Are alpha-gliadins glycosylated?

Structural Basis of IgE Binding to α- and γ-Gliadins: Contribution of Disulfide Bonds and Repetitive and Nonrepetitive Domains

Wheat products cause IgE-mediated allergies. The present study aimed to decipher the molecular basis of α- and γ-gliadin allergenicity. Gliadins and their domains, the repetitive N-terminal and the nonrepetitive C-terminal domains, were cloned and expressed in Escherichia coli. Their secondary structures and their IgE binding capacity were compared with those of natural proteins before and after reduction/alkylation. Allergenicity was evaluated with sera from patients who had a wheat food allergy or baker's asthma. The secondary structures of natural and recombinant proteins were slightly different. Compared with natural gliadins, recombinant proteins retained IgE binding but with reduced reactivity. Reduction/alkylation decreased IgE binding for both natural and recombinant gliadins. Although more continuous epitopes were identified in the N-terminal domains of α- and γ-gliadins, both the N-terminal and C-terminal domains contributed to IgE binding. As for other members of the prolamin superfamily, disulfide bonds appear to be of high importance for IgE binding.

Gluten quality of bread wheat is associated with activity of RabD GTPases

In the developing endosperm of bread wheat (Triticum aestivum), seed storage proteins are produced on the rough endoplasmic reticulum (ER) and transported to protein bodies, specialized vacuoles for the storage of protein. The functionally important gluten proteins of wheat are transported by two distinct routes to the protein bodies where they are stored: vesicles that bud directly off the ER and transport through the Golgi. However, little is known about the processing of glutenin and gliadin proteins during these steps or the possible impact on their properties. In plants, the RabD GTPases mediate ER-to-Golgi vesicle transport. Available sequence information for Rab GTPases in Arabidopsis, rice, Brachypodium and bread wheat was compiled and compared to identify wheat RabD orthologs. Partial genetic sequences were assembled using the first draft of the Chinese Spring wheat genome. A suitable candidate gene from the RabD clade (TaRabD2a) was chosen for down-regulation by RNA interference (RNAi), and an RNAi construct was used to transform wheat plants. All four available RabD genes were shown by qRT-PCR to be down-regulated in the transgenic developing endosperm. The transgenic grain was found to produce flour with significantly altered processing properties when measured by farinograph and extensograph. SE-HPLC found that a smaller proportion of HMW-GS and large proportion of LMW-GS are incorporated into the glutenin macropolymer in the transgenic dough. Lower protein content but a similar protein profile on SDS-PAGE was seen in the transgenic grain.

Barley γ3-hordein: glycosylation at an atypical site, disulfide bridge analysis, and reactivity with IgE from patients allergic to wheat

Post translational modifications of a seed storage protein, barley γ3-hordein, were determined using immunochemical and mass spectrometry methods. IgE reactivity towards this protein was measured using sera from patients diagnosed with allergies to wheat. N-glycosylation was found at an atypical Asn-Leu-Cys site. The observed glycan contains xylose. This indicates that at least some γ3-hordein molecules trafficked through the Golgi apparatus. Disulfide bridges in native γ3-hordein were almost the same as those found in wheat γ46-gliadin, except the bridge involving the cysteine included in the glycosylation site. IgE reacted more strongly towards the recombinant than the natural γ3-hordein protein. IgE binding to γ3-hordein increased when the protein sample was reduced. Glycosylation and disulfide bridges therefore decrease epitope accessibility. Thus the IgE from patients sensitized to wheat cross-react with γ3-hordein due to sequence homology with wheat allergens rather than through shared carbohydrate determinants.

Mass spectrometry analysis of gliadins in celiac disease

In recent years, scientific research on wheat gluten proteins has followed three main directions aimed at (1) finding relationships between individual genetic alleles coding for gliadins, high or low molecular weight glutenin subunits, and the viscoelastic dough properties of flour-derived products such as pasta and bread; (2) identifying prolamins and derived peptides involved in celiac disease, a pathological condition in which the small intestine of genetically predisposed individuals is reversibly damaged; and (3) developing and validating sensitive and specific methods for detecting trace amounts of gluten proteins in gluten-free foods for celiac disease patients. In this review, the main aspects of current and perspective applications of mass spectrometry and proteomic technologies to the structural characterization of gliadins are presented, with focus on issues related to detection, identification, and quantification of intact gliadins, as well as gliadin-derived peptides relevant to the biochemical, immunological, and toxicological aspects of celiac disease.

Transglutaminase-independent binding of gliadin to intestinal brush border membrane and GM1 ganglioside

Anti-ganglioside antibodies have been described in celiac disease or gluten sensitivity, in conjunction with the presence of central and peripheral nervous system deficits. The observed antibody reactivity to gangliosides is postulated to be related to the anti-gliadin immune response, either through antigenic mimicry, or by formation of gliadin-ganglioside complexes and haptenization. We examined the possibility of the presence of ganglioside-like epitopes in gliadin, as well as the potential for complex formation between gliadin and GM1 ganglioside. Low levels of glycosylation were present in gliadin, but ganglioside-like carbohydrate epitopes were not detected. However, gliadin was found to bind to GM1 ganglioside and to the GM1-rich intestinal brush border membrane. The described complex formation and possible haptenization of GM1 by gliadin may be responsible for driving the anti-ganglioside antibody response in some patients with gluten sensitivity. Furthermore, binding of gliadin to GM1 on the intestinal epithelium might have a role in the anti-gliadin immune response and contribute to the intestinal inflammatory reaction in celiac disease.