Extensive homology between the subunits of the phytohemagglutinin mitogenic proteins derived from Phaseolus vulgaris

Molecular and cellular characterization of European sea bass CD3ε+ T lymphocytes and their modulation by microalgal feed supplementation

The CD3 coreceptor is a master T cell surface marker, and genes encoding CD3ζ, γδ, and ε chains have been reported in several teleost fish. Here, a complete cDNA sequence of CD3ɛ chain was identified from a sea bass (Dicentrarchus labrax L.) gill transcriptome. Its basal expression was quantified in both lymphoid and non-lymphoid organs of sea bass juveniles with real-time qPCR analysis. After either in vitro stimulation of head kidney leukocytes with the T-cell mitogen phytohaemagglutinin or in vivo stimulation with an orally administered Vibrio anguillarum vaccine, CD3ε expression levels increased in head kidney leukocytes, confirming that CD3ε T cells may play important roles in fish systemic protection against pathogens. Further, three peptides were designed on the CD3ɛ cytoplasmic tail region and employed as immunogens for antibody production in rabbit. One antiserum so obtained, named RACD3/1, immunostained a band of the expected size in a western blot of a sea bass thymocyte lysate. The distribution of CD3ε⁺ lymphocyte population in the lymphoid organs and mucosal tissues was addressed in healthy fish by IHC. In decreasing percentage order, CD3ε⁺ lymphocytes were detected by flow cytometry in thymus, peripheral blood leukocytes, gills, head kidney, gut, and spleen. Finally, a significant in vivo enhancement of CD3ε⁺ T intestinal lymphocytes was found in fish fed on diets in which 100% fish meal was replaced by the microalgae Nannochloropsis sp. biomass. These results indicate that CD3ε⁺ T cells are involved in nutritional immune responses.

Calcium redistribution contributes to the hard-to-cook phenotype and increases PHA-L lectin thermal stability in common bean low phytic acid 1 mutant seeds

Seed phytic acid reduces mineral bioavailability by chelating minerals. Consumption of common bean seeds with the low phytic acid 1 (lpa1) mutation improved iron status in human trials but caused adverse gastrointestinal effects, presumably due to increased stability of lectin phytohemagglutinin L (PHA-L) compared to the wild type (wt). A hard-to-cook (HTC) defect observed in lpa1 seeds intensified this problem. We quantified the HTC phenotype of lpa1 common beans with three genetic backgrounds. The HTC phenotype in the lpa1 black bean line correlated with the redistribution of calcium particularly in the cell walls, providing support for the "phytase-phytate-pectin" theory of the HTC mechanism. Furthermore, the excess of free cations in the lpa1 mutation in combination with different PHA alleles affected the stability of PHA-L lectin.

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.

Expression of Lectins in Heterologous Systems

Lectins are proteins that have the ability to recognize and bind in a reversible and specific way to free carbohydrates or glycoconjugates of cell membranes. For these reasons, they have been extensively used in a wide range of industrial and pharmacological applications. Currently, there is great interest in their production on a large scale. Unfortunately, conventional techniques do not provide the appropriate platform for this purpose and therefore, the heterologous production of lectins in different organisms has become the preferred method in many cases. Such systems have the advantage of providing better yields as well as more homogeneous and better-defined properties for the resultant products. However, an inappropriate choice of the expression system can cause important structural alterations that have repercussions on their biological activity since the specificity may lay in their post-translational processing, which depends largely on the producing organism. The present review aims to examine the most representative studies in the area, exposing the four most frequently used systems (bacteria, yeasts, plants and animal cells), with the intention of providing the necessary information to determine the strategy to follow in each case as well as their respective advantages and disadvantages.

Favin versus concanavalin A: Circularly permuted amino acid sequences

We have determined the tentative amino acid sequence of the beta chain (M(r) 20,000) of the lectin favin. In previous studies, we have shown that the alpha chain (M(r) 5600) of this lectin is homologous to a region in the middle of the concanavalin A (Con A) sequence (residues 70-119). Now we present evidence that the beta chain is homologous to two discrete segments of Con A. The homology begins at residue 120 of Con A, extends to the COOH terminus (residue 237) and continues without interruption through the NH(2)-terminal 69 residues of Con A. Together, the alpha and beta chains of favin account for a polypeptide chain equivalent in size to that of Con A. The comparison of the two proteins thus reveals a circular permutation of extensive homologous sequences. The favin molecule contains residues identical to many of the residues postulated to be involved in sugar binding by Con A, and contains all of the direct metal ligands as well as residues homologous to most of the residues that form the beta-pleated sheets of Con A. These homologies suggest that the three-dimensional structures of the two lectins are likely to be very similar. Moreover, favin appears to be even more closely related in primary structure and sugar specificity to the lectins from pea and lentil, raising the possibility that all of these lectins may have structures that resemble Con A. Some of these similarities may also extend to the lectins from soybean, peanut, and red kidney bean, which have different sugar specificities but share sequence homologies with the favin beta chain.

