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

Sambucus nigra L. (fam. Viburnaceae) in Sicily: Distribution, Ecology, Traditional Use and Therapeutic Properties

Sambucus nigra, the elderberry, has long been used for its medicinal properties in treating numerous diseases. Based on this traditional knowledge, its different pharmacological activities have been the focus of active research. All parts of the tree have long been used in traditional medicine, that is, the bark, the leaves, the flowers and the fruit. This study, carried out in Sicily (Italy), concerns the traditional uses of elder against human diseases. In order to trace the history of man's interaction with elder on the island, multidisciplinary research was carried out, aiming at (1) presenting a comprehensive overview of elderberry's applications and activities and (2) bridging traditional knowledge (uses and beliefs) with modern science, i.e., the most recent scientific findings in the biomedical and pharmacological fields. A rigorous literature review of scientific (and other local) reports on the elderberry tree and its application in food, health and household applications was undertaken. This article also provides a synthetic and updated picture of the ecology and distribution of S. nigra in Sicily. The elderberry is quite widespread in Sicily, yet its distribution is discontinuous. It prefers hedges, riparian woodlands, forest margins and clearings and is rather common along the watercourses flowing in the canyons of the Hyblaean Plateau, in the Madonie Mts. and in Enna province. Indeed, many old plants are often found near sacred places and rural houses, suggesting that in the past, it was extensively planted on purpose for its multiple uses. The complementary data obtained from multidisciplinary research confirm the usefulness of this approach in building a comprehensive and correct picture of the distribution of the most common woody species, for which the available knowledge is often fragmentary and imprecise.

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.

Profilings of subproteomes of lectin-binding proteins of nine Bothrops venoms reveal variability driven by different glycan types

Snake venom proteomes have long been investigated to explore a multitude of biologically active components that are used for prey capture and defense, and are involved in the pathological effects observed upon mammalian envenomation. Glycosylation is a major protein post-translational modification in venoms and contributes to the diversification of proteomes. We have shown that Bothrops venoms are markedly defined by their content of glycoproteins, and that most N-glycan structures of eight Bothrops venoms contain sialic acid, while bisected N-acetylglucosamine was identified in Bothrops cotiara venom. To further investigate the mechanisms involved in the generation of different venoms by related snakes, here the glycoproteomes of nine Bothrops venoms (Bothrops atrox, B. cotiara, Bothrops erythromelas, Bothrops fonsecai, B. insularis, Bothrops jararaca, Bothrops jararacussu, Bothrops moojeni and Bothrops neuwiedi) were comparatively analyzed by enrichment with three lectins of different specificities, recognizing bisecting N-acetylglucosamine- and sialic acid-containing glycoproteins, and mass spectrometry. The lectin capture strategy generated venom fractions enriched with several glycoproteins, including metalloprotease, serine protease, and L- amino acid oxidase, in addition to various types of low abundant enzymes. The different contents of lectin-enriched proteins underscore novel aspects of the variability of the glycoprotein subproteomes of Bothrops venoms and point to the role of distinct types of glycan chains in generating different venoms by closely related snake species.

Sequence, Structure, and Binding Site Analysis of Kirkiin in Comparison with Ricin and Other Type 2 RIPs

Kirkiin is a new type 2 ribosome-inactivating protein (RIP) purified from the caudex of Adenia kirkii with a cytotoxicity compared to that of stenodactylin. The high toxicity of RIPs from Adenia genus plants makes them interesting tools for biotechnology and therapeutic applications, particularly in cancer therapy. The complete amino acid sequence and 3D structure prediction of kirkiin are here reported. Gene sequence analysis revealed that kirkiin is encoded by a 1572 bp open reading frame, corresponding to 524 amino acid residues, without introns. The amino acid sequence analysis showed a high degree of identity with other Adenia RIPs. The 3D structure of kirkiin preserves the overall folding of type 2 RIPs. The key amino acids of the active site, described for ricin and other RIPs, are also conserved in the kirkiin A chain. Sugar affinity studies and docking experiments revealed that both the 1α and 2γ sites of the kirkiin B chain exhibit binding activity toward lactose and D-galactose, being lower than ricin. The replacement of His246 in the kirkiin 2γ site instead of Tyr248 in ricin causes a different structure arrangement that could explain the lower sugar affinity of kirkiin with respect to ricin.

