Identification of a new form of lipid transfer protein (LTP1) in wheat seeds

Features and Possible Applications of Plant Lipid-Binding and Transfer Proteins

In plants, lipid trafficking within and inside the cell is carried out by lipid-binding and transfer proteins. Ligands for these proteins are building and signaling lipid molecules, secondary metabolites with different biological activities due to which they perform diverse functions in plants. Many different classes of such lipid-binding and transfer proteins have been found, but the most common and represented in plants are lipid transfer proteins (LTPs), pathogenesis-related class 10 (PR-10) proteins, acyl-CoA-binding proteins (ACBPs), and puroindolines (PINs). A low degree of amino acid sequence homology but similar spatial structures containing an internal hydrophobic cavity are common features of these classes of proteins. In this review, we summarize the latest known data on the features of these protein classes with particular focus on their ability to bind and transfer lipid ligands. We analyzed the structural features of these proteins, the diversity of their possible ligands, the key amino acids participating in ligand binding, the currently known mechanisms of ligand binding and transferring, as well as prospects for possible application.

Critical structural elements for the antigenicity of wheat allergen LTP1 (Tri a 14) revealed by site-directed mutagenesis

Lipid transfer proteins (LTPs) were identified as allergens in a large variety of pollens and foods, including cereals. LTPs belong to the prolamin superfamily and display an α-helical fold, with a bundle of four α-helices held together by four disulfide bonds. Wheat LTP1 is involved in allergic reactions to food. To identify critical structural elements of antibody binding to wheat LTP1, we used site-directed mutagenesis on wheat recombinant LTP1 to target: (i) sequence conservation and/or structure flexibility or (ii) each disulfide bond. We evaluated the modifications induced by these mutations on LTP1 secondary structure by synchrotron radiation circular dichroism and on its antigenicity with patient's sera and with mouse monoclonal antibodies. Disruption of the C28-C73 disulfide bond significantly affected IgE-binding and caused protein denaturation, while removing C13-C27 bond decreased LTP1 antigenicity and slightly modified LTP1 overall folding. In addition, we showed Lys72 to be a key residue; the K72A mutation did not affect global folding but modified the local 3D structure of LTP1 and strongly reduced IgE-binding. This work revealed a cluster of residues (C13, C27, C28, C73 and K72), four of which embedded in disulfide bonds, which play a critical role in LTP1 antigenicity.

Optimised purification and characterisation of lipid transfer protein 1 (LTP1) and its lipid-bound isoform LTP1b from barley malt

In beer brewing, brewers worldwide strive to obtain product consistency in terms of flavour, colour and foam. Important proteins contributing to beer foam are lipid transfer proteins (LTPs), in particular LTP1 and its lipid-bound isoform LTP1b, which are known to transport lipids in vivo and prevent lipids from destabilising the beer foam. LTP1 and LTP1b were successfully purified using only five purification steps with a high purified protein yield (160 mg LTP1 and LTP1b from 200 g barley). Circular dichroism of LTP1 and LTP1b confirmed that both proteins are highly tolerant to high temperatures (>90 °C) and are pH stable, particularly at a neutral to a more basic pH. Only LTP1 exhibited antiyeast and thermo-stable lytic activity, while LTP1b was inactive, indicating that the fatty acid moiety compromised the antimicrobial activity of LTP1. This lack in antiyeast activity and the positive foam properties of LTP1b would benefit beer fermentation and quality.

Construction and analysis of a plant non-specific lipid transfer protein database (nsLTPDB)

BACKGROUND

Plant non-specific lipid transfer proteins (nsLTPs) are small and basic proteins. Recently, nsLTPs have been reported involved in many physiological functions such as mediating phospholipid transfer, participating in plant defence activity against bacterial and fungal pathogens, and enhancing cell wall extension in tobacco. However, the lipid transfer mechanism of nsLTPs is still unclear, and comprehensive information of nsLTPs is difficult to obtain.

METHODS

In this study, we identified 595 nsLTPs from 121 different species and constructed an nsLTPs database--nsLTPDB--which comprises the sequence information, structures, relevant literatures, and biological data of all plant nsLTPs http://nsltpdb.life.nthu.edu.tw/.