Physiology of the prion protein

Prion diseases are transmissible spongiform encephalopathies (TSEs), attributed to conformational conversion of the cellular prion protein (PrP(C)) into an abnormal conformer that accumulates in the brain. Understanding the pathogenesis of TSEs requires the identification of functional properties of PrP(C). Here we examine the physiological functions of PrP(C) at the systemic, cellular, and molecular level. Current data show that both the expression and the engagement of PrP(C) with a variety of ligands modulate the following: 1) functions of the nervous and immune systems, including memory and inflammatory reactions; 2) cell proliferation, differentiation, and sensitivity to programmed cell death both in the nervous and immune systems, as well as in various cell lines; 3) the activity of numerous signal transduction pathways, including cAMP/protein kinase A, mitogen-activated protein kinase, phosphatidylinositol 3-kinase/Akt pathways, as well as soluble non-receptor tyrosine kinases; and 4) trafficking of PrP(C) both laterally among distinct plasma membrane domains, and along endocytic pathways, on top of continuous, rapid recycling. A unified view of these functional properties indicates that the prion protein is a dynamic cell surface platform for the assembly of signaling modules, based on which selective interactions with many ligands and transmembrane signaling pathways translate into wide-range consequences upon both physiology and behavior.

Large-scale production, purification, and characterisation of recombinant Phaseolus vulgaris phytohemagglutinin E-form expressed in the methylotrophic yeast Pichia pastoris

The kidney bean lectin Phaseolus vulgaris phytohemagglutinin E-form (PHA-E) was expressed and secreted by the methylotrophic yeast Pichia pastoris. To optimise yields of PHA-E, transformants of P. pastoris were selected for high-level production of the recombinant protein. A scaleable process for the production and purification of gram quantities of recombinant PHA-E is reported. PHA-E was secreted at approximately 100 mg/L at the 2- and 200-L scale and was purified to 95% homogeneity in a single step using cation-exchange chromatography. The purified recombinant PHA-E consists of four forms with molecular masses between 28.5 and 31.5 kDa, as assessed by MALDI-TOF, whereas its native counterpart has a molecular mass of approximately 30.5 kDa. Endoglycosidase treatment revealed that the range in size of the recombinant protein was attributed to differences in the nature of the N-linked oligosaccharides bound to the protein. The primary amino acid sequence of the recombinant PHA-E was found to be identical to the native protein and to have an agglutination activity similar to that of native PHA-E. The data presented here suggest that, using P. pastoris, gram quantities of a recombinant phytohemagglutinin E-form can be produced and that the recombinant protein is similar to the protein synthesised in plants with respect to structure and biological activity.

Functional phytohemagglutinin (PHA) and Galanthus nivalis agglutinin (GNA) expressed in Pichia pastoris correct N-terminal processing and secretion of heterologous proteins expressed using the PHA-E signal peptide

Phytohemagglutinin (Phaseolus vulgaris agglutinin; PHA; E- and L-forms) and snowdrop lectin (Galanthus nivalis agglutinin; GNA) were expressed in Pichia pastoris using native signal peptides, or the Saccharomyces alpha-factor preprosequence, to direct proteins into the secretory pathway. PHA and GNA were present as soluble, functional proteins in culture supernatants when expressed from constructs containing the alpha-factor preprosequence. The recombinant lectins, purified by affinity chromatography, agglutinated rabbit erythrocytes at concentrations similar to the respective native lectins. However, incomplete processing of the signal sequence resulted in PHA-E, PHA-L and GNA with heterogenous N-termini, with the majority of the protein containing N-terminal extensions derived from the alpha-factor prosequence. Polypeptides in which most of the alpha-factor prosequence was present were also glycosylated. Inclusion of Glu-Ala repeats at the C-terminal end of the alpha-factor preprosequence led to efficient processing N-terminal to the Glu-Ala sequence, but inefficient removal of the repeats themselves, resulting in polypeptides with heterogenous N-termini still containing N-terminal extensions. In contrast, PHA expressed with the native signal peptide was secreted, correctly processed, and also fully functional. No expression of GNA from a construct containing the native GNA signal peptide was observed. The PHA-E signal peptide directed correct processing and secretion of both GNA and green fluorescent protein (GFP) when used in expression constructs, and is suggested to have general utility for synthesis of correctly processed proteins in Pichia.

