Ebulin 1, a nontoxic novel type 2 ribosome-inactivating protein from Sambucus ebulus L. leaves

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.

Ebulin l Is Internalized in Cells by Both Clathrin-Dependent and -Independent Mechanisms and Does Not Require Clathrin or Dynamin for Intoxication

Ebulin l is an A-B toxin, and despite the presence of a B chain, this toxin displays much less toxicity to cells than the potent A-B toxin ricin. Here, we studied the binding, mechanisms of endocytosis, and intracellular pathway followed by ebulin l and compared it with ricin. COS-1 cells and HeLa cells with inducible synthesis of a mutant dynamin (K44A) were used in this study. The transport of these toxins was measured using radioactively or fluorescently labeled toxins. The data show that ebulin l binds to cells to a lesser extent than ricin. Moreover, the expression of mutant dynamin does not affect the endocytosis, degradation, or toxicity of ebulin l. However, the inhibition of clathrin-coated pit formation by acidification of the cytosol reduced ebulin l endocytosis but not toxicity. Remarkably, unlike ricin, ebulin l is not transported through the Golgi apparatus to intoxicate the cells and ebulin l induces apoptosis as the predominant cell death mechanism. Therefore, after binding to cells, ebulin l is taken up by clathrin-dependent and -independent endocytosis into the endosomal/lysosomal system, but there is no apparent role for clathrin and dynamin in productive intracellular routing leading to intoxication.

Molecular cloning and in-depth bioinformatics analysis of type II ribosome-inactivating protein isolated from Sambucus ebulus

Plant ribosome-inactivating proteins (RIPs) are N-glycosidases which inhibit protein synthesis through depurination of the ribosomal RNA sequence. Type II RIPs are heterodimer proteins which can bind to cell surfaces. The cytotoxicity of these RIPs is different. Sambucus spp. are a rich source of RIP proteins with different properties. In the present study, a type II RIP was isolated from S. ebulus plant that grows widely in the north of Iran, and different bioinformatics tools were used for the evaluation of physicochemical, functional and 3D protein characteristics. The results showed significant differences among isolated RIP and other Sambucus RIP proteins. The study of these differences can not only expand our insight into the functioning mechanisms of plant RIPs but also provide information about a novel RIP protein with potential biological applications.

New ribosome-inactivating proteins and other proteins with protein synthesis-inhibiting activities

Ribosome-inactivating proteins (RIPs) consist of three varieties. Type 1 RIPs are single-chained and approximately 30-kDa in molecular weight. Type 2 RIPs are double-chained and composed of a type 1 RIP chain and a lectin chain. Type III RIPs, such as maize b-32 barley and JIP60 which are produced as single-domain proenzymes, possess an N-terminal domain corresponding to the A domain of RIPs and fused to a C-terminal domain. In addition to the aforementioned three types of RIPs originating from flowering plants, there are recently discovered proteins and peptides with ribosome-inactivating and protein synthesis inhibitory activities but which are endowed with characteristics such as molecular weights distinctive from those of the regular RIPs. These new/unusual RIPs discussed in the present review encompass metazoan RIPs from Anopheles and Culex mosquitos, antimicrobial peptides derived from RIP of the pokeweed Phytolacca dioica, maize RIP (a type III RIP derived from a precursor form), RIPs from the garden pea and the kelp. In addition, RIPs with a molecular weight smaller than those of regular type 1 RIPs are produced by plants in the Cucurbitaceae family including the bitter gourd, bottle gourd, sponge gourd, ridge gourd, wax gourd, hairy gourd, pumpkin, and Chinese cucumber. A small type II RIP from camphor tree (Cinnamomum camphora) seeds and a snake gourd type II RIP with its catalytic chain cleaved into two have been reported. RIPs produced from mushrooms including the golden needle mushroom, king tuber mushroom, straw mushroom, and puffball mushroom are also discussed in addition to a type II RIP from the mushroom Polyporus umbellatus. Bacterial (Spiroplasma) RIPs associated with the fruitfly, Shiga toxin, and Streptomyces coelicolor RIP are also dealt with. The aforementioned proteins display a diversity of molecular weights, amino acid sequences, and mechanisms of action. Some of them are endowed with exploitable antipathogenic activities.

Crystal structures of a β-trefoil lectin from Entamoeba histolytica in monomeric and a novel disulfide bond-mediated dimeric forms

β-Trefoil lectins are galactose/N-acetyl galactosamine specific lectins, which are widely distributed across all kingdoms of life and are known to perform several important functions. However, there is no report available on the characterization of these lectins from protozoans. We have performed structural and biophysical studies on a β-trefoil lectin from Entamoeba histolytica (EntTref), which exists as a mixture of monomers and dimers in solution. Further, we have determined the affinities of EntTref for rhamnose, galactose and different galactose-linked sugars. We obtained the crystal structure of EntTref in a sugar-free form (EntTref_apo) and a rhamnose-bound form (EntTref_rham). A novel Cys residue-mediated dimerization was revealed in the crystal structure of EntTref_apo while the structure of EntTref_rham provided the structural basis for the recognition of rhamnose by a β-trefoil lectin for the first time. To the best of our knowledge, this is the only report of the structural, functional and biophysical characterization of a β-trefoil lectin from a protozoan source and the first report of Cys-mediated dimerization in this class of lectins.

