Ribosome-inactivating proteins from plants

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.

Direct evidence for the deoxyribonuclease activity of the plant ribosome inactivating protein gelonin

Several plant ribotoxins, including gelonin, were reported to have additional weak nuclease activities on supercoiled DNA. The potential contribution of this activity to their cytotoxicity has not been given serious consideration due to concerns about contaminating nucleases in the protein preparations. We now report the degradation of single-stranded DNA by preparations of native plant gelonin and recombinant gelonin produced in E. coli. The DNase activity of both preparations is similarly modulated by zinc. An SDS-PAGE DNase assay identifies gelonin as the polypeptide responsible for deoxyribonuclease activity.

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.

Molecular, biological, and preliminary structural analysis of recombinant bryodin 1, a ribosome-inactivating protein from the plant Bryonia dioica

Bryonia dioica (Cucurbitaceae family) produces at least two type I ribosome-inactivating proteins, bryodin 1 (BD1) and bryodin 2 (BD2). A cDNA sequence encoding BD1 was isolated from B. dioica leaf mRNA using degenerative oligonucleotides and codes for a 22 amino acid signal peptide followed by a protein of 267 residues. Expression of two recombinant BD1 (rBD1) forms in Escherichia coli yielded proteins of 267 (to the natural stop codon) and 247 amino acids (to the putative cleavage site yielding the mature protein) that had identical protein synthesis inhibition activity as compared to native BD1. The substitution of Lys for Glu at position 189 near the active site reduced the ability of rBD1 to inhibit protein synthesis by 10-fold. Toxicologic analysis showed that rBD1 was well tolerated in rodents with LD50 values of 40 mg/kg in mice and >25 mg/kg in rats. A crystal of mature rBD1 protein was used to collect X-ray diffraction data to 2.1 A resolution. The rBD1 crystal structure was solved and showed extensive homology with other type I RIPs and A chains of type II RIPs. The studies described here demonstrate that rBD1 retains full biologic activity and serve as a guide for using this potent, yet nontoxic, RIP in the construction of single-chain immunotoxin fusion proteins.

Complete amino-acid sequence of PD-S2, a new ribosome-inactivating protein from seeds of Phytolacca dioica L

The primary structure has been determined for PD-S2, a new type 1 ribosome-inactivating protein (RIP), isolated from the seeds of Phytolacca dioica L. PD-S2 has 265 amino-acid residues, and a molecular mass of 29586 Da. The polypeptide chain contains four amino-acid residues more than PAP-S, a type-I RIP isolated from the seeds of the taxonomically related plant Phytolacca americana L. We have compared the amino-acid sequence of PD-S2 with those of two other RIPs with known three-dimensional structure: PAP-S and ricin A-chain (RTA), the active chain of the best known type-2 RIP. This analysis shows an identity of 76% and 33% with PAP-S and RTA respectively, and a similarity of 82% and 54%. Comparison with the PAP sequence, isolated from leaves of P. americana, shows an even higher identity (80%) and similarity (87%). Furthermore, the amino-acid residues reported in other RIPs to be invariant and participate in the definition of the active site (Tyr-76, Tyr-127, Glu-179, Arg-182 and Trp-211; PD-S2 numbering) are all present. Asn-74, Arg-138, Gln-175, and Glu-208 are also conserved, while Asn-209 is substituted by Glu, all residues located in the active-site cleft of RIPs (Tahirov, T.H., Lu, T.-H., Liaw, Y.-C., Chen, J.L. and Lin, J.Y. (1995) Crystal structure of abrin-a at 2.14 A, J. Mol. Biol. 250, 354-367). The polypeptide chain of PD-S2 contains two N-glycosylation sites at Asn-112 and Asn-120, the second of which appears to be linked to sugars. Like PAP-S, PD-S2 does not contain free sulfhydryl groups. The four cysteinyl residues of the two proteins have corresponding sequence positions, most likely with identical S-S pairing.

Animal lectins

Protein and lipid glycosylation is no longer considered as a topic whose appeal is restricted to a limited number of analytical experts perseveringly pursuing the comprehensive cataloguing of structural variants. It is in fact arousing curiosity in various areas of basic and applied bioscience. Well founded by the conspicuous coding potential of the sugar part of cellular glycoconjugates which surpasses the storage capacity of oligonucleotide- or oligopeptide-based code systems, recognition of distinct oligosaccharide ligands by endogenous receptors, i.e. lectins and sugar-binding enzymes or antibodies, is increasingly being discovered to play salient roles in animal physiology. Having inevitably started with a descriptive stage, research on animal lectins has now undubitably reached maturity. Besides listing the current categories for lectin classification and providing presentations of the individual families and their presently delineated physiological significance, this review places special emphasis on tracing common structural and functional themes which appear to reverberate in nominally separated lectin and animal categories as well as lines of research which may come to fruition for medical sciences.

Isolation of a novel plant lectin with an unusual specificity from Calystegia sepium

A novel plant lectin has been isolated from the rhizomes of Calystegia sepium (hedge bindweed) and partially characterized. The lectin is a dimeric protein composed of two identical non-covalently linked subunits of 16 kDa. Hapten inhibition studies indicate that the novel lectin is best inhibited by maltose and mannose and hence exhibits a sugar binding specificity that differs in some respects from that of all previously isolated plant lectins. Mitogenicity tests have shown that the Calystegia lectin is a powerful T-cell mitogen. Affinity purification of human, plant and fungal glycoproteins on immobilized C. sepium lectin demonstrates that this novel lectin can be used for the isolation of glycoconjugates from various sources. Moreover, it can be expected that by virtue of its distinct specificity, the new lectin will become an important tool in glycobiology.

