Wheat-germ agglutinin is synthesized as a glycosylated precursor

Wheat Germ Agglutinin—From Toxicity to Biomedical Applications

Wheat germ agglutinin is a hevein class N-Acetylglucosamine–binding protein with specific toxicity and biomedical potential. It is extractable from wheat germ—a low-value byproduct of the wheat industry—using well–established extraction methods based on salt precipitation and affinity chromatography. Due to its N-Acetylglucosamine affinity, wheat germ agglutinin exhibits antifungal properties as well as cytotoxic properties. Its anticancer properties have been demonstrated for various cancer cells, and toxicity mechanisms are well described. Wheat germ agglutinin has been demonstrated as a viable solution for various biomedical and therapeutic applications, such as chemotherapy, targeted drug delivery, antibiotic-resistant bacteria monitoring and elimination. This is performed mostly in conjunction with nanoparticles, liposomes, and other carrier mechanisms via surface functionalization. Combined with abundant wheat byproduct sources, wheat germ agglutinin has the potential to improve the biomedical field considerably.

Wheat germ agglutinin is a biomarker of whole grain content in wheat flour and pasta

When consumed as whole grain, wheat has a high nutrient density that contributes to a healthy diet. Yet, products labeled as whole wheat can still contain a substantial amount of refined grain leading to the confusion for consumers, so a method was designed to determine the whole grain status within wheat-based foods. Wheat germ agglutinin (WGA), a lectin found in the germ tissue of wheat kernels, was evaluated as a biomarker of whole grain wheat. WGA content strongly correlated with the percentage of whole wheat within premade mixtures of whole and refined (white) flours. Then, commercial flours labeled as whole wheat were tested for WGA content and found to contain up to 40% less WGA compared to a whole grain standard. Commercial pasta products labeled as whole wheat were also tested for WGA content and found to contain up to 90% less WGA compared to a whole grain standard. The differences in WGA content were not likely due to varietal differences alone, as the WGA content in common varieties used in domestic wheat flour production varied less than 25%. The levels of other constituents in wheat kernels, including starch, mineral, phytate, and total protein, were not different among the commercial whole wheat flours and pasta products. WGA is a unique biomarker that can identify wheat products with the highest whole grain content. PRACTICAL ABSTRACT: Whole grain wheat has a high nutrient density that can be part of a healthy diet, but products labeled as whole wheat can still contain some refined grain. Wheat germ agglutinin (WGA) was tested as a biomarker to measure whole grain status in wheat-based foods and revealed that some commercial whole wheat flour and pasta contained unexpectedly lower levels of the WGA biomarker compared to a whole grain standard. WGA may therefore be a useful way to test for whole grain wheat content.

The life of phi: the development of phi thickenings in roots of the orchids of the genus Miltoniopsis

Phi thickenings, bands of secondary wall thickenings that reinforce the primary wall of root cortical cells in a wide range of species, are described for the first time in the epiphytic orchid Miltoniopsis. As with phi thickenings found in other plants, the phi thickenings in Miltoniopsis contain highly aligned cellulose running along the lengths of the thickenings, and are lignified but not suberized. Using a combination of histological and immunocytochemical techniques, thickening development can be categorized into three different stages. Microtubules align lengthwise along the thickening during early and intermediate stages of development, and callose is deposited within the thickening in a pattern similar to the microtubules. These developing thickenings also label with the fluorescently tagged lectin wheat germ agglutinin (WGA). These associations with microtubules and callose, and the WGA labeling, all disappear when the phi thickenings are mature. This pattern of callose and WGA deposition show changes in the thickened cell wall composition and may shed light on the function of phi thickenings in plant roots, a role for which has yet to be established.

Visualizing selective neural pathways with WGA transgene: combination of neuroanatomy with gene technology

Functional logic employed by the nervous system for information processing resides mainly in the wiring patterns among specific types of neurons. Therefore, detailed knowledge on neuronal networks is essential for understanding a wide range of brain functions. A powerful and long-awaited method for analyzing the neuronal connectivity patterns would be to deliver tracers selectively to specific types of neurons and at the same time to label transsynaptically their axonal target neurons. For this purpose, we took advantage of a unique property of plant lectin, wheat germ agglutinin (WGA), which has been used as a transsynaptic tracer in classical neuroanatomical studies. We developed a novel genetic strategy that employs WGA cDNA as a transgene, for the visualization of selective and functional neural pathways in the nervous system. In this article, I will introduce several examples of neural pathways visualized with the WGA transgene and discuss about its further refinement and applications.

