Does not address how RAP dissociates from receptors after ER transit\", \"Secreted/extracellular pool not characterized here\"]\n    },\n    {\n      \"year\": 1995,\n      \"claim\": \"Synthesis of field data assigned RAP a dual function as an ER folding chaperone for megalin and a surface ligand-binding inhibitor, framing its role in receptor biogenesis.\",\n      \"evidence\": \"Review integrating co-purification, ligand-inhibition, and ER localization data\",\n      \"pmids\": [\"7579068\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Review-level synthesis rather than new primary data\", \"Mechanism separating folding from inhibition not resolved\"]\n    },\n    {\n      \"year\": 1996,\n      \"claim\": \"Whether RAP itself is regulated was unknown; reconstituted assays showed it binds calmodulin Ca2+/pH-dependently and is phosphorylated on D2/D3 serines by CaM-kinase II and casein kinase II, reversed by calcineurin.\",\n      \"evidence\": \"In vitro kinase/phosphatase and calmodulin-binding assays with domain fragments, confirmed by 32P labeling in cells\",\n      \"pmids\": [\"8861945\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Functional effect of phosphorylation on receptor binding not established\", \"Cellular trigger for phosphorylation unknown\"]\n    },\n    {\n      \"year\": 1996,\n      \"claim\": \"RAP's role in lipoprotein metabolism was addressed by quantifying its contribution to LPL clearance, distinguishing an LRP-mediated RAP-sensitive route from a dominant proteoglycan route.\",\n      \"evidence\": \"125I-LPL degradation assays in adipocytes with RAP inhibition, heparinase/PI-PLC treatment, and 35SO4 proteoglycan labeling\",\n      \"pmids\": [\"8978495\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Identifies pathway contribution rather than direct molecular mechanism\", \"Single lab, single cell type\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"RAP-sensitive ligand handling was extended to transthyretin, implicating an unidentified LDL-receptor-family member in hepatic TTR internalization.\",\n      \"evidence\": \"Uptake competition with RAP and cross-linking of 125I-TTR in hepatoma lines and primary hepatocytes\",\n      \"pmids\": [\"11278770\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Receptor identity not established\", \"Single lab\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"A second review consolidated the escort-protein model, asserting RAP prevents premature ligand interactions and promotes folding independently of ligand inhibition.\",\n      \"evidence\": \"Review of co-purification, transport, and ER localization data\",\n      \"pmids\": [\"11580203\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Review synthesis, no new primary data\", \"Separation of folding vs. inhibition functions not directly tested here\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Whether RAP could move across barriers was tested; pharmacokinetic and cell-based assays established saturable, megalin-mediated transcytosis delivering RAP to brain parenchyma.\",\n      \"evidence\": \"125I-RAP pharmacokinetics, in situ brain perfusion, capillary depletion, and megalin-transfected MDCK transport assays\",\n      \"pmids\": [\"15383619\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiological cargo/relevance of RAP transcytosis unclear\", \"Does not address fate of RAP in parenchyma\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"The structural basis for RAP's conformational versatility was resolved as three independent three-helix bundles connected by flexible linkers.\",\n      \"evidence\": \"NMR structure of domain 2 plus SANS with simulated annealing for overall architecture\",\n      \"pmids\": [\"17656581\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No co-structure with a receptor\", \"Conformational states during binding not directly visualized\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"RAP-LRP engagement was shown to have signaling output, stimulating axon growth via ERK2 independently of APP while potentiating APP's inhibitory effect.\",\n      \"evidence\": \"Neurite outgrowth in WT vs. APP-knockout neurons with RAP/sAPPa treatment and ERK2 readout\",\n      \"pmids\": [\"23061396\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Receptor mediating the ERK2 response not pinpointed\", \"Single lab, single study\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"A causal human disease link was established by finding a homozygous LRPAP1 frameshift co-segregating with autosomal recessive early-onset high myopia.\",\n      \"evidence\": \"Whole-exome sequencing of high-myopia patients, Sanger confirmation, family co-segregation, control absence\",\n      \"pmids\": [\"25525168\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No functional assay in this study\", \"Mechanism linking LRPAP1 loss to myopia not addressed here\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"An unexpected role as a B-cell receptor autoantigen was demonstrated, with LRPAP1 driving chronic BCR signaling in a subset of mantle cell lymphomas.\",\n      \"evidence\": \"Recombinant BCR expression from MCL patients/cell lines, antigen screening, BCR activation, and LRPAP1-ETA' immunotoxin killing\",\n      \"pmids\": [\"29955130\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism of LRPAP1 autoantigen exposure unknown\", \"Single lab\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"The mechanism behind LRPAP1-linked myopia was defined in zebrafish: lrpap1 loss activates TGF-\\u03b2 signaling and scleral apoptosis, with bidirectional pharmacological rescue confirming the pathway.\",\n      \"evidence\": \"CRISPR lrpap1 knockout zebrafish with refraction, scleral EM, RNA-seq, TUNEL, and TGF-\\u03b2 agonist/inhibitor rescue\",\n      \"pmids\": [\"36261846\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct molecular link between RAP loss and TGF-\\u03b2 activation not defined\", \"Mammalian confirmation absent\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"A secreted extracellular function was uncovered: LRPAP1 binds IFNAR1 via its N-terminus to drive its ubiquitination and degradation, a route co-opted by viral proteases for immune evasion.\",\n      \"evidence\": \"Binding, ubiquitination, and degradation assays in vitro/ex vivo/in vivo, N-terminal peptide mapping, a2M inhibition, and multi-virus infection assays\",\n      \"pmids\": [\"37743411\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Ubiquitin ligase mediating IFNAR1 degradation not identified\", \"How secreted LRPAP1 traffics to IFNAR1 unresolved\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How LRPAP1's intracellular chaperone role and its secreted extracellular signaling/degradation roles are coordinated, and which receptors mediate its diverse phenotypes, remain unresolved.\",\n      \"evidence\": \"No single study integrates ER chaperone, transcytosis, neuronal signaling, IFNAR1 degradation, and myopia mechanisms\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No co-structure of RAP with any receptor\", \"E3 ligase for IFNAR1 unknown\", \"Direct mechanistic bridge from RAP loss to scleral TGF-\\u03b2 activation undefined\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0044183\", \"supporting_discovery_ids\": [2, 4]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [0, 2, 10]},\n      {\"term_id\": \"GO:0140313\", \"supporting_discovery_ids\": [0, 2]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [1, 2, 4]},\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [10]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [2, 4]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [10]},\n      {\"term_id\": \"R-HSA-9609507\", \"supporting_discovery_ids\": [5]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"LRP2\", \"LRP1\", \"IFNAR1\", \"CALM1\", \"CAMK2\", \"TTR\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":7,"faith_total":7,"faith_pct":100.0}}