Ebulin-RP, a novel member of the Ebulin gene family with low cytotoxicity as a result of deficient sugar binding domains

BACKGROUND

Sambucus ebulus is a rich source of ribosome-inactivating proteins (RIPs) and RIP-related lectins generated from multiple genes. These proteins differ in their structure, enzymatic activity and sugar binding specificity.

METHODS

We have purified and characterized ebulin-RP from S. ebulus leaves and determined the amino acid sequence by cDNA cloning. Cytotoxicity was studied in a variety of cancer cells and a comparative study of the ability of ebulin-RP to bind sugars using "in vitro" and "in silico" approaches was performed.

RESULTS

Ebulin-RP is a novel heterodimeric type 2 RIP present in S. ebulus leaves together with the type 2 RIP ebulin l, which displayed rRNA N-glycosidase activity but unlike ebulin l, lacked functional sugar binding domains. As a consequence of changes in its B-chain, ebulin-RP displayed lower cytotoxicity than ebulin l towards cancer cells and induced apoptosis as the predominant pattern of cell death.

CONCLUSIONS

Ebulin-RP is a novel member of the ebulin gene family with low cytotoxicity as a result of deficient sugar binding domains. Type 2 RIP genes from Sambucus have evolved to render proteins with different sugar affinities that may be related to different biological activities and could result in an advantage for the plant.

GENERAL SIGNIFICANCE

The ebulin family of RIPs and lectins can serve as a good model for studying the evolutionary process which may have occurred in RIPs. The lack of cytotoxicity of ebulin-RP makes it a good candidate as a toxic moiety in the construction of immunotoxins and conjugates directed against specific targets.

Biological Effects and Clinical Applications of Dwarf Elder ( Sambucus ebulus L): A Review

Dwarf elder ( Sambucus ebulus L) is one of the best known medicinal herbs since ancient times. In view of its benefits as a widely applicable phytomedicine, it is still used in folk medicine of different parts of the world. In addition to its nutritional values, dwarf elder contains different phytochemicals among which flavonoids and lectins are responsible for most of its therapeutic effects. Dwarf elder has been used for different ailments including: joint pains, cold, wounds, and infections. Nevertheless, recent evidence has revealed its potentials for making attempts at treating cancer and metabolic disorders. This review aimed to provide a comprehensive description of dwarf elder regarding its traditional uses and modern findings which may contribute to the development of novel natural-based therapeutic agents.

Glypican-3 Targeting Immunotoxins for the Treatment of Liver Cancer

Hepatocellular carcinoma (HCC) is the most common form of primary liver cancer, yet no effective therapeutics exist. This review provides an overview of the recent development of recombinant immunotoxins for the treatment of glypican-3 (GPC3) expressing HCC. GPC3 is a cell surface heparan sulfate proteoglycan that is overexpressed in HCC, but is absent from normal adult human tissues. Treatment of HCC with anti-GPC3 immunotoxins represents a new therapeutic option. Using phage display and hybridoma technologies, three high affinity antibodies (HN3, HS20 and YP7) have been generated against GPC3. Two of these antibodies (HN3 and HS20) have demonstrated the ability to inhibit Wnt/Yap signaling, leading to a reduction in liver cancer cell proliferation. By combining the HN3 antibody capable of inhibiting Wnt/Yap signaling with the protein synthesis inhibitory domain of the Pseudomonas exotoxin, a recombinant immunotoxin that exhibits a dual inhibitory mechanism was generated. This immunotoxin was found to be highly effective in the treatment of human HCCs in mouse xenograft models. Engineering of the toxin fragment to reduce the level of immunogenicity is currently being explored. The development of immunotoxins provides opportunities for novel liver cancer therapies.

Structural studies on a non-toxic homologue of type II RIPs from bitter gourd: Molecular basis of non-toxicity, conformational selection and glycan structure

The structures of nine independent crystals of bitter gourd seed lectin (BGSL), a non-toxic homologue of type II RIPs, and its sugar complexes have been determined. The four-chain, two-fold symmetric, protein is made up of two identical two-chain modules, each consisting of a catalytic chain and a lectin chain, connected by a disulphide bridge. The lectin chain is made up of two domains. Each domain carries a carbohydrate binding site in type II RIPs of known structure. BGSL has a sugar binding site only on one domain, thus impairing its interaction at the cell surface. The adenine binding site in the catalytic chain is defective. Thus, defects in sugar binding as well as adenine binding appear to contribute to the non-toxicity of the lectin. The plasticity of the molecule is mainly caused by the presence of two possible well defined conformations of a surface loop in the lectin chain. One of them is chosen in the sugar complexes, in a case of conformational selection, as the chosen conformation facilitates an additional interaction with the sugar, involving an arginyl residue in the loop. The N-glycosylation of the lectin involves a plant-specific glycan while that in toxic type II RIPs of known structure involves a glycan which is animal as well as plant specific.

A new age for biomedical applications of Ribosome Inactivating Proteins (RIPs): from bioconjugate to nanoconstructs

Ribosome-inactivating proteins (RIPs) are enzymes (3.2.2.22) that possess N-glycosilase activity that irreversibly inhibits protein synthesis. RIPs have been found in plants, fungi, algae, and bacteria; their biological role is still under investigation, even if it has been recognized their role in plant defence against predators and viruses. Nevertheless, several studies on these toxins have been performed to evaluate their applicability in the biomedical field making RIPs selectively toxic towards target cells. Indeed, these molecules are extensively used to produce chimeric biomolecules, such as immunotoxins or protein/peptides conjugates. However, to date, clinical use of most of these bioconiujates has been limited by toxicity and immunogenicity. More recently, material sciences have provided a wide range of nanomaterials to be used as excellent vehicles for toxin-delivery, since they are characterized by improved stability, solubility, and in vivo pharmacokinetics. This review discusses progresses in the development of RIPs bioconjugates, with particular attention to the recent use of nanomaterials, whose appropriate design opens up a broad range of different possibilities to the use of RIPs in novel therapeutic approaches in human diseases.