Plant lectins: occurrence, biochemistry, functions and applications

Growing insights into the many roles of glycoconjugates in biorecognition as ligands for lectins indicates a need to compare plant and animal lectins. Furthermore, the popularity of plant lectins as laboratory tools for glycan detection and characterization is an incentive to start this review with a brief introduction to landmarks in the history of lectinology. Based on carbohydrate recognition by lectins, initially described for concanavalin A in 1936, the chemical nature of the ABH-blood group system was unraveled, which was a key factor in introducing the term lectin in 1954. How these versatile probes are produced in plants and how they are swiftly and efficiently purified are outlined, and insights into the diversity of plant lectin structures are also given. The current status of understanding their functions calls for dividing them into external activities, such as harmful effects on aggressors, and internal roles, for example in the transport and assembly of appropriate ligands, or in the targeting of enzymatic activities. As stated above, attention is given to intriguing parallels in structural/functional aspects of plant and animal lectins as well as to explaining caveats and concerns regarding their application in crop protection or in tumor therapy by immunomodulation. Integrating the research from these two lectin superfamilies, the concepts are discussed on the role of information-bearing glycan epitopes and functional consequences of lectin binding as translation of the sugar code (functional glycomics).

Processing, targeting, and antifungal activity of stinging nettle agglutinin in transgenic tobacco

The gene encoding the precursor to stinging nettle (Urtica dioica L. ) isolectin I was introduced into tobacco (Nicotiana tabacum). In transgenic plants this precursor was processed to mature-sized lectin. The mature isolectin is deposited intracellularly, most likely in the vacuoles. A gene construct lacking the C-terminal 25 amino acids was also introduced in tobacco to study the role of the C terminus in subcellular trafficking. In tobacco plants that expressed this construct, the mutant precursor was correctly processed and the mature isolectin was targeted to the intercellular space. These results indicate the presence of a C-terminal signal for intracellular retention of stinging nettle lectin and most likely for sorting of the lectin to the vacuoles. In addition, correct processing of this lectin did not depend on vacuolar deposition. Isolectin I purified from tobacco displayed identical biological activities as isolectin I isolated from stinging nettle. In vitro antifungal assays on germinated spores of the fungi Botrytis cinerea, Trichoderma viride, and Colletotrichum lindemuthianum revealed that growth inhibition by stinging nettle isolectin I occurs at a specific phase of fungal growth and is temporal, suggesting that the fungi had an adaptation mechanism.

A genetic approach to visualization of multisynaptic neural pathways using plant lectin transgene

The wiring patterns among various types of neurons via specific synaptic connections are the basis of functional logic employed by the brain for information processing. This study introduces a powerful method of analyzing the neuronal connectivity patterns by delivering a tracer selectively to specific types of neurons while simultaneously transsynaptically labeling their target neurons. We developed a novel genetic approach introducing cDNA for a plant lectin, wheat germ agglutinin (WGA), as a transgene under the control of specific promoter elements. Using this method, we demonstrate three examples of visualization of specific transsynaptic neural pathways: the mouse cerebellar efferent pathways, the mouse olfactory pathways, and the Drosophila visual pathways. This strategy should greatly facilitate studies on the anatomical and functional organization of the developing and mature nervous system.

Protein glycosylation. Structural and functional aspects

During the last decade, there have been enormous advances in our knowledge of glycoproteins and the stage has been set for the biotechnological production of many of them for therapeutic use. These advances are reviewed, with special emphasis on the structure and function of the glycoproteins (excluding the proteoglycans). Current methods for structural analysis of glycoproteins are surveyed, as are novel carbohydrate-peptide linking groups, and mono- and oligo-saccharide constituents found in these macromolecules. The possible roles of the carbohydrate units in modulating the physicochemical and biological properties of the parent proteins are discussed, and evidence is presented on their roles as recognition determinants between molecules and cells, or cell and cells. Finally, examples are given of changes that occur in the carbohydrates of soluble and cell-surface glycoproteins during differentiation, growth and malignancy, which further highlight the important role of these substances in health and disease.

