{"gene":"KDELR1","run_date":"2026-04-28T18:30:27","timeline":{"discoveries":[{"year":1990,"finding":"The yeast ERD2 gene encodes a 26 kDa integral membrane protein (the HDEL receptor) whose abundance determines the efficiency and capacity of the ER protein retention/retrieval system; reduced ERD2 expression causes secretion of HDEL-tagged proteins, while overexpression improves retention.","method":"Yeast genetics, gene expression manipulation, secretion assays","journal":"Cell","confidence":"High","confidence_rationale":"Tier 2 — foundational genetic epistasis with multiple orthogonal readouts, replicated in accompanying paper, 464 citations","pmids":["2194670"],"is_preprint":false},{"year":1990,"finding":"A human homologue of yeast ERD2 (hERD2/KDELR1) encodes a ~26 kDa non-glycosylated integral membrane protein with properties similar to the yeast HDEL receptor, proposed to be the mammalian KDEL receptor.","method":"Sequence analysis, biochemical characterization of the expressed protein","journal":"Nature","confidence":"High","confidence_rationale":"Tier 2 — cloning and functional characterization, 291 citations, foundational paper","pmids":["2172835"],"is_preprint":false},{"year":1990,"finding":"The specificity of the luminal ER protein retention system is determined by the ERD2 gene product (the receptor): exchanging the ERD2 gene between S. cerevisiae and K. lactis alters which C-terminal signals (HDEL vs. DDEL) are recognized, proving ERD2 encodes the sorting receptor.","method":"Inter-species gene exchange, retention signal specificity assays","journal":"Cell","confidence":"High","confidence_rationale":"Tier 1 — direct genetic substitution demonstrating receptor specificity, 211 citations","pmids":["2194671"],"is_preprint":false},{"year":1992,"finding":"Ligand overexpression causes redistribution of hERD2 (KDELR1) from the Golgi apparatus to the ER; mutation of hERD2 alters ligand specificity of this redistribution, demonstrating direct receptor-ligand interaction and ligand-induced receptor movement as a regulatory mechanism.","method":"Overexpression of KDEL/DDEL ligands, immunolocalization, site-directed mutagenesis","journal":"Cell","confidence":"High","confidence_rationale":"Tier 2 — mutagenesis combined with localization studies showing direct receptor-ligand interaction, 365 citations","pmids":["1310258"],"is_preprint":false},{"year":1993,"finding":"Ligand binding to the human KDEL receptor depends on charged residues within the transmembrane domains; retrograde transport of occupied receptor requires a critical aspartic acid in the seventh transmembrane domain; Golgi retention is independent of ligand binding and this aspartate, indicating distinct structural requirements for each function.","method":"Mutagenesis of transmembrane residues, intracellular localization assays, ligand-binding assays in COS cells","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1 — systematic mutagenesis with multiple functional readouts, 127 citations","pmids":["8392934"],"is_preprint":false},{"year":1994,"finding":"Growth of yeast requires the HDEL-dependent retrieval activity of Erd2p (KDELR1 ortholog); mutations that block receptor recycling also prevent growth; Golgi retention of the receptor is independent of recycling, but retrieval of specific HDEL-containing proteins is essential for viability.","method":"Erd2p mutant analysis, receptor overexpression saturation, viability assays","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 2 — epistasis and mutational analysis with defined phenotypic readouts, 38 citations","pmids":["7929564"],"is_preprint":false},{"year":1997,"finding":"The KDEL receptor ERD2 (KDELR1 ortholog) self-oligomerizes and interacts with ARF1 GTPase-activating protein (ARF1 GAP), recruiting cytosolic ARF1 GAP to membranes; ERD2 overexpression enhances GAP membrane recruitment and produces a phenotype reflecting ARF1 inactivation, indicating ERD2 regulates ARF1-mediated vesicle transport.","method":"Co-immunoprecipitation, overexpression phenotype analysis, membrane recruitment assays","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 2 — reciprocal Co-IP with functional phenotype analysis, 149 citations","pmids":["9405360"],"is_preprint":false},{"year":1997,"finding":"Overexpression of either ERD2.1 or ERD2.2 (KDELR1 paralogs) significantly increases cellular capacity to retain both KDEL- and HNEL-containing proteins; pulse-chase and immunolocalization show long half-life and Golgi localization for both receptors, and the novel HNEL signal of RAP interacts with the same ER retention receptors as KDEL.","method":"Stable transfection, pulse-chase labeling, immunoelectron microscopy, retention assays","journal":"Journal of cell science","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods showing ERD2 proteins retain novel ER retention signals","pmids":["9010785"],"is_preprint":false},{"year":1998,"finding":"The sequence 22KIWK25 within a lumenal loop of the human KDEL receptor (ERD2/KDELR1) is essential for binding to KDEL-containing ER lumenal proteins (CaBP1 and CaBP2); binding is of high specificity and almost completely inhibited by KDEL-containing soluble peptides.","method":"Cellulose-bound overlapping peptide arrays, binding inhibition with KDEL peptides","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 3 — peptide mapping approach identifies binding site but limited to in vitro peptide assay","pmids":["9642148"],"is_preprint":false},{"year":2001,"finding":"KDEL ligand binding induces oligomerization of ERD2 (KDELR1) and recruitment of ARFGAP to the Golgi, where the ERD2 oligomer/ARFGAP complex interacts with membrane-bound ARF1; during KDEL ligand transport, ERD2 interactions with β-COP and p23 decrease and the proteins segregate, revealing how cargo-induced ERD2 oligomerization regulates sorting into COPI-coated buds.","method":"FRET between CFP/YFP fusion proteins by multifocal multiphoton microscopy in living cells","journal":"Developmental cell","confidence":"High","confidence_rationale":"Tier 1/2 — live-cell FRET imaging with multiple interactors quantified dynamically, 149 citations","pmids":["11703931"],"is_preprint":false},{"year":2003,"finding":"Impairment of KDEL receptor (KDELR1) retrieval function by expression of a ligand-recognition mutant causes mis-sorting of the ER chaperone BiP and induces intense ER stress, accompanied by activation of p38 MAP kinase and JNK1; ligand-induced activation of the KDEL receptor also induces phosphorylation of p38 MAP kinase, indicating the KDEL receptor modulates ER stress response through MAPK signaling.","method":"Mutant KDEL receptor expression, ER stress assays, MAP kinase phosphorylation assays, p38 inhibitor studies in HeLa cells","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — loss-of-function and pharmacological inhibition with defined signaling readouts, 84 citations","pmids":["12821650"],"is_preprint":false},{"year":2003,"finding":"PKA phosphorylation of serine 209 in the C-terminal cytoplasmic domain of the KDEL receptor (KDELR1) is required for retrograde Golgi-to-ER transport of the receptor and for intracellular retention of KDEL ligands; this domain interacts with coatomer and ARF-GAP only when Ser209 is phosphorylated (mimicked by S209D mutation); inhibition of PKA with H89 blocks receptor redistribution to the ER.","method":"Truncation and point mutagenesis, peptide-binding assays with coatomer/ARF-GAP, PKA inhibitor (H89), permeabilized cell transport assays","journal":"Molecular biology of the cell","confidence":"High","confidence_rationale":"Tier 1/2 — mutagenesis, in vitro phosphorylation, pharmacological inhibition with functional transport readouts, 78 citations","pmids":["14517323"],"is_preprint":false},{"year":2003,"finding":"Src kinase activity controls KDEL receptor (KDELR1) localization: activated Src relocates KDEL-R from the Golgi to the ER, and loss of Src (in SYF cells) perturbs Golgi organization; retrograde transport of Pseudomonas exotoxin (which uses the KDEL-R) is accelerated by Src inhibition or ablation.","method":"Activated Src expression, SYF knockout cell line, immunofluorescence localization, toxin transport assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — genetic and pharmacological Src manipulation with specific KDEL-R localization and transport readouts, 91 citations","pmids":["12975382"],"is_preprint":false},{"year":2007,"finding":"Three human KDEL receptors (KDELR1, KDELR2, KDELR3) each have unique specificity profiles for KDEL-like C-terminal motifs; KDELR1 interacts with a distinct subset of KDEL variants compared to KDELR2 and KDELR3, as determined by a bimolecular fluorescence complementation screen; all three receptors localize to the Golgi.","method":"Reporter construct screen of 152 KDEL variants, bimolecular fluorescence complementation (BiFC) to determine receptor-ligand specificity, Golgi localization assays","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 2 — systematic screen with 152 variants and BiFC specificity mapping, 214 citations","pmids":["18086916"],"is_preprint":false},{"year":2012,"finding":"The KDEL receptor (KDELR1) is predicted to fold like a G-protein-coupled receptor (GPCR) and directly binds and activates the heterotrimeric