The S-poor prolamins of wheat, barley and rye

Deamidated Gliadin Peptides as Targets for Celiac Disease‐Specific Antibodies

Celiac disease (CD) is an (auto)immunologically mediated intestinal intolerance against proteins from wheat (gluten) and related cereal proteins. Tissue transglutaminase (tTG) has been identified as the autoantigen in CD. Although ultimate diagnosis is based on histological analysis of small intestinal mucosa obtained via tissue biopsy, assessment of autoantibodies can provide substantial help in the evaluation of CD. Gliadin antibodies are directed against the native disease-provoking cereal proteins. Despite their initial usefulness, these antibodies have lost diagnostic importance due to their poor specificity and sensitivity as CD markers. Recently, it was found, however, that gliadin antibodies from sera of patients with active CD preferentially recognized deamidated gliadin peptides. The use of deamidated gliadin peptides in immunoassays has significantly improved the usefulness of gliadin antibodies in diagnosis of CD to that observed with autoantibody assay methods (endomysium antibodies, antibodies against tTG). The antibody epitopes (B-cell epitopes) reflect substrate specificity of tTG and resemble peptide sequences known to be strongly T-cell stimulatory (T-cell epitopes) in CD. The assay applying deamidated gliadin peptides measures a new species of antibodies, which is different from conventional gliadin antibodies as well as from autoantibodies and will likely provide new information on pathophysiological mechanisms of CD.

Effects of enzymatic deamidation by protein-glutaminase on structure and functional properties of wheat gluten

Protein-glutaminase (PG) purified from Chryseobacterium proteolyticum was used to investigate its deamidation effects on wheat gluten. Water-insoluble gluten was able to be deamidated to the extent of deamidation degree (DD) 72% in 200 mM sodium phosphate buffer (pH 7) at 40 degrees C for 30 h. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis exhibited an upper shift of gluten bands with only deamidation for 1.5-2.0 h (DD 35-45%) compared to the bands of nondeamidated gluten. Results of Fourier transform infrared analysis revealed alterations in secondary structure of gluten by PG deamidation. The assignment within amide I region showed decreases in both inter- (around 1695 cm(-1)) and intramolecular beta-sheets (around 1680 cm(-1)) by deamidation suggesting the deterioration of the aggregation ability of gluten molecules. Solubility and emulsification properties of gluten at pH 7 were improved by deamidation, while both properties at pH 3 were deteriorated by deamidation. Enzyme-linked immunosorbent assay identified that allergenicity of deamidated gluten as compared to the nondeamidated cohorts was decreased remarkably as the deamidation time was prolonged.

Nanomechanical force measurements of gliadin protein interactions

The strength and nature of interactions between monomeric gliadin proteins involving alpha-alpha, omega-omega, and alpha-omega interactions in 0.01M acetic acid, and the effect of urea has been investigated. It was shown by means of nanomechanical force measurements that the stretching events in the separation curve after adhesive phenomena originated from proteins. These stretching events displayed different responses of the alpha- and omega-gliadins to urea. While 2M urea caused the more globular alpha-gliadins to unfold, the beta-turn-rich omega-gliadins remained fairly stable even in 8M urea. This suggests different roles for gliadins in the formation of dough; while the omega-gliadins are still in a compact structure being responsible for the viscous flow, the alpha-gliadins have already started to participate in forming the network in dough.

Endosperm structure affects the malting quality of barley (Hordeum vulgare L.)

Twenty-seven barley (Hordeum vulgare L.) samples collected from growing sites in Scandinavia in 2001 and 2002 were examined to study the effect of endosperm structure on malting behavior. Samples were micromalted, and several malt characteristics were measured. Samples were classified as having a mealier or steelier endosperm on the basis of light transflectance (LTm). Because endosperm structure is greatly dependent on protein content, three barley sample pairs with similar protein contents were chosen for further analysis. During malting, the steelier barley samples produced less root mass, but showed higher respiration losses and higher activities of starch-hydrolyzing enzymes. Malts made from steelier barley had a less friable structure, with more urea-soluble D hordein and more free amino nitrogen and soluble protein. The reason for these differences may lie in the structure or localization of the hordeins as well as the possible effects of endosperm packing on water uptake and movement of enzymes.

