Characterization and molecular cloning of Sambucus nigra agglutinin V (nigrin b), a GalNAc-specific type-2 ribosome-inactivating protein from the bark of elderberry (Sambucus nigra)

Expanding role of ribosome-inactivating proteins: From toxins to therapeutics

Ribosome-inactivating proteins (RIPs) are toxic proteins with N-glycosidase activity. RIPs exert their action by removing a specific purine from 28S rRNA, thereby, irreversibly inhibiting the process of protein synthesis. RIPs can target both prokaryotic and eukaryotic cells. In bacteria, the production of RIPs aid in the process of pathogenesis whereas, in plants, the production of these toxins has been attributed to bolster defense against insects, viral, bacterial and fungal pathogens. In recent years, RIPs have been engineered to target a particular cell type, this has fueled various experiments testing the potential role of RIPs in many biomedical applications like anti-viral and anti-tumor therapies in animals as well as anti-pest agents in engineered plants. In this review, we present a comprehensive study of various RIPs, their mode of action, their significance in various fields involving plants and animals. Their potential as treatment options for plant infections and animal diseases is also discussed.

Structure and Biological Properties of Ribosome-Inactivating Proteins and Lectins from Elder (Sambucus nigra L.) Leaves

Ribosome-inactivating proteins (RIPs) are a group of proteins with rRNA N-glycosylase activity that catalyze the removal of a specific adenine located in the sarcin–ricin loop of the large ribosomal RNA, which leads to the irreversible inhibition of protein synthesis and, consequently, cell death. The case of elderberry (Sambucus nigra L.) is unique, since more than 20 RIPs and related lectins have been isolated and characterized from the flowers, seeds, fruits, and bark of this plant. However, these kinds of proteins have never been isolated from elderberry leaves. In this work, we have purified RIPs and lectins from the leaves of this shrub, studying their main physicochemical characteristics, sequences, and biological properties. In elderberry leaves, we found one type 2 RIP and two related lectins that are specific for galactose, four type 2 RIPs that fail to agglutinate erythrocytes, and one type 1 RIP. Several of these proteins are homologous to others found elsewhere in the plant. The diversity of RIPs and lectins in the different elderberry tissues, and the different biological activities of these proteins, which have a high degree of homology with each other, constitute an excellent source of proteins that are of great interest in diagnostics, experimental therapy, and agriculture.

Elderberries—A Source of Bioactive Compounds with Antiviral Action

In the current context, when more and more unknown pathogens appear, healthy eating and supplementing it with natural products play an increasingly important role in maintaining the health of the body. The European black elder (Sambucus nigra), found in abundance in the spontaneous flora, can provide us, as a raw material, elderberries, which have been known for thousands of years as having nutritional and healing properties. The phytotherapeutic principles found in elderberry fruits give them antiviral, antibacterial and antidiabetic properties, antitumor potential, antioxidant, antidepressant and immune boosting properties, as well as a certain impacts on obesity and metabolic dysfunctions. Polyphenols and lectins give elderberry fruits the ability to inhibit coronaviruses, which is a topic of great interest in our times. This article summarizes the existing data regarding the chemical composition, active principles and biopharmaceutical properties of elderberries, as well as their use.

A Review of the Antiviral Properties of Black Elder (Sambucus nigra L.) Products

Black elder (Sambucus nigra L.) has a long ethnobotanical history across many disparate cultures as a treatment for viral infection and is currently one of the most-used medicinal plants worldwide. Until recently, however, substantial scientific research concerning its antiviral properties has been lacking. Here, we evaluate the state of current scientific research concerning the use of elderberry extract and related products as antivirals, particularly in the treatment of influenza, as well as their safety and health impacts as dietary supplements. While the extent of black elder's antiviral effects are not well known, antiviral and antimicrobial properties have been demonstrated in these extracts, and the safety of black elder is reflected by the United States Food and Drug Administration approval as generally recognized as safe. A deficit of studies comparing these S. nigra products and standard antiviral medications makes informed and detailed recommendations for use of S. nigra extracts in medical applications currently impractical. Copyright © 2017 John Wiley & Sons, Ltd.

The Cytotoxicity of Elderberry Ribosome-Inactivating Proteins Is Not Solely Determined by Their Protein Translation Inhibition Activity

Although the protein translation inhibition activity of ribosome inactivating proteins (RIPs) is well documented, little is known about the contribution of the lectin chain to the biological activity of these proteins. In this study, we compared the in vitro and intracellular activity of several S. nigra (elderberry) RIPs and non-RIP lectins. Our data demonstrate that RIPs from elderberry are much more toxic to HeLa cells than to primary fibroblasts. Differences in the cytotoxicity between the elderberry proteins correlated with differences in glycan specificity of their lectin domain, cellular uptake efficiency and intracellular destination. Despite the fact that the bulk of the RIPs accumulated in the lysosomes and partly in the Golgi apparatus, we could demonstrate effective inhibition of protein synthesis in cellula. As we also observed cytotoxicity for non-RIP lectins, it is clear that the lectin chain triggers additional pathways heralding cell death. Our data suggest that one of these pathways involves the induction of autophagy.