G-protein Gαq/11; this Gαq/11 activation regulates transport through the Golgi complex, revealing an unexpected GPCR-like signaling mode for the KDEL receptor.","method":"GPCR structural prediction, G-protein binding and activation assays, Golgi transport assays","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 2 — G-protein binding demonstrated biochemically with functional transport readouts, 103 citations","pmids":["22580821"],"is_preprint":false},{"year":2014,"finding":"ER-to-Golgi cargo transport activates the KDEL receptor (KDELR1) at the Golgi, which triggers a signaling cascade involving Gs protein, adenylyl cyclase, phosphodiesterase isoforms, and PKA, leading to phosphorylation of transport machinery proteins; this induces retrograde traffic to the ER to balance anterograde flux; additionally, the KDEL receptor activates CREB1 and other transcription factors that upregulate transport-related genes.","method":"Pharmacological perturbations of signaling components, phosphorylation assays, transcription factor activation assays","journal":"Developmental cell","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods dissecting the signaling cascade with functional transport and transcriptional readouts, 92 citations","pmids":["25117681"],"is_preprint":false},{"year":2015,"finding":"A recessive missense allele of KDELR1 in mice causes cell-intrinsic lymphopenia; homozygous mutant and CRISPR/Cas9 frameshift T cells show reduced TCR surface expression, increased CD44, and impaired viral clearance; the phenotype can be partially corrected by an MHC class I-restricted TCR transgene, revealing a nonredundant role for KDELR1 in lymphocyte homeostasis.","method":"Mouse genetics (ENU missense and CRISPR/Cas9 frameshift alleles), bone marrow chimeras for cell-intrinsic analysis, TCR transgene rescue, flow cytometry","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 — two independent loss-of-function alleles with cell-intrinsic phenotype and transgenic rescue","pmids":["26438836"],"is_preprint":false},{"year":2015,"finding":"KDELR1 regulates integrated stress responses (ISR) to promote naive T-cell survival; in mice with nonfunctional KDELR1 (T-Red mice), naive T cells show excessive ISR and undergo apoptosis; strong TCR-mediated signals suppress ISR, and surviving naive T cells in KDELR1-deficient mice express higher CD5 and exhibit higher TCR affinity, demonstrating that KDELR1 deficiency-induced ISR can be counteracted by strong TCR signals.","method":"T-Red mouse model, TCR transgenic rescue, tetramer dissociation assay, altered peptide ligand stimulation, ISR markers","journal":"International immunology","confidence":"High","confidence_rationale":"Tier 2 — in vivo genetic model with TCR affinity measurements and multiple functional readouts","pmids":["26489882"],"is_preprint":false},{"year":2019,"finding":"KDELR1 knockout (HAP1 cells) causes increased secretion of the ER-resident protein PDI, decreased cell viability under ER stress, transcriptional upregulation of genes involved in cell adhesion and ECM composition, and impaired cell adhesion capacity that is partially rescued by collagen/laminin coating, indicating KDELR1 is required for ER homeostasis and normal cell adhesion.","method":"KDELR1 knockout cell line, whole transcriptome analysis, in vitro adhesion assays, PDI secretion assay, ER stress viability assay","journal":"Scientific reports","confidence":"Medium","confidence_rationale":"Tier 2/3 — KO with transcriptomics and functional adhesion assay but limited pathway mechanistic detail","pmids":["31337861"],"is_preprint":false},{"year":2024,"finding":"KDELR1 contributes to chondrosarcoma drug resistance and malignant behavior through the Integrin-PLCγ-YAP1 (Hippo) signaling axis; mass spectrometry proteomics and transcriptomics revealed KDELR1 modulates Hippo-YAP pathway activity in chondrosarcoma cells, affecting ECM formation and chemotherapy resistance.","method":"Single-cell transcriptomics, mass spectrometry proteomics, KDELR1 knockdown/overexpression with proliferation and drug resistance assays","journal":"Cell death & disease","confidence":"Medium","confidence_rationale":"Tier 2/3 — multi-omics pathway identification with functional knockdown/OE, but mechanistic details of the Integrin-PLCγ-YAP1 axis for KDELR1 are limited to correlative proteomics","pmids":["39715773"],"is_preprint":false},{"year":2026,"finding":"KDELR1 and KDELR3 have opposite effects on AGR2 (a mucin folding assistant) production: KDELR1 downregulation decreases AGR2 transcripts while KDELR3 silencing dramatically increases them; silencing ERp44 (but not other ER residents) phenocopies KDELR3 knockdown, suggesting AGR2 regulation by KDELR3 depends on ERp44-KDELR3 interactions; this defines a novel regulatory circuit controlling early secretory pathway composition distinct from the unfolded protein response.","method":"siRNA silencing of KDELR1, KDELR2, KDELR3, and ER residents; transcriptional readout of AGR2; phenocopy experiments with ERp44 silencing","journal":"Cellular and molecular life sciences : CMLS","confidence":"Medium","confidence_rationale":"Tier 2/3 — genetic epistasis by silencing with defined transcriptional readout, but mechanistic basis of KDELR1-specific effect on AGR2 not fully elucidated","pmids":["41706164"],"is_preprint":false}],"current_model":"KDELR1 (human ERD2) is a seven-transmembrane Golgi-resident integral membrane protein that functions as the KDEL-sequence receptor, recognizing escaped ER-lumenal proteins via charged transmembrane residues and a lumenal 22KIWK25 motif; ligand binding triggers receptor oligomerization, ARFGAP recruitment, and PKA-dependent phosphorylation of Ser209 in its cytoplasmic C-terminus to enable COPI-mediated retrograde Golgi-to-ER transport; it additionally acts as a GPCR-like sensor that activates Gαq/11 and Gs-PKA signaling cascades to maintain ER-Golgi transport homeostasis, modulates ER stress responses via p38/JNK MAPK signaling, and plays a nonredundant cell-intrinsic role in lymphocyte survival by suppressing excessive integrated stress responses, with emerging evidence for regulation of cell adhesion and, in opposition to KDELR3, control of AGR2 production in the early secretory pathway."},"narrative":{"teleology":[{"year":1990,"claim":"Identification of ERD2/KDELR1 as the ER retention receptor resolved how cells retrieve escaped lumenal proteins: the receptor's abundance sets system capacity, and species-specific ERD2 exchange switches retention-signal specificity, proving direct receptor–ligand determination.","evidence":"Yeast ERD2 gene manipulation, secretion assays, and inter-species gene exchange between S. cerevisiae and K. lactis; cloning and biochemical characterization of human homologue","pmids":["2194670","2194671","2172835"],"confidence":"High","gaps":["Structural basis of KDEL signal recognition not determined","How receptor cycles between Golgi and ER was unknown","Redundancy among mammalian KDELR paralogs unaddressed"]},{"year":1993,"claim":"Systematic mutagenesis established that ligand binding requires charged transmembrane residues and that retrograde transport depends on a specific aspartate in TM7, separating Golgi retention from recycling functions mechanistically.","evidence":"Transmembrane point mutagenesis in COS cells with ligand-binding and localization readouts; ligand overexpression-induced receptor redistribution and specificity-altering mutations","pmids":["8392934","1310258","7929564"],"confidence":"High","gaps":["Identity of coat machinery coupling the receptor to COPI vesicles was unclear","Lumenal residues involved in ligand binding not yet mapped"]},{"year":1998,"claim":"Mapping of the lumenal ligand-binding site to the 22KIWK25 motif defined the minimal receptor determinant contacting KDEL-containing proteins.","evidence":"Cellulose-bound overlapping peptide arrays with soluble KDEL peptide competition","pmids":["9642148"],"confidence":"Medium","gaps":["In vitro peptide assay only; not confirmed by crystallography or in-cell crosslinking","Contribution of individual residues within KIWK not dissected"]},{"year":2001,"claim":"Live-cell FRET imaging revealed that ligand binding induces receptor oligomerization and recruits ARFGAP to Golgi membranes, establishing the mechanistic link between cargo recognition and COPI bud formation.","evidence":"CFP/YFP FRET with multifocal multiphoton microscopy measuring interactions among ERD2, ARFGAP, ARF1, β-COP, and p23 in living cells","pmids":["11703931","9405360"],"confidence":"High","gaps":["Stoichiometry of the oligomer and precise ARFGAP interaction surface unknown","Whether oligomerization is required for signaling (vs. transport) not tested"]},{"year":2003,"claim":"Two parallel regulatory inputs were defined: PKA-dependent Ser209 phosphorylation is required for coatomer/ARFGAP engagement and retrograde transport, while Src kinase activity controls receptor steady-state distribution between Golgi and ER.","evidence":"Point and truncation mutagenesis, PKA inhibitor H89, permeabilized-cell transport assays; activated Src expression and SYF knockout cells with toxin transport readouts","pmids":["14517323","12975382"],"confidence":"High","gaps":["Direct Src phosphorylation