Cloning and molecular characterization of B-hordeins from Hordeum chilense (Roem. et Schult.)

One of the main limitations of cereal breeding is the lack of genetic variability within cultivated crops. Hordeum chilense is a wild relative of Hordeum vulgare, which has been successfully used in the synthesis of amphiploids by crossing with Triticum spp. Among the agronomic traits of these new amphiploids, the allelic variation in the endosperm storage proteins and their influence on breadmaking and malting quality are of special interest. B-hordeins are sulfur rich prolamins, which account for 70-80% of the total hordein fraction in barley. In this work, rapid amplification of cDNA ends by PCR (RACE-PCR) has been used for the cloning of the full-length open reading frame (ORF) of six sequences of B3-hordeins from two lines of H. chilense. Two consensus sequences of 813 and 822 bp for the H1 and H7 lines, respectively, were determined by alignment of all the sequences generated. Between both lines, differences involving single base changes, which could correspond to single nucleotide polymorphisms (SNP), insertions and deletions were observed. Of these differences, only six out of the 13 within the ORF caused a change of amino acid. Two insertions/deletions of 9 and 12 bp were also observed between both lines. The derived amino acid sequences showed a similar structure to the B-hordeins from cultivated barley and other prolamins. The repetitive region is based on the repetition of the motif PQQPFPQQ. The copy number of the B3-hordeins was estimated as a minimum of nine and five copies for the H1 and H7 lines, respectively. The expression profile of the B-hordeins through the developing endosperm is also described in this work. This study of the storage proteins of H. chilense is a useful contribution to the knowledge of the genetic diversity available in wild relatives of cultivated barley. In addition, the origin of the different prolamins can be better understood with an in-depth knowledge of its wild equivalent.

ATR-FT/IR study on the interactions between gliadins and dextrin and their effects on protein secondary structure

The effects of heat treatment and dextrin addition on the secondary structure of gliadins were investigated by means of attenuated total reflection Fourier transform infrared spectroscopy (ATR-FT/IR). Gliadins and gliadin/dextrin mixtures (before and after thermal treatment) were prepared as a dried protein film on the ATR-FT/IR zinc selenide cell plate and equilibrated at a water activity (a(w)) of 0.06. The results show that gliadins undergo conformational changes upon thermal treatment both in the absence and in the presence of dextrin. In particular, in the thermally treated gliadins, the decrease of the band at around 1651 cm(-)(1) and the increase of the bands at around 1628 and 1690 cm(-)(1) suggest a loss of alpha-helix structure and a higher content of protein aggregates. The same trend was observed in the presence of dextrin. Concerning the interactions between gliadins and dextrin, gliadin/dextrin mixtures show variations in the amide I region compared to native gliadins (e.g., an increase of the band at 1645 cm(-)(1) and the absence of the band at around 1668 cm(-)(1)) that might be due to hydrogen bond formation between gliadins and dextrin. It was also found that the spectrum of gliadin/dextrin mixtures was less affected by the hydration state than that of native gliadins, as observed from the differential spectra obtained by subtraction of the spectrum obtained at a(w) = 0.06 (driest condition tested) from the spectrum of the sample equilibrated at a(w) = 0.84. This could be due to the fact that C=O and N-H groups of gliadins are engaged to form hydrogen bonds with the hydroxyl groups of dextrin, and so they are not perturbed by the presence of water molecules. Finally, water activity effects on the secondary structure of gliadins are also discussed.

Fast omega-gliadin is a major allergen in wheat-dependent exercise-induced anaphylaxis

BACKGROUND

Wheat-dependent exercise-induced anaphylaxis is an anaphylaxy induced by physical exercise after ingestion of wheat. An immediate-type hypersensitivity to water/salt-insoluble fraction of wheat proteins (gluten) has been considered to underlie in this disease.

OBJECTIVE

The aim of the study is to determine the major allergen in Japanese patients with wheat-dependent exercise-induced anaphylaxis by using a panel of purified wheat gliadins and glutenins.

