CH1

UniProt ID: P30438
Organism: Felis catus
Review Status: DRAFT
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Gene Description

Chain 1 (alpha chain) of the major cat allergen Fel d 1, a secreted secretoglobin (uteroglobin-family) glycoprotein of the domestic cat. The mature 70-residue chain 1 pairs with chain 2 (CH2) through three interchain disulfide bonds to form a heterodimer; two heterodimers associate non-covalently into the ~35-38 kDa Fel d 1 heterotetramer. The all-alpha secretoglobin fold closely resembles uteroglobin and encloses an internal hydrophobic cavity; the tetramer binds calcium ions at defined sites and binds small hydrophobic ligands, including fatty acids and steroids (e.g. lauric acid and the steroid pheromone androsterone). Fel d 1 also binds 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, anal glands, skin and fur, with production regulated by testosterone. Its endogenous biological function in the cat is unresolved, with proposed roles in transport/presentation of lipophilic signaling molecules (pheromones), epithelial protection and immunoregulation. It is the dominant cat allergen, recognized by IgE in the great majority of cat-allergic people.

Existing Annotations Review

GO Term Evidence Action Reason
GO:0005496 steroid binding
IBA
GO_REF:0000033 		KEEP AS NON CORE
Summary: Phylogenetic (IBA) inference of steroid binding from the secretoglobin/ uteroglobin family. This is corroborated by ligand-binding data: Fel d 1 is reported to bind fatty acids and steroids with good affinity (best ligands lauric acid and androsterone, a volatile steroid pheromone), consistent with the internal hydrophobic cavity of the secretoglobin fold. The term is therefore a genuine activity, but likely one facet of a broader small- hydrophobic-ligand binding / transport role rather than the single core function, so it is retained as non-core.
Reason: Beyond the family-level inference, Fel d 1 has reported affinity for steroids (androsterone) and fatty acids (lauric acid), so steroid binding is supported rather than spurious. It is kept as non-core because it represents one of several ligand-binding activities and the physiological role of this binding (e.g. pheromone transport) is not established.
Supporting Evidence:
PMID:34026578
binding with good affinity to some fatty acids and steroids, the best ligands being lauric acid
PMID:12851385
Fel d 1 that could bind an endogenous ligand
GO:0005576 extracellular region
IBA
GO_REF:0000033 		ACCEPT
Summary: Fel d 1 is a secreted protein, consistent with the extracellular region annotation. It is produced mainly by sebaceous glands and is also found in saliva, anal glands, skin and fur, and is ubiquitous in cat-inhabited environments.
Reason: The secretoglobin Fel d 1 is experimentally established as a secreted protein, so localization to the extracellular region is well supported.
Supporting Evidence:
PMID:29643919
It is now recognized that the sebaceous glands, and not saliva, are the main production site
GO:0005496 steroid binding
IEA
GO_REF:0000118 		KEEP AS NON CORE
Summary: TreeGrafter (IEA) duplicate of the steroid-binding inference. As for the IBA annotation, this is corroborated by reported binding of Fel d 1 to steroids and fatty acids, so it is retained as a non-core ligand-binding activity.
Reason: Steroid binding is supported by ligand-binding data (androsterone, lauric acid) in addition to the family inference, but is one of several ligand- binding activities and not the single core function.
Supporting Evidence:
PMID:34026578
binding with good affinity to some fatty acids and steroids, the best ligands being lauric acid
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.
Reason: Fel d 1 is a secreted secretoglobin; the extracellular region location is correct and corroborated 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 crystal structure of the tetrameric form of Fel d 1 resolves two distinct calcium-binding sites, and UniProt records three calcium-coordinating residues (68, 71, 74) in chain 1. Calcium binding is the only ligand-binding activity experimentally demonstrated for Fel d 1 and is not currently captured in the GOA annotations.
Reason: Direct structural evidence supports calcium ion binding by the Fel d 1 tetramer, with chain 1 contributing calcium-coordinating residues. This is a better-supported molecular function than the family-propagated steroid binding term and should be added.
Supporting Evidence:
PMID:17543334
structure of tetrameric Fel d 1 reveals two different calcium-binding sites
file:FELCA/CH1/CH1-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). Fel d 1 directly binds the TLR4 agonist lipopolysaccharide (LPS) and is proposed to act via a lipid-transfer mechanism (CD14/MD2- dependent), enhancing innate immune signaling. This is the best-characterized molecular activity of the protein and is not captured in GOA.
Reason: Direct experimental evidence (Herre et al. 2013) shows Fel d 1 binds LPS, explaining its ability to enhance TLR4/TLR2 signaling. Largely demonstrated in the context of human innate-immune amplification/allergenicity rather than confirmed cat physiology, but it is a bona fide molecular function.
Supporting Evidence:
PMID:23878318
bind to the TLR4 agonist LPS
file:FELCA/CH1/CH1-deep-research-falcon.md
facilitating lipid transfer to CD14 and the TLR signaling complex
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, Fel d 1 substantially enhances signaling through TLR4 (and TLR2), potentiating innate immune (TNF-alpha) responses to bacterial lipid ligands.
Reason: Herre et al. 2013 showed Fel d 1 enhances TLR4/TLR2 signaling in response to lipid agonists. This immunomodulatory activity is the proposed basis of its allergenicity; well supported as a molecular activity though distinct from the cat's own (unknown) physiology.
Supporting Evidence:
PMID:23878318
enhances signaling through the innate receptors TLR4 and TLR2

Core Functions

Calcium-binding subunit of the secreted Fel d 1 secretoglobin heterotetramer. Chain 1 (this protein) dimerizes with chain 2 via interchain disulfide bonds; the assembled tetramer binds calcium ions at defined sites and functions in the extracellular space. The endogenous physiological role of the complex is not established.

Molecular Function:
calcium ion binding
Cellular Locations:
extracellular region
Supporting Evidence:
  • PMID:17543334
    structure of tetrameric Fel d 1 reveals two different calcium-binding sites
  • file:FELCA/CH1/CH1-uniprot.txt
    Chelates calciums ions and may inhibit the activity of

Immunomodulatory lipid-binding activity. Fel d 1 directly binds bacterial lipopolysaccharide (LPS) and, via a CD14/MD2-dependent lipid-transfer mechanism, enhances signaling through the innate immune receptors TLR4 and TLR2, potentiating pro-inflammatory responses to lipid ligands. This is the best-characterized molecular activity of the protein, demonstrated chiefly in the context of human innate-immune amplification and allergic sensitization.

Molecular Function:
lipopolysaccharide binding
Directly Involved In:
positive regulation of toll-like receptor 4 signaling pathway
Cellular Locations:
extracellular region
Supporting Evidence:

References

GO_REF:0000033
Annotation inferences using phylogenetic trees
GO_REF:0000118
TreeGrafter-generated GO annotations
GO_REF:0000120
Combined Automated Annotation using Multiple IEA Methods
PMID:29643919
An update on molecular cat allergens: Fel d 1 and what else? Chapter 1: Fel d 1, the major cat allergen.
  • Fel d 1 is a secreted secretoglobin (not a lipocalin) and the major cat allergen; its endogenous biological function is still unknown.
    "The biological function of Fel d 1 is still unknown"
  • Sebaceous glands, rather than saliva, are the main production site of Fel d 1.
    "It is now recognized that the sebaceous glands, and not saliva, are the main production site"
PMID:12851385
The crystal structure of the major cat allergen Fel d 1, a member of the secretoglobin family.
  • Fel d 1 is an all-alpha secretoglobin structurally similar to uteroglobin, with an internal asymmetric cavity that could bind an amphipathic endogenous ligand.
    "Fel d 1 that could bind an endogenous ligand"
PMID:17543334
Structural characterization of the tetrameric form of the major cat allergen Fel d 1.
  • The Fel d 1 (1+2) tetramer crystal structure reveals two distinct calcium-binding sites; PLA2 modulation via calcium sequestration is speculated by analogy to uteroglobin.
    "structure of tetrameric Fel d 1 reveals two different calcium-binding sites"
PMID:23878318
Allergens as immunomodulatory proteins: the cat dander protein Fel d 1 enhances TLR activation by lipid ligands.
  • Fel d 1 binds the TLR4 agonist LPS and substantially enhances signaling through TLR4 and TLR2 via a lipid-transfer (CD14/MD2-dependent) mechanism.
    "bind to the TLR4 agonist LPS"
  • Fel d 1 enhances TLR4/TLR2 innate immune signaling in response to lipid ligands.
    "enhances signaling through the innate receptors TLR4 and TLR2"
PMID:34026578
Molecular diagnosis in cat allergy.
  • Fel d 1 binds fatty acids and steroids with good affinity, the best ligands being lauric acid and androsterone (a volatile steroid pheromone).
    "binding with good affinity to some fatty acids and steroids, the best ligands being lauric acid"
PMID:19844127
The major cat allergen, Fel d 1, in diagnosis and therapy.
file:FELCA/CH1/CH1-deep-research-falcon.md
Falcon deep research report for CH1 (Fel d 1 chain 1)