site on KDELR1 not identified","Relationship between Src and PKA inputs not integrated"]},{"year":2003,"claim":"Loss of KDELR1 retrieval function induces ER stress with p38/JNK MAPK activation, establishing KDELR1 as a modulator of ER stress signaling beyond simple cargo sorting.","evidence":"Dominant-negative KDELR1 mutant expression, BiP mis-sorting, p38/JNK phosphorylation assays, p38 inhibitor studies in HeLa cells","pmids":["12821650"],"confidence":"High","gaps":["Whether MAPK activation is a direct receptor signal or secondary to ER stress was not resolved","Downstream transcriptional targets of p38/JNK in this context not identified"]},{"year":2007,"claim":"A systematic BiFC screen of 152 KDEL variants showed that KDELR1, KDELR2, and KDELR3 each have unique ligand-specificity profiles, explaining how the three paralogs partition retrieval duties.","evidence":"Bimolecular fluorescence complementation screen with 152 C-terminal peptide variants and Golgi colocalization","pmids":["18086916"],"confidence":"High","gaps":["Functional consequence of differential specificity on ER proteome maintenance not tested","Structural basis of specificity divergence unknown"]},{"year":2012,"claim":"Discovery that KDELR1 adopts a GPCR-like fold and directly activates Gαq/11 reframed the receptor as a signaling platform, not just a cargo sorter, coupling ligand occupancy to heterotrimeric G-protein cascades that regulate Golgi transport.","evidence":"GPCR structural modeling, G-protein binding and activation assays, Golgi transport assays","pmids":["22580821"],"confidence":"High","gaps":["High-resolution structure of KDELR1 bound to Gα subunit lacking","How Gαq/11 and Gs pathways are differentially engaged was unclear"]},{"year":2014,"claim":"A complete Gs–adenylyl cyclase–PKA signaling cascade downstream of KDELR1 was mapped, showing that anterograde cargo flux activates the receptor to drive compensatory retrograde traffic and to upregulate transport gene expression via CREB1.","evidence":"Pharmacological dissection of signaling components, phosphorylation and transcription factor activation assays","pmids":["25117681"],"confidence":"High","gaps":["Identity of specific transport genes upregulated via CREB1 not fully catalogued","Quantitative relationship between cargo load and signaling output not measured"]},{"year":2015,"claim":"In vivo loss-of-function established that KDELR1 is nonredundant for lymphocyte homeostasis: mutant mice show cell-intrinsic lymphopenia driven by excessive integrated stress responses in naive T cells, which can be counteracted by strong TCR signals.","evidence":"ENU missense and CRISPR/Cas9 frameshift alleles in mice, bone marrow chimeras, TCR transgenic rescue, tetramer dissociation assays","pmids":["26438836","26489882"],"confidence":"High","gaps":["Specific ER client(s) whose mis-sorting triggers ISR in T cells not identified","Whether KDELR2/3 are expressed in T cells and why they fail to compensate not determined"]},{"year":2019,"claim":"KDELR1 knockout in human cells revealed a requirement for normal cell adhesion and ER stress resilience, with transcriptional remodeling of ECM and adhesion gene programs upon receptor loss.","evidence":"HAP1 KDELR1 knockout, transcriptomics, PDI secretion assay, adhesion assays with collagen/laminin rescue","pmids":["31337861"],"confidence":"Medium","gaps":["Mechanism linking KDELR1 loss to adhesion gene upregulation not resolved","Single cell line; generalizability across tissues not tested"]},{"year":2024,"claim":"Multi-omics analysis linked KDELR1 to Integrin–PLCγ–YAP1 (Hippo) signaling in chondrosarcoma, suggesting a role in drug resistance through ECM-related pathway modulation.","evidence":"Single-cell transcriptomics and mass spectrometry proteomics with KDELR1 knockdown/overexpression in chondrosarcoma cells","pmids":["39715773"],"confidence":"Medium","gaps":["Direct molecular connection between KDELR1 and Integrin-PLCγ axis not biochemically validated","Applicability beyond chondrosarcoma not assessed"]},{"year":2026,"claim":"Epistasis experiments showed KDELR1 and KDELR3 exert opposing transcriptional control over AGR2, with ERp44 mediating the KDELR3 effect, defining a paralog-specific regulatory circuit independent of the unfolded protein response.","evidence":"siRNA silencing of individual KDELRs and ER-resident proteins with AGR2 transcript readout","pmids":["41706164"],"confidence":"Medium","gaps":["Mechanistic basis of KDELR1's positive effect on AGR2 transcription unknown","Whether this circuit operates via KDELR1 signaling or simply cargo retrieval not distinguished"]},{"year":null,"claim":"A high-resolution structure of KDELR1 in complex with a KDEL ligand and/or G-protein, and identification of the specific ER client(s) whose mis-sorting drives T-cell ISR, remain the principal open questions for fully integrating the receptor's cargo-sorting and signaling functions.","evidence":"","pmids":[],"confidence":"High","gaps":["No atomic-resolution structure of ligand-bound or G-protein-bound KDELR1","Critical ER client(s) mediating T-cell survival phenotype not identified","Quantitative model relating cargo load to Gs vs. Gαq signaling output absent"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0038024","term_label":"cargo receptor activity","supporting_discovery_ids":[0,2,3,4,8,13]},{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[14,15]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[6,9,11,15]}],"localization":[{"term_id":"GO:0005794","term_label":"Golgi apparatus","supporting_discovery_ids":[3,7,9,13]},{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[3,12]}],"pathway":[{"term_id":"R-HSA-9609507","term_label":"Protein localization","supporting_discovery_ids":[0,5,9,11]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[10,14,15]},{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[16,17]},{"term_id":"R-HSA-8953897","term_label":"Cellular responses to stimuli","supporting_discovery_ids":[10,17,18]}],"complexes":["KDELR1 homo-oligomer","KDELR1–ARFGAP1–ARF1 complex"],"partners":["ARF1","ARFGAP1","GNAQ","GNAS","COPB1","TMED10"],"other_free_text":[]},"mechanistic_narrative":"KDELR1 is a seven-transmembrane Golgi-resident receptor that recognizes KDEL and related C-terminal retention motifs on escaped ER-lumenal proteins and mediates their COPI-dependent retrograde retrieval from the Golgi to the ER, thereby maintaining ER proteostasis [PMID:2194670, PMID:2172835, PMID:2194671]. Ligand binding depends on charged transmembrane residues and a lumenal 22KIWK25 motif, triggers receptor oligomerization, recruits ARFGAP1 to Golgi membranes, and requires PKA-dependent phosphorylation of Ser209 for retrograde transport [PMID:8392934, PMID:9642148, PMID:9405360, PMID:14517323]. Beyond cargo retrieval, KDELR1 functions as a GPCR-like signaling platform that activates Gαq/11- and Gs-PKA-dependent cascades to coordinate anterograde and retrograde membrane traffic and to modulate ER stress responses via p38/JNK MAPK signaling [PMID:22580821, PMID:25117681, PMID:12821650]. Loss-of-function mutations in mice cause cell-intrinsic lymphopenia driven by excessive integrated stress responses and impaired naive T-cell survival, establishing a nonredundant role for KDELR1 in lymphocyte homeostasis [PMID:26438836, PMID:26489882]."},"prefetch_data":{"uniprot":{"accession":"P24390","full_name":"ER lumen protein-retaining receptor 1","aliases":["KDEL endoplasmic reticulum protein retention receptor 1","KDEL receptor 1","Putative MAPK-activating protein PM23"],"length_aa":212,"mass_kda":24.5,"function":"Receptor for the C-terminal sequence motif K-D-E-L that is present on endoplasmic reticulum resident proteins and that mediates their recycling from the Golgi back to the endoplasmic reticulum","subcellular_location":"Golgi apparatus membrane; Cytoplasmic vesicle, COPI-coated vesicle membrane; Endoplasmic reticulum membrane; Endoplasmic reticulum-Golgi intermediate compartment membrane","url":"https://www.uniprot.org/uniprotkb/P24390/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/KDELR1","classification":"Not Classified","n_dependent_lines":18,"n_total_lines":1208,"dependency_fraction":0.014900662251655629},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/KDELR1","total_profiled":1310},"omim":[{"mim_id":"620983","title":"IMMUNODEFICIENCY 128; IMD128","url":"https://www.omim.org/entry/620983"},{"mim_id":"619900","title":"KDEL ENDOPLASMIC RETICULUM PROTEIN RETENTION RECEPTOR 3; KDELR3","url":"https://www.omim.org/entry/619900"},{"mim_id":"615525","title":"COATOMER PROTEIN COMPLEX, SUBUNIT GAMMA-1; COPG1","url":"https://www.omim.org/entry/615525"},{"mim_id":"614905","title":"SORTING NEXIN 8; SNX8","url":"https://www.omim.org/entry/614905"},{"mim_id":"614694","title":"REGULATION OF NUCLEAR PRE-mRNA DOMAIN-CONTAINING PROTEIN 1B; RPRD1B","url":"https://www.omim.org/entry/614694"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Golgi apparatus","reliability":"Supported"},{"location":"Vesicles","reliability":"Additional"},{"location":"Cytosol","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/KDELR1"},"hgnc":{"alias_symbol":["ERD2.1","ERD2","HDEL"],"prev_symbol":[]},"alphafold":{"accession":"P24390","domains":[{"cath_id":"-","chopping":"2-202","consensus_level":"high","plddt":93.4302,"start":2,"end":202}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P24390","model_url":"https://alphafold.ebi.ac.uk/files/AF-P24390-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P24390-F1-predicted_aligned_error_v6.png","plddt_mean":92.5},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=KDELR1","jax_strain_url":"https://www.jax.org/strain/search?query=KDELR1"},"sequence":{"accession":"P24390","fasta_url":"https://rest.uniprot.org/uniprotkb/P24390.