In-depth glycoproteomic characterization of γ-conglutin by high-resolution accurate mass spectrometry

Protein Concentration Affects the Food Allergen γ-Conglutin Uptake and Bacteria-Induced Cytokine Production in Dendritic Cells

γ-Conglutin (γ-C) from lupin seeds has been identified as a potent allergen with cross reactivity to peanuts. Here, we investigated how γ-C affected the response in bone marrow-derived dendritic cells (DCs) to bacterial stimuli. γ-C enhanced L. acidophilus NCFM (LaNCFM)-induced IL-12, IL-10, and IL-23 dose-dependently. In contrast, together with E. coli Nissle or LPS, γ-C reduced the production of IL-12 but not of IL-23 and IL-10. Enzyme-hydrolyzed γ-C also enhanced LaNCFM-induced IL-12 and IL-23 production. All preparations induced ROS production in the DCs. The mannose receptor ligands mannan and dextran and the clathrin inhibitor monodansylcadaverine partly inhibited the endocytosis of γ-C. Kunitz trypsin inhibitor and the scavenger receptor ligand polyG also enhanced LaNCFM-induced IL-12, indicating the involvement of receptors other than C-type lectin receptors. The endocytosis of labeled γ-C increased dose-dependently by addition of unlabeled γ-C, which coincided with γ-C's tendency to aggregate. Taken together, γ-C aggregation affects endocytosis and affects the cytokine production induced by gram-positive and gram-negative bacteria differently. We suggest that γ-C is taken up by the same mechanism as other food proteins but due to aggregation is present in higher concentration in the DCs. This could influence the resulting T-cell response in a microbial stimuli-dependent way.

A not-glycosylated isoform of γ-conglutin, a hexameric glycoprotein of Lupinus albus seed, participates in the oligomerization equilibrium

γ-conglutin (γ-C) is a hexameric glycoprotein accumulated in lupin seeds and has long been considered as a storage protein. Recently, it has been investigated for its possible postprandial glycaemic regulating action in human nutrition and for its physiological role in plant defence. The quaternary structure of γ-C results from the assembly of six monomers in reversible pH-dependent association/dissociation equilibrium. Our working hypothesis was that the γ-C hexamer is made up of glycosylated subunits in association with not-glycosylated isoforms, that seem to have 'escaped' the correct glycosylation process in the Golgi. Here we describe the isolation of not-glycosylated γ-C monomers in native condition by two in tandem lectin-based affinity chromatography and the characterization of their oligomerization capacity. We report, for the first time, the observation that a plant multimeric protein may be formed by identical polypeptide chains that have undergone different post-translational modifications. All obtained considered, the results strongly suggest that the not-glycosylated isoform can also take part in the oligomerization equilibrium of the protein.

Basic 7S globulin in plants

Soybean seed basic 7S globulin (Bg7S)-like proteins are found in many plant species. Bg7S was originally thought to be a major seed storage protein but was later found to be multifunctional, with stress response, antibacterial activity, hormone receptor-like activity. Moreover, functional differences between Bg7S proteins from legumes and other plants have been revealed. In non-leguminous plants, Bg7S molecules inhibit the invasion of pathogenic microorganisms. However, although leguminous plants have a peptide called leg-insulin that can bind to Bg7S, non-leguminous plants do not have leginsulin. Bg7S in leguminous plants and other plants may have evolved in functionally different directions. Several homologs of Bg7S in plants are reported, but there is no homolog of this protein in peas, suggesting that the pea evolution might have followed a different route when compared to other leguminous plants. Although the functions of Bg7S are well documented in plants, recent studies suggest that this protein is also important in controlling blood glucose level, blood pressure and plasma cholesterol level, and cancer cell antiproliferative actions.

