{"gene":"TEPSIN","run_date":"2026-06-10T10:51:54","timeline":{"discoveries":[{"year":2012,"finding":"Tepsin was identified as the first accessory protein of the AP-4 coat complex, co-purifying with AP-4-coated vesicles from HeLa cells as determined by multivariate SILAC-based quantitative mass spectrometry and siRNA knockdown profiling.","method":"SILAC-based quantitative mass spectrometry, siRNA knockdown, principal component analysis of coated vesicle fractions","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal quantitative proteomic methods with knockdown controls, replicated by subsequent studies","pmids":["22472443"],"is_preprint":false},{"year":2014,"finding":"Recruitment of tepsin to the membrane is abolished when AP-4 complex assembly is disrupted by loss-of-function mutations in AP4S1, demonstrating that tepsin membrane recruitment depends on intact AP-4 complex.","method":"Patient fibroblast cell line analysis; immunofluorescence/Western blot showing loss of AP-4 subunits and tepsin membrane recruitment upon AP4S1 frameshift/nonsense mutations","journal":"Human molecular genetics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — cellular loss-of-function in patient-derived cells with defined molecular phenotype, single lab","pmids":["25552650"],"is_preprint":false},{"year":2015,"finding":"Tepsin contains two phylogenetically conserved peptide motifs in its unstructured C-terminus—[GS]LFXG[ML]X[LV] and S[AV]F[SA]FLN—that interact with the C-terminal ear/appendage domains of the β4 and ε subunits of AP-4, respectively. Both interactions are required for efficient association of tepsin with AP-4 and for tepsin recruitment to the TGN. Bivalent interaction increases avidity and may cross-link AP-4 heterotetramers to contribute to coat assembly.","method":"Protein interaction assays (GST pulldown, yeast two-hybrid, phage display), structure-based mutagenesis, cellular localization assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — multiple orthogonal binding assays plus mutagenesis in vitro and in cells, replicated by independent lab (PMID:26756312)","pmids":["26542808"],"is_preprint":false},{"year":2016,"finding":"The tepsin C-terminal LFxG[M/L]x[L/V] motif binds directly and specifically to the AP-4 β4 appendage domain. NMR chemical shift mapping defined the binding site on the β4 appendage surface. Point mutations in either the tepsin motif or the cognate β4 surface abolish in vitro binding and greatly reduce (but do not completely abolish) tepsin–AP-4 interaction in cells, suggesting additional interaction sites exist.","method":"NMR chemical shift mapping, in vitro binding assays, point mutagenesis, co-immunoprecipitation in cells","journal":"Traffic (Copenhagen, Denmark)","confidence":"High","confidence_rationale":"Tier 1 / Strong — NMR structure-guided mutagenesis combined with in vitro binding and cellular co-IP, validated independently (PMID:26542808)","pmids":["26756312"],"is_preprint":false},{"year":2017,"finding":"X-ray crystal structures of the tepsin ENTH and VHS/ENTH-like domains revealed that: (1) the tepsin ENTH domain lacks helix0, helix8, and a lipid-binding pocket present in epsin1/2/3, explaining why tepsin requires AP-4 for membrane recruitment rather than binding lipids directly; (2) the tepsin VHS domain lacks helix8 and does not mediate known VHS functions such as recognition of dileucine-based cargo motifs or ubiquitin binding.","method":"X-ray crystallography, biochemical/biophysical binding assays, phylogenetic and comparative genomic analysis","journal":"Traffic (Copenhagen, Denmark)","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structures of both domains with biochemical validation of functional inferences, single lab but multiple orthogonal methods","pmids":["28691777"],"is_preprint":false},{"year":2024,"finding":"Tepsin directly binds LC3B (preferentially over other mammalian ATG8 family members) via a canonical LC3-Interacting Region (LIR) motif, with micromolar affinity at the LC3B LIR docking site. Loss of tepsin in cultured cells dysregulates ATG9A export from the TGN and ATG9A distribution at the cell periphery. Tepsin depletion increases autophagosome volume and number without affecting autophagic flux. Reintroduction of wild-type tepsin partially rescues ATG9A trafficking defects, while tepsin with a mutated LIR motif or missing N-terminus