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P24390/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P24390"}},"corpus_meta":[{"pmid":"2194670","id":"PMC_2194670","title":"ERD2, a yeast gene required for the receptor-mediated retrieval of luminal ER proteins from the secretory pathway.","date":"1990","source":"Cell","url":"https://pubmed.ncbi.nlm.nih.gov/2194670","citation_count":464,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"2172835","id":"PMC_2172835","title":"A human homologue of the yeast HDEL receptor.","date":"1990","source":"Nature","url":"https://pubmed.ncbi.nlm.nih.gov/2172835","citation_count":291,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"2194671","id":"PMC_2194671","title":"The ERD2 gene determines the specificity of the luminal ER protein retention system.","date":"1990","source":"Cell","url":"https://pubmed.ncbi.nlm.nih.gov/2194671","citation_count":211,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"8910335","id":"PMC_8910335","title":"Endoplasmic reticulum glucosidase II is composed of a catalytic subunit, conserved from yeast to mammals, and a tightly bound noncatalytic HDEL-containing subunit.","date":"1996","source":"The 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pathway; its abundance determines the efficiency and capacity of the retention system.\",\n      \"method\": \"Yeast genetics, ERD2 overexpression/underexpression, biochemical retention assays\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — foundational genetic and biochemical study, 464 citations, replicated across accompanying papers\",\n      \"pmids\": [\"2194670\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1990,\n      \"finding\": \"The ERD2 gene determines the ligand-binding specificity of the ER retention system; swapping ERD2 between S. cerevisiae and K. lactis transfers recognition specificity from HDEL to DDEL, demonstrating ERD2 encodes the sorting receptor.\",\n      \"method\": \"Interspecies ERD2 gene swap, retention assays with HDEL/DDEL-tagged proteins\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct genetic epistasis with defined molecular specificity, 211 citations\",\n      \"pmids\": [\"2194671\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1990,\n      \"finding\": \"Human KDELR1 encodes a protein similar in sequence, size (~26 kDa), and properties to yeast ERD2, is not glycosylated, and is proposed to be the human KDEL receptor for ER luminal protein retrieval.\",\n      \"method\": \"cDNA cloning, sequence analysis, biochemical characterization (size, glycosylation status)\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — molecular cloning with biochemical validation, 291 citations, foundational identification paper\",\n      \"pmids\": [\"2172835\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1992,\n      \"finding\": \"ERD2 (yeast) is essential for normal Golgi function and growth; complete deletion of ERD2 causes functional and morphological perturbation of the Golgi apparatus, and suppressor genes (SED1/SEC12/SED4/DPM1) can rescue growth by modulating membrane flow balance.\",\n      \"method\": \"Yeast genetics, erd2 deletion, suppressor (SED gene) isolation and sequencing, morphological analysis\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis with morphological readout, 101 citations\",\n      \"pmids\": [\"1327759\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1994,\n      \"finding\": \"KDELR1/ERD2 function requires both its ability to recycle (mutations blocking recycling prevent growth) and its HDEL-binding activity; mutations that block recycling also prevent growth, and mere recycling of a ligand-binding-inactive form is insufficient, indicating that HDEL-dependent retrieval of specific proteins is required for Golgi homeostasis.\",\n      \"method\": \"ERD2 mutagenesis, HDEL-saturation experiments, yeast growth assays, BiP secretion assays\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — structure-function mutagenesis combined with functional assays, multiple orthogonal methods\",\n      \"pmids\": [\"7929564\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"The KDEL receptor ERD2 self-oligomerizes and interacts with ARF1 GAP, recruiting it to membranes; ERD2 overexpression enhances ARF1 GAP membrane recruitment and produces a phenotype reflecting ARF1 inactivation, placing KDELR1 as a regulator of ARF1-mediated membrane traffic.\",\n      \"method\": \"Co-immunoprecipitation, overexpression phenotype analysis, membrane recruitment assays\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal interaction shown with functional phenotype, 149 citations\",\n      \"pmids\": [\"9405360\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"ERD2 proteins (ERD2.1 and ERD2.2) localize predominantly to the Golgi and have a long half-life; overexpression of either ERD2.1 or ERD2.2 significantly increases the capacity of cells to retain both KDEL- and HNEL-containing proteins, and ERD2 mediates ER retention of the non-canonical HNEL signal of RAP.\",\n      \"method\": \"Stable transfection, pulse-chase labeling, immunoelectron microscopy, ERD2 overexpression retention assays\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (immunoEM, pulse-chase, overexpression) in a single study\",\n      \"pmids\": [\"9010785\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"The human KDEL receptor (ERD2) binds KDEL-tagged ER luminal proteins (CaBP1/CaBP2) via a specific lumenal loop sequence 22KIWK25; binding is inhibited by soluble KDEL peptides and is of high specificity.\",\n      \"method\": \"Cellulose-bound overlapping peptide scanning of full ERD2 sequence, competitive inhibition with KDEL peptides\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — peptide-binding mapping, single study, no mutagenesis confirmation in intact protein\",\n      \"pmids\": [\"9642148\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"KDELR1 is required cell-intrinsically for normal lymphocyte homeostasis in mice; Kdelr1 missense and frameshift mutant mice show lymphopenia with reduced T-cell receptor expression and increased CD44 on T cells, and fail to clear a self-limiting viral infection, demonstrating a nonredundant cellular function.\",\n      \"method\": \"ENU mutagenesis missense allele, CRISPR/Cas9 frameshift allele, bone marrow chimeras (cell-intrinsic test), TCR transgene rescue, viral infection assays\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — two independent alleles (ENU + CRISPR), cell-intrinsic rescue experiment, multiple phenotypic readouts\",\n      \"pmids\": [\"26438836\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"KDELR1 regulates integrated stress responses (ISR) to promote naive T-cell survival; in mice with nonfunctional KDELR1, naive T cells show excessive ISR and apoptosis, which is rescued by strong TCR-mediated signals, linking KDELR1 to ISR regulation in lymphocytes.\",\n      \"method\": \"T-Red mouse model (nonfunctional KDELR1), TCR transgene rescue, tetramer dissociation assay, altered peptide ligand stimulation, apoptosis assays\",\n      \"journal\": \"International immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic mouse model with multiple functional assays, single lab\",\n      \"pmids\": [\"26489882\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"KDELR1 knockout in HAP1 cells leads to increased secretion of ER-resident PDI, decreased cell viability under ER stress, and impaired cell adhesion capacity, demonstrating KDELR1 is required for maintaining ER protein retention and cellular homeostasis.\",\n      \"method\": \"KDELR1 CRISPR knockout, transcriptome analysis, PDI secretion assay, cell adhesion assay, ER stress viability assay\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — KO with multiple functional readouts (PDI secretion, adhesion, ER stress), single lab\",\n      \"pmids\": [\"31337861\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"KDELR1 regulates drug resistance and malignant behavior in chondrosarcoma via the Integrin-PLCγ-YAP1 (Hippo) axis, as revealed by mass spectrometry proteomics and transcriptomics following KDELR1 manipulation.\",\n      \"method\": \"Mass spectrometry proteomics, transcriptomics, loss-of-function experiments in chondrosarcoma cells\",\n      \"journal\": \"Cell death & disease\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — pathway placement by omics without direct biochemical reconstitution, single study\",\n      \"pmids\": [\"39715773\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"While KDELR2 plays a major role in client retrieval, KDELR1 and KDELR3 regulate AGR2 (mucin folding assistant) production in opposite ways: KDELR1 downregulation decreases AGR2 transcripts, whereas KDELR3 downregulation increases them; ERp44-KDELR3 interaction specifically mediates this regulation, identifying a regulatory circuit in the early secretory pathway distinct from the unfolded protein response.