ITIH1 is a liver-enriched secreted plasma glycoprotein that is processed to inter-alpha-trypsin inhibitor heavy chain H1 (HC1). In plasma, HC1 is ester-linked through the chondroitin-sulfate chain of bikunin and assembles with HC2 and bikunin in inter-alpha-inhibitor. HC1 itself is noninhibitory; the complex's serine-protease inhibition resides in bikunin. TSG-6-dependent transesterification can transfer HC1 from bikunin chondroitin sulfate to hyaluronan, producing an HA-associated heavy-chain pool that participates in organization of HA-rich extracellular matrices. Its VWFA domain contains a magnesium-binding MIDAS motif that contributes to metal-dependent recognition by TSG-6 before heavy-chain transfer.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0004867 serine-type endopeptidase inhibitor activity | IEA GO_REF:0000002 | REMOVE | Summary: InterPro transfers the inhibitor activity associated with the inter-alpha-inhibitor name and complex to the ITIH1 heavy chain. Reason: Direct biochemical separation shows that HC1 and HC2 are noninhibitory and that the distinct bikunin chain supplies trypsin-inhibitor activity. This is a complex-to-component overpropagation, not an intrinsic HC1 activity. Supporting Evidence: PMID:2476436 Inter-alpha-trypsin inhibitor contains noninhibitory heavy chains of 65,000 and 70,000 Da PMID:16873769 bikunin in its free state, but not when associated with HCs, is a relevant protease inhibitor |
| GO:0005576 extracellular region | IEA GO_REF:0000044 | ACCEPT | Summary: UniProt subcellular-location mapping correctly places the secreted HC1 precursor and mature plasma protein outside the cell. Reason: ITIH1 has a cleaved signal peptide and is a soluble secreted plasma protein, so extracellular region is correct. GO has since obsoleted GO:0005615 extracellular space, the replacement previously proposed here, and merged it into GO:0005576 (replaced_by), so this term is now the correct location term. The duplicate NAS annotation is handled consistently. Supporting Evidence: file:human/ITIH1/ITIH1-uniprot.txt SUBCELLULAR LOCATION: Secreted. |
| GO:0030212 hyaluronan metabolic process | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro assigns the heavy-chain family to hyaluronan metabolism. Reason: Direct studies identify HC1 as a serum-derived HA-associated heavy chain and show dynamic transfer of bikunin-complex heavy chains onto and between HA molecules. This is a core biological process for HC1. Supporting Evidence: PMID:20463016 can be transferred to different hyaluronan (HA) molecules by TSG-6/HC2 PMID:7504674 participation of the COOH-terminal half of ITI with an amphipathic alpha helix structure in the HA binding file:human/ITIH1/ITIH1-deep-research-manual.md HC1 is therefore best understood as a transferable extracellular heavy-chain component that decorates and helps organize HA matrices |
| GO:0030246 carbohydrate binding | EXP PMID:7513643 Chondroitin sulphate covalently cross-links the three polype... | MODIFY | Summary: The source demonstrates that chondroitin sulfate covalently links HC1, HC2, and bikunin; it does not assay a general noncovalent carbohydrate-binding activity. Reason: The broad term conflates covalent glycosaminoglycan assembly with binding. Independent direct HC1 studies identify the biologically relevant and much more specific HA association, so hyaluronic acid binding is the appropriate replacement. Proposed replacements: hyaluronic acid binding Supporting Evidence: PMID:7513643 the different peptide chains constituting ITI are associated by the new cross-link described as the protein-glycosaminoglycan-protein cross-link PMID:7504674 participation of the COOH-terminal half of ITI with an amphipathic alpha helix structure in the HA binding |