Polynucleotide:adenosine glycosidase activity of ribosome-inactivating proteins: effect on DNA, RNA and poly(A)

Ribosome-inactivating proteins (RIP) are a family of plant enzymes for which a unique activity was determined: rRNAN-glycosidase at a specific universally conserved position, A4324in the case of rat ribosomes. Recently we have shown that the RIP from Saponaria officinalis have a much wider substrate specificity: they are actually polynucleotide:adenosine glycosidases. Here we extend studies on substrate specificity to most known RIP: 52 purified proteins, both type 1 (single-chain) and type 2 (two chain, an enzymatic chain and a lectin chain) were examined for adenine release on various substrates including RNAs from different sources, DNA, and poly(A). All RIP depurinated extensively DNA and some released adenine from all adenine-containing polynucleotides tested. From experimental evidence the entire class of plant proteins, up to now called ribosome-inactivating proteins, may be classified as polynucleotide:adenosine glycosidases. The newly identified substrates may be implicated in the biological role(s) of RIP.

Fluorescence spectroscopic study of the interaction of saporin with phospholipid vesicles

Saporin-S6 (SO-6) is a type-1 ribosome-inactivating protein purified from the seeds of Saponaria officinalis. The fluorescence characteristics of SO-6 were studied in the presence and absence of phospholipids. The interaction of SO-6 with DMPG or DMPC vesicles results in a decrease in the fluorescence emission intensity of tryptophan without any shift in the emission maximal wavelength. The results of fluorescence titration indicate that DMPG/SO-6 saturation molar ratio is 100: 1, but the binding of DMPC with SO-6 does not reach a saturating plot. A shielding of the tryptophan fluorescence from quenching by acrylamide on interaction with the phospholipids was observed, and this shielding was more pronounced in the presence of DMPG. The interaction of SO-6 with DMPG vesicles is stronger in the liquid-crystalline phase than in the gel phase. Extrinsic fluorescence studies indicated that the interaction of the protein with DMPG vesicles does not modify the phase transition temperature of the lipid, but decreases the amplitude of the change of fluorescence anisotropy associated with the co-operative melting of 1,6-diphenyl-1,3,5-hexatriene (DPH)-labelled vesicles. These results indicate that both electrostatic and hydrophobic components are involved in the SO-6-lipid vesicle interaction.

Immunoconjugates made of an anti-EGF receptor monoclonal antibody and type 1 ribosome-inactivating proteins from Saponaria ocymoides or Vaccaria pyramidata

The present paper describes two immunoconjugates consisting of an anti-epidermal growth factor receptor (EGFR) monoclonal antibody (MAb), named Mint5, covalently linked to the type 1 ribosome-inactivating proteins (RIPs) ocymoidine (Ocy) and pyramidatine (Pyra) from Saponaria ocymoides and Vaccaria pyramidata respectively. Both antibody and toxins are shown to retain their respective biological properties upon chemical conjugation. The immunoconjugates exert specific inhibition of EGFR expressing target cell proliferation and protein synthesis in in vitro assays and also inhibit the growth of grafted human tumour cells in nude mice.

Purification, characterization and subcellular localization of a type-1 ribosome-inactivating protein from the sarcocarp of Cucurbita pepo

The flesh of the fruit of Cucurbita pepo contains a type-1 ribosome-inactivating protein (RIP), which we named pepocin. Pepocin was purified to apparent homogeneity by acid fractionation, ion-exchange chromatography and adsorption chromatography. The protein was found to have a molecular mass of 26 kDa and a pI of about 9.9. It does not contain glycosidic linkages. The protein inhibits protein synthesis in a rabbit-reticulocyte lysate with an IC50 (concentration causing 50% inhibition) of 15.4 pM, and depurinates 28S rRNA in the ribosomes of the lysate in a manner identical to that of ricin A-chain and other RIP. The enzyme is also active on wheat-germ ribosomes and on Escherichia coli ribosomes. The sequence of the N-terminal 20 amino acids of the protein reveals a close relationship to other RIP. Immunoelectron-microscopic localization of pepocin in the sarcocarp shows that the protein is predominantly localized in intercellular spaces. In addition, the immunolocalized signals are observed in leaf intercellular spaces.

Complete amino acid sequence of the A chain of mistletoe lectin I

The complete amino acid sequence of the A chain of mistletoe lectin I was determined via Edman degradation sequencing of the N-terminus and tryptic and endoproteinase Asp-N overlapping fragments, amino acid analysis and MALDI-MS. The data obtained show a great homology with the chains of ribosome-inactivating proteins such as ricin and abrin with 111 (abrin-a) and 103 (ricin-D) amino acid residues conserved, respectively. The knowledge of the primary structure of MLA will have a fundamental impact on elucidating the biological function of medically applied mistletoe lectins on a molecular basis.

Molecular cloning of the mitogenic mannose/maltose-specific rhizome lectin from Calystegia sepium

cDNA clones encoding the mitogenic mannose/maltose-specific lectin from the rhizomes of hedge bindweed (Calystegia sepium) have been isolated and sequenced. Comparison of the deduced amino acid sequence and the molecular weight of the lectin subunit as determined by mass spectrometry indicated that the mature protein comprises the entire open reading frame of the cDNA, which implies that the primary translation product contains no signal peptide and is not proteolytically processed. Searches in the databases revealed sequence homology with the previously described lectins from the taxonomically unrelated Moraceae species Artocarpus integrifolia and Maclura pomifera.