\",\n      \"method\": \"siRNA silencing of KDELR1/KDELR3/ERp44, transcriptomic analysis, functional interaction studies\",\n      \"journal\": \"Cellular and molecular life sciences : CMLS\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — multiple siRNA knockdowns with defined transcriptional readout, mechanistic circuit proposed, single lab\",\n      \"pmids\": [\"41706164\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"KDELR1 is an integral membrane KDEL receptor concentrated in the Golgi that retrieves escaped ER-luminal proteins bearing C-terminal KDEL signals by binding them through a specific lumenal loop sequence (22KIWK25), returning them to the ER via retrograde transport; it also self-oligomerizes to recruit ARF1 GAP to membranes to regulate ARF1-mediated vesicular traffic, and has nonredundant roles in lymphocyte homeostasis by suppressing integrated stress responses, with emerging evidence for specialized regulatory functions (e.g., controlling AGR2 production and Integrin-Hippo-YAP1 signaling) distinct from the canonical ER retention role shared by its paralogs.\"\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1990,\n      \"finding\": \"The yeast ERD2 gene encodes a 26 kDa integral membrane protein (the HDEL receptor) whose abundance determines the efficiency and capacity of the ER protein retention/retrieval system; reduced ERD2 expression causes secretion of HDEL-tagged proteins, while overexpression improves retention.\",\n      \"method\": \"Yeast genetics, gene expression manipulation, secretion assays\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — foundational genetic epistasis with multiple orthogonal readouts, replicated in accompanying paper, 464 citations\",\n      \"pmids\": [\"2194670\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1990,\n      \"finding\": \"A human homologue of yeast ERD2 (hERD2/KDELR1) encodes a ~26 kDa non-glycosylated integral membrane protein with properties similar to the yeast HDEL receptor, proposed to be the mammalian KDEL receptor.\",\n      \"method\": \"Sequence analysis, biochemical characterization of the expressed protein\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — cloning and functional characterization, 291 citations, foundational paper\",\n      \"pmids\": [\"2172835\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1990,\n      \"finding\": \"The specificity of the luminal ER protein retention system is determined by the ERD2 gene product (the receptor): exchanging the ERD2 gene between S. cerevisiae and K. lactis alters which C-terminal signals (HDEL vs. DDEL) are recognized, proving ERD2 encodes the sorting receptor.\",\n      \"method\": \"Inter-species gene exchange, retention signal specificity assays\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct genetic substitution demonstrating receptor specificity, 211 citations\",\n      \"pmids\": [\"2194671\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1992,\n      \"finding\": \"Ligand overexpression causes redistribution of hERD2 (KDELR1) from the Golgi apparatus to the ER; mutation of hERD2 alters ligand specificity of this redistribution, demonstrating direct receptor-ligand interaction and ligand-induced receptor movement as a regulatory mechanism.\",\n      \"method\": \"Overexpression of KDEL/DDEL ligands, immunolocalization, site-directed mutagenesis\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — mutagenesis combined with localization studies showing direct receptor-ligand interaction, 365 citations\",\n      \"pmids\": [\"1310258\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1993,\n      \"finding\": \"Ligand binding to the human KDEL receptor depends on charged residues within the transmembrane domains; retrograde transport of occupied receptor requires a critical aspartic acid in the seventh transmembrane domain; Golgi retention is independent of ligand binding and this aspartate, indicating distinct structural requirements for each function.\",\n      \"method\": \"Mutagenesis of transmembrane residues, intracellular localization assays, ligand-binding assays in COS cells\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — systematic mutagenesis with multiple functional readouts, 127 citations\",\n      \"pmids\": [\"8392934\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1994,\n      \"finding\": \"Growth of yeast requires the HDEL-dependent retrieval activity of Erd2p (KDELR1 ortholog); mutations that block receptor recycling also prevent growth; Golgi retention of the receptor is independent of recycling, but retrieval of specific HDEL-containing proteins is essential for viability.\",\n      \"method\": \"Erd2p mutant analysis, receptor overexpression saturation, viability assays\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — epistasis and mutational analysis with defined phenotypic readouts, 38 citations\",\n      \"pmids\": [\"7929564\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"The KDEL receptor ERD2 (KDELR1 ortholog) self-oligomerizes and interacts with ARF1 GTPase-activating protein (ARF1 GAP), recruiting cytosolic ARF1 GAP to membranes; ERD2 overexpression enhances GAP membrane recruitment and produces a phenotype reflecting ARF1 inactivation, indicating ERD2 regulates ARF1-mediated vesicle transport.\",\n      \"method\": \"Co-immunoprecipitation, overexpression phenotype analysis, membrane recruitment assays\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal Co-IP with functional phenotype analysis, 149 citations\",\n      \"pmids\": [\"9405360\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"Overexpression of either ERD2.1 or ERD2.2 (KDELR1 paralogs) significantly increases cellular capacity to retain both KDEL- and HNEL-containing proteins; pulse-chase and immunolocalization show long half-life and Golgi localization for both receptors, and the novel HNEL signal of RAP interacts with the same ER retention receptors as KDEL.\",\n      \"method\": \"Stable transfection, pulse-chase labeling, immunoelectron microscopy, retention assays\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods showing ERD2 proteins retain novel ER retention signals\",\n      \"pmids\": [\"9010785\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"The sequence 22KIWK25 within a lumenal loop of the human KDEL receptor (ERD2/KDELR1) is essential for binding to KDEL-containing ER lumenal proteins (CaBP1 and CaBP2); binding is of high specificity and almost completely inhibited by KDEL-containing soluble peptides.\",\n      \"method\": \"Cellulose-bound overlapping peptide arrays, binding inhibition with KDEL peptides\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — peptide mapping approach identifies binding site but limited to in vitro peptide assay\",\n      \"pmids\": [\"9642148\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"KDEL ligand binding induces oligomerization of ERD2 (KDELR1) and recruitment of ARFGAP to the Golgi, where the ERD2 oligomer/ARFGAP complex interacts with membrane-bound ARF1; during KDEL ligand transport, ERD2 interactions with β-COP and p23 decrease and the proteins segregate, revealing how cargo-induced ERD2 oligomerization regulates sorting into COPI-coated buds.\",\n      \"method\": \"FRET between CFP/YFP fusion proteins by multifocal multiphoton microscopy in living cells\",\n      \"journal\": \"Developmental cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1/2 — live-cell FRET imaging with multiple interactors quantified dynamically, 149 citations\",\n      \"pmids\": [\"11703931\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Impairment of KDEL receptor (KDELR1) retrieval function by expression of a ligand-recognition mutant causes mis-sorting of the ER chaperone BiP and induces intense ER stress, accompanied by activation of p38 MAP kinase and JNK1; ligand-induced activation of the KDEL receptor also induces phosphorylation of p38 MAP kinase, indicating the KDEL receptor modulates ER stress response through MAPK signaling.\",\n      \"method\": \"Mutant KDEL receptor expression, ER stress assays, MAP kinase phosphorylation assays, p38 inhibitor studies in HeLa cells\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — loss-of-function and pharmacological inhibition with defined signaling readouts, 84 citations\",\n      \"pmids\": [\"12821650\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"PKA phosphorylation of serine 209 in the C-terminal cytoplasmic domain of the KDEL receptor (KDELR1) is required for retrograde Golgi-to-ER transport of the receptor and for intracellular retention of KDEL ligands; this domain interacts with coatomer and ARF-GAP only when Ser209 is phosphorylated (mimicked by S209D mutation); inhibition of PKA with H89 blocks receptor redistribution to the ER.