Fel d 1

The major allergen in cats, Fel d 1, belongs to the secretoglobin family of proteins and is, worldwide, one of the major causes of allergic asthma, as it induces IgE responses in 90 to 95% of those allergic to cats and in 60 to 90% of total allergenic activity to cat hair. Symptoms range from mild rhinitis and conjunctivitis to life-threatening asthmatic responses. The structure of Fel d 1 displays the location of three previously defined Fel d 1 IgE epitopes on the surface of the protein ^[1].

It is produced mainly by the salivary and sebaceous glands, it is also present in the perianal, lacrimal and squamous epithelial cells and is attached to the cat's hair through the habit of licking ^[1][2].

Function

The biological function of Fel d 1 is still unknown, however it is thought to have a pheromone/chemical signaling function^[2].

Structure

Fel d 1 is a tetrameric glycoprotein of 35 to 39 kDa, by size exclusion chromatography, formed by two identical heterodimers of about 18 kDa, noncovalently linked. These heterodimers are totally α-helical, formed by 8 helices, H1-H4 and H5-H8, corresponding to the 2 and 1 chains, respectively^[1][3].

Chains 1 and 2 are two antiparallel polypeptides, linked via 3 interchain disulfide bonds, formed between cysteine residues at positions Cys3-Cys73, Cys44-Cys48, and Cys70-Cys7, at chains 1 and 2, respectively, in chains 1 and 2, respectively^[1][3].

Chain 1 has about 8kDa, composed of a residue of 70 amino acids and chain 2 has about 10 kDa, which can be composed of a residue of 90 amino acids, found preferably in the sebaceous glands, or composed of a residue of 92 amino acids, which is expressed by the salivary glands. Its glycan portion is found in chain 2 and the recombinant structure of Fel d 1 reveals that the N33 residue is located in the loop connecting the H2 and H3 helices and that the side chain is exposed to the solvent^[1][3].

Figure 1.General structure of a Fel d 1 monomer, shown in two distinct orientations, rotated 90° about the vertical axis.

In the Fel d 1 tetramer, three Ca²⁺ binding sites were identified [4], as in Figure 2, where the Ca²⁺ are indicated as red balls. Two Ca²⁺ binding sites are equivalent and are found symmetrically located on either side of the dimer and the third is found within the dimerization interface (Figure 2 (a) and (b)). The equivalent Ca²⁺ binds to the carbonyl groups of residues Asp46 and Met49, as well as to four and three water molecules in the A and B subunits, respectively (Figure 2 (c)) and the Ca²⁺ located at the dimerization interface binds to the OD1 atoms of residues Asn89 (in subunit A), Asn89 (B) and Asp130 (B), as well as to the carbonyl group of residue Ile125 (A) and three molecules of water (Figure 2 (d))^[4].

References

1. ↑ ^{1.0 1.1 1.2 1.3 1.4 1.5} KAISER, Liselotte; GRÖNLUND, Hans; SANDALOVA, Tatyana; LJUNGGREN, Hans-Gustaf; HAGE-HAMSTEN, Marianne van; ACHOUR, Adnane; SCHNEIDER, Gunter. The Crystal Structure of the Major Cat Allergen Fel d 1, a Member of the Secretoglobin Family. The Journal of Biological Chemistry, [s. l.], v. 278, n. 39, p. 37730–37735, 2003. doi: https://doi.org/10.1074/jbc.M304740200.
2. ↑ ^{2.0 2.1} WANDALSEN, Gustavo Falbo; SANO, Flavio; SOLÉ, Dirceu. Alérgenos do gato nas alergias respiratórias: situação atual e novas perspectivas. Asma, alergia e imunologia, São Paulo, SP, Brasil, v. 4, n. 1, p. 61-71, 10 mar. 2020. doi: http://dx.doi.org/10.5935/2526-5393.20200004.
3. ↑ ^{3.0 3.1 3.2} GRÖNLUND H, Saarne T, Gafvelin G, van Hage M: The Major Cat Allergen, Fel d 1, in Diagnosis and Therapy. Int Arch Allergy Immunol 2010;151:265-274. doi: 10.1159/000250435.
4. ↑ KAISER, Liselotte; VELICKOVIC, Tanja Cirkovic; BADIA-MARTINEZ, Daniel; ADEDOYIN, Justus; THUNBERG, Sarah; HALLÉN, Dan; BERNDT, Kurt; GRÖNLUND, Hans; GAFVELIN, Guro; HAGE, Marianne van; ACHOUR, Adnane. Structural Characterization of the Tetrameric form of the Major Cat Allergen Fel d 1. Journal of Molecular Biology, [s. l.], v. 370, ed. 4, p. 714-727, 2007. doi: https://doi.org/10.1016/j.jmb.2007.04.074.