A- and B-subunit variant distribution in the holoprotein variants of protein toxin abrin: variants of abrins I and III have constant toxic A subunits and variant lectin B subunits

The cytotoxic lectin abrin shows more than 30 variant forms (R. Hegde, T. K. Maiti, and S. K. Podder, 1991, Anal. Biochem. 194, 101-109). The lectin B subunit as cause for variance in abrins I and III was detected by a combination of one- and two-dimensional electrophoresis and Western blotting. Intriguingly, in abrin I but not in abrin III, association of a single A subunit with the variant B subunits shifts the holoprotein pI toward the alkaline side indicating that the subunit association involves neutralization of few negative charges. The B-subunit variants of abrins I and III overlap in their pI, and the A-subunit association gives the holoproteins a distinctness on isoelectric focusing gel. The results were also confirmed by analyzing the pH titration curves. These differences in the subunit association pattern between abrins I and III are in corroboration with the previously observed differences in the kinetics of protein synthesis inactivation and accessibility of the disulfide bridge to reducing agents in the presence or absence of putative receptor (R. Hegde, A. Karande, and S. K. Podder, 1993 Eur. J. Biochem. 215, 411-419). Further, the genetic origin of variance was confirmed by peptide mapping of the individual subunit variants. Considering a theoretical value of 0.1 to 0.2 pI/charge, a 15-17 charge difference could be predicted between the variants of two extreme pIs. The fact that the A subunits are not shared between the groups was taken to interpret that the protein synthesized as prepro form is processed posttranslationally and the processing takes place only after the disulfide bond formation between A and B subunits. The N-terminal 16 amino acids of A subunits of abrins I and III showed 26% dissimilarity. The A subunits of abrins I and III did not react with concanavalin A, indicating that the heterogeneity in the molecular weight is because of differential processing but not because of glycosylation.

Hard-to-cook phenomenon in common beans--a review

Legumes are one of the world's most important sources of food supply, especially in developing countries, in terms of food energy as well as nutrients. Common beans are a good source of proteins, vitamins (thiamine, riboflavin, niacin, vitamin B6) and certain minerals (Ca, Fe, Cu, Zn, P, K, and Mg). They are an excellent source of complex carbohydrates and polyunsaturated free fatty acids (linoleic, linolenic). However, common beans have several undesirable attributes, such as long cooking times, being enzyme inhibitors, phytates, flatus factors, and phenolic compounds, having a "beany" flavor, and being lectins and allergens, which should be removed or eliminated for effective utilization. Grain quality of common beans is determined by factors such as acceptability by the consumer, soaking characteristics, cooking quality, and nutritive value. Acceptability characteristics include a wide variety of attributes, such as grain size, shape, color, appearance, stability under storage conditions, cooking properties, quality of the product obtained, and flavor. Storage of common beans under adverse conditions of high temperature and high humidity renders them susceptible to a hardening phenomenon, also known as the hard-to-cook (HTC) defect. Beans with this defect are characterized by extended cooking times for cotyledon softening, are less acceptable to the consumer, and are of lower nutritive value. Mechanisms involved in the HTC defect have not been elucidated satisfactorily. Attempts to provide a definitive explanation of this phenomenon have not been successful. The most important hypotheses that have been proposed to explain the cause of bean hardening are (1) lipid oxidation and/or polymerization, (2) formation of insoluble pectates, (3) lignification of middle lamella, and (4) multiple mechanisms. Most researchers have reported that the defect develops in the cotyledons. Recently, some authors have suggested that the seed coat plays a significant role in the process of common bean hardening. A better knowledge of cotyledon and seed coat microstructure may lead to a better understanding of the causes of seed hardness. In order to prevent the development of the HTC defect several procedures have been proposed: (1) appropriate storage, (2) controlled atmospheres, and (3) pretreatments. Probably, the most workable solution to the hardening phenomenon may be the development of materials less prone to HTC phenomenon. Decreasing cooking time, increasing nutritive value, and improving sensory properties of seeds with HTC defect would have great nutritional and economical impact.(ABSTRACT TRUNCATED AT 400 WORDS)

Studies on the variants of the protein toxins ricin and abrin

This study elucidates some structural and biological features of galactose-binding variants of the cytotoxic proteins ricin and abrin. An isolation procedure is reported for ricin variants from Ricinus communis seeds by using lactamyl-Sepharose affinity matrix, similar to that reported previously for variants of abrin from Abrus precatorius seeds [Hegde, R., Maiti, T. K. & Podder, S. K. (1991) Anal. Biochem. 194, 101-109]. Ricin variants, subfractionated on carboxymethyl-Sepharose CL-6B ion-exchange chromatography, were characterized further by SDS/PAGE, IEF and a binding assay. Based on the immunological cross-reactivity of antibody raised against a single variant of each of ricin and abrin, it was established that all the variants of the corresponding type are immunologically indistinguishable. Analysis of protein titration curves on an immobilized pH gradient indicated that variants of abrin I differ from other abrin variants, mainly in their acidic groups and that variance in ricin is a cause of charge substitution. Detection of subunit variants of proteins by two-dimensional gel electrophoresis showed that there are twice as many subunit variants as there are variants of holoproteins, suggesting that each variant has a set of subunit variants, which, although homologous, are not identical to the subunits of any other variant with respect to pI. Seeds obtained from polymorphic species of R. communis showed no difference in the profile of toxin variants, as analyzed by isoelectric focussing. Toxin variants obtained from red and white varieties of A. precatorius, however, showed some difference in the number of variants as well as in their relative intensities. Furthermore, variants analyzed from several single seeds of A. precatorius red type revealed a controlled distribution of lectin variants in three specific groups, indicating an involvement of at least three genes in the production of Abrus lectins. The complete absence or presence of variants in each group suggested a post-translational differential proteolytic processing, a secondary event in the production of abrin variants.

