{"gene":"LTBP1","run_date":"2026-06-10T02:59:50","timeline":{"discoveries":[{"year":1996,"finding":"The third 8-Cys (TB3) repeat of LTBP-1 binds covalently to the LAP region of TGF-β1 via disulfide bond exchange; specifically, Cys33 of β1-LAP is required for the covalent association. The N-terminal region (first ~400 aa) of LTBP-1 associates covalently with the ECM. This was the first demonstration of an extracellular protein module capable of exchanging cysteine disulfide bonds with a heterologous ligand.","method":"Co-expression of TGF-β1 and LTBP-1 fragments in mammalian cells with signal peptide constructs, immunoblotting of secreted fusion protein complexes, site-directed mutagenesis of LAP Cys33","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1 / Strong — reconstitution in mammalian expression system plus mutagenesis identifying specific cysteine residue; foundational study replicated conceptually by multiple subsequent structure papers","pmids":["8617200"],"is_preprint":false},{"year":2003,"finding":"NMR solution structure of TB3 from LTBP-1 reveals that a two-residue insertion (absent in fibrillin-1 TB domains) increases solvent accessibility of the disulfide bond linking the 2nd and 6th cysteines, making it the exchangeable bond responsible for covalent LAP association. Site-directed mutagenesis confirmed this is the only disulfide that can be removed without perturbing the TB domain fold. A ring of negatively charged residues surrounds this disulfide.","method":"NMR solution structure determination, site-directed mutagenesis, homology modelling of TB3 isoforms","journal":"Journal of molecular biology","confidence":"High","confidence_rationale":"Tier 1 / Strong — NMR structure with mutagenesis validation, multiple orthogonal methods in one study","pmids":["14607119"],"is_preprint":false},{"year":2001,"finding":"LTBP-1 contains three distinct ECM-interacting domains: regions containing the first (hybrid) 8-Cys domain, the second 8-Cys domain, and the fourth 8-Cys domain each independently bind fibroblast ECM. N-terminal fragments bind more readily. Each fragment can competitively inhibit association of native LTBP-1 with the ECM, and binding resists sodium deoxycholate treatment suggesting strong/covalent interactions.","method":"Recombinant fragment production in mammalian expression system, binding assays to cultured fibroblast ECM and isolated matrices, competitive inhibition assays, sodium deoxycholate resistance assay","journal":"Journal of cell science","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple recombinant fragments tested with multiple binding assays and competition experiments; replicated across different cell types","pmids":["11112702"],"is_preprint":false},{"year":2005,"finding":"A 24 amino acid sequence in the hinge domain of LTBP-1 is required for integrin αvβ6-mediated activation of latent TGF-β. This hinge region associates with fibronectin. Fibronectin-null cells minimally activate latent TGF-β and poorly incorporate the active hinge sequence into their matrix. Cells lacking the fibronectin receptor α5β1 also exhibit defective αvβ6-mediated latent TGF-β activation and decreased matrix incorporation of LTBP-1.","method":"Peptide binding assays, cell-based activation assays using fibronectin-null cells and α5β1-deficient cells, matrix incorporation assays","journal":"FASEB journal","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic null cells plus biochemical binding assays, multiple orthogonal experiments identifying specific sequence and fibronectin dependency","pmids":["16260650"],"is_preprint":false},{"year":2008,"finding":"MT1-MMP (MMP-14) proteolytically processes ECM-bound LTBP-1 to release latent TGF-β complexes from the subendothelial matrix. This process requires PKC and ERK1/2 signaling and is coupled to PMA-induced MT1-MMP expression upregulation. Neither secreted MMPs nor the uPA/plasmin system contributed to LTBP-1 release.","method":"Lentiviral shRNA gene silencing of MT1-MMP, metalloproteinase inhibitors (TIMP-2, TIMP-3, TIMP-1), uPA/plasmin inhibitors, endothelial cell PMA activation assays","journal":"Experimental cell research","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic silencing plus pharmacological inhibitors with multiple controls distinguishing specific protease, replicated with orthogonal methods","pmids":["18602101"],"is_preprint":false},{"year":2007,"finding":"LTBP-1 and LTBP-2 compete for binding to the same or closely adjacent site on the amino-terminal region of fibrillin-1. The major fibrillin-1 binding site on LTBP-1 resides near its C-terminus. The interaction is Ca²⁺-dependent (abolished by EDTA). A C-terminal fragment of LTBP-2 blocked LTBP-1 binding to fibrillin-1 and vice versa, indicating overlapping binding sites.","method":"Solid phase binding assays, overlay blotting, competitive binding assays with recombinant C-terminal fragments, EDTA/Ca²⁺ manipulation","journal":"Matrix biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — solid phase and competitive binding assays with recombinant fragments, single lab but multiple orthogonal binding methods","pmids":["17293099"],"is_preprint":false},{"year":2019,"finding":"Under hypoxia, AMPK phosphorylates PTPS at Thr58, which promotes PTPS binding to LTBP1 and drives iNOS-mediated S-nitrosylation of LTBP1 within a PTPS/iNOS/LTBP1 complex. LTBP1 S-nitrosylation leads to proteasome-dependent LTBP1 protein degradation, impairing TGF-β secretion and thereby maintaining tumor cell growth.","method":"Co-immunoprecipitation to identify PTPS-LTBP1 complex, S-nitrosylation assays, proteasome inhibitor experiments, AMPK kinase assays, LTBP1 stability assays under hypoxia","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal biochemical methods (Co-IP, S-nitrosylation assay, kinase assay, proteasome inhibition) in a single rigorous study with functional readout","pmids":["31628042"],"is_preprint":false},{"year":2000,"finding":"The third 8-Cys (CR3) domain of LTBP-1 contains a conserved N-glycosylation site that is modified with complex and hybrid glycans. Glycosylation status was characterized by MALDI-TOF mass spectrometry and enzymatic analysis in insect cell expression systems.","method":"MALDI-TOF mass spectrometry, enzymatic glycan analysis, recombinant protein expression in Sf9 and High-Five insect cells","journal":"Biochemistry","confidence":"Medium","confidence_rationale":"Tier 1 / Moderate — mass spectrometric characterization plus enzymatic analysis of glycosylation, single lab study","pmids":["10677208"],"is_preprint":false},{"year":2006,"finding":"LTBP-1 contributes to TGF-β1 activation in mouse embryonic fibroblasts by influencing the activities of plasminogen activator/plasmin, elastase, and thrombospondin-1, and by modulating MMP-2 activity. siRNA knockdown of LTBP-1 reduced active TGF-β1 levels and reduced PA/plasmin and elastase activities without significantly affecting their mRNA levels.","method":"siRNA knockdown of LTBP-1, TGF-β1 neutralizing antibody, recombinant TGF-β1 addition, protease activity assays, protease-specific inhibitors","journal":"Journal of cellular biochemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — siRNA knockdown with multiple protease activity readouts and pharmacological controls, single lab","pmids":["16187295"],"is_preprint":false},{"year":2008,"finding":"AhR (dioxin receptor) represses Ltbp-1 transcription by recruiting HDAC2 to the Ltbp-1 promoter, which maintains histone deacetylation and prevents pCREB1(Ser133) binding. In AhR-null cells, absence of HDAC2 at the promoter and increased pCREB1 binding leads to Ltbp-1 overexpression. HDAC2 siRNA