Peptidase inhibitor 16 is a glycosylated CAP-superfamily ectoprotein that can be displayed at the plasma membrane through a GPI anchor and is also recovered as a soluble protein in serum and urine. Human biochemical studies establish inhibition of matrix metalloproteinase 2 (MMP2) and cathepsin K, placing PI16 in extracellular control of proteolysis. In coronary endothelial cells, PI16 is induced by laminar shear, suppressed by inflammatory cytokines, and limits MMP activity and cell migration. PI16 is also expressed by fibroblast and immune-cell subsets; mouse studies connect its cathepsin K inhibition to reduced chemerin activation in the myocardium and associate fibroblast-derived Pi16 with cardiomyocyte growth and neuropathic-pain responses. The mechanisms producing membrane-tethered versus soluble PI16 and the breadth of its physiological protease targets remain unresolved.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005576 extracellular region | IBA GO_REF:0000033 | ACCEPT | Summary: The phylogenetic annotation places active PI16 in the extracellular region, consistent with direct recovery of human PI16 from serum and with its extracellular protease-inhibitor activities. Reason: Although this broad PANTHER node spans divergent CAP-family proteins, the localization is independently and directly supported for PI16. Human PI16 was purified from serum, inhibits extracellular MMP2, and can be displayed as a GPI-anchored ectoprotein. Extracellular region therefore captures a genuine core site of action. Supporting Evidence: PMID:15344909 PSPBP (PSP94-binding protein) was purified from human serum |
| GO:0060090 molecular adaptor activity | IBA GO_REF:0000033 | REMOVE | Summary: This IBA is propagated at the root CAP-family node from a single mouse Glipr1l1 seed rather than from PI16 or a PI16 ortholog. Reason: The local PAINT trace assigns molecular adaptor activity at PTHR10334 node PTN000036124 using only MGI:MGI:1916536, which is mouse Glipr1l1, a sperm-fusion protein. That node spans deeply divergent animal, fungal, and plant CAP proteins. No PI16 study demonstrates adaptor activity; direct human experiments instead establish MMP2 and cathepsin K inhibitor activities. The root-level IBA is therefore over-propagated on biological grounds and is replaced by specific NEW inhibitor annotations. Propagation Review Root cause: PROPAGATION BAD Failure modes: WRONG ORTHOLOG OR PARALOG FUNCTIONAL DIVERGENCE Sources checked: PANTHER:PTN000036124 · broad cysteine-rich secretory protein-related family node SUPPORTS SOURCE BUT NOT TARGET The node spans divergent CAP-family subfamilies; a single reproductive adaptor seed is not sufficient to infer adaptor activity for PI16. MGI:MGI:1916536 · mouse Glipr1l1 SUPPORTS SOURCE BUT NOT TARGET Mouse Glipr1l1 supports a sperm-fusion adaptor role, but it is a different mammalian CAP-family subfamily from PI16. Supporting Evidence: file:human/PI16/PI16-deep-research-manual.md The cached PANTHER trace places GO:0060090 molecular adaptor activity at the broad PTHR10334 node PTN000036124 from only MGI:MGI:1916536, mouse `Glipr1l1`. |
| GO:0005576 extracellular region | IEA GO_REF:0000120 | ACCEPT | Summary: Combined automated annotation places PI16 in the extracellular region, in agreement with its signal peptide, serum/urine recovery, and extracellular inhibitor activity. Reason: The broad location is correct for both soluble PI16 and the extracellular domain of GPI-anchored PI16. Multiple direct human studies independently support extracellular exposure, so the electronic annotation is reliable. Supporting Evidence: PMID:15344909 PSPBP (PSP94-binding protein) was purified from human serum |
| GO:0005515 protein binding | IPI PMID:32296183 A reference map of the human binary protein interactome. | MARK AS OVER ANNOTATED | Summary: A proteome-scale human binary-interactome study contributes six PI16 WITH/FROM partners in the current GOA group. Reason: These high-throughput binary interactions may provide partner hypotheses, but generic protein binding does not describe PI16's biochemical function and the cached main article does not expose the PI16 supplementary rows. Specific interactions should be retained as interaction data and validated in the relevant secretory or membrane context rather than promoted as a core GO molecular function. |
| GO:0005515 protein binding | IPI PMID:35922511 A physical wiring diagram for the human immune system. | MARK AS OVER ANNOTATED | Summary: The immune-surface wiring study reports a PI16 interaction with TNFRSF21 in GOA, derived from a systematic extracellular-domain interaction platform. Reason: The partner-specific result may be biologically interesting, but generic protein binding is uninformative and does not establish whether PI16 acts as a TNFRSF21 ligand in vivo. This annotation should not be treated as a core molecular activity without partner-specific functional evidence. |
| GO:0005576 extracellular region | EXP PMID:15344909 Identification, purification and characterization of a novel... | ACCEPT | Summary: PI16/PSPBP was purified and N-terminally sequenced from human serum. Reason: Direct purification from serum, together with detection of the glycosylated mature protein, establishes an extracellular soluble pool of human PI16. The term is broad but accurate. Supporting Evidence: PMID:15344909 PSPBP (PSP94-binding protein) was purified from human serum |