Expression and secretion of wheat germ agglutinin by Saccharomyces cerevisiae

Genes encoding pre-protein and prepro-protein of wheat germ agglutinin isolectin 2 (WGA2) were chemically synthesized and expressed in the yeast Saccharomyces cerevisiae under the control of the ENO1 promoter. Yeast harboring either a pre-WGA2 or a prepro-WGA2 gene expression plasmid secreted a mature form of WGA2 into the culture medium. The amount of WGA2 secreted by the strain KS58-2Ddel, which has a ssl1 mutation causing a supersecretion of human lysozyme [Suzuki, K., Ichikawa, K. & Jigami, Y. (1989) Mol. Gen. Genet. 219, 58-64], was 20-fold greater than that secreted by the wild-type strain KK4. The recombinant WGA2 from the cells containing the prepro-WGA2 gene expression plasmid was purified to homogeneity by a three-step ion-exchange chromatography scheme. As in wheat, the N-terminal signal peptide of recombinant WGA2 purified from yeast culture was processed to form an N-terminal 5-oxoprolyl (pyroglutamyl) residue. Likewise, we found that the C-terminal pro-region of recombinant WGA2 had also been processed in yeast. Using electrospray ionization mass spectrometry, we found the processed C-terminus to be heterogeneous in both recombinant WGA2 purified from yeast and in authentic WGA2. The major component of the recombinant WGA2 contained two additional amino acids at its C-terminus compared to that of authentic WGA2. In spite of this difference in the C-terminus, the recombinant WGA2 exhibited a sugar binding activity that was indistinguishable from that of authentic WGA2.

Isolation and characterization of pro-barley lectin expressed in Escherichia coli

Lectins are a class of proteins with specific carbohydrate-binding properties found in a wide variety of plants and animals. Gramineae lectins are presumably defense-related proteins in plants that exert their effect by binding to N-acetylglucosamine. Barley lectin is a vacuolar protein synthesized with an amino-terminal signal sequence for entering the secretory pathway and a carboxyl-terminal propeptide necessary for proper targeting to the vacuole. To analyze the three-dimensional structure of barley lectin with the carboxyl-terminal extension and to investigate whether the conversion of the prolectin into the mature molecule leads to a conformational change, the precursor and the mature forms of barley lectin were expressed in Escherichia coli. Both proteins accumulated in denatured form in inclusion bodies were solubilized in 8 M urea and renatured in a redox buffer system. Active pro- and mature barley lectins were purified to homogeneity by affinity chromatography.

Abscisic acid enhances the transcription of wheat-germ agglutinin mRNA without altering its tissue-specific expression

We used RNA gel-blot analysis, in-situ hybridization, and nuclear run-on transcription to examine the effects of exogenous abscisic acid (ABA) on the spatial distribution of mRNA for the lectin wheat-germ agglutinin (WGA) in developing wheat (Triticum aestivum L. cv. Marshall) embryos and seedlings. When analyzed by RNA gel blots, both developing embryos and seedlings exhibited higher steady-state levels of WGA mRNA after ABA treatment. As determined by in-situ hybridization, incubation of developing embryos in 0.1 mM ABA resulted in accumulation of WGA mRNA in the epidermal and subepidermal cell layers of the radicle and seminal roots and throughout the rootcap and coleorhiza. This spatial distribution was identical to that in control embryos. Nuclear run-on transcription assays indicated that at least part of this increase is attributable to transcriptional induction. Thus, exogenous ABA is capable of inducing increased WGA mRNA accumulation only in cells where it is expressed during normal embryogenesis. When seeds were germinated in the absence of ABA, WGA mRNA was detected only in the rootcap. In contrast, seeds imbibed and germinated in the presence of ABA for 3 d exhibited a spatial distribution of WGA mRNA similar to that observed in developing embryos treated with ABA. In contrast, when ABA was added to 3-d-old seedlings, WGA mRNA was not detected in regions of the root beyond the rootcap. We conclude that exogenous ABA, when applied continuously from imbibition, causes retention of the embryo-specific pattern of WGA mRNA distribution and that the spatial pattern of WGA mRNA expression in roots does not change when ABA is added after germination.