Lupine allergens: Clinical relevance, molecular characterization, cross-reactivity, and detection strategies

Lupine is commonly utilized as a technological food and ingredient in a great variety of processed products (snacks, bakery, meat, and dairy products) principally owing to its nutritional value and technological properties. However, its ingestion, even at trace amounts (in the range of mg protein per kg of food), can lead to severe adverse reactions in allergic individuals. Lupine belongs to the Leguminosae family, having the conglutins (α-, β-, δ-, and γ-) as allergens, among other proteins. Cross-sensitization of lupine-sensitized individuals with other legume species, mainly peanut, can occur, but the associated clinical reactivity is still unclear. The protection of the sensitized individuals should depend on an avoidance diet, which should rely on the compliance of food labeling and, as such, on their verification by analytical methods. Food processing, such as heat treatments, has an important influence on the structural properties of lupine proteins, altering their detectability and allergenicity. In this review, different aspects related with lupine allergy are described, namely, the overall prevalence, clinical relevance, diagnosis, and treatment. The characterization of lupine allergens and their potential cross-reactivity with other legumes are critically discussed. The effects of food matrix, processing, and digestibility on lupine proteins, as well as the available analytical tools for detecting lupine at trace levels in foods, are also herein emphasized.

Lupinus albus γ-Conglutin, a Protein Structurally Related to GH12 Xyloglucan-Specific Endo-Glucanase Inhibitor Proteins (XEGIPs), Shows Inhibitory Activity against GH2 β-Mannosidase

γ-conglutin (γC) is a major protein of Lupinus albus seeds, but its function is still unknown. It shares high structural similarity with xyloglucan-specific endo-glucanase inhibitor proteins (XEGIPs) and, to a lesser extent, with Triticum aestivum endoxylanase inhibitors (TAXI-I), active against fungal glycoside hydrolases GH12 and GH11, respectively. However, γC lacks both these inhibitory activities. Since β-galactomannans are major components of the cell walls of endosperm in several legume plants, we tested the inhibitory activity of γC against a GH2 β-mannosidase (EC 3.2.1.25). γC was actually able to inhibit the enzyme, and this effect was enhanced by the presence of zinc ions. The stoichiometry of the γC/enzyme interaction was 1:1, and the calculated K_i was 1.55 μM. To obtain further insights into the interaction between γC and β-mannosidase, an in silico structural bioinformatic approach was followed, including some docking analyses. By and large, this work describes experimental findings that highlight new scenarios for understanding the natural role of γC. Although structural predictions can leave space for speculative interpretations, the full complexity of the data reported in this work allows one to hypothesize mechanisms of action for the basis of inhibition. At least two mechanisms seem plausible, both involving lupin-γC-peculiar structures.

Advances in the Biology of Seed and Vegetative Storage Proteins Based on Two-Dimensional Electrophoresis Coupled to Mass Spectrometry

Seed storage proteins play a fundamental role in plant reproduction and human nutrition. They accumulate during seed development as reserve material for germination and seedling growth and are a major source of dietary protein for human consumption. Storage proteins encompass multiple isoforms encoded by multi-gene families that undergo abundant glycosylations and phosphorylations. Two-dimensional electrophoresis (2-DE) is a proteomic tool especially suitable for the characterization of storage proteins because of their peculiar characteristics. In particular, storage proteins are soluble multimeric proteins highly represented in the seed proteome that contain polypeptides of molecular mass between 10 and 130 kDa. In addition, high-resolution profiles can be achieved by applying targeted 2-DE protocols. 2-DE coupled with mass spectrometry (MS) has traditionally been the methodology of choice in numerous studies on the biology of storage proteins in a wide diversity of plants. 2-DE-based reference maps have decisively contributed to the current state of our knowledge about storage proteins in multiple key aspects, including identification of isoforms and quantification of their relative abundance, identification of phosphorylated isoforms and assessment of their phosphorylation status, and dynamic changes of isoforms during seed development and germination both qualitatively and quantitatively. These advances have translated into relevant information about meaningful traits in seed breeding such as protein quality, longevity, gluten and allergen content, stress response and antifungal, antibacterial, and insect susceptibility. This review addresses progress on the biology of storage proteins and application areas in seed breeding using 2-DE-based maps.

