{"gene":"AMN","run_date":"2026-06-09T22:02:43","timeline":{"discoveries":[{"year":2003,"finding":"AMN (amnionless) and cubilin form a tight complex (cubam) where AMN binds to the amino-terminal third of cubilin, directs cubilin trafficking from early biosynthetic compartments to the cell surface and endosomes, and enables intrinsic factor-cobalamin endocytosis and lysosomal degradation of intrinsic factor. Neither protein alone confers ligand endocytosis in transfected cells.","method":"Co-purification by IF-cobalamin affinity and nondenaturing gel filtration chromatography; cotransfection of AMN and truncated cubilin constructs in polarized epithelial cells; subcellular localization imaging; endocytosis/lysosomal degradation assays","journal":"Blood","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — biochemical co-purification, functional reconstitution in transfected cells with multiple orthogonal readouts (trafficking, endocytosis, lysosomal degradation), replicated concept across multiple subsequent studies","pmids":["14576052"],"is_preprint":false},{"year":2001,"finding":"The amnionless (Amn) gene encodes a novel type I transmembrane protein expressed exclusively in extraembryonic visceral endoderm during gastrulation; its extracellular region contains a cysteine-rich domain with similarity to BMP-binding domains in chordin and its Drosophila homolog Sog. Loss of Amn function in the visceral endoderm specifically impairs middle primitive streak formation, resulting in absence of non-axial trunk mesoderm.","method":"Positional cloning of insertional mouse mutation; in situ hybridization for expression pattern; chimera analysis (amn−/− ES cells ↔ wild-type blastocysts); histological and molecular phenotypic characterization","journal":"Nature genetics","confidence":"High","confidence_rationale":"Tier 2 / Strong — loss-of-function mouse mutation with defined cellular phenotype, chimera rescue experiments, replicated across two independent studies (PMIDs 11279523 and 9851841)","pmids":["11279523"],"is_preprint":false},{"year":1998,"finding":"The amn gene product is required specifically in extraembryonic tissues (visceral endoderm) for the generation of middle primitive streak derivatives; chimera analysis demonstrated that wild-type ES cells cannot rescue the mutant defect when the visceral endoderm lacks amn function, placing Amn action in the visceral endoderm signaling pathway during gastrulation.","method":"Wild-type ES cell ↔ amn−/− blastocyst chimera analysis; histological and molecular marker analysis of gastrulation-stage embryos","journal":"Developmental biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic epistasis via chimera analysis with defined tissue-of-action, replicated and extended in later studies","pmids":["9851841"],"is_preprint":false},{"year":2003,"finding":"Homozygous mutations affecting exons 1–4 of human AMN cause selective malabsorption of vitamin B12 (Imerslund-Gräsbeck syndrome) in otherwise normal individuals, demonstrating that the 5′ end of AMN is dispensable for embryonic development but necessary for intestinal vitamin B12 absorption. When the 5′ end is truncated, translation initiates from alternative downstream start codons.","method":"Human genetic analysis (mutation identification in IGS patients); RT-PCR demonstration of alternative translation initiation","journal":"Nature genetics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — human genetics with functional inference from mutation analysis and alternative initiation demonstration, single study","pmids":["12590260"],"is_preprint":false},{"year":2004,"finding":"In vivo, Amn is required for cell-surface localization and endocytic function of cubilin in kidney proximal tubules and visceral endoderm; Amn−/− chimeric mice exhibit selective proteinuria of cubilin ligands, and cubilin is not properly localized to the cell surface in Amn−/− tissues.","method":"Amn−/− ES cell ↔ wild-type blastocyst chimera analysis; immunolocalization of cubilin in embryonic and adult tissues; urinary protein analysis","journal":"Development","confidence":"High","confidence_rationale":"Tier 2 / Strong — in vivo genetic loss-of-function with direct immunolocalization and functional urinary phenotype, consistent with in vitro data from PMID 14576052","pmids":["15342463"],"is_preprint":false},{"year":2005,"finding":"In vivo, AMN dysfunction abrogates cubilin processing and targeting to the apical brush-border membrane in the intestine and kidney, as demonstrated in dogs with AMN mutations (in-frame deletion in exon 10 or disruption of the translation initiation codon), recapitulating human Imerslund-Gräsbeck syndrome. These essential features were reproduced in a heterologous cell-transfection system.","method":"Canine genetic model; in vivo immunofluorescence and Western blotting of intestinal and renal tissues; heterologous cell transfection validation","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 / Strong — in vivo animal model with direct protein localization assays, validated in cell transfection system, orthogonal to mouse and human data","pmids":["15845892"],"is_preprint":false},{"year":2005,"finding":"AMN interacts with the EGF domains of cubilin (not solely the N-terminal domain), is responsible for membrane attachment and ER export of the cubam complex, and prevents secretion of cubilin into the medium. Apical sorting of cubilin is mediated by its own extracellular region in a glycosylation-dependent manner, and inhibition of glycosylation (tunicamycin) abolished apical sorting preference.","method":"Transient transfection of AMN and cubilin truncation mutants in polarized MDCK cells; co-immunoprecipitation; confocal colocalization; tunicamycin glycosylation inhibition","journal":"Journal of the American Society of Nephrology","confidence":"High","confidence_rationale":"Tier 2 / Moderate — reciprocal co-IP, multiple cubilin truncation constructs, functional