A systemic antiviral resistance-inducing protein isolated from Clerodendrum inerme Gaertn. is a polynucleotide:adenosine glycosidase (ribosome-inactivating protein)

Two systemic antiviral resistance-inducing proteins, CIP-29 and CIP-34, isolated from Clerodendrum inerme Gaertn. leaves, were tested for ribosome-inactivating properties. It was found that CIP-29 has the characteristics of a polynucleotide:adenosine glycosidase (ribosome-inactivating protein), in that it inhibits protein synthesis both in cell-free systems and, at higher concentrations, in cells, and releases adenine from ribosomes, RNA, poly(A) and DNA. As compared with other known RIPs, CIP-29 deadenylates DNA at a high rate, and induces systemic antiviral resistance in susceptible plants.

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.

Induction of apoptosis by ribosome-inactivating proteins and related immunotoxins

Immunotoxins have been prepared with 3 ribosome-inactivating proteins (RIPs), namely, momordin, pokeweed anti-viral protein from seeds (PAP-S) and saporin, linked to the Ber-H2 monoclonal antibody directed against the CD30 antigen of human lymphocytes. Either the RIPs or the immunotoxins induced apoptosis in the CD30+ L540 cell line, as shown by the morphological aspects of the cells, by the DNA fragmentation visible at the electrophoresis, and by the formation of DNA breaks evidenced by 2 cytofluorometric techniques (propidium-iodide staining and fluoresceine-isothiocyanate conjugate dUTP incorporation). The AC50 (concentration causing apoptosis in 50% of the cells) is in the range 10(-8) to 10(-7) M in the case of RIPs, and 10(-11) to 10(-10) M in the case of the immunotoxins.

Polynucleotide: adenosine glycosidase activity of saporin-L1: effect on DNA, RNA and poly(A)

The ribosome-inactivating proteins (RIPs) are a family of plant enzymes for which a unique activity has been determined: rRNA N-glycosidase, which removes adenine at a specific universally conserved position (A4324 in the case of rat ribosomes). Here we report that saporin-L1, a RIP from the leaves of Saponaria officinalis, recognizes other substrates, including RNAs from different sources, DNA and poly(A). Saporin-L1 depurinated DNA extensively and released adenine from all adenine-containing polynucleotides tested. Adenine was the only base released from DNA or artificial polynucleotides. The characteristics of the reactions catalysed by saporin-L1 have been determined: optimal pH and temperature, ionic requirements, and the kinetic parameters Km and kcat. The reaction proceeded without cofactors, at low ionic strength, in the absence of Mg2+ and K+. Saporin-L1 had no activity towards various adenine-containing non-polynucleotide compounds (cytokinins, cofactors, nucleotides). This plant protein may now be classified as a polynucleotide: adenosine glycosidase.

Demonstration of ribonuclease activity in the plant ribosome-inactivating proteins alpha- and beta-momorcharins

Alpha- and beta-momorcharins, ribosome-inactivating proteins from Momordica charantia seeds, were utilized in this investigation. Ribonucleolytic cleavage was observed after naked rRNA was incubated with either momorcharin. Beta-momorcharin, and to a lesser extent alpha-momorcharin, also acted on tRNA to release acid-soluble UV-absorbing products. Such activity was optimal at pH around 5.5. Using polyhomoribonucleotides as substrate, it was found that the momorcharins preferentially acted on polyU, but exerted negligible effects on polyA, polyC and polyG. Chromatographic analysis of the reaction product indicated that mono and/or oligo-ribonucleotides, but not free base, were generated from polyU, suggesting that the enzymatic action involved ribonucleolytic cleavage. Similar to the results obtained with tRNA as substrate, beta-momorcharin was about 15-fold more active than alpha-momorcharin on polyU.

The plant translational apparatus

Protein synthesis in both eukaryotic and prokaryotic cells is a complex process requiring a large number of macromolecules: initiation factors, elongation factors, termination factors, ribosomes, mRNA, amino-acylsynthetases and tRNAs. This review focuses on our current knowledge of protein synthesis in higher plants.

Lectin binding and uptake in human (myelo)monocytic cell lines: HL60 and U937

The terminal carbohydrate residues of the human (myelo) monocytic cell lines HL60 and two subclones of U937 were investigated by the use of lectins. Several terminal carbohydrate residues, including N-acetylglucosamine and carbohydrates of the complex type, were detected on all three cell lines. Except for galactose residues, the two subclones of U937 had almost identical terminal carbohydrate residues. The differences between the two U937 subclones and HL60 were more pronounced, the latter expressing fucose residues, which might be part of the CD15 cell adhesion molecules. Some of the differences of the carbohydrate residues between the cell lines could be attributed to their differentiation within the myelomonocytic cell lineage. Lectins that bound to the cell surface were internalized via either clathrin-coated pits and vesicles or nonspecific endocytosis, indicating that functionally different classes of cell surface glycoproteins are involved in lectin binding and internalization.

