{"gene":"SDF4","run_date":"2026-06-10T07:46:30","timeline":{"discoveries":[{"year":1996,"finding":"Cab45 (SDF4) is a soluble, lumenal Golgi-resident calcium-binding protein with six EF-hand motifs and an N-terminal signal sequence but no membrane-anchor sequences, making it the first known soluble protein resident in the Golgi lumen.","method":"Subcellular fractionation, immunofluorescence localization, Ca2+-binding assays, sequence analysis of signal sequence and EF-hand motifs","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — direct localization by fractionation and imaging, Ca2+-binding demonstrated biochemically, replicated by multiple subsequent studies","pmids":["8609160"],"is_preprint":false},{"year":2007,"finding":"A cytosolic splice variant of Cab45 (Cab45b) interacts with Munc18b in a Ca2+-dependent manner; Cab45b co-immunoprecipitates with Munc18b, syntaxin 2, and syntaxin 3 in pancreatic acini, and antibodies against Cab45b inhibit Ca2+-induced amylase release from permeabilized acini, establishing a role in zymogen exocytosis.","method":"Yeast two-hybrid screen, in vitro binding assay, 45Ca2+ binding assay, co-immunoprecipitation, antibody inhibition of amylase secretion from streptolysin-O-permeabilized acini","journal":"Molecular biology of the cell","confidence":"High","confidence_rationale":"Tier 2 / Moderate — reciprocal Co-IP, in vitro binding, functional antibody inhibition assay with dose-dependent readout; single lab but multiple orthogonal methods","pmids":["17442889"],"is_preprint":false},{"year":2012,"finding":"Cab45 is required for SPCA1 (secretory pathway calcium ATPase 1)-dependent Ca2+ import into the TGN lumen and binds secretory cargo in a Ca2+-dependent reaction, functioning as a cargo sorter at the trans-Golgi network.","method":"RNAi knockdown, Ca2+ imaging, co-immunoprecipitation of Cab45 with secretory cargo, functional secretion assays","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — loss-of-function with specific cargo-sorting phenotype, Ca2+-dependent binding demonstrated, independently confirmed by subsequent studies","pmids":["23266954"],"is_preprint":false},{"year":2016,"finding":"Cab45 reversibly assembles into oligomers in the presence of Ca2+; these oligomers specifically bind secretory cargo proteins (COMP and LyzC) in vitro in a Ca2+-dependent manner; mutation of Ca2+-binding sites in Cab45 impairs both oligomerization and cargo sorting; superresolution microscopy shows Cab45 colocalizes with secretory cargo and SPCA1 in specific TGN microdomains.","method":"In vitro oligomerization assay, in vitro Ca2+-dependent cargo binding, Ca2+-binding site mutagenesis, STED superresolution microscopy, secretion assay in cells","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 1 / Strong — reconstituted Ca2+-dependent oligomerization and cargo binding in vitro, validated by mutagenesis and superresolution imaging in cells; multiple orthogonal methods in one rigorous study","pmids":["27138253"],"is_preprint":false},{"year":2020,"finding":"Cab45 is phosphorylated by the Golgi-specific kinase Fam20C; phosphomimetic mutation of Cab45 promotes its translocation into TGN-derived vesicles, increases export of the Cab45 client LyzC, and fine-tunes Cab45 oligomerization and TGN retention.","method":"Mass spectrometry identification of phosphorylation site, phosphomimetic mutagenesis, vesicle budding/secretion assays, oligomerization analysis","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 1 / Moderate — phosphorylation site identified by MS, mechanistic consequences tested by mutagenesis and functional secretion assays; single lab with multiple orthogonal methods","pmids":["32422653"],"is_preprint":false},{"year":2021,"finding":"SDF4 (Cab45) interacts with CXCR4 in endothelial cells and triggers VEGFD expression through activation of ERK1/2 and p38 pathways; SDF4 is transcriptionally induced by CEBPD in cancer-associated fibroblasts in response to cisplatin/5-FU treatment.","method":"Co-immunoprecipitation (SDF4–CXCR4 interaction), Western blot for ERK1/2 and p38 phosphorylation, in vitro tube formation assay, in vivo angiogenesis assay, promoter/ChIP analysis of CEBPD on SDF4","journal":"Cell death discovery","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — Co-IP for CXCR4 interaction, signaling pathway activation shown by phospho-blots, functional secretion phenotype; single lab, limited mechanistic depth on direct binding","pmids":["33953165"],"is_preprint":false},{"year":2023,"finding":"Cab45 deficiency causes hypersecretion of lysosomal hydrolases, specifically unprocessed precursors of prosaposin (PSAP) and progranulin (PGRN), and