ITIH4 is a liver-produced, secreted plasma glycoprotein of the inter-alpha-inhibitor heavy-chain family. Its VIT and von Willebrand factor A domains precede an H4-specific C-terminal region containing a proline-rich protease-susceptible segment. Cleavage by MASP-1, MASP-2, plasma kallikrein, or other serine proteases enables ITIH4 to form divalent-cation-dependent, noncovalent inhibitory complexes that sterically restrict access of large protein substrates while retaining protease activity toward small substrates. Through this bait-and-trap mechanism, ITIH4 inhibits intravascular serine proteases and suppresses lectin-pathway complement activation. ITIH4 is also an IL-6-inducible type II acute-phase protein. Unlike ITIH1, ITIH2, and ITIH3, ITIH4 lacks the conserved C-terminal sequence required for covalent linkage to bikunin and is not incorporated into the hyaluronan-associated inter-alpha-inhibitor proteoglycan complexes.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0004867 serine-type endopeptidase inhibitor activity | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro transfers serine-type endopeptidase inhibitor activity from the ITI heavy-chain C-terminal family signature. Reason: Independent direct human-protein experiments validate the term. Cleaved ITIH4 forms noncovalent inhibitory complexes with MASP-1, MASP-2, and plasma kallikrein and suppresses cleavage of physiological protein substrates. The unusual steric mechanism still meets the inhibitor-activity definition. Supporting Evidence: PMID:33523981 Here, we show that ITIH4 is cleaved by several human proteases within a protease-susceptible region, enabling ITIH4 to function as a protease inhibitor. file:human/ITIH4/ITIH4-deep-research-manual.md The core demonstrated molecular function is extracellular serine-type endopeptidase inhibition. |
| GO:0005576 extracellular region | IEA GO_REF:0000120 | ACCEPT | Summary: Combined automated annotation predicts extracellular localization from orthology and the reviewed secretory record. Reason: A cleaved signal peptide, direct purification from plasma, and recovery of newly synthesized ITIH4 in culture medium establish a soluble secreted 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. Supporting Evidence: PMID:7775381 The N-terminal 28 residues corresponded to a signal peptide for secretion. PMID:7947966 A 120 kDa plasma protein, which is susceptible to plasma kallikrein, was purified from human plasma |
| GO:0030212 hyaluronan metabolic process | IEA GO_REF:0000002 | REMOVE | Summary: InterPro propagates hyaluronan metabolism from the ITI heavy-chain family. Reason: This family transfer conflates ITIH4 with the bikunin-linked heavy chains. ITIH4 lacks the conserved C-terminal consensus required for covalent bikunin linkage, is not incorporated into proteoglycan complexes, and was directly purified as a protein without bikunin. No direct ITIH4 evidence supports hyaluronan transfer, binding, or metabolism. Supporting Evidence: PMID:33523981 ITIH4 lacks a C-terminal consensus sequence, which is present in all other ITIHs, that is required for the formation of bikunin-ITIH complexes. PMID:7541790 we concluded that GP120 was not a complex with bikunin |
| GO:0031012 extracellular matrix | HDA PMID:28675934 Characterization of the Extracellular Matrix of Normal and D... | KEEP AS NON CORE | Summary: ITIH4 was recovered in a large-scale proteomic analysis of ECM-enriched normal and diseased human tissue fractions. Reason: The HDA supports an ECM-associated context for secreted ITIH4 but does not establish it as a structural ECM component. ITIH4 is explicitly excluded from the bikunin-linked proteoglycan complexes characteristic of other ITIH heavy chains, so this localization should not drive a hyaluronan function. Supporting Evidence: PMID:28675934 analyze ECM peptides by mass spectrometry PMID:33523981 Consequently, ITIH4 is not incorporated into proteoglycan complexes |