Immunotoxins containing A-chain of mistletoe lectin I are more active than immunotoxins with ricin A-chain

Conjugates of anti-CD25 monoclonal antibodies against cell surface IL-2 receptor with MLIA and RTA were prepared and investigated. Both of the immunotoxins had high specific cytotoxic activity on target cells. The IC50 value of the anti-CD25/MLIA immunotoxin was 15-fold greater than that of the anti-CD25/RTA. Previous studies of the anti-CD5 immunotoxins with MLIA and RTA showed that the anti-CD5/MLIA IT was 80-fold more active than anti-CD5/RTA IT [Tonevitsky et al. (1991) Int. J. Immunopharmacol. 13, 1037-1041]. The surface hydrophobicity of the MLI A-chain was 4-fold higher than that of the ricin A-chain as estimated by binding with ANS. In model experiments with small unilamellar DMPC liposomes, MLIA but not RTA increased the turbidity of liposome suspensions at pH 4.5. Our results indicate that the greater cytotoxic activity of the MLI A-chain immunotoxin probably provided a higher surface hydrophobicity of the protein and the ability to interact with phospholipid membranes.

A highly efficient procedure for purifying the ribosome-inactivating proteins alpha- and beta-momorcharins from Momordica charantia seeds, N-terminal sequence comparison and establishment of their N-glycosidase activity

A new purification scheme, involving two successive ion exchange chromatographic steps on DEAE-cellulose and Mono-S FPLC, was developed for the isolation of the ribosome-inactivating proteins, alpha- and beta-momorcharins, from the Chinese herb Kuquazi (seeds of Momordica charantia). This simple and rapid procedure yielded 3.1 and 1.7 mg of alpha- and beta-momorcharins, respectively, from 2.5 g of decorticated seeds in only two days. The N-terminal amino acid sequence of beta-momorcharin was found to be DVNFDLSTATAKTYTKFIED. It differed from that of alpha-momorcharin (DVSFRLSGADPRSYGMFIKD) in 10 out of the 20 positions investigated. Like other ribosome-inactivating proteins, the purified momorcharins showed specific N-glycosidase activity at nanomolar concentrations, when rRNA from rabbit reticulocyte lysate was used as substrate. The N-glycosidase activity of both momorcharins was optimal at pH7, not inhibited by K+ and not appreciably affected by NH4+. The activity of alpha-momorcharin was not drastically altered by Mn2+ but (1-10mM) Mn2+ inhibited the activity of beta-momorcharin by about 40%.

Morphology of ricin and abrin exposed endothelial cells is consistent with apoptotic cell death

Cultures of bovine pulmonary endothelial (BPE) cells were exposed to LC70 doses of ricin or abrin (15.5 and 4.5 pM respectively) over a period of up to 40 h. The viability of the cultures (as determined by the neutral red (NR) dye retention assay) declined after 6 h exposure to the toxins. From 15 h onwards, cellular material in toxin exposed cultures became detached from the substratum of the culture vessels. Hoffman modulation contrast photomicrography showed that this process was due to ricin and abrin exposed cells collapsing into membrane bound vesicles which retained the NR dye, became detached and floated into the medium. These apoptotic-like structural changes were further investigated by transmission electron microscopy (TEM) and by agarose gel electrophoresis of DNA from control and exposed cultures. Many of the characteristic changes associated with apoptotic cell death were seen using TEM, including heterochromatin condensation at the nuclear periphery, crenulation of the nuclear membrane and progressive degeneration of residual nuclear and cytoplasmic structures. The plasma membrane of many cells remained intact, and contained nuclear and cytoplasmic debris. Agarose gel electrophoresis of DNA extracted from toxin-treated cells revealed oligonucleosome sized DNA fragments, characteristic of apoptosis, from adherent cells at 7 h and both adherent and floating populations when harvested from 15 h; DNA from unexposed control cells did not show this fragmentation. The identification of apoptosis as being a significant additional mechanism of toxicity following exposure to ricin and abrin holotoxins raises the possibility of developing new therapeutic strategies against poisoning by these phytotoxins.

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)

The molecular structure of the Sambucus nigra agglutinin V (SNAV), which has been described previously as a type-2 ribosome-inactivating protein called nigrin b, has been studied in detail by analysis of the purified protein combined with cDNA cloning and molecular modelling. Native SNAV is a dimer of two [A-s-s-B] pairs. Hapten inhibition assays indicated that GalNAc is a 20-fold more potent inhibitor of SNAV than Gal. A cDNA clone encoding SNAV was isolated from a cDNA library constructed with mRNA from the bark. Sequence analysis of this cDNA revealed a striking similarity to the recently cloned NeuAc alpha-2,6-gal/GalNAc-specific S. nigra bark agglutinin I (SNAI) and to the previously sequenced type-2 ribosome-inactivating proteins from Ricinus communis and Abrus precatorius. In addition, molecular modelling of SNAV further suggested that its structure closely resembles that of ricin. The N-terminal sequence of the B chain of SNAV also shows a marked similarity with the polypeptide of the previously described GalNAc-specific s. nigra bark agglutinin II (SNAII), which unlike SNAV and SNAI has no ribosome-inactivating activity. It appears, therefore, that elderberry bark contains at least two different type-2 ribosome-inactivating proteins and a lectin built up of subunits which are closely related to the B chain of SNAV.