METHODS

Water/salt-insoluble wheat proteins, alpha-gliadin, beta-gliadin, gamma-gliadin, fast omega-gliadin, slow omega-gliadin, high molecular weight glutenin and low molecular weight glutenin, were purified, and five patients with wheat-dependent exercise-induced anaphylaxis, whose diagnose had been determined by positive-challenge test, were evaluated for skin prick test, dot-blotting test and CAP-RAST inhibition test by using these purified wheat proteins.

RESULTS

The fast omega-gliadin was the most potent allergen among these water/salt-insoluble proteins when evaluated by skin prick test and dot-blotting test. Fast and slow omega-gliadin, and gamma-gliadin caused dose-dependent inhibition of the serum IgE-binding to solid-phase gluten in the patients. The incubation with fast omega-gliadin of the patient's serum caused dose-dependent inhibition in the IgE-binding to gamma-gliadin as well as slow omega-gliadin, indicating a cross-reactivity of these proteins in IgE-binding.

CONCLUSION

We concluded that fast omega-gliadin is a major allergen among these water/salt-insoluble proteins for wheat-dependent exercise-induced anaphylaxis in Japanese patients, and IgE against fast omega-gliadin cross-reacts to gamma-gliadin and slow omega-gliadin.

Cloning and characterization of a gamma-3 hordein mRNA (cDNA) from Hordeum chilense (Roem. et Schult.)

Hordeum chilense is a wild relative of H. vulgare, cultivated barley, that has been successfully used in the synthesis of amphiploids by crossing with Triticum spp. These amphiploids-named generically x Tritordeum-have been tested under field conditions, and one of them, the hexaploid tritordeum obtained following chromosome doubling of the hybrid H. chilense x T. turgidum, shows traits of interest inherited from the barley parent. Of great interest is the allelic variation observed in the endosperm storage proteins and their influence on the breadmaking and malting quality of tritordeum. We report here two mRNA (cDNA) sequences for a gamma-3 hordein from two accession lines of H. chilense, H1 and H7, and their characterization by quantitative real time (QRT)-PCR in the developing endosperm. Sequences were obtained by rapid amplification of cDNA ends and "edge-to-edge" amplification of open reading frames from cDNA of H. chilense. Eight putative single nucleotide polymorphisms and one codon insertion were identified in the sequences of the H1 and H7 gamma-3 hordeins. The deduced amino acid sequences showed similar features to that of the gamma-3 hordein and gamma-gliadins from barley and wheat, respectively. While the repetitive motif (PQQQPF) is similar to that of the gamma-3 hordein from H. vulgare, there are 19 motif repeats in H. vulgare, whereas H. chilense shows 15 tandem repeats. The transcription of the genes encoding for the gamma-3 hordein were monitored by QRT-PCR: in both lines maximum transcription occurred 12 days after flowering.

Similarities of omega gliadins from Triticum urartu to those encoded on chromosome 1A of hexaploid wheat and evidence for their post-translational processing

The omega-gliadins encoded on chromosome 1 of the A genome were purified from Triticum aestivum L. (2n=6 x=42, AABBDD) cv. Butte86, nullisomic 1D-tetrasomic 1A of cv. Chinese Spring (CS N1DT1A), and the diploid T. urartu (2n=2 x=14, AA ). Reverse-phase high-performance liquid chromatography combined with sodium dodecyl sulfate-polyacrylamide gel electrophoresis of gliadin extracts from CS nullisomic-tetrasomic (NT) lines confirmed the assignment to chromosome 1A. The purified omega-gliadins were characterized by mass spectrometry and N-terminal sequencing. The 1A-encoded omega-gliadins were smaller than 1B- or 1D-encoded omega-gliadins. The N-terminal amino acid sequences for 1A omega-gliadin mature peptides were nearly identical to those for the T. urartu omega-gliadins and were more similar to 1D omega-gliadin sequences than to sequences for T. monococum omega-gliadins, barley C-hordeins, or rye omega-secalins. They diverged greatly from the N-terminal sequences for the 1B omega-gliadins. The data suggest that T. urartu is the A-genome donor, and that post-translational cleavage by an asparaginyl endoprotease produces those omega-gliadins with N-terminal sequences beginning with KEL.