apical targeting assay, glycosylation inhibition experiment in one rigorous study","pmids":["15976000"],"is_preprint":false},{"year":2008,"finding":"Cubilin interacts with both AMN and megalin in renal brush-border membranes; three distinct regions of cubilin (113-residue N-terminus, CUB domains 12–17, and 22–27) bind megalin in a Ca2+-dependent manner. Both megalin and AMN interactions are required for intracellular stability of cubilin: silencing either megalin or AMN in opossum kidney cells reduced cubilin staining by 85–90% and decreased cubilin half-life 2-fold.","method":"In vitro Ca2+-dependent binding assays with cubilin fragments; immunoprecipitation and immunoblotting of renal brush-border membrane extracts; ligand-affinity chromatography; gene silencing (siRNA) with immunohistochemistry and turnover studies","journal":"Biochemical Journal","confidence":"High","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal methods (in vitro binding, co-IP, affinity chromatography, gene silencing with functional readout) in one study","pmids":["17990981"],"is_preprint":false},{"year":2010,"finding":"The cytosolic domain of AMN contains two FXNPXF motifs that are functionally redundant; both signals can individually mediate endocytosis of the cubam complex through direct interaction with the clathrin-associated sorting proteins Dab2 and ARH, as demonstrated by sequential mutagenesis and yeast two-hybrid analysis.","method":"Sequential mutagenesis of AMN FXNPXF motifs; functional endocytosis assays with AMN mutant panel; yeast two-hybrid analysis of AMN–Dab2 and AMN–ARH interactions","journal":"Traffic","confidence":"High","confidence_rationale":"Tier 1–2 / Moderate — mutagenesis combined with functional endocytosis assays and yeast two-hybrid interaction mapping in one study","pmids":["20088845"],"is_preprint":false},{"year":2012,"finding":"Drosophila orthologs of cubilin (dCubilin) and amnionless (dAMN) are specifically expressed in nephrocytes and function as co-receptors for protein reabsorption. Human AMN expressed in Drosophila nephrocytes rescues defective protein uptake caused by dAMN knockdown, demonstrating evolutionary conservation of the Cubilin/AMN co-receptor function.","method":"Drosophila genetic screen; tissue-specific RNAi knockdown; transgenic rescue with human AMN; fluorescent protein uptake assays; electron microscopy of nephrocyte ultrastructure","journal":"Journal of the American Society of Nephrology","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic loss-of-function with defined cellular phenotype, cross-species rescue experiment with human AMN, orthogonal structural and functional readouts","pmids":["23264686"],"is_preprint":false},{"year":2018,"finding":"AMN-mediated glycosylation of cubilin is necessary (but interaction between cubilin and AMN is not sufficient) for cell-surface expression of the cubam complex in renal and intestinal cells. Quantitative mass spectrometry and mutagenesis identified N-linked glycosylation of at least 4 specific cubilin residues as required for its plasma membrane targeting. IGS-causing AMN missense mutations cause ER retention of both AMN and cubilin.","method":"Quantitative cell-surface targeting assay in cultured renal and intestinal cells; mass spectrometry glycosylation mapping; site-directed mutagenesis of glycosylation sites; immunofluorescence of patient renal proximal tubular cells","journal":"Scientific reports","confidence":"High","confidence_rationale":"Tier 1–2 / Moderate — mutagenesis, mass spectrometry, functional targeting assay, and patient tissue validation in one rigorous study","pmids":["29402915"],"is_preprint":false},{"year":2011,"finding":"In humans with compound heterozygous AMN mutations (premature stop codon in exon 7), cubilin luminal expression is dramatically decreased in the absence of any CUBN mutation, despite normal intrinsic factor-binding affinity of residual receptor, establishing that AMN is essential for correct luminal expression of cubilin in vivo.","method":"Genetic mutation analysis; measurement of cubilin receptor activity in urine; intrinsic factor-binding affinity assay","journal":"Haematologica","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — human genetics with functional urinary receptor assay, single study, limited mechanistic depth","pmids":["21750092"],"is_preprint":false},{"year":2022,"finding":"In Drosophila nephrocytes, store-operated calcium entry (SOCE) via Stim/Orai mediates the localization of the endocytic co-receptor Amnionless at the nephrocyte surface; RNAi knockdown of Stim or Orai reduced Amnionless abundance and disrupted albumin binding and accumulation.","method":"GCaMP6 calcium reporter live imaging in Drosophila nephrocytes; RNAi knockdown of SOCE genes (Stim, Orai); pharmacological inhibition (EGTA, 2-APB); fluorescent albumin uptake assays; immunofluorescence","journal":"Journal of insect physiology","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — genetic knockdown with functional and localization readouts in a Drosophila ortholog model, single study","pmids":["36341969"],"is_preprint":false}],"current_model":"AMN (amnionless) encodes a type I transmembrane protein that forms the cubam complex with cubilin by binding to cubilin's EGF domains; within this complex, AMN serves as the membrane anchor that drives ER export, apical trafficking, and N-linked glycosylation of cubilin required for plasma membrane targeting, while its cytoplasmic FXNPXF motifs recruit the clathrin-associated adaptors Dab2 and ARH to mediate clathrin-dependent endocytosis of cubam ligands including intrinsic factor-cobalamin (vitamin B12) in the intestine and low-molecular-weight proteins in the kidney proximal tubule; AMN also stabilizes cubilin protein in the cell, and its loss—whether by mutation in humans/dogs or gene knockout in mice—abrogates cubilin surface expression and causes Imerslund-Gräsbeck syndrome, while in early mouse development AMN acts in the visceral endoderm to direct middle primitive streak formation."