Solution structure of the strawberry allergen Fra a 1

Allergenicity and structural properties of new Cor a 1 isoallergens from hazel identified in different plant tissues

The hazel allergen Cor a 1 is a PR-10 protein, closely related to the major birch pollen allergen Bet v 1. Hazel allergies are caused by cross-reactive IgE antibodies originally directed against Bet v 1. Despite the importance of PR-10 proteins in allergy development, their function and localization in the plant remain largely elusive. Therefore, the presence of Cor a 1 mRNA and proteins was investigated in different tissues, i.e., the female flower, immature and mature nuts, catkins, and pollen. Four yet unknown Cor a 1 isoallergens, i.e., Cor a 1.0501-1.0801, and one new Cor a 1.03 variant were discovered and characterized. Depending on the isoallergen, the occurrence and level of mRNA expression varied in different tissues, suggesting different functions. Interestingly, Cor a 1.04 previously thought to be only present in nuts, was also detected in catkins and pollen. The corresponding Cor a 1 genes were expressed in Escherichia coli. The purified proteins were analysed by CD and NMR spectroscopy. Immunoblots and ELISAs to determine their allergenic potential showed that the new proteins reacted positively with sera from patients allergic to birch, hazel and elder pollen and were recognized as novel isoallergens/variants by the WHO/IUIS Allergen Nomenclature Sub-Committee.

Expression and purification of 15N-labeled Fra a 1, a strawberry allergen, to prepare samples for NMR measurements

Raw strawberries contain allergens that cause oral allergic syndrome. Fra a 1 is one of the major allergens in strawberries and might decrease their allergenicity by heating, likely due to structural changes in the allergen leading to decreased recognition of the allergens in the oral cavity. In the present study, to understand the relationship between allergen structure and allergenicity, the expression and purification of 15N-labeled Fra a 1 were examined and the sample was used for NMR analysis. Two isoforms, Fra a 1.01 and Fra a 1.02, were used and expressed in E. coli BL21(DE3) in M9 minimal medium. Fra a 1.02 was purified as a single protein by using the GST tag approach, whereas histidine×6-tag (his6-tag) Fra a 1.02 was obtained both as the full-length (∼20 kDa) and a truncated (∼18 kDa) form. On the other hand, his6-tag Fra a 1.01 was purified as a homogeneous protein. ¹⁵N-labeled HSQC NMR spectra suggested that Fra a 1.02 was thermally denatured at lower temperatures than Fra a 1.01, despite the high amino acid sequence homology (79.4%) of these isoforms. Furthermore, the samples in the present study allowed us to analyze ligand binding that probably affects structural stability. In conclusion, GST tag was effective for obtaining a homogeneous protein when his6-tag failed to give a single form, and the present study provided a sample that could be used for NMR studies of the details of the allergenicity and structure of Fra a 1.

Review: Biological functions of major latex-like proteins in plants

Major latex-like proteins (MLPs) have been identified in dicots and monocots. They are members of the birch pollen allergen Bet v 1 family as well as pathogenesis-related proteins class 10. MLPs have two main features. One is binding affinity toward various hydrophobic compounds, such as long-chain fatty acids, steroids, and systemic acquired resistance signals, via its internal hydrophobic cavity or hydrophobic residues on its surface. MLPs transport such compounds to other organs via phloem and xylem vessels and contribute to the expression of physiologically important ligands' activity in the particular organs. The second feature is responses to abiotic and biotic stresses. MLPs are involved in drought and salt tolerance through the mediation of plant hormone signaling pathways. MLPs generate resistance against pathogens by the induction of pathogenesis-related protein genes. Therefore, MLPs play crucial roles in drought and salt tolerance and resistance against pathogens. However, knowledge of MLPs is fragmented, and an overview of them is needed. Herein, we summarize the current knowledge of the biological functions of MLPs, which to our knowledge, is the first review about MLPs that has been reported.