increased Ltbp-1 expression and histone acetylation in AhR-expressing cells.","method":"Chromatin immunoprecipitation (ChIP), RNA interference (RNAi/siRNA), reporter gene assays, AhR overexpression, site-directed mutagenesis of promoter elements","journal":"Journal of molecular biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — ChIP, RNAi, reporter gene, and mutagenesis all in one study; multiple orthogonal methods establishing epigenetic mechanism","pmids":["18508077"],"is_preprint":false},{"year":2021,"finding":"POGLUT2 and POGLUT3 O-glucosylate over half of the EGF repeats on LTBP1 (as well as fibrillin-1 and -2). These enzymes can distinguish folded versus unfolded EGF repeats. O-glucosylation by POGLUT2/3 plays a role in secretion of fibrillin-1; reduced secretion was observed in single and double knockout cells.","method":"Mass spectrometry analysis of O-glucosylation sites, POGLUT2/POGLUT3 single and double knockout HEK293T cells, in vitro secretion assays","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — mass spectrometry plus genetic knockout plus functional secretion assay, but LTBP1 secretion phenotype not directly demonstrated (shown for FBN1)","pmids":["34411563"],"is_preprint":false},{"year":2022,"finding":"LTBP1 promotes the incorporation of fibrillin-1 and fibrillin-2 into the extracellular matrix in vitro. This function is differentially exerted by the two isoforms (LTBP-1S and LTBP-1L), revealing a TGF-β-independent function of LTBP1 in ECM assembly.","method":"In vitro cell culture assays of fibrillin incorporation, comparison of LTBP1S vs LTBP1L isoform activities, ECM fractionation","journal":"Matrix biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct functional assays in cell culture with isoform comparison, single lab study","pmids":["35452817"],"is_preprint":false},{"year":2014,"finding":"NMR spectroscopic analysis of the LTBP1 C-terminus reveals that the four canonical domains (cbEGF14, TB3, EGF3, cbEGF15) adopt canonical folds but largely lack the rigid interdomain interactions seen in fibrillin; three interdomain regions act as flexible linkers allowing wide motion. The EGF3-cbEGF15 pair has a well-defined interdomain interface. This 'knotted rope' flexibility may facilitate ECM interactions and accessibility to proteases.","method":"NMR spectroscopy of overlapping C-terminal LTBP1 fragments, 15N relaxation studies for domain dynamics","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 1 / Moderate — NMR structural data with relaxation studies, single lab; functional implications inferred from structure","pmids":["24489852"],"is_preprint":false},{"year":2024,"finding":"Lactate released from PLLA is taken up by fibroblasts via MCT1, leading to KAT8-mediated lactylation of LTBP1 at lysine 752 (K752), which increases collagen I and collagen III protein levels in fibroblasts.","method":"Western blotting, immunofluorescence, lactylation site mapping, KAT8 inhibition, MCT1 transporter assays, PLLA treatment of fibroblasts and aged mouse skin","journal":"International journal of biological macromolecules","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — specific PTM site identified with functional readout, single lab study with in vitro and in vivo components","pmids":["39102921"],"is_preprint":false},{"year":2002,"finding":"Xenopus LTBP-1 (xLTBP-1) is expressed in the Spemann organizer and potentiates the activity of activin and nodal in animal cap assays. The potentiation did not require covalent association with activin, as conditioned medium containing both activin and LTBP-1 enhanced activin's effect, suggesting LTBP-1 can non-covalently modulate TGF-β family member activity.","method":"Xenopus animal cap assay, in situ hybridization for spatial expression, conditioned medium experiments","journal":"Developmental biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional animal cap assay with conditioned medium experiments; ortholog study in Xenopus with domain-compatible protein","pmids":["12142025"],"is_preprint":false},{"year":2026,"finding":"MCCC2 directly interacts with LTBP1 (identified by LC-MS, validated by Co-IP and GST pulldown). MCCC2 competitively inhibits SMURF1-mediated ubiquitination and degradation of LTBP1, thereby stabilizing LTBP1 and activating TGF-β signaling to promote prostate cancer bone metastasis.","method":"Liquid chromatography-mass spectrometry, co-immunoprecipitation, GST pulldown assay, ubiquitination assay, in vitro migration/invasion assays, in vivo bone metastasis models","journal":"Oncogene","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal Co-IP and GST pulldown for interaction, ubiquitination assay for mechanism, single lab study","pmids":["42251191"],"is_preprint":false},{"year":1998,"finding":"LTBP-1 co-localizes with fibrillin-containing microfibrils in normal human skin and localizes to these structures even during early de novo formation of the microfibrillar apparatus in skin regenerating from keratinocyte autografts. This establishes LTBP-1 as a component of fibrillin microfibrils and shows it targets latent TGF-β1 to the cutaneous microfibrillar apparatus as a repository.","method":"Immunohistochemistry and immunofluorescence localization in normal and regenerating human skin, co-localization with fibrillin antibodies","journal":"The Journal of investigative dermatology","confidence":"Medium","confidence_rationale":"Tier 3 / Strong — direct immunolocalization in tissue, replicated across normal and regenerating skin; establishes ECM compartmentalization","pmids":["9764833"],"is_preprint":false},{"year":2025,"finding":"LTA4H induces HNRNPA1 phosphorylation, enhancing LTA4H-HNRNPA1 interaction and functionally inhibiting HNRNPA1's role in regulating Ltbp1 mRNA maturation and processing in the nucleus. LTA4H deficiency upregulates LTBP1 expression and downstream TGF-β secretion/activation, promoting CD206+ macrophage polarization.","method":"Co-immunoprecipitation of LTA4H and HNRNPA1, phosphorylation assays, nuclear mRNA processing assays, TGF-β secretion/activation assays, macrophage polarization assays","journal":"Cell reports. Medicine","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP and functional mRNA processing assays establishing post-transcriptional regulation, single lab study","pmids":["40056904"],"is_preprint":false},{"year":2024,"finding":"In IPF lung tissue, fibulin-1 co-localizes with both OPG and LTBP1; proximity ligation assays confirmed close proximity of fibulin-1 to LTBP1 but NOT of OPG directly to LTBP1, suggesting fibulin-1 bridges OPG and LTBP1 in a trimeric ECM complex in interstitial lung tissue.","method":"Immunofluorescence co-localization, proximity ligation assay in IPF and control lung tissue, fibulin-1 knockout mouse tissue analysis","journal":"bioRxiv","confidence":"Low","confidence_rationale":"Tier 3 / Weak — proximity ligation and co-localization in tissue sections, preprint, single lab, no reconstitution or biochemical pulldown of the trimeric complex","pmids":[],"is_preprint":true},{"year":1999,"finding":"Two LTBP-1 isoforms (LTBP-1L and LTBP-1S) are transcribed from independent functional promoters in a cell type-specific manner. LTBP-1L uses an upstream promoter while LTBP-1S uses a downstream one; LTBP-1L transcript is alternatively spliced into an internal splice acceptor inside exon 1 of LTBP-1S. TGF-β1 induction of LTBP-1 isoforms appears to occur by post-transcriptional mechanisms, since TGF-β1 failed to stimulate LTBP-1 reporter gene constructs.","method":"Genomic sequencing, reporter gene analysis with deletion constructs, Northern blotting, promoter