| GO:0005515 protein binding | IPI PMID:26468290 Metal Ion-dependent Heavy Chain Transfer Activity of TSG-6 M... | MARK AS OVER ANNOTATED | Summary: Recombinant HC1 directly interacts with TNFAIP6/TSG-6 through both Link- and metal-dependent CUB-domain contacts. Reason: The interaction is real and mechanistically important, but generic protein binding is uninformative and does not describe HC1's role in the transfer reaction. No partner-specific GO molecular-function term captures this interface, so the evidence is retained in the synthesis rather than as a core generic binding term. Supporting Evidence: PMID:26468290 WT CUB_C interacted with rHC1 in the presence of Ca2+ |
| GO:0005515 protein binding | IPI PMID:16873769 TSG-6 potentiates the antitissue kallikrein activity of inte... | MARK AS OVER ANNOTATED | Summary: The source analyzes TSG-6, inter-alpha-inhibitor heavy chains, and bikunin during complex remodeling and bikunin release. Reason: Complex membership and experimentally observed contacts are biologically valid, but generic protein binding obscures the distinct covalent glycosaminoglycan assembly and HC-transfer mechanism. Supporting Evidence: PMID:16873769 forms covalent complexes with heavy chains (HCs) from |
| GO:0005515 protein binding | IPI PMID:20463016 The TSG-6/HC2-mediated transfer is a dynamic process shuffli... | MARK AS OVER ANNOTATED | Summary: The study detects heavy-chain association with bikunin chondroitin sulfate and TSG-6-dependent covalent intermediates during HC transfer. Reason: The contacts are authentic but generic protein binding is not an informative functional description. Hyaluronic acid binding and hyaluronan metabolism capture the demonstrated HC1 biology more directly. Supporting Evidence: PMID:20463016 The heavy chain (HC) subunits of the bikunin proteins are covalently attached |
| GO:0005540 hyaluronic acid binding | IDA PMID:20463016 The TSG-6/HC2-mediated transfer is a dynamic process shuffli... | ACCEPT | Summary: Direct biochemical work demonstrates formation and exchange of heavy-chain-HA products, while the original SHAP study identifies human HC1 among HA-associated serum heavy chains. Reason: HA association is the best-supported specific molecular function for HC1 and is central to its extracellular-matrix role. The annotation is retained while acknowledging that much of the measured association is covalent after TSG-6-mediated transfer. Supporting Evidence: PMID:20463016 HCs transferred to HA may function as HC donors in subsequent transfer reactions PMID:7504674 participation of the COOH-terminal half of ITI with an amphipathic alpha helix structure in the HA binding |
| GO:0031012 extracellular matrix | HDA PMID:28675934 Characterization of the Extracellular Matrix of Normal and D... | ACCEPT | Summary: ITIH1 was recovered in proteomic characterization of normal and diseased tissue extracellular matrices. Reason: The HDA detection agrees with independent biochemical evidence that HC1 becomes covalently attached to HA in extracellular matrices, making ECM a functionally relevant rather than incidental location. Supporting Evidence: PMID:28675934 Characterization of the Extracellular Matrix of Normal and Diseased Tissues |
| GO:0031012 extracellular matrix | HDA PMID:25037231 Extracellular matrix signatures of human primary metastatic ... | ACCEPT | Summary: ITIH1 was recovered in ECM proteomics of primary metastatic colon cancers and liver metastases. Reason: Although the source is a disease proteome rather than an ITIH1 mechanism study, the localization is consistent with direct HC1-HA matrix biology and with the independent ECM HDA dataset. Supporting Evidence: PMID:25037231 Extracellular matrix signatures of human primary metastatic colon cancers |
| GO:0070062 extracellular exosome | HDA PMID:23533145 In-depth proteomic analyses of exosomes isolated from expres... | KEEP AS NON CORE | Summary: ITIH1 was detected in pooled expressed-prostatic-secretion urine exosome preparations. Reason: The HDA observation is retained, but ITIH1 is an abundant soluble plasma protein and could co-purify with extracellular vesicles. The study does not establish vesicle incorporation or an exosomal function. Supporting Evidence: PMID:23533145 In pooled EPS-urine exosome samples, ~900 proteins were detected. |