RESULTS

Meanwhile, bioinformatics and statistics methods were implemented to develop a classification method for nsLTPs based on the patterns of the eight highly-conserved cysteine residues, and to suggest strict Prosite-styled patterns for Type I and Type II nsLTPs. The pattern of Type I is C X2 V X5-7 C [V, L, I] × Y [L, A, V] X8-13 CC × G X12 D × [Q, K, R] X2 CXC X16-21 P X2 C X13-15C, and that of Type II is C X4 L X2 C X9-11 P [S, T] X2 CC X5 Q X2-4 C[L, F]C X2 [A, L, I] × [D, N] P X10-12 [K, R] X4-5 C X3-4 P X0-2 C. Moreover, we referred the Prosite-styled patterns to the experimental mutagenesis data that previously established by our group, and found that the residues with higher conservation played an important role in the structural stability or lipid binding ability of nsLTPs.

CONCLUSIONS

Taken together, this research has suggested potential residues that might be essential to modulate the structural and functional properties of plant nsLTPs. Finally, we proposed some biologically important sites of the nsLTPs, which are described by using a new Prosite-styled pattern that we defined.

Immunoglobulin-E-binding epitopes of wheat allergens in patients with food allergy to wheat and in mice experimentally sensitized to wheat proteins

BACKGROUND

At present, B cell epitopes involved in food allergy to wheat are known only for a few allergens and a few categories of patients.

OBJECTIVE

To characterize the epitopes of different wheat kernel allergens: α-, γ, ω2, and ω5-gliadin, a low-molecular-weight (LMW) glutenin subunit, and a lipid transfer protein (LTP1) recognized by allergic patients and by sensitized mice and provide further understanding of the role of structure in determining allergic response.

METHODS

Sera were obtained from 39 patients suffering from food allergy to wheat. BALB/c mice were sensitized to gliadins or LTP1 by intraperitoneal immunizations. Continuous epitopes bound by IgE were delineated by the Pepscan technique. The response to reduced, alkylated LTP1 was compared with that of the native form to evaluate the importance of protein folding on IgE reactivity.

RESULTS

Few continuous epitopes of LTP1 reacted with IgE from allergic patients and mice, but one of them was common to several patients and sensitized mice. The unfolded protein was not recognized by either patient or mouse IgE, emphasizing the major role of LTP1 folding and discontinuous epitopes in IgE-binding. In contrast, many continuous epitopes were detected by patient and mouse IgE especially for an ω5-gliadin, which is an unstructured protein, and to a lesser extent, for the other gliadins and a LMW-glutenin subunit.

CONCLUSION AND CLINICAL RELEVANCE

The conformation of LTP1 appeared to have a strong impact on the type of IgE-binding epitopes elicited by this protein in both man and mouse. The responses in mice sensitized to gliadins or LTP1 were sufficiently comparable with the human response in terms of IgE-binding epitopes to provide support for the use of the mouse model in further investigations.

The crystal structure of oxylipin-conjugated barley LTP1 highlights the unique plasticity of the hydrophobic cavity of these plant lipid-binding proteins

The barley lipid transfer protein (LTP1) adducted by an alpha-ketol, (9-hydroxy-10-oxo-12(Z)-octadecenoic acid) exhibits an unexpected high lipid transfer activity. The crystal structure of this oxylipin-adducted LTP1, (LTP1b) was determined at 1.8A resolution. The covalently bound oxylipin was partly exposed at the surface of the protein and partly buried within the hydrophobic cavity. The structure of the oxylipidated LTP1 emphasizes the unique plasticity of the hydrophobic cavity of these plant lipid-binding proteins when compared to the other members of the family. The plasticity of the hydrophobic cavity and increase of its surface hydrophobicity induced by the oxylipin account for the improvement of the lipid transfer activity of LTP1b. These observations open new perspectives to explore the different biological functions of LTPs, including their allergenic properties.

Unexpected localization of a lipid transfer protein in germinating sunflower seeds

Plant lipid transfer proteins (LTPs) are low-molecular-mass proteins whose biological function still remains elusive. They are synthesized with a signal peptide that drives them to the secretory pathway. We have previously described the occurrence of an apoplastic LTP named Ha-AP10, present in sunflower seeds. Using a biochemical approach we now demonstrate that a fraction of Ha-AP10 is perispherically bound to membranes of germinating seeds. Purification of plasma membranes revealed the presence of Ha-AP10 in this fraction. Fluorimmunolocalization studies on germinating sunflower seeds demonstrated that in addition to the apoplastic and plasma membrane localization, Ha-AP10 is also present intracellularlly associated to unidentified structures. This varied distribution of Ha-AP10 in sunflower seeds may give novel clues to understand the role of LTPs in seed physiology.

