Chain 2 (beta chain) of the major cat allergen Fel d 1, a secreted secretoglobin (uteroglobin-family) glycoprotein of the domestic cat. The mature ~92-residue chain 2 is N-glycosylated (Asn50) and pairs with chain 1 (CH1) through three interchain disulfide bonds to form a heterodimer; two heterodimers associate non-covalently into the ~35-38 kDa Fel d 1 heterotetramer. Chain 2 contributes calcium-coordinating residues to the tetramer's calcium-binding sites and forms part of the uteroglobin-like all-alpha fold enclosing an internal hydrophobic cavity. The assembled Fel d 1 binds small hydrophobic ligands (fatty acids and steroids, e.g. lauric acid and the steroid pheromone androsterone) and bacterial lipopolysaccharide, and by a CD14/MD2-dependent lipid-transfer mechanism enhances TLR4/TLR2 innate immune signaling. It is secreted, produced chiefly by sebaceous glands and also found in saliva, with production regulated by testosterone; alternatively spliced long (salivary-gland-preferential) and short (skin-preferential) forms exist. Together with chain 1 it constitutes the dominant cat allergen, recognized by IgE in the great majority of cat-allergic people; its endogenous biological function in the cat is unresolved.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005576 extracellular region | IEA GO_REF:0000120 | ACCEPT | Summary: Automated subcellular-location annotation (UniProt "Secreted") consistent with the experimentally established secretion of Fel d 1. Chain 2 is found in saliva and sebaceous gland secretions, with sebaceous glands being the main production site. Reason: Fel d 1 is a secreted secretoglobin; localization of chain 2 to the extracellular region is well supported by experimental localization to saliva and sebaceous gland secretions. Supporting Evidence: PMID:29643919 It is now recognized that the sebaceous glands, and not saliva, are the main production site |
| GO:0005509 calcium ion binding | IDA PMID:17543334 Structural characterization of the tetrameric form of the ma... | NEW | Summary: NEW (proposed). The tetrameric Fel d 1 crystal structure resolves two distinct calcium-binding sites, and UniProt records three calcium-coordinating residues (36, 72, 77) within chain 2. Calcium binding is the only ligand-binding activity experimentally demonstrated for Fel d 1 and is not currently captured in the GOA annotations for chain 2. Reason: Direct structural evidence supports calcium ion binding by the Fel d 1 tetramer, with chain 2 contributing calcium-coordinating residues. This is a well-supported molecular function that should be added. Supporting Evidence: PMID:17543334 structure of tetrameric Fel d 1 reveals two different calcium-binding sites file:FELCA/CH2/CH2-uniprot.txt Chelates calciums ions and may inhibit the activity of |
| GO:0001530 lipopolysaccharide binding | IDA PMID:23878318 Allergens as immunomodulatory proteins: the cat dander prote... | NEW | Summary: NEW (proposed). The assembled Fel d 1 protein directly binds the TLR4 agonist lipopolysaccharide (LPS) and acts as a lipid-transfer platform; chain 2 is an integral subunit of this complex. Notably, the enhancement is independent of glycosylation status, so chain 2's N-glycan is not required. Not in GOA. Reason: Herre et al. 2013 show Fel d 1 binds LPS; chain 2 contributes to the complex that carries this activity (UniProt assigns the same FUNCTION to both chains). Demonstrated chiefly in the context of human innate-immune amplification. Supporting Evidence: PMID:23878318 bind to the TLR4 agonist LPS file:FELCA/CH2/CH2-deep-research-falcon.md independent of glycosylation status |
| GO:0034145 positive regulation of toll-like receptor 4 signaling pathway | IDA PMID:23878318 Allergens as immunomodulatory proteins: the cat dander prote... | NEW | Summary: NEW (proposed). By binding LPS and transferring it to the TLR4 receptor complex, the Fel d 1 protein enhances TLR4 (and TLR2) signaling and innate immune (TNF-alpha) responses; chain 2 contributes as a subunit of the complex. Reason: Herre et al. 2013 showed Fel d 1 enhances TLR4/TLR2 signaling. This immunomodulatory activity is the proposed basis of allergenicity; chain 2 is part of the functional unit. Supporting Evidence: PMID:23878318 enhances signaling through the innate receptors TLR4 and TLR2 |
Loading supporting content…
Download this section (compressed HTML)Q: What is the endogenous amphipathic ligand (if any) that occupies the internal cavity of the Fel d 1 tetramer, and does chain 2 contribute to its binding?
Q: Does Fel d 1 modulate calcium-dependent phospholipase A2 activity in vivo, as has been speculated by analogy to uteroglobin?
Q: Do the alternatively spliced long (salivary) and short (skin) forms of chain 2 confer any functional difference, or only tissue-specific expression?
Experiment: Perform untargeted lipidomics/metabolomics on ligands co-purifying with native Fel d 1 isolated from cat sebaceous secretions, and confirm direct binding and affinity of candidate ligands to recombinant Fel d 1 by isothermal titration calorimetry and co-crystallization.
Hypothesis: The Fel d 1 internal cavity binds a specific endogenous amphipathic ligand (e.g. a steroid, fatty acid, or pheromone).
Type: ligand identification / biophysical binding assay
Experiment: Reconstitute a calcium-dependent PLA2 activity assay in the presence and absence of folded recombinant Fel d 1 (and Ca2+-binding-site mutants in chain 2) to test for PLA2 inhibition and its calcium dependence.
Hypothesis: Calcium binding by the Fel d 1 tetramer enables sequestration that inhibits calcium-dependent phospholipase A2 activity.
Type: enzymatic / biochemical assay
Loading supporting content…
Download this section (compressed HTML)Loading supporting content…
Download this section (compressed HTML)Loading supporting content…
Download this section (compressed HTML)