| GO:0072562 blood microparticle | HDA PMID:22516433 Proteomic analysis of microvesicles from plasma of healthy d... | KEEP AS NON CORE | Summary: ITIH1 was recovered in a proteomic survey of healthy-donor plasma microvesicle preparations. Reason: The source HDA observation is valid, but it does not distinguish stable microparticle association from adsorption or co-purification of an abundant plasma protein. This is not a defining functional location. Supporting Evidence: PMID:22516433 Proteomic analysis of microvesicles from plasma of healthy donors reveals high individual variability. |
| GO:0005576 extracellular region | NAS PMID:14718574 The human plasma proteome: a nonredundant list developed by ... | ACCEPT | Summary: The plasma-proteome source supports extracellular occurrence of secreted ITIH1. Reason: The location is correct for the secreted soluble plasma pool. GO has since obsoleted GO:0005615 extracellular space, the replacement previously proposed here, and merged it into GO:0005576 (replaced_by), so this term is now the correct location term. This matches the action on the duplicate IEA annotation. Supporting Evidence: file:human/ITIH1/ITIH1-uniprot.txt SUBCELLULAR LOCATION: Secreted. |
| GO:0005509 calcium ion binding | TAS PMID:2446322 Isolation and characterization of cDNAs encoding the heavy c... | MODIFY | Summary: The 1987 cDNA paper noted only sequence-predicted potential calcium-binding sites in the heavy chain. Reason: Potential sites do not establish calcium binding. A modern study of recombinant human HC1 instead directly identifies a magnesium-containing MIDAS motif in the VWFA domain and shows that mutation of its coordinating Asp298 abolishes metal binding and the TSG-6 interaction. Magnesium ion binding is therefore the evidence-based replacement. Proposed replacements: magnesium ion binding Supporting Evidence: PMID:2446322 The H chain contains potential calcium-binding sites PMID:26468290 its von Willebrand factor A domain contains a Mg2+-containing MIDAS motif |
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Download this section (compressed HTML)Q: Does HC1 bind HA noncovalently before or after transfer, or does the current hyaluronic-acid-binding annotation principally describe covalent HC1-HA attachment?
Q: Which human tissues preferentially deploy HC1 rather than HC2 in HC-HA matrices, and do the two heavy chains confer distinct matrix mechanics or receptor interactions?
Q: Do the three annotated ITIH1 isoforms differ in secretion, processing, bikunin loading, or TSG-6-mediated transfer?
Experiment: Compare purified glycosylated human HC1 wild type, D298A, and conservative MIDAS mutants by direct magnesium-binding measurements, SPR against full-length TSG-6 and isolated domains, and reconstituted HC-to-HA transfer assays.
Hypothesis: The HC1 VWFA MIDAS motif couples magnesium binding to TSG-6 recognition without itself conferring the catalytic transferase activity.
Type: Quantitative metal-binding, interaction, and transfer biochemistry
Experiment: Reconstitute HA films with defined bikunin-HC1, bikunin-HC2, or mixed inter-alpha-inhibitor substrates plus TSG-6 and pentraxin-3; quantify heavy-chain transfer, cross-link density, viscoelasticity, and cell adhesion using isoform-specific mass spectrometry and biophysical measurements.
Hypothesis: HC1 and HC2 generate compositionally and mechanically distinct HC-HA matrices.
Type: Defined extracellular-matrix reconstitution and mechanics
Experiment: Express each endogenous human isoform in a hepatocyte model lacking ITIH1, then compare secretion, propeptide cleavage, chondroitin-sulfate attachment, inter-alpha-inhibitor assembly, and TSG-6-dependent HA transfer with isoform-specific rescue.
Hypothesis: ITIH1 isoforms differ in precursor processing or entry into the bikunin/HA transfer pathway.
Type: Isoform-resolved hepatocyte biochemistry
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