results in aberrant perinuclear accumulation of lysosomes, revealing a role for Cab45 in lysosomal protein targeting and lysosomal positioning.","method":"hiSPECs secretome analysis of Cab45-deficient cells, immunofluorescence of lysosomal markers, Western blot for PSAP and PGRN processing","journal":"Traffic (Copenhagen, Denmark)","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — precise secretome quantification by hiSPECs technology and imaging; single lab, novel client identification with functional phenotype","pmids":["36398980"],"is_preprint":false},{"year":2023,"finding":"In polarized epithelial MDCK cells, Cab45 and TGN Ca2+ levels controlled by SPCA1 drive polarized apical secretion of soluble cargoes (secretory form of placental alkaline phosphatase and endogenous clusterin/Gp80); knockdown of Cab45 or expression of an inactive SPCA1 mutant impairs apical exocytosis.","method":"Cab45 siRNA knockdown, overexpression of inactive SPCA1 mutant, secretion assays tracking apical vs. basolateral secretion in polarized MDCK cells","journal":"Molecular biology of the cell","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — loss-of-function with specific directional secretion phenotype, dominant-negative validation; single lab, single cell model","pmids":["37163315"],"is_preprint":false},{"year":2016,"finding":"Cab45-G (Golgi isoform) overexpression promotes cancer cell migration, upregulates MMP-2 and MMP-7 expression, increases phospho-ERK levels, and modulates secretion of extracellular matrix proteins fibronectin and fibulin; knockdown reduces MMP expression.","method":"Overexpression and siRNA knockdown of Cab45-G, migration assay, qRT-PCR for MMP-2/MMP-7, Western blot for phospho-ERK, conditioned medium proteomics for fibronectin/fibulin","journal":"International journal of biological sciences","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single lab, no direct mechanistic link between Cab45-G and ERK pathway established; primarily phenotypic overexpression/knockdown data","pmids":["27194945"],"is_preprint":false},{"year":2024,"finding":"Perturbation of sphingolipid biosynthesis pathways specifically induces secretion of SDF4 (Cab45) while suppressing secretion of its known cargo proteins, suggesting a mechanistic link between sphingolipid composition and Cab45 TGN retention/export.","method":"Mass spectrometry-based secretome profiling of hepatocyte model cells treated with sphingolipid biosynthesis inhibitors; thermal proteome profiling","journal":"bioRxiv","confidence":"Low","confidence_rationale":"Tier 3 / Weak — secretome profiling in drug-treated cells, preprint, no direct mechanistic follow-up on SDF4 specifically; single lab","pmids":[],"is_preprint":true}],"current_model":"SDF4/Cab45 is a soluble, lumenal Ca2+-binding protein (six EF-hand motifs) resident in the trans-Golgi network that, upon transient Ca2+ influx mediated by SPCA1, reversibly oligomerizes and binds specific secretory cargo proteins (e.g., COMP, LyzC, clusterin) to sort them into sphingomyelin-rich TGN-derived vesicles for secretion; its activity is tuned by phosphorylation by the Golgi kinase Fam20C, which promotes vesicle export, and loss of Cab45 additionally causes mistargeting of lysosomal hydrolases (progranulin, prosaposin) and aberrant lysosomal positioning; a cytosolic splice variant (Cab45b) interacts with Munc18b in a Ca2+-dependent manner and participates in regulated exocytosis in pancreatic acini."},"narrative":{"mechanistic_narrative":"SDF4/Cab45 is a soluble, lumenal calcium-binding protein resident in the trans-Golgi network that acts as a Ca2+-dependent sorting receptor for soluble secretory cargo [PMID:8609160, PMID:23266954]. It was the first soluble protein shown to reside in the Golgi lumen, harboring six EF-hand motifs and an N-terminal signal sequence but no membrane anchor [PMID:8609160]. Cab45 functions downstream of the secretory pathway Ca2+ ATPase SPCA1, which imports Ca2+ into the TGN lumen; upon Ca2+ binding, Cab45 reversibly assembles into oligomers that capture specific cargo proteins such as COMP and lysozyme C, and mutation of its Ca2+-binding sites abolishes both oligomerization and cargo sorting [PMID:23266954, PMID:27138253]. Cab45 colocalizes with SPCA1 and its clients in discrete TGN microdomains, where it directs cargo into TGN-derived secretory vesicles [PMID:27138253]. This activity is tuned by phosphorylation by the Golgi kinase Fam20C, which promotes Cab45 translocation into budding vesicles and enhances client export [PMID:32422653]. Beyond conventional secretion, Cab45 governs the correct targeting of lysosomal hydrolase precursors, as its