| GO:0005576 extracellular region | EXP PMID:17062675 Prognostic value of prostate secretory protein of 94 amino a... | ACCEPT | Summary: PI16/PSPBP was quantitatively measured in pretreatment human serum from a prostate-cancer cohort. Reason: The serum ELISA study independently confirms extracellular circulating PI16. Its biomarker association is not a core function, but the localization evidence is direct and consistent with the original purification study. Supporting Evidence: PMID:17062675 Total PSP94, free PSP94, and the PSPBP were quantified in the pretreatment serum |
| GO:0005576 extracellular region | EXP PMID:22171320 Human urinary glycoproteomics; attachment site specific anal... | ACCEPT | Summary: UniProt cites this human urinary glycoproteomics study for mass-spectrometric identification and glycosylation of PI16 in urine. Reason: The cached abstract does not list individual proteins, but the UniProt curator had access to the complete proteomic dataset. Urinary recovery is also concordant with independently demonstrated serum secretion and extracellular exposure. The annotation is accepted while deferring to the source curator rather than claiming that the abstract alone verifies the PI16 peptide row. Supporting Evidence: file:human/PI16/PI16-uniprot.txt SUBCELLULAR LOCATION, GLYCOSYLATION, STRUCTURE OF CARBOHYDRATES, AND IDENTIFICATION BY MASS SPECTROMETRY. |
| GO:0008191 metalloendopeptidase inhibitor activity | IDA PMID:27996045 PI16 is a shear stress and inflammation-regulated inhibitor ... | NEW | Summary: Proposed direct human annotation: recombinant PI16 potently inhibits recombinant MMP2, and PI16 overexpression reduces MMP activity released by human coronary endothelial cells. Reason: This term captures a demonstrated molecular activity and is substantially more informative than the missing generic peptidase-inhibitor annotation. Phage display and competition experiments further map PI16 recognition to an exposed MMP2 loop near the active site. Supporting Evidence: PMID:27996045 recombinant PI16 produced in E. coli showed a strong inhibition of recombinant MMP2 activity |
| GO:0004869 cysteine-type endopeptidase inhibitor activity | IDA PMID:30365157 Skin-homing CD8(+) T cells preferentially express GPI-anchor... | NEW | Summary: Proposed direct human annotation: inhibitor screening and pull-down assays identify cathepsin K as a PI16 target. Reason: Cathepsin K is a cysteine-type endopeptidase, making this the appropriate specific GO molecular-function term. Independent mouse cardiac experiments corroborate cathepsin K inhibition and connect it to reduced pro-chemerin activation. Supporting Evidence: PMID:30365157 Inhibitor screening and pull-down experiments confirmed that PI16 inhibits cathepsin K PMID:27539859 Purified recombinant PI16 efficiently inhibited cathepsin K, a chemerin-activating protease, in vitro |
| GO:0005886 plasma membrane | IDA PMID:30365157 Skin-homing CD8(+) T cells preferentially express GPI-anchor... | NEW | Summary: Proposed direct human annotation: PI16 is a GPI-anchored plasma-membrane protein on a subset of skin-homing CD8 T cells. Reason: Flow-cytometric and biochemical experiments directly place human PI16 at the plasma membrane and establish GPI anchoring. This context-specific membrane pool complements, rather than contradicts, soluble PI16 detected in serum and urine. Supporting Evidence: PMID:30365157 PI16 was confined to the plasma membrane, was GPI-anchored |
| GO:0010951 negative regulation of endopeptidase activity | IDA PMID:27996045 PI16 is a shear stress and inflammation-regulated inhibitor ... | NEW | Summary: Proposed human process annotation: PI16 directly inhibits MMP2 and cathepsin K, reducing extracellular endopeptidase activity. Reason: Direct biochemical assays establish inhibition of two endopeptidases from distinct catalytic classes. This process term summarizes the shared effect without extending the target spectrum beyond MMP2 and cathepsin K. Supporting Evidence: PMID:27996045 recombinant PI16 produced in E. coli showed a strong inhibition of recombinant MMP2 activity PMID:30365157 Inhibitor screening and pull-down experiments confirmed that PI16 inhibits cathepsin K |
| GO:0010596 negative regulation of endothelial cell migration | IDA PMID:27996045 PI16 is a shear stress and inflammation-regulated inhibitor ... | NEW | Summary: Proposed direct human annotation: PI16 overexpression reduces migration of primary human coronary artery endothelial cells. Reason: The scratch-wound experiment directly demonstrates a negative effect on endothelial migration and links it to reduced extracellular MMP activity. This is a vascular context of PI16's inhibitor function, not a claim that PI16 universally suppresses movement of all cell types. Supporting Evidence: PMID:27996045 adenoviral overexpression of PI16 significantly reduced the migration of HCAECs into a ‘scratch wound’ in vitro |
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Download this section (compressed HTML)Q: What structural determinants allow the PI16 CAP domain to inhibit both the metalloprotease MMP2 and the cysteine protease cathepsin K, and what is its complete physiological target spectrum?