Suggested Questions for Experts

Q: What is the endogenous amphipathic ligand (if any) that occupies the internal cavity of the Fel d 1 tetramer, and does chain 1 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: Is the physiological role of Fel d 1 in the cat one of skin/epithelial barrier protection, lipid/pheromone transport, or something else?

Suggested Experiments

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) 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

Deep Research

Falcon

(CH1-deep-research-falcon.md)
Functional Annotation of CH1 (Fel d 1 Chain 1) — UniProt P30438, *Felis catus* Falcon

The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.

You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.

We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.

We are interested in where in or outside the cell the gene product carries out its function.

We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.

Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.

Functional Annotation of CH1 (Fel d 1 Chain 1) — UniProt P30438, Felis catus

Gene Identity and Nomenclature

The gene symbol CH1 in Felis catus (domestic cat) encodes chain 1 of the major allergen Fel d 1 (UniProt accession P30438). This protein is described as "Major allergen I polypeptide chain 1" and belongs to the secretoglobin family, featuring a characteristic uteroglobin domain. In the literature, the gene product is almost universally referred to as Fel d 1 rather than by the gene symbol CH1 (gronlund2010themajorcat pages 1-2). The functional protein Fel d 1 is a heterodimer comprising two distinct polypeptide chains: chain 1 (encoded by CH1) and chain 2 (encoded by CH2), which associate via three inter-chain disulfide bonds (gronlund2010themajorcat pages 1-2, popescu2021moleculardiagnosisin pages 2-4). This report describes the biology of the CH1 gene product in the context of the mature Fel d 1 complex.

The following table summarizes the key properties of the CH1/Fel d 1 gene product:

Property Summary Evidence
Protein name Fel d 1; major cat allergen; UniProt P30438 corresponds specifically to chain 1 of the Fel d 1 complex (major allergen I polypeptide chain 1). (gronlund2010themajorcat pages 2-3, gronlund2010themajorcat pages 1-2)
Gene names CH1 encodes chain 1; Fel d 1 is produced as a two-chain secretoglobin complex comprising chain 1 and chain 2, commonly discussed together as the CH1/CH2 gene product. (gronlund2010themajorcat pages 1-2, brackett2022newfrontiersprecise pages 3-4)
UniProt accession P30438 (chain 1 / CH1). (gronlund2010themajorcat pages 2-3, gronlund2010themajorcat pages 1-2)
Organism Felis catus (domestic cat). (popescu2021moleculardiagnosisin pages 2-4, satyaraj2019keepthecat pages 1-2)
Protein family Secretoglobin family; uteroglobin-like protein rather than a lipocalin; homologous to rabbit uteroglobin and human Clara cell protein/CC16. (popescu2021moleculardiagnosisin pages 2-4, gronlund2010themajorcat pages 2-3)
Molecular weight Acidic glycoprotein; apparent mass 35–39 kDa by size-exclusion chromatography; quaternary structure consists of two non-covalently linked ~18-kDa heterodimers. (gronlund2010themajorcat pages 1-2)
Structure: overall architecture Small tetrameric secretoglobin composed of two heterodimers; all-helical protein with 8 helices and an internal ligand-binding cavity; tetramer contains three calcium-binding sites. (gronlund2010themajorcat pages 2-3, popescu2021moleculardiagnosisin pages 2-4)
Structure: chain 1 Chain 1 is an ~8-kDa, 70-aa polypeptide; structurally similar to chain 2 despite limited sequence identity; chain 1 contains prominent T-cell epitopes and is the UniProt P30438 product. (gronlund2010themajorcat pages 1-2, gronlund2010themajorcat pages 3-3)
Structure: chain 2 Chain 2 is an ~10-kDa glycopeptide, covalently linked to chain 1 by three inter-chain disulfide bonds; carries the N-linked tri-antennary glycan; skin-associated form is ~90 aa and salivary-gland form ~92 aa. (gronlund2010themajorcat pages 1-2, popescu2021moleculardiagnosisin pages 2-4)
Ligands Binds fatty acids and steroids; reported optimal ligands include lauric acid and androsterone. Structural cavity is consistent with small hydrophobic ligand binding typical of secretoglobins. (popescu2021moleculardiagnosisin pages 2-4, karn2025abroadgenome pages 9-10)
Tissue expression sites Produced by sebaceous glands, salivary glands, lacrimal glands, anal sacs, and perianal glands; co-expressed in skin and salivary glands. (popescu2021moleculardiagnosisin pages 2-4, gronlund2010themajorcat pages 2-3, gronlund2010themajorcat pages 1-2, brackett2022newfrontiersprecise pages 2-3)
Expression/regulation Produced by all cats, but levels vary widely; expression is testosterone-regulated; intact males produce more Fel d 1, and neutering reduces production by roughly 3–5-fold. (popescu2021moleculardiagnosisin pages 4-5)
Biological function (putative) Native function in cats remains not definitively established. Evidence supports putative roles in epithelial/skin defense, immunoregulation, and intra-species chemical communication; ligand binding suggests a role in transport/presentation of hydrophobic signaling molecules. (popescu2021moleculardiagnosisin pages 2-4, gronlund2010themajorcat pages 2-3, brackett2022newfrontiersprecise pages 2-3)
Biochemical/immunological activity Experimentally shown to bind LPS and enhance TLR4 and TLR2 signaling via CD14/MD2-dependent lipid transfer, increasing innate immune activation; this is the best-defined molecular activity, though demonstrated mainly in the context of allergenicity rather than confirmed cat physiology. (herre2013allergensasimmunomodulatory pages 6-7, herre2013allergensasimmunomodulatory pages 1-2, herre2013allergensasimmunomodulatory pages 4-6, herre2013allergensasimmunomodulatory pages 10-14)
Subcellular localization Secreted extracellular protein localized to secretory epithelia and glandular secretions; deposited on skin, hair follicles, and fur during grooming; shed into the environment with hair and dander; present at high levels on fur, lower in saliva, and minimal in urine. (popescu2021moleculardiagnosisin pages 2-4, gronlund2010themajorcat pages 2-3, popescu2021moleculardiagnosisin pages 4-5)

Table: This table summarizes the verified identity, structure, ligands, expression, localization, and putative biological function of the Fel d 1 CH1/CH2 secretoglobin complex in domestic cats. It is useful as a compact reference for functional annotation of UniProt P30438/CH1 in the context of the full Fel d 1 heterotetramer.

Protein Structure

Primary and Quaternary Architecture

Fel d 1 is an acidic glycoprotein with an apparent molecular weight of 35–39 kDa by size-exclusion chromatography (gronlund2010themajorcat pages 1-2). The native protein is a tetramer consisting of two non-covalently linked ~18-kDa heterodimers. Each heterodimer contains two antiparallel polypeptide chains: chain 1 (~8 kDa, 70 amino acids) and chain 2 (~10 kDa), covalently linked by three inter-chain disulfide bonds (gronlund2010themajorcat pages 1-2). Chain 2 carries the glycan moiety of the protein, a complex N-linked tri-antennary carbohydrate structure accounting for 10–20% of the molecular weight (gronlund2010themajorcat pages 1-2). Despite only 10–15% sequence identity between chains 1 and 2, the two polypeptides are structurally similar (gronlund2010themajorcat pages 2-3).