Cytotoxic activity of chimeric protein PD-L4UWSCI(tr) does not appear be affected by specificity of inhibition mediated by anti-protease WSCI domain

In a previously study, a type 1 ribosome inactivating protein (PD-L4) and a wheat subtilisin/chymotrypsin inhibitor (WSCI) were engineered into a chimeric protein (PD-L4UWSCI) that presented in addition to the same properties of both domains an intriguing selective cytotoxic action on murine tumor cells. This finding supported the idea that the protection of C-terminal region of PD-L4 could amplify its cytotoxic action by virtue of a greater resistance to proteases. Several authors indeed revealed that the cytotoxicity of RIPs depends not only on the intracellular routing, but also on the intrinsic resistance to proteolysis. In this regard in the present work we have produced a variant of chimeric protein, named PD-L4UWSCI(tr), changing the inhibitory specificity of WSCI domain. The purpose of this approach was to check if the cytotoxicity of the chimeric protein was altered depending on the properties of protease inhibitor domain or by a different fold of whole protein. Data collected supposedly indicate that WSCI domain contributes to cytotoxicity of chimeric protein exclusively from a structural point of view.

Ebulin from dwarf elder (Sambucus ebulus L.): a mini-review

Sambucus ebulus L. (dwarf elder) is a medicinal plant, the usefulness of which also as food is restricted due to its toxicity. In the last few years, both the chemistry and pharmacology of Sambucus ebulus L. have been investigated. Among the structural and functional proteins present in the plant, sugar-binding proteins (lectins) with or without anti-ribosomal activity and single chain ribosome-inactivating proteins (RIPs) have been isolated. RIPs are enzymes (E.C. 3.2.2.22) that display N-glycosidase activity on the 28S rRNA subunit, leading to the inhibition of protein synthesis by arresting the step of polypeptide chain elongation. The biological role of all these proteins is as yet unknown. The evidence suggests that they could be involved in the defense of the plant against predators and viruses or/and a nitrogen store, with an impact on the nutritional characteristics and food safety. In this mini-review we describe all the isoforms of ebulin that have to date been isolated from dwarf elder, as well as their functional characteristics and potential uses, whilst highlighting concern regarding ebulin toxicity.

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.

Isolation and molecular characterization of two lectins from dwarf elder (Sambucus ebulus L.) blossoms related to the Sam n1 allergen

Sambucus species contain a number of lectins with and without antiribosomal activity. Here, we show that dwarf elder (Sambucus ebulus L.) blossoms express two d-galactose-binding lectins that were isolated and purified by affinity chromatography and gel filtration. These proteins, which we named ebulin blo (A-B toxin) and SELblo (B-B lectin)—blo from blossoms—were subjected to molecular characterization and analysis by MALDI-TOF mass spectrometry and tryptic peptide fingerprinting. Both lectins share a high degree of amino acid sequence homology with Sambucus lectins related to the Sam n1 allergen. Ebulin blo, but not SELblo, was highly toxic by nasal instillation to mice. Overall, our results suggested that both lectins would belong to an allergen family exemplified by Sam n1 and could trigger allergy responses. Furthermore, they raise a concern about ebulin blo toxicity.

Differential sensitivity of D-galactose-binding lectins from fruits of dwarf elder (Sambucus ebulus L.) to a simulated gastric fluid

Some lectins from Sambucus spp. share amino acid sequences with the pollen Sam n1 allergen. The lectins ebulin f and SELfd from the early stages of growth were isolated and subjected to analysis by MALDI-TOF mass spectrometry, tryptic peptide fingerprinting, molecular characterization and pepsin digestibility. The molecular mass (33.214) and other structural features of the Sam n1 allergen fit best with a monomeric lectin like SELlm (Mr 34.2 kDa) found in shoots of dwarf elder. Ebulin f toxicity to mice was higher intraperitoneally than orally at the same dose (5mg/kg body weight). In contrast SELfd at the same dose lacks of apparent toxicity. Ebulin f, but not SELfd, undergoes extensive pepsin proteolysis, which could explain the differences in toxicity. The present study supports our hypothesis that the Sam n1 allergen could be a sequence-related monomeric lectin like SELlm present in shoots of Sambucus ebulus rather than ebulin.

Enhanced cytotoxic activity of a bifunctional chimeric protein containing a type 1 ribosome-inactivating protein and a serine protease inhibitor

Both ribosome-inactivating proteins (RIPs) and plant proteinase inhibitors, belong to protein families known to regulate cellular homeostasis and likely involved in plant defense. Nevertheless the interest in these protein classes is due to their potential use for the treatment of several important human diseases such as cancer. Thus, in the present study, type 1 ribosome-inactivating protein and wheat subtilisin/chymotrypsin inhibitor, were engineered into a chimeric protein with cytotoxic action selective for murine tumor cells, while lacking any appreciable toxicity on murine normal cells. This chimeric protein selectively sensitizes to apoptotic death cells derived from Simian-virus-40-transformed mouse fibroblasts (SVT2 cells). The cytotoxicity of this new recombinant product has been detected also on three different human malignant cells. Therefore action on tumor cells of this protein could represent a potentially very attractive novel tool for anticancer drug design.