\",\n      \"method\": \"Truncation and point mutagenesis, peptide-binding assays with coatomer/ARF-GAP, PKA inhibitor (H89), permeabilized cell transport assays\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1/2 — mutagenesis, in vitro phosphorylation, pharmacological inhibition with functional transport readouts, 78 citations\",\n      \"pmids\": [\"14517323\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Src kinase activity controls KDEL receptor (KDELR1) localization: activated Src relocates KDEL-R from the Golgi to the ER, and loss of Src (in SYF cells) perturbs Golgi organization; retrograde transport of Pseudomonas exotoxin (which uses the KDEL-R) is accelerated by Src inhibition or ablation.\",\n      \"method\": \"Activated Src expression, SYF knockout cell line, immunofluorescence localization, toxin transport assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic and pharmacological Src manipulation with specific KDEL-R localization and transport readouts, 91 citations\",\n      \"pmids\": [\"12975382\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Three human KDEL receptors (KDELR1, KDELR2, KDELR3) each have unique specificity profiles for KDEL-like C-terminal motifs; KDELR1 interacts with a distinct subset of KDEL variants compared to KDELR2 and KDELR3, as determined by a bimolecular fluorescence complementation screen; all three receptors localize to the Golgi.\",\n      \"method\": \"Reporter construct screen of 152 KDEL variants, bimolecular fluorescence complementation (BiFC) to determine receptor-ligand specificity, Golgi localization assays\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — systematic screen with 152 variants and BiFC specificity mapping, 214 citations\",\n      \"pmids\": [\"18086916\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"The KDEL receptor (KDELR1) is predicted to fold like a G-protein-coupled receptor (GPCR) and directly binds and activates the heterotrimeric G-protein Gαq/11; this Gαq/11 activation regulates transport through the Golgi complex, revealing an unexpected GPCR-like signaling mode for the KDEL receptor.\",\n      \"method\": \"GPCR structural prediction, G-protein binding and activation assays, Golgi transport assays\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — G-protein binding demonstrated biochemically with functional transport readouts, 103 citations\",\n      \"pmids\": [\"22580821\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"ER-to-Golgi cargo transport activates the KDEL receptor (KDELR1) at the Golgi, which triggers a signaling cascade involving Gs protein, adenylyl cyclase, phosphodiesterase isoforms, and PKA, leading to phosphorylation of transport machinery proteins; this induces retrograde traffic to the ER to balance anterograde flux; additionally, the KDEL receptor activates CREB1 and other transcription factors that upregulate transport-related genes.\",\n      \"method\": \"Pharmacological perturbations of signaling components, phosphorylation assays, transcription factor activation assays\",\n      \"journal\": \"Developmental cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods dissecting the signaling cascade with functional transport and transcriptional readouts, 92 citations\",\n      \"pmids\": [\"25117681\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"A recessive missense allele of KDELR1 in mice causes cell-intrinsic lymphopenia; homozygous mutant and CRISPR/Cas9 frameshift T cells show reduced TCR surface expression, increased CD44, and impaired viral clearance; the phenotype can be partially corrected by an MHC class I-restricted TCR transgene, revealing a nonredundant role for KDELR1 in lymphocyte homeostasis.\",\n      \"method\": \"Mouse genetics (ENU missense and CRISPR/Cas9 frameshift alleles), bone marrow chimeras for cell-intrinsic analysis, TCR transgene rescue, flow cytometry\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — two independent loss-of-function alleles with cell-intrinsic phenotype and transgenic rescue\",\n      \"pmids\": [\"26438836\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"KDELR1 regulates integrated stress responses (ISR) to promote naive T-cell survival; in mice with nonfunctional KDELR1 (T-Red mice), naive T cells show excessive ISR and undergo apoptosis; strong TCR-mediated signals suppress ISR, and surviving naive T cells in KDELR1-deficient mice express higher CD5 and exhibit higher TCR affinity, demonstrating that KDELR1 deficiency-induced ISR can be counteracted by strong TCR signals.\",\n      \"method\": \"T-Red mouse model, TCR transgenic rescue, tetramer dissociation assay, altered peptide ligand stimulation, ISR markers\",\n      \"journal\": \"International immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vivo genetic model with TCR affinity measurements and multiple functional readouts\",\n      \"pmids\": [\"26489882\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"KDELR1 knockout (HAP1 cells) causes increased secretion of the ER-resident protein PDI, decreased cell viability under ER stress, transcriptional upregulation of genes involved in cell adhesion and ECM composition, and impaired cell adhesion capacity that is partially rescued by collagen/laminin coating, indicating KDELR1 is required for ER homeostasis and normal cell adhesion.\",\n      \"method\": \"KDELR1 knockout cell line, whole transcriptome analysis, in vitro adhesion assays, PDI secretion assay, ER stress viability assay\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2/3 — KO with transcriptomics and functional adhesion assay but limited pathway mechanistic detail\",\n      \"pmids\": [\"31337861\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"KDELR1 contributes to chondrosarcoma drug resistance and malignant behavior through the Integrin-PLCγ-YAP1 (Hippo) signaling axis; mass spectrometry proteomics and transcriptomics revealed KDELR1 modulates Hippo-YAP pathway activity in chondrosarcoma cells, affecting ECM formation and chemotherapy resistance.\",\n      \"method\": \"Single-cell transcriptomics, mass spectrometry proteomics, KDELR1 knockdown/overexpression with proliferation and drug resistance assays\",\n      \"journal\": \"Cell death & disease\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2/3 — multi-omics pathway identification with functional knockdown/OE, but mechanistic details of the Integrin-PLCγ-YAP1 axis for KDELR1 are limited to correlative proteomics\",\n      \"pmids\": [\"39715773\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"KDELR1 and KDELR3 have opposite effects on AGR2 (a mucin folding assistant) production: KDELR1 downregulation decreases AGR2 transcripts while KDELR3 silencing dramatically increases them; silencing ERp44 (but not other ER residents) phenocopies KDELR3 knockdown, suggesting AGR2 regulation by KDELR3 depends on ERp44-KDELR3 interactions; this defines a novel regulatory circuit controlling early secretory pathway composition distinct from the unfolded protein response.\",\n      \"method\": \"siRNA silencing of KDELR1, KDELR2, KDELR3, and ER residents; transcriptional readout of AGR2; phenocopy experiments with ERp44 silencing\",\n      \"journal\": \"Cellular and molecular life sciences : CMLS\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2/3 — genetic epistasis by silencing with defined transcriptional readout, but mechanistic basis of KDELR1-specific effect on AGR2 not fully elucidated\",\n      \"pmids\": [\"41706164\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"KDELR1 (human ERD2) is a seven-transmembrane Golgi-resident integral membrane protein that functions as the KDEL-sequence receptor, recognizing escaped ER-lumenal proteins via charged transmembrane residues and a lumenal 22KIWK25 motif; ligand binding triggers receptor oligomerization, ARFGAP recruitment, and PKA-dependent phosphorylation of Ser209 in its cytoplasmic C-terminus to enable COPI-mediated retrograde Golgi-to-ER transport; it additionally acts as a GPCR-like sensor that activates Gαq/11 and Gs-PKA signaling cascades to maintain ER-Golgi transport homeostasis, modulates ER stress responses via p38/JNK MAPK signaling, and plays a nonredundant cell-intrinsic role in lymphocyte survival by suppressing excessive integrated stress responses, with emerging evidence for regulation of cell adhesion and, in opposition to KDELR3, control of AGR2 production in the early secretory pathway.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"KDELR1 is a Golgi-localized integral membrane receptor that retrieves ER-resident luminal proteins bearing C-terminal KDEL-type retention signals from the secretory pathway and returns them to the ER via retrograde transport, thereby maintaining ER and Golgi homeostasis. Identified through its yeast ortholog ERD2, KDELR1 binds KDEL-tagged clients via a luminal loop sequence (22KIWK25), with its abundance directly determining retrieval capacity; loss of ERD2/KDELR1 causes Golgi dysfunction, secretion of ER-resident proteins such as PDI, and impaired cell viability under ER stress [PMID:2194670, PMID:7929564, PMID:31337861]. KDELR1 self-oligomerizes and recruits ARF1 GAP to membranes, positioning it as a regulator of ARF1-dependent vesicular trafficking in addition to its cargo-retrieval function [PMID:9405360]. Beyond canonical ER retention, KDELR1 has a cell-intrinsic, nonredundant role in lymphocyte homeostasis: loss-of-function mutations in mice cause lymphopenia, excessive integrated stress responses, and naive T-cell apoptosis [PMID:26438836, PMID:26489882].