Intestinal translocation capabilities of wheat allergens using the Caco-2 cell line

Because intestinal absorption of food protein can trigger an allergic reaction, the effect of wheat proteins on intestinal epithelial cell permeability was evaluated and the abilities of these proteins in native or pepsin-hydrolyzed state to cross the epithelial cell monolayer were compared. Enterocytic monolayers were established by culturing Caco-2 cells, a model of enterocytes, on permeable supports that separate the apical and basal compartments. Proteins were added into the apical compartment, and the transepithelial resistance (TER) was measured; proteins that crossed the cell monolayer were detected in the basal medium by ELISA. Wheat proteins did not alter the cell monolayer. TER and Caco-2 cell viability were conserved, and the passage of dextran was prevented. Native and pepsin-hydrolyzed forms of omega5-gliadin and lipid transfer proteins were detected in the basal medium. The results suggest that these two major allergens in food allergy to wheat were able to cross the cell monolayer by the transcellular route.

The Triticum aestivum non-specific lipid transfer protein (TaLtp) gene family: comparative promoter activity of six TaLtp genes in transgenic rice

Plant non-specific lipid transfer proteins (nsLTPs) are encoded by a multigene family and support physiological functions, which remain unclear. We adapted an efficient ligation-mediated polymerase chain reaction (LM-PCR) procedure that enabled isolation of 22 novel Triticum aestivum nsLtp (TaLtp) genes encoding types 1 and 2 nsLTPs. A phylogenetic tree clustered the wheat nsLTPs into ten subfamilies comprising 1-7 members. We also studied the activity of four type 1 and two type 2 TaLtp gene promoters in transgenic rice using the 1-Glucuronidase reporter gene. The activities of the six promoters displayed both overlapping and distinct features in rice. In vegetative organs, these promoters were active in leaves and root vascular tissues while no beta-Glucuronidase (GUS) activity was detected in stems. In flowers, the GUS activity driven by the TaLtp7.2a, TaLtp9.1a, TaLtp9.2d, and TaLtp9.3e gene promoters was associated with vascular tissues in glumes and in the extremities of anther filaments whereas only the TaLtp9.4a gene promoter was active in anther epidermal cells. In developing grains, GUS activity and GUS immunolocalization data evidenced complex patterns of activity of the TaLtp7.1a, TaLtp9.2d, and TaLtp9.4a gene promoters in embryo scutellum and in the grain epicarp cell layer. In contrast, GUS activity driven by TaLtp7.2a, TaLtp9.1a, and TaLtp9.3e promoters was restricted to the vascular bundle of the embryo scutellum. This diversity of TaLtp gene promoter activity supports the hypothesis that the encoded TaLTPs possess distinct functions in planta.

Specific Adduction of Plant Lipid Transfer Protein by an Allene Oxide Generated by 9-Lipoxygenase and Allene Oxide Synthase

Lipid transfer proteins (LTPs) are ubiquitous plant lipid-binding proteins that have been associated with multiple developmental and stress responses. Although LTPs typically bind fatty acids and fatty acid derivatives in a non-covalent way, studies on the LTPs of barley seeds have identified an abundantly occurring covalently modified form, LTP1b, the lipid ligand of which has resisted clarification. In the present study, this adduct was identified as the alpha-ketol 9-hydroxy-10-oxo-12(Z)-octadecenoic acid. Further studies on the formation of LTP1b demonstrated that the ligand was introduced by nucleophilic attack of the free carboxylate group of the Asp-7 residue of the protein at carbon-9 of the allene oxide fatty acid 9(S),10-epoxy-10,12(Z)-octadecadienoic acid. This reactive oxylipin was produced in barley seeds by oxygenation of linoleic acid by 9-lipoxygenase followed by dehydration of the resulting hydroperoxide by allene oxide synthase. The generation of protein-oxylipin adducts represents a new function for plant allene oxide synthases, enzymes that have earlier been implicated mainly in the biosynthesis of the jasmonate family of plant hormones. Additionally, the LTP-allene oxide synthase interaction opens new perspectives regarding the roles of LTPs in the signaling of plant defense and development.