Three-Dimensional Structure and Uteroglobin Fold

The crystal structure of Fel d 1, solved in 2003 by Kaiser et al., revealed an all-helical protein containing 8 helices with the characteristic uteroglobin (UG) fold — a four-helix bundle forming a boomerang-shaped structure (gronlund2010themajorcat pages 2-3). This fold is shared among all secretoglobin family members and creates a central hydrophobic cavity capable of harboring small ligands (gronlund2010themajorcat pages 2-3). Dimer formation generates two pockets of different sizes: a smaller 350-ų compartment and a larger 750-ų compartment, with the larger pocket formed by dislocation of amino acids creating a cavity entrance (gronlund2010themajorcat pages 2-3). The tetrameric form contains three well-defined calcium-binding sites, a feature consistent with other uteroglobin structures (gronlund2010themajorcat pages 2-3, gronlund2010themajorcat pages 3-3).

Chain 1 (the CH1 product, P30438) shares approximately 30% sequence identity with rabbit uteroglobin, while chain 2 shows lower (10–15%) sequence homology (gronlund2010themajorcat pages 2-3). Notably, Fel d 1 is distinguished from most other mammalian allergens in being a uteroglobin-like protein rather than a lipocalin (gronlund2010themajorcat pages 2-3, gronlund2010themajorcat pages 3-3). Chain 2 exists in tissue-specific isoforms: a shorter 90-amino acid version is preferentially found in skin, while a longer 92-amino acid version is expressed in salivary glands (gronlund2010themajorcat pages 1-2).

Biological Function

Native Function in the Cat

The precise biological function of Fel d 1 in cat physiology remains formally unestablished (popescu2021moleculardiagnosisin pages 2-4, gronlund2010themajorcat pages 2-3). However, converging lines of evidence suggest several putative roles:

  1. Lipid and steroid transport/presentation: Fel d 1 binds fatty acids and steroids, with lauric acid and androsterone identified as optimal ligands (popescu2021moleculardiagnosisin pages 2-4). This is consistent with the hydrophobic ligand-binding cavity characteristic of the secretoglobin fold, which in other family members (e.g., uteroglobin) binds progesterone and phospholipids (karn2025abroadgenome pages 9-10, karn2025abroadgenome pages 13-14). The production of Fel d 1 is under testosterone control, with intact male cats producing significantly more than castrated males, and this reduction being reversible by exogenous testosterone (popescu2021moleculardiagnosisin pages 4-5). These observations suggest a hormonal signaling or transport function.

  2. Intra-species chemical communication: The testosterone-dependent expression pattern, secretion by multiple exocrine glands, and ability to bind volatile steroids like androsterone suggest a role in pheromone-like chemical communication between cats (popescu2021moleculardiagnosisin pages 2-4). This is supported by evolutionary parallels: mouse salivary androgen-binding protein (ABP), another secretoglobin, functions in reproductive isolation and species recognition (karn2025abroadgenome pages 14-15).

  3. Epithelial defense and immunoregulation: Fel d 1 has been proposed to play a protective role for cat skin and may be involved in local immunoregulation (popescu2021moleculardiagnosisin pages 2-4, brackett2022newfrontiersprecise pages 2-3).

Immunomodulatory Activity: Lipid Transfer and TLR Enhancement

The best-characterized molecular activity of Fel d 1 was demonstrated by Herre et al. (2013), who showed that Fel d 1 functions as an immunomodulatory protein (IMP) that enhances innate immune signaling. Specifically, recombinant Fel d 1 increased LPS-induced TLR4 signaling approximately 15-fold in HEK293 cells and also enhanced TLR2 signaling in response to lipoteichoic acid (LTA), but did not modify TLR5 signaling (herre2013allergensasimmunomodulatory pages 4-6). The mechanism involves Fel d 1 directly binding lipopolysaccharide (LPS) and facilitating lipid transfer to CD14 and the TLR signaling complex, rather than acting as a co-receptor mimic like the dust mite allergen Der p 2 (herre2013allergensasimmunomodulatory pages 6-7, herre2013allergensasimmunomodulatory pages 1-2). Enhancement is dependent on both MD2 and CD14 co-receptors and is independent of glycosylation (herre2013allergensasimmunomodulatory pages 6-7). In primary macrophages and human PBMCs, Fel d 1 potentiated TNFα production in response to bacterial lipid ligands (LPS, LTA, Pam2CSK4, Pam3CSK4), with effects confirmed to operate through TLR4 by ablation in TLR4-deficient cells (herre2013allergensasimmunomodulatory pages 10-14). This lipid-binding and immunomodulatory activity explains, at least in part, the mechanism by which Fel d 1 promotes allergic sensitization in humans — by amplifying innate immune responses to environmental lipid pathogen-associated molecular patterns (PAMPs) at low concentrations (herre2013allergensasimmunomodulatory pages 14-17).

Evolutionary Context: The Secretoglobin Superfamily

Secretoglobins (SCGBs) are small, dimeric, cytokine-like proteins with a characteristic four-helix bundle (UG fold) and conserved ligand-binding residues (karn2025abroadgenome pages 4-6). A comprehensive genome survey by Karn and Laukaitis (2025) revealed that SCGBs are not uniquely mammalian but are widespread in amniotes, including turtles, crocodilians, lizards, and birds, suggesting their origin in the Carboniferous Period (~320 MYA) (karn2025abroadgenome pages 1-2). The family binds retinoids, phospholipids, steroids, and calcium (karn2025abroadgenome pages 1-2, karn2025abroadgenome pages 9-10). Uteroglobin (SCGB1A1), the prototypical family member, is induced by progesterone and secreted into reproductive and airway luminal fluid, where it binds sperm surface proteins and modulates motility (karn2025abroadgenome pages 9-10). Despite extensive research, no clear function has been established for any SCGB, though they are dysregulated in lung disease, kidney disease, inflammation, and cancer (karn2025abroadgenome pages 2-4). The deep evolutionary conservation and functional diversity of SCGBs strongly suggest a fundamental, yet poorly characterized, biological role common to all amniotes (karn2025abroadgenome pages 1-2).

Strikingly, the brachial gland exudate secretion protein (BGEsp) of slow lorises (Nycticebus spp.) — a 17.6-kDa secretoglobin heterodimer — shows strong structural homology to Fel d 1 rather than to primate proteins, representing an unusual case of convergent evolution between lorises and felids (fitzpatrick2023slowlymakingsense pages 7-8). In lorises, this secretoglobin participates in a two-step venom system with functions in ectoparasite defense and intraspecific competition, and may also serve as an olfactory chemical signal (fitzpatrick2023slowlymakingsense pages 11-13, fitzpatrick2023slowlymakingsense pages 22-23). This convergence further supports the hypothesis that Fel d 1 may play roles in chemical communication and defense in cats.

Tissue Expression and Localization

Fel d 1 is a secreted extracellular protein produced by multiple glandular tissues in cats. Production sites include sebaceous glands, salivary glands, lacrimal glands, anal sacs, and perianal glands (popescu2021moleculardiagnosisin pages 2-4, brackett2022newfrontiersprecise pages 2-3). Chain 1 and chain 2 are co-expressed in both skin and salivary glands, though with tissue-specific chain 2 length variants (gronlund2010themajorcat pages 1-2). The allergen is deposited onto skin, hair follicles, and fur during grooming, and is subsequently shed into the environment with hair and dander (popescu2021moleculardiagnosisin pages 2-4, gronlund2010themajorcat pages 2-3). Levels on fur are highest at the neck region (ranging from 1 to 1,770 μg/g), with lower levels in saliva and minimal concentrations in urine (popescu2021moleculardiagnosisin pages 4-5).