Isolation and Partial Characterization of a Seed Lectin from Tepary Bean that Delays Bruchid Beetle Development

Four isolectin forms of a seed lectin from mature seed of tepary bean (Phaseolus acutifolius) were isolated using solubility fractionation, affinity chromatography, and high performance liquid chromatography. The subunits are polypeptides with an apparent molecular mass of 30,000 daltons. The 30 kilodalton subunits are produced starting approximately 13 days after flowering and subsequently comprise a major fraction of the proteins found in the mature seed. The amino terminus of each isolectin fraction was determined to be highly homologous with that of the subunits of common bean (Phaseolus vulgaris L.) phytohemagglutinin (PHA). The tepary isolectin cross-reacts with both erythroagglutinating and leucoagglutinating subunits of PHA antibodies, although differential cross-reactivity was noted. A seed protein fraction enriched in tepary bean lectin was found to be toxic to bean bruchid beetles (Acanthoscelides obtectus), when incorporated into their diets at incremental concentrations from (1-5% w/w) above that of PHA concentrations in mature seeds of the susceptible common bean variety "Red Kidney."

The amino acid sequence of Erythrina corallodendron lectin and its homology with other legume lectins

The primary sequence of Erythrina corallodendron lectin was deduced from analysis of the peptides derived from the lectin by digestion with trypsin, chymotrypsin, Staphylococcus aureus V8 protease, elastase and lysylendopeptidase-C, and of fragments generated by cleavage of the lectin with dilute formic acid in 6 M guanidine hydrochloride. Purification of the individual peptides was achieved by gel filtration, followed by reverse phase HPLC. The glycosylation site (Asn17-Leu18-Thr19) was deduced from analysis of the glycopeptide isolated from a pronase digest of the lectin before and after deglycosylation of the glycopeptide with endoglycosidase F. Comparison of the sequence of 244 residues thus obtained with those of 9 other legume lectins revealed extensive homologies, including 39 invariant positions and 60 partial identities. These data provide further evidence for the conservation of the lectin gene in leguminous plants.

Transport of proteins to the plant vacuole is not by bulk flow through the secretory system, and requires positive sorting information

Plant cells, like other eukaryotic cells, use the secretory pathway to target proteins to the vacuolar/lysosomal compartment and to the extracellular space. We wished to determine whether the presence of a hydrophobic signal peptide would result in the transport of a reporter protein to vacuoles by bulk flow; to investigate this question, we expressed a chimeric gene in transgenic tobacco. The chimeric gene, Phalb, used for this study consists of the 1,188-bp 5' upstream sequence and the hydrophobic signal sequence of a vacuolar seed protein phytohemagglutinin, and the coding sequence of a cytosolic seed albumin (PA2). The chimeric protein PHALB cross-reacted with antibodies to PA2 and was found in the seeds of the transgenic plants (approximately 0.7% of total protein), but not in the leaves, roots, or flowers. Immunoblot analyses of seed extracts revealed four glycosylated polypeptides ranging in molecular weight from 29,000 to 32,000. The four polypeptides are glycoforms of a single polypeptide of Mr 27,000, and the heterogeneity is due to the presence of high mannose and endoglycosidase H-resistant glycans. The PHALB products reacted with an antiserum specific for complex plant glycans indicating that the glycans had been modified in the Golgi apparatus. Subcellular fractionation of glycerol extracts of mature seeds showed that only small amounts of PHALB accumulated in the protein storage vacuoles of the tobacco seeds. In homogenates made in an isotonic medium, very little PHALB was associated with the organelle fraction containing the endoplasmic reticulum and Golgi apparatus; most of it was in the soluble fraction. We conclude that PHALB passed through the Golgi apparatus, but did not arrive in the vacuoles. Transport to vacuoles is not by a bulk-flow mechanism, once proteins have entered the secretory system, and requires information beyond that provided by a hydrophobic signal peptide.