Ribosome-inactivating and related proteins

Ribosome-inactivating proteins (RIPs) are toxins that act as N-glycosidases (EC 3.2.2.22). They are mainly produced by plants and classified as type 1 RIPs and type 2 RIPs. There are also RIPs and RIP related proteins that cannot be grouped into the classical type 1 and type 2 RIPs because of their different sizes, structures or functions. In addition, there is still not a uniform nomenclature or classification existing for RIPs. In this review, we give the current status of all known plant RIPs and we make a suggestion about how to unify those RIPs and RIP related proteins that cannot be classified as type 1 or type 2 RIPs.

Elderberries: a source of ribosome-inactivating proteins with lectin activity

Sambucus (Adoxaceae) species have been used for both food and medicine purposes. Among these, Sambucus nigra L. (black elder), Sambucus ebulus L. (dwarf elder), and Sambucus sieboldiana L. are the most relevant species studied. Their use has been somewhat restricted due to the presence of bioactive proteins or/and low molecular weight compounds whose ingestion could trigger deleterious effects. Over the last few years, the chemical and pharmacological characteristics of Sambucus species have been investigated. Among the proteins present in Sambucus species both type 1, and type 2 ribosome-inactivating proteins (RIPs), and hololectins have been reported. The biological role played by these proteins remains unknown, although they are conjectured to be involved in defending plants against insect predators and viruses. These proteins might have an important impact on the nutritional characteristics and food safety of elderberries. Type 2 RIPs are able to interact with gut cells of insects and mammals triggering a number of specific and mostly unknown cell signals in the gut mucosa that could significantly affect animal physiology. In this paper, we describe all known RIPs that have been isolated to date from Sambucus species, and comment on their antiviral and entomotoxic effects, as well as their potential uses.

Comparative analysis of carbohydrate binding properties of Sambucus nigra lectins and ribosome-inactivating proteins

In the past three decades a lot of research has been done on the extended family of carbohydrate-binding proteins from Sambucus nigra, including several so-called type 2 RIPs as well as hololectins. Although all these proteins have been studied for their carbohydrate-binding properties using hapten inhibition assays, detailed carbohydrate specificity studies have only been performed for a few Sambucus proteins. In particular SNA-I, has been studied extensively. Because of its unique binding characteristics this lectin was developed as an important tool in glycoconjugate research to detect sialic acid containing glycoconjugates. At present much less information is available with respect to the detailed carbohydrate binding specificity of other S. nigra lectins and RIPs, and as a consequence their applications remain limited. In this paper we report a comparative analysis of several lectins from S. nigra using the glycan microarray technology. Ultimately a better understanding of the ligands for each lectin can contribute to new/more applications for these lectins in glycoconjugate research. Furthermore, the data from glycan microarray analyses combined with the previously obtained sequence information can help to explain how evolution within a single lectin family eventually yielded a set of carbohydrate-binding proteins with a very broad specificity range.

Sambucus nigra extracts inhibit infectious bronchitis virus at an early point during replication

Background

Infectious bronchitis virus (IBV) is a pathogenic chicken coronavirus. Currently, vaccination against IBV is only partially protective; therefore, better preventions and treatments are needed. Plants produce antimicrobial secondary compounds, which may be a source for novel anti-viral drugs. Non-cytotoxic, crude ethanol extracts of Rhodiola rosea roots, Nigella sativa seeds, and Sambucus nigra fruit were tested for anti-IBV activity, since these safe, widely used plant tissues contain polyphenol derivatives that inhibit other viruses.

Results

Dose–response cytotoxicity curves on Vero cells using trypan blue staining determined the highest non-cytotoxic concentrations of each plant extract. To screen for IBV inhibition, cells and virus were pretreated with extracts, followed by infection in the presence of extract. Viral cytopathic effect was assessed visually following an additional 24 h incubation with extract. Cells and supernatants were harvested separately and virus titers were quantified by plaque assay. Variations of this screening protocol determined the effects of a number of shortened S. nigra extract treatments. Finally, S. nigra extract-treated virions were visualized by transmission electron microscopy with negative staining.

Virus titers from infected cells treated with R. rosea and N. sativa extracts were not substantially different from infected cells treated with solvent alone. However, treatment with S. nigra extracts reduced virus titers by four orders of magnitude at a multiplicity of infection (MOI) of 1 in a dose-responsive manner. Infection at a low MOI reduced viral titers by six orders of magnitude and pretreatment of virus was necessary, but not sufficient, for full virus inhibition. Electron microscopy of virions treated with S. nigra extract showed compromised envelopes and the presence of membrane vesicles, which suggested a mechanism of action.

Conclusions

These results demonstrate that S. nigra extract can inhibit IBV at an early point in infection, probably by rendering the virus non-infectious. They also suggest that future studies using S. nigra extract to treat or prevent IBV or other coronaviruses are warranted.