fails to fully rescue ATG9A distribution.","method":"In silico LIR motif prediction, recombinant protein biochemistry (pulldown, calorimetry), structural modeling, siRNA knockdown, fluorescence microscopy (mRFP-GFP-LC3B reporter), rescue experiments with LIR-mutant tepsin","journal":"Molecular biology of the cell","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — calorimetry plus structural modeling plus mutagenesis plus cellular loss-of-function/rescue with defined phenotypic readouts, multiple orthogonal methods in one study","pmids":["38381558"],"is_preprint":false},{"year":2022,"finding":"Loss of tepsin in CRISPR-edited zebrafish embryos causes abnormal head morphology and neural necrosis, and alters expression levels and patterns of autophagy genes atg9a and map1lc3b, linking tepsin function to AP-4-dependent ATG9A trafficking and autophagy in a developmental context.","method":"CRISPR-ExoCas9 knockout in zebrafish, morphological analysis at 24 hpf, gene expression analysis","journal":"Advances in biological regulation","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vivo loss-of-function with defined morphological and molecular phenotypes, single lab","pmids":["36642642"],"is_preprint":false},{"year":2025,"finding":"Computational modeling (AlphaFold Multimer) combined with bio-layer interferometry (BLI) and biochemical experiments identified three additional LC3B-binding motifs beyond the canonical LIR in tepsin's disordered regions, all engaging the LC3B LIR docking site. All four motifs must be mutated to abrogate LC3B binding in vitro. Stoichiometry data indicate one tepsin molecule likely binds two LC3B molecules simultaneously, suggesting multivalent LC3B engagement that could dynamically modulate binding strength in response to LC3B membrane concentrations.","method":"AlphaFold Multimer structural modeling, bio-layer interferometry (BLI), biochemical binding assays, mutagenesis, thermodynamic/kinetic analysis","journal":"Advances in biological regulation","confidence":"Medium","confidence_rationale":"Tier 1–2 / Moderate — structural modeling plus BLI and mutagenesis in single lab, builds directly on prior peer-reviewed findings","pmids":["41198464"],"is_preprint":false}],"current_model":"Tepsin is the only known accessory protein of the AP-4 non-clathrin coat at the trans-Golgi network (TGN); it is recruited to membranes exclusively through AP-4 (not via direct lipid binding, as its ENTH domain lacks a lipid-binding pocket), using two conserved C-terminal peptide motifs that engage the β4 and ε appendage domains of AP-4 in a bivalent interaction; tepsin further directly binds LC3B via multiple LIR motifs (with micromolar affinity) and this interaction is required for proper ATG9A cargo export from the TGN and delivery to the cell periphery, linking tepsin to autophagosome biogenesis."},"narrative":{"mechanistic_narrative":"Tepsin is the only known dedicated accessory protein of the AP-4 non-clathrin coat that operates at the trans-Golgi network, where it is recruited to membranes through the AP-4 complex rather than through direct lipid binding [PMID:22472443, PMID:25552650]. Its recruitment is strictly AP-4-dependent: disruption of AP-4 assembly through loss-of-function mutation of the AP4S1 subunit abolishes tepsin membrane association [PMID:25552650], and two phylogenetically conserved C-terminal peptide motifs engage the β4 and ε appendage/ear domains of AP-4 in a bivalent interaction that increases avidity and can cross-link AP-4 heterotetramers during coat assembly [PMID:26542808, PMID:26756312]. Structurally, tepsin's ENTH and VHS domains are degenerate—lacking the lipid-binding pocket of epsins and the cargo/ubiquitin-recognition surfaces of canonical VHS domains—accounting for its dependence on AP-4 for membrane localization [PMID:28691777]. Beyond its scaffolding role, tepsin directly binds LC3B through a canonical LIR motif plus additional disordered-region motifs that engage the LC3B docking site with micromolar affinity in a multivalent manner [PMID:38381558, PMID:41198464], and this interaction is required for proper ATG9A export from the TGN and its peripheral distribution, linking tepsin to autophagosome biogenesis [PMID:38381558]. Loss of tepsin in zebrafish causes abnormal head morphology and neural necrosis with altered atg9a and map1lc3b expression, situating