Correct glycosylation, Golgi-processing, and targeting to protein bodies of the vacuolar protein phytohemagglutinin in transgenic tobacco

We used a heterologous system (transgenic Nicotiana tabacum L.) to investigate the processing, assembly and targeting of phytohemagglutinin (PHA), the lectin of the common bean, Phaseolus vulgaris L. In the bean, this glycoprotein accumulates in the protein bodies of the storage parenchyma cells in the cotyledons, and each polypeptide has a high-mannose glycan attached to Asn12 and a complex glycan on Asn60. The gene for PHA-L, dlec2, with 1200 basepairs (bp) 5' upstream and 1600 bp 3' downstream from the coding sequence was introduced into tobacco using Agrobacterium-mediated transformation (T. Voelker et al., 1987, EMBO J. 6, 3571-3577). Examination of thin sections of tobacco seeds by immunocytochemistry with antibodies against PHA showed that PHA-L accumulated in the amorphous matrix of the protein bodies in the embryo and endosperm. This localization was confirmed using a non-aqueous method to isolate the protein bodies from mature tobacco seeds. The biochemical analysis of tobacco PHA indicated that the signal peptide had been correctly removed, and that the polypeptides formed 6.4 S oligomers; tobacco PHA had a high-mannose glycan at Asn12 and a complex glycan at Asn60. The presence of the complex glycan shows that transport to the protein bodies was mediated by the Golgi complex. At seed maturity, a substantial portion of the PHA-L remained associated with the endoplasmic reticulum and the Golgi complex, as indicated by fractionation experiments using aqueous media and the presence of two high-mannose glycans on some of the polypeptides. Taken together, these data show that insertion of the nascent PHA into the endoplasmic reticulum, signal peptide processing, glycosylation, assembly into oligomers, glycan modification in the Golgi, and targeting of the protein occur faithfully in this heterologous system, although transport may not be as efficient as in bean cotyledons.

The plant vacuolar protein, phytohemagglutinin, is transported to the vacuole of transgenic yeast

Phytohemagglutinin (PHA), the major seed lectin of the common bean, Phaseolus vulgaris, accumulates in the parenchyma cells of the cotyledons. It has been previously shown that PHA is cotranslationally inserted into the endoplasmic reticulum with cleavage of the NH2-terminal signal peptide. Two N-linked oligosaccharide side chains are added, one of which is modified to a complex type in the Golgi apparatus. PHA is then deposited in membrane-bound protein storage vacuoles which are biochemically and functionally equivalent to the vacuoles of yeast cells and the lysosomes of animal cells. We wished to determine whether yeast cells would recognize the vacuolar sorting determinant of PHA and target the protein to the yeast vacuole. We have expressed the gene for leukoagglutinating PHA (PHA-L) in yeast under control of the yeast acid phosphatase (PHO5) promoter. Under control of this promoter, PHA-L accumulates to 0.1% of the total yeast protein. PHA-L produced in yeast is glycosylated as expected for a yeast vacuolar glycoprotein. Cell fractionation studies show that PHA-L is efficiently transported to the yeast vacuole. This is the first demonstration that vacuolar targeting information is recognized between two highly divergent species. A small proportion of yeast PHA-L is secreted which may be due to inefficient recognition of the vacuolar sorting signal because of the presence of an uncleaved signal peptide on a subset of the PHA-L polypeptides. This system can now be used to identify the vacuolar sorting determinant of a plant vacuolar protein.

cDNA cloning and in vitro synthesis of the Dolichos biflorus seed lectin

The Dolichos biflorus seed lectin contains two structurally related subunits. A cDNA library was constructed using RNA isolated from D. biflorus seeds actively synthesizing the seed lectin. The library was expressed in Escherichia coli using a lambda Charon 16 vector, and lectin-specific antiserum was used to isolate a seed lectin cDNA. Hybridization of the D. biflorus seed lectin cDNA to RNA isolated from seeds actively producing both lectin subunits identifies a single-size RNA of 1100 bases. An oligodeoxyribonucleotide probe, constructed from an amino acid sequence common to both lectin subunits, detects the same size RNA. Translation of seed mRNA in vitro and immunoprecipitation of translation products using a lectin-specific antiserum yields a single polypeptide of slightly higher molecular mass than the largest seed lectin subunit. This seed lectin precursor is indistinguishable from a polypeptide synthesized from mRNA hybrid selected by the seed lectin cDNA. These data support the existence of a single polypeptide precursor for both subunit types of the D. biflorus seed lectin and suggest that differences between the subunit types arise by posttranslational processing.