Use of ribosome-inactivating proteins from Sambucus for the construction of immunotoxins and conjugates for cancer therapy

The type 2 ribosome-inactivating proteins (RIPs) isolated from some species belonging to the Sambucus genus, have the characteristic that although being even more active than ricin inhibiting protein synthesis in cell-free extracts, they lack the high toxicity of ricin and related type 2 RIPs to intact cells and animals. This is due to the fact that after internalization, they follow a different intracellular pathway that does not allow them to reach the cytosolic ribosomes. The lack of toxicity of type 2 RIPs from Sambucus make them good candidates as toxic moieties in the construction of immunotoxins and conjugates directed against specific targets. Up to now they have been conjugated with either transferrin or anti-CD105 to target either transferrin receptor- or endoglin-overexpressing cells, respectively.

Internalization of Sambucus nigra agglutinins I and II in insect midgut CF-203 cells

In this project, the uptake mechanisms and localization of two lectins from Sambucus nigra, further referred to as S. nigra agglutinin (SNA)-I and SNA-II, into insect midgut CF-203 cells were studied. SNA-I is a chimeric lectin belonging to the class of ribosome-inactivating proteins, whereas SNA-II is a hololectin devoid of enzymatic activity. Internalization of the fluorescein isothiocyanate-labeled lectin was investigated using confocal microscopy. Both lectins were internalized into the cytoplasm of CF-203 cells at similar rates. Preexposure of the insect midgut cells to specific inhibitors of clathrin- and caveolae-mediated endocytosis resulted in an inhibition of lectin uptake in CF-203 cells and caspase-induced cytotoxicity caused by SNA-I and SNA-II, confirming the involvement of both endocytosis pathways. Further studies demonstrated that the uptake mechanism(s) for both lectins required phosphoinositide 3-kinases, but did not depend on the actin cytoskeleton. Since the hololectin SNA-II apparently uses a similar endocytosis pathway as the chimerolectin SNA-I, it can be concluded that the endocytosis process mainly relies on the carbohydrate-binding activity of the lectins under investigation. © 2011 Wiley Periodicals, Inc.

Entomotoxic action of Sambucus nigra agglutinin I in Acyrthosiphon pisum aphids and Spodoptera exigua caterpillars through caspase-3-like-dependent apoptosis

In this project, the toxicity and mechanism of action of the ricin-B-related lectin SNA-I from elderberry (Sambucus nigra) in the pea aphid (Acyrthosiphon pisum) and the beet armyworm (Spodoptera exigua), two important pest insects in agriculture, were studied. SNA-I is a chimeric lectin belonging to the class of ribosome-inactivating proteins and consists of an A-chain with N-glycosidase activity and a carbohydrate-binding B-chain. Incorporation of 2 mg/ml of SNA-I in the diet of neonates and adults of A. pisum caused 40-46% mortality within 2 days, while in third instars of S. exigua, the larval biomass was significantly reduced by 12% after feeding for 3 days on a diet containing 5 mg/g of SNA-I. Interestingly, extracts of the (mid)gut of treated A. pisum and S. exigua demonstrated DNA fragmentation and this was accompanied with an increase in caspase-3-like activity. The involvement of cell death or apoptosis in the entomotoxicity of SNA-I through induction of caspase-3-like activity was also confirmed by addition of the permeable caspase-3 inhibitor III in the diet, leading to a rescue of the treated aphid neonates. Finally, similar to the chimeric lectin SNA-I, the hololectin SNA-II, consisting of two carbohydrate-binding B-chains caused high mortality to neonate A. pisum aphids with an LC₅₀ of 1.59 mg/ml, suggesting that the entomotoxic action of the lectins under study mainly relies on their carbohydrate-binding activity.

Exposure of insect midgut cells to Sambucus nigra L. agglutinins I and II causes cell death via caspase-dependent apoptosis

Sambucus nigra agglutinins I and II, further referred to as SNA-I and SNA-II, are two ricin-related lectins from elderberry. SNA-I is a chimeric lectin composed of an A-chain with enzymatic activity and a B-chain with carbohydrate-binding activity, and therefore belongs to the group of type 2 ribosome-inactivating proteins. In contrast, SNA-II consists only of carbohydrate-binding B-chains. The physiological effect of SNA-I was tested on different insect cell lines (midgut, ovary, fat body, embryo). In sensitive midgut CF-203 cells, SNA-I induced cell death with typical characteristics such as cell shrinkage, plasma membrane blebbing, nuclear condensation and DNA fragmentation. The effect was dose-dependent with 50% death of 4-day-exposed cells at 3nM. SNA-I exposure induced caspase-3 like activities, suggesting that SNA-I can induce the apoptotic pathway. Interestingly, the hololectin SNA-II also induced apoptosis in CF-203 cells at similar doses with the same physiological events. SNA-I and SNA-II both induced caspase-dependent apoptosis at low concentrations (nM order), leading to typical symptoms of cell death in sensitive cells. This effect seems independent from the catalytic activity of the A-chain, but depends on the carbohydrate-binding B-chain.