mapping","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reporter gene assays plus genomic sequencing establishing independent promoters; negative result for direct TGF-β transcriptional regulation confirmed by reporter assay","pmids":["10551816"],"is_preprint":false}],"current_model":"LTBP1 is a large extracellular matrix glycoprotein that covalently associates with the LAP propeptide of TGF-β via a disulfide bond exchange mediated specifically by its third 8-Cys (TB3) domain, targeting the large latent TGF-β complex to the ECM through multiple matrix-binding domains; latent TGF-β can be released by MT1-MMP proteolysis of LTBP1 or activated by integrin αvβ6 via a hinge-domain sequence requiring fibronectin as a matrix co-factor; LTBP1 stability is regulated post-translationally by PTPS-driven iNOS-mediated S-nitrosylation (leading to proteasomal degradation), by SMURF1-mediated ubiquitination (antagonized by MCCC2), and by KAT8-mediated lactylation at K752; additionally, LTBP1 independently promotes fibrillin-1/2 incorporation into the ECM and its EGF repeats are O-glucosylated by POGLUT2/3, while its transcription is epigenetically repressed by AhR-recruited HDAC2 at the promoter."},"narrative":{"mechanistic_narrative":"LTBP1 is a large extracellular matrix glycoprotein that controls the latency, matrix sequestration, and activation of TGF-β by covalently tethering the latent cytokine to the ECM. Its third 8-Cys (TB3) domain forms a covalent disulfide-exchange bond with Cys33 of the TGF-β1 LAP propeptide — the first demonstrated case of an extracellular module exchanging cysteine disulfide bonds with a heterologous ligand — a reaction enabled by a TB3-specific two-residue insertion that exposes the exchangeable 2nd–6th cysteine bond [PMID:8617200, PMID:14607119]. Multiple N-terminal and internal 8-Cys domains independently anchor LTBP1 to fibroblast matrix [PMID:11112702], and the protein co-localizes with fibrillin microfibrils, binding fibrillin-1 through a Ca²⁺-dependent C-terminal site shared with LTBP-2, thereby targeting latent TGF-β to the microfibrillar apparatus as a tissue repository [PMID:17293099, PMID:9764833]. Beyond TGF-β sequestration, LTBP1 independently promotes incorporation of fibrillin-1 and fibrillin-2 into the ECM, a function differing between the LTBP-1S and LTBP-1L isoforms [PMID:35452817]. Latent TGF-β is mobilized either by MT1-MMP (MMP-14) proteolysis of matrix-bound LTBP1 [PMID:18602101] or by integrin αvβ6 acting on a 24-residue hinge sequence whose activity requires fibronectin and the α5β1 receptor as matrix co-factors [PMID:16260650]. LTBP1 abundance is further set post-translationally: AMPK-activated PTPS drives iNOS-mediated S-nitrosylation and proteasomal degradation under hypoxia [PMID:31628042], SMURF1-mediated ubiquitination targets it for degradation and is antagonized by MCCC2 [PMID:42251191], and KAT8-mediated lactylation at K752 promotes collagen production [PMID:39102921]. Transcription is epigenetically repressed by AhR-recruited HDAC2 at the Ltbp-1 promoter [PMID:18508077], and the EGF repeats are O-glucosylated by POGLUT2/3 [PMID:34411563].","teleology":[{"year":1996,"claim":"Established the molecular basis by which LTBP1 captures latent TGF-β, showing the TB3 domain forms a covalent bond with the LAP propeptide rather than a non-covalent association.","evidence":"Co-expression of TGF-β1 and LTBP-1 fragments in mammalian cells with immunoblotting and mutagenesis of LAP Cys33","pmids":["8617200"],"confidence":"High","gaps":["Did not resolve the structural feature making the TB3 bond exchangeable","Did not map the ECM-anchoring determinants beyond the N-terminal region"]},{"year":1999,"claim":"Resolved why LTBP1 exists in two forms by mapping independent cell-type-specific promoters generating LTBP-1L and LTBP-1S, and showed TGF-β1 induction is post-transcriptional.","evidence":"Genomic sequencing, reporter gene deletion constructs, Northern blotting, promoter mapping","pmids":["10551816"],"confidence":"Medium","gaps":["Functional differences between isoforms not defined","Post-transcriptional mechanism of TGF-β induction not identified"]},{"year":2000,"claim":"Characterized post-translational modification of the TB3 region, identifying a conserved N-glycosylation site bearing complex/hybrid glycans.","evidence":"MALDI-TOF mass spectrometry and enzymatic glycan analysis in insect cell expression","pmids":["10677208"],"confidence":"Medium","gaps":["Functional consequence of glycosylation for LAP binding untested","Insect-cell glycans may not reflect mammalian processing"]},{"year":2001,"claim":"Defined how LTBP1 is anchored in the matrix, showing three distinct 8-Cys domains independently and strongly bind fibroblast ECM.","evidence":"Recombinant fragment binding to fibroblast matrices, competitive inhibition, deoxycholate-resistance assays","pmids":["11112702"],"confidence":"High","gaps":["Specific ECM ligands of each domain not identified","Nature of the deoxycholate-resistant bond not biochemically defined"]},{"year":2002,"claim":"Revealed a non-covalent mode of TGF-β family modulation, showing LTBP-1 potentiates activin/nodal in development without requiring covalent binding.","evidence":"Xenopus animal cap assays, in situ hybridization, conditioned medium experiments","pmids":["12142025"],"confidence":"Medium","gaps":["Mechanism of non-covalent potentiation unknown","Relevance to mammalian LTBP1 not established"]},{"year":2003,"claim":"Explained at atomic resolution why TB3 alone among LTBP1 8-Cys domains exchanges disulfides, attributing it to a two-residue insertion exposing the 2nd–6th cysteine bond.","evidence":"NMR solution structure with site-directed mutagenesis and homology modelling","pmids":["14607119"],"confidence":"High","gaps":["Structure of the TB3-LAP covalent complex not solved","Role of the surrounding charged ring in ligand selection untested"]},{"year":2005,"claim":"Identified the activation determinant, showing a 24-aa hinge sequence mediates integrin αvβ6 activation of latent TGF-β and requires fibronectin as a matrix co-factor.","evidence":"Peptide binding, fibronectin-null and α5β1-deficient cell activation and matrix incorporation assays","pmids":["16260650"],"confidence":"High","gaps":["Mechanical vs conformational mode of integrin activation not resolved","How fibronectin physically positions the hinge unclear"]},{"year":2006,"claim":"Linked LTBP1 to proteolytic activation pathways, showing its knockdown reduces active TGF-β1 via plasmin, elastase, thrombospondin-1, and MMP-2 activities.","evidence":"siRNA knockdown in mouse embryonic fibroblasts with protease activity assays and inhibitors","pmids":["16187295"],"confidence":"Medium","gaps":["Direct vs indirect coupling to each protease not distinguished","Mechanism by which LTBP1 modulates protease activity unknown"]},{"year":2007,"claim":"Mapped the LTBP1-fibrillin interaction, locating the major fibrillin-1 binding site near the LTBP1 C-terminus and showing LTBP-1 and LTBP-2 compete for an overlapping site.","evidence":"Solid phase and competitive binding assays with recombinant fragments and Ca²⁺/EDTA manipulation","pmids":["17293099"],"confidence":"Medium","gaps":["Single-lab in vitro binding without cellular confirmation","Functional consequence of LTBP-1/LTBP-2 competition in vivo unknown"]},{"year":2008,"claim":"Identified a protease that releases matrix-bound latent TGF-β, showing MT1-MMP processing of LTBP-1 via PKC/ERK signaling mediates release from subendothelial matrix.","evidence":"shRNA silencing of MT1-MMP, TIMP and uPA/plasmin inhibitors, endothelial PMA activation assays","pmids":["18602101"],"confidence":"High","gaps":["Exact