Stability of barley and malt lipid transfer protein 1 (LTP1) toward heating and reducing agents: relationships with the brewing process

Barley lipid transfer protein (LTP1) is a heat-stable and protease-resistant albumin that concentrates in beer, where it participates in the formation and stability of beer foam. Whereas the barley LTP1 does not display any foaming properties, the corresponding beer protein is surface-active. Such an improvement is related to glycation by Maillard reactions on malting, acylation on mashing, and structural unfolding on brewing. The structural stability of purified barley and glycated malt LTP1 toward heating has been analyzed. Whatever the modification, lipid adduction or glycation, barley LTP1s are highly stable proteins that resisted temperatures up to 100 degrees C. Unfolding of LTP1 occurred only when heating was conducted in the presence of a reducing agent. In the presence of sodium sulfite, the lipid-adducted barley and malt LTP1 displayed higher heat stability than the nonadducted protein. Glycation had no or weak effect on heat-induced unfolding. Finally, it was shown that unfolding occurred on wort boiling before fermentation and that the reducing conditions are provided by malt extract.

Diversity of wheat anti-microbial peptides

From seeds of Triticum kiharae Dorof. et Migusch., 24 novel anti-microbial peptides were isolated and characterized by a combination of three-step HPLC (affinity, size-exclusion and reversed-phase) with matrix-assisted laser-desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry and Edman degradation. Based on sequence similarity and cysteine motifs, partially sequenced peptides were assigned to 7 families: defensins, thionins, lipid-transfer proteins, hevein-like peptides, knottin-like peptides, glycine-rich peptides, and MBP-1 homologs. A novel subfamily of defensins consisting of 6 peptides and a new family of glycine-rich (8 peptides with different repeat motifs) were identified. Three 6-cysteine knottin-like peptides represented by N- and C-terminally truncated variants revealed no sequence homology to any known plant anti-microbial peptides. A new 8-cysteine hevein-like peptide and three 4-cysteine peptides homologous to MBP-1 from maize were isolated. This is the first communication on the occurrence of nearly all families of plant anti-microbial peptides in a single species.

Probing heat-stable water-soluble proteins from barley to malt and beer

Proteins determine the quality of barley in malting and brewing end-uses. In this regard, water-soluble barley proteins play a major role in the formation, stability, and texture of head foams. Our objective was to survey the barley seed proteins that could be involved in the foaming properties of beer. Therefore, two-dimensional (2-D) electrophoresis and mass spectrometry were combined to highlight the barley proteins that could resist the heating treatments occurring during malting and brewing processes. As expected, from barley to malt and to beer, most of the heat-stable proteins are disulfide-rich proteins, implicated in the defense of plants against their bio-aggressors, e.g., serpin-like chymotrypsin inhibitors (protein Z), amylase and amylase-protease inhibitors, and lipid transfer proteins (LTP1 and LTP2). For LTP1s, the complex pattern displayed in 2-D electrophoresis could be related to some chemical modifications already described elsewhere, such as acylation or glycation through Maillard reactions, which occur on malting. Our proteomics approach allowed the identification of the numerous proteins present in beer in addition to the major ones already described. The involvement of these proteins in the quality of beer foam can now be evaluated.

Barley lipid transfer protein, LTP1, contains a new type of lipid-like post-translational modification

In plants a group of proteins termed nonspecific lipid transfer proteins are found. These proteins bind and catalyze transfer of lipids in vitro, but their in vivo function is unknown. They have been suggested to be involved in different aspects of plant physiology and cell biology, including the formation of cutin and involvement in stress and pathogen responses, but there is yet no direct demonstration of an in vivo function. We have found and characterized a novel post-translational modification of the barley nonspecific lipid transfer protein, LTP1. The protein-modification bond is of a new type in which an aspartic acid in LTP1 is bound to the modification through what most likely is an ester bond. The chemical structure of the modification has been characterized by means of two-dimensional homo- and heteronuclear nuclear magnetic resonance spectroscopy as well as mass spectrometry and is found to be lipid-like in nature. The modification does not resemble any standard lipid post-translational modification but is similar to a compound with known antimicrobial activity.