Structural basis for sugar recognition, including the Tn carcinoma antigen, by the lectin SNA-II from Sambucus nigra

Bark of elderberry (Sambucus nigra) contains a galactose (Gal)/N-acetylgalactosamine (GalNAc)-specific lectin (SNA-II) corresponding to slightly truncated B-chains of a genuine Type-II ribosome-inactivating protein (Type-II RIPs, SNA-V), found in the same species. The three-dimensional X-ray structure of SNA-II has been determined in two distinct crystal forms, hexagonal and tetragonal, at 1.90 A and 1.35 A, respectively. In both crystal forms, the SNA-II molecule folds into two linked beta-trefoil domains, with an overall conformation similar to that of the B-chains of ricin and other Type-II RIPs. Glycosylation is observed at four sites along the polypeptide chain, accounting for 14 saccharide units. The high-resolution structures of SNA-II in complex with Gal and five Gal-related saccharides (GalNAc, lactose, alpha1-methylgalactose, fucose, and the carcinoma-specific Tn antigen) were determined at 1.55 A resolution or better. Binding is observed in two saccharide-binding sites for most of the sugars: a conserved aspartate residue interacts simultaneously with the O3 and O4 atoms of saccharides. In one of the binding sites, additional interactions with the protein involve the O6 atom. Analytical gel filtration, small angle X-ray scattering studies and crystal packing analysis indicate that, although some oligomeric species are present, the monomeric species predominate in solution.

Cloning and expression of mistletoe lectin III B-subunit

Aqueous extracts of mistletoe (Viscum album L.) contain toxic proteins (lectins) MLI (viscumin), MLII, and MLIII. We previously cloned the gene encoding MLIII precursor. In the present study, a gene fragment encoding the carbohydrate-binding subunit of mistletoe toxic lectin MLIII was cloned and expressed in Escherichia coli cells. The structure and immunochemical properties of recombinant MLIII B-subunit were investigated using a panel of monoclonal antibodies against ML-toxins. Sugar-binding activity of recombinant MLIII B-subunit was determined by ELISA. Amino acid sequence analysis of the cloned MLIII compared with known mistletoe toxins and other ribosome-inactivating type II proteins (ricin, abrin a, and nigrin b B-subunits) revealed essential features of the recombinant MLIIIB primary structure that could determine sugar specificity of the lectin as well as immunomodulating and anti-tumor properties of mistletoe extracts.

Differences in amino acid sequences of mistletoe lectin I and III B-subunits determining carbohydrate binding specificity

Toxic lectins of European mistletoe Viscum album L.--MLI (viscumin), MLII and MLIII--are present in water extracts of this plant. Earlier we have cloned the full-length gene of MLIII precursor [A.G. Tonevitsky, I.I. Agapov, I.B. Pevzner, N.V. Maluchenko, M.M. Mojsenovich, U. Pfueller, M.P. Kirpichnikov, (2004) Biochemistry (Mosc.), 69 (6), 790-800, in press]. Here for the first time we report the cloning and expression in Escherichia coli cells of MLIII gene fragment encoding the carbohydrate-binding subunit. We have proved with our panel of monoclonal antibodies against ML toxins that the cloned fragment encoded MLIII B-subunit. The immunochemical and sugar-binding activities of renatured recombinant MLIII B-subunit were demonstrated in ELISA and ELLA, respectively. The comparative analysis of amino acid sequences of the cloned rMLIIIB and the B-subunits of other type II RIPs--MLI, ricin, abrin and nigrin b--was performed, revealing the main differences in primary structure of MLI and MLIII B-chains, which could determine their sugar specificity. The antigenicity analysis of MLI and MLIII B-subunits showed one epitope 25RDDDFRDGNQ34 in MLIB that is absent in MLIIIB sequence. The role of the toxic lectins and their subunits in immunological properties of mistletoe extracts is discussed.

Cytotoxic ribosome-inactivating lectins from plants

A class of heterodimeric plant proteins consisting of a carbohydrate-binding B-chain and an enzymatic A-chain which act on ribosomes to inhibit protein synthesis are amongst the most toxic substances known. The best known example of such a toxic lectin is ricin, produced by the seeds of the castor oil plant, Ricinnus communis. For ricin to reach its substrate in the cytosol, it must be endocytosed, transported through the endomembrane system to reach the compartment from which it is translocated into the cytosol, and there avoid degradation making it possible for a few molecules to inactivate a large proportion of the ribosomes and hence kill the cell. Cell entry by ricin involves the following steps: (i) binding to cell-surface glycolipids and glycoproteins bearing beta-1,4-linked galactose residues through the lectin activity of the B-chain (RTB); (ii) uptake by endocytosis and entry into early endosomes; (iii) transfer by vesicular transport to the trans-Golgi network; (iv) retrograde vesicular transport through the Golgi complex and into the endoplasmic reticulum (ER); (v) reduction of the disulfide bond connecting the A- and B-chains; (vi) a partial unfolding of the A-chain (RTA) to enable it to translocate across the ER membrane via the Sec61p translocon using the pathway normally followed by misfolded ER proteins for targeting to the ER-associated degradation (ERAD) machinery; (vi) refolding in the cytosol into a protease-resistant, enzymatically active structure; (vii) interaction with the sarcin-ricin domain (SRD) of the large ribosome subunit RNA followed by cleavage of a single N-glycosidic bond in the RNA to generate a depurinated, inactive ribosome. In addition to the highly specific action on ribosomes, ricin and related ribosome-inactivating proteins (RIPs) have a less specific action in vitro on DNA and RNA substrates releasing multiple adenine, and in some instances, guanine residues. This polynucleotide:adenosine glycosidase activity has been implicated in the general antiviral, and specifically, the anti HIV-1 activity of several single-chain RIPs which are homologous to the A-chains of the heterodimeric lectins. However, in the absence of clear cause and effect evidence in vivo, such claims should be regarded with caution.