},"narrative":{"mechanistic_narrative":"AMN (amnionless) encodes a type I transmembrane protein that partners with cubilin to form the endocytic co-receptor complex cubam, governing receptor-mediated uptake of specific ligands in absorptive epithelia [PMID:14576052]. AMN binds cubilin's amino-terminal region and EGF domains and serves as the membrane anchor that drives ER export, prevents secretion of cubilin, and directs cubilin trafficking to the apical surface and endosomes—functions neither protein performs alone [PMID:14576052, PMID:15976000]. Cell-surface delivery of cubam additionally requires AMN-dependent N-linked glycosylation of specific cubilin residues, and IGS-causing AMN missense mutations cause ER retention of both proteins [PMID:29402915]. AMN is also required for the intracellular stability and luminal expression of cubilin in vivo, acting together with megalin to control cubilin half-life [PMID:17990981, PMID:21750092]. The cytosolic tail of AMN carries two functionally redundant FXNPXF motifs that recruit the clathrin-associated sorting adaptors Dab2 and ARH to mediate clathrin-dependent endocytosis of cubam ligands, including intrinsic factor-cobalamin (vitamin B12) [PMID:14576052, PMID:20088845]. Loss of AMN function—by mutation in humans and dogs or knockout in mice—abrogates cubilin surface targeting and causes selective vitamin B12 malabsorption and proteinuria characteristic of Imerslund-Gräsbeck syndrome [PMID:12590260, PMID:15342463, PMID:15845892]. Independently of its endocytic role, AMN acts in the extraembryonic visceral endoderm during gastrulation to direct middle primitive streak formation and generation of non-axial trunk mesoderm [PMID:11279523, PMID:9851841]; the 5' end of AMN required for development is dispensable for B12 absorption [PMID:12590260].","teleology":[{"year":1998,"claim":"Established that AMN function is required specifically in the extraembryonic visceral endoderm—not the embryo proper—for gastrulation, defining its tissue of action by genetic epistasis.","evidence":"Wild-type ES cell ↔ amn−/− blastocyst chimera analysis with marker-based phenotyping of gastrulation embryos","pmids":["9851841"],"confidence":"High","gaps":["Molecular mechanism of the developmental defect not defined","No link yet to a receptor or endocytic function"]},{"year":2001,"claim":"Identified AMN as a novel type I transmembrane protein with a chordin/Sog-like cysteine-rich extracellular domain, and showed its loss specifically blocks middle primitive streak formation.","evidence":"Positional cloning of an insertional mouse mutation, in situ hybridization, and chimera/phenotypic analysis","pmids":["11279523"],"confidence":"High","gaps":["Whether the BMP-binding-like domain functions in signaling untested","No biochemical partner identified at this stage"]},{"year":2003,"claim":"Connected AMN biochemically to cubilin, revealing the cubam complex as the unit that confers ligand endocytosis and cubilin trafficking to the cell surface—neither protein suffices alone.","evidence":"IF-cobalamin affinity co-purification, gel filtration, cotransfection of AMN with truncated cubilin in polarized epithelia, and endocytosis/degradation assays","pmids":["14576052"],"confidence":"High","gaps":["Cytoplasmic endocytic signal not yet mapped","Glycosylation requirement not addressed"]},{"year":2003,"claim":"Defined the human disease link, showing AMN mutations cause Imerslund-Gräsbeck syndrome and that the developmentally essential 5' end is dispensable for B12 absorption via alternative downstream translation initiation.","evidence":"Mutation identification in IGS patients and RT-PCR demonstration of alternative translation initiation","pmids":["12590260"],"confidence":"Medium","gaps":["Single study","Functional consequence inferred rather than directly reconstituted for each allele"]},{"year":2004,"claim":"Demonstrated in vivo that AMN is required for cubilin cell-surface localization and endocytic function, linking AMN loss to selective proteinuria of cubilin ligands.","evidence":"Amn−/− ↔ wild-type chimeric mice with cubilin immunolocalization and urinary protein analysis","pmids":["15342463"],"confidence":"High","gaps":["Trafficking step blocked not resolved at the molecular level","Did not distinguish glycosylation from anchoring defects"]},{"year":2005,"claim":"Mapped AMN's structural role: it binds cubilin EGF domains, provides membrane attachment and ER export, and prevents cubilin secretion, while apical sorting depends on cubilin's glycosylated extracellular region.","evidence":"Cubilin truncation mutants and AMN co-transfection in polarized MDCK cells with co-IP, confocal colocalization, and tunicamycin glycosylation inhibition","pmids":["15976000"],"confidence":"High","gaps":["Specific glycosylation sites not yet identified","Endocytic adaptor recruitment not defined"]},{"year":2005,"claim":"Validated the cubilin-targeting role of AMN across species using a canine genetic model, showing AMN dysfunction abrogates cubilin processing and apical brush-border targeting and recapitulates human IGS.","evidence":"Canine AMN mutants with intestinal/renal immunofluorescence and immunoblotting, plus heterologous transfection","pmids":["15845892"],"confidence":"High","gaps":["Molecular basis of processing defect for each mutation not dissected"]},{"year":2008,"claim":"Showed cubilin requires both AMN and megalin for intracellular stability, establishing AMN as a determinant of cubilin half-life rather than only trafficking.","evidence":"In vitro Ca2+-dependent binding of cubilin fragments, brush-border co-IP, affinity chromatography, and siRNA silencing with turnover studies in opossum kidney cells","pmids":["17990981"],"confidence":"High","gaps":["Mechanism by which AMN stabilizes cubilin not defined","Relative contributions of AMN vs megalin