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Structural characteristics of lipocalin allergens: Crystal structure of the immunogenic dog allergen Can f 6.

ABSTRACT: Lipocalins represent the most important protein family of the mammalian respiratory allergens. Four of the seven named dog allergens are lipocalins: Can f 1, Can f 2, Can f 4, and Can f 6. We present the structure of Can f 6 along with data on the biophysical and biological activity of this protein in comparison with other animal lipocalins. The Can f 6 structure displays the classic lipocalin calyx-shaped ligand binding cavity within a central ?-barrel similar to other lipocalins. Despite low sequence identity between the different dog lipocalin proteins, there is a high degree of structural similarity. On the other hand, Can f 6 has a similar primary sequence to cat, horse, mouse lipocalins as well as a structure that may underlie their cross reactivity. Interestingly, the entrance to the ligand binding pocket is capped by a His instead of the usually seen Tyr that may help select its natural ligand binding partner. Our highly pure recombinant Can f 6 is able to bind to human IgE (hIgE) demonstrating biological antigenicity.

SUBMITTER: Clayton GM

PROVIDER: S-EPMC6746357 | biostudies-literature | 2019

REPOSITORIES: biostudies-literature

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Structural characteristics of lipocalin allergens: Crystal structure of the immunogenic dog allergen Can f 6.

Clayton Gina M GM White Janice J Lee Schuyler S Kappler John W JW Chan Sanny K SK

PloS one 20190916 9

Lipocalins represent the most important protein family of the mammalian respiratory allergens. Four of the seven named dog allergens are lipocalins: Can f 1, Can f 2, Can f 4, and Can f 6. We present the structure of Can f 6 along with data on the biophysical and biological activity of this protein in comparison with other animal lipocalins. The Can f 6 structure displays the classic lipocalin calyx-shaped ligand binding cavity within a central β-barrel similar to other lipocalins. Despite low s ...[more]

PMID: 31525203

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Project description:Calcium oxalate raphide crystals are found in bundles in intravacuolar membrane chambers of specialized idioblasts cells of most plant families including many crop plants and are found in most tissues. Raphides are needle-shaped, often with barbed ends, and their morphology and biomineralization pattern have been a focus of much research. In the family Araceae, raphides have been proposed to cause acridity in crops such as taro (Colocasia esculenta (L.) Schott). Acridity is irritation that causes swelling of lips, mouth and throat, as well as itchiness and pain when raw or insufficiently cooked tissues are eaten; this response varies among taro consumers. Since raphides in some other foods do not cause acridity, and since acridity can be inactivated by cooking and/or protease treatment, it is possible that a toxin or allergen-like compound is associated with the crystals. Using two-dimensional (2D) gel electrophoresis and mass spectrometry (MS) peptide sequencing of selected peptides from purified raphides and the results from taro apex transcriptome sequencing, we have shown that profilins, known allergens, are present on raphides. One of the five raphide profilins genes was highly expressed in the apex and had a 17-amino-acid insert that significantly increased that profilin’s epitope peak. This insert was predicted to be coiled and exposed on the surface of the folded peptide. A second profilin had a 2-amino-acid insert that also had a greater B-cell epitope prediction. The taro profilins showed 83 to 92% similarity to known characterized profilins while other raphide peptides showed greater than 90% similarity to other higher plants. The presence on the raphides of peptides normally associated with mitochrondria (ATP synthase), chloroplasts (chaperonin cpn 60 kDa), cytoplasm (actin, profilin) and vacuole (V-type ATPase) indicates a multistage biocrystallation process ending with possible invagination of the tonoplast and addition of mucilage that may be derived from the Golgi. Actin might play a crucial role in the generation of the needle-like raphides. We also showed that commercial allergen test strips for hazelnuts, where profilin is a secondary allergen, have potential for screening in a plant breeding program to reduce acridity and during food processing to avoid over-cooking.

Project description:A major part of important mammalian respiratory allergens belongs to the lipocalin family of proteins. By this time, 19 respiratory mammalian lipocalin allergens have been registered in the WHO/IUIS Allergen Nomenclature Database. Originally, lipocalins, small extracellular proteins (molecular mass ca. 20 kDa), were characterized as transport proteins but they are currently known to exert a variety of biological functions. The three-dimensional structure of lipocalins is well-preserved, and lipocalin allergens can exhibit high amino acid identities, in several cases more than 50%. Lipocalins contain an internal ligand-binding site where they can harbor small principally hydrophobic molecules. Another characteristic feature is their capacity to bind to specific cell-surface receptors. In all, the physicochemical properties of lipocalin allergens do not offer any straightforward explanations for their allergenicity. Allergic sensitization begins at epithelial barriers where diverse insults through pattern recognition receptors awaken innate immunity. This front-line response is manifested by epithelial barrier-associated cytokines which together with other components of immunity can initiate the sensitization process. In the following, the crucial factor in allergic sensitization is interleukin (IL)-4 which is needed for stabilizing and promoting the type 2 immune response. The source for IL-4 has been searched widely. Candidates for it may be non-professional antigen-presenting cells, such as basophils or mast cells, as well as CD4+ T cells. The synthesis of IL-4 by CD4+ T cells requires T cell receptor engagement, i.e., the recognition of allergen peptides, which also provides the specificity for sensitization. Lipocalin and innate immunity-associated cell-surface receptors are implicated in facilitating the access of lipocalin allergens into the immune system. However, the significance of this for allergic sensitization is unclear, as the recognition by these receptors has been found to produce conflicting results. As to potential adjuvants associated with mammalian lipocalin allergens, the hydrophobic ligands transported by lipocalins have not been reported to enhance sensitization while it is justified to suppose that lipopolysaccharide plays a role in it. Taken together, type 2 immunity to lipocalin allergens appears to be a harmful immune response resulting from a combination of signals involving both the innate and adaptive immunities.

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Structural characteristics of lipocalin allergens: Crystal structure of the immunogenic dog allergen Can f 6.

Publications

Structural characteristics of lipocalin allergens: Crystal structure of the immunogenic dog allergen Can f 6.

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