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.

A case of neofunctionalization of a Putranjiva roxburghii PNP protein to trypsin inhibitor by disruption of PNP-UDP domain through an insert containing inhibitory site

The attainment of new function by a protein is achieved through convergent/divergent evolution. In present work, the sequence analysis of a 34kDa protein from Putranjiva roxburghii, earlier reported as a potent trypsin inhibitor, showed resemblance to some of the wound inducible and vegetative storage proteins. A detailed sequence analysis revealed that these proteins belong to PNP-UDP family. In case of P. roxburghii protein, an approximately 46 residue insert disrupts the PNP domain. Similar disruption of PNP domain is observed in related plant proteins. The characterization of recombinant full length and truncated (without 46 residue insert) forms of P. roxburghii PNP family protein (PRpnp) unraveled that trypsin inhibitory active site is located within the insert. The truncated form containing uninterrupted PNP domain showed strong PNP enzymatic activity where it hydrolyzed the N-glycosidic bond of inosine and guanosine. The full length protein, however, showed weak PNP enzyme activity which may be due to presence of the insert. These results indicate towards the neofunctionalization of PRpnp to a potent trypsin inhibitor through an insert containing inhibitory residue to cater to the needs of plant defense. The similar wound inducible and vegetative storage proteins may have also evolved due to evolutionary needs.

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.

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.

Stability of AtVSP in the insect digestive canal determines its defensive capability

We have previously demonstrated that Arabidopsis vegetative storage protein (AtVSP) is an acid phosphatase that has anti-insect activity in in vitro feeding assays [Liu et al., 2005. Plant Physiology 139, 1545-1556]. To investigate the functionality of AtVSP in planta as an anti-insect defense protein, we produced AtVSP-overexpressing as well as AtVSP-silenced transgenic Arabidopsis lines, and evaluated impact on the polyphagous American grasshopper Schistocerca americana. Grasshoppers showed no significant difference in weight gain and growth rate when feeding on wild type, overexpressing, or silenced lines, respectively. In addition, AtVSP protein was undetectable in either the midgut or frass of grasshoppers reared on transgenic plants suggesting that AtVSP was unable to withstand proteolytic degradation. To determine the stability of the AtVSP protein in grasshopper digestive canal, midgut extracts from various nymphal stages were incubated with bacterially expressed AtVSP for different periods of time. AtVSP was hydrolyzed rapidly by grasshopper midgut extract, in stark contrast with its fate when incubated with cowpea bruchid midgut extract. Multiple proteases have been detected in the midgut of grasshoppers, which may play important roles in determining the insect response to AtVSP. Results indicate that stability of an anti-insect protein in insect guts is a crucial property integral to the defense protein.

Use of glycan microarrays to explore specificity of glycan-binding proteins

Microarrays of defined glycans represent a high throughput approach to determining the specificity of lectins, or more generally glycan-binding proteins (GBPs). The utility of a glycan microarray is directly related to the number and variety of the glycans available on the printed surface for interrogation by GBPs. The Consortium for Functional Glycomics (CFG), funded by the National Institute of General Medical Sciences (NIGMS), has generated a glycan microarray available to the public as an investigator-driven resource, where hundreds of GBPs have been analyzed. Here we describe the methods generally used by the CFG to prepare glycan arrays and interrogate them with GBPs. We also describe our new approach to normalizing glycan microarray data derived from concentration-dependent analyses of GBP binding, and the application of this approach with the plant lectin Sambucus nigra agglutinin (SNA-I) and human galectin-8. The use of glycan microarrays with this approach readily generates a prediction of the glycan determinants required for high affinity binding by a GBP.