Cloning and characterization of the genes encoding toxic lectins in mistletoe (Viscum album L)

Leaves of mistletoe (Viscum album L) contain three toxic lectins (type 2 ribosome-inactivating proteins) MLI, MLII, and MLIII, differing in molecular mass and carbohydrate specificity. Clones, containing sequences of three gene variants designated ml1p, ml2p, and ml3p, were obtained using PCR amplification from cDNA and from mistletoe genomic DNA. The quantitative ratio of the ml1p, ml2p, and ml3p genes in genomic DNA was found to be 1.5 : 1 : 4, respectively, whereas the ratio of their mRNA was 50 : 10 : 1. The quantitative prevalence of the ml1p transcript correlates well with the observation that MLI is quantitatively dominant over MLII and MLIII in the mistletoe extract. The sequences of the proteins encoded by the ml1p, ml2p, and ml3p genes are identical to MLI by 98, 88, and 77%, respectively. The similarity to MLI of the amino acid sequence encoded by the gene ml1p, the quantitative prevalent of its mRNA, as well as structural properties of the B-chain indicate that the gene, ml1p, corresponds to MLI. Western blot analysis of recombinant A-chains encoded by the three variants of mlp genes with the monoclonal antibody MNA4 having differential affinity to MLI, MLII and MLIII A-chains suggests that the ml2p and ml3p genes correspond to MLII and MLIII, respectively. Structural differences in the carbohydrate-binding sites of the B-subunits of ML1p, ML2p, and ML3p probably explain the difference in sugar specificity of MLI, MLII and MLIII.

Analysis of the in planta antiviral activity of elderberry ribosome-inactivating proteins

Although the type-2 ribosome-inactivating proteins (SNA-I, SNA-V, SNLRP) from elderberry (Sambucus nigra L.) are all devoid of rRNA N-glycosylase activity towards plant ribosomes, some of them clearly show polynucleotide-adenosine glycosylase activity towards tobacco mosaic virus RNA. This particular substrate specificity was exploited to further unravel the mechanism underlying the in planta antiviral activity of ribosome-inactivating proteins. Transgenic tobacco (Nicotiana tabacum L. cv Samsun NN) plants expressing the elderberry ribosome-inactivating proteins were generated and challenged with tobacco mosaic virus in order to analyze their antiviral properties. Although some transgenic plants clearly showed antiviral activity, no clear correlation was observed between in planta antiviral activity of transgenic tobacco lines expressing the different ribosome-inactivating proteins and the in vitro polynucleotide-adenosine glycosylase activity of the respective proteins towards tobacco mosaic virus genomic RNA. However, our results suggest that the in planta antiviral activity of some ribosome-inactivating proteins may rely on a direct mechanism on the virus. In addition, it is evident that the working mechanism proposed for pokeweed antiviral protein cannot be extrapolated to elderberry ribosome-inactivating proteins because the expression of SNA-V is not accompanied by induction of pathogenesis-related proteins.

Crystal structure at 3 A of mistletoe lectin I, a dimeric type-II ribosome-inactivating protein, complexed with galactose

The X-ray structure of mistletoe lectin I (MLI), a type-II ribosome-inactivating protein (RIP), cocrystallized with galactose is described. The model was refined at 3.0 A resolution to an R-factor of 19.9% using 21 899 reflections, with Rfree 24.0%. MLI forms a homodimer (A-B)2 in the crystal, as it does in solution at high concentration. The dimer is formed through contacts between the N-terminal domains of two B-chains involving weak polar and non-polar interactions. Consequently, the overall arrangement of sugar-binding sites in MLI differs from those in monomeric type-II RIPs: two N-terminal sugar-binding sites are 15 A apart on one side of the dimer, and two C-terminal sugar-binding sites are 87 A apart on the other side. Galactose binding is achieved by common hydrogen bonds for the two binding sites via hydroxy groups 3-OH and 4-OH and hydrophobic contact by an aromatic ring. In addition, at the N-terminal site 2-OH forms hydrogen bonds with Asp27 and Lys41, and at the C-terminal site 3-OH and 6-OH undergo water-mediated interactions and C5 has a hydrophobic contact. MLI is a galactose-specific lectin and shows little affinity for N-acetylgalactosamine. The reason for this is discussed. Structural differences among the RIPs investigated in this study (their quaternary structures, location of sugar-binding sites, and fine sugar specificities of their B-chains, which could have diverged through evolution from a two-domain protein) may affect the binding sites, and consequently the cellular transport processes and biological responses of these toxins.