Comparison of the refined crystal structures of two wheat germ isolectins

The crystal structures of two closely related members of the multigene family of wheat lectins (isolectins 1 and 2) have been compared. These isolectins differ at five sequence positions, one being located in the saccharide binding site modulating ligand affinity. Crystals of the two isolectins are closely isomorphous (space group C2). The atomic models are based on structure refinement at 1.8 A resolution in the case of isolectin 2 (WGA2) and 2.0 A resolution in the case of isolectin 1 (WGA1). Refinement results for WGA1, recently completed with a crystallographic R-factor of 16.5% (Fo greater than 3 sigma (Fo)), are presented. Examination of a difference Fourier map, [FWGA2-FWGA1], at 2.0 A resolution and direct superposition of the two models indicated an overall close match of the two structures. Local differences are observed in the region of residues 44 to 69, where three sequence differences occur, and at highly mobile external residues on the surface. The average positional discrepancy (root-mean-square delta r) for corresponding protein atoms in the two crystal structures is 0.64 A for independent protomer I and 0.61 A for protomer II (0.29 A and 0.30 A for main-chain atoms). The mean atomic temperature factors are very similar 20.9 versus 22.0 A2). Regions of high flexibility coincide in the two isolectin structures. Of the 210 water sites identified in WGA1, 144 have corresponding positions in WGA2. A set of 51 well-ordered sites was found to be identical in the two independent environments in both structures, and was considered to be important for structure stabilization. Both of the unique sugar binding sites superimpose very closely, exhibiting root-mean-square positional differences ranging from 0.29 A to 0.42 A. The side-chains of the critical tyrosine residues, Tyr73 (P-site) and Tyr159 (S-site), superimpose best, while other highly flexible aromatic groups (Tyr64 and Trp150) and several water sites display large differences in position (0.5 to 1.0 A) and high temperature factors. The aromatic side-chains of Tyr66 in WGA1 and His66 in WGA2 are oriented similarly.

Sequence variability in three wheat germ agglutinin isolectins: products of multiple genes in polyploid wheat

Three highly homologous wheat germ isolectins (95-97%) are distinct gene products in hexaploid wheat. The amino acid sequences of two of these [wheat germ agglutinin 1 (WGA1) and 2 (WGA2)] are compared with sequence data derived from a complementary DNA (cDNA) clone for the third isolectin (WGA3). This comparison includes three corrections to earlier amino acid sequence data of both WGA1 and WGA2 at positions 109 (from Ser to Phe), 134 (from Gly to Lys), and 150 (from Gly to Trp). These reassignments are based on new results from crystal structure refinement and amino acid sequence data of WGA1, as well as the recently determined nucleotide sequence of WGA3. In addition, the C-terminal residue of WGA1 has been revised to Gly171 and now differs from WGA2 (Ala171). Four other positions, Asn9, Ala53, Gly119, and Ser123, at which WGA1 and WGA2 are identical but differ from the DNA sequence of WGA3, were also reinvestigated by amino acid sequencing techniques and confirmed. Variability among the three isolectins is observed at a total of 10 sequence positions: 9, 53, 56, 59, 66, 93, 109, 119, 123, and 171. Pairwise comparisons indicate that WGA3 deviates to a much larger extent from WGA1 (at eight positions) and from WGA2 (at seven positions) than the latter from one another (at five positions). Eight of the 10 mutations are equally distributed between domains B and C, the two interior and more highly conserved of the four WGA domains (A, B, C, D). Correlation of the variable residues with the three-dimensional structure indicates that all except the two previously described B-domain residues, 56 and 59 (Wright and Olafsdottir 1986), are easily accommodated at the dimer surface. WGA3 displays a higher degree of inter-domain similarity than found in WGA1 and WGA2. Of the seven variable positions that are located in the domain core (residues 3-31), five are in perfect agreement with our earlier predicted domain ancestor sequence. This suggests that of the three isolectins WGA3 is most closely related to the common ancestral molecule.

Cell-type-specific expression of a wheat-germ agglutinin gene in embryos and young seedlings of Triticum aestivum

We have investigated the levels of cell-specific expression of wheat-germ agglutinin (WGA) during the development of embryos and in 3-d-old seedlings. Southern blot analysis of genomic DNA derived from hexaploid and diploid wheat (Triticum) species indicates that the isolated cDNA (complementary DNA) clone is specific for isolectin B (WGA-B). Specific accumulation of transcript for WGA-B was determined by RNA blot analysis and in-situ hybridization. The WGA-B mRNA increased tenfold during embryogenesis between 10 and 40 d post-anthesis and then declined again as the seed reached maturity. The root tips of 3-d-old seedlings contained approximately threefold more WGA-B mRNA than mature seeds. In-situ hybridization experiments showed that during embryogenesis, WGA-B mRNA was present only in the epidermal layers of the radicle and the coleorhiza, while in 3-d-old seedlings it was found in the root-cap cells. To analyze de-novo transcription of WGA-B mRNA a new variation of in-situ hybridization was developed. RNAs from developing embryos and seedlings were labeled with [(3)H]uridine and then hybridized in-situ with unlabeled antisense and sense transcripts. These experiments demonstrated that accumulation of WGA-B mRNA in embryos and 3-d-old seedlings resulted, at least partially, from de-novo transcription. All cells containing WGA-B mRNA also contained WGA, as shown by immunocytochemistry.