Suggested experts: Stephen J. White, Zoltán Pós
Q: Which PI16 isoforms and post-translational processing events determine GPI-anchored plasma-membrane display versus release into serum, urine, or tissue interstitium?
Suggested experts: Zoltán Pós, Jonathan R. Reeves
Q: Do PI16-dependent control of chemerin activation, cardiomyocyte growth, and endothelial-barrier permeability observed in mice operate in human tissues, and which effects require MMP2 versus cathepsin K inhibition?
Suggested experts: Stefan Engelhardt, Annemieke Kavelaars
Experiment: Express mammalian-glycosylated human PI16 isoforms and CAP-domain mutants, then measure binding kinetics and inhibition constants against a focused panel of MMPs, cysteine cathepsins, and unrelated extracellular proteases.
Hypothesis: PI16 directly inhibits a restricted target set centered on MMP2 and cathepsin K through separable CAP-domain interaction surfaces.
Type: recombinant protein biochemistry, enzyme kinetics, and structure-guided mutagenesis
Experiment: Endogenously tag each PI16 isoform in primary human fibroblasts, coronary endothelial cells, and skin-homing T cells; combine phosphatidylinositol-specific phospholipase C treatment, pulse-chase labeling, secretome proteomics, and C-terminal anchor-site mutagenesis.
Hypothesis: Cell type and isoform-specific GPI-anchor processing determine the balance between membrane-tethered and soluble PI16 pools.
Type: isoform-resolved genome editing, cell-surface biochemistry, and quantitative secretomics
Experiment: Compare wild-type and PI16-null human fibroblast/endothelial co-cultures under laminar shear or inflammatory cytokines, with rescue by wild-type PI16 and mutants selectively defective in MMP2 or cathepsin K inhibition. Quantify protease activity, endothelial migration, barrier permeability, and chemerin proteoforms.
Hypothesis: PI16 restrains MMP2-dependent endothelial migration at baseline, while loss or inflammatory down-regulation shifts proteolysis and barrier behavior; chemerin processing depends specifically on cathepsin K inhibition.
Type: CRISPR genetics, vascular co-culture, protease reporters, and targeted proteomics
What is not known — curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: The physiological protease-target spectrum and structural inhibition mechanism of human PI16 are incompletely defined.
OPEN BIOLOGY MF_DARK
What is known: Direct experiments support MMP2 and cathepsin K inhibition, but CAP-family membership does not justify extrapolation to all metalloproteases or cysteine peptidases.
Significance: Defining target selectivity is required to understand PI16's molecular function across vascular, cardiac, immune, and fibroblast contexts.
What would resolve it: Quantitative enzyme panels, direct binding measurements, PI16-protease structures, and separation-of-function mutants should establish specificity and mechanism.
Provenance (the field's own admissions):
Gap: The relationship between GPI-anchored and soluble human PI16 pools is unknown.
OPEN BIOLOGY
What is known: Human PI16 is directly observed at the GPI-anchored plasma membrane and is independently purified from serum and detected in urine, but the responsible isoforms and release mechanisms have not been resolved.
Significance: Membrane retention versus release will determine which proteases, ligands, and neighboring cells PI16 can regulate.
What would resolve it: Isoform-specific endogenous tagging, anchor-site mapping, shedding assays, and matched surface/secretome measurements across relevant human cell types are needed.
Provenance (the field's own admissions):
Gap: The human physiological relevance of PI16-dependent chemerin processing, cardiomyocyte growth control, and neuropathic-pain signaling is not established.
OPEN BIOLOGY
What is known: These causal phenotypes are supported primarily by mouse genetics; direct human work establishes protease inhibition and vascular cell behavior but not the corresponding organism-level outcomes.
Significance: Establishing conservation is necessary before extending mouse tissue-level process annotations to human PI16.
What would resolve it: Human organoids, primary-cell co-cultures, population proteogenetics, and separation-of-function rescue should test conservation without overinterpreting expression-marker associations.
Provenance (the field's own admissions):
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