Immunotoxins: an update

The use of immunotoxins (ITs) in the therapy of cancer, graft-vs-host disease (GvHD), autoimmune diseases, and AIDS has been ongoing for the past two decades. ITs contain a targeting moiety for delivery and a toxic moiety for cytotoxicity. Theoretically, one molecule of a toxin, routed to the appropriate cellular compartment, will be lethal to a cell. Newly developed MoAbs, toxins, and molecular biological technologies have enabled researchers to construct ITs that can effectively kill many different cell types. In fact, phase I/II clinical trials have given promising results. Although nonspecific toxicity and immunogenicity still limit the use of IT therapy, these agents hold enormous promise in an optimal setting to treat minimal disease.

Activities associated with the presence of ribosome-inactivating proteins increase in senescent and stressed leaves

The ribosome-inactivating proteins (RIPs) from Hura crepitans and Phytolacca americana release adenine from herring sperm DNA. Leaf extracts from these plants show the same enzymatic activities as the RIPs. The translation inhibitory activity and the activity on DNA are both increased in the leaves of both plants during senescence or when subjected to heat or osmotic stress. It is proposed that a physiological role of RIPs could be to intervene in the death of plant cells.

Molecular cloning of two different mannose-binding lectins from tulip bulbs

Two lectins were isolated from the bulbs of Tulipa cv. Apeldoorn and their corresponding cDNA clones analyzed. The first, called TxLMII (second mannose-binding Tulipa hybrid lectin), is a novel mannose-binding tulip lectin. Based on its molecular structure, carbohydrate-binding specificity and amino acid sequence, TxLMII belongs to the superfamily of mannose-binding monocot lectins which are also found in representatives of the plant families Amaryllidaceae, Alliaceae, Orchidaceae and Araceae. Molecular cloning of the second lectin, called TxLCI (first Tulipa hybrid lectin with complex specificity), allowed determination unambiguously of the molecular structure of this previously described protein. In addition, evidence is presented that each TxLCI subunit possesses a mannose-binding site and an N-acetylgalactosamine-binding site, which act independently of each other. Both binding sites are located in a separate domain of the lectin polypeptide. Since the first domain of TxLCI shows sequence similarity to TxLMII, it is suggested that their genes evolved from a common ancestor.

Toxicity of ribosome-inactivating proteins-containing immunotoxins to a human bladder carcinoma cell line

Immunotoxins were prepared by linking the type 1 ribosome-inactivating proteins (RIP) momordin I, pokeweed antiviral protein from seeds (PAP-S) and saporin-S6 to the 48-127 monoclonal antibody (MAb) recognising a glycoprotein (gp54) expressed on all human bladder tumours tested and on human bladder carcinoma cell lines, in particular on the T24 cell line. T24 cells required a 2 hr contact with immunotoxins to ensure binding and endocytosis. A time course of exposure, followed by further incubation without the immunotoxins, showed that maximum inhibition of protein synthesis by T24 cells was reached after 2 hr of contact followed by 3 days without the immunotoxins. Under optimal conditions, 48-127/RIP immunotoxins at nanomolar concentrations inhibited by 50% protein synthesis of target T24 cells. No toxicity was observed if (i) target cells were treated with non-conjugated RIP, (ii) target cells were treated with momordin I- or PAP-S-containing immunotoxins made with an irrelevant antibody and (iii) a non-target cell line was treated with the same 2 RIP conjugated to 48-127 antibody. The in vitro selective toxicity of these immunotoxins encourages further studies in view of a possible use in clinical trials for the local therapy of human bladder carcinomas.

Sialylated oligosaccharide-specific plant lectin from Japanese elderberry (Sambucus sieboldiana) bark tissue has a homologous structure to type II ribosome-inactivating proteins, ricin and abrin. cDNA cloning and molecular modeling study

Bark lectins from the elderberry species belonging to the genus Sambucus have a unique carbohydrate binding specificity for sialylated glycoconjugates containing NeuAc(alpha 2-6)Gal/GalNAc sequence. To elucidate the structure of the elderberry lectin, a cDNA library was constructed from the mRNA isolated from the bark tissue of Japanese elderberry (Sambucus sieboldiana) with lambda gt11 phage and screened with anti-S. sieboldiana agglutinin (SSA) antibody. The nucleotide sequence of a cDNA clone encoding full-length SSA (LecSSA1) showed the presence of an open reading frame with 1902 base pairs, which corresponded to 570 amino acid residues. This open reading frame encoded a signal peptide and a linker region (19 amino acid residues) between the two subunits of SSA, the hydrophobic (A-chain) and hydrophilic (B-chain) subunits. This indicates that SSA is synthesized as a preproprotein and post-translationally cleaved into two mature subunits. Homology searching as well as molecular modeling studies unexpectedly revealed that each subunit of SSA has a highly homologous structure to the galactose-specific lectin subunit and ribosome-inactivating subunit of plant toxic proteins such as ricin and abrin, indicating a close evolutionary relationship between these carbohydrate-binding proteins.

The NeuAc(alpha-2,6)-Gal/GalNAc-binding lectin from elderberry (Sambucus nigra) bark, a type-2 ribosome-inactivating protein with an unusual specificity and structure

The cDNA encoding the NeuAc(alpha-2,6)Gal/GalNAc binding lectin from elderberry (Sambucus nigra) bark (SNAI) was isolated from a cDNA library constructed with mRNA from the bark. Sequence analysis of this lectin cDNA revealed a striking similarity to the previously sequenced type-2 ribosome-inactivating proteins from Ricinus communis and Abrus precatorius. Molecular modelling of SNAI further indicated that its structure closely resembles that of ricin. Since SNAI strongly inhibits cell-free protein synthesis in a rabbit reticulocyte lysate it presumably is a type-2 ribosome-inactivating protein. However, SNAI differs from all previously described type-2 ribosome-inactivating proteins by its specificity towards NeuAc(alpha-2,6)Gal/GalNAc and its unusual molecular structure.