All cats produce Fel d 1 regardless of breed, age, hair length, sex, or housing conditions, but individual variation is substantial — up to an 80-fold difference in salivary Fel d 1 levels between cats (satyaraj2019keepthecat pages 1-2). Production is androgen-regulated: intact male cats produce 3–5 times more Fel d 1 than neutered males, and this reduction can be reversed by exogenous testosterone administration (popescu2021moleculardiagnosisin pages 4-5). Urine is not a significant source, though hormonal status affects urinary levels in intact male cats (popescu2021moleculardiagnosisin pages 4-5). Once shed, Fel d 1 becomes airborne as particles from dried saliva and dandruff, and is passively transferred to environments lacking cats (homes, vehicles, public buildings) on clothing (gronlund2010themajorcat pages 2-3, satyaraj2019keepthecat pages 1-2).

Allergenicity and Clinical Significance

Fel d 1 accounts for 60–96% of cat dander's allergenic activity, with approximately 88–96% of IgE responses to cat allergens directed against Fel d 1, and roughly 95% of cat-allergic patients producing IgE antibodies to this protein (popescu2021moleculardiagnosisin pages 4-5, brackett2022newfrontiersprecise pages 3-4, gronlund2010themajorcat pages 3-3). It is the immunodominant allergen in cat allergy, which affects approximately 1 in 5 adults worldwide (satyaraj2019keepthecat pages 1-2). A recent comprehensive study by Trifonova et al. (2023) confirmed that Fel d 1, along with Fel d 4 and Fel d 7, was recognized by more than 65% of patients with respiratory allergy to cats, and induced basophil degranulation at hundred-fold lower concentrations than other cat allergens (trifonova2023allergenicactivityof pages 1-2, trifonova2023allergenicactivityof pages 15-16).

Recent Developments and Therapeutic Approaches

Several innovative strategies are being pursued to reduce Fel d 1 exposure or mitigate its allergenic effects:

  1. CRISPR gene editing: Brackett et al. (2022) demonstrated CRISPR-based knockout of CH1 and CH2 genes in feline kidney epithelial cells with editing efficiencies of 5–55%, with no off-target mutations detected (brackett2022newfrontiersprecise pages 3-4). This approach aims to create hypoallergenic cats by eliminating Fel d 1 at the genetic source (brackett2022newfrontiersprecise pages 1-2).

  2. Anti-Fel d 1 antibody approaches: Polyclonal anti-Fel d 1 antibodies delivered through cat food achieved a 47% reduction in haircoat allergen levels, while immunization of cats with anti-Fel d 1 resulted in approximately 50% reduction in tear extract allergen and 30% reduction in allergic patient symptoms (brackett2022newfrontiersprecise pages 2-3).

  3. Recombinant IgG4 therapy: Passive immunization of cat-allergic patients using Fel d 1-specific recombinant human IgG4 antibodies has been demonstrated to be clinically effective (trifonova2023allergenicactivityof pages 1-2).

  4. Hypoallergen design: Engineered Fel d 1 variants with disrupted disulfide bridges show 400–900 times lower IgE-binding capacity while preserving T-cell activation, serving as candidates for allergen-specific immunotherapy (gronlund2010themajorcat pages 5-6).

  5. Peptide immunotherapy: Second-generation vaccines consisting of overlapping T-cell epitope peptides demonstrated reduced late-phase skin reactions, decreased allergen-stimulated proliferation, and increased IL-10 production (gronlund2010themajorcat pages 5-6).

Summary

The CH1 gene in Felis catus encodes chain 1 of Fel d 1, a secreted secretoglobin protein with the characteristic uteroglobin fold. The mature Fel d 1 is a tetrameric heterodimer complex with an internal hydrophobic cavity that binds fatty acids (lauric acid) and steroids (androsterone). While its native biological function in cats is not definitively established, evidence supports roles in lipid transport/presentation, chemical communication, and epithelial defense, consistent with the broader secretoglobin family. Its best-characterized molecular activity is the enhancement of TLR4/TLR2 innate immune signaling through lipid binding and transfer — a property that underlies its exceptional allergenicity in humans. Fel d 1 is produced by multiple exocrine glands under testosterone regulation and is deposited onto cat fur and subsequently dispersed into the environment. It is the dominant allergen in cat allergy, recognized by over 90% of sensitized patients, and is the target of emerging genetic, immunological, and pharmacological strategies to reduce allergen exposure.

References

  1. (gronlund2010themajorcat pages 1-2): Hans Grönlund, Tiiu Saarne, Guro Gafvelin, and Marianne van Hage. The major cat allergen, fel d 1, in diagnosis and therapy. International Archives of Allergy and Immunology, 151:265-274, Oct 2010. URL: https://doi.org/10.1159/000250435, doi:10.1159/000250435. This article has 154 citations and is from a peer-reviewed journal.

  2. (popescu2021moleculardiagnosisin pages 2-4): Florin-Dan Popescu, Carmen Saviana Ganea, Carmen Panaitescu, and Mariana Vieru. Molecular diagnosis in cat allergy. World Journal of Methodology, 11:46-60, May 2021. URL: https://doi.org/10.5662/wjm.v11.i3.46, doi:10.5662/wjm.v11.i3.46. This article has 35 citations.

  3. (gronlund2010themajorcat pages 2-3): Hans Grönlund, Tiiu Saarne, Guro Gafvelin, and Marianne van Hage. The major cat allergen, fel d 1, in diagnosis and therapy. International Archives of Allergy and Immunology, 151:265-274, Oct 2010. URL: https://doi.org/10.1159/000250435, doi:10.1159/000250435. This article has 154 citations and is from a peer-reviewed journal.

  4. (brackett2022newfrontiersprecise pages 3-4): Nicole F. Brackett, Anna Pomés, and Martin D. Chapman. New frontiers: precise editing of allergen genes using crispr. Frontiers in Allergy, Jan 2022. URL: https://doi.org/10.3389/falgy.2021.821107, doi:10.3389/falgy.2021.821107. This article has 40 citations and is from a peer-reviewed journal.

  5. (satyaraj2019keepthecat pages 1-2): Ebenezer Satyaraj, Harold James Wedner, and Jean Bousquet. Keep the cat, change the care pathway: a transformational approach to managing fel d 1, the major cat allergen. Allergy, 74:5-17, Sep 2019. URL: https://doi.org/10.1111/all.14013, doi:10.1111/all.14013. This article has 103 citations and is from a highest quality peer-reviewed journal.

  6. (gronlund2010themajorcat pages 3-3): Hans Grönlund, Tiiu Saarne, Guro Gafvelin, and Marianne van Hage. The major cat allergen, fel d 1, in diagnosis and therapy. International Archives of Allergy and Immunology, 151:265-274, Oct 2010. URL: https://doi.org/10.1159/000250435, doi:10.1159/000250435. This article has 154 citations and is from a peer-reviewed journal.

  7. (karn2025abroadgenome pages 9-10): Robert C Karn and Christina M Laukaitis. A broad genome survey reveals widespread presence of secretoglobin genes in squamate and archosaur reptiles that flowered into diversity in mammals. Genome Biology and Evolution, Feb 2025. URL: https://doi.org/10.1093/gbe/evaf024, doi:10.1093/gbe/evaf024. This article has 0 citations and is from a domain leading peer-reviewed journal.

  8. (brackett2022newfrontiersprecise pages 2-3): Nicole F. Brackett, Anna Pomés, and Martin D. Chapman. New frontiers: precise editing of allergen genes using crispr. Frontiers in Allergy, Jan 2022. URL: https://doi.org/10.3389/falgy.2021.821107, doi:10.3389/falgy.2021.821107. This article has 40 citations and is from a peer-reviewed journal.

  9. (popescu2021moleculardiagnosisin pages 4-5): Florin-Dan Popescu, Carmen Saviana Ganea, Carmen Panaitescu, and Mariana Vieru. Molecular diagnosis in cat allergy. World Journal of Methodology, 11:46-60, May 2021. URL: https://doi.org/10.5662/wjm.v11.i3.46, doi:10.5662/wjm.v11.i3.46. This article has 35 citations.

