ITIH6 is a 1,313-amino-acid, tissue-restricted member of the inter-alpha-trypsin inhibitor heavy-chain family. It is synthesized with an N-terminal signal peptide and contains VIT, von Willebrand factor type A, and ITI-heavy-chain C-terminal domains, separated by an unusually expanded central region that distinguishes ITIH6 from other family members. ITIH6 protein is abundant in selected glandular epithelia, including salivary gland, kidney, colon, and stomach. A conserved candidate heavy-chain cleavage motif makes extracellular-matrix and hyaluronan functions structurally plausible, but secretion of the mature protein, coupling to bikunin, hyaluronan transfer or binding, and any intrinsic molecular activity have not been demonstrated experimentally.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0004867 serine-type endopeptidase inhibitor activity | IEA GO_REF:0000002 | REMOVE | Summary: InterPro mapping assigns protease-inhibitor activity from the ITIH-family C-terminal domain, but no ITIH6 experiment demonstrates protease inhibition. Reason: This inference conflates the activity of the bikunin light chain or assembled inter-alpha-inhibitor with an ITIH heavy-chain polypeptide. Direct biochemical chain analysis found that human inter-alpha-inhibitor has one shared inhibitory 30-kDa chain and explicitly described its heavy chains as noninhibitory. ITIH6 lacks a Kunitz/bikunin inhibitor domain and has never been assayed as an inhibitor. Supporting Evidence: PMID:2476436 Inter-alpha-trypsin inhibitor contains noninhibitory heavy chains of 65,000 and 70,000 Da file:human/ITIH6/ITIH6-deep-research-manual.md ITIH6 itself has neither a bikunin/Kunitz inhibitor domain nor a direct inhibition assay. |
| GO:0005576 extracellular region | IEA GO_REF:0000044 | ACCEPT | Summary: UniProt subcellular-location mapping predicts that the signal-peptide-bearing ITIH6 precursor enters the secretory pathway and is secreted. Reason: ITIH6 has a clear N-terminal signal peptide, was discovered in a secreted-protein cDNA initiative, and belongs to a family of extracellular proteins. These data support the broad extracellular-region IEA without asserting a particular fluid, matrix compartment, or experimentally resolved mature form. Direct ITIH6 secretion and extracellular processing remain useful validation targets. Supporting Evidence: file:human/ITIH6/ITIH6-uniprot.txt FT SIGNAL 1..23 file:human/ITIH6/ITIH6-uniprot.txt CC -!- SUBCELLULAR LOCATION: Secreted {ECO:0000250}. |
| GO:0030212 hyaluronan metabolic process | IEA GO_REF:0000002 | UNDECIDED | Summary: InterPro transfers a hyaluronan-process annotation from ITIH-family architecture. ITIH6 has a conserved candidate heavy-chain cleavage motif, but its HA biology has not been tested directly. Reason: The 2025 first characterization treats ITIH6 interaction with HA and ECM stabilization as a structurally motivated hypothesis and explicitly calls for future functional studies. No experiment establishes ITIH6 processing, bikunin or chondroitin-sulfate coupling, TSG-6-dependent transfer, HA binding, or an effect on HA abundance or organization. The conserved motif prevents confident rejection, but family membership alone is insufficient for acceptance. Supporting Evidence: PMID:41109368 a previously uncharacterized member of the ITI heavy chain (ITIH) family file:human/ITIH6/ITIH6-deep-research-manual.md No experiment in the 2025 paper demonstrates ITIH6 secretion, incorporation into a bikunin-containing complex, covalent linkage to chondroitin sulfate, transfer to hyaluronan, or hyaluronan binding. |
| GO:0005515 protein binding | IPI PMID:32296183 A reference map of the human binary protein interactome. | MARK AS OVER ANNOTATED | Summary: HuRI reports eight verified Y2H pairs for ITIH6: ANKS1A, EVX2, KPRP, KRTAP6-1, SMIM29, TLX3, VENTX, and ZNF414. Reason: The high-throughput binary interactions are valid records of the screening assay, but `protein binding` is functionally uninformative. The study does not establish endogenous complexes, secretion-compatible colocalization, or a physiological consequence for any ITIH6 pair, and no more specific shared activity can be inferred from this heterogeneous partner set. Supporting Evidence: PMID:32296183 To map the reference interactome, we performed nine screens of Space III file:human/ITIH6/ITIH6-notes.md The grouped `protein binding` annotation represents eight HuRI yeast-two-hybrid pairs |
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Download this section (compressed HTML)Q: Is endogenous ITIH6 secreted as a full-length protein or cleaved at the predicted DPHFVI motif, and what is the mature C terminus?