Differential up-regulation by tRNAs of ribosome-inactivating proteins

Some plant ribosome-inactivating proteins (RIPs) with RNA-N-glycosidase activity on 28S RNA require, for the inactivation of ribosomes, the presence of macromolecular cofactors present in post-ribosomal supernatants. In the case of gelonin one of the cofactors is tRNATrp lacking one or two nucleotides at the 3'-CCA end [Brigotti, M., Carnicelli, D., Alvergna, P., Pallanca, A., Lorenzetti, R., Denaro, M., Sperti, S. and Montanaro, L. (1995) Biochem. J. 310, 249-253]. In the present study it is shown that tRNAs are involved in the up-regulation of all the cofactor-requiring RIPs up to now identified (agrostin, barley RIP, PAP and tritin, besides gelonin). Polyacrylamide gel electrophoresis shows that tRNA fractions with different mobilities stimulate different RIPs. With the identification of agrostin, the cofactor-requiring RIPs (italics) add to five out of a total of thirteen investigated: barley RIP, bryodin-R, gelonin, lychnin, momordin, momorcochin-S, PAP, saporin-6, tritin [Carnicelli, D., Brigotti, M., Montanaro, L. and Sperti, S. (1992) Biochem. Biophys. Res. Commun. 182, 579-582], agrostin, luffin, trichokirin and trichosanthin (present study).

Targeting of type 1 ribosome-inactivating proteins to CD30+ or CD25+ hematologic neoplasias by bispecific antibodies

In this study, we compared the ability of different bispecific monoclonal antibodies (BsmAb) and immunotoxins to deliver the type 1 ribosome-inactivating proteins (RIP) saporin and gelonin through the CD25 or CD30 target molecules to Hodgkin's lymphoma cells. An anti-CD25/antisaporin and an anti-CD30/antisaporin BsmAb enhanced the toxicity of the relevant RIP against the CD25+CD30+ L540 Hodgkin's lymphoma cell line, although targeting by anti-CD30 BsmAb appeared eight times more efficient. Two anti-CD30/antigelonin BsmAb, reacting with different epitopes of the gelonin molecule, were able to enhance gelonin toxicity against L540 cells and had a synergistic effect when used in combination. Among CD25-CD30+ Hodgkin's lymphoma lines, which were resistant to targeting by anti-CD25/saporin BsmAb, one (L428) was sensitive to both gelonin and saporin delivered by anti-CD30 BsmAb. Another CD25-CD30+ cell line (COLE) was completely resistant to the toxic effect of gelonin targeted by the two synergistic BsmAb, as well as to an anti-CD30/gelonin immunotoxin. However, these cells were partially sensitive to saporin delivered by an anti-CD30/anti-saporin BsmAb, and they were efficiently killed by an anti-CD30/saporin immunotoxin. These results indicate that heterogeneity in the sensitivity to certain RIP, such as gelonin, exists among tumor cells of the same histotype.

Simultaneous existence of two types of ribosome-inactivating proteins in the seeds of Cinnamonum camphora--characterization of the enzymatic activities of these cytotoxic proteins

Two types of ribosome-inactivating proteins (RIPs), camphorin and cinnamomin, have been purified from the seeds of Cinnamomum camphora. Camphorin is a type I RIP with a molecular mass of 23 kDa. Cinnamomin is a type II RIP having three isoforms, its molecular mass is around 61 kDa. The A- and B-chain derived from cinnamomin isoforms have similar mobilities in SDS-polyacrylamide gel. Camphorin and cinnamomin are both glycoproteins. In phylogenesis, it is particularly significant to find the simultaneous existence of type I and type II RIP as well as three isoforms of the latter in the same organ of a higher plant. The molecular mechanism of action of these two RIPs on mammalian ribosomes was demonstrated to be the specific RNA N-glycosidase activity. They modify 28S RNA among the four species of ribosomal RNAs. The cleavage site is the adenosine at position 4324 (rat liver 28S rRNA) embedded in the highly conserved 'R/S domain'. It is the first report that an intact type II RIP (cinnamomin) exhibits RNA N-glycosidase activity. Additionally, camphorin and cinnamomin can cleave supercoiled double-stranded DNA into nicked and linear forms (Ling et al. (1994) FEBS Lett. 345, 143-146).

Suicide retrograde transport of volkensin in cerebellar afferents: direct evidence, neuronal lesions and comparison with ricin

Volkensin and ricin, either free or conjugated with colloidal gold, were injected into the cerebellar cortex of rats. The inferior olive and pontine nuclei were examined to verify the retrograde axonal transport of these two toxins, and the consequent neuronal damage. No evidence was obtained of a retrograde axonal transport of ricin in these pathways. Injection of gold-conjugated volkensin in the cerebellar cortex resulted in retrogradely labelled neurones in the inferior olive after 3 h, and in the pontine nuclei after 6 h. Degenerative changes were very severe in the retrogradely labelled neurones 48 h after the gold-conjugated volkensin injection. In the Nissl-stained material, neuronal degeneration started to be evident in the inferior olive 12 h, and in pontine nuclei 6 h, after volkensin injection. The neuronal degeneration in both the inferior olive and pons increased up to 4 days after the injection. These findings provide direct evidence of the retrograde axonal transport of volkensin in the central nervous system, and the time course of the consequent degenerative changes in the afferents to the cerebellar cortex.