MT1-MMP cleavage sites in LTBP-1 not mapped","In vivo relevance beyond endothelial cells untested"]},{"year":2008,"claim":"Defined transcriptional control of LTBP1, showing AhR represses Ltbp-1 by recruiting HDAC2 and blocking pCREB1 binding at the promoter.","evidence":"ChIP, RNAi, reporter assays, AhR overexpression, promoter mutagenesis","pmids":["18508077"],"confidence":"High","gaps":["Physiological signals driving AhR occupancy not defined","Downstream consequences for TGF-β output not quantified"]},{"year":2014,"claim":"Provided a dynamic structural picture, showing the LTBP1 C-terminal domains are connected by flexible linkers unlike rigid fibrillin, a 'knotted rope' architecture.","evidence":"NMR spectroscopy with 15N relaxation on overlapping C-terminal fragments","pmids":["24489852"],"confidence":"Medium","gaps":["Functional role of flexibility in ECM binding inferred, not demonstrated","Full-length conformation not determined"]},{"year":2019,"claim":"Uncovered hypoxic post-translational regulation, showing AMPK-phosphorylated PTPS drives iNOS-mediated S-nitrosylation and proteasomal degradation of LTBP1 to limit TGF-β secretion.","evidence":"Co-IP, S-nitrosylation assays, AMPK kinase assays, proteasome inhibition under hypoxia","pmids":["31628042"],"confidence":"High","gaps":["S-nitrosylation site(s) on LTBP1 not mapped","Which E3 ligase executes degradation not identified"]},{"year":2021,"claim":"Identified enzymes modifying LTBP1 EGF repeats, showing POGLUT2/3 O-glucosylate over half its EGF repeats and distinguish folded from unfolded domains.","evidence":"Mass spectrometry of O-glucosylation sites and POGLUT2/3 knockout HEK293T secretion assays","pmids":["34411563"],"confidence":"Medium","gaps":["LTBP1 secretion phenotype shown for FBN1, not directly for LTBP1","Effect of O-glucosylation on TGF-β sequestration untested"]},{"year":2022,"claim":"Established a TGF-β-independent role, showing LTBP1 promotes fibrillin-1/2 incorporation into the ECM with isoform-specific activity.","evidence":"In vitro fibrillin incorporation assays comparing LTBP-1S vs LTBP-1L, ECM fractionation","pmids":["35452817"],"confidence":"Medium","gaps":["Molecular basis of isoform difference not defined","In vivo contribution to microfibril assembly not confirmed"]},{"year":2024,"claim":"Revealed metabolic regulation by lactylation, showing lactate uptake via MCT1 drives KAT8-mediated LTBP1 K752 lactylation that increases collagen I/III.","evidence":"Western blotting, lactylation site mapping, KAT8 inhibition, MCT1 assays, PLLA treatment in vitro and in aged mouse skin","pmids":["39102921"],"confidence":"Medium","gaps":["Mechanistic link between K752 lactylation and collagen induction unclear","Single-lab study without independent confirmation"]},{"year":2025,"claim":"Identified nuclear post-transcriptional control, showing LTA4H phosphorylates HNRNPA1 to inhibit Ltbp1 mRNA processing, with LTA4H loss raising LTBP1 and TGF-β to drive macrophage polarization.","evidence":"Co-IP of LTA4H/HNRNPA1, phosphorylation and nuclear mRNA processing assays, TGF-β and macrophage polarization assays","pmids":["40056904"],"confidence":"Medium","gaps":["Direct HNRNPA1 binding sites on Ltbp1 pre-mRNA not mapped","Single-lab study"]},{"year":2026,"claim":"Identified a stabilizer of LTBP1, showing MCCC2 directly binds LTBP1 and competitively blocks SMURF1-mediated ubiquitination to activate TGF-β and promote prostate cancer bone metastasis.","evidence":"LC-MS, reciprocal Co-IP, GST pulldown, ubiquitination assays, migration/invasion and in vivo bone metastasis models","pmids":["42251191"],"confidence":"Medium","gaps":["SMURF1 ubiquitination sites on LTBP1 not mapped","Single-lab study"]},{"year":null,"claim":"It remains unresolved how the many post-translational and transcriptional inputs (S-nitrosylation, ubiquitination, lactylation, O-glucosylation, AhR/HDAC2 repression) are integrated to set LTBP1 levels and tune TGF-β bioavailability in specific tissues.","evidence":"","pmids":[],"confidence":"Low","gaps":["No unified quantitative model of LTBP1 regulation","Tissue-specific hierarchy among regulators undefined","Structure of the LTBP1-latent TGF-β-fibrillin matrix complex not solved"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140313","term_label":"molecular sequestering activity","supporting_discovery_ids":[0,1,16]},{"term_id":"GO:0005198","term_label":"structural molecule activity","supporting_discovery_ids":[2,11,16]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[3,14]}],"localization":[{"term_id":"GO:0031012","term_label":"extracellular matrix","supporting_discovery_ids":[2,11,16]},{"term_id":"GO:0005576","term_label":"extracellular region","supporting_discovery_ids":[0,16]}],"pathway":[{"term_id":"R-HSA-1474244","term_label":"Extracellular matrix organization","supporting_discovery_ids":[2,11,16]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[3,4,6]}],"complexes":["large latent TGF-β complex","fibrillin microfibrils","PTPS/iNOS/LTBP1 complex"],"partners":["TGFB1","FBN1","LTBP2","ITGB6","FN1","MMP14","SMURF1","MCCC2"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q14766","full_name":"Latent-transforming growth factor beta-binding protein 1","aliases":["Transforming growth factor beta-1-binding protein 1","TGF-beta1-BP-1"],"length_aa":1721,"mass_kda":186.8,"function":"Key regulator of transforming growth factor beta (TGFB1, TGFB2 and TGFB3) that controls TGF-beta activation by maintaining it in a latent state during storage in extracellular space (PubMed:2022183, PubMed:8617200, PubMed:8939931). Associates specifically via disulfide bonds with the Latency-associated peptide (LAP), which is the regulatory chain of TGF-beta, and regulates integrin-dependent activation of TGF-beta (PubMed:15184403, PubMed:8617200, PubMed:8939931). Outcompeted by LRRC32/GARP for binding to LAP regulatory chain of TGF-beta (PubMed:22278742)","subcellular_location":"Secreted; Secreted, extracellular space, extracellular matrix","url":"https://www.uniprot.org/uniprotkb/Q14766/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/LTBP1","classification":"Not Classified","n_dependent_lines":0,"n_total_lines":1208,"dependency_fraction":0.0},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/LTBP1","total_profiled":1310},"omim":[{"mim_id":"619451","title":"CUTIS LAXA, AUTOSOMAL RECESSIVE, TYPE IIE; ARCL2E","url":"https://www.omim.org/entry/619451"},{"mim_id":"617135","title":"L3MBTL HISTONE METHYL-LYSINE-BINDING PROTEIN 4; L3MBTL4","url":"https://www.omim.org/entry/617135"},{"mim_id":"612277","title":"ADAMTS-LIKE PROTEIN 2; ADAMTSL2","url":"https://www.omim.org/entry/612277"},{"mim_id":"604710","title":"LATENT TRANSFORMING GROWTH FACTOR-BETA-BINDING PROTEIN 4; LTBP4","url":"https://www.omim.org/entry/604710"},{"mim_id":"602091","title":"LATENT TRANSFORMING GROWTH FACTOR-BETA-BINDING PROTEIN 2; LTBP2","url":"https://www.omim.org/entry/602091"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in many","driving_tissues":[{"tissue":"blood vessel","ntpm":382.8}],"url":"https://www.proteinatlas.org/search/LTBP1"},"hgnc":{"alias_symbol":[],"prev_symbol":[]},"alphafold":{"accession":"Q14766","domains":[{"cath_id":"-","chopping":"351-386_401-435_485-501","consensus_level":"medium","plddt":61.2778,"start":351,"end":501},{"cath_id":"3.90.290.10","chopping":"673-738","consensus_level":"medium","plddt":80.5455,"start":673,"end":738},{"cath_id":"3.90.290.10","chopping":"1348-1410_1424-1469","consensus_level":"high","plddt":82.6774,"start":1348,"end":1469},{"cath_id":"3.90.290.10","chopping":"1523-1583","consensus_level":"medium","plddt":79.5797,"start":1523,"end":1583}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q14766","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q14766-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q14766-F1-predicted_aligned_error_v6.png","plddt_mean":58.88},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=LTBP1","jax_strain_url":"https://www.jax.org/strain/search?query=LTBP1"},"sequence":{"accession":"Q14766","fasta_url":"https://rest.uniprot.org/uniprotkb/Q14766.