Expression of Sambucus nigra agglutinin (SNA-I') from elderberry bark in transgenic tobacco plants results in enhanced resistance to different insect species

Tobacco plants (Nicotiana tabacum cv Samsun NN) have been transformed with the gene encoding the type-2 ribosome-inactivating protein (RIP) SNA-I' from elderberry (Sambucus nigra) under the control of the Cauliflower Mosaic Virus 35S promoter. Previous research confirmed that these plants synthesize, correctly process and assemble a fully active RIP. Variability in protein expression was observed within the transgenic lines. The effects of the type-2 RIP SNA-I' delivered through a leaf feeding assay were evaluated in the laboratory on two economically important pest insects belonging to the orders of Hemiptera, the tobacco aphid (Myzus nicotianae) and Lepidoptera, the beet armyworm (Spodoptera exigua). In the experiment with aphids, significant effects were observed on the life parameters, such as survival, intrinsic rate of increase, net reproductive rate, mean generation time and mean daily offspring, whereas with caterpillars significant reduction in fresh weight as well as retardation in development were observed. In addition, significant increases in mortality were noted for insects fed on the transgenic lines as compared to wild type plants. This information provides further support for RIPs having a role in plant resistance to insect pest species.

Arabidopsis vegetative storage protein is an anti-insect acid phosphatase

Indirect evidence previously suggested that Arabidopsis (Arabidopsis thaliana) vegetative storage protein (VSP) could play a role in defense against herbivorous insects. To test this hypothesis, other AtVSP-like sequences in Arabidopsis were identified through a Basic Local Alignment Search Tool search, and their transcriptional profiles were investigated. In response to methyl jasmonate application or phosphate starvation, AtVSP and AtVSP-like genes exhibited differential expression patterns, suggesting distinct roles played by each member. Arabidopsis VSP2 (AtVSP2), a gene induced by wounding, methyl jasmonate, insect feeding, and phosphate deprivation, was selected for bacterial expression and functional characterization. The recombinant protein exhibited a divalent cation-dependent phosphatase activity in the acid pH range. When incorporated into the diets of three coleopteran and dipteran insects that have acidic gut lumen, recombinant AtVSP2 significantly delayed development of the insects and increased their mortality. To further determine the biochemical basis of the anti-insect activity of the protein, the nucleophilic aspartic acid-119 residue at the conserved DXDXT signature motif was substituted by glutamic acid via site-directed mutagenesis. This single-amino acid alteration did not compromise the protein's secondary or tertiary structure, but resulted in complete loss of its acid phosphatase activity as well as its anti-insect activity. Collectively, we conclude that AtVSP2 is an anti-insect protein and that its defense function is correlated with its acid phosphatase activity.

Complete structure determination ofN-acetyl-D-galactosamine-binding mistletoe lectin-3 fromViscum album L. album

The primary structure of the B chain of the N-acetyl-D-galactosamine-recognizing mistletoe lectin-3 (ML-3B) has been deduced from proteolytic digest peptides of the purified glycoprotein, their HPLC-separation and Edman degradation and confirmation of the peptide sequences by MALDI-MS. ML-3B consists of 262 amino acid residues including 10 cysteine moieties. The structure and linkage of the carbohydrate side chains, connected to two N-glycosylation sites at positions Asn(95) and Asn(135) of the lectin, were determined by a combination of glycosidase treatment and MALDI-MS of corresponding glycopeptide fragments. The sequence alignment reveals a high homology with other B chains of type-II RIPs, although there are remarkable differences in the D-galactose-specific mistletoe lectin-1B chain. The recently published primary structure of the mistletoe lectin-3A chain1 and the now available primary sequence of the 3B chain allowed the construction of a preliminary homology model of ML-3. The model demonstrates, unequivocally, that ML-3 is a member of the type-II RIP family with rigid conservation of the enzymatic active site of the A chain and an identical overall protein fold. Specific amino acid residue exchanges and the different glycosylation pattern in comparison with ML-1 are discussed and related to the properties of the two glycoproteins. The knowledge of the complete primary structure of mistletoe lectin-3 is a major contribution towards more insight into the mechanism of the biological activity of commercial mistletoe preparations.

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.