loss causes hypersecretion of unprocessed prosaposin and progranulin and aberrant perinuclear lysosome positioning [PMID:36398980], and it drives polarized apical secretion of soluble cargo in epithelial cells [PMID:37163315]. A cytosolic splice variant, Cab45b, binds Munc18b, syntaxin 2, and syntaxin 3 in a Ca2+-dependent manner and is required for regulated zymogen exocytosis in pancreatic acini [PMID:17442889].","teleology":[{"year":1996,"claim":"Established that a soluble protein could permanently reside in the Golgi lumen, defining Cab45 as an EF-hand Ca2+-binding protein without a membrane anchor and raising the question of its functional role.","evidence":"Subcellular fractionation, immunofluorescence, Ca2+-binding assays and sequence analysis","pmids":["8609160"],"confidence":"High","gaps":["No function assigned at this stage","Mechanism of lumenal retention without a membrane anchor unresolved"]},{"year":2007,"claim":"Identified a cytosolic splice variant (Cab45b) operating in regulated exocytosis, showing the locus encodes functionally distinct isoforms beyond the Golgi-lumenal form.","evidence":"Yeast two-hybrid, in vitro and Ca2+ binding assays, reciprocal Co-IP, and antibody inhibition of amylase secretion in permeabilized pancreatic acini","pmids":["17442889"],"confidence":"High","gaps":["Does not address how the cytosolic isoform is generated and targeted","Structural basis of Ca2+-dependent Munc18b binding unknown"]},{"year":2012,"claim":"Defined Cab45's core function as a TGN cargo sorter operating downstream of SPCA1-mediated Ca2+ import, linking lumenal Ca2+ to cargo selection.","evidence":"RNAi knockdown, Ca2+ imaging, Co-IP of Cab45 with secretory cargo, secretion assays","pmids":["23266954"],"confidence":"High","gaps":["Molecular mechanism of cargo capture not yet reconstituted","Cargo specificity determinants undefined"]},{"year":2016,"claim":"Provided the biochemical mechanism: Ca2+ triggers reversible Cab45 oligomerization that drives specific cargo binding in TGN microdomains, validated by Ca2+-site mutagenesis.","evidence":"In vitro oligomerization and Ca2+-dependent cargo-binding assays, Ca2+-binding site mutagenesis, STED superresolution microscopy, cellular secretion assays","pmids":["27138253"],"confidence":"High","gaps":["Structure of the oligomer-cargo complex unresolved","How distinct clients are discriminated remains unclear"]},{"year":2020,"claim":"Showed Cab45 activity is post-translationally regulated, with Fam20C phosphorylation tuning oligomerization, TGN retention, and vesicle export.","evidence":"MS phosphosite identification, phosphomimetic mutagenesis, vesicle budding/secretion and oligomerization assays","pmids":["32422653"],"confidence":"High","gaps":["Signals controlling Fam20C phosphorylation of Cab45 unknown","Whether phosphorylation alters cargo selectivity not addressed"]},{"year":2023,"claim":"Extended Cab45's role to lysosomal hydrolase targeting and organelle positioning, identifying new clients (prosaposin, progranulin) beyond conventional secretory cargo.","evidence":"hiSPECs secretome analysis of Cab45-deficient cells, lysosomal marker immunofluorescence, processing Western blots","pmids":["36398980"],"confidence":"Medium","gaps":["Direct binding of Cab45 to PSAP/PGRN not demonstrated","Link between hydrolase mistargeting and lysosome mispositioning mechanistically unresolved"]},{"year":2023,"claim":"Demonstrated that the SPCA1–Cab45 axis directs polarized apical secretion, showing the sorting machinery operates in a directional context in epithelia.","evidence":"Cab45 siRNA, dominant-negative SPCA1, apical vs basolateral secretion assays in polarized MDCK cells","pmids":["37163315"],"confidence":"Medium","gaps":["How Cab45 sorting is coupled to apical polarity machinery unknown","Single epithelial cell model"]},{"year":2021,"claim":"Reported a non-canonical extracellular/signaling role in which SDF4 engages CXCR4 to drive VEGFD expression via ERK1/2 and p38, with transcriptional induction by CEBPD in cancer-associated fibroblasts.","evidence":"Co-IP, phospho-blots, tube formation and in vivo angiogenesis assays, CEBPD promoter/ChIP analysis","pmids":["33953165"],"confidence":"Medium","gaps":["Direct SDF4–CXCR4 binding interface not defined","How a Golgi-lumenal protein reaches CXCR4 mechanistically unclear"]},{"year":null,"claim":"How sphingolipid composition controls Cab45 TGN retention versus export, and how this is integrated with Ca2+ and phosphorylation signals, remains open.","evidence":"Secretome and thermal proteome profiling in sphingolipid-inhibitor-treated cells (preprint)","pmids":[],"confidence":"Low","gaps":["Preprint, no