this AP-4/autophagy axis in a developmental context [PMID:36642642].","teleology":[{"year":2012,"claim":"Established that the AP-4 coat, unlike other adaptor complexes, possesses a dedicated accessory protein, identifying tepsin as the first such component and defining a starting point for AP-4 coat biology.","evidence":"SILAC quantitative mass spectrometry of AP-4-coated vesicle fractions with siRNA knockdown profiling in HeLa cells","pmids":["22472443"],"confidence":"High","gaps":["Did not define how tepsin associates with AP-4 mechanistically","No cargo or downstream function established"]},{"year":2014,"claim":"Determined that tepsin's membrane localization is not autonomous but requires an intact AP-4 complex, establishing the hierarchy of coat assembly.","evidence":"Patient-derived fibroblasts carrying AP4S1 loss-of-function mutations, with immunofluorescence/Western blot of AP-4 subunits and tepsin","pmids":["25552650"],"confidence":"Medium","gaps":["Single lab, patient-cell context","Did not map the molecular interface mediating recruitment"]},{"year":2015,"claim":"Defined the molecular basis of tepsin–AP-4 binding by identifying two conserved C-terminal motifs that engage the β4 and ε appendage domains, showing a bivalent interaction that could drive coat assembly.","evidence":"GST pulldown, yeast two-hybrid, phage display, structure-based mutagenesis and cellular localization assays","pmids":["26542808"],"confidence":"High","gaps":["Structural detail of the binding surfaces not resolved","Functional consequence of cross-linking for vesicle formation untested"]},{"year":2016,"claim":"Mapped the tepsin LFxG[M/L]x[L/V] motif directly onto the β4 appendage surface, confirming specificity but revealing residual cellular binding that implied additional interaction sites.","evidence":"NMR chemical shift mapping, in vitro binding, point mutagenesis and co-immunoprecipitation in cells","pmids":["26756312"],"confidence":"High","gaps":["Identity of the residual ε-mediated contribution not structurally resolved here","Did not address physiological cargo"]},{"year":2017,"claim":"Explained why tepsin depends on AP-4 by solving structures showing its ENTH and VHS domains are degenerate, lacking lipid-binding and cargo/ubiquitin-recognition features of canonical epsin/VHS proteins.","evidence":"X-ray crystallography of tepsin ENTH and VHS domains with biochemical binding assays and comparative genomics","pmids":["28691777"],"confidence":"High","gaps":["Functional role of the degenerate domains, if any, remained undefined","No interactor identified for these domains"]},{"year":2022,"claim":"Placed tepsin function in a whole-organism developmental context, linking its loss to neural defects and dysregulated autophagy gene expression in vivo.","evidence":"CRISPR knockout in zebrafish embryos with morphological and gene-expression analysis at 24 hpf","pmids":["36642642"],"confidence":"Medium","gaps":["Causal mechanism linking tepsin loss to neural necrosis not resolved","Single lab, indirect link to ATG9A trafficking"]},{"year":2024,"claim":"Connected tepsin to autophagosome biogenesis by demonstrating direct LIR-mediated LC3B binding required for ATG9A export from the TGN and peripheral distribution.","evidence":"Recombinant pulldown and calorimetry, structural modeling, siRNA knockdown with mRFP-GFP-LC3B reporter and LIR-mutant rescue experiments","pmids":["38381558"],"confidence":"High","gaps":["Only partial rescue achieved by wild-type reintroduction","How ATG9A export couples to LC3B binding mechanistically unresolved","Effect on autophagic flux absent despite altered autophagosome number"]},{"year":2025,"claim":"Refined the tepsin–LC3B interaction as multivalent, identifying three additional LC3B-binding motifs and stoichiometry consistent with one tepsin engaging two LC3B molecules.","evidence":"AlphaFold Multimer modeling, bio-layer interferometry, biochemical binding and mutagenesis with thermodynamic/kinetic analysis","pmids":["41198464"],"confidence":"Medium","gaps":["Functional relevance of multivalency in cells not directly tested","Modeling-derived motifs require structural confirmation","Single lab"]},{"year":null,"claim":"How tepsin mechanistically coordinates AP-4 coat assembly with LC3B-dependent ATG9A export, and whether its bivalent AP-4 binding and multivalent LC3B engagement are temporally