not separated"]},{"year":2010,"claim":"Identified the cytoplasmic endocytic code: two redundant FXNPXF motifs in AMN that directly recruit the clathrin-associated adaptors Dab2 and ARH to drive cubam internalization.","evidence":"Sequential mutagenesis of AMN motifs, functional endocytosis assays, and yeast two-hybrid mapping of AMN–Dab2/ARH interactions","pmids":["20088845"],"confidence":"High","gaps":["Whether both adaptors act in the same tissue context unresolved","Reciprocal validation of adaptor interactions in native cells not shown"]},{"year":2011,"claim":"Confirmed in human patients that AMN is essential for luminal cubilin expression independent of CUBN mutation, even when residual receptor retains intrinsic factor binding.","evidence":"Genetic analysis of compound heterozygous AMN mutations with urinary cubilin receptor activity and IF-binding affinity assays","pmids":["21750092"],"confidence":"Medium","gaps":["Single study with limited mechanistic depth","Quantitative trafficking defect not measured"]},{"year":2012,"claim":"Demonstrated evolutionary conservation of the cubilin/AMN co-receptor function, with human AMN rescuing protein-uptake defects in Drosophila nephrocytes lacking dAMN.","evidence":"Drosophila tissue-specific RNAi, transgenic rescue with human AMN, fluorescent uptake assays, and nephrocyte electron microscopy","pmids":["23264686"],"confidence":"High","gaps":["Conservation of glycosylation and adaptor mechanisms in flies not tested"]},{"year":2018,"claim":"Resolved the glycosylation requirement, showing AMN-mediated N-linked glycosylation of at least four specific cubilin residues is necessary for cubam surface targeting and that AMN–cubilin interaction alone is insufficient.","evidence":"Quantitative surface-targeting assays in renal/intestinal cells, mass spectrometry glycosylation mapping, site-directed mutagenesis, and patient proximal tubule immunofluorescence","pmids":["29402915"],"confidence":"High","gaps":["How AMN promotes cubilin glycosylation mechanistically unknown","Whether glycosylation defect is the sole cause of ER retention for all alleles unclear"]},{"year":2022,"claim":"Linked calcium signaling to AMN surface localization, showing store-operated calcium entry via Stim/Orai is required to maintain Amnionless at the nephrocyte surface.","evidence":"GCaMP6 calcium imaging, Stim/Orai RNAi, pharmacological SOCE inhibition, and albumin uptake assays in Drosophila nephrocytes","pmids":["36341969"],"confidence":"Medium","gaps":["Single study in a fly ortholog","Whether SOCE regulates mammalian AMN trafficking untested","Direct molecular link between calcium entry and AMN localization not defined"]},{"year":null,"claim":"The molecular mechanism by which AMN promotes cubilin N-linked glycosylation and ER export, and whether AMN's developmental visceral-endoderm role is mechanistically distinct from its endocytic co-receptor function, remain unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No structural model of the cubam complex","Chordin/Sog-like domain function in development not tested biochemically","Mechanism coupling AMN to the glycosylation machinery unknown"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0038024","term_label":"cargo receptor activity","supporting_discovery_ids":[0,4,9]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[8]},{"term_id":"GO:0140313","term_label":"molecular sequestering activity","supporting_discovery_ids":[6,7,10]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[0,4,6]},{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[6,10]},{"term_id":"GO:0005768","term_label":"endosome","supporting_discovery_ids":[0]}],"pathway":[{"term_id":"R-HSA-5653656","term_label":"Vesicle-mediated transport","supporting_discovery_ids":[0,8]},{"term_id":"R-HSA-382551","term_label":"Transport of small molecules","supporting_discovery_ids":[0,4]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[1,2]}],"complexes":["cubam (cubilin–AMN co-receptor complex)"],"partners":["CUBN","DAB2","LRP2","ARH"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q9BXJ7","full_name":"Protein amnionless","aliases":[],"length_aa":453,"mass_kda":47.8,"function":"Membrane-bound component of the endocytic receptor formed by AMN and CUBN (PubMed:14576052, PubMed:29402915, PubMed:30523278). Required for normal CUBN glycosylation and trafficking to the cell surface (PubMed:14576052, PubMed:29402915). The complex formed by AMN and CUBN is required for efficient absorption of vitamin B12 (PubMed:12590260, PubMed:14576052, PubMed:26040326). Required for normal CUBN-mediated protein transport in the kidney (Probable)","subcellular_location":"Secreted","url":"https://www.uniprot.org/uniprotkb/Q9BXJ7/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/AMN","classification":"Not Classified","n_dependent_lines":2,"n_total_lines":1208,"dependency_fraction":0.0016556291390728477},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/AMN","total_profiled":1310},"omim":[{"mim_id":"618882","title":"IMERSLUND-GRASBECK SYNDROME 2; IGS2","url":"https://www.omim.org/entry/618882"},{"mim_id":"614362","title":"ACYL-CoA SYNTHETASE, BUBBLEGUM FAMILY, MEMBER 1; ACSBG1","url":"https://www.omim.org/entry/614362"},{"mim_id":"614071","title":"MYOCARDIAL ZONULA ADHERENS PROTEIN; MYZAP","url":"https://www.omim.org/entry/614071"},{"mim_id":"609342","title":"COBALAMIN-BINDING INTRINSIC FACTOR; CBLIF","url":"https://www.omim.org/entry/609342"},{"mim_id":"605799","title":"AMNION-ASSOCIATED TRANSMEMBRANE PROTEIN; AMN","url":"https://www.omim.org/entry/605799"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Group enriched","tissue_distribution":"Detected in many","driving_tissues":[{"tissue":"intestine","ntpm":86.9},{"tissue":"kidney","ntpm":52.4},{"tissue":"liver","ntpm":55.2}],"url":"https://www.proteinatlas.org/search/AMN"},"hgnc":{"alias_symbol":["amnionless"],"prev_symbol":[]},"alphafold":{"accession":"Q9BXJ7","domains":[{"cath_id":"-","chopping":"25-104","consensus_level":"medium","plddt":89.7134,"start":25,"end":104},{"cath_id":"-","chopping":"119-226","consensus_level":"medium","plddt":92.8303,"start":119,"end":226},{"cath_id":"3.30.70,3.30.70","chopping":"235-350","consensus_level":"high","plddt":91.5153,"start":235,"end":350}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9BXJ7","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9BXJ7-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9BXJ7-F1-predicted_aligned_error_v6.png","plddt_mean":80.5},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=AMN","jax_strain_url":"https://www.jax.org/strain/search?query=AMN"},"sequence":{"accession":"Q9BXJ7","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9BXJ7.