Toxicological studies on plant proteins: a review

Nowadays, toxicological studies are contributing to human health more than ever. Reports on the toxicological studies of plant proteins, which are continuously growing in number in the literature, have been reviewed. Two important aspects are discussed: dietary safety evaluation, including toxicity tests and the maximum daily intake allowance, and the appropriate proportion in our daily diets of proteins from traditional foods and of new proteins from plant sources not traditionally employed as foods. Water hyacinth leaf proteins, sweet lupin proteins and canola proteins have not been shown to be toxic, although they are not traditionally employed as food proteins. These findings are very important for exploiting valuable new protein sources that are suitable for human or animal consumption and applicable to the food industry. Acutely toxic proteins, including lectins, ribosome-inactivating proteins, inhibitors of proteolytic enzymes and glycohydro-lases, have been isolated from plant materials and identified. Their toxicities and molecular characteristics have been described. The toxicity of proteins depends upon their specific native structures. Once they are denatured by appropriate treatment, such as heating, their toxicity can be reduced or even eliminated. These findings indicate that raw materials that contain this kind of toxic protein are not edible. However, after proper processing, they may be suitable for human or animal consumption. Although the toxicities of type 2 ribosome-inactivating proteins reported by different authors vary, the maximum dosages are still trace amounts.

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.

Glycine-rich RNA binding protein of Oryza sativa inhibits growth of M15 E. coli cells

Background

Plant glycine-rich RNA binding proteins have been implicated to have roles in diverse abiotic stresses.

Findings

E. coli M15 cells transformed with full-length rice glycine-rich RNA binding protein4 (OsGR-RBP4), truncated rice glycine-rich RNA binding protein4 (OsGR-RBP4ΔC) and rice FK506 binding protein (OsFKBP20) were analyzed for growth profiles using both broth and solid media. Expression of OsGR-RBP4 and OsGR-RBP4ΔC proteins caused specific, inhibitory effect on growth of recombinant M15 E. coli cells. The bacterial inhibition was shown to be time and incubation temperature dependent. Removal of the inducer, IPTG, resulted in re-growth of the cells, indicating that effect of the foreign proteins was of reversible nature. Although noted at different levels of dilution factors, addition of purified Os-GR-RBP4 and OsGR-RBP4ΔC showed a similar inhibitory effect as seen with expression inside the bacterial cells.

Conclusions

Expression of eukaryotic, stress-associated OsGR-RBP4 protein in prokaryotic E. coli M15 cells proves injurious to the growth of the bacterial cells. E. coli genome does not appear to encode for any protein that has significant homology to OsGR-RBP4 protein. Therefore, the mechanism of inhibition appears to be due to some illegitimate interactions of the OsGR-RBP4 with possibly the RNA species of the trans-host bacterial cells. The detailed mechanism underlying this inhibition remains to be worked out.

Wound healing potential of Sambucus ebulus L. leaves and isolation of an active component, quercetin 3-O-glucoside

ETHNOPHARMACOLOGICAL RELEVANCE

The leaves of Sambucus ebulus L. are used in Turkish folk medicine for treatment of high fever, rheumatic pains, snake bites and wounds.

AIM OF THE STUDY

Four different solvent extracts by using n-hexane, diethyl ether, ethyl acetate and methanol were prepared from the leaves of Sambucus ebulus in order to investigate the claimed wound healing activity in vivo and isolation of active component(s) from the active extract through the bioassay-guided fractionation procedures.

MATERIALS AND METHODS

Linear incision and circular excision wound models on rats and mice were employed. The methanol extract was fractionated by successive chromatographic techniques. Wound healing activity of each fraction was investigated following the bioassay-guided fractionation procedures. Moreover, the tissue samples were examined histopathologically.

RESULTS

Significant wound healing activity was observed for the ointment prepared with methanol extract at 1% concentration. The methanol extract treated groups of animals showed 84.3% contraction in circular excision model, which was close to contraction value of the reference drug Madecassol (100%). On the other hand, a significant increase (43.7%) in the wound tensile strength was determined with the same extract on incision wound model. Subfractions showed significant but reduced wound healing activity on both in vivo wound models. A flavonoid derivative "quercetin 3-O-glucoside" was isolated and determined as one of the active component of active final subfraction. The results of histopathological examination supported the outcome of linear incision and circular excision wound models.

CONCLUSION

The experimental data revealed that the methanolic extract of Sambucus ebulus leaves displayed remarkable wound healing activity.

Elicitor-dependent expression of the ribosome-inactivating protein beetin is developmentally regulated

BE27 and BE29 are two forms of beetin, a virus-inducible type 1 ribosome-inactivating protein isolated from leaves of Beta vulgaris L. Western blot analysis revealed the presence of beetin forms in adult plants but not in germ or young plants, indicating that the expression of these proteins is developmentally regulated. While beetins are expressed only in adult plants, their transcripts are present through all stages of development. In addition, the treatment of B. vulgaris leaves with mediators of plant-acquired resistance such as salicylic acid and hydrogen peroxide promoted the expression of beetin by induction of its transcript, but only in adult plants. The plant expresses three mRNAs which differ only in their 3' untranslated region. All these observations suggest a dual regulation of beetin expression, i.e. at the post-transcriptional and transcriptional levels. Additionally, total RNA isolated from leaves treated with hydrogen peroxide, which express high levels of active beetin, is not de-adenylated by endogenous beetin, nor in vitro by the addition of BE27, thus suggesting that sugar beet ribosomes are resistant to beetin.