coupled during vesicle formation, remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No integrated structural model of tepsin bridging AP-4 and LC3B","Cargo selectivity determinants for ATG9A export undefined","Physiological role of multivalent LC3B binding untested in cells"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[0,2,3,5]}],"localization":[{"term_id":"GO:0005794","term_label":"Golgi apparatus","supporting_discovery_ids":[1,2,5]}],"pathway":[{"term_id":"R-HSA-9612973","term_label":"Autophagy","supporting_discovery_ids":[5,6]},{"term_id":"R-HSA-5653656","term_label":"Vesicle-mediated transport","supporting_discovery_ids":[0,5]}],"complexes":["AP-4 coat"],"partners":["AP4B1","AP4E1","LC3B","AP4S1"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q96N21","full_name":"AP-4 complex accessory subunit Tepsin","aliases":["ENTH domain-containing protein 2","Epsin for AP-4","Tetra-epsin"],"length_aa":525,"mass_kda":55.1,"function":"Associates with the adapter-like complex 4 (AP-4) and may therefore play a role in vesicular trafficking of proteins at the trans-Golgi network","subcellular_location":"Golgi apparatus, trans-Golgi network membrane; Cytoplasmic vesicle; Cytoplasm, cytosol","url":"https://www.uniprot.org/uniprotkb/Q96N21/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/TEPSIN","classification":"Not Classified","n_dependent_lines":161,"n_total_lines":1208,"dependency_fraction":0.13327814569536423},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/TEPSIN","total_profiled":1310},"omim":[],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Vesicles","reliability":"Supported"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/TEPSIN"},"hgnc":{"alias_symbol":["FLJ31528"],"prev_symbol":["C17orf56","ENTHD2"]},"alphafold":{"accession":"Q96N21","domains":[{"cath_id":"1.25.40.90","chopping":"8-133","consensus_level":"high","plddt":96.9926,"start":8,"end":133},{"cath_id":"1.25.40,1.25.40","chopping":"243-353","consensus_level":"high","plddt":93.8583,"start":243,"end":353}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q96N21","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q96N21-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q96N21-F1-predicted_aligned_error_v6.png","plddt_mean":67.81},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=TEPSIN","jax_strain_url":"https://www.jax.org/strain/search?query=TEPSIN"},"sequence":{"accession":"Q96N21","fasta_url":"https://rest.uniprot.org/uniprotkb/Q96N21.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q96N21/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q96N21"}},"corpus_meta":[{"pmid":"22472443","id":"PMC_22472443","title":"Multivariate proteomic profiling identifies novel accessory proteins of coated vesicles.","date":"2012","source":"The Journal of cell biology","url":"https://pubmed.ncbi.nlm.nih.gov/22472443","citation_count":140,"is_preprint":false},{"pmid":"25552650","id":"PMC_25552650","title":"Recessive loss-of-function mutations in AP4S1 cause mild fever-sensitive seizures, developmental delay and spastic paraplegia through loss of AP-4 complex assembly.","date":"2014","source":"Human molecular genetics","url":"https://pubmed.ncbi.nlm.nih.gov/25552650","citation_count":49,"is_preprint":false},{"pmid":"18256691","id":"PMC_18256691","title":"A promoter sequence variant of ZNF750 is linked with familial psoriasis.","date":"2008","source":"The Journal of investigative dermatology","url":"https://pubmed.ncbi.nlm.nih.gov/18256691","citation_count":31,"is_preprint":false},{"pmid":"26756312","id":"PMC_26756312","title":"Molecular Basis for the Interaction Between AP4 β4 and its Accessory Protein, Tepsin.","date":"2016","source":"Traffic (Copenhagen, Denmark)","url":"https://pubmed.ncbi.nlm.nih.gov/26756312","citation_count":24,"is_preprint":false},{"pmid":"31395793","id":"PMC_31395793","title":"Frailty in middle age is associated with frailty status and race-specific changes to the transcriptome.","date":"2019","source":"Aging","url":"https://pubmed.ncbi.nlm.nih.gov/31395793","citation_count":24,"is_preprint":false},{"pmid":"26542808","id":"PMC_26542808","title":"Bivalent Motif-Ear Interactions Mediate the Association of the Accessory Protein Tepsin with the AP-4 Adaptor Complex.","date":"2015","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/26542808","citation_count":22,"is_preprint":false},{"pmid":"33084855","id":"PMC_33084855","title":"The