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9BXJ7/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9BXJ7"}},"corpus_meta":[{"pmid":"14576052","id":"PMC_14576052","title":"The 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Neither protein alone confers ligand endocytosis in transfected cells.\",\n      \"method\": \"Co-purification by IF-cobalamin affinity and nondenaturing gel filtration chromatography; cotransfection of AMN and truncated cubilin constructs in polarized epithelial cells; subcellular localization imaging; endocytosis/lysosomal degradation assays\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — biochemical co-purification, functional reconstitution in transfected cells with multiple orthogonal readouts (trafficking, endocytosis, lysosomal degradation), replicated concept across multiple subsequent studies\",\n      \"pmids\": [\"14576052\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"The amnionless (Amn) gene encodes a novel type I transmembrane protein expressed exclusively in extraembryonic visceral endoderm during gastrulation; its extracellular region contains a cysteine-rich domain with similarity to BMP-binding domains in chordin and its Drosophila homolog Sog. Loss of Amn function in the visceral endoderm specifically impairs middle primitive streak formation, resulting in absence of non-axial trunk mesoderm.\",\n      \"method\": \"Positional cloning of insertional mouse mutation; in situ hybridization for expression pattern; chimera analysis (amn−/− ES cells ↔ wild-type blastocysts); histological and molecular phenotypic characterization\",\n      \"journal\": \"Nature genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — loss-of-function mouse mutation with defined cellular phenotype, chimera rescue experiments, replicated across two independent studies (PMIDs 11279523 and 9851841)\",\n      \"pmids\": [\"11279523\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"The amn gene product is required specifically in extraembryonic tissues (visceral endoderm) for the generation of middle primitive streak derivatives; chimera analysis demonstrated that wild-type ES cells cannot rescue the mutant defect when the visceral endoderm lacks amn function, placing Amn action in the visceral endoderm signaling pathway during gastrulation.\",\n      \"method\": \"Wild-type ES cell ↔ amn−/− blastocyst chimera analysis; histological and molecular marker analysis of gastrulation-stage embryos\",\n      \"journal\": \"Developmental biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic epistasis via chimera analysis with defined tissue-of-action, replicated and extended in later studies\",\n      \"pmids\": [\"9851841\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Homozygous mutations affecting exons 1–4 of human AMN cause selective malabsorption of vitamin B12 (Imerslund-Gräsbeck syndrome) in otherwise normal individuals, demonstrating that the 5′ end of AMN is dispensable for embryonic development but necessary for intestinal vitamin B12 absorption. When the 5′ end is truncated, translation initiates from alternative downstream start codons.\",\n      \"method\": \"Human genetic analysis (mutation identification in IGS patients); RT-PCR demonstration of alternative translation initiation\",\n      \"journal\": \"Nature genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — human genetics with functional inference from mutation analysis and alternative initiation demonstration, single study\",\n      \"pmids\": [\"12590260\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"In vivo, Amn is required for cell-surface localization and endocytic function of cubilin in kidney proximal tubules and visceral endoderm; Amn−/− chimeric mice exhibit selective proteinuria of cubilin ligands, and cubilin is not properly localized to the cell surface in Amn−/− tissues.\",\n      \"method\": \"Amn−/− ES cell ↔ wild-type blastocyst chimera analysis; immunolocalization of cubilin in embryonic and adult tissues; urinary protein analysis\",\n      \"journal\": \"Development\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — in vivo genetic loss-of-function with direct immunolocalization and functional urinary phenotype, consistent with in vitro data from PMID 14576052\",\n      \"pmids\": [\"15342463\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"In vivo, AMN dysfunction abrogates cubilin processing and targeting to the apical brush-border membrane in the intestine and kidney, as demonstrated in dogs with AMN mutations (in-frame deletion in exon 10 or disruption of the translation initiation codon), recapitulating human Imerslund-Gräsbeck syndrome. These essential features were reproduced in a heterologous cell-transfection system.\",\n      \"method\": \"Canine genetic model; in vivo immunofluorescence and Western blotting of intestinal and renal tissues; heterologous cell transfection validation\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — in vivo animal model with direct protein localization assays, validated in cell transfection system, orthogonal to mouse and human data\",\n      \"pmids\": [\"15845892\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"AMN interacts with the EGF domains of cubilin (not solely the N-terminal domain), is responsible for membrane attachment and ER export of the cubam complex, and prevents secretion of cubilin into the medium. Apical sorting of cubilin is mediated by its own extracellular region in a glycosylation-dependent manner, and inhibition of glycosylation (tunicamycin) abolished apical sorting preference.