Q: Does ITIH6 form a covalent complex with bikunin chondroitin sulfate, either alone or together with another ITIH heavy chain?
Q: Can TSG-6 transfer processed ITIH6 to hyaluronan, and does the expanded central region alter HA binding, matrix organization, or interaction partners?
Q: Which, if any, HuRI interaction partners form endogenous complexes with ITIH6 in its native glandular expression contexts?
Experiment: Endogenously tag ITIH6 in a validated glandular epithelial model and analyze matched cell lysates, conditioned medium, and extracellular matrix by pulse-chase, immunoprecipitation, N-/C-terminomics, and intact-mass spectrometry. Use signal-peptide and cleavage-motif mutants as controls.
Hypothesis: ITIH6 is secreted and proteolytically processed at its conserved candidate heavy-chain cleavage motif.
Type: Endogenous secretion and proteolytic-processing analysis
Experiment: Reconstitute assembly with purified ITIH6, AMBP-derived bikunin proteoglycan, and candidate processing machinery, then add TSG-6 and isotopically labeled HA. Detect ester-linked ITIH6-CS and ITIH6-HA products by glycosidase sensitivity, intact mass, linkage-site LC-MS/MS, and orthogonal HA pull-down. Include ITIH1/ITIH2 positive controls and cleavage-motif-mutant ITIH6 negative controls.
Hypothesis: Processed ITIH6 is covalently coupled to bikunin chondroitin sulfate and can be transferred to hyaluronan by TSG-6.
Type: Bikunin coupling and TSG-6-dependent hyaluronan-transfer reconstitution
Experiment: In ITIH6-positive salivary, renal, gastric, and colonic epithelial models, combine endogenous affinity purification, extracellular proximity labeling, and cross-linking mass spectrometry with knockout controls. Validate candidate partners reciprocally and test whether they depend on secretion or heavy-chain processing.
Hypothesis: ITIH6 has a native extracellular interaction network distinct from the heterogeneous HuRI Y2H partner set.
Type: Context-specific endogenous interactome mapping
What is not known — curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: It is unknown whether ITIH6 is processed into a canonical heavy chain and covalently incorporated into a bikunin-containing proteoglycan complex.
OPEN BIOLOGYCURATION MF_DARK
What is known: Sequence analysis predicts a conserved cleavage motif, but neither the cleavage site nor a terminal-Asp linkage to bikunin chondroitin sulfate has been mapped experimentally.
Significance: Resolving processing and complex assembly is necessary before ITIH6 can be treated as a canonical transferable inter-alpha-inhibitor heavy chain.
What would resolve it: Map endogenous ITIH6 termini by N-/C-terminomics and intact-mass spectrometry, test co-purification and covalent glycosaminoglycan linkage with bikunin, and repeat the analysis after AMBP or candidate processing-enzyme perturbation.
Provenance (the field's own admissions):
Gap: Hyaluronan binding, TSG-6-mediated ITIH6 transfer, and any effect on extracellular-matrix organization remain untested.
OPEN BIOLOGYCURATION BP_DARK
What is known: HA/ECM activity is inferred from family architecture and a candidate cleavage motif; the first ITIH6-focused study explicitly presents it as a hypothesis.
Significance: These experiments are required to adjudicate the current hyaluronan metabolic process IEA and to define a specific positive molecular function.
What would resolve it: Reconstitute ITIH6 processing and transfer with purified bikunin proteoglycan, TSG-6, and HA; map covalent products by mass spectrometry and test direct HA association with orthogonal biochemical assays.
Provenance (the field's own admissions):
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