role of AP-4 in cargo export from the trans-Golgi network and hereditary spastic paraplegia.","date":"2020","source":"Biochemical Society transactions","url":"https://pubmed.ncbi.nlm.nih.gov/33084855","citation_count":16,"is_preprint":false},{"pmid":"28691777","id":"PMC_28691777","title":"Structure and evolution of ENTH and VHS/ENTH-like domains in tepsin.","date":"2017","source":"Traffic (Copenhagen, Denmark)","url":"https://pubmed.ncbi.nlm.nih.gov/28691777","citation_count":14,"is_preprint":false},{"pmid":"38381558","id":"PMC_38381558","title":"Tepsin binds LC3B to promote ATG9A trafficking and delivery.","date":"2024","source":"Molecular biology of the cell","url":"https://pubmed.ncbi.nlm.nih.gov/38381558","citation_count":5,"is_preprint":false},{"pmid":"36642642","id":"PMC_36642642","title":"AP-4 loss in CRISPR-edited zebrafish affects early embryo development.","date":"2022","source":"Advances in biological regulation","url":"https://pubmed.ncbi.nlm.nih.gov/36642642","citation_count":4,"is_preprint":false},{"pmid":"40586818","id":"PMC_40586818","title":"Characterization of the malaria parasite Plasmodium falciparum Tepsin homolog.","date":"2025","source":"Microbiology spectrum","url":"https://pubmed.ncbi.nlm.nih.gov/40586818","citation_count":3,"is_preprint":false},{"pmid":"37502979","id":"PMC_37502979","title":"Tepsin binds LC3B to promote ATG9A export and delivery at the cell periphery.","date":"2023","source":"bioRxiv : the preprint server for biology","url":"https://pubmed.ncbi.nlm.nih.gov/37502979","citation_count":2,"is_preprint":false},{"pmid":"41082687","id":"PMC_41082687","title":"Vesicle adaptors in malaria parasites show conservation and flexibility of protein sorting machinery.","date":"2025","source":"The Journal of cell biology","url":"https://pubmed.ncbi.nlm.nih.gov/41082687","citation_count":2,"is_preprint":false},{"pmid":"41082686","id":"PMC_41082686","title":"Tepsin and AP4 mediate transport from the trans-Golgi to the plant-like vacuole in toxoplasma.","date":"2025","source":"The Journal of cell biology","url":"https://pubmed.ncbi.nlm.nih.gov/41082686","citation_count":1,"is_preprint":false},{"pmid":"41198464","id":"PMC_41198464","title":"The AP-4 accessory protein tepsin exhibits multivalent binding to LC3B.","date":"2025","source":"Advances in biological regulation","url":"https://pubmed.ncbi.nlm.nih.gov/41198464","citation_count":1,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":9007,"output_tokens":2247,"usd":0.030363,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":9404,"output_tokens":2840,"usd":0.05901,"stage2_stop_reason":"end_turn"},"total_usd":0.089373,"stage1_batch_id":"msgbatch_017qqVz33Ga63SCDwzb35XPb","stage2_batch_id":"msgbatch_01Scr2Ei9x4HJMiRGxwrmvLK","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2012,\n      \"finding\": \"Tepsin was identified as the first accessory protein of the AP-4 coat complex, co-purifying with AP-4-coated vesicles from HeLa cells as determined by multivariate SILAC-based quantitative mass spectrometry and siRNA knockdown profiling.\",\n      \"method\": \"SILAC-based quantitative mass spectrometry, siRNA knockdown, principal component analysis of coated vesicle fractions\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal quantitative proteomic methods with knockdown controls, replicated by subsequent studies\",\n      \"pmids\": [\"22472443\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Recruitment of tepsin to the membrane is abolished when AP-4 complex assembly is disrupted by loss-of-function mutations in AP4S1, demonstrating that tepsin membrane recruitment depends on intact AP-4 complex.\",\n      \"method\": \"Patient fibroblast cell line analysis; immunofluorescence/Western blot showing loss of AP-4 subunits and tepsin membrane recruitment upon AP4S1 frameshift/nonsense mutations\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — cellular loss-of-function in patient-derived cells with defined molecular phenotype, single lab\",\n      \"pmids\": [\"25552650\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Tepsin contains two phylogenetically conserved peptide motifs in its unstructured C-terminus—[GS]LFXG[ML]X[LV] and S[AV]F[SA]FLN—that interact with the C-terminal ear/appendage domains of the β4 and ε subunits of AP-4, respectively. Both interactions are required for efficient association of tepsin with AP-4 and for tepsin recruitment to the TGN. Bivalent interaction increases avidity and may cross-link AP-4 heterotetramers to contribute to coat assembly.