direct mechanistic follow-up on SDF4","Causal link between sphingolipid composition and Cab45 export untested"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[2,3]}],"localization":[{"term_id":"GO:0005794","term_label":"Golgi apparatus","supporting_discovery_ids":[0]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[1]}],"pathway":[{"term_id":"R-HSA-5653656","term_label":"Vesicle-mediated transport","supporting_discovery_ids":[2,3,4]},{"term_id":"R-HSA-9609507","term_label":"Protein localization","supporting_discovery_ids":[6,7]}],"complexes":[],"partners":["SPCA1","FAM20C","MUNC18B","STX2","STX3","CXCR4"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q9BRK5","full_name":"45 kDa calcium-binding protein","aliases":["Stromal cell-derived factor 4","SDF-4"],"length_aa":362,"mass_kda":41.8,"function":"May regulate calcium-dependent activities in the endoplasmic reticulum lumen or post-ER compartment Isoform 5 may be involved in the exocytosis of zymogens by pancreatic acini","subcellular_location":"Cytoplasm; Cell membrane; Cell projection, bleb","url":"https://www.uniprot.org/uniprotkb/Q9BRK5/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/SDF4","classification":"Not Classified","n_dependent_lines":4,"n_total_lines":1208,"dependency_fraction":0.0033112582781456954},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/SDF4","total_profiled":1310},"omim":[{"mim_id":"614282","title":"STROMAL CELL-DERIVED FACTOR 4; SDF4","url":"https://www.omim.org/entry/614282"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Golgi apparatus","reliability":"Supported"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/SDF4"},"hgnc":{"alias_symbol":["Cab45"],"prev_symbol":[]},"alphafold":{"accession":"Q9BRK5","domains":[{"cath_id":"-","chopping":"157-216","consensus_level":"medium","plddt":93.7897,"start":157,"end":216}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9BRK5","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9BRK5-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9BRK5-F1-predicted_aligned_error_v6.png","plddt_mean":80.62},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=SDF4","jax_strain_url":"https://www.jax.org/strain/search?query=SDF4"},"sequence":{"accession":"Q9BRK5","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9BRK5.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9BRK5/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9BRK5"}},"corpus_meta":[{"pmid":"8609160","id":"PMC_8609160","title":"Cab45, a novel (Ca2+)-binding protein localized to the Golgi lumen.","date":"1996","source":"The Journal of cell biology","url":"https://pubmed.ncbi.nlm.nih.gov/8609160","citation_count":130,"is_preprint":false},{"pmid":"23266954","id":"PMC_23266954","title":"Cab45 is required for Ca(2+)-dependent secretory cargo sorting at the trans-Golgi network.","date":"2012","source":"The Journal of cell biology","url":"https://pubmed.ncbi.nlm.nih.gov/23266954","citation_count":77,"is_preprint":false},{"pmid":"27138253","id":"PMC_27138253","title":"Secretory cargo sorting by Ca2+-dependent Cab45 oligomerization at the trans-Golgi network.","date":"2016","source":"The Journal of cell biology","url":"https://pubmed.ncbi.nlm.nih.gov/27138253","citation_count":42,"is_preprint":false},{"pmid":"33953165","id":"PMC_33953165","title":"Fibroblast CEBPD/SDF4 axis in response to chemotherapy-induced angiogenesis through CXCR4.","date":"2021","source":"Cell death discovery","url":"https://pubmed.ncbi.nlm.nih.gov/33953165","citation_count":29,"is_preprint":false},{"pmid":"17442889","id":"PMC_17442889","title":"A cytosolic splice variant of Cab45 interacts with Munc18b and impacts on amylase secretion by pancreatic acini.","date":"2007","source":"Molecular biology of the cell","url":"https://pubmed.ncbi.nlm.nih.gov/17442889","citation_count":29,"is_preprint":false},{"pmid":"32422653","id":"PMC_32422653","title":"Fam20C regulates protein secretion by Cab45 phosphorylation.","date":"2020","source":"The Journal of cell biology","url":"https://pubmed.ncbi.nlm.nih.gov/32422653","citation_count":18,"is_preprint":false},{"pmid":"28372832","id":"PMC_28372832","title":"Cab45-Unraveling key features of a novel secretory cargo sorter at the trans-Golgi network.","date":"2017","source":"European journal of cell biology","url":"https://pubmed.ncbi.nlm.nih.gov/28372832","citation_count":18,"is_preprint":false},{"pmid":"27194945","id":"PMC_27194945","title":"A Novel Role of Cab45-G in Mediating Cell Migration in Cancer Cells.","date":"2016","source":"International journal of biological sciences","url":"https://pubmed.ncbi.nlm.nih.gov/27194945","citation_count":15,"is_preprint":false},{"pmid":"9254016","id":"PMC_9254016","title":"Sequence