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q14766/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q14766"}},"corpus_meta":[{"pmid":"8617200","id":"PMC_8617200","title":"Association 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The N-terminal region (first ~400 aa) of LTBP-1 associates covalently with the ECM. This was the first demonstration of an extracellular protein module capable of exchanging cysteine disulfide bonds with a heterologous ligand.\",\n      \"method\": \"Co-expression of TGF-β1 and LTBP-1 fragments in mammalian cells with signal peptide constructs, immunoblotting of secreted fusion protein complexes, site-directed mutagenesis of LAP Cys33\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — reconstitution in mammalian expression system plus mutagenesis identifying specific cysteine residue; foundational study replicated conceptually by multiple subsequent structure papers\",\n      \"pmids\": [\"8617200\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"NMR solution structure of TB3 from LTBP-1 reveals that a two-residue insertion (absent in fibrillin-1 TB domains) increases solvent accessibility of the disulfide bond linking the 2nd and 6th cysteines, making it the exchangeable bond responsible for covalent LAP association. Site-directed mutagenesis confirmed this is the only disulfide that can be removed without perturbing the TB domain fold. A ring of negatively charged residues surrounds this disulfide.\",\n      \"method\": \"NMR solution structure determination, site-directed mutagenesis, homology modelling of TB3 isoforms\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — NMR structure with mutagenesis validation, multiple orthogonal methods in one study\",\n      \"pmids\": [\"14607119\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"LTBP-1 contains three distinct ECM-interacting domains: regions containing the first (hybrid) 8-Cys domain, the second 8-Cys domain, and the fourth 8-Cys domain each independently bind fibroblast ECM. N-terminal fragments bind more readily. Each fragment can competitively inhibit association of native LTBP-1 with the ECM, and binding resists sodium deoxycholate treatment suggesting strong/covalent interactions.\",\n      \"method\": \"Recombinant fragment production in mammalian expression system, binding assays to cultured fibroblast ECM and isolated matrices, competitive inhibition assays, sodium deoxycholate resistance assay\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple recombinant fragments tested with multiple binding assays and competition experiments; replicated across different cell types\",\n      \"pmids\": [\"11112702\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"A 24 amino acid sequence in the hinge domain of LTBP-1 is required for integrin αvβ6-mediated activation of latent TGF-β. This hinge region associates with fibronectin. Fibronectin-null cells minimally activate latent TGF-β and poorly incorporate the active hinge sequence into their matrix. Cells lacking the fibronectin receptor α5β1 also exhibit defective αvβ6-mediated latent TGF-β activation and decreased matrix incorporation of LTBP-1.\",\n      \"method\": \"Peptide binding assays, cell-based activation assays using fibronectin-null cells and α5β1-deficient cells, matrix incorporation assays\",\n      \"journal\": \"FASEB journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic null cells plus biochemical binding assays, multiple orthogonal experiments identifying specific sequence and fibronectin dependency\",\n      \"pmids\": [\"16260650\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"MT1-MMP (MMP-14) proteolytically processes ECM-bound LTBP-1 to release latent TGF-β complexes from the subendothelial matrix. This process requires PKC and ERK1/2 signaling and is coupled to PMA-induced MT1-MMP expression upregulation. Neither secreted MMPs nor the uPA/plasmin system contributed to LTBP-1 release.\",\n      \"method\": \"Lentiviral shRNA gene silencing of MT1-MMP, metalloproteinase inhibitors (TIMP-2, TIMP-3, TIMP-1), uPA/plasmin inhibitors, endothelial cell PMA activation assays\",\n      \"journal\": \"Experimental cell research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic silencing plus pharmacological inhibitors with multiple controls distinguishing specific protease, replicated with orthogonal methods\",\n      \"pmids\": [\"18602101\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"LTBP-1 and LTBP-2 compete for binding to the same or closely adjacent site on the amino-terminal region of fibrillin-1. The major fibrillin-1 binding site on LTBP-1 resides near its C-terminus. The interaction is Ca²⁺-dependent (abolished by EDTA). A C-terminal fragment of LTBP-2 blocked LTBP-1 binding to fibrillin-1 and vice versa, indicating overlapping binding sites.\",\n      \"method\": \"Solid phase binding assays, overlay blotting, competitive binding assays with recombinant C-terminal fragments, EDTA/Ca²⁺ manipulation\",\n      \"journal\": \"Matrix biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — solid phase and competitive binding assays with recombinant fragments, single lab but multiple orthogonal binding methods\",\n      \"pmids\": [\"17293099\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Under hypoxia, AMPK phosphorylates PTPS at Thr58, which promotes PTPS binding to LTBP1 and drives iNOS-mediated S-nitrosylation of LTBP1 within a PTPS/iNOS/LTBP1 complex. LTBP1 S-nitrosylation leads to proteasome-dependent LTBP1 protein degradation, impairing TGF-β secretion and thereby maintaining tumor cell growth.\",\n      \"method\": \"Co-immunoprecipitation to identify PTPS-LTBP1 complex, S-nitrosylation assays, proteasome inhibitor experiments, AMPK kinase assays, LTBP1 stability assays under hypoxia\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal biochemical methods (Co-IP, S-nitrosylation assay, kinase assay, proteasome inhibition) in a single rigorous study with functional readout\",\n      \"pmids\": [\"31628042\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"The third 8-Cys (CR3) domain of LTBP-1 contains a conserved N-glycosylation site that is modified with complex and hybrid glycans. Glycosylation status was characterized by MALDI-TOF mass spectrometry and enzymatic analysis in insect cell expression systems.\",\n      \"method\": \"MALDI-TOF mass spectrometry, enzymatic glycan analysis, recombinant protein expression in Sf9 and High-Five insect cells\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — mass spectrometric characterization plus enzymatic analysis of glycosylation, single lab study\",\n      \"pmids\": [\"10677208\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"LTBP-1 contributes to TGF-β1 activation in mouse embryonic fibroblasts by influencing the activities of plasminogen activator/plasmin, elastase, and thrombospondin-1, and by modulating MMP-2 activity. siRNA knockdown of LTBP-1 reduced active TGF-β1 levels and reduced PA/plasmin and elastase activities without significantly affecting their mRNA levels.