Proteomics for exploiting diversity of lupin seed storage proteins and their use as nutraceuticals for health and welfare

Lupins have a variety of both traditional and modern uses. In the last decade, reports assessing the benefits of lupin seed proteins have proliferated and, nowadays, the pharmaceutical industry is interested in lupin proteins for human health. Modern genomics and proteomics have hugely contributed to describing the diversity of lupin storage genes and, above all, proteins. Most of these studies have been centered on few edible lupin species. However, Lupinus genus comprises hundreds of species spread throughout the Old and New Worlds, and these resources have been scarcely explored and exploited. We present here a detailed review of the literature on the potential of lupin seed proteins as nutraceuticals, and the use of -omic tools to analyze seed storage polypeptides in main edible lupins and their diversity at the Lupinus inter- and intra-species level. In this sense, proteomics, more than any other, has been a key approach. Proteomics has shown that lupin seed protein diversity, where post-translational modifications yield a large number of peptide variants with a potential concern in bioactivity, goes far beyond gene diversity. The future extended use of second and third generation proteomics should definitely help to go deeper into coverage and characterization of lupin seed proteome.

BIOLOGICAL SIGNIFICANCE

Some important topics concerning storage proteins from lupin seeds are presented and analyzed in an integrated way in this review. Proteomic approaches have been essential in characterizing lupin seed protein diversity, which goes far beyond gene diversity since the protein level adds to the latter differential proteolytic cleavage of conglutin pro-proteins and a diverse array of glycosylation forms and sites. Proteomics has also proved helpful for screening and studying Lupinus germplasm with the future aim of exploiting and improving food production, quality, and nutritional values.

Structure of γ-conglutin: insight into the quaternary structure of 7S basic globulins from legumes

γ-Conglutin from lupin seeds is an unusual 7S basic globulin protein. It is capable of reducing glycaemia in mammals, but the structural basis of this activity is not known. γ-Conglutin shares a high level of structural homology with glycoside hydrolase inhibitor proteins, although it lacks any kind of inhibitory activity against plant cell-wall degradation enzymes. In addition, γ-conglutin displays a less pronounced structural similarity to pepsin-like aspartic proteases, but it is proteolytically dysfunctional. Only one structural study of a legume 7S basic globulin, that isolated from soybean, has been reported to date. The quaternary assembly of soybean 7S basic globulin (Bg7S) is arranged as a cruciform-shaped tetramer comprised of two superposed dimers. Here, the crystal structure of γ-conglutin isolated from Lupinus angustifolius seeds (LangC) is presented. The polypeptide chain of LangC is post-translationally cleaved into α and β subunits but retains its covalent integrity owing to a disulfide bridge. The protomers of LangC undergo an intricate quaternary assembly, resulting in a ring-like hexamer with noncrystallographic D3 symmetry. The twofold-related dimers are similar to those in Bg7S but their assembly is different as a consequence of mutations in a β-strand that is involved in intermolecular β-sheet formation in γ-conglutin. Structural elucidation of γ-conglutin will help to explain its physiological role, especially in the evolutionary context, and will guide further research into the hypoglycaemic activity of this protein in humans, with potential consequences for novel antidiabetic therapies.

Are vicilins another major class of legume lectins?

Legume lectins comprise a structurally related, Ca/Mn-dependent, widespread, abundant and well characterized lectin family when compared to the large number of lectins from other sources described in the literature. Strangely enough, no specific function has been assigned to them aside from a possible role in storage and/or defense. Using a recent and fine-tuned methodology capable of specific lectin identification, β-conglutin, Vicia faba vicilin and β-lathyrin, the vicilin storage globulins from Lupinus albus, V. faba and Lathyrus sativus, respectively, were shown to be capable of affinity binding to thoroughly washed erythrocyte membranes and of specific elution with appropriate sugars. Based on this evidence and on sparse data published in the literature, a second family of legume lectins is proposed: the 7S family of storage proteins from leguminous seeds, or family II of legume lectins. These lectins are also structurally related, widespread and well characterized. In addition, they self-aggregate in a Ca/Mg, electrostatic dependent manner and are even more abundant than the family I of legume lectins. Using the same evidence, reserve and defense roles may be attributed to family II of legume lectins.