Structural Bases for the Allergenicity of Fra a 1.02 in Strawberry Fruits

Although strawberries are highly appreciated fruits, their intake can induce allergic reactions in atopic patients. These reactions can be due to the patient's previous sensitization to the major birch pollen allergen Bet v 1, by which IgE generated in response to Bet v 1 cross-reacts with the structurally related strawberry Fra a 1 protein family. Fra a 1.02 is the most expressed paralog in ripe strawberries and is highly allergenic. To better understand the molecular mechanisms regulating this allergic response, we have determined the three-dimensional structure of Fra a 1.02 and four site-directed mutants that were designed based on their positions in potential epitopes. Fra a 1.02 and mutants conform to the START fold. We show that the cross-reactivity of all the mutant variants to IgE from patients allergic to Bet v 1 was significantly reduced without altering the conserved structural fold, so that they could potentially be used as hypoallergenic Fra a 1 variants for the generation of vaccines against strawberry allergy in atopic patients.

Identification of a natural ligand of the hazel allergen Cor a 1

Hazelnut is one of the most frequent causes of food allergy. The major hazel allergen in Northern Europe is Cor a 1, which is homologous to the major birch pollen allergen Bet v 1. Both allergens belong to the pathogenesis related class PR-10. We determined the solution structure of Cor a 1.0401 from hazelnut and identified a natural ligand of the protein. The structure reveals the protein fold characteristic for PR-10 family members, which consists of a seven-stranded antiparallel β-sheet, two short α-helices arranged in V-shape and a long C-terminal α-helix encompassing a hydrophobic pocket. However, despite the structural similarities between Cor a 1 and Bet v 1, they bind different ligands. We have shown previously that Bet v 1 binds to quercetin-3-O-sophoroside. Here, we isolated Cor a 1 from hazel pollen and identified the bound ligand, quercetin-3-O-(2"-O-β-D-glucopyranosyl)-β-D-galactopyranoside, by mass spectrometry and nuclear magnetic resonance spectroscopy (NMR). NMR experiments were performed to confirm binding. Remarkably, although it has been shown that PR-10 allergens show promiscuous binding behaviour in vitro, we can demonstrate that Cor a 1.0401 and Bet v 1.0101 exhibit highly selective binding for their specific ligand but not for the respective ligand of the other allergen.

pH and Heat Resistance of the Major Celery Allergen Api g 1

SCOPE

The major celery allergen Api g 1 is a member of the pathogenesis-related 10 class protein family. This study aims to investigate the impact of heat and pH on the native protein conformation required for Immunoglobulin E (IgE) recognition.

METHODS AND RESULTS

Spectroscopic methods, MS and IgE-binding analyses are used to study the effects of pH and thermal treatment on Api g 1.0101. Heat processing results in a loss of the native protein fold via denaturation, oligomerization, and precipitation along with a subsequent reduction of IgE recognition. The induced effects and timescales are strongly pH dependent. While Api g 1 refolds partially into an IgE-binding conformation at physiological pH, acidic pH treatment leads to the formation of structurally heat-resistant, IgE-reactive oligomers. Thermal processing in the presence of a celery matrix or at pH conditions close to the isoelectric point (pI = 4.63) of Api g 1.0101 results in almost instant precipitation.

CONCLUSION

This study demonstrates that Api g 1.0101 is not intrinsically susceptible to heat treatment in vitro. However, the pH and the celery matrix strongly influence the stability of Api g 1.0101 and might be the main reasons for the observed temperature lability of this important food allergen.

Structural basis for cross-reactivity and conformation fluctuation of the major beech pollen allergen Fag s 1

Fag s 1 is a member of the Pathogen Related protein family 10 (PR-10) and can elicit cross-reaction with IgE antibodies produced against the birch pollen allergen Bet v 1. The Nuclear Magnetic Resonance (NMR) structure of Fag s 1 is presented along with its dynamic properties. It shares 66% identity with Bet v 1 and exhibits the expected three α-helices and seven β-sheets arranged as a semi-beta barrel and exposing the residues mapped as the Bet v 1 IgE epitope. The structural dynamics of Fag s 1 were monitored on the fast and intermediate timescales, using relaxation rates. The complex dynamics of Fag s 1 are closely related to the internal cavity, and they modulate IgE and ligand binding.