\",\n      \"method\": \"siRNA knockdown of LTBP-1, TGF-β1 neutralizing antibody, recombinant TGF-β1 addition, protease activity assays, protease-specific inhibitors\",\n      \"journal\": \"Journal of cellular biochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — siRNA knockdown with multiple protease activity readouts and pharmacological controls, single lab\",\n      \"pmids\": [\"16187295\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"AhR (dioxin receptor) represses Ltbp-1 transcription by recruiting HDAC2 to the Ltbp-1 promoter, which maintains histone deacetylation and prevents pCREB1(Ser133) binding. In AhR-null cells, absence of HDAC2 at the promoter and increased pCREB1 binding leads to Ltbp-1 overexpression. HDAC2 siRNA increased Ltbp-1 expression and histone acetylation in AhR-expressing cells.\",\n      \"method\": \"Chromatin immunoprecipitation (ChIP), RNA interference (RNAi/siRNA), reporter gene assays, AhR overexpression, site-directed mutagenesis of promoter elements\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — ChIP, RNAi, reporter gene, and mutagenesis all in one study; multiple orthogonal methods establishing epigenetic mechanism\",\n      \"pmids\": [\"18508077\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"POGLUT2 and POGLUT3 O-glucosylate over half of the EGF repeats on LTBP1 (as well as fibrillin-1 and -2). These enzymes can distinguish folded versus unfolded EGF repeats. O-glucosylation by POGLUT2/3 plays a role in secretion of fibrillin-1; reduced secretion was observed in single and double knockout cells.\",\n      \"method\": \"Mass spectrometry analysis of O-glucosylation sites, POGLUT2/POGLUT3 single and double knockout HEK293T cells, in vitro secretion assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — mass spectrometry plus genetic knockout plus functional secretion assay, but LTBP1 secretion phenotype not directly demonstrated (shown for FBN1)\",\n      \"pmids\": [\"34411563\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"LTBP1 promotes the incorporation of fibrillin-1 and fibrillin-2 into the extracellular matrix in vitro. This function is differentially exerted by the two isoforms (LTBP-1S and LTBP-1L), revealing a TGF-β-independent function of LTBP1 in ECM assembly.\",\n      \"method\": \"In vitro cell culture assays of fibrillin incorporation, comparison of LTBP1S vs LTBP1L isoform activities, ECM fractionation\",\n      \"journal\": \"Matrix biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct functional assays in cell culture with isoform comparison, single lab study\",\n      \"pmids\": [\"35452817\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"NMR spectroscopic analysis of the LTBP1 C-terminus reveals that the four canonical domains (cbEGF14, TB3, EGF3, cbEGF15) adopt canonical folds but largely lack the rigid interdomain interactions seen in fibrillin; three interdomain regions act as flexible linkers allowing wide motion. The EGF3-cbEGF15 pair has a well-defined interdomain interface. This 'knotted rope' flexibility may facilitate ECM interactions and accessibility to proteases.\",\n      \"method\": \"NMR spectroscopy of overlapping C-terminal LTBP1 fragments, 15N relaxation studies for domain dynamics\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — NMR structural data with relaxation studies, single lab; functional implications inferred from structure\",\n      \"pmids\": [\"24489852\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Lactate released from PLLA is taken up by fibroblasts via MCT1, leading to KAT8-mediated lactylation of LTBP1 at lysine 752 (K752), which increases collagen I and collagen III protein levels in fibroblasts.\",\n      \"method\": \"Western blotting, immunofluorescence, lactylation site mapping, KAT8 inhibition, MCT1 transporter assays, PLLA treatment of fibroblasts and aged mouse skin\",\n      \"journal\": \"International journal of biological macromolecules\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — specific PTM site identified with functional readout, single lab study with in vitro and in vivo components\",\n      \"pmids\": [\"39102921\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Xenopus LTBP-1 (xLTBP-1) is expressed in the Spemann organizer and potentiates the activity of activin and nodal in animal cap assays. The potentiation did not require covalent association with activin, as conditioned medium containing both activin and LTBP-1 enhanced activin's effect, suggesting LTBP-1 can non-covalently modulate TGF-β family member activity.\",\n      \"method\": \"Xenopus animal cap assay, in situ hybridization for spatial expression, conditioned medium experiments\",\n      \"journal\": \"Developmental biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional animal cap assay with conditioned medium experiments; ortholog study in Xenopus with domain-compatible protein\",\n      \"pmids\": [\"12142025\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"MCCC2 directly interacts with LTBP1 (identified by LC-MS, validated by Co-IP and GST pulldown). MCCC2 competitively inhibits SMURF1-mediated ubiquitination and degradation of LTBP1, thereby stabilizing LTBP1 and activating TGF-β signaling to promote prostate cancer bone metastasis.\",\n      \"method\": \"Liquid chromatography-mass spectrometry, co-immunoprecipitation, GST pulldown assay, ubiquitination assay, in vitro migration/invasion assays, in vivo bone metastasis models\",\n      \"journal\": \"Oncogene\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal Co-IP and GST pulldown for interaction, ubiquitination assay for mechanism, single lab study\",\n      \"pmids\": [\"42251191\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"LTBP-1 co-localizes with fibrillin-containing microfibrils in normal human skin and localizes to these structures even during early de novo formation of the microfibrillar apparatus in skin regenerating from keratinocyte autografts. This establishes LTBP-1 as a component of fibrillin microfibrils and shows it targets latent TGF-β1 to the cutaneous microfibrillar apparatus as a repository.\",\n      \"method\": \"Immunohistochemistry and immunofluorescence localization in normal and regenerating human skin, co-localization with fibrillin antibodies\",\n      \"journal\": \"The Journal of investigative dermatology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Strong — direct immunolocalization in tissue, replicated across normal and regenerating skin; establishes ECM compartmentalization\",\n      \"pmids\": [\"9764833\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"LTA4H induces HNRNPA1 phosphorylation, enhancing LTA4H-HNRNPA1 interaction and functionally inhibiting HNRNPA1's role in regulating Ltbp1 mRNA maturation and processing in the nucleus. LTA4H deficiency upregulates LTBP1 expression and downstream TGF-β secretion/activation, promoting CD206+ macrophage polarization.\",\n      \"method\": \"Co-immunoprecipitation of LTA4H and HNRNPA1, phosphorylation assays, nuclear mRNA processing assays, TGF-β secretion/activation assays, macrophage polarization assays\",\n      \"journal\": \"Cell reports. Medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP and functional mRNA processing assays establishing post-transcriptional regulation, single lab study\",\n      \"pmids\": [\"40056904\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"In IPF lung tissue, fibulin-1 co-localizes with both OPG and LTBP1; proximity ligation assays confirmed close proximity of fibulin-1 to LTBP1 but NOT of OPG directly to LTBP1, suggesting fibulin-1 bridges OPG and LTBP1 in a trimeric ECM complex in interstitial lung tissue.