\",\n      \"method\": \"Protein interaction assays (GST pulldown, yeast two-hybrid, phage display), structure-based mutagenesis, cellular localization assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — multiple orthogonal binding assays plus mutagenesis in vitro and in cells, replicated by independent lab (PMID:26756312)\",\n      \"pmids\": [\"26542808\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"The tepsin C-terminal LFxG[M/L]x[L/V] motif binds directly and specifically to the AP-4 β4 appendage domain. NMR chemical shift mapping defined the binding site on the β4 appendage surface. Point mutations in either the tepsin motif or the cognate β4 surface abolish in vitro binding and greatly reduce (but do not completely abolish) tepsin–AP-4 interaction in cells, suggesting additional interaction sites exist.\",\n      \"method\": \"NMR chemical shift mapping, in vitro binding assays, point mutagenesis, co-immunoprecipitation in cells\",\n      \"journal\": \"Traffic (Copenhagen, Denmark)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — NMR structure-guided mutagenesis combined with in vitro binding and cellular co-IP, validated independently (PMID:26542808)\",\n      \"pmids\": [\"26756312\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"X-ray crystal structures of the tepsin ENTH and VHS/ENTH-like domains revealed that: (1) the tepsin ENTH domain lacks helix0, helix8, and a lipid-binding pocket present in epsin1/2/3, explaining why tepsin requires AP-4 for membrane recruitment rather than binding lipids directly; (2) the tepsin VHS domain lacks helix8 and does not mediate known VHS functions such as recognition of dileucine-based cargo motifs or ubiquitin binding.\",\n      \"method\": \"X-ray crystallography, biochemical/biophysical binding assays, phylogenetic and comparative genomic analysis\",\n      \"journal\": \"Traffic (Copenhagen, Denmark)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structures of both domains with biochemical validation of functional inferences, single lab but multiple orthogonal methods\",\n      \"pmids\": [\"28691777\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Tepsin directly binds LC3B (preferentially over other mammalian ATG8 family members) via a canonical LC3-Interacting Region (LIR) motif, with micromolar affinity at the LC3B LIR docking site. Loss of tepsin in cultured cells dysregulates ATG9A export from the TGN and ATG9A distribution at the cell periphery. Tepsin depletion increases autophagosome volume and number without affecting autophagic flux. Reintroduction of wild-type tepsin partially rescues ATG9A trafficking defects, while tepsin with a mutated LIR motif or missing N-terminus fails to fully rescue ATG9A distribution.\",\n      \"method\": \"In silico LIR motif prediction, recombinant protein biochemistry (pulldown, calorimetry), structural modeling, siRNA knockdown, fluorescence microscopy (mRFP-GFP-LC3B reporter), rescue experiments with LIR-mutant tepsin\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — calorimetry plus structural modeling plus mutagenesis plus cellular loss-of-function/rescue with defined phenotypic readouts, multiple orthogonal methods in one study\",\n      \"pmids\": [\"38381558\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Loss of tepsin in CRISPR-edited zebrafish embryos causes abnormal head morphology and neural necrosis, and alters expression levels and patterns of autophagy genes atg9a and map1lc3b, linking tepsin function to AP-4-dependent ATG9A trafficking and autophagy in a developmental context.\",\n      \"method\": \"CRISPR-ExoCas9 knockout in zebrafish, morphological analysis at 24 hpf, gene expression analysis\",\n      \"journal\": \"Advances in biological regulation\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vivo loss-of-function with defined morphological and molecular phenotypes, single lab\",\n      \"pmids\": [\"36642642\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Computational modeling (AlphaFold Multimer) combined with bio-layer interferometry (BLI) and biochemical experiments identified three additional LC3B-binding motifs beyond the canonical LIR in tepsin's disordered regions, all engaging the LC3B LIR docking site. All four motifs must be mutated to abrogate LC3B binding in vitro. Stoichiometry data indicate one tepsin molecule likely binds two LC3B molecules simultaneously, suggesting multivalent LC3B engagement that could dynamically modulate binding strength in response to LC3B membrane concentrations.