| GO:0031012 extracellular matrix | HDA PMID:25037231 Extracellular matrix signatures of human primary metastatic ... | KEEP AS NON CORE | Summary: ITIH4 was recovered in ECM-enriched proteomic fractions from human colon, colon cancer, liver, or liver-metastasis samples. Reason: This is a valid context-specific proteomic localization, but soluble plasma proteins and ECM-associated factors can occur in enriched fractions. It does not override direct evidence that ITIH4 lacks bikunin-linked HA-complex biology. Supporting Evidence: PMID:25037231 We have used enrichment of extracellular matrix (ECM) from human patient samples and proteomics PMID:33523981 ITIH4 is not incorporated into proteoglycan complexes |
| GO:0005576 extracellular region | TAS Reactome:R-HSA-481009 | ACCEPT | Summary: Reactome models release of the ITIH4 70-kDa chain from platelet dense granules into the extracellular region. Reason: The event's extracellular output agrees with direct evidence that ITIH4 is a secreted plasma protein, independently of uncertainty about the platelet-storage step. 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. Supporting Evidence: PMID:7947966 A 120 kDa plasma protein, which is susceptible to plasma kallikrein, was purified from human plasma |
| GO:0031089 platelet dense granule lumen | TAS Reactome:R-HSA-481009 | UNDECIDED | Summary: Reactome explicitly places residues 29-661 of ITIH4 in its platelet dense-granule-content input set. Reason: The current Reactome entity and compartment were verified, but its accessible event literature documents dense-granule content generally and does not provide ITIH4-specific primary evidence. The curated localization should not be rejected from incomplete evidence, but it cannot yet be promoted to a verified core functional location. |
| GO:0070062 extracellular exosome | HDA PMID:23533145 In-depth proteomic analyses of exosomes isolated from expres... | KEEP AS NON CORE | Summary: ITIH4 was detected in a large-scale EPS-urine exosome preparation. Reason: This is a legitimate HDA dataset observation but not a core location. The source explicitly notes that some proteins in these preparations may also be soluble, a substantial caveat for abundant secreted plasma ITIH4. Supporting Evidence: PMID:23533145 some of these proteins could also exist as a soluble form |
| GO:0072562 blood microparticle | HDA PMID:22516433 Proteomic analysis of microvesicles from plasma of healthy d... | KEEP AS NON CORE | Summary: ITIH4 was detected among proteins associated with purified plasma microvesicles from healthy donors. Reason: The proteomic recovery supports a contextual blood-microparticle association, but it does not establish that vesicle packaging is required for ITIH4's inhibitor function. The robust core pool is soluble plasma/extracellular space. Supporting Evidence: PMID:22516433 We have detected 161 microvesicle-associated proteins |
| GO:0070062 extracellular exosome | HDA PMID:19056867 Large-scale proteomics and phosphoproteomics of urinary exos... | KEEP AS NON CORE | Summary: ITIH4 was identified in a large-scale human urinary-exosome proteomic dataset. Reason: The HDA is retained as a vesicle-preparation observation, but it does not show intraluminal incorporation or a functional exosome role. Secreted plasma/urinary ITIH4 can co-purify with extracellular vesicles. Supporting Evidence: PMID:19056867 analysis identified 1132 proteins unambiguously PMID:23533145 some of these proteins could also exist as a soluble form |
| GO:0005515 protein binding | IPI PMID:16271702 BIP co-chaperone MTJ1/ERDJ1 interacts with inter-alpha-tryps... | MARK AS OVER ANNOTATED | Summary: ITIH4's C-terminal region binds the SANT2 domain of DNAJC1/MTJ1, and the proteins co-immunoprecipitate from human liver extracts. Reason: The interaction is direct and may protect ITIH4 from kallikrein processing, but generic protein binding is uninformative and does not describe ITIH4's core inhibitor activity. No sufficiently specific GO MF term is established by the accessible abstract. Supporting Evidence: PMID:16271702 ITIH4 and MTJ1 co-immunoprecipitate from total liver protein extracts |
| GO:0034097 response to cytokine | IEP PMID:10486281 ITIH4 serum concentration increases during acute-phase proce... | KEEP AS NON CORE | Summary: IL-6 dose-dependently increases ITIH4 mRNA and radiolabeled protein in HepG2 culture medium, whereas IL-1 beta and TNF-alpha do not. Reason: The expression-response evidence is direct and cytokine-selective, so the annotation is valid. It describes regulation of ITIH4 abundance rather than the protein's core extracellular protease-inhibitor activity. Supporting Evidence: PMID:10486281 HepG2 cell line we have observed up-regulation of ITIH4 mRNA expression upon dose-response treatments with interleukin-6 (IL-6) |