  10. (herre2013allergensasimmunomodulatory pages 6-7): Jurgen Herre, Hans Grönlund, Heather Brooks, Lee Hopkins, Lisa Waggoner, Ben Murton, Monique Gangloff, Olaniyi Opaleye, Edwin R. Chilvers, Kate Fitzgerald, Nick Gay, Tom Monie, and Clare Bryant. Allergens as immunomodulatory proteins: the cat dander protein fel d 1 enhances tlr activation by lipid ligands. The Journal of Immunology, 191:1529-1535, Aug 2013. URL: https://doi.org/10.4049/jimmunol.1300284, doi:10.4049/jimmunol.1300284. This article has 131 citations.

  11. (herre2013allergensasimmunomodulatory pages 1-2): Jurgen Herre, Hans Grönlund, Heather Brooks, Lee Hopkins, Lisa Waggoner, Ben Murton, Monique Gangloff, Olaniyi Opaleye, Edwin R. Chilvers, Kate Fitzgerald, Nick Gay, Tom Monie, and Clare Bryant. Allergens as immunomodulatory proteins: the cat dander protein fel d 1 enhances tlr activation by lipid ligands. The Journal of Immunology, 191:1529-1535, Aug 2013. URL: https://doi.org/10.4049/jimmunol.1300284, doi:10.4049/jimmunol.1300284. This article has 131 citations.

  12. (herre2013allergensasimmunomodulatory pages 4-6): Jurgen Herre, Hans Grönlund, Heather Brooks, Lee Hopkins, Lisa Waggoner, Ben Murton, Monique Gangloff, Olaniyi Opaleye, Edwin R. Chilvers, Kate Fitzgerald, Nick Gay, Tom Monie, and Clare Bryant. Allergens as immunomodulatory proteins: the cat dander protein fel d 1 enhances tlr activation by lipid ligands. The Journal of Immunology, 191:1529-1535, Aug 2013. URL: https://doi.org/10.4049/jimmunol.1300284, doi:10.4049/jimmunol.1300284. This article has 131 citations.

  13. (herre2013allergensasimmunomodulatory pages 10-14): Jurgen Herre, Hans Grönlund, Heather Brooks, Lee Hopkins, Lisa Waggoner, Ben Murton, Monique Gangloff, Olaniyi Opaleye, Edwin R. Chilvers, Kate Fitzgerald, Nick Gay, Tom Monie, and Clare Bryant. Allergens as immunomodulatory proteins: the cat dander protein fel d 1 enhances tlr activation by lipid ligands. The Journal of Immunology, 191:1529-1535, Aug 2013. URL: https://doi.org/10.4049/jimmunol.1300284, doi:10.4049/jimmunol.1300284. This article has 131 citations.

  14. (karn2025abroadgenome pages 13-14): Robert C Karn and Christina M Laukaitis. A broad genome survey reveals widespread presence of secretoglobin genes in squamate and archosaur reptiles that flowered into diversity in mammals. Genome Biology and Evolution, Feb 2025. URL: https://doi.org/10.1093/gbe/evaf024, doi:10.1093/gbe/evaf024. This article has 0 citations and is from a domain leading peer-reviewed journal.

  15. (karn2025abroadgenome pages 14-15): Robert C Karn and Christina M Laukaitis. A broad genome survey reveals widespread presence of secretoglobin genes in squamate and archosaur reptiles that flowered into diversity in mammals. Genome Biology and Evolution, Feb 2025. URL: https://doi.org/10.1093/gbe/evaf024, doi:10.1093/gbe/evaf024. This article has 0 citations and is from a domain leading peer-reviewed journal.

  16. (herre2013allergensasimmunomodulatory pages 14-17): Jurgen Herre, Hans Grönlund, Heather Brooks, Lee Hopkins, Lisa Waggoner, Ben Murton, Monique Gangloff, Olaniyi Opaleye, Edwin R. Chilvers, Kate Fitzgerald, Nick Gay, Tom Monie, and Clare Bryant. Allergens as immunomodulatory proteins: the cat dander protein fel d 1 enhances tlr activation by lipid ligands. The Journal of Immunology, 191:1529-1535, Aug 2013. URL: https://doi.org/10.4049/jimmunol.1300284, doi:10.4049/jimmunol.1300284. This article has 131 citations.

  17. (karn2025abroadgenome pages 4-6): Robert C Karn and Christina M Laukaitis. A broad genome survey reveals widespread presence of secretoglobin genes in squamate and archosaur reptiles that flowered into diversity in mammals. Genome Biology and Evolution, Feb 2025. URL: https://doi.org/10.1093/gbe/evaf024, doi:10.1093/gbe/evaf024. This article has 0 citations and is from a domain leading peer-reviewed journal.

  18. (karn2025abroadgenome pages 1-2): Robert C Karn and Christina M Laukaitis. A broad genome survey reveals widespread presence of secretoglobin genes in squamate and archosaur reptiles that flowered into diversity in mammals. Genome Biology and Evolution, Feb 2025. URL: https://doi.org/10.1093/gbe/evaf024, doi:10.1093/gbe/evaf024. This article has 0 citations and is from a domain leading peer-reviewed journal.

  19. (karn2025abroadgenome pages 2-4): Robert C Karn and Christina M Laukaitis. A broad genome survey reveals widespread presence of secretoglobin genes in squamate and archosaur reptiles that flowered into diversity in mammals. Genome Biology and Evolution, Feb 2025. URL: https://doi.org/10.1093/gbe/evaf024, doi:10.1093/gbe/evaf024. This article has 0 citations and is from a domain leading peer-reviewed journal.

  20. (fitzpatrick2023slowlymakingsense pages 7-8): Leah Lucy Joscelyne Fitzpatrick, Rodrigo Ligabue-Braun, and K. Anne-Isola Nekaris. Slowly making sense: a review of the two-step venom system within slow (nycticebus spp.) and pygmy lorises (xanthonycticebus spp.). Toxins, 15:514, Aug 2023. URL: https://doi.org/10.3390/toxins15090514, doi:10.3390/toxins15090514. This article has 8 citations.

  21. (fitzpatrick2023slowlymakingsense pages 11-13): Leah Lucy Joscelyne Fitzpatrick, Rodrigo Ligabue-Braun, and K. Anne-Isola Nekaris. Slowly making sense: a review of the two-step venom system within slow (nycticebus spp.) and pygmy lorises (xanthonycticebus spp.). Toxins, 15:514, Aug 2023. URL: https://doi.org/10.3390/toxins15090514, doi:10.3390/toxins15090514. This article has 8 citations.

  22. (fitzpatrick2023slowlymakingsense pages 22-23): Leah Lucy Joscelyne Fitzpatrick, Rodrigo Ligabue-Braun, and K. Anne-Isola Nekaris. Slowly making sense: a review of the two-step venom system within slow (nycticebus spp.) and pygmy lorises (xanthonycticebus spp.). Toxins, 15:514, Aug 2023. URL: https://doi.org/10.3390/toxins15090514, doi:10.3390/toxins15090514. This article has 8 citations.

  23. (trifonova2023allergenicactivityof pages 1-2): Daria Trifonova, Mirela Curin, Ksenja Riabova, Antonina Karsonova, Walter Keller, Hans Grönlund, Ulrika Käck, Jon R. Konradsen, Marianne van Hage, Alexander Karaulov, and Rudolf Valenta. Allergenic activity of individual cat allergen molecules. International Journal of Molecular Sciences, 24:16729, Nov 2023. URL: https://doi.org/10.3390/ijms242316729, doi:10.3390/ijms242316729. This article has 26 citations.

  24. (trifonova2023allergenicactivityof pages 15-16): Daria Trifonova, Mirela Curin, Ksenja Riabova, Antonina Karsonova, Walter Keller, Hans Grönlund, Ulrika Käck, Jon R. Konradsen, Marianne van Hage, Alexander Karaulov, and Rudolf Valenta. Allergenic activity of individual cat allergen molecules. International Journal of Molecular Sciences, 24:16729, Nov 2023. URL: https://doi.org/10.3390/ijms242316729, doi:10.3390/ijms242316729. This article has 26 citations.