Structural studies of wheat flour glutenin polymers by CD spectroscopy

A dissolution procedure of unreduced glutenin polymers of three wheat flour varieties (WRU 6981, Alisei 1, and Alisei 2) by sonication in the presence of SDS (sodium dodecyl sulphate), after the elimination of albumins, globulins, and gliadins, was achieved, and the molecular weight distribution of glutenin polymers obtained by this method was measured by matrix assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry. A structural study by CD spectroscopy at different temperatures of WRU 6981 glutenin polymer and of 1Ax1 high-M(r) (relative molecular mass) glutenin subunit, which is the only high-M(r) subunit contained in WRU 6981 flour, was undertaken to understand if the information obtained from the single subunit were applicable to the total polymer. CD spectroscopy also has been employed to study the glutenin polymers obtained by Alisei 1 and Alisei 2 wheat flours; Alisei 1 biotype contained 1Bx7 and 1Dx2+1Dy12 high-M(r) subunits, whereas the Alisei 2 biotype contained only 1Bx7 and 1Dy12 subunits. A conformational study was undertaken by CD spectroscopy at different temperatures and in the presence of some chemical denaturant agents, such as urea and sodium dodecyl sulphate, in order to obtain information about their intrinsic stability and to verify if the 1Dx2 subunit presence determined a different structural behavior between Alisei 1 and Alisei 2 polymers. MALDI-TOF mass spectrometric experiments showed that the glutenin polymers molecular weights were in the mass range of 500000-5000000. CD spectra indicated that a single conformational state did not predominate in the temperature range studied but equilibrium between two distinct conformational states existed; moreover, all the changes induced by urea and by SDS followed a multistep transition process.

Conformational studies of wheat flour high relative molecular mass glutenin subunits by circular dichroism spectroscopy

Conformational studies of 1Dx2, 1Bx7, and 1Dy12 high relative molecular mass glutenin subunits, extracted from Alisei 1 flour, are reported. Circular dichroism (CD) spectroscopy is employed to study their conformational polymorphism induced by urea and by urea in the presence of 1% sodium dodecyl sulfate (SDS). The CD spectra indicate that SDS promotes ordered structures. The addition of urea to the SDS-acetate solution of 1Dx2, 1Bx7, and 1Dy12 subunits eliminates the effect of SDS. Its addition to the acetate solution of proteins induces conformational transitions to form a poly-L-proline II-like structure. All the changes induced by urea follow a multistep transition process that is typical of proteins consisting of different domains.

Prediction of protein cleavage sites by the barley cysteine endoproteases EP-A and EP-B based on the kinetics of synthetic peptide hydrolysis

Hordeins, the natural substrates of barley (Hordeum vulgare) cysteine endoproteases (EPs), were isolated as protein bodies and degraded by purified EP-B from green barley malt. Cleavage specificity was determined by synthesizing internally quenched, fluorogenic tetrapeptide substrates of the general formula 2-aminobenzoyl-P(2)-P(1)-P(1)'-P(2)' 1-tyrosine(NO(2))-aspartate. The barley EPs preferred neutral amino acids with large aliphatic and nonpolar (leucine, valine, isoleucine, and methionine) or aromatic (phenylalanine, tyrosine, and tryptophan) side chains at P(2), and showed less specificity at P(1), although asparagine, aspartate, valine, and isoleucine were particularly unfavorable. Peptides with proline at P(1) or P(1)' were extremely poor substrates. Cleavage sites with EP-A and EP-B preferred substrate sequences are found in hordeins, their natural substrates. The substrate specificity of EP-B with synthetic peptides was used successfully to predict the cleavage sites in the C-terminal extension of barley beta-amylase. When all of the primary cleavage sites in C hordein, which occur mainly in the N- and C-terminal domains, were removed by site-directed mutagenesis, the resulting protein was degraded 112 times more slowly than wild-type C hordein. We suggest that removal of the C hordein terminal domains is necessary for unfolding of the beta-reverse turn helix of the central repeat domain, which then becomes more susceptible to proteolytic attack by EP-B.