| GO:0006953 acute-phase response | IEP PMID:19263524 Inter-alpha-trypsin inhibitor heavy chain 4 is a novel marke... | KEEP AS NON CORE | Summary: Circulating ITIH4 was strongly depleted during acute ischemic stroke and returned toward normal with clinical improvement. Reason: The source establishes disease-associated abundance dynamics, and an independent human study directly classifies ITIH4 as a type II acute-phase protein. Acute-phase deployment is important physiological context but is distinct from the core inhibitor mechanism. Supporting Evidence: PMID:19263524 The ITIH4 protein was completely absent in AIS patients as compared to those in the control group, and serum levels returned to normal in AIS patients as their condition improved PMID:10486281 The results presented here indicate that ITIH4 is a type II acute-phase protein in humans. |
| GO:0005576 extracellular region | NAS PMID:14718574 The human plasma proteome: a nonredundant list developed by ... | ACCEPT | Summary: The human plasma-proteome compilation supports an extracellular pool of ITIH4. Reason: Direct plasma purification and the cleaved N-terminal signal peptide support a soluble extracellular localization. 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. Supporting Evidence: PMID:14718574 We have merged four different views of the human plasma proteome PMID:7947966 A 120 kDa plasma protein, which is susceptible to plasma kallikrein, was purified from human plasma |
| GO:0004866 endopeptidase inhibitor activity | TAS PMID:9756925 Human inter-alpha-trypsin inhibitor heavy chain H3 gene. Gen... | MODIFY | Summary: The legacy TAS assigns broad endopeptidase inhibitor activity but cites a paper about ITIH3 gene organization and promoter regulation. Reason: The citation is miscited for ITIH4 inhibitor activity, but the biological essence is independently correct. Direct modern assays demonstrate inhibition specifically of serine-type endopeptidases, making GO:0004867 the better term and PMID:33523981 the appropriate evidence source. Proposed replacements: serine-type endopeptidase inhibitor activity Supporting Evidence: PMID:9756925 To understand more about the human inter-alpha-trypsin inhibitor heavy chain H3 (ITIH3) expression PMID:33523981 Thus, we demonstrate that ITIH4 functions as a protease inhibitor by a previously undescribed inhibitory mechanism. |
| GO:0001869 negative regulation of complement activation, lectin pathway | IDA PMID:33523981 ITIH4 acts as a protease inhibitor by a novel inhibitory mec... | NEW | Summary: Recombinant ITIH4 dose-dependently suppresses lectin-pathway C4 and C3 deposition in human serum through inhibition of MASP-1 and MASP-2. Reason: Direct serum-complement assays establish inhibition at both C4 and C3 deposition stages, and pathway-selective experiments identify the lectin pathway. This is the most specific supported biological-process term. Supporting Evidence: PMID:33523981 ITIH4 efficiently and dose-dependently inhibited C4 and C3 deposition |
| GO:1900004 negative regulation of serine-type endopeptidase activity | IDA PMID:33523981 ITIH4 acts as a protease inhibitor by a novel inhibitory mec... | NEW | Summary: ITIH4 directly inhibits MASP-1, MASP-2, and plasma kallikrein cleavage of physiologically relevant protein substrates. Reason: Purified-component assays demonstrate dose-dependent inhibition of three serine-type endopeptidases. The process term complements the direct inhibitor MF by representing the observed negative regulation of their activities. Supporting Evidence: PMID:33523981 In summary, ITIH4 inhibited the activity of both MASP-1 and MASP-2 against two distinct, highly-relevant protein substrates but not against smaller peptide substrates. |
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Download this section (compressed HTML)Q: Which endogenous protease-ITIH4 complexes predominate in healthy plasma and during complement- or kallikrein-driven inflammation?
Q: Does ITIH4 selectively regulate lectin-pathway complement in vivo, or do its effects extend to other complement and coagulation protease cascades?