Mutational analysis of the carbohydrate-binding activity of the NeuAc(alpha-2,6)Gal/GalNAc-specific type 2 ribosome-inactivating protein from elderberry (Sambucus nigra) fruits

Sambucus nigra agglutinin I (SNA-I) is a type 2 ribosome-inactivating protein. Site-directed mutagenesis was used to mimic the conversion of the highly active B-chain of fruit-specific SNA (SNA-If) into the completely inactive B-chain of the closely related and naturally occurring loss-of-activity mutant called S. nigra agglutinin lectin-related protein. In the first mutant SNA-If-M1 the high-affinity site 2 of SNA-If was disrupted by replacing the presumed critical residue Asp231 with Glu231. In the double mutant SNA-If-M2, site 1 of SNA-If-M1 was also disrupted by substituting the presumed critical residue Asn48 with Ser48. The parent type 2 ribosome-inactivating protein and both mutants were expressed in Nicotiana tabacum Samsun NN and the recombinant proteins were purified and analysed. Recombinant SNA-If agglutinated rabbit erythrocytes equally well as SNA-If, but both mutants were completely inactive in this test. Binding assays to immobilized galactose and fetuin revealed that the mutation Asp231-->Glu231 reduces the affinity of the B-chain for galactose and fetuin by more than 50%. Furthermore, the introduction of the second mutation Asn48-->Ser48 reduces the binding activity to less than 20% of the original activity.

A complex fruit-specific type-2 ribosome-inactivating protein from elderberry (Sambucus nigra) is correctly processed and assembled in transgenic tobacco plants

Fruits of elderberry (Sambucus nigra) express small quantities of a type-2 ribosome-inactivating protein with an exclusive specificity towards the NeuAc(alpha2,6)Gal/GalNAc disaccharide and a unique molecular structure typified by the occurrence of a disulfide bridge between the B-chains of two adjacent protomers. A cDNA clone encoding this so-called Sambucus nigra fruit specific agglutinin I (SNA-If) was isolated and expressed in tobacco (Samsun NN) under the control of the 35S cauliflower mosaic virus promoter. Characterization of the purified protein indicated that the recombinant SNA-If from tobacco leaves has the same molecular structure and biological activities as native SNA-If from elderberry fruits, demonstrating that transgenic tobacco plants are fully capable of expressing and correctly processing and assembling a type-2 ribosome-inactivating protein with a complex molecular structure. None of the transformants showed a phenotypic effect, indicating that the ectopically expressed SNA-If does not affect the viability of the tobacco cells. Bioassays further demonstrated that none of the transgenic lines exhibited a decreased sensitivity to infection with tobacco mosaic virus suggesting that the elderberry type-2 RIP SNA-If does not act as an antiviral agent in planta.

The Sambucus nigra type-2 ribosome-inactivating protein SNA-I' exhibits in planta antiviral activity in transgenic tobacco

Transgenic tobacco (Samsun NN) plants transformed with a cDNA clone encoding SNA-I' from Sambucus nigra synthesize, and correctly process and assemble, a fully active type-2 ribosome-inactivating protein. Expression of SNA-I' under the control of the 35S cauliflower mosaic virus promoter enhances the plant's resistance against infection with tobacco mosaic virus. In contrast to type-1 ribosome-inactivating proteins, the expression of SNA-I' does not affect the growth and fertility of the transgenic plants and is not accompanied by an increased expression of pathogenesis-related proteins indicating that its antiviral activity most probably differs from that of pokeweed antiviral protein.

2.8-A crystal structure of a nontoxic type-II ribosome-inactivating protein, ebulin l

Ebulin l is a type-II ribosome-inactivating protein (RIP) isolated from the leaves of Sambucus ebulus L. As with other type-II RIP, ebulin is a disulfide-linked heterodimer composed of a toxic A chain and a galactoside-specific lectin B chain. A normal level of ribosome-inactivating N-glycosidase activity, characteristic of the A chain of type-II RIP, has been demonstrated for ebulin l. However, ebulin is considered a nontoxic type-II RIP due to a reduced cytotoxicity on whole cells and animals as compared with other toxic type-II RIP like ricin. The molecular cloning, amino acid sequence, and the crystal structure of ebulin l are presented and compared with ricin. Ebulin l is shown to bind an A-chain substrate analogue, pteroic acid, in the same manner as ricin. The galactoside-binding ability of ebulin l is demonstrated crystallographically with a complex of the B chain with galactose and with lactose. The negligible cytotoxicity of ebulin l is apparently due to a reduced affinity for galactosides. An altered mode of galactoside binding in the 2gamma subdomain of the lectin B chain primarily causes the reduced affinity.