Effect of the Strawberry Genotype, Cultivation and Processing on the Fra a 1 Allergen Content

Birch pollen allergic patients show cross-reactivity to vegetables and fruits, including strawberries (Fragaria × ananassa). The objective of this study was to quantify the level of the Fra a 1 protein, a Bet v 1-homologous protein in strawberry fruits by a newly developed ELISA, and determine the effect of genotype, cultivation and food processing on the allergen amount. An indirect competitive ELISA using a specific polyclonal anti-Fra a 1.02 antibody was established and revealed high variability in Fra a 1 levels within 20 different genotypes ranging from 0.67 to 3.97 µg/g fresh weight. Mature fruits of red-, white- and yellow-fruited strawberry cultivars showed similar Fra a 1 concentrations. Compared to fresh strawberries, oven and solar-dried fruits contained slightly lower levels due to thermal treatment during processing. SDS-PAGE and Western blot analysis demonstrated degradation of recombinant Fra a 1.02 after prolonged (>10 min) thermal treatment at 99 °C. In conclusion, the genotype strongly determined the Fra a 1 quantity in strawberries and the color of the mature fruits does not relate to the amount of the PR10-protein. Cultivation conditions (organic and conventional farming) do not affect the Fra a 1 level, and seasonal effects were minor.

Analysis of major paralogs encoding the Fra a 1 allergen based on their organ-specificity in Fragaria × ananassa

Fra a 1 protein in strawberry causes oral allergic syndrome. Over 39 Fra a 1 paralogs have been identified in strawberry genome. Fra a 1.01 is major accumulating protein in edible organs. Strawberry fruits contain allergenic proteins that cause oral allergic syndrome. The hypothesized major allergen is Fra a 1, an ortholog of the birch pollen allergen protein Bet v 1. We organized Fra a 1 genes and analyzed their localizations at the transcriptional and translational levels. In total, 15 new Fra a 1 proteins were identified from the genomic database, increasing the total number of Fra a 1 to 30 proteins encoded by 39 genes. Fra a 1.02 was mostly expressed in receptacles, and Fra a 1.01 in achenes, when analyzed by RNA sequencing. Immunoblotting showed that the Fra a 1.01 protein was broadly accumulated in strawberry organs, while the Fra a 1.02 protein was mostly expressed in receptacles. Recombinant Fra a 1.01 strongly reacted with human IgE. The mRNA and protein expression levels of Fra a 1 did not correlate, indicating the importance of protein levels when evaluating the abundance of allergens in strawberry. Based on the localizations, accumulation levels and reactivity to human IgE, we determined that Fra a 1.01 was the most important allergen, followed by Fra a 1.02, and then other Fra a 1 proteins. The information obtained here will be useful for selecting the target Fra a 1 paralogs when breeding hypoallergenic strawberry.

Fra a 1.02 Is the Most Potent Isoform of the Bet v 1-like Allergen in Strawberry Fruit

The strawberry fruit proteins Fra a 1.01E-1.08 are homologues of the major birch pollen allergen Bet v 1. Three of the proteins are known to have essential biological functions in pigment formation during fruit ripening and seem to be responsible for allergic reactions to strawberry fruit. We evaluated the cross-reactive allergenic potential of these putative strawberry allergens in patients allergic to birch pollen. Activation of basophils of eight atopic patients was studied using different concentrations of Fra a 1 isoforms. Bet v 1a was used as control and as atopic patient selection criterion. Although Fra a 1.01E-1.08 have amino acid sequence identities of 74.5-97.5% with Fra a 1.02, the basophil activation mediated by the eight Fra a 1 proteins differed substantially. Fra a 1.03 and Fra a 1.02 showed the highest activation of basophils, 73 and 66% of total basophils, respectively. On the basis of the high relative expression of the gene Fra a 1.02 in ripe strawberry fruits of allergenic varieties, Fra a 1.02 was identified as the main strawberry allergen of the Bet v 1 superfamily. Knowledge of the allergenic potential of Fra a 1.02/1.03 will help to improve food safety and can serve as a valuable marker for the development of red-fruited hypoallergenic strawberry cultivars.

Structural and bioinformatic analysis of the kiwifruit allergen Act d 11, a member of the family of ripening-related proteins