  25. (brackett2022newfrontiersprecise pages 1-2): Nicole F. Brackett, Anna Pomés, and Martin D. Chapman. New frontiers: precise editing of allergen genes using crispr. Frontiers in Allergy, Jan 2022. URL: https://doi.org/10.3389/falgy.2021.821107, doi:10.3389/falgy.2021.821107. This article has 40 citations and is from a peer-reviewed journal.

  26. (gronlund2010themajorcat pages 5-6): Hans Grönlund, Tiiu Saarne, Guro Gafvelin, and Marianne van Hage. The major cat allergen, fel d 1, in diagnosis and therapy. International Archives of Allergy and Immunology, 151:265-274, Oct 2010. URL: https://doi.org/10.1159/000250435, doi:10.1159/000250435. This article has 154 citations and is from a peer-reviewed journal.

📚 Additional Documentation

Notes

(CH1-notes.md)

Fel d 1 chain 1 (CH1, UniProt P30438) — curation notes

Curation triggered by the review article PMC5891966 / PMID:29643919
(Bonnet et al. 2018, Allergy Asthma Clin Immunol, "An update on molecular cat
allergens: Fel d 1 and what else? Chapter 1: Fel d 1, the major cat allergen").

Identity

  • UniProt P30438, FEL1A_FELCA, gene name CH1 (chain 1 / α-chain).
  • 92 aa precursor; signal 1–22, mature chain 23–92 (~70 aa, ~8 kDa).
  • The complete Fel d 1 allergen is a ~35–38 kDa heterotetramer: two heterodimers,
    each a chain 1 (CH1, this gene, P30438) + chain 2 (CH2, P30440) pair, the two
    chains covalently linked by three interchain disulfide bonds; encoded by two
    separate genes PMID:29643919.

Family / fold

  • Member of the secretoglobin (uteroglobin) family; all-α fold of eight
    helices PMID:29643919.
  • Crystal structure shows striking similarity to uteroglobin (a steroid-inducible,
    cytokine-like, anti-inflammatory/immunomodulatory molecule)
    PMID:12851385.

Molecular function (what is actually known vs inferred)

  • Calcium ion binding — experimentally supported. The tetramer crystal
    structure reveals two distinct Ca2+-binding sites; UniProt records three Ca2+
    binding residues (68, 71, 74)
    PMID:17543334.
  • Steroid/ligand binding — NOT demonstrated, only a structural possibility.
    The structures show an internal asymmetric cavity that could accommodate an
    amphipathic endogenous ligand; no steroid (or other ligand) has been shown to
    bind PMID:12851385.
    The IBA/IEA "steroid binding" GO annotation is a family-level (uteroglobin)
    inference, not evidence for this protein → treated as over-annotation.
  • Phospholipase A2 modulation — speculation. By analogy to uteroglobin, Fel d 1
    is speculated to modulate PLA2 by sequestering Ca2+
    PMID:17543334.
    UniProt summarizes this as "Chelates calciums ions and may inhibit the activity of
    calcium-dependent phospholipase A2." Too tentative for a confident MF assignment.
  • Biological function genuinely unknown. Proposed (unproven) roles: skin
    protection (uteroglobin analogy) or transport of lipids/steroids/hormones/
    pheromones PMID:29643919.

Localization / expression

  • Secreted protein [UniProt SUBCELLULAR LOCATION: Secreted].
  • Produced mainly in sebaceous glands (not saliva, as once thought); also in
    saliva, anal glands, skin and fur
    PMID:29643919.
  • → "extracellular region" (GO:0005576) annotations are appropriate.

Allergen biology (context, not core evolved function)

  • Major cat allergen; >80–95% of cat-allergic patients have anti-Fel d 1 IgE;
    accounts for 60–90% of cat allergenic activity; binds IgE; drives Th2 responses;
    recognized by the mannose receptor on antigen-presenting cells
    PMID:29643919.
  • IgE binding (GO:0019863) is a real molecular activity but reflects allergenicity
    toward humans, not the protein's endogenous evolved function in cat.

Curation decisions (existing GOA annotations)

  1. GO:0005496 steroid binding (IBA, GO_REF:0000033) → MARK_AS_OVER_ANNOTATED
    (family inference; no direct evidence Fel d 1 binds steroids).
  2. GO:0005576 extracellular region (IBA, GO_REF:0000033) → ACCEPT (secreted).
  3. GO:0005496 steroid binding (IEA, GO_REF:0000118) → MARK_AS_OVER_ANNOTATED.
  4. GO:0005576 extracellular region (IEA, GO_REF:0000120) → ACCEPT (secreted).

core_functions: documented MF = calcium ion binding (GO:0005509) in the
extracellular region; in vivo biological role recorded as a knowledge gap.

Key references

  • PMID:29643919 — review (curation target), full text in PMC.
  • PMID:12851385 — monomeric crystal structure; secretoglobin/uteroglobin fold; cavity.
  • PMID:17543334 — tetramer crystal structure; two Ca2+-binding sites; PLA2 speculation.
  • PMID:1946388 — Fel d 1 protein sequence + cDNA cloning.
  • PMID:1572548 — genomic structure of the two Fel d 1 genes.
  • PMID:1712068 — biochemical structure of the major cat allergen.
  • PMID:6747135 — Cat allergen 1 biochemical/antigenic/allergenic properties.

Update: Falcon deep research integration (CH1-deep-research-falcon.md)

Ran FutureHouse Falcon deep research (34 citations). Key additions beyond the
original PMC5891966-based review, with primary sources fetched and verified:

New molecular activity — LPS binding + TLR4/TLR2 enhancement (experimental)

Herre et al. 2013 (PMID:23878318, full text in PMC) is the best-characterized
molecular activity of Fel d 1:
- "Fel d 1 substantially enhances signaling through the innate receptors TLR4 and TLR2"
PMID:23878318.
- Fel d 1 binds the TLR4 agonist LPS, acting via a lipid-transfer mechanism (CD14/MD2-
dependent) rather than by mimicking MD2
PMID:23878318.
→ Supports NEW annotations: lipopolysaccharide binding (GO:0001530) and positive
regulation of toll-like receptor 4 signaling pathway (GO:0034145). This is shown
largely in the context of human allergic sensitization/innate amplification, not
confirmed cat physiology, but it is a bona fide molecular activity of the protein.

Ligand binding revises the "steroid binding" assessment

Popescu et al. 2021 (PMID:34026578) reports that Fel d 1 binds fatty acids and
steroids with good affinity:
- PMID:34026578
(lauric acid; androsterone — a volatile steroid pheromone).
→ The IBA/IEA "steroid binding" (GO:0005496) is therefore NOT merely a baseless
family inference; there is reported ligand-binding evidence. Revised from
MARK_AS_OVER_ANNOTATED to KEEP_AS_NON_CORE: a real activity, plausibly part of a
broader small-hydrophobic-ligand binding / transport (pheromone) role, but not the
single core function. Fatty acid binding (lauric acid) is also reported.

Native function still unknown; expression is androgen-regulated

  • Native biological function in the cat remains formally unestablished
    (Grönlund 2010 PMID:19844127; Popescu 2021); proposed roles: lipid/steroid (pheromone)
    transport, chemical communication, epithelial/skin defense, immunoregulation.
  • Production is testosterone-regulated (intact males >> neutered); secreted by
    sebaceous, salivary, lacrimal glands and anal sacs.
  • Secretoglobin superfamily binds retinoids, phospholipids, steroids and calcium
    (Karn & Laukaitis 2025); no SCGB has a firmly established function.

Newly added/verified references

  • PMID:23878318 — Herre 2013, Fel d 1 enhances TLR activation by lipid ligands (PMC full text).
  • PMID:19844127 — Grönlund 2010, major cat allergen Fel d 1 in diagnosis and therapy (review).
  • PMID:34026578 — Popescu 2021, molecular diagnosis in cat allergy (review; ligand data).
  • file:FELCA/CH1/CH1-deep-research-falcon.md — Falcon deep research report.