Transient occurrence of an ebulin-related D-galactose-lectin in shoots of Sambucus ebulus L

Young shoots of Sambucus ebulus L. contain a monomeric d-galactose binding lectin (SELlm), which disappears upon shoot development, and was previously undetected since it co-purifies with the non-toxic type 2 ribosome-inactivating protein ebulin l and the dimeric lectin SELld. Molecular cloning of cDNA coding for SELlm and mass spectrometry analysis revealed a protein with a molecular mass of 34,239 Da, which displays 80%, 77% and 45% of amino acid sequence identity with the ebulin l-B chain, SELld and ricin-B chain, respectively. Furthermore, the cloned precursor, with respect to the ebulin l precursor is truncated and contains the signal peptide, a piece of the A chain, a piece of the connecting peptide and the B chain. Further processing yields the lectin protein, which contains only the B chain. Despite the fact that SELlm displays the same d-galactose-binding sites than ricin, it was found that the lectin has different binding properties to D-galactose-containing matrix than ricin. Notably, and unlike ricin, the binding of SELlm and other Sambucus lectins to such matrix was maximum in range of 0-10 degrees C and abolished at 20 degrees C.

The role of plant defence proteins in fungal pathogenesis

SUMMARY It is becoming increasingly evident that a plant-pathogen interaction may be compared to an open warfare, whose major weapons are proteins synthesized by both organisms. These weapons were gradually developed in what must have been a multimillion-year evolutionary game of ping-pong. The outcome of each battle results in the establishment of resistance or pathogenesis. The plethora of resistance mechanisms exhibited by plants may be grouped into constitutive and inducible, and range from morphological to structural and chemical defences. Most of these mechanisms are defensive, exhibiting a passive role, but some are highly active against pathogens, using as major targets the fungal cell wall, the plasma membrane or intracellular targets. A considerable overlap exists between pathogenesis-related (PR) proteins and antifungal proteins. However, many of the now considered 17 families of PR proteins do not present any known role as antipathogen activity, whereas among the 13 classes of antifungal proteins, most are not PR proteins. Discovery of novel antifungal proteins and peptides continues at a rapid pace. In their long coevolution with plants, phytopathogens have evolved ways to avoid or circumvent the plant defence weaponry. These include protection of fungal structures from plant defence reactions, inhibition of elicitor-induced plant defence responses and suppression of plant defences. A detailed understanding of the molecular events that take place during a plant-pathogen interaction is an essential goal for disease control in the future.

Molecular cloning and functional identification of a ribosome inactivating/antiviral protein from leaves of post-flowering stage of Celosia cristata and its expression in E. coli

A full-length cDNA clone, encoding a ribosome inactivating/antiviral protein (RIP/AVP) was isolated from the cDNA library of post-flowering stage of Celosia cristata leaves. The full-length cDNA consisted of 1015 nucleotides, with an open reading frame encoding 283 amino acids. The deduced amino acid sequence had a putative active site domain conserved in other ribosome inactivating/antiviral proteins (RIPs/AVPs). The coding region of the cDNA was amplified by polymerase chain reaction (PCR), cloned and expressed in Escherichia coli as recombinant protein of 72 kDa. The expressed fusion product was confirmed by Western analysis and purification by affinity chromatography. Both the recombinant protein (reCCP-27) and purified expressed protein (eCCP-27) inhibited translation in rabbit reticulocytes showing IC50 values at 95 ng and 45 ng, respectively. The native purified nCCP-27 has IC50 at 25 ng. The purified product also showed N-glycosidase activity towards tobacco ribosomes and antiviral activity towards tobacco mosaic virus (TMV) and sunnhemp rosette virus (SRV).

Purification, characterization and cloning of antiviral/ribosome inactivating protein from Amaranthus tricolor leaves

An antiviral protein (AVP), imparting high level of resistance against sunnhemp rosette virus (SRV) was purified from the dried leaves of Amaranthus tricolor. The purified protein (AAP-27) exhibited approximately 98% inhibition of local lesion formation at a concentration range of approximately 30 microg ml(-1). The protein was found to be highly basic glycoprotein monomer (pI approximately 9.8) of Mr 27 kDa, with neutral sugar content of 4%. The purified protein exhibited N-glycosidase and RNase activities. We have also isolated full-length cDNA clone, encoding this protein designated as A. tricolor antiviral protein-1 (AAP-1). Two primers, one designed on the basis of N-terminal sequence of the purified protein and the other from the conserved active peptides of other AVPs/RIPs were used for PCR amplification of double stranded cDNA, isolated from the leaves of A. tricolor. The amplified fragment was used as a probe for library screening. The isolated full-length cDNA consisted of 1058 nucleotides with an open reading frame encoding a polypeptide of 297 amino acids. The deduced amino acid sequence of AAP-1 has a putative active domain conserved in other AVPs/RIPs and shows varying homology to the RIPs from other plant species.