Purification and characterization of four isoforms of Himalayan mistletoe ribosome-inactivating protein from Viscum album having unique sugar affinity

Ribosome-inactivating proteins having antitumor and immunomodulatory properties constitute the active principle of widely used mistletoe therapy in Europe. This is the first report of the four isoforms of Himalayan mistletoe ribosome-inactivating proteins (HmRips) from Viscum album parasitized on wild apple inhabiting NW Himalayas. HmRips were purified by affinity chromatography and four isoforms were separated by ion-exchange chromatography. HmRip 1, 2, 3, and 4 have isoelectric points of 6.6, 6.1, 5.2, and 4.7, respectively. Disulfide linked toxin and lectin subunits of HmRip 1 and 2 isoforms have molecular weights of 28 and 34kDa while those of HmRip 3 and 4 have 28 and 32kDa. The isoforms lacked blood group specificity and showed positive activity with seven mammalian erythrocyte types but did not show any activity with avian erythrocyte type. Lectin activity of HmRips remained unchanged for a wide range of temperatures (0-65 degrees C) and pH (3-9). Unlike other type II Rips, the HmRip 1, 2, and 4 showed unique affinity towards l-rhamnose, meso-inositol, and l-arabinose while HmRip 3 has specificity to gal/galNAc. Sugar binding studies with 22 sugars also suggested that the C-4 hydroxyl of galactose might be the critical site involved in sugar binding of HmRips. Type II Rips are known to be galactoside specific and do not have affinity for l-rhamnose and meso-inositol. However, HmRip 1, 2, and 4 having equal affinity for galactose and l-rhamnose do not strictly fit into any of the four structural classes of the lectins and represent a new class of type II Rips and plant lectins.

Complete structure determination of the A chain of mistletoe lectin III fromViscum albumL. ssp.album

The complete primary structure of the A chain of mistletoe lectin III (ML3A), a type II ribosome-inactivating protein, was determined using proteolytic digests of ML3A, HPLC separation of the peptides, Edman degration and MALDI-MS. Based on our results, ML3A consists of 254 amino acid residues, showing a high homology to the A chain of isolectin ML1 with only 24 amino acid residue exchanges. A striking important structural difference compared with ML1A is the lack of the single N-glycosylation site in ML3A due to an amino acid exchange at position 112 (ML1A: NL112GS ==> ML3A: T112GS). The alignment of ML3A with the A chains of ML1, isoabrins, ricin D, Ricinus communis agglutinin and three lectins, identified from the Korean mistletoe Viscum album ssp. coloratum, demonstrates the rigid conservation of all amino acid residues, responsible for the RNA-N-glycosidase activity as reported for ricin D. In addition, the fully determined primary structure of ML3A will give further information about the biological mechanism of mistletoe lectin therapy.

The size, shape and specificity of the sugar-binding site of the jacalin-related lectins is profoundly affected by the proteolytic cleavage of the subunits

Mannose-specific lectins with high sequence similarity to jacalin and the Maclura pomifera agglutinin have been isolated from species belonging to the families Moraceae, Convolvulaceae, Brassicaceae, Asteraceae, Poaceae and Musaceae. Although these novel mannose-specific lectins are undoubtedly related to the galactose-specific Moraceae lectins there are several important differences. Apart from the obvious differences in specificity, the mannose- and galactose-specific jacalin-related lectins differ in what concerns their biosynthesis and processing, intracellular location and degree of oligomerization of the protomers. Taking into consideration that the mannose-specific lectins are widely distributed in higher plants, whereas their galactose-specific counterparts are confined to a subgroup of the Moraceae sp. one can reasonably assume that the galactose-specific Moraceae lectins are a small-side group of the main family. The major change that took place in the structure of the binding site of the diverging Moraceae lectins concerns a proteolytic cleavage close to the N-terminus of the protomer. To corroborate the impact of this change, the specificity of jacalin was re-investigated using surface plasmon resonance analysis. This approach revealed that in addition to galactose and N -acetylgalactosamine, the carbohydrate-binding specificity of jacalin extends to mannose, glucose, N -acetylmuramic acid and N -acetylneuraminic acid. Owing to this broad carbohydrate-binding specificity, jacalin is capable of recognizing complex glycans from plant pathogens or predators.

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.

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.

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.

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.