of a human cDNA encoding Cab45, a Ca2+-binding protein with six EF-hand motifs.","date":"1997","source":"DNA sequence : the journal of DNA sequencing and mapping","url":"https://pubmed.ncbi.nlm.nih.gov/9254016","citation_count":10,"is_preprint":false},{"pmid":"36606322","id":"PMC_36606322","title":"LINC00173 facilitates tumor progression by stimulating RAB1B-mediated PA2G4 and SDF4 secretion in nasopharyngeal carcinoma.","date":"2023","source":"Molecular oncology","url":"https://pubmed.ncbi.nlm.nih.gov/36606322","citation_count":8,"is_preprint":false},{"pmid":"18355758","id":"PMC_18355758","title":"The ethanol response gene Cab45 can modulate the impairment elicited by ethanol and ultraviolet in PC12 cells.","date":"2008","source":"Journal of genetics and genomics = Yi chuan xue bao","url":"https://pubmed.ncbi.nlm.nih.gov/18355758","citation_count":8,"is_preprint":false},{"pmid":"36398980","id":"PMC_36398980","title":"Cab45 deficiency leads to the mistargeting of progranulin and prosaposin and aberrant lysosomal positioning.","date":"2023","source":"Traffic (Copenhagen, Denmark)","url":"https://pubmed.ncbi.nlm.nih.gov/36398980","citation_count":3,"is_preprint":false},{"pmid":"37163315","id":"PMC_37163315","title":"Calcium-binding Cab45 regulates the polarized apical secretion of soluble proteins in epithelial cells.","date":"2023","source":"Molecular biology of the cell","url":"https://pubmed.ncbi.nlm.nih.gov/37163315","citation_count":2,"is_preprint":false},{"pmid":"41464845","id":"PMC_41464845","title":"The Molecular Relationship Between SDF4 and Thiol/Disulfide Homeostasis and Cardiac Injury Markers in Serum and Pericardial Fluid of Patients Undergoing Open-Heart Surgery.","date":"2025","source":"Journal of clinical medicine","url":"https://pubmed.ncbi.nlm.nih.gov/41464845","citation_count":0,"is_preprint":false},{"pmid":null,"id":"bio_10.1101_2025.08.13.25333571","title":"Circulating SDF4 as a Potential Early Detection Biomarker for Pancreatic Cancer","date":"2025-08-16","source":"bioRxiv","url":"https://doi.org/10.1101/2025.08.13.25333571","citation_count":0,"is_preprint":true},{"pmid":null,"id":"bio_10.1101_2025.02.27.25323006","title":"Integrating Genome-wide and Epigenome-wide Associations for Antipsychotic Induced Extrapyramidal Side Effects","date":"2025-02-28","source":"bioRxiv","url":"https://doi.org/10.1101/2025.02.27.25323006","citation_count":0,"is_preprint":true},{"pmid":null,"id":"bio_10.1101_2024.10.16.618637","title":"Pharmacoproteomic profiling identifies secreted markers for aberrant drug action","date":"2024-10-18","source":"bioRxiv","url":"https://doi.org/10.1101/2024.10.16.618637","citation_count":0,"is_preprint":true}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":9621,"output_tokens":2703,"usd":0.034704,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":9961,"output_tokens":2966,"usd":0.061977,"stage2_stop_reason":"end_turn"},"total_usd":0.096681,"stage1_batch_id":"msgbatch_013LWZZT6MWaqB9qyTSeQBmU","stage2_batch_id":"msgbatch_0194ZzQaTGNpruoNJBbEMbt3","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1996,\n      \"finding\": \"Cab45 (SDF4) is a soluble, lumenal Golgi-resident calcium-binding protein with six EF-hand motifs and an N-terminal signal sequence but no membrane-anchor sequences, making it the first known soluble protein resident in the Golgi lumen.\",\n      \"method\": \"Subcellular fractionation, immunofluorescence localization, Ca2+-binding assays, sequence analysis of signal sequence and EF-hand motifs\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — direct localization by fractionation and imaging, Ca2+-binding demonstrated biochemically, replicated by multiple subsequent studies\",\n      \"pmids\": [\"8609160\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"A cytosolic splice variant of Cab45 (Cab45b) interacts with Munc18b in a Ca2+-dependent manner; Cab45b co-immunoprecipitates with Munc18b, syntaxin 2, and syntaxin 3 in pancreatic acini, and antibodies against Cab45b inhibit Ca2+-induced amylase release from permeabilized acini, establishing a role in zymogen exocytosis.\",\n      \"method\": \"Yeast two-hybrid screen, in vitro binding assay, 45Ca2+ binding assay, co-immunoprecipitation, antibody inhibition of amylase secretion from streptolysin-O-permeabilized acini\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal Co-IP, in vitro binding, functional antibody inhibition assay with dose-dependent readout; single lab but multiple orthogonal methods\",\n      \"pmids\": [\"17442889\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Cab45 is required for SPCA1 (secretory pathway calcium ATPase 1)-dependent Ca2+ import into the TGN lumen and binds secretory cargo in a Ca2+-dependent reaction, functioning as a cargo sorter at the trans-Golgi network.