\",\n      \"method\": \"Transient transfection of AMN and cubilin truncation mutants in polarized MDCK cells; co-immunoprecipitation; confocal colocalization; tunicamycin glycosylation inhibition\",\n      \"journal\": \"Journal of the American Society of Nephrology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal co-IP, multiple cubilin truncation constructs, functional apical targeting assay, glycosylation inhibition experiment in one rigorous study\",\n      \"pmids\": [\"15976000\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Cubilin interacts with both AMN and megalin in renal brush-border membranes; three distinct regions of cubilin (113-residue N-terminus, CUB domains 12–17, and 22–27) bind megalin in a Ca2+-dependent manner. Both megalin and AMN interactions are required for intracellular stability of cubilin: silencing either megalin or AMN in opossum kidney cells reduced cubilin staining by 85–90% and decreased cubilin half-life 2-fold.\",\n      \"method\": \"In vitro Ca2+-dependent binding assays with cubilin fragments; immunoprecipitation and immunoblotting of renal brush-border membrane extracts; ligand-affinity chromatography; gene silencing (siRNA) with immunohistochemistry and turnover studies\",\n      \"journal\": \"Biochemical Journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal methods (in vitro binding, co-IP, affinity chromatography, gene silencing with functional readout) in one study\",\n      \"pmids\": [\"17990981\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"The cytosolic domain of AMN contains two FXNPXF motifs that are functionally redundant; both signals can individually mediate endocytosis of the cubam complex through direct interaction with the clathrin-associated sorting proteins Dab2 and ARH, as demonstrated by sequential mutagenesis and yeast two-hybrid analysis.\",\n      \"method\": \"Sequential mutagenesis of AMN FXNPXF motifs; functional endocytosis assays with AMN mutant panel; yeast two-hybrid analysis of AMN–Dab2 and AMN–ARH interactions\",\n      \"journal\": \"Traffic\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Moderate — mutagenesis combined with functional endocytosis assays and yeast two-hybrid interaction mapping in one study\",\n      \"pmids\": [\"20088845\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Drosophila orthologs of cubilin (dCubilin) and amnionless (dAMN) are specifically expressed in nephrocytes and function as co-receptors for protein reabsorption. Human AMN expressed in Drosophila nephrocytes rescues defective protein uptake caused by dAMN knockdown, demonstrating evolutionary conservation of the Cubilin/AMN co-receptor function.\",\n      \"method\": \"Drosophila genetic screen; tissue-specific RNAi knockdown; transgenic rescue with human AMN; fluorescent protein uptake assays; electron microscopy of nephrocyte ultrastructure\",\n      \"journal\": \"Journal of the American Society of Nephrology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic loss-of-function with defined cellular phenotype, cross-species rescue experiment with human AMN, orthogonal structural and functional readouts\",\n      \"pmids\": [\"23264686\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"AMN-mediated glycosylation of cubilin is necessary (but interaction between cubilin and AMN is not sufficient) for cell-surface expression of the cubam complex in renal and intestinal cells. Quantitative mass spectrometry and mutagenesis identified N-linked glycosylation of at least 4 specific cubilin residues as required for its plasma membrane targeting. IGS-causing AMN missense mutations cause ER retention of both AMN and cubilin.\",\n      \"method\": \"Quantitative cell-surface targeting assay in cultured renal and intestinal cells; mass spectrometry glycosylation mapping; site-directed mutagenesis of glycosylation sites; immunofluorescence of patient renal proximal tubular cells\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Moderate — mutagenesis, mass spectrometry, functional targeting assay, and patient tissue validation in one rigorous study\",\n      \"pmids\": [\"29402915\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"In humans with compound heterozygous AMN mutations (premature stop codon in exon 7), cubilin luminal expression is dramatically decreased in the absence of any CUBN mutation, despite normal intrinsic factor-binding affinity of residual receptor, establishing that AMN is essential for correct luminal expression of cubilin in vivo.\",\n      \"method\": \"Genetic mutation analysis; measurement of cubilin receptor activity in urine; intrinsic factor-binding affinity assay\",\n      \"journal\": \"Haematologica\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — human genetics with functional urinary receptor assay, single study, limited mechanistic depth\",\n      \"pmids\": [\"21750092\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"In Drosophila nephrocytes, store-operated calcium entry (SOCE) via Stim/Orai mediates the localization of the endocytic co-receptor Amnionless at the nephrocyte surface; RNAi knockdown of Stim or Orai reduced Amnionless abundance and disrupted albumin binding and accumulation.