Bacterial expression of biologically active recombinant musarmin 1 from bulbs of Muscari armeniacum L. and Miller

Musarmins are type 1 ribosome-inactivating proteins with N-glycosidase activity on the 28 S rRNA that are present in bulbs of Muscari armeniacum L. and Miller at rather low concentrations. In the present work, a cDNA fragment coding for musarmin 1 was sub-cloned and expressed in Escherichia coli. The recombinant protein (rMU1) was synthesised as a polypeptide of 295 amino acids that was delivered to the periplasm and processed. Recombinant musarmin 1 present in the periplam has two forms: insoluble with a molecular mass of 29,423 and soluble with a molecular mass of 29,117 because of a small proteolytic shortening with respect to the insoluble one, presumably in the C-terminal. The yield of protein homogeneous by polyacrylamide gel electrophoresis was 23mgl-1 of bacterial culture. The recombinant musarmin 1 forms isolated from both the soluble and the insoluble (upon refolding) fractions retained full translational inhibitory and 28 S rRNA N-glycosidase activities as compared with the native protein. The recombinant protein displayed great stability towards trypsin, collagenase, rat plasma and rat liver protein extract, but was sensitive to the action of papain and proteinase K. The easy availability and full activity of the recombinant musarmin 1 makes it a good candidate for the preparation of immunotoxins for targeted therapy and for the construction of transgenic plants expressing it as antipathogenic agent.

Enzymatic activity of toxic and non-toxic type 2 ribosome-inactivating proteins

Ribosome-inactivating proteins (RIPs) display adenine polynucleotide glycosylase activity on different nucleic acid substrates, which at the ribosomal level is responsible for the arrest of protein synthesis. Some type 2 RIPs, namely ricin and related proteins, are extremely toxic to mammalian cells and animals whilst other type 2 RIPs (non-toxic type 2 RIPs) display three to four logs less toxicity. We studied whether a correlation exists between toxicity on cells and enzymatic activity on nucleic acids. All type 2 RIPs differ in their depurinating activity on the different substrates with differences of up to one to two logs. The toxicity of type 2 RIPs is independent of their enzymatic activity on nucleic acids or on ribosomes.

Biochemical characterization and antifungal activity of an endo-1,3-beta-glucanase of Paenibacillus sp. isolated from garden soil

A 44-kDa 1,3-beta-glucanase was purified from the culture medium of a Paenibacillus strain with a 28-fold increase in specific activity with 31% recovery. The purified enzyme preferentially catalyzes the hydrolysis of glucans with 1,3-beta-linkage and has an endolytic mode of action. The enzyme also showed binding activity to various insoluble polysaccharides including unhydrolyzable substrates such as xylan and cellulose. The antifungal activity of this Paenibacillus enzyme and a previously purified 1,3-beta-glucanase from Streptomyces sioyaensis were examined in this study. Both enzymes had the ability to damage the cell-wall structures of the growing mycelia of phytopathogenic fungi Pythium aphanidermatum and Rhizoctonic solani AG-4. Nonetheless, the Paenibacillus enzyme had a much stronger effect on inhibiting the growth of fungi tested.

Musarmins: three single-chain ribosome-inactivating protein isoforms from bulbs of Muscari armeniacum L. and Miller

Three new ribosome-inactivating protein (RIP; EC 3.2.2.22) isoforms that we have named musarmins (MUs) 1, 2 and 3 have been isolated from the bulbs of Muscari armeniacum L. and Miller by ion-exchange chromatography and gel filtration. Analysis by electrophoresis revealed that they are single-chain proteins and mass spectrometry analysis afforded Mr values of 28,708, 30,003 and 27,626 for MUs 1, 2 and 3, respectively. Musarmins strongly inhibited protein synthesis carried out by mammalian ribosomes, with IC50 values in the 0.14-0.24nM range but not that carried out by plant cell-free systems or HeLa cells. MUs promote the single depurination of rabbit reticulocyte 28S rRNA. cDNA cloning of genes coding for musarmins revealed that they contain open reading frames of 298, 294 and 295 aminoacids for MU1, MU2 and MU3, respectively. Mature MU1, MU2 and MU3 contain 277, 273 and 273 aminoacids, respectively suggesting post-translational C-terminal processing. An untranslated mRNA coding for an ORF very similar to that of MU3 was detected in leaves. Each of the four MU genes contains an intron. In contrast to other RIPs, MUs are present only in bulbs and are not induced in leaves either by senescence, or by treatment of leaves with H2O2 or salicylic acid, or by growth in darkness. Therefore, these proteins could play a non-vital role in plants; for instance, as anti-pathogens and protective agents only in some stages of the plant life cycle (237).

Plant toxic proteins with insecticidal properties. A review on their potentialities as bioinsecticides

To meet the demands for food of the expanding world population, there is need of new ways for protecting plant crops against predators and pathogens while avoiding the use of environmentally aggressive chemicals. A milestone in this field was the introduction into crop plants of genes expressing Bacillus thuringiensis entomotoxic proteins. In spite of the success of this new technology, however, there are difficulties for acceptance of these 'anti-natural' products by the consumers and some concerns about its biosafety in mammals. An alternative could be exploring the plant's own defense mechanisms, by manipulating the expression of their endogenous defense proteins, or introducing an insect control gene derived from another plant. This review deals with the biochemical features and mechanisms of actions of plant proteins supposedly involved in defense mechanisms against insects, including lectins, ribosome-inactivating proteins, enzymes inhibitors, arcelins, chitinases, ureases, and modified storage proteins. The potentialities of genetic engineering of plants with increased resistance to insect predation relying on the repertoire of genes found in plants are also discussed. Several different genes encoding plant entomotoxic proteins have been introduced into crop genomes and many of these insect resistant plants are now being tested in field conditions or awaiting commercialization.