\",\n      \"method\": \"RNAi knockdown, Ca2+ imaging, co-immunoprecipitation of Cab45 with secretory cargo, functional secretion assays\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — loss-of-function with specific cargo-sorting phenotype, Ca2+-dependent binding demonstrated, independently confirmed by subsequent studies\",\n      \"pmids\": [\"23266954\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Cab45 reversibly assembles into oligomers in the presence of Ca2+; these oligomers specifically bind secretory cargo proteins (COMP and LyzC) in vitro in a Ca2+-dependent manner; mutation of Ca2+-binding sites in Cab45 impairs both oligomerization and cargo sorting; superresolution microscopy shows Cab45 colocalizes with secretory cargo and SPCA1 in specific TGN microdomains.\",\n      \"method\": \"In vitro oligomerization assay, in vitro Ca2+-dependent cargo binding, Ca2+-binding site mutagenesis, STED superresolution microscopy, secretion assay in cells\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — reconstituted Ca2+-dependent oligomerization and cargo binding in vitro, validated by mutagenesis and superresolution imaging in cells; multiple orthogonal methods in one rigorous study\",\n      \"pmids\": [\"27138253\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Cab45 is phosphorylated by the Golgi-specific kinase Fam20C; phosphomimetic mutation of Cab45 promotes its translocation into TGN-derived vesicles, increases export of the Cab45 client LyzC, and fine-tunes Cab45 oligomerization and TGN retention.\",\n      \"method\": \"Mass spectrometry identification of phosphorylation site, phosphomimetic mutagenesis, vesicle budding/secretion assays, oligomerization analysis\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — phosphorylation site identified by MS, mechanistic consequences tested by mutagenesis and functional secretion assays; single lab with multiple orthogonal methods\",\n      \"pmids\": [\"32422653\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"SDF4 (Cab45) interacts with CXCR4 in endothelial cells and triggers VEGFD expression through activation of ERK1/2 and p38 pathways; SDF4 is transcriptionally induced by CEBPD in cancer-associated fibroblasts in response to cisplatin/5-FU treatment.\",\n      \"method\": \"Co-immunoprecipitation (SDF4–CXCR4 interaction), Western blot for ERK1/2 and p38 phosphorylation, in vitro tube formation assay, in vivo angiogenesis assay, promoter/ChIP analysis of CEBPD on SDF4\",\n      \"journal\": \"Cell death discovery\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — Co-IP for CXCR4 interaction, signaling pathway activation shown by phospho-blots, functional secretion phenotype; single lab, limited mechanistic depth on direct binding\",\n      \"pmids\": [\"33953165\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Cab45 deficiency causes hypersecretion of lysosomal hydrolases, specifically unprocessed precursors of prosaposin (PSAP) and progranulin (PGRN), and results in aberrant perinuclear accumulation of lysosomes, revealing a role for Cab45 in lysosomal protein targeting and lysosomal positioning.\",\n      \"method\": \"hiSPECs secretome analysis of Cab45-deficient cells, immunofluorescence of lysosomal markers, Western blot for PSAP and PGRN processing\",\n      \"journal\": \"Traffic (Copenhagen, Denmark)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — precise secretome quantification by hiSPECs technology and imaging; single lab, novel client identification with functional phenotype\",\n      \"pmids\": [\"36398980\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"In polarized epithelial MDCK cells, Cab45 and TGN Ca2+ levels controlled by SPCA1 drive polarized apical secretion of soluble cargoes (secretory form of placental alkaline phosphatase and endogenous clusterin/Gp80); knockdown of Cab45 or expression of an inactive SPCA1 mutant impairs apical exocytosis.\",\n      \"method\": \"Cab45 siRNA knockdown, overexpression of inactive SPCA1 mutant, secretion assays tracking apical vs. basolateral secretion in polarized MDCK cells\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — loss-of-function with specific directional secretion phenotype, dominant-negative validation; single lab, single cell model\",\n      \"pmids\": [\"37163315\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Cab45-G (Golgi isoform) overexpression promotes cancer cell migration, upregulates MMP-2 and MMP-7 expression, increases phospho-ERK levels, and modulates secretion of extracellular matrix proteins fibronectin and fibulin; knockdown reduces MMP expression.