\",\n      \"method\": \"GCaMP6 calcium reporter live imaging in Drosophila nephrocytes; RNAi knockdown of SOCE genes (Stim, Orai); pharmacological inhibition (EGTA, 2-APB); fluorescent albumin uptake assays; immunofluorescence\",\n      \"journal\": \"Journal of insect physiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — genetic knockdown with functional and localization readouts in a Drosophila ortholog model, single study\",\n      \"pmids\": [\"36341969\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"AMN (amnionless) encodes a type I transmembrane protein that forms the cubam complex with cubilin by binding to cubilin's EGF domains; within this complex, AMN serves as the membrane anchor that drives ER export, apical trafficking, and N-linked glycosylation of cubilin required for plasma membrane targeting, while its cytoplasmic FXNPXF motifs recruit the clathrin-associated adaptors Dab2 and ARH to mediate clathrin-dependent endocytosis of cubam ligands including intrinsic factor-cobalamin (vitamin B12) in the intestine and low-molecular-weight proteins in the kidney proximal tubule; AMN also stabilizes cubilin protein in the cell, and its loss—whether by mutation in humans/dogs or gene knockout in mice—abrogates cubilin surface expression and causes Imerslund-Gräsbeck syndrome, while in early mouse development AMN acts in the visceral endoderm to direct middle primitive streak formation.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"AMN (amnionless) encodes a type I transmembrane protein that partners with cubilin to form the endocytic co-receptor complex cubam, governing receptor-mediated uptake of specific ligands in absorptive epithelia [#0]. AMN binds cubilin's amino-terminal region and EGF domains and serves as the membrane anchor that drives ER export, prevents secretion of cubilin, and directs cubilin trafficking to the apical surface and endosomes—functions neither protein performs alone [#0, #6]. Cell-surface delivery of cubam additionally requires AMN-dependent N-linked glycosylation of specific cubilin residues, and IGS-causing AMN missense mutations cause ER retention of both proteins [#10]. AMN is also required for the intracellular stability and luminal expression of cubilin in vivo, acting together with megalin to control cubilin half-life [#7, #11]. The cytosolic tail of AMN carries two functionally redundant FXNPXF motifs that recruit the clathrin-associated sorting adaptors Dab2 and ARH to mediate clathrin-dependent endocytosis of cubam ligands, including intrinsic factor-cobalamin (vitamin B12) [#0, #8]. Loss of AMN function—by mutation in humans and dogs or knockout in mice—abrogates cubilin surface targeting and causes selective vitamin B12 malabsorption and proteinuria characteristic of Imerslund-Gräsbeck syndrome [#3, #4, #5]. Independently of its endocytic role, AMN acts in the extraembryonic visceral endoderm during gastrulation to direct middle primitive streak formation and generation of non-axial trunk mesoderm [#1, #2]; the 5' end of AMN required for development is dispensable for B12 absorption [#3].\",\n  \"teleology\": [\n    {\n      \"year\": 1998,\n      \"claim\": \"Established that AMN function is required specifically in the extraembryonic visceral endoderm—not the embryo proper—for gastrulation, defining its tissue of action by genetic epistasis.\",\n      \"evidence\": \"Wild-type ES cell ↔ amn−/− blastocyst chimera analysis with marker-based phenotyping of gastrulation embryos\",\n      \"pmids\": [\"9851841\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular mechanism of the developmental defect not defined\", \"No link yet to a receptor or endocytic function\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Identified AMN as a novel type I transmembrane protein with a chordin/Sog-like cysteine-rich extracellular domain, and showed its loss specifically blocks middle primitive streak formation.\",\n      \"evidence\": \"Positional cloning of an insertional mouse mutation, in situ hybridization, and chimera/phenotypic analysis\",\n      \"pmids\": [\"11279523\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether the BMP-binding-like domain functions in signaling untested\", \"No biochemical partner identified at this stage\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Connected AMN biochemically to cubilin, revealing the cubam complex as the unit that confers ligand endocytosis and cubilin trafficking to the cell surface—neither protein suffices alone.\",\n      \"evidence\": \"IF-cobalamin affinity co-purification, gel filtration, cotransfection of AMN with truncated cubilin in polarized epithelia, and endocytosis/degradation assays\",\n      \"pmids\": [\"14576052\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Cytoplasmic endocytic signal not yet mapped\", \"Glycosylation requirement not addressed\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Defined the human disease link, showing AMN mutations cause Imerslund-Gräsbeck syndrome and that the developmentally essential 5' end is dispensable for B12 absorption via alternative downstream translation initiation.\",\n      \"evidence\": \"Mutation identification in IGS patients and RT-PCR demonstration of alternative translation initiation\",\n      \"pmids\": [\"12590260\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single study\", \"Functional consequence inferred rather than directly reconstituted for each allele\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Demonstrated in vivo that AMN is required for cubilin cell-surface localization and endocytic function, linking AMN loss to selective proteinuria of cubilin ligands.\",\n      \"evidence\": \"Amn−/− ↔ wild-type chimeric mice with cubilin immunolocalization and urinary protein analysis\",\n      \"pmids\": [\"15342463\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Trafficking step blocked not resolved at the molecular level\", \"Did not distinguish glycosylation from anchoring defects\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Mapped AMN's structural role: it binds cubilin EGF domains, provides membrane attachment and ER export, and prevents cubilin secretion, while apical sorting depends on cubilin's glycosylated extracellular region.