Targeting cancer cells with transferrin conjugates containing the non-toxic type 2 ribosome-inactivating proteins nigrin b or ebulin l

Nigrin b and ebulin l are type 2 ribosome-inactivating proteins (RIPs) with 10(4) times less cellular and in vivo toxicity than ricin that are currently being considered for the construction of anti-cancer conjugates. Here we provide evidence that both RIPs can be used for the construction of conjugates directed to a target such as the transferrin receptor (TfR), which is over-expressed in cancer cells. Nigrin b- and ebulin l-transferrin conjugates were constructed with no substantial reduction in the translational inhibitory molecular activity of either RIPs. Conjugation with transferrin decreased the IC(50) of the proteins from 3 x 10(-7)M (nigrin b) and 1.5 x 10(-8)M (ebulin l) to 3.5 x 10(-10)M in HeLa cells. Thus, both conjugates could be considered as useful tools for targeting TfR-over-expressing cancer cells.

Expression of recombinant trichosanthin, a ribosome-inactivating protein, in transgenic tobacco

Trichosanthin (TCS) is an antiviral plant defense protein, classified as a type-I ribosome-inactivating protein, found in the root tuber and leaves of the medicinal plant Trichosanthes kirilowii. It is processed from a larger precursor protein, containing a 23 amino acid amino (N)-terminal sequence (pre sequence) and a 19 amino acid carboxy (C)-terminal extension (pro sequence). Various constructs of the TCS gene were expressed in transgenic tobacco plants to determine the effects of the amino- and carboxy-coding gene sequences on TCS expression and host toxicity in plants. The maximum TCS expression levels of 2.7% of total soluble protein (0.05% of total dry weight) were obtained in transgenic tobacco plants carrying the complete prepro-TCS gene sequence under the Cauliflower mosaic virus 35S RNA promoter. The N-terminal sequence matched the native TCS sequence indicating that the T. kirilowii signal sequence was properly processed in tobacco and the protein translation inhibitory activity of purified rTCS was similar to native TCS. One hundred-fold lower expression levels and phenotypic aberrations were evident in plants expressing the gene constructs without the C-terminal coding sequence. Transgenic tobacco plants expressing recombinant TCS exhibited delayed symptoms of systemic infection following exposure to Cucumber mosaic virus and Tobacco mosaic virus (TMV). Local lesion assays using extracts from the infected transgenic plants indicated reduced levels of TMV compared with nontransgenic controls.

The low lysine content of ricin A chain reduces the risk of proteolytic degradation after translocation from the endoplasmic reticulum to the cytosol

Several protein toxins, including the A chain of ricin (RTA), enter mammalian cells by endocytosis and subsequently reach their cytosolic substrates by translocation across the endoplasmic reticulum (ER) membrane. To achieve this export, such toxins exploit the ER-associated protein degradation (ERAD) pathway but must escape, at least in part, the normal degradative fate of ERAD substrates. Toxins that translocate from the ER have an unusually low lysine content. Since lysyl residues are potential ubiquitination sites, it has been proposed that this paucity of lysines reduces the chance of ubiquitination and subsequent ubiquitin-mediated proteasomal degradation [Hazes, B., and Read, R. J. (1997) Biochemistry 36, 11051-11054]. Here we provide experimental support for this hypothesis. The two lysyl residues within RTA were changed to arginyl residues. Their replacement in RTA did not have a significant stabilizing effect, suggesting that the endogenous lysyl residues are not the usual sites for ubiquitin attachment. However, when four additional lysines were introduced into RTA in a way that did not compromise the activity, structure, or stability of the toxin, degradation was significantly enhanced. Enhanced degradation resulted from ubiquitination that predisposed the toxin to proteasomal degradation. Treatment with the proteasome inhibitor clasto-lactacystin beta-lactone increased the cytotoxicity of the lysine-rich RTA to a level approaching that of wild-type ricin. The introduction of four additional lysyl residues into a second ribosome-inactivating protein, abrin A chain, also dramatically decreased the cytotoxicity of the holotoxin compared to wild-type abrin. This effect could also be reversed by proteasomal inhibition. Our data support the hypothesis that the evolution of a low lysine content is a degradation-avoidance strategy for toxins that retrotranslocate from the ER.

Sensitivity of cancer cell lines to the novel non-toxic type 2 ribosome-inactivating protein nigrin b

The cytotoxicity of the type 2 ribosome-inactivating proteins (RIPs) ricin and nigrin b was determined in a variety of cancer cells. Nigrin b, considered to be a novel non-toxic type 2 RIP as compared with ricin, was approximately 10(4)-10(5) times less toxic than ricin in all cancer cells studied, with the exception of melanoma cells. Cancer cells displayed considerable heterogeneity in their sensitivity to ricin, melanoma cells being the least sensitive. Rabbit polyclonal anti-nigrin b antibodies did not cross-react with ricin as analyzed by enzyme-linked immunosorbent assays. The low non-specific toxicity of nigrin b as compared with that of ricin and the lack of immunological cross-reaction between anti-nigrin b antibodies and ricin supports the use of nigrin b in the construction of cytotoxic conjugates as an alternative to ricin when anti-ricin antibodies are produced during cancer therapy.