\",\n      \"method\": \"Overexpression and siRNA knockdown of Cab45-G, migration assay, qRT-PCR for MMP-2/MMP-7, Western blot for phospho-ERK, conditioned medium proteomics for fibronectin/fibulin\",\n      \"journal\": \"International journal of biological sciences\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single lab, no direct mechanistic link between Cab45-G and ERK pathway established; primarily phenotypic overexpression/knockdown data\",\n      \"pmids\": [\"27194945\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Perturbation of sphingolipid biosynthesis pathways specifically induces secretion of SDF4 (Cab45) while suppressing secretion of its known cargo proteins, suggesting a mechanistic link between sphingolipid composition and Cab45 TGN retention/export.\",\n      \"method\": \"Mass spectrometry-based secretome profiling of hepatocyte model cells treated with sphingolipid biosynthesis inhibitors; thermal proteome profiling\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — secretome profiling in drug-treated cells, preprint, no direct mechanistic follow-up on SDF4 specifically; single lab\",\n      \"pmids\": [],\n      \"is_preprint\": true\n    }\n  ],\n  \"current_model\": \"SDF4/Cab45 is a soluble, lumenal Ca2+-binding protein (six EF-hand motifs) resident in the trans-Golgi network that, upon transient Ca2+ influx mediated by SPCA1, reversibly oligomerizes and binds specific secretory cargo proteins (e.g., COMP, LyzC, clusterin) to sort them into sphingomyelin-rich TGN-derived vesicles for secretion; its activity is tuned by phosphorylation by the Golgi kinase Fam20C, which promotes vesicle export, and loss of Cab45 additionally causes mistargeting of lysosomal hydrolases (progranulin, prosaposin) and aberrant lysosomal positioning; a cytosolic splice variant (Cab45b) interacts with Munc18b in a Ca2+-dependent manner and participates in regulated exocytosis in pancreatic acini.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"SDF4/Cab45 is a soluble, lumenal calcium-binding protein resident in the trans-Golgi network that acts as a Ca2+-dependent sorting receptor for soluble secretory cargo [#0, #2]. It was the first soluble protein shown to reside in the Golgi lumen, harboring six EF-hand motifs and an N-terminal signal sequence but no membrane anchor [#0]. Cab45 functions downstream of the secretory pathway Ca2+ ATPase SPCA1, which imports Ca2+ into the TGN lumen; upon Ca2+ binding, Cab45 reversibly assembles into oligomers that capture specific cargo proteins such as COMP and lysozyme C, and mutation of its Ca2+-binding sites abolishes both oligomerization and cargo sorting [#2, #3]. Cab45 colocalizes with SPCA1 and its clients in discrete TGN microdomains, where it directs cargo into TGN-derived secretory vesicles [#3]. This activity is tuned by phosphorylation by the Golgi kinase Fam20C, which promotes Cab45 translocation into budding vesicles and enhances client export [#4]. Beyond conventional secretion, Cab45 governs the correct targeting of lysosomal hydrolase precursors, as its loss causes hypersecretion of unprocessed prosaposin and progranulin and aberrant perinuclear lysosome positioning [#6], and it drives polarized apical secretion of soluble cargo in epithelial cells [#7]. A cytosolic splice variant, Cab45b, binds Munc18b, syntaxin 2, and syntaxin 3 in a Ca2+-dependent manner and is required for regulated zymogen exocytosis in pancreatic acini [#1].\",\n  \"teleology\": [\n    {\n      \"year\": 1996,\n      \"claim\": \"Established that a soluble protein could permanently reside in the Golgi lumen, defining Cab45 as an EF-hand Ca2+-binding protein without a membrane anchor and raising the question of its functional role.\",\n      \"evidence\": \"Subcellular fractionation, immunofluorescence, Ca2+-binding assays and sequence analysis\",\n      \"pmids\": [\"8609160\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No function assigned at this stage\", \"Mechanism of lumenal retention without a membrane anchor unresolved\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Identified a cytosolic splice variant (Cab45b) operating in regulated exocytosis, showing the locus encodes functionally distinct isoforms beyond the Golgi-lumenal form.