The position of the oligosaccharide side-chains of phytohemagglutinin and their accessibility to glycosidases determines their subsequent processing in the Golgi

Phytohemagglutinin (PHA), the glycoprotein lectin of Phaseolus vulgaris has two types of asparagine-linked oligosaccharides per polypeptide: a high-mannose chain with the formula (Man)8-9(GlcNAc)2 on Asn12 and a modified chain with fewer mannose residues and additional fucose and xylose residues on Asn60. Glycosylation of PHA is a cotranslational process, which occurs in the endoplasmic reticulum, and newly synthesized PHA has two high-mannose chains. Transport of PHA to the protein bodies via the Golgi complex is accompanied by the modification of one of the two high-mannose chains. Why is only one chain modified, while the other remains in the high-mannose configuration? By determining the effect of digestion with various glycosidases (alpha-mannosidase, endo-beta-N-acetylglucosaminidase H and endo-beta-N-acetylglucosaminidase F) on native and denatured PHA we obtained evidence consistent with the interpretation that the accessibility of oligosaccharide chains to modifying enzymes is of major importance in determining whether a high-mannose chain becomes modified or not. The high-mannose chain of mature undenatured PHA is only partially accessible to glycosidases, while PHA obtained from the endoplasmic reticulum has one high-mannose chain, which is readily accessible to alpha-mannosidase and endoglycosidases H and F. We show that this readily accessible chain is in the same position on the polypeptide (Asn60) as the modified oligosaccharide on mature PHA. Thus, accessibility of the oligosaccharide side-chains to processing enzymes in the Golgi determines whether a particular oligosaccharide side-chain is processed or not.

Griffonia simplicifolia I isolectin as a functionally monovalent probe for use in flow cytometry

The inherent tendency of lectins to agglutinate cells has limited their use as reagents for the detection of carbohydrate groups on cell surfaces by flow cytometry. In the current study, we demonstrate a method for the use of a fluoresceinated tetrameric isolectin (Griffonia simplicifolia I-A3B, FITC-GS I-A3B) as a functionally monovalent, nonagglutinating probe in flow cytometry. This isolectin contains three A subunits and one B subunit. Both types of subunits bind alpha-D-galactopyranosyl (alpha-D-galp-) end groups with similar affinities; however, the A subunits have a 1,000-fold greater affinity for N-Acetyl-D-galactosamine (GalNAc) than does the B subunit. The addition of low (1-2 mM) concentrations of GalNAc to the FITC-GS I-A3B isolectin results in blockage of the three A subunits without significantly affecting the B subunit; this yields a functionally monovalent probe for the detection of cell surface alpha-D-Galp end groups. This approach has been used to examine two types of cells: Ehrlich ascites tumor cells and rat alveolar macrophages, both of which are known to express cell surface alpha-D-Galp end groups. Lectin binding, as determined by number of positive cells and fluorescence intensity, was dependent upon concentration of the lectin and haptenic sugar. Specificity of the staining was demonstrated by the ability of methyl alpha-D-galactopyranoside (Met alpha-D-Galp) to abolish the binding of the lectin to the cells. Elimination of both GalNAc and Met alpha-D-Galp from the staining solution resulted in agglutination of the cells, indicating that the A subunits were active in the absence of GalNAc.

Sambucus nigra agglutinin is located in protein bodies in the phloem parenchyma of the bark

The bark of some young woody stems contains storage proteins which are subject to an annual rhythm: they accumulate in the autumn and are mobilized in the spring. We show here that the bark phoem-parenchyma cells of Sambucus nigra L. contain numerous protein bodies, and that the bark lectin (S. nigra agglutinin) which undergoes an annual rhythm is localized in these protein bodies. The protein bodies in the cotyledons of legume seeds also contain lectin, indicating that lectins may be storage compounds themselves or may have a function in storage and-or mobilization processes.

A one-step procedure for isolation and resolution of the Phaseolus vulgaris isolectins by affinity chromatography

A new fast-working one-step method for the resolution of the Phaseolus vulgaris isolectins is described which requires inexpensive materials.

Characterization of two Phaseolus vulgaris phytohemagglutinin genes closely linked on the chromosome

A lambda 1059 library of Phaseolus vulgaris cv. 'Tendergreen' DNA was screened with a cloned lectin-like cDNA. Among the phages selected was clone lambda B10 containing two complete lectin genes in the same orientation approximately 4 kb apart. The DNA sequences of the lectin genes and their flanking regions have been determined and their transcriptional initiation sites were located by S1 nuclease mapping. On the basis of the deduced amino acid sequences and compositions and the mol. wts. of their encoded glycoproteins, the genes, dlec1 and dlec2, are predicted to encode erythro- and leucoagglutinating phytohemagglutinins (PHA-E and PHA-L), respectively. The mRNA coding regions of dlec1 and dlec2 are 90% homologous, suggesting an origin involving duplication of an ancestral gene. Both lectin genes are intronless and have at least two ATG codons in a short (11-14 bp) 5'-untranslated region. Most of their 5'-untranslated regions consist of alternating pyrimidines and purines (RY repeats). Upstream sequences are also highly conserved between dlec1 and dlec2, including stretches of nine or more alternating R and Y residues. RY repeats of such length are not found within the protein coding portion of dlec1, dlec2 or a Phaseolus lectinlike gene previously described. Overlapping double (dlec1) or triple (dlec2) polyadenylation addition signals are found and there is an unusually high degree of homology (84%) between their 3'-untranslated regions.(ABSTRACT TRUNCATED AT 250 WORDS)