\",\n      \"evidence\": \"Cubilin truncation mutants and AMN co-transfection in polarized MDCK cells with co-IP, confocal colocalization, and tunicamycin glycosylation inhibition\",\n      \"pmids\": [\"15976000\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Specific glycosylation sites not yet identified\", \"Endocytic adaptor recruitment not defined\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Validated the cubilin-targeting role of AMN across species using a canine genetic model, showing AMN dysfunction abrogates cubilin processing and apical brush-border targeting and recapitulates human IGS.\",\n      \"evidence\": \"Canine AMN mutants with intestinal/renal immunofluorescence and immunoblotting, plus heterologous transfection\",\n      \"pmids\": [\"15845892\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular basis of processing defect for each mutation not dissected\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Showed cubilin requires both AMN and megalin for intracellular stability, establishing AMN as a determinant of cubilin half-life rather than only trafficking.\",\n      \"evidence\": \"In vitro Ca2+-dependent binding of cubilin fragments, brush-border co-IP, affinity chromatography, and siRNA silencing with turnover studies in opossum kidney cells\",\n      \"pmids\": [\"17990981\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism by which AMN stabilizes cubilin not defined\", \"Relative contributions of AMN vs megalin not separated\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Identified the cytoplasmic endocytic code: two redundant FXNPXF motifs in AMN that directly recruit the clathrin-associated adaptors Dab2 and ARH to drive cubam internalization.\",\n      \"evidence\": \"Sequential mutagenesis of AMN motifs, functional endocytosis assays, and yeast two-hybrid mapping of AMN–Dab2/ARH interactions\",\n      \"pmids\": [\"20088845\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether both adaptors act in the same tissue context unresolved\", \"Reciprocal validation of adaptor interactions in native cells not shown\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Confirmed in human patients that AMN is essential for luminal cubilin expression independent of CUBN mutation, even when residual receptor retains intrinsic factor binding.\",\n      \"evidence\": \"Genetic analysis of compound heterozygous AMN mutations with urinary cubilin receptor activity and IF-binding affinity assays\",\n      \"pmids\": [\"21750092\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single study with limited mechanistic depth\", \"Quantitative trafficking defect not measured\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Demonstrated evolutionary conservation of the cubilin/AMN co-receptor function, with human AMN rescuing protein-uptake defects in Drosophila nephrocytes lacking dAMN.\",\n      \"evidence\": \"Drosophila tissue-specific RNAi, transgenic rescue with human AMN, fluorescent uptake assays, and nephrocyte electron microscopy\",\n      \"pmids\": [\"23264686\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Conservation of glycosylation and adaptor mechanisms in flies not tested\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Resolved the glycosylation requirement, showing AMN-mediated N-linked glycosylation of at least four specific cubilin residues is necessary for cubam surface targeting and that AMN–cubilin interaction alone is insufficient.\",\n      \"evidence\": \"Quantitative surface-targeting assays in renal/intestinal cells, mass spectrometry glycosylation mapping, site-directed mutagenesis, and patient proximal tubule immunofluorescence\",\n      \"pmids\": [\"29402915\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How AMN promotes cubilin glycosylation mechanistically unknown\", \"Whether glycosylation defect is the sole cause of ER retention for all alleles unclear\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Linked calcium signaling to AMN surface localization, showing store-operated calcium entry via Stim/Orai is required to maintain Amnionless at the nephrocyte surface.\",\n      \"evidence\": \"GCaMP6 calcium imaging, Stim/Orai RNAi, pharmacological SOCE inhibition, and albumin uptake assays in Drosophila nephrocytes\",\n      \"pmids\": [\"36341969\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single study in a fly ortholog\", \"Whether SOCE regulates mammalian AMN trafficking untested\", \"Direct molecular link between calcium entry and AMN localization not defined\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The molecular mechanism by which AMN promotes cubilin N-linked glycosylation and ER export, and whether AMN's developmental visceral-endoderm role is mechanistically distinct from its endocytic co-receptor function, remain unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No structural model of the cubam complex\", \"Chordin/Sog-like domain function in development not tested biochemically\", \"Mechanism coupling AMN to the glycosylation machinery unknown\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0038024\", \"supporting_discovery_ids\": [0, 4, 9]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [8]},\n      {\"term_id\": \"GO:0140313\", \"supporting_discovery_ids\": [6, 7, 10]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 4, 6]},\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [6, 10]},\n      {\"term_id\": \"GO:0005768\", \"supporting_discovery_ids\": [0]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-5653656\", \"supporting_discovery_ids\": [0, 8]},\n      {\"term_id\": \"R-HSA-382551\", \"supporting_discovery_ids\": [0, 4]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [1, 2]}\n    ],\n    \"complexes\": [\"cubam (cubilin–AMN co-receptor complex)\"],\n    \"partners\": [\"CUBN\", \"DAB2\", \"LRP2\", \"ARH\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":7,"faith_total":7,"faith_pct":100.0}}