thaumatin and allergenic/antifungal thaumatin-like proteins: plant homologs
This subfamily is represented by the sweet-tasting protein thaumatin from the African berry Thaumatococcus daniellii, allergenic/antifungal Thaumatin-like proteins (TLPs), and related plant proteins. TLPs are involved in host defense and a wide range of developmental processes in fungi, plants, and animals. Plant TLPs are classified as pathogenesis-related (PR) protein family 5 (PR5), their expression is induced by environmental stresses such as pathogen/pest attack, drought and cold. TLPs in this subfamily include such proteins as zeamatin, found in high concentrations in cereal seeds, and osmotin, a salt-induced protein in osmotically stressed plants. Several members of the plant TLP family have been reported as food allergens from fruits (i.e., cherry, Pru av 2; bell pepper, Cap a1; tomatoes, Lyc e NP24) and pollen allergens from conifers (i.e., mountain cedar, Jun a 3; Arizona cypress, Cup a3; Japanese cedar, Cry j3). Thaumatin and TLPs are three-domain, crescent-fold structures with either an electronegative, electropositive, or neutral cleft occurring between domains I and II. It has been proposed that the antifungal activity of plant PR5 proteins relies on the strong electronegative character of this cleft. IgE-binding epitopes of mountain Cedar (Juniperus ashei) allergen Jun a 3, which interact with pooled IgE from patients suffering allergenic response to this allergen, were mainly located on the helical domain II; the best-conserved IgE-binding epitope predicted for TLPs corresponds to this region. Some TLPs hydrolyze the beta-1,3-glucans of the type commonly found in fungal walls. Most TLPs contain 16 conserved Cys residues. A deletion within the third domain (domain II) of the Triticum aestivum thaumatin-like xylanase inhibitor is observed, thus, only 10 conserved Cys residues are present within this smaller TLP and similar homologs.
Comment:Thaumatin-like proteins are crescent-fold structures with either an electronegative, electropositive, or neutral cleft occurring between domains I and II.
Comment:It has been proposed that the antifungal activity of plant PR5 proteins (pathogenesis-related protein family 5) relies on the strong electronegative character of the cleft with conserved cleft residues: Arg, Glu, Asp, and Asp.
Comment:Sweet-tasting protein thaumatin has an electropositively charged cleft (with conserved cleft residues Lys, Glu, Asp, and Lys, instead of Arg, Glu, Asp, and Asp) and lacks antifungal activity.
Jiyao W et al.(2023). "The conserved domain database in 2023.",