\",\n      \"method\": \"AlphaFold Multimer structural modeling, bio-layer interferometry (BLI), biochemical binding assays, mutagenesis, thermodynamic/kinetic analysis\",\n      \"journal\": \"Advances in biological regulation\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1–2 / Moderate — structural modeling plus BLI and mutagenesis in single lab, builds directly on prior peer-reviewed findings\",\n      \"pmids\": [\"41198464\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"Tepsin is the only known accessory protein of the AP-4 non-clathrin coat at the trans-Golgi network (TGN); it is recruited to membranes exclusively through AP-4 (not via direct lipid binding, as its ENTH domain lacks a lipid-binding pocket), using two conserved C-terminal peptide motifs that engage the β4 and ε appendage domains of AP-4 in a bivalent interaction; tepsin further directly binds LC3B via multiple LIR motifs (with micromolar affinity) and this interaction is required for proper ATG9A cargo export from the TGN and delivery to the cell periphery, linking tepsin to autophagosome biogenesis.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"Tepsin is the only known dedicated accessory protein of the AP-4 non-clathrin coat that operates at the trans-Golgi network, where it is recruited to membranes through the AP-4 complex rather than through direct lipid binding [#0, #1]. Its recruitment is strictly AP-4-dependent: disruption of AP-4 assembly through loss-of-function mutation of the AP4S1 subunit abolishes tepsin membrane association [#1], and two phylogenetically conserved C-terminal peptide motifs engage the β4 and ε appendage/ear domains of AP-4 in a bivalent interaction that increases avidity and can cross-link AP-4 heterotetramers during coat assembly [#2, #3]. Structurally, tepsin's ENTH and VHS domains are degenerate—lacking the lipid-binding pocket of epsins and the cargo/ubiquitin-recognition surfaces of canonical VHS domains—accounting for its dependence on AP-4 for membrane localization [#4]. Beyond its scaffolding role, tepsin directly binds LC3B through a canonical LIR motif plus additional disordered-region motifs that engage the LC3B docking site with micromolar affinity in a multivalent manner [#5, #7], and this interaction is required for proper ATG9A export from the TGN and its peripheral distribution, linking tepsin to autophagosome biogenesis [#5]. Loss of tepsin in zebrafish causes abnormal head morphology and neural necrosis with altered atg9a and map1lc3b expression, situating this AP-4/autophagy axis in a developmental context [#6].\",\n  \"teleology\": [\n    {\n      \"year\": 2012,\n      \"claim\": \"Established that the AP-4 coat, unlike other adaptor complexes, possesses a dedicated accessory protein, identifying tepsin as the first such component and defining a starting point for AP-4 coat biology.\",\n      \"evidence\": \"SILAC quantitative mass spectrometry of AP-4-coated vesicle fractions with siRNA knockdown profiling in HeLa cells\",\n      \"pmids\": [\"22472443\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not define how tepsin associates with AP-4 mechanistically\", \"No cargo or downstream function established\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Determined that tepsin's membrane localization is not autonomous but requires an intact AP-4 complex, establishing the hierarchy of coat assembly.\",\n      \"evidence\": \"Patient-derived fibroblasts carrying AP4S1 loss-of-function mutations, with immunofluorescence/Western blot of AP-4 subunits and tepsin\",\n      \"pmids\": [\"25552650\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab, patient-cell context\", \"Did not map the molecular interface mediating recruitment\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Defined the molecular basis of tepsin–AP-4 binding by identifying two conserved C-terminal motifs that engage the β4 and ε appendage domains, showing a bivalent interaction that could drive coat assembly.