\",\n      \"method\": \"Immunofluorescence co-localization, proximity ligation assay in IPF and control lung tissue, fibulin-1 knockout mouse tissue analysis\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — proximity ligation and co-localization in tissue sections, preprint, single lab, no reconstitution or biochemical pulldown of the trimeric complex\",\n      \"pmids\": [],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"Two LTBP-1 isoforms (LTBP-1L and LTBP-1S) are transcribed from independent functional promoters in a cell type-specific manner. LTBP-1L uses an upstream promoter while LTBP-1S uses a downstream one; LTBP-1L transcript is alternatively spliced into an internal splice acceptor inside exon 1 of LTBP-1S. TGF-β1 induction of LTBP-1 isoforms appears to occur by post-transcriptional mechanisms, since TGF-β1 failed to stimulate LTBP-1 reporter gene constructs.\",\n      \"method\": \"Genomic sequencing, reporter gene analysis with deletion constructs, Northern blotting, promoter mapping\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reporter gene assays plus genomic sequencing establishing independent promoters; negative result for direct TGF-β transcriptional regulation confirmed by reporter assay\",\n      \"pmids\": [\"10551816\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"LTBP1 is a large extracellular matrix glycoprotein that covalently associates with the LAP propeptide of TGF-β via a disulfide bond exchange mediated specifically by its third 8-Cys (TB3) domain, targeting the large latent TGF-β complex to the ECM through multiple matrix-binding domains; latent TGF-β can be released by MT1-MMP proteolysis of LTBP1 or activated by integrin αvβ6 via a hinge-domain sequence requiring fibronectin as a matrix co-factor; LTBP1 stability is regulated post-translationally by PTPS-driven iNOS-mediated S-nitrosylation (leading to proteasomal degradation), by SMURF1-mediated ubiquitination (antagonized by MCCC2), and by KAT8-mediated lactylation at K752; additionally, LTBP1 independently promotes fibrillin-1/2 incorporation into the ECM and its EGF repeats are O-glucosylated by POGLUT2/3, while its transcription is epigenetically repressed by AhR-recruited HDAC2 at the promoter.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"LTBP1 is a large extracellular matrix glycoprotein that controls the latency, matrix sequestration, and activation of TGF-\\u03b2 by covalently tethering the latent cytokine to the ECM. Its third 8-Cys (TB3) domain forms a covalent disulfide-exchange bond with Cys33 of the TGF-\\u03b21 LAP propeptide \\u2014 the first demonstrated case of an extracellular module exchanging cysteine disulfide bonds with a heterologous ligand \\u2014 a reaction enabled by a TB3-specific two-residue insertion that exposes the exchangeable 2nd\\u20136th cysteine bond [#0, #1]. Multiple N-terminal and internal 8-Cys domains independently anchor LTBP1 to fibroblast matrix [#2], and the protein co-localizes with fibrillin microfibrils, binding fibrillin-1 through a Ca\\u00b2\\u207a-dependent C-terminal site shared with LTBP-2, thereby targeting latent TGF-\\u03b2 to the microfibrillar apparatus as a tissue repository [#5, #16]. Beyond TGF-\\u03b2 sequestration, LTBP1 independently promotes incorporation of fibrillin-1 and fibrillin-2 into the ECM, a function differing between the LTBP-1S and LTBP-1L isoforms [#11]. Latent TGF-\\u03b2 is mobilized either by MT1-MMP (MMP-14) proteolysis of matrix-bound LTBP1 [#4] or by integrin \\u03b1v\\u03b26 acting on a 24-residue hinge sequence whose activity requires fibronectin and the \\u03b15\\u03b21 receptor as matrix co-factors [#3]. LTBP1 abundance is further set post-translationally: AMPK-activated PTPS drives iNOS-mediated S-nitrosylation and proteasomal degradation under hypoxia [#6], SMURF1-mediated ubiquitination targets it for degradation and is antagonized by MCCC2 [#15], and KAT8-mediated lactylation at K752 promotes collagen production [#13]. Transcription is epigenetically repressed by AhR-recruited HDAC2 at the Ltbp-1 promoter [#9], and the EGF repeats are O-glucosylated by POGLUT2/3 [#10].\"\n,\n  \"teleology\": [\n    {\n      \"year\": 1996,\n      \"claim\": \"Established the molecular basis by which LTBP1 captures latent TGF-\\u03b2, showing the TB3 domain forms a covalent bond with the LAP propeptide rather than a non-covalent association.\",\n      \"evidence\": \"Co-expression of TGF-\\u03b21 and LTBP-1 fragments in mammalian cells with immunoblotting and mutagenesis of LAP Cys33\",\n      \"pmids\": [\"8617200\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not resolve the structural feature making the TB3 bond exchangeable\", \"Did not map the ECM-anchoring determinants beyond the N-terminal region\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Resolved why LTBP1 exists in two forms by mapping independent cell-type-specific promoters generating LTBP-1L and LTBP-1S, and showed TGF-\\u03b21 induction is post-transcriptional.\",\n      \"evidence\": \"Genomic sequencing, reporter gene deletion constructs, Northern blotting, promoter mapping\",\n      \"pmids\": [\"10551816\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Functional differences between isoforms not defined\", \"Post-transcriptional mechanism of TGF-\\u03b2 induction not identified\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Characterized post-translational modification of the TB3 region, identifying a conserved N-glycosylation site bearing complex/hybrid glycans.\",\n      \"evidence\": \"MALDI-TOF mass spectrometry and enzymatic glycan analysis in insect cell expression\",\n      \"pmids\": [\"10677208\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Functional consequence of glycosylation for LAP binding untested\", \"Insect-cell glycans may not reflect mammalian processing\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Defined how LTBP1 is anchored in the matrix, showing three distinct 8-Cys domains independently and strongly bind fibroblast ECM.\",\n      \"evidence\": \"Recombinant fragment binding to fibroblast matrices, competitive inhibition, deoxycholate-resistance assays\",\n      \"pmids\": [\"11112702\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Specific ECM ligands of each domain not identified\", \"Nature of the deoxycholate-resistant bond not biochemically defined\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Revealed a non-covalent mode of TGF-\\u03b2 family modulation, showing LTBP-1 potentiates activin/nodal in development without requiring covalent binding.\",\n      \"evidence\": \"Xenopus animal cap assays, in situ hybridization, conditioned medium experiments\",\n      \"pmids\": [\"12142025\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism of non-covalent potentiation unknown\", \"Relevance to mammalian LTBP1 not established\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Explained at atomic resolution why TB3 alone among LTBP1 8-Cys domains exchanges disulfides, attributing it to a two-residue insertion exposing the 2nd\\u20136th cysteine bond.\",\n      \"evidence\": \"NMR solution structure with site-directed mutagenesis and homology modelling\",\n      \"pmids\": [\"14607119\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structure of the TB3-LAP covalent complex not solved\", \"Role of the surrounding charged ring in ligand selection untested\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Identified the activation determinant, showing a 24-aa hinge sequence mediates integrin \\u03b1v\\u03b26 activation of latent TGF-\\u03b2 and requires fibronectin as a matrix co-factor.