Characterization of the sugar-binding specificity of the toxic lectins isolated from Abrus pulchellus seeds

The sugar-binding specificity of the toxic lectins from Abrus pulchellus seeds was investigated by combination of affinity chromatography of glycopeptides and oligosaccharides of well-defined structures on a lectin-Sepharose column and measurement of the kinetic interactions in real time towards immobilized glycoproteins. The lectins showed strong affinity for a series of bi- and triantennary N-acetyllactosamine type glycans. The related asialo-oligosaccharides interact more strongly with the lectins. The best recognized structures were asialo-glycopeptides from fetuin. Accordingly, the kinetic interaction with immobilized asialofetuin was by far the most pronounced. Human and bovine lactotransferrins and human serotransferrin interacted to a lesser extent. The interaction with asialofetuin was inhibited by galactose in a dose dependent manner. Lactose, N-acetyllactosamine and lacto-N-biose exhibited similar degree of inhibition while N-acetylgalactosamine was a poor inhibitor. These results suggested that the carbohydrate-binding site of the Abrus pulchellus lectins was specific for galactose and possess a remarkable affinity for the sequences lactose [beta-D-Gal-(1-->4)-D-Glc], N-acetyllactosamine [beta-D-Gal-(1-->4)-D-GlcNAc] and lacto-N-biose [beta-D-Gal-(1-->3)-D-GlcNAc].

Classification of plant lectins in families of structurally and evolutionary related proteins

The majority of plant lectins can be classified in seven families of structurally and evolutionary related proteins. Within a given lectin family most but not necessarily all members are built up of protomers with a similar primary structure and overall 3-D fold. The overall structure of the native lectins is not only determined by the structure of the protomers but depends also on the degree of oligomerization and in some cases on the post-translational processing of the lectin precursors. In general, lectin families are fairly homogeneous for what concerns the overall specificity of the individual lectins, which illustrates that the 3-D structure of the binding site has been conserved during evolution. In the case of the jacalin-related lectins the occurrence of a mannose- and galactose-binding subfamily can be explained by the fact that a post-translational cleavage of the protomers (of the galactose-binding subfamily) yields a slightly altered binding site. Unlike the other families, the legume lectins display a wide range of specificites, which is clearly reflected in the occurrence of sugar-binding sites with a different 3-D structure.

Characterization and molecular cloning of two different type 2 ribosome-inactivating proteins from the monocotyledonous plant Polygonatum multiflorum

Leaves of the monocotyledonous plant Polygonatum multiflorum L. (Solomon's seal) contain besides a monocot mannose-binding lectin two galactose/N-acetylgalactosamine (Gal/GalNAc)-binding type 2 ribosome-inactivating proteins (RIPs). Both RIPs were purified using a combination of classical protein purification techniques and affinity chromatography. Although both RIPs consist of protomers of 65 kDa, the P. multiflorum RIP monomer (PMRIPm) occurs as a monomer of approximately 60 kDa, whereas the tetramer (PMRIPt) is a tetramer of 240 kDa. Both RIPs exhibit similar RNA N-glycosidase activity but differ in their specific agglutination activity and carbohydrate-binding specificity, PMRIPt being a GalNAc-specific lectin whereas PMRIPm is Gal/GalNAc-specific. Toxicity tests indicated that both Polygonatum RIPs exhibit a very low cytotoxicity towards human and animal cells. Analysis of the genomic clones encoding both RIPs revealed a high degree of sequence similarity to other type 2 RIPs. Molecular modelling confirmed that both Polygonatum RIPs have a similar structure to ricin.

A gene encoding a hevein-like protein from elderberry fruits is homologous to PR-4 and class V chitinase genes

We isolated SN-HLPf (Sambucus nigra hevein-like fruit protein), a hevein-like chitin-binding protein, from mature elderberry fruits. Cloning of the corresponding gene demonstrated that SN-HLPf is synthesized as a chimeric precursor consisting of an N-terminal chitin-binding domain corresponding to the mature elderberry protein and an unrelated C-terminal domain. Sequence comparisons indicated that the N-terminal domain of this precursor has high sequence similarity with the N-terminal domain of class I PR-4 (pathogenesis-related) proteins, whereas the C terminus is most closely related to that of class V chitinases. On the basis of these sequence homologies the gene encoding SN-HLPf can be considered a hybrid between a PR-4 and a class V chitinase gene.

Elderberry (Sambucus nigra) contains truncated Neu5Ac(alpha-2,6)Gal/GalNAc-binding type 2 ribosome-inactivating proteins

Analysis of affinity-purified preparations of the fetuin-binding proteins from elderberry bark and fruits revealed besides the previously reported Neu5Ac(alpha-2,6)Gal/GalNAc-specific type 2 ribosome-inactivating proteins (RIP) the occurrence of single chain proteins of 22 kDa, which according to their N-terminal amino acid sequence correspond to the second part of the B chain of the respective type 2 RIP. Both proteins are very similar except that the polypeptides of the fruit lectin are 10 amino acid residues longer than these from the bark lectin. Our findings not only demonstrate the occurrence of carbohydrate-binding fragments of type 2 RIP but also provide further evidence that type 2 RIP genes give rise to complex mixtures of type 2 RIP/lectins in elderberry.