\",\n  \"teleology\": [\n    {\n      \"year\": 1990,\n      \"claim\": \"Identification of ERD2/KDELR1 as the sorting receptor for ER-luminal protein retrieval resolved how KDEL/HDEL-bearing proteins are retained: ERD2 encodes the receptor whose abundance sets retrieval capacity, and interspecies gene swaps proved it directly determines ligand specificity.\",\n      \"evidence\": \"Yeast ERD2 overexpression/deletion with retention assays; S. cerevisiae/K. lactis ERD2 swap transferring HDEL vs. DDEL recognition; cloning of human KDELR1 cDNA\",\n      \"pmids\": [\"2194670\", \"2194671\", \"2172835\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Binding affinity and stoichiometry of receptor–ligand interaction not measured\", \"Topology and membrane insertion mechanism not experimentally determined\"]\n    },\n    {\n      \"year\": 1992,\n      \"claim\": \"Establishing that ERD2 is essential for Golgi integrity showed the receptor's function extends beyond retrieval to general organelle homeostasis, as complete deletion perturbs Golgi morphology and growth, with suppressor genes restoring viability by rebalancing membrane flow.\",\n      \"evidence\": \"Yeast erd2Δ deletion, suppressor isolation (SED1/SEC12/SED4/DPM1), morphological analysis\",\n      \"pmids\": [\"1327759\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether Golgi dysfunction reflects loss of specific client retrieval or a general trafficking imbalance remained unresolved\"]\n    },\n    {\n      \"year\": 1994,\n      \"claim\": \"Structure–function mutagenesis demonstrated that both recycling competence and HDEL-binding activity are independently required for growth, proving that the essential function is HDEL-dependent retrieval of specific ER proteins, not receptor cycling per se.\",\n      \"evidence\": \"ERD2 point mutants tested for recycling, HDEL binding, yeast growth, and BiP secretion\",\n      \"pmids\": [\"7929564\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of the essential retrieved proteins whose loss causes lethality was not determined\"]\n    },\n    {\n      \"year\": 1997,\n      \"claim\": \"Discovery that ERD2 self-oligomerizes and recruits ARF1 GAP to membranes revealed a signaling function beyond cargo binding, linking the KDEL receptor to regulation of ARF1-dependent vesicular trafficking.\",\n      \"evidence\": \"Co-immunoprecipitation of ERD2 oligomers and ARF1 GAP; overexpression phenotype mimicking ARF1 inactivation\",\n      \"pmids\": [\"9405360\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct structural basis of oligomerization unknown\", \"Whether ARF1 GAP recruitment is ligand-dependent was not tested\"]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Mapping the KDEL-binding site to the luminal loop residues 22KIWK25 provided the first molecular-level identification of the ligand-recognition determinant on the receptor.\",\n      \"evidence\": \"Cellulose-bound overlapping peptide scanning of full ERD2 sequence, competitive inhibition with soluble KDEL peptides\",\n      \"pmids\": [\"9642148\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Binding site not confirmed by mutagenesis in intact receptor\", \"pH-dependent binding mechanism not addressed\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Mouse genetic studies revealed a nonredundant, cell-intrinsic role for KDELR1 in lymphocyte homeostasis and integrated stress response suppression, showing that loss causes lymphopenia, excessive ISR-driven apoptosis in naive T cells, and impaired viral clearance.\",\n      \"evidence\": \"ENU missense and CRISPR frameshift Kdelr1 alleles in mice, bone marrow chimeras, TCR transgene rescue, viral infection assays, apoptosis and ISR measurements\",\n      \"pmids\": [\"26438836\", \"26489882\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Which KDELR1-retrieved client(s) are responsible for ISR suppression in T cells is unknown\", \"Whether KDELR2/KDELR3 compensate partially in lymphocytes was not directly tested\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"KDELR1 knockout in human cells confirmed its requirement for ER protein retention and cellular homeostasis: loss increased PDI secretion, impaired adhesion, and reduced viability under ER stress.\",\n      \"evidence\": \"CRISPR KO in HAP1 cells, PDI secretion quantification, cell adhesion and ER-stress viability assays, transcriptomics\",\n      \"pmids\": [\"31337861\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Relative contributions of KDELR1 vs. KDELR2/KDELR3 not delineated in this system\", \"Mechanism linking KDELR1 loss to adhesion defects not biochemically dissected\"]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"Differential regulation of AGR2 production by KDELR1 and KDELR3 uncovered a paralog-specific transcriptional regulatory circuit in the early secretory pathway operating independently of the unfolded protein response.\",\n      \"evidence\": \"siRNA silencing of KDELR1/KDELR3/ERp44 with transcriptomic readout in cultured cells\",\n      \"pmids\": [\"41706164\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism by which KDELR1 positively regulates AGR2 transcription is not defined\", \"Whether this regulation operates in vivo is untested\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The structural basis of pH-dependent KDEL binding, the identity of KDELR1-retrieved clients critical for T-cell survival and ISR suppression, and the mechanism by which KDELR1 signals to transcriptional programs (e.g., AGR2, Hippo-YAP1) remain unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No high-resolution structure of KDELR1 bound to KDEL ligand in the timeline\", \"Specific ER-resident proteins whose retrieval prevents ISR activation in lymphocytes not identified\", \"Signaling cascade linking KDELR1 to transcriptional regulation of AGR2 or YAP1 not biochemically reconstituted\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0038024\", \"supporting_discovery_ids\": [0, 1, 2, 7]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [5]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005794\", \"supporting_discovery_ids\": [3, 6]},\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [0, 4]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-9609507\", \"supporting_discovery_ids\": [0, 1, 4, 6]},\n      {\"term_id\": \"R-HSA-5653656\", \"supporting_discovery_ids\": [4, 5]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [8, 9]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"ARF1GAP\",\n      \"ERp44\",\n      \"AGR2\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"mechanistic_narrative\": \"KDELR1 is a seven-transmembrane Golgi-resident receptor that recognizes KDEL and related C-terminal retention motifs on escaped ER-lumenal proteins and mediates their COPI-dependent retrograde retrieval from the Golgi to the ER, thereby maintaining ER proteostasis [PMID:2194670, PMID:2172835, PMID:2194671]. Ligand binding depends on charged transmembrane residues and a lumenal 22KIWK25 motif, triggers receptor oligomerization, recruits ARFGAP1 to Golgi membranes, and requires PKA-dependent phosphorylation of Ser209 for retrograde transport [PMID:8392934, PMID:9642148, PMID:9405360, PMID:14517323]. Beyond cargo retrieval, KDELR1 functions as a GPCR-like signaling platform that activates Gαq/11- and Gs-PKA-dependent cascades to coordinate anterograde and retrograde membrane traffic and to modulate ER stress responses via p38/JNK MAPK signaling [PMID:22580821, PMID:25117681, PMID:12821650]. Loss-of-function mutations in mice cause cell-intrinsic lymphopenia driven by excessive integrated stress responses and impaired naive T-cell survival, establishing a nonredundant role for KDELR1 in lymphocyte homeostasis [PMID:26438836, PMID:26489882].\",\n  \"teleology\": [\n    {\n      \"year\": 1990,\n      \"claim\": \"Identification of ERD2/KDELR1 as the ER retention receptor resolved how cells retrieve escaped lumenal proteins: the receptor's abundance sets system capacity, and species-specific ERD2 exchange switches retention-signal specificity, proving direct receptor–ligand determination.\",\n      \"evidence\": \"Yeast ERD2 gene manipulation, secretion assays, and inter-species gene exchange between S. cerevisiae and K. lactis; cloning and biochemical characterization of human homologue\",\n      \"pmids\": [\"2194670\", \"2194671\", \"2172835\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Structural basis of KDEL signal recognition not determined\",\n        \"How receptor cycles between Golgi and ER was unknown\",\n        \"Redundancy among mammalian KDELR paralogs unaddressed\"\n      ]\n    },\n    {\n      \"year\": 1993,\n      \"claim\": \"Systematic mutagenesis established that ligand binding requires charged transmembrane residues and that retrograde transport depends on a specific aspartate in TM7, separating Golgi retention from recycling functions mechanistically.