Q: Do the proline-rich peptide and 35-kDa C-terminal chain have independent receptors or signaling activities after ITIH4 cleavage?
Q: Is ITIH4 actively packaged in platelet dense granules or extracellular vesicles, or does soluble plasma ITIH4 co-purify with these compartments?
Experiment: Compare lectin-pathway C4/C3 deposition and protease-complex abundance in ITIH4-depleted human serum reconstituted with wild-type, cleavage-resistant, or VWA metal-binding-deficient ITIH4. Confirm complexes by targeted native mass spectrometry and neoepitope immunoassays.
Hypothesis: Endogenous ITIH4 restrains lectin-pathway complement activation by forming cleavage-dependent complexes with MASP-1 and MASP-2 in plasma.
Type: Human-serum depletion/reconstitution and mechanistic proteomics
Experiment: Measure bradykinin release and HMWK cleavage kinetics in contact-system reconstitutions containing wild-type or cleavage/complex-formation mutants, then validate in endothelial permeability assays.
Hypothesis: Plasma kallikrein cleavage converts ITIH4 from a soluble substrate into an inhibitor that limits high-molecular-weight kininogen processing.
Type: Reconstituted kallikrein-kinin biochemistry and functional vascular assay
Experiment: Generate authentic proline-rich and 35-kDa fragments, screen extracellular receptor binding and inflammatory signaling, and use cleavage-resistant ITIH4 plus fragment add-back to separate inhibitor from fragment-mediated phenotypes.
Hypothesis: ITIH4 proteolytic fragments have biological activities distinct from the intact inhibitor complex.
Type: Fragment-resolved ligand screening and separation-of-function rescue
Experiment: Localize endogenous ITIH4 chains in resting and activated platelets by immunogold electron microscopy, granule fractionation, protease protection, and stimulus-dependent release, using plasma carryover controls.
Hypothesis: ITIH4 is selectively stored and released from a platelet granule compartment.
Type: Platelet subcellular localization and regulated-secretion analysis
What is not known — curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: The physiological occupancy, lifetime, and tissue distribution of endogenous ITIH4-protease inhibitory complexes are not established.
OPEN BIOLOGY MF_DARK
What is known: Purified proteins and serum assays directly establish complex formation and inhibition, but the abundance of each complex in healthy and inflamed human plasma has not been measured comprehensively.
Significance: Quantifying endogenous complexes is necessary to establish which serine proteases are the dominant physiological ITIH4 targets.
What would resolve it: Develop cleavage-neoepitope and protease-ITIH4 complex assays, then perform targeted proteomics across healthy plasma and defined inflammatory states.
Provenance (the field's own admissions):
Gap: The functions of the released proline-rich peptide and the 35-kDa C-terminal ITIH4 chain remain uncertain.
OPEN BIOLOGY BP_DARK
What is known: Proteolytic processing and fragment boundaries are established, but proposed bioactivity of the released peptide is not supported by a defined receptor, molecular target, or separation-of-function phenotype.
Significance: Fragment-specific functions could explain context-dependent inflammatory and biomarker behavior not accounted for by intact-protein protease inhibition.
What would resolve it: Purify authentic fragments, identify binding partners and receptor responses, and compare cleavage-resistant versus fragment-rescue alleles in inflammatory models.
Provenance (the field's own admissions):
Gap: It is unclear whether ITIH4 is bona fide extracellular-vesicle cargo and whether the 70-kDa chain is truly stored in platelet dense granules.
OPEN BIOLOGYCURATION CC_DARK
What is known: Proteomic datasets and Reactome place ITIH4 in vesicle/dense-granule contexts, but accessible evidence does not establish vesicle topology or ITIH4-specific dense-granule storage by orthogonal methods.
Significance: Resolving trafficking would distinguish regulated vesicular delivery from co-purification of an abundant soluble plasma protein.
What would resolve it: Use density gradients, size-exclusion chromatography, protease protection, detergent sensitivity, immunogold microscopy, and platelet-granule release assays with chain-specific ITIH4 antibodies.
Provenance (the field's own admissions):
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