Identification of microphases in mixed alpha- and omega-gliadin protein films investigated by atomic force microscopy

Pure and mixed films of alpha- and omega-gliadins were studied by tapping mode atomic force microscopy (AFM). The technique was sensitive to the chemistry of the surface properties of the films, allowing imaging of the mixed gliadin phases at different ratios. In addition to the study of the phases at the micrometer level, higher resolution images allowed visualization of the protein films at the molecular level. These studies may have relevance to the formation of phases in developing protein bodies in grain, where gliadins and glutenins are deposited together. It has been assumed that the protein bodies consist of a random network of proteins; these studies indicate that microphases could be present in protein bodies. The technique provides novel methods for studying mixed biopolymer systems.

Separation and characterization of barley (Hordeum vulgare L.) hordeins by free zone capillary electrophoresis

Extraction conditions, separation conditions, and capillary rinsing protocols were optimized for the separation of barley hordeins by free zone capillary electrophoresis. Stable hordein extracts were obtained with a single 5 min extraction after the albumins and globulins were removed. Hordeins had to be reduced for optimal resolution. Optimum separation conditions for hordein separations were 100 mM phosphate-glycine buffer containing 20% acetonitrile and 0.05% hydroxypropylmethylcellulose. The addition of zwitterionic sulfobetaine detergents containing hydrocarbon tails of eight and ten carbons slightly improved the resolution of the separations, but not enough to warrant their use on a routine basis. The migration positions of the hordein subclasses were determined by two- dimensional reversed-phase high-performance liquid chromatography x free zone capillary electrophoresis mapping. The hordein subclasses formed clusters similar to those of wheat gliadins. Separation-to-separation repeatability was good, with migration time relative standard deviations < 1% for a 15-run period. For routine discrimination of cultivars, a 2 min post-separation rinse with 500 mM acetic acid was necessary to prevent protein build-up on the capillary walls. An example of successfully differentiating barley cultivars using this technique is shown.

Small angle X-ray scattering of wheat seed-storage proteins: alpha-, gamma- and omega-gliadins and the high molecular weight (HMW) subunits of glutenin

Small angle X-ray scattering in solution was performed on seed-storage proteins from wheat. Three different groups of gliadins (alpha-, gamma- and omega-) and a high molecular weight (HMW) subunit of glutenin (1Bx20) were studied to determine molecular size parameters. All the gliadins could be modelled as prolate ellipsoids with extended conformations. The HMW subunit existed as a highly extended rod-like particle in solution with a length of about 69 nm and a diameter of about 6.4 nm. Specific aggregation effects were observed which may reflect mechanisms of self-assembly that contribute to the unique viscoelastic properties of wheat dough.

Biochemical and genetic characterisation of a D glutenin subunit encoded at the Glu-B3 locus

The SDS-PAGE pattern of reduced and alkylated glutenins from the bread wheat cultivar Prinqual presents a subunit (named d4) in the mobility zone of the omega -gliadins that only appears under reduced conditions. This subunit was isolated and characterised at the biochemical and genetic levels. Subunit d4 was shown to form disulphide aggregates with glutenins and had an acidic pI. These characteristics correspond to those of the D glutenin subunits. The N-terminal amino acid sequence of subunit d4 was coincident with the SRL sequence type characteristic of omega -gliadins encoded by genes on the 1B chromosome, and confirms the similarity between D glutenin subunits and omega -gliadins. The genetic study of subunit d4 was performed in the F2 progeny from the 'Prinqual' x 'Ablaca' cross, based on four prolamin loci: Glu-B1, Glu-B3, Gli-B1, and Gli-B5. The recombinants found between Glu-B3 and Gli-B1 demonstrated that subunit d4 was encoded at the Glu-B3 locus, and reinforces the hypothesis of the duplication of prolamin gene clusters in wheat. A preliminary study of the effect of subunit d4 on gluten strength showed that lines with the Glu-B3 allele from 'Prinqual', which includes subunit d4, had a significantly higher sedimentation volume than those with the allele from 'Ablaca'.Key words: wheat gluten proteins, D glutenin subunits, amino acid sequence, linkage mapping, complex loci duplication.