\",\n      \"evidence\": \"Transmembrane point mutagenesis in COS cells with ligand-binding and localization readouts; ligand overexpression-induced receptor redistribution and specificity-altering mutations\",\n      \"pmids\": [\"8392934\", \"1310258\", \"7929564\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Identity of coat machinery coupling the receptor to COPI vesicles was unclear\",\n        \"Lumenal residues involved in ligand binding not yet mapped\"\n      ]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Mapping of the lumenal ligand-binding site to the 22KIWK25 motif defined the minimal receptor determinant contacting KDEL-containing proteins.\",\n      \"evidence\": \"Cellulose-bound overlapping peptide arrays with soluble KDEL peptide competition\",\n      \"pmids\": [\"9642148\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"In vitro peptide assay only; not confirmed by crystallography or in-cell crosslinking\",\n        \"Contribution of individual residues within KIWK not dissected\"\n      ]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Live-cell FRET imaging revealed that ligand binding induces receptor oligomerization and recruits ARFGAP to Golgi membranes, establishing the mechanistic link between cargo recognition and COPI bud formation.\",\n      \"evidence\": \"CFP/YFP FRET with multifocal multiphoton microscopy measuring interactions among ERD2, ARFGAP, ARF1, β-COP, and p23 in living cells\",\n      \"pmids\": [\"11703931\", \"9405360\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Stoichiometry of the oligomer and precise ARFGAP interaction surface unknown\",\n        \"Whether oligomerization is required for signaling (vs. transport) not tested\"\n      ]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Two parallel regulatory inputs were defined: PKA-dependent Ser209 phosphorylation is required for coatomer/ARFGAP engagement and retrograde transport, while Src kinase activity controls receptor steady-state distribution between Golgi and ER.\",\n      \"evidence\": \"Point and truncation mutagenesis, PKA inhibitor H89, permeabilized-cell transport assays; activated Src expression and SYF knockout cells with toxin transport readouts\",\n      \"pmids\": [\"14517323\", \"12975382\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Direct Src phosphorylation site on KDELR1 not identified\",\n        \"Relationship between Src and PKA inputs not integrated\"\n      ]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Loss of KDELR1 retrieval function induces ER stress with p38/JNK MAPK activation, establishing KDELR1 as a modulator of ER stress signaling beyond simple cargo sorting.\",\n      \"evidence\": \"Dominant-negative KDELR1 mutant expression, BiP mis-sorting, p38/JNK phosphorylation assays, p38 inhibitor studies in HeLa cells\",\n      \"pmids\": [\"12821650\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Whether MAPK activation is a direct receptor signal or secondary to ER stress was not resolved\",\n        \"Downstream transcriptional targets of p38/JNK in this context not identified\"\n      ]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"A systematic BiFC screen of 152 KDEL variants showed that KDELR1, KDELR2, and KDELR3 each have unique ligand-specificity profiles, explaining how the three paralogs partition retrieval duties.\",\n      \"evidence\": \"Bimolecular fluorescence complementation screen with 152 C-terminal peptide variants and Golgi colocalization\",\n      \"pmids\": [\"18086916\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Functional consequence of differential specificity on ER proteome maintenance not tested\",\n        \"Structural basis of specificity divergence unknown\"\n      ]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Discovery that KDELR1 adopts a GPCR-like fold and directly activates Gαq/11 reframed the receptor as a signaling platform, not just a cargo sorter, coupling ligand occupancy to heterotrimeric G-protein cascades that regulate Golgi transport.\",\n      \"evidence\": \"GPCR structural modeling, G-protein binding and activation assays, Golgi transport assays\",\n      \"pmids\": [\"22580821\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"High-resolution structure of KDELR1 bound to Gα subunit lacking\",\n        \"How Gαq/11 and Gs pathways are differentially engaged was unclear\"\n      ]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"A complete Gs–adenylyl cyclase–PKA signaling cascade downstream of KDELR1 was mapped, showing that anterograde cargo flux activates the receptor to drive compensatory retrograde traffic and to upregulate transport gene expression via CREB1.\",\n      \"evidence\": \"Pharmacological dissection of signaling components, phosphorylation and transcription factor activation assays\",\n      \"pmids\": [\"25117681\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Identity of specific transport genes upregulated via CREB1 not fully catalogued\",\n        \"Quantitative relationship between cargo load and signaling output not measured\"\n      ]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"In vivo loss-of-function established that KDELR1 is nonredundant for lymphocyte homeostasis: mutant mice show cell-intrinsic lymphopenia driven by excessive integrated stress responses in naive T cells, which can be counteracted by strong TCR signals.\",\n      \"evidence\": \"ENU missense and CRISPR/Cas9 frameshift alleles in mice, bone marrow chimeras, TCR transgenic rescue, tetramer dissociation assays\",\n      \"pmids\": [\"26438836\", \"26489882\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Specific ER client(s) whose mis-sorting triggers ISR in T cells not identified\",\n        \"Whether KDELR2/3 are expressed in T cells and why they fail to compensate not determined\"\n      ]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"KDELR1 knockout in human cells revealed a requirement for normal cell adhesion and ER stress resilience, with transcriptional remodeling of ECM and adhesion gene programs upon receptor loss.\",\n      \"evidence\": \"HAP1 KDELR1 knockout, transcriptomics, PDI secretion assay, adhesion assays with collagen/laminin rescue\",\n      \"pmids\": [\"31337861\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Mechanism linking KDELR1 loss to adhesion gene upregulation not resolved\",\n        \"Single cell line; generalizability across tissues not tested\"\n      ]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Multi-omics analysis linked KDELR1 to Integrin–PLCγ–YAP1 (Hippo) signaling in chondrosarcoma, suggesting a role in drug resistance through ECM-related pathway modulation.\",\n      \"evidence\": \"Single-cell transcriptomics and mass spectrometry proteomics with KDELR1 knockdown/overexpression in chondrosarcoma cells\",\n      \"pmids\": [\"39715773\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Direct molecular connection between KDELR1 and Integrin-PLCγ axis not biochemically validated\",\n        \"Applicability beyond chondrosarcoma not assessed\"\n      ]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"Epistasis experiments showed KDELR1 and KDELR3 exert opposing transcriptional control over AGR2, with ERp44 mediating the KDELR3 effect, defining a paralog-specific regulatory circuit independent of the unfolded protein response.\",\n      \"evidence\": \"siRNA silencing of individual KDELRs and ER-resident proteins with AGR2 transcript readout\",\n      \"pmids\": [\"41706164\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Mechanistic basis of KDELR1's positive effect on AGR2 transcription unknown\",\n        \"Whether this circuit operates via KDELR1 signaling or simply cargo retrieval not distinguished\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"A high-resolution structure of KDELR1 in complex with a KDEL ligand and/or G-protein, and identification of the specific ER client(s) whose mis-sorting drives T-cell ISR, remain the principal open questions for fully integrating the receptor's cargo-sorting and signaling functions.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"No atomic-resolution structure of ligand-bound or G-protein-bound KDELR1\",\n        \"Critical ER client(s) mediating T-cell survival phenotype not identified\",\n        \"Quantitative model relating cargo load to Gs vs. Gαq signaling output absent\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0038024\", \"supporting_discovery_ids\": [0, 2, 3, 4, 8, 13]},\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [14, 15]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [6, 9, 11, 15]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005794\", \"supporting_discovery_ids\": [3, 7, 9, 13]},\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [3, 12]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"GO:R-HSA-5653656\", \"supporting_discovery_ids\": [0, 3, 6, 9, 11, 15]},\n      {\"term_id\": \"R-HSA-9609507\", \"supporting_discovery_ids\": [0, 5, 9, 11]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [10, 14, 15]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [16, 17]},\n      {\"term_id\": \"R-HSA-8953897\", \"supporting_discovery_ids\": [10, 17, 18]}\n    ],\n    \"complexes\": [\n      \"KDELR1 homo-oligomer\",\n      \"KDELR1–ARFGAP1–ARF1 complex\"\n    ],\n    \"partners\": [\n      \"ARF1\",\n      \"ARFGAP1\",\n      \"GNAQ\",\n      \"GNAS\",\n      \"COPB1\",\n      \"TMED10\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}