\",\n      \"evidence\": \"GST pulldown, yeast two-hybrid, phage display, structure-based mutagenesis and cellular localization assays\",\n      \"pmids\": [\"26542808\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural detail of the binding surfaces not resolved\", \"Functional consequence of cross-linking for vesicle formation untested\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Mapped the tepsin LFxG[M/L]x[L/V] motif directly onto the β4 appendage surface, confirming specificity but revealing residual cellular binding that implied additional interaction sites.\",\n      \"evidence\": \"NMR chemical shift mapping, in vitro binding, point mutagenesis and co-immunoprecipitation in cells\",\n      \"pmids\": [\"26756312\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of the residual ε-mediated contribution not structurally resolved here\", \"Did not address physiological cargo\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Explained why tepsin depends on AP-4 by solving structures showing its ENTH and VHS domains are degenerate, lacking lipid-binding and cargo/ubiquitin-recognition features of canonical epsin/VHS proteins.\",\n      \"evidence\": \"X-ray crystallography of tepsin ENTH and VHS domains with biochemical binding assays and comparative genomics\",\n      \"pmids\": [\"28691777\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Functional role of the degenerate domains, if any, remained undefined\", \"No interactor identified for these domains\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Placed tepsin function in a whole-organism developmental context, linking its loss to neural defects and dysregulated autophagy gene expression in vivo.\",\n      \"evidence\": \"CRISPR knockout in zebrafish embryos with morphological and gene-expression analysis at 24 hpf\",\n      \"pmids\": [\"36642642\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Causal mechanism linking tepsin loss to neural necrosis not resolved\", \"Single lab, indirect link to ATG9A trafficking\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Connected tepsin to autophagosome biogenesis by demonstrating direct LIR-mediated LC3B binding required for ATG9A export from the TGN and peripheral distribution.\",\n      \"evidence\": \"Recombinant pulldown and calorimetry, structural modeling, siRNA knockdown with mRFP-GFP-LC3B reporter and LIR-mutant rescue experiments\",\n      \"pmids\": [\"38381558\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Only partial rescue achieved by wild-type reintroduction\", \"How ATG9A export couples to LC3B binding mechanistically unresolved\", \"Effect on autophagic flux absent despite altered autophagosome number\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Refined the tepsin–LC3B interaction as multivalent, identifying three additional LC3B-binding motifs and stoichiometry consistent with one tepsin engaging two LC3B molecules.\",\n      \"evidence\": \"AlphaFold Multimer modeling, bio-layer interferometry, biochemical binding and mutagenesis with thermodynamic/kinetic analysis\",\n      \"pmids\": [\"41198464\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Functional relevance of multivalency in cells not directly tested\", \"Modeling-derived motifs require structural confirmation\", \"Single lab\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How tepsin mechanistically coordinates AP-4 coat assembly with LC3B-dependent ATG9A export, and whether its bivalent AP-4 binding and multivalent LC3B engagement are temporally coupled during vesicle formation, remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No integrated structural model of tepsin bridging AP-4 and LC3B\", \"Cargo selectivity determinants for ATG9A export undefined\", \"Physiological role of multivalent LC3B binding untested in cells\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [0, 2, 3, 5]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005794\", \"supporting_discovery_ids\": [1, 2, 5]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-9612973\", \"supporting_discovery_ids\": [5, 6]},\n      {\"term_id\": \"R-HSA-5653656\", \"supporting_discovery_ids\": [0, 5]}\n    ],\n    \"complexes\": [\"AP-4 coat\"],\n    \"partners\": [\"AP4B1\", \"AP4E1\", \"LC3B\", \"AP4S1\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":5,"faith_total":5,"faith_pct":100.0}}