The allergen Act d 11, also known as kirola, is a 17 kDa protein expressed in large amounts in ripe green and yellow-fleshed kiwifruit. Ten percent of all kiwifruit-allergic individuals produce IgE specific for the protein. Using X-ray crystallography, we determined the first three-dimensional structures of Act d 11, produced from both recombinant expression in Escherichia coli and from the natural source (kiwifruit). While Act d 11 is immunologically correlated with the birch pollen allergen Bet v 1 and other members of the pathogenesis-related protein family 10 (PR-10), it has low sequence similarity to PR-10 proteins. By sequence Act d 11 appears instead to belong to the major latex/ripening-related (MLP/RRP) family, but analysis of the crystal structures shows that Act d 11 has a fold very similar to that of Bet v 1 and other PR-10 related allergens regardless of the low sequence identity. The structures of both the natural and recombinant protein include an unidentified ligand, which is relatively small (about 250 Da by mass spectrometry experiments) and most likely contains an aromatic ring. The ligand-binding cavity in Act d 11 is also significantly smaller than those in PR-10 proteins. The binding of the ligand, which we were not able to unambiguously identify, results in conformational changes in the protein that may have physiological and immunological implications. Interestingly, the residue corresponding to Glu45 in Bet v 1 (Glu46), which is important for IgE binding to the birch pollen allergen, is conserved in Act d 11, even though it is not in other allergens with significantly higher sequence identity to Bet v 1. We suggest that the so-called Gly-rich loop (or P-loop), which is conserved in all PR-10 allergens, may be responsible for IgE cross-reactivity between Bet v 1 and Act d 11.

The strawberry pathogenesis-related 10 (PR-10) Fra a proteins control flavonoid biosynthesis by binding to metabolic intermediates

Pathogenesis-related 10 (PR-10) proteins are involved in many aspects of plant biology but their molecular function is still unclear. They are related by sequence and structural homology to mammalian lipid transport and plant abscisic acid receptor proteins and are predicted to have cavities for ligand binding. Recently, three new members of the PR-10 family, the Fra a proteins, have been identified in strawberry, where they are required for the activity of the flavonoid biosynthesis pathway, which is essential for the development of color and flavor in fruits. Here, we show that Fra a proteins bind natural flavonoids with different selectivity and affinities in the low μm range. The structural analysis of Fra a 1 E and a Fra a 3-catechin complex indicates that loops L3, L5, and L7 surrounding the ligand-binding cavity show significant flexibility in the apo forms but close over the ligand in the Fra a 3-catechin complex. Our findings provide mechanistic insight on the function of Fra a proteins and suggest that PR-10 proteins, which are widespread in plants, may play a role in the control of secondary metabolic pathways by binding to metabolic intermediates.

Purification, crystallization and preliminary X-ray analysis of the strawberry allergens Fra a 1E and Fra a 3 in the presence of catechin

The strawberry Fra a proteins belong to the pathogenesis-related PR-10 protein family and share a common fold with the Bet v 1 major pollen allergen and the START/PYR/PYL proteins, which are characterized by the presence of a central cavity and are often involved in the binding of a variety of natural compounds. The Fra a proteins play a key role in the control of flavonoid biosynthesis in strawberries and are essential for pigment formation in fruits. In order to understand Fra a protein function, full-length Fra a 1E and Fra a 3 cDNAs were cloned and expressed in Escherichia coli, and the proteins were purified to homogeneity using metal-affinity chromatography. Diffraction-quality crystals of Fra a 1E and of Fra a 3 in the presence of (+)-catechin were obtained by the sitting-drop vapour-diffusion method. X-ray diffraction data from single crystals of Fra a 1E and Fra a 3 were processed to 2.2 and 3.0 Å resolution in space groups P212121 and P2221, with unit-cell parameters a = 70.02, b = 74.42, c = 84.04 Å and a = 137.91, b = 206.61, c = 174.7 Å for Fra a 1E and Fra a 3, respectively.

Structural and functional aspects of PR-10 proteins

Physical, chemical and biological stress factors, such as microbial infection, upregulate the transcription levels of a number of plant genes, coding for the so-called pathogenesis-related (PR) proteins. For PR proteins of class-10 (PR-10), the biological function remains unclear, despite two decades of scientific research. PR-10 proteins have a wide distribution throughout the plant kingdom and the class members share size and secondary structure organization. Throughout the years, we and other groups have determined the structures of a number of PR-10 proteins, both in the crystalline state by X-ray diffraction and in solution by NMR spectroscopy. Despite the accumulating structural information, our understanding of PR-10 function is still limited. PR-10 proteins are rather small (~ 160 amino acids) with a fold consisting of three α helices and seven antiparallel β strands. These structural elements enclose a large hydrophobic cavity that is most probably the key to their functional relevance. Also, the outer surface of these proteins is of extreme interest, as epitopes from a PR-10 subclass cause allergic reactions in humans.