3'-immature tRNA(Trp) is required for ribosome inactivation by gelonin,a plant RNA N-glycosidase

Inactivation of ribosomes by gelonin, a ribosome-inactivating protein with RNA N-glycosidase activity on 28 S rRNA, requires macromolecular cofactors present in post-ribosomal supernatants. One of these cofactors has been purified from a rat liver extract and identified as an RNA about 70 nt long which in sequence analysis shows a high level of similarity with mammalian (bovine) tRNA(Trp). The pattern of the sequencing gel is consistent with the co-existence in the preparation of two 3'-immature tRNA(Trp) species, missing only A75, or both A75 and C74. In the presence of ATP, CTP and tRNA nucleotidyltransferase, the gelonin-stimulating RNA is a good acceptor of tryptophan. An oligodeoxynucleotide complementary to positions 55 to 72 of mammalian (bovine) tRNA(Trp) hybridizes with the gelonin-stimulating RNA as demonstrated by gel mobility shift and ribonuclease H digestion. The oligodeoxynucleotide-directed ribonuclease H treatment also abolishes the gelonin-promoting activity of crude preparations of RNA, giving strong evidence that the only active RNA is a tRNA(Trp)-like molecule.

Differential sensitivity of CD30+ neoplastic cells to gelonin delivered by anti-CD30/anti-gelonin bispecific antibodies

Lymphocyte activation antigens, such as CD30, represent suitable target molecules for antibody-driven drug delivery in haemopoietic malignancies. A ribosome-inactivating protein (RIP) type 1 of potential interest for mAb targeting is gelonin, which displays a lower toxicity, as compared to other RIPs. In this study, two anti-CD30/antigelonin bispecific monoclonal antibodies (bimAbs), secreted by hybrid hybridomas, were used to deliver this RIP to CD30+ tumour cells. The two bimAbs, termed D4 and A18, were produced using the same anti-CD30 mAb and two anti-gelonin mAbs, directed to unrelated epitopes of the gelonin molecule. These bimAbs enhanced gelonin toxicity (IC50 5 x 10(-8) M, in the absence of mAbs) against the CD30+ L540 Hodgkin's lymphoma cell line in a protein synthesis inhibition assay. Thus, in the presence of 10(-9) M D4 bimAb, protein synthesis was inhibited with an IC50 of 5 x 10(-10) M as gelonin, whereas with A18 bimAb the IC50 was 8 x 10(-11) M. More interestingly, the combined use of the two bimAbs had a synergistic effect, since the IC50 of gelonin reached 6 x 10(-12) M. Among CD30 tumour cell lines, the Hodgkin's lymphoma L428 was also sensitive to gelonin delivered by bimAbs (IC50 6 x 10(-11) M), whereas the COLE Hodgkin's cell line and the T-ALL Jurkat were completely resistant to the toxic effect of gelonin and bimAbs. COLE and Jurkat cells were also resistant to a gelonin/anti-CD30 conventional immunotoxin, whereas they were sensitive to a saporin/anti-CD30 immunotoxin. This suggests that the resistance to gelonin is not related to a lack of internalization through the CD30 molecule but is associated with some property of the RIP.

Biochemical, histochemical and cell biological investigations on the actions of mistletoe lectins I, II and III with human breast cancer cell lines

Cytotoxicity of the mistletoe lectins I, II and III towards six human breast cancer cell lines was assessed using the Mossman assay. In addition, binding of the three mistletoe lectins to the separated membrane glycoproteins of these cell lines, the binding and uptake of these lectins into the cells in tissue culture and the binding of the lectins to histological preparations of these cell lines were analysed. The results indicate that there are quantitative differences concerning the toxicity of these three lectins towards the different cell lines. Furthermore, the lectin binding pattern in the cell lines differed. In Western blots, several membrane glycoproteins were labelled by the lectins. Our results indicate subtle differences between the three lectins with regard to the parameters mentioned above; however, the toxicity of all three lectins from mistletoe is so similar that they all seem suitable for the construction of immunotoxins.

Ricin A chain can be chemically cross-linked to the mammalian ribosomal proteins L9 and L10e

Indirect immunofluorescence studies revealed that when fixed, permeabilized cultured human cells were incubated with ricin A chain, the toxin molecule localized in a staining pattern indicative of binding to the endoplasmic reticulum and to nucleoli. Chemical cross-linking experiments were performed to identify the cellular components that mediated the binding of ricin A chain. Conjugates were formed between 125I-labeled ricin A chain and two proteins present in preparations of total cell membranes and in samples of purified mammalian ribosomes. Specificity of the ricin A chain-ribosome interaction was demonstrated by inhibition of formation of the complexes by excess unlabeled ricin A chain, but not by excess unlabeled gelonin, another ribosome-inactivating protein. Complexes of ricin A chain cross-linked to the ribosomal proteins were purified and subjected to proteolytic digestion with trypsin. Amino acid sequencing of internal tryptic peptides enabled identification of the ricin A chain-binding proteins as L9 and L10e of the mammalian large ribosomal subunit.