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.

The pH-dependent interaction of cinnamomin with lipid membranes investigated by fluorescence methods

Cinnamomin, a new type II ribosome-inactivating protein (RIP), was found to be able to induce the release of calcein loaded in lecithin small unilamellar vesicles and the fusion or aggregation of the lecithin liposomes. Such induction could be promoted severalfold by a pH 5.0 environment, a condition similar to that in endocytic vesicles. Lowering the pH from 7.5 to 5.0 evoked conformational changes of cinnamomin and unmasked its hydrophobic areas, including the exposure of 1-anilino-8-naphthalenesulfonate (1,8-ANS) binding sites of the molecule. Some tryptophan residues with affinity to acrylamide were demonstrated to participate in the lipid-protein interaction. The pH dependent fusogenicity of type II RIP might suggest its in vivo function as a fusogen to exert its cytotoxicity.

Mistletoe lectin dissociates into catalytic and binding subunits before translocation across the membrane to the cytoplasm

Hybridomas producing monoclonal antibodies (mAbs) against the mistletoe lectin A-chain (MLA) were obtained to investigate the intracellular routing and translocation of ribosome-inactivating proteins. Anti-MLA mAb MNA5 did not bind the holotoxin but interacted with isolated MLA. This epitope was not recognized upon MLA denaturation or conjugation of MLA with the ricin binding subunit (RTB). Furthermore, the mAbs did not appreciably react with a panel of MLA synthetic octapeptides linked to the surface of polyethylene pins. A study of the cytotoxicity of mistletoe lectin, ricin, and chimeric toxin MLA/RTB for the hybridomas revealed that interchain disulfide bond reduction and subunit dissociation are required for cytotoxic activity of mistletoe lectin.

Differences in cytotoxicity of native and engineered RIPs can be used to assess their ability to reach the cytoplasm

Ricin is a heterodimeric cytotoxin composed of RTB, a galactose binding lectin, and RTA, an enzymatic N-glycosidase. The toxin is endocytosed, and after intracellular routing, RTA is translocated to the cytoplasm where it inactivates ribosomes resulting in a loss of host cell protein synthesis and cell death. We show for the first time that the cytotoxicity against cultured T cells by several RTA mutants is directly proportional to the enzyme activity of RTA, suggesting this is a reliable system to measure translocation effects. Large discrepancies between cytotoxicity and enzyme action for a given pair of toxins are therefore attributable to differences in cell binding, uptake, or membrane translocation. Fluid phase uptake and cytotoxicity of isolated RTA are essentially identical to that of the single chain toxin PAP. This important finding suggests that RTA, and the A chain of class 2 RIPs in general, has not evolved special translocation signals to complement the increased target cell binding facilitated by RTB. Experiments with the lectin RCA and with ebulin suggest those toxins have diminished cytotoxicity probably mediated by comparative deficiencies in B chain binding. Addition of a KDEL sequence to RTA increases fluid phase uptake, consistent with the notion that transport to the ER is important for cytotoxicity. Fusion of MBP or GST to the amino terminus of RTA has little effect on enzyme action or cytotoxicity. This result is not altered by protease inhibitors, suggesting the fusion proteins are probably not cleaved prior to translocation of the toxic A chain and implying that the toxins can carry large passenger proteins into the cytoplasm, an observation with interesting potential for analytical and therapeutic chemistry.

Constitutive and inducible type 1 ribosome-inactivating proteins (RIPs) in elderberry (Sambucus nigra L.)

Two novel highly basic type 1 (single chain) ribosome-inactivating proteins (RIPs) with N-glycosidase activity have been found in elderberries (the fruits of Sambucus nigra L.). Mass spectrometry of these RIPs, which we named nigritins f1 and f2, gave Mr values of 24095 and 23 565, respectively. Both proteins strongly inhibited protein synthesis in rabbit reticulocyte lysates but were inactive against plant ribosomes. Both nigritins have a similar topological activity on pBlueScript SK+ DNA as that displayed by dianthin 30. Nigritin f1 is a constitutive RIP since it is present in both green and mature intact elderberries at nearly the same proportion with respect to total fruit protein. By contrast, nigritin f2 is inducible and only appeared in mature intact elderberries. Elderberries also contain two isoforms of a basic nigrin equivalent to the recently found basic nigrin b in elder bark (De Benito et al., FEBS Letters 413 (1997) 85-91). Our results indicate that probably not all plant RIPs exert the same biological function and that this may be determined by the physiological state of the tissue.

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.

Ribosome-inactivating lectins with polynucleotide:adenosine glycosidase activity

Lectins from Aegopodium podagraria (APA), Bryonia dioica (BDA), Galanthus nivalis (GNA), Iris hybrid (IRA) and Sambucus nigra (SNAI), and a new lectin-related protein from Sambucus nigra (SNLRP) were studied to ascertain whether they had the properties of ribosome-inactivating proteins (RIP). IRA and SNLRP inhibited protein synthesis by a cell-free system and, at much higher concentrations, by cells and had polynucleotide:adenosine glycosidase activity, thus behaving like non-toxic type 2 (two chain) RIP. APA and SNAI had much less activity, and BDA and GNA did not inhibit protein synthesis.

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.