\",\n      \"evidence\": \"Peptide binding, fibronectin-null and \\u03b15\\u03b21-deficient cell activation and matrix incorporation assays\",\n      \"pmids\": [\"16260650\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanical vs conformational mode of integrin activation not resolved\", \"How fibronectin physically positions the hinge unclear\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Linked LTBP1 to proteolytic activation pathways, showing its knockdown reduces active TGF-\\u03b21 via plasmin, elastase, thrombospondin-1, and MMP-2 activities.\",\n      \"evidence\": \"siRNA knockdown in mouse embryonic fibroblasts with protease activity assays and inhibitors\",\n      \"pmids\": [\"16187295\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct vs indirect coupling to each protease not distinguished\", \"Mechanism by which LTBP1 modulates protease activity unknown\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Mapped the LTBP1-fibrillin interaction, locating the major fibrillin-1 binding site near the LTBP1 C-terminus and showing LTBP-1 and LTBP-2 compete for an overlapping site.\",\n      \"evidence\": \"Solid phase and competitive binding assays with recombinant fragments and Ca\\u00b2\\u207a/EDTA manipulation\",\n      \"pmids\": [\"17293099\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab in vitro binding without cellular confirmation\", \"Functional consequence of LTBP-1/LTBP-2 competition in vivo unknown\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Identified a protease that releases matrix-bound latent TGF-\\u03b2, showing MT1-MMP processing of LTBP-1 via PKC/ERK signaling mediates release from subendothelial matrix.\",\n      \"evidence\": \"shRNA silencing of MT1-MMP, TIMP and uPA/plasmin inhibitors, endothelial PMA activation assays\",\n      \"pmids\": [\"18602101\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Exact MT1-MMP cleavage sites in LTBP-1 not mapped\", \"In vivo relevance beyond endothelial cells untested\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Defined transcriptional control of LTBP1, showing AhR represses Ltbp-1 by recruiting HDAC2 and blocking pCREB1 binding at the promoter.\",\n      \"evidence\": \"ChIP, RNAi, reporter assays, AhR overexpression, promoter mutagenesis\",\n      \"pmids\": [\"18508077\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiological signals driving AhR occupancy not defined\", \"Downstream consequences for TGF-\\u03b2 output not quantified\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Provided a dynamic structural picture, showing the LTBP1 C-terminal domains are connected by flexible linkers unlike rigid fibrillin, a 'knotted rope' architecture.\",\n      \"evidence\": \"NMR spectroscopy with 15N relaxation on overlapping C-terminal fragments\",\n      \"pmids\": [\"24489852\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Functional role of flexibility in ECM binding inferred, not demonstrated\", \"Full-length conformation not determined\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Uncovered hypoxic post-translational regulation, showing AMPK-phosphorylated PTPS drives iNOS-mediated S-nitrosylation and proteasomal degradation of LTBP1 to limit TGF-\\u03b2 secretion.\",\n      \"evidence\": \"Co-IP, S-nitrosylation assays, AMPK kinase assays, proteasome inhibition under hypoxia\",\n      \"pmids\": [\"31628042\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"S-nitrosylation site(s) on LTBP1 not mapped\", \"Which E3 ligase executes degradation not identified\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Identified enzymes modifying LTBP1 EGF repeats, showing POGLUT2/3 O-glucosylate over half its EGF repeats and distinguish folded from unfolded domains.\",\n      \"evidence\": \"Mass spectrometry of O-glucosylation sites and POGLUT2/3 knockout HEK293T secretion assays\",\n      \"pmids\": [\"34411563\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"LTBP1 secretion phenotype shown for FBN1, not directly for LTBP1\", \"Effect of O-glucosylation on TGF-\\u03b2 sequestration untested\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Established a TGF-\\u03b2-independent role, showing LTBP1 promotes fibrillin-1/2 incorporation into the ECM with isoform-specific activity.\",\n      \"evidence\": \"In vitro fibrillin incorporation assays comparing LTBP-1S vs LTBP-1L, ECM fractionation\",\n      \"pmids\": [\"35452817\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Molecular basis of isoform difference not defined\", \"In vivo contribution to microfibril assembly not confirmed\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Revealed metabolic regulation by lactylation, showing lactate uptake via MCT1 drives KAT8-mediated LTBP1 K752 lactylation that increases collagen I/III.\",\n      \"evidence\": \"Western blotting, lactylation site mapping, KAT8 inhibition, MCT1 assays, PLLA treatment in vitro and in aged mouse skin\",\n      \"pmids\": [\"39102921\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanistic link between K752 lactylation and collagen induction unclear\", \"Single-lab study without independent confirmation\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Identified nuclear post-transcriptional control, showing LTA4H phosphorylates HNRNPA1 to inhibit Ltbp1 mRNA processing, with LTA4H loss raising LTBP1 and TGF-\\u03b2 to drive macrophage polarization.\",\n      \"evidence\": \"Co-IP of LTA4H/HNRNPA1, phosphorylation and nuclear mRNA processing assays, TGF-\\u03b2 and macrophage polarization assays\",\n      \"pmids\": [\"40056904\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct HNRNPA1 binding sites on Ltbp1 pre-mRNA not mapped\", \"Single-lab study\"]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"Identified a stabilizer of LTBP1, showing MCCC2 directly binds LTBP1 and competitively blocks SMURF1-mediated ubiquitination to activate TGF-\\u03b2 and promote prostate cancer bone metastasis.\",\n      \"evidence\": \"LC-MS, reciprocal Co-IP, GST pulldown, ubiquitination assays, migration/invasion and in vivo bone metastasis models\",\n      \"pmids\": [\"42251191\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"SMURF1 ubiquitination sites on LTBP1 not mapped\", \"Single-lab study\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"It remains unresolved how the many post-translational and transcriptional inputs (S-nitrosylation, ubiquitination, lactylation, O-glucosylation, AhR/HDAC2 repression) are integrated to set LTBP1 levels and tune TGF-\\u03b2 bioavailability in specific tissues.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No unified quantitative model of LTBP1 regulation\", \"Tissue-specific hierarchy among regulators undefined\", \"Structure of the LTBP1-latent TGF-\\u03b2-fibrillin matrix complex not solved\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140313\", \"supporting_discovery_ids\": [0, 1, 16]},\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [2, 11, 16]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [3, 14]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0031012\", \"supporting_discovery_ids\": [2, 11, 16]},\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [0, 16]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1474244\", \"supporting_discovery_ids\": [2, 11, 16]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [3, 4, 6]}\n    ],\n    \"complexes\": [\n      \"large latent TGF-\\u03b2 complex\",\n      \"fibrillin microfibrils\",\n      \"PTPS/iNOS/LTBP1 complex\"\n    ],\n    \"partners\": [\n      \"TGFB1\",\n      \"FBN1\",\n      \"LTBP2\",\n      \"ITGB6\",\n      \"FN1\",\n      \"MMP14\",\n      \"SMURF1\",\n      \"MCCC2\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":6,"faith_total":6,"faith_pct":100.0}}