Delivery of the ribosome-inactivating protein, gelonin, to lymphoma cells via CD22 and CD38 using bispecific antibodies

It is well established that bispecific antibodies (BsAbs) can be used effectively in targeting the ribosome-inactivating protein (RIP), saporin, against neoplastic B cells. We have now extended this delivery system for use with gelonin. By measuring antigen-binding characteristics and epitope mapping a panel of anti-gelonin MAbs using the IAsys resonant mirror bisensor, we were able to rapidly select the most suitable for making BaAbs. The Fab' fragments from these MAbs were chemically conjugated with Fab' from either anti-CD22 or anti-CD38. Cytotoxicity assays showed that BsAbs were highly efficient at delivering gelonin to cultured Daudi cells and achieved levels of toxicity which correlated closely with the affinity of the BsAbs. Using pairs of anti-CD22 BsAbs we were able to generate bivalent BsAb-gelonin complexes which achieved IC50 values of 2 x 10(-11) M gelonin, a potency which is equivalent to that reached by saporin in this targeting system. However, because gelonin is 5-10 times less toxic than saporin, the therapeutic ratio for gelonin is superior, making it potentially a more useful agent for human treatment. Cytotoxicity assays and kinetic analysis showed that targeting gelonin via CD38 was 2-5 times less effective than delivery through CD22. However, with a pair of BsAbs designed to co-target gelonin via CD22 and CD38, the cytotoxicity achieved equalled that obtained with a pair of anti-CD22 BsAbs (IC50 = 1 x 10(-11) M). This important result suggests that the anti-CD38 helps bind the gelonin to the cell and is then 'dragged' or 'piggy-backed' into the cell by the anti-CD22 BsAb. The implication of these findings for cancer therapy is discussed.

The role of lectins in plant defence

Recent progress in the search for the physiological role of plant lectins supports the idea that some of these proteins are involved in the defence mechanisms of the plant. To place the evidence in favour of such a defensive role in a broad perspective, a short overview is given of the most important plant pathogens and predators. In addition, the solutions that plants have developed to resist the continuous threat of a hostile environment are briefly discussed in relation to the protective role of proteins in general. The presumed involvement of plant lectins in defence mechanisms is first inferred from an analysis of the biochemical, physiological, cellular biological and molecular biological properties of plant lectins. Subsequently, the available experimental evidence for the involvement of lectins in the plant's defence against viruses, bacteria, fungi and herbivorous invertebrates and vertebrates is discussed in some detail. Since the defensive role of plant lectins is determined largely by their ability to recognize and bind foreign glycans, a brief discussion is given of how the basically protective properties of these proteins can be exploited for histochemical applications in biological and biomedical research.

Ribosome-inactivating proteins (RNA N-glycosidases) from the seeds of Saponaria ocymoides and Vaccaria pyramidata

From the seeds of the Caryophyllaceae Saponaria ocymoides and Vaccaria pyramidata two proteins were purified which have the properties of the type-1 (single-chain) ribosome-inactivating proteins [reviewed by Barbieri, L., Battelli, M. G. & Stirpe, F. (1993) Ribosome-inactivating proteins from plants, Biochim. Biophys. Acta 1154, 237-282]. The proteins have molecular masses of 30.2 kDa (S. ocymoides) and 28.0 kDa (V. pyramidata) and pI greater than 9.5, their N-terminal amino acid sequences are similar to those of saporin-S6 and dianthin 30, ribosome-inactivating proteins from other Caryophyllaceae, and they partially cross-react with sera against these proteins. Both proteins inhibit protein synthesis by a rabbit-reticulocyte lysate with IC50 (concentrations giving 50% inhibition) below 10(-10) M, have a smaller effect on poly(U)-directed phenylalanine polymerisation by rat liver ribosomes (nanomolar IC50, approximately) and on protein synthesis by various cell lines (IC50 ranging from 4 nM to > 3000 nM) and possess rRNA N-glycosidase activity, releasing 1 mol adenine/ribosome.

Ebulitins: a new family of type 1 ribosome-inactivating proteins (rRNA N-glycosidases) from leaves of Sambucus ebulus L. that coexist with the type 2 ribosome-inactivating protein ebulin 1

A new family of single chain (type 1) ribosome-inactivating proteins (RIPs), that we have named ebulitins, have been found in mature leaves of Sambucus ebulus L., a caprifoliaceae plant also known to contain a non-toxic two chain (type 2) RIP named ebulin I in its leaves. Ebulitins are basic proteins of M(r) 32,000, 29,000 and 29,000 for ebulitins alpha, beta and gamma, respectively. The simultaneous presence of different basic type 1 and acidic type 2 RIPs in the same plant and in the same tissue is described here for the first time and opens a new door in research into RIPs.

Anti-CD30 immunotoxins with native and recombinant dianthin 30

Immunotoxins were prepared with a Ber-H2 (anti-CD30) monoclonal antibody and native or recombinant dianthin 30, a ribosome-inactivating protein from Dianthus caryophyllus (carnation). Both immunotoxins selectively inhibited protein synthesis by CD30+ cell lines D430B (lymphoblastoid, infected with Epstein-Barr virus), L428 and L540 (both from Hodgkin's lymphoma). IC50 values (concentrations, as dianthin, causing 50% inhibition) ranged from 324 pM to 479 pM (immunotoxin with native dianthin 30) or from 45 pM to 182 pM (immunotoxin with recombinant dianthin 30). The effect of either immunotoxin on protein synthesis by the CD30+ cell line K562 (from a chronic myeloid leukaemia) was not different from that of free dianthin (IC50 higher than nM).