\",\n      \"evidence\": \"Yeast two-hybrid, in vitro and Ca2+ binding assays, reciprocal Co-IP, and antibody inhibition of amylase secretion in permeabilized pancreatic acini\",\n      \"pmids\": [\"17442889\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Does not address how the cytosolic isoform is generated and targeted\", \"Structural basis of Ca2+-dependent Munc18b binding unknown\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Defined Cab45's core function as a TGN cargo sorter operating downstream of SPCA1-mediated Ca2+ import, linking lumenal Ca2+ to cargo selection.\",\n      \"evidence\": \"RNAi knockdown, Ca2+ imaging, Co-IP of Cab45 with secretory cargo, secretion assays\",\n      \"pmids\": [\"23266954\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular mechanism of cargo capture not yet reconstituted\", \"Cargo specificity determinants undefined\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Provided the biochemical mechanism: Ca2+ triggers reversible Cab45 oligomerization that drives specific cargo binding in TGN microdomains, validated by Ca2+-site mutagenesis.\",\n      \"evidence\": \"In vitro oligomerization and Ca2+-dependent cargo-binding assays, Ca2+-binding site mutagenesis, STED superresolution microscopy, cellular secretion assays\",\n      \"pmids\": [\"27138253\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structure of the oligomer-cargo complex unresolved\", \"How distinct clients are discriminated remains unclear\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Showed Cab45 activity is post-translationally regulated, with Fam20C phosphorylation tuning oligomerization, TGN retention, and vesicle export.\",\n      \"evidence\": \"MS phosphosite identification, phosphomimetic mutagenesis, vesicle budding/secretion and oligomerization assays\",\n      \"pmids\": [\"32422653\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Signals controlling Fam20C phosphorylation of Cab45 unknown\", \"Whether phosphorylation alters cargo selectivity not addressed\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Extended Cab45's role to lysosomal hydrolase targeting and organelle positioning, identifying new clients (prosaposin, progranulin) beyond conventional secretory cargo.\",\n      \"evidence\": \"hiSPECs secretome analysis of Cab45-deficient cells, lysosomal marker immunofluorescence, processing Western blots\",\n      \"pmids\": [\"36398980\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct binding of Cab45 to PSAP/PGRN not demonstrated\", \"Link between hydrolase mistargeting and lysosome mispositioning mechanistically unresolved\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Demonstrated that the SPCA1–Cab45 axis directs polarized apical secretion, showing the sorting machinery operates in a directional context in epithelia.\",\n      \"evidence\": \"Cab45 siRNA, dominant-negative SPCA1, apical vs basolateral secretion assays in polarized MDCK cells\",\n      \"pmids\": [\"37163315\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"How Cab45 sorting is coupled to apical polarity machinery unknown\", \"Single epithelial cell model\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Reported a non-canonical extracellular/signaling role in which SDF4 engages CXCR4 to drive VEGFD expression via ERK1/2 and p38, with transcriptional induction by CEBPD in cancer-associated fibroblasts.\",\n      \"evidence\": \"Co-IP, phospho-blots, tube formation and in vivo angiogenesis assays, CEBPD promoter/ChIP analysis\",\n      \"pmids\": [\"33953165\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct SDF4–CXCR4 binding interface not defined\", \"How a Golgi-lumenal protein reaches CXCR4 mechanistically unclear\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How sphingolipid composition controls Cab45 TGN retention versus export, and how this is integrated with Ca2+ and phosphorylation signals, remains open.\",\n      \"evidence\": \"Secretome and thermal proteome profiling in sphingolipid-inhibitor-treated cells (preprint)\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"Preprint, no direct mechanistic follow-up on SDF4\", \"Causal link between sphingolipid composition and Cab45 export untested\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005509\", \"supporting_discovery_ids\": [0]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [2, 3]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005794\", \"supporting_discovery_ids\": [0]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [1]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-5653656\", \"supporting_discovery_ids\": [2, 3, 4]},\n      {\"term_id\": \"R-HSA-9609507\", \"supporting_discovery_ids\": [6, 7]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"SPCA1\", \"FAM20C\", \"Munc18b\", \"STX2\", \"STX3\", \"CXCR4\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":7,"faith_total":7,"faith_pct":100.0}}