Cystatin-A (stefin A) of the domestic cat, the allergen Fel d 3. A member of the cystatin (type 1 stefin) family, it is a small, single-domain, intracellular cysteine-protease inhibitor that reversibly and tightly binds the active site of papain-like (C1) cysteine proteases such as cathepsins, thereby negatively regulating proteolysis. Predominantly cytosolic, stefin A is also a structural precursor of the keratinocyte cornified envelope, crosslinked into the cornified layer during epidermal differentiation. It is a minor cat allergen that binds IgE.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005829 cytosol | IBA GO_REF:0000033 | ACCEPT | Summary: Stefin A is a cytosolic (intracellular) protease inhibitor; cytosolic localization is its principal functional compartment. Reason: Consistent with the documented intracellular/cytoplasmic localization of cystatin A. Supporting Evidence: file:FELCA/CSTA/CSTA-uniprot.txt SUBCELLULAR LOCATION: Cytoplasm |
| GO:0004869 cysteine-type endopeptidase inhibitor activity | IBA GO_REF:0000033 | ACCEPT | Summary: The defining molecular function of cystatin A: reversible, tight-binding inhibition of papain-like cysteine endopeptidases (e.g. cathepsins B, H, L). Reason: Core, family-defining function strongly supported by phylogeny and the curated UniProt function. Supporting Evidence: file:FELCA/CSTA/CSTA-uniprot.txt This is an intracellular thiol proteinase inhibitor. |
| GO:0004866 endopeptidase inhibitor activity | IEA GO_REF:0000002 | MARK AS OVER ANNOTATED | Summary: Generic parent of the specific cysteine-type endopeptidase inhibitor activity already annotated. Reason: Redundant broad parent; the specific cysteine-type term (GO:0004869) is the informative annotation. |
| GO:0004869 cysteine-type endopeptidase inhibitor activity | IEA GO_REF:0000120 | ACCEPT | Summary: Electronic duplicate of the core cysteine-protease inhibitor activity. Reason: Consistent with the family-defining inhibitory function. Supporting Evidence: file:FELCA/CSTA/CSTA-uniprot.txt This is an intracellular thiol proteinase inhibitor. |
| GO:0005737 cytoplasm | IEA GO_REF:0000120 | ACCEPT | Summary: UniProt records cytoplasmic localization for stefin A. Reason: Directly supported by the curated subcellular location. Supporting Evidence: file:FELCA/CSTA/CSTA-uniprot.txt SUBCELLULAR LOCATION: Cytoplasm |
| GO:0005829 cytosol | IEA GO_REF:0000118 | ACCEPT | Summary: Electronic cytosol annotation, consistent with the intracellular localization. Reason: Consistent with documented cytoplasmic/cytosolic localization. |
| GO:0001533 cornified envelope | IEA GO_REF:0000107 | ACCEPT | Summary: Stefin A is a well-known component/precursor of the keratinocyte cornified envelope, crosslinked into the cornified layer during epidermal differentiation. Reason: Consistent with the established role of cystatin A in the cornified envelope of skin. |
| GO:0002020 protease binding | IEA GO_REF:0000107 | KEEP AS NON CORE | Summary: Cystatin A binds the target protease active site; protease binding is the mechanistic basis of its inhibitory activity rather than a separate function. Reason: Mechanistically part of the inhibitor activity; retained as supporting/non-core. |
| GO:0005576 extracellular region | IEA GO_REF:0000107 | KEEP AS NON CORE | Summary: Stefin A is principally intracellular; it appears extracellularly in the skin/ cornified layer and as an environmental allergen, but the extracellular region is not its primary functional compartment. Reason: Secondary localization; the core functional compartment is the cytosol. |
| GO:0030216 keratinocyte differentiation | IEA GO_REF:0000107 | KEEP AS NON CORE | Summary: Consistent with stefin A's contribution to epidermal/keratinocyte differentiation via the cornified envelope. Reason: Plausible developmental role tied to the cornified-envelope function; electronic, non-core. |
| GO:0045861 negative regulation of proteolysis | IEA GO_REF:0000107 | ACCEPT | Summary: The direct biological-process consequence of cysteine-protease inhibition by stefin A. Reason: Directly follows from the core inhibitor activity. Supporting Evidence: file:FELCA/CSTA/CSTA-uniprot.txt This is an intracellular thiol proteinase inhibitor. |
| GO:0098609 cell-cell adhesion | IEA GO_REF:0000107 | KEEP AS NON CORE | Summary: Electronically transferred cell-cell adhesion association; not a well-established function of cystatin A and tangential to its protease-inhibitor and cornified-envelope roles. Reason: Weakly supported electronic annotation; retained as non-core pending evidence. |
| GO:1904090 peptidase inhibitor complex | IEA GO_REF:0000107 | KEEP AS NON CORE | Summary: Refers to the stefin A:protease inhibitory complex; consistent with the inhibitor activity. Reason: Describes the transient inhibitor-protease complex; supporting/non-core. |
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Download this section (compressed HTML)Q: Which cat cysteine proteases (cathepsin orthologs) are the physiological targets of Fel d 3, and does its inhibitory profile match human stefin A?
Experiment: Express recombinant Fel d 3 and measure Ki against papain and cathepsins B/H/L in vitro; confirm the wedge-type active-site interaction by co-structure or mutagenesis of the conserved QxVxG motif.
Hypothesis: Fel d 3 inhibits papain-family cysteine proteases with the canonical stefin-A specificity.
Type: enzymatic / biochemical inhibition assay
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