Isolation and partial characterization of a novel and uncommon two-chain 64-kDa ribosome-inactivating protein from the bark of elder (Sambucus nigra L.)

A novel, strongly basic, two-chain ribosome-inactivating protein (RIP) with an apparent Mr of 64000 by SDS-PAGE and 63469 by mass spectrometry analysis, that we have named basic nigrin b, has been found in the bark of elder (Sambucus nigra L.). The new protein does not agglutinate red blood cells, even at high concentrations and displays an unusually and extremely high activity towards animal ribosomes (IC50 of 18 pg/ml for translation by rabbit reticulocyte lysates). However, it is inactive against plant and HeLa cells protein synthesis. Our functional and structural data are consistent with a heterodimeric structure for basic nigrin b of the type A-B*, B* being a truncated lectin lacking functional binding domains equivalent to the B (lectin) chain of the type 2 RIP SNA I and nigrin b present also in elder bark.

Elderberry (Sambucus nigra) bark contains two structurally different Neu5Ac(alpha2,6)Gal/GalNAc-binding type 2 ribosome-inactivating proteins

A second NeuAc(alpha2,6)Gal/GalNAc binding type 2 ribosome-inactivating protein (RIP), called SNAI' has been isolated from elderberry (Sambucus nigra) bark. SNAI' is a minor bark protein which closely resembles the previously described major Neu5Ac(alpha2,6)Gal/GalNAc binding type 2 RIP called SNAI with respect to its carbohydrate-binding specificity and ribosome-inactivating activity but has a different molecular structure. Molecular cloning revealed that the deduced amino acid sequence of SNAI' is highly similar to that of SNAI and that the difference in molecular structure between both proteins relies on a single cysteine residue present in the B chain of SNAI but absent from SNAI'. The isolation of SNAI' not only identifies a minor bark protein as a type 2 RIP but also further emphasizes the complexity of the type 2 RIP/lectin mixture present in the bark of elderberry.

Isolation, cDNA cloning, biological properties, and carbohydrate binding specificity of sieboldin-b, a type II ribosome-inactivating protein from the bark of Japanese elderberry (Sambucus sieboldiana)

A type II ribosome-inactivating protein (RIP) was isolated from the bark tissue of Japanese elderberry (Sambucus sieboldiana) and named sieboldin-b. Sieboldin-b is a heterodimeric protein consisting of 27- and 33-kDa subunits and showed strong ribosome-inactivating activity in vitro but did not show in vivo toxicity. The amino acid sequence of sieboldin-b deduced from the structure of the cDNA showed that both subunits of sieboldin-b are encoded on a single precursor polypeptide. Sieboldin-b has a structure homologous with the Neu5Ac(alpha 2-6)Gal/GalNAc-specific bark lectin from S. sieboldiana (SSA) and also typical type II RIPs such as ricin and abrin. Detailed analyses of carbohydrate binding properties of sieboldin-b revealed that sieboldin-b binds to Gal/GalNAc, similar to ricin/abrin, in spite of its highly homologous structure with SSA. The biological properties of these toxins/lectins are compared, and the possible explanation for such diversity is discussed.

Isolation and molecular cloning of a novel type 2 ribosome-inactivating protein with an inactive B chain from elderberry (Sambucus nigra) bark

One of the predominant proteins in the bark of elderberry (Sambucus nigra) has been identified as a novel type 2 ribosome-inactivating protein that exhibits a normal RNA N-glycosidase activity, but is devoid of carbohydrate binding activity. Sequence analysis of the corresponding cDNA clones revealed a striking homology to the previously cloned bark lectins from elderberry, suggesting that the new protein is a lectin-related protein. Molecular modeling of the protein confirmed that its A chain is fully active, whereas its B chain contains two functionally inactive carbohydrate-binding sites. These findings not only demonstrate for the first time the occurrence of a type 2 ribosome-inactivating protein with an inactive B chain, but also offer interesting perspectives for the synthesis of immunotoxins with an improved selectivity.

Isolation and partial characterization of a small chitin-binding lectin from mistletoe (Viscum album)

A novel lectin, called VisalbCBA, was isolated from European mistletoe (Viscum album). This lectin differs completely from the classical galactose/N-acetylgalactosamine-binding mistletoe lectins MLI, MLII and MLIII. Biochemical analyses indicated that VisalbCBA is a dimeric protein composed of two identical subunits of approx. 10 kDa. VisalbCBA exhibits specificity towards oligomers of N-acetylglucosamine and shows sequence homology to the previously isolated chitin-binding plant proteins. Although VisalbCBA is less toxic than the other mistletoe lectins, it definitely exhibits cytotoxic properties. The possible involvement of VisalbCBA in the biological and therapeutic effects of mistletoe is discussed.