📄 View Raw YAML

 
id: P30438
gene_symbol: CH1
product_type: PROTEIN
status: DRAFT
taxon:
  id: NCBITaxon:9685
  label: Felis catus
description: >-
  Chain 1 (alpha chain) of the major cat allergen Fel d 1, a secreted
  secretoglobin (uteroglobin-family) glycoprotein of the domestic cat. The
  mature 70-residue chain 1 pairs with chain 2 (CH2) through three interchain
  disulfide bonds to form a heterodimer; two heterodimers associate
  non-covalently into the ~35-38 kDa Fel d 1 heterotetramer. The all-alpha
  secretoglobin fold closely resembles uteroglobin and encloses an internal
  hydrophobic cavity; the tetramer binds calcium ions at defined sites and binds
  small hydrophobic ligands, including fatty acids and steroids (e.g. lauric acid
  and the steroid pheromone androsterone). Fel d 1 also binds 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, anal glands, skin and fur, with
  production regulated by testosterone. Its endogenous biological function in the
  cat is unresolved, with proposed roles in transport/presentation of lipophilic
  signaling molecules (pheromones), epithelial protection and immunoregulation. It
  is the dominant cat allergen, recognized by IgE in the great majority of
  cat-allergic people.
alternative_products:
- name: 1 (Leader A, Major)
  id: P30438-1
- name: 2 (Leader B, Minor)
  id: P30438-2
  sequence_note: VSP_015665
existing_annotations:
- term:
    id: GO:0005496
    label: steroid binding
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: enables
  review:
    summary: >-
      Phylogenetic (IBA) inference of steroid binding from the secretoglobin/
      uteroglobin family. This is corroborated by ligand-binding data: Fel d 1 is
      reported to bind fatty acids and steroids with good affinity (best ligands
      lauric acid and androsterone, a volatile steroid pheromone), consistent
      with the internal hydrophobic cavity of the secretoglobin fold. The term is
      therefore a genuine activity, but likely one facet of a broader small-
      hydrophobic-ligand binding / transport role rather than the single core
      function, so it is retained as non-core.
    action: KEEP_AS_NON_CORE
    reason: >-
      Beyond the family-level inference, Fel d 1 has reported affinity for
      steroids (androsterone) and fatty acids (lauric acid), so steroid binding
      is supported rather than spurious. It is kept as non-core because it
      represents one of several ligand-binding activities and the physiological
      role of this binding (e.g. pheromone transport) is not established.
    supported_by:
    - reference_id: PMID:34026578
      supporting_text: binding with good affinity to some fatty acids and steroids, the best ligands being lauric acid
    - reference_id: PMID:12851385
      supporting_text: Fel d 1 that could bind an endogenous ligand
- term:
    id: GO:0005576
    label: extracellular region
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: is_active_in
  review:
    summary: >-
      Fel d 1 is a secreted protein, consistent with the extracellular region
      annotation. It is produced mainly by sebaceous glands and is also found in
      saliva, anal glands, skin and fur, and is ubiquitous in cat-inhabited
      environments.
    action: ACCEPT
    reason: >-
      The secretoglobin Fel d 1 is experimentally established as a secreted
      protein, so localization to the extracellular region is well supported.
    supported_by:
    - reference_id: PMID:29643919
      supporting_text: It is now recognized that the sebaceous glands, and not saliva, are the main production site
- term:
    id: GO:0005496
    label: steroid binding
  evidence_type: IEA
  original_reference_id: GO_REF:0000118
  qualifier: enables
  review:
    summary: >-
      TreeGrafter (IEA) duplicate of the steroid-binding inference. As for the IBA
      annotation, this is corroborated by reported binding of Fel d 1 to steroids
      and fatty acids, so it is retained as a non-core ligand-binding activity.
    action: KEEP_AS_NON_CORE
    reason: >-
      Steroid binding is supported by ligand-binding data (androsterone, lauric
      acid) in addition to the family inference, but is one of several ligand-
      binding activities and not the single core function.
    supported_by:
    - reference_id: PMID:34026578
      supporting_text: binding with good affinity to some fatty acids and steroids, the best ligands being lauric acid
- term:
    id: GO:0005576
    label: extracellular region
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: located_in
  review:
    summary: >-
      Automated subcellular-location annotation (UniProt "Secreted") consistent
      with the experimentally established secretion of Fel d 1.
    action: ACCEPT
    reason: >-
      Fel d 1 is a secreted secretoglobin; the extracellular region location is
      correct and corroborated by experimental localization to saliva and
      sebaceous gland secretions.
    supported_by:
    - reference_id: PMID:29643919
      supporting_text: It is now recognized that the sebaceous glands, and not saliva, are the main production site
- term:
    id: GO:0005509
    label: calcium ion binding
  evidence_type: IDA
  original_reference_id: PMID:17543334
  qualifier: enables
  review:
    summary: >-
      NEW (proposed). The crystal structure of the tetrameric form of Fel d 1
      resolves two distinct calcium-binding sites, and UniProt records three
      calcium-coordinating residues (68, 71, 74) in chain 1. Calcium binding is
      the only ligand-binding activity experimentally demonstrated for Fel d 1
      and is not currently captured in the GOA annotations.
    action: NEW
    reason: >-
      Direct structural evidence supports calcium ion binding by the Fel d 1
      tetramer, with chain 1 contributing calcium-coordinating residues. This is
      a better-supported molecular function than the family-propagated steroid
      binding term and should be added.
    supported_by:
    - reference_id: PMID:17543334
      supporting_text: structure of tetrameric Fel d 1 reveals two different calcium-binding sites
    - reference_id: file:FELCA/CH1/CH1-uniprot.txt
      supporting_text: Chelates calciums ions and may inhibit the activity of
- term:
    id: GO:0001530
    label: lipopolysaccharide binding
  evidence_type: IDA
  original_reference_id: PMID:23878318
  qualifier: enables
  review:
    summary: >-
      NEW (proposed). Fel d 1 directly binds the TLR4 agonist lipopolysaccharide
      (LPS) and is proposed to act via a lipid-transfer mechanism (CD14/MD2-
      dependent), enhancing innate immune signaling. This is the best-characterized
      molecular activity of the protein and is not captured in GOA.
    action: NEW
    reason: >-
      Direct experimental evidence (Herre et al. 2013) shows Fel d 1 binds LPS,
      explaining its ability to enhance TLR4/TLR2 signaling. Largely demonstrated
      in the context of human innate-immune amplification/allergenicity rather
      than confirmed cat physiology, but it is a bona fide molecular function.
    supported_by:
    - reference_id: PMID:23878318
      supporting_text: bind to the TLR4 agonist LPS
    - reference_id: file:FELCA/CH1/CH1-deep-research-falcon.md
      supporting_text: facilitating lipid transfer to CD14 and the TLR signaling complex
- term:
    id: GO:0034145
    label: positive regulation of toll-like receptor 4 signaling pathway
  evidence_type: IDA
  original_reference_id: PMID:23878318
  qualifier: involved_in
  review:
    summary: >-
      NEW (proposed). By binding LPS and transferring it to the TLR4 receptor
      complex, Fel d 1 substantially enhances signaling through TLR4 (and TLR2),
      potentiating innate immune (TNF-alpha) responses to bacterial lipid ligands.
    action: NEW
    reason: >-
      Herre et al. 2013 showed Fel d 1 enhances TLR4/TLR2 signaling in response to
      lipid agonists. This immunomodulatory activity is the proposed basis of its
      allergenicity; well supported as a molecular activity though distinct from
      the cat's own (unknown) physiology.
    supported_by:
    - reference_id: PMID:23878318
      supporting_text: enhances signaling through the innate receptors TLR4 and TLR2
core_functions:
- description: >-
    Calcium-binding subunit of the secreted Fel d 1 secretoglobin heterotetramer.
    Chain 1 (this protein) dimerizes with chain 2 via interchain disulfide bonds;
    the assembled tetramer binds calcium ions at defined sites and functions in
    the extracellular space. The endogenous physiological role of the complex is