Bean lectins : 5. Quantitative genetic variation in seed lectins of Phaseolus vulgaris L. and its relationship to qualitative lectin variation

Seeds of forty bean cultivars having different lectin types based on two-dimensional isoelectric focusing-sodium dodecyl sulfate polyacrylamide gel electrophoresis (IEF-SDS/PAGE) were analyzed for quantities of lectin, phaseolin and total protein. Significant differences were found among groups of cultivars with different lectin types for the quantity of lectin and phaseolin. Cultivars with more complex lectin types based on IEF-SDS/PAGE tended to have higher quantities of lectin and lower quantities of phaseolin than cultivars with simple lectin types. An association between lectin type and the quantity of lectin and phaseolin was found also in the seeds of F2 plants that segregated in a Mendelian fashion for two lectin types. Seeds from plants with the complex lectin type had more lectin and less phaseolin than seeds from plants with the simple lectin type. Therefore, the genes controlling qualitative lectin variation also may influence the quantitative variation of lectin and phaseolin. The results of this study are related to other studies on the quantitative variation for seed proteins and to the possible molecular basis for variation in the quantity of lectins in beans.

Biosynthesis and processing of phytohemagglutinin in developing bean cotyledons

Phytohemagglutinin (PHA) is a family of tetrameric isolectins which accumulate in the protein bodies of developing Phaseolus vulgaris cotyledons. Each tetramer contains erythroagglutinating (E) or lymphocyte-mitogenic (L) subunits, or a combination of both. The subunits have Mr around 33000, E being slightly larger than L. Phytohemagglutinin is a glycoprotein, and its carbohydrate moiety contains N-acetylglucosamine, mannose, fucose and xylose, indicating that this protein has complex oligosaccharide sidechains. Several steps in the biosynthesis and in the cotranslational and post-translational processing of the glycopolypeptides of PHA have been identified. The polypeptides of PHA are synthesized by polysomes attached to the endoplasmic reticulum. The glycosylation of the polypeptides is a cotranslational process, in which each PHA polypeptide usually acquires two oligosaccharide sidechains. The oligosaccharides of PHA isolated from the endoplasmic reticulum are susceptible to digestion with alpha-mannosidase and endo-beta-N-acetylglucosaminidase H indicating that they are of the high-mannose type. In the presence of tunicamycin two unglycosylated polypeptides of PHA are synthesized, indicating that the differences in Mr between the E and L subunits of PHA are not due to differences in glycosylation alone. Transport of PHA to the protein bodies is mediated by the Golgi apparatus where at least part of the oligosaccharide chains of PHA are modified [ Chrispeels , M. J. (1983) Planta ( Berl .) 157, 454-461, and 158, 140-151]. The modified oligosaccharide chains of PHA are then gradually trimmed to a smaller size when the protein is already in the protein bodies. This processing results in an increase in the mobility of the PHA subunits in denaturing polyacrylamide gels.

Molecular cloning of Phaseolus vulgaris lectin mRNA and use of cDNA as a probe to estimate lectin transcript levels in various tissues

The complete amino acid sequence of a Phaseolus vulgaris cv. Tendergreen lectin was deduced from the nucleotide sequence of a cDNA plasmid. The lectin appears to be synthesized as a prepeptide consisting of a signal sequence of at least 20 residues and a mature protein of 223 amino acids. The 5' region of the lectin cDNA contains four AUG codons, one of which may initiate a 58 amino acid peptide out of phase with lectin translation. Lectin transcripts 1.1 kb long begin accumulating in cotyledons by the 9 mm stage, increasing in amount until about the 16 mm stage, after which their levels dramatically decrease. In addition, larger lectin-specific transcripts are detected in mid-maturation phase cotyledons. A 1.1 kb RNA species in leaves from plants filling their pods hybridizes with lectin cDNA, but no hybridizing RNA is found in primary leaves or roots.

Bean lectins : Part 2: Relationship between qualitative lectin variation in Phaseolus vulgaris L. and previous observations on purified bean lectins

The relationship between the polypeptide composition and the agglutination behaviour of the lectin-containing G2/albumin protein groups has allowed the identification of the active lectin polypeptides in different cultivars of Phaseolus vulgaris (Brown et al. accompanying paper). These results were used to ascertain the particular G2/albumin group contained in the various lectin sources used previously for the purification of lectin proteins. Many studies were found to have included lectin sources which contained the same G2/albumin pattern (TG2) and this common denominator has permitted the direct comparison of the properties reported for these purified lectins. Thus, much of the extensive literature on bean lectins is concurred.