    not established.
  molecular_function:
    id: GO:0005509
    label: calcium ion binding
  supported_by:
  - reference_id: PMID:17543334
    supporting_text: structure of tetrameric Fel d 1 reveals two different calcium-binding sites
  - reference_id: file:FELCA/CH1/CH1-uniprot.txt
    supporting_text: Chelates calciums ions and may inhibit the activity of
  locations:
  - id: GO:0005576
    label: extracellular region
  knowledge_gaps:
  - gap_statement: >-
      The endogenous biological function of Fel d 1 is undetermined: it is not
      known whether the protein acts in skin/epithelial barrier protection, in
      transport of a lipophilic ligand, or in another role.
    boundary: >-
      Established: Fel d 1 is a secreted secretoglobin heterotetramer with a
      uteroglobin-like fold, calcium-binding sites, and an internal amphipathic
      cavity. Unknown: the physiological process it serves in the cat.
    gap_kind:
    - BIOLOGY
    provenance:
    - reference_id: PMID:29643919
      supporting_text: The biological function of Fel d 1 is still unknown
  - gap_statement: >-
      The physiological ligand and the in-cat role of Fel d 1's ligand binding are
      unconfirmed. Candidate ligands (lauric acid, androsterone) and LPS binding
      are reported in vitro, but which ligand the protein carries in vivo, and to
      what biological end, is not established.
    boundary: >-
      Established: the secretoglobin cavity binds small hydrophobic ligands and
      Fel d 1 binds fatty acids, steroids and LPS in vitro. Unknown: the
      physiological ligand/cargo and the biological process this serves in the cat.
    gap_kind:
    - BIOLOGY
    provenance:
    - reference_id: PMID:12851385
      supporting_text: Fel d 1 that could bind an endogenous ligand
- description: >-
    Immunomodulatory lipid-binding activity. Fel d 1 directly binds bacterial
    lipopolysaccharide (LPS) and, via a CD14/MD2-dependent lipid-transfer
    mechanism, enhances signaling through the innate immune receptors TLR4 and
    TLR2, potentiating pro-inflammatory responses to lipid ligands. This is the
    best-characterized molecular activity of the protein, demonstrated chiefly in
    the context of human innate-immune amplification and allergic sensitization.
  molecular_function:
    id: GO:0001530
    label: lipopolysaccharide binding
  directly_involved_in:
  - id: GO:0034145
    label: positive regulation of toll-like receptor 4 signaling pathway
  supported_by:
  - reference_id: PMID:23878318
    supporting_text: bind to the TLR4 agonist LPS
  - reference_id: PMID:23878318
    supporting_text: enhances signaling through the innate receptors TLR4 and TLR2
  locations:
  - id: GO:0005576
    label: extracellular region
proposed_new_terms: []
suggested_questions:
- question: >-
    What is the endogenous amphipathic ligand (if any) that occupies the internal
    cavity of the Fel d 1 tetramer, and does chain 1 contribute to its binding?
  experts: []
- question: >-
    Does Fel d 1 modulate calcium-dependent phospholipase A2 activity in vivo, as
    has been speculated by analogy to uteroglobin?
  experts: []
- question: >-
    Is the physiological role of Fel d 1 in the cat one of skin/epithelial barrier
    protection, lipid/pheromone transport, or something else?
  experts: []
suggested_experiments:
- hypothesis: >-
    The Fel d 1 internal cavity binds a specific endogenous amphipathic ligand
    (e.g. a steroid, fatty acid, or pheromone).
  description: >-
    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.
  experiment_type: ligand identification / biophysical binding assay
- hypothesis: >-
    Calcium binding by the Fel d 1 tetramer enables sequestration that inhibits
    calcium-dependent phospholipase A2 activity.
  description: >-
    Reconstitute a calcium-dependent PLA2 activity assay in the presence and
    absence of folded recombinant Fel d 1 (and Ca2+-binding-site mutants) to test
    for PLA2 inhibition and its calcium dependence.
  experiment_type: enzymatic / biochemical assay
references:
- id: GO_REF:0000033
  title: Annotation inferences using phylogenetic trees
  findings: []
- id: GO_REF:0000118
  title: TreeGrafter-generated GO annotations
  findings: []
- id: GO_REF:0000120
  title: Combined Automated Annotation using Multiple IEA Methods
  findings: []
- id: PMID:29643919
  title: 'An update on molecular cat allergens: Fel d 1 and what else? Chapter 1: Fel d 1, the major cat allergen.'
  findings:
  - statement: >-
      Fel d 1 is a secreted secretoglobin (not a lipocalin) and the major cat
      allergen; its endogenous biological function is still unknown.
    supporting_text: The biological function of Fel d 1 is still unknown
  - statement: Sebaceous glands, rather than saliva, are the main production site of Fel d 1.
    supporting_text: It is now recognized that the sebaceous glands, and not saliva, are the main production site
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: >-
      PMC full text reviewed; this is the curation-target review and accurately
      summarizes Fel d 1 structure, secretoglobin family membership, expression
      and unknown function.
- id: PMID:12851385
  title: The crystal structure of the major cat allergen Fel d 1, a member of the secretoglobin family.
  findings:
  - statement: >-
      Fel d 1 is an all-alpha secretoglobin structurally similar to uteroglobin,
      with an internal asymmetric cavity that could bind an amphipathic endogenous
      ligand.
    supporting_text: Fel d 1 that could bind an endogenous ligand
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: >-
      Abstract-only cache; establishes the secretoglobin/uteroglobin fold and the
      internal ligand-binding cavity. Used to argue steroid binding is an
      unverified family-level inference.
- id: PMID:17543334
  title: Structural characterization of the tetrameric form of the major cat allergen Fel d 1.
  findings:
  - statement: >-
      The Fel d 1 (1+2) tetramer crystal structure reveals two distinct
      calcium-binding sites; PLA2 modulation via calcium sequestration is
      speculated by analogy to uteroglobin.
    supporting_text: structure of tetrameric Fel d 1 reveals two different calcium-binding sites
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: >-
      Abstract-only cache; primary evidence for calcium ion binding by the Fel d 1
      tetramer (core_functions). PLA2 inhibition is explicitly framed as
      speculation.
- id: PMID:23878318
  title: 'Allergens as immunomodulatory proteins: the cat dander protein Fel d 1 enhances TLR activation by lipid ligands.'
  findings:
  - statement: >-
      Fel d 1 binds the TLR4 agonist LPS and substantially enhances signaling
      through TLR4 and TLR2 via a lipid-transfer (CD14/MD2-dependent) mechanism.
    supporting_text: bind to the TLR4 agonist LPS
  - statement: Fel d 1 enhances TLR4/TLR2 innate immune signaling in response to lipid ligands.
    supporting_text: enhances signaling through the innate receptors TLR4 and TLR2
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: >-
      PMC full text reviewed; primary experimental evidence for LPS binding and
      enhancement of TLR4/TLR2 signaling. Surfaced by Falcon deep research.
- id: PMID:34026578
  title: Molecular diagnosis in cat allergy.
  findings:
  - statement: >-
      Fel d 1 binds fatty acids and steroids with good affinity, the best ligands
      being lauric acid and androsterone (a volatile steroid pheromone).
    supporting_text: binding with good affinity to some fatty acids and steroids, the best ligands being lauric acid
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: >-
      PMC full-text review; supports steroid/fatty-acid ligand binding and the
      pheromone-transport hypothesis. Surfaced by Falcon deep research.
- id: PMID:19844127
  title: The major cat allergen, Fel d 1, in diagnosis and therapy.
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: >-
      Authoritative review of Fel d 1 structure (uteroglobin fold, calcium sites,
      hydrophobic cavity), expression and therapy. Surfaced by Falcon deep research.
- id: file:FELCA/CH1/CH1-deep-research-falcon.md
  title: Falcon deep research report for CH1 (Fel d 1 chain 1)
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: >-
      FutureHouse Falcon deep research (34 citations). Claims were verified against
      the cited primary literature (Herre 2013, Popescu 2021, Gronlund 2010) before
      use; treated as a pointer to those sources, which carry the supporting_text.