Immunocytochemical localisation of lectins in cells of Phaseolus vulgaris L. seeds

Antibodies against pure E4- and L4-lectins from the seeds of Phaseolus vulgaris L. raised in rabbits were made monospecific by immunoaffinity chromatography on E4- or L4-lectin Sepharose 4B columns. Localisation of lectins in bean seeds was investigated by indirect immunofluorescence and by electron microscopy on sections stained with colloidal gold particles coated with monospecific anti-E4- and anti-L4-IgG. In parenchyma cells from the cotyledons both E- and L-type lectins were found inside the protein bodies. Apparently the matrix of all protein bodies contained both types of lectins. On the other hand in vascular and in axis cells the two types of lectins were localised in the cytoplasm, outside the protein bodies. Thus these findings suggest different roles for the lectins: in cotyledons this may be a specific form of N storage, while in vascular and axis cells lectins may have a more direct metabolic part to play.

Cell-surface discoidin in aggregating cells of Dictyostelium discoideum

Both discoidin I and discoidin II have been detected on the surface of aggregating (10 h developmental stage) cells of Dictyostelium discoideum NC4 by radioiodination of the cell-surface followed by immunoprecipitation and sodium dodecyl sulphate/polyacrylamide-gel-electrophoretic analysis. Approx. 92% of cell-surface discoidin I and 72% of cell-surface discoidin II can be eluted with 0.5 M-galactose, showing that most of each endogenous lectin is not present as integral membrane protein but rather is bound to cell-surface discoidin receptors. Two-dimensional polyacrylamide-gel-electrophoretic analysis of discoidin I suggests that the native tetramer may be a hetero-multimer composed of both Ia and Ib subunits. Cell-surface discoidin I also contains both types of subunit, but it is not clear whether both subunits have corresponding cell-surface receptors.

Isolation of the Maclura pomifera hemagglutinin on a deoxymelibiotol affinity support and preliminary characterization by buffer electrofocusing and high-performance liquid chromatography

Maclurin, the potent non-specific blood-group hemagglutinin present in extracts of Maclura pomifera, has been purified by a new biospecific affinity-chromatographic procedure. Additional studies have indicated that this hemagglutinin occurs as five closely related tetrameric protein isoforms derived from two non-covalently-linked polypeptide chains, mol. wts. ca. 10,000 and 13,000 respectively. Buffer electrofocusing fractionated the lectin into 12 components; the major isolectin exhibited an isoelectric point at pH 4.8.

Mitogenic leukoagglutinin from Phaseolus vulgaris binds to a pentasaccharide unit in N-acetyllactosamine-type glycoprotein glycans

The carbohydrate binding specificity of leukoagglutinin (La; Phaseolus vulgaris isolectin L4) was studied by using quantitative precipitation and precipitation-inhibition. A series of purified glycopeptides and synthetic oligosaccharides were used as inhibitors. The minimal structural unit required for La binding was the disaccharide GlcNac(1 leads to beta 2)Man. Additions for this basic unit of different sugar residues gave a positive or negative contribution to binding. The most complementary structure was the pentasaccharide (formula: see text). This pentasaccharide units occurs in tetraantennary N-acetyllactosamine-type glycoprotein glycans. Glycoproteins containing such structures were accordingly precipitated by La. Selected glycopeptides and oligosaccharides were also tested as inhibitors of La-induced DNA synthesis in human lymphocytes. The pattern of inhibition was essentially the same as that obtained by precipitation-inhibition, indicating that binding to lymphocytes via the carbohydrate binding site of the lectin is an essential step in the activation process.

Linkage relationships between genes controlling seed proteins in French bean

The inheritance of phaseolin and globulin-2 (G2)/albumin polypeptides was investigated in crosses involving varieties which exhibited the three electrophoretic banding patterns of phaseolin found in French bean. 'Total' seed protein extracts of single seeds of the F1 and F2 generations from the crosses: 'Sanilac' × 'Contender', 'BBL 240' × 'Contender', and 'Sanilac' × 'BBL 240' were analyzed by two-dimensional electrophoresis. Segregation of the genes controlling phaseolin and G2/albumin polypeptides, and those controlling a further five groups of seed proteins (A, B, D, E, and F) were observed. No recombinant electrophoretic phenotypes were seen for phaseolin or G2/albumin polypeptides suggesting that the genes controlling each of these groups of polypeptides are closely linked and segregate like single Mendelian genes. The phaseolin genes and G2/albumin genes were not linked to each other. The group of genes controlling phaseolin polypeptides were linked to those controlling group B proteins, and those controlling G2/albumin polypeptides were linked to those controlling group F proteins.