The Biological Action and Structural Characterization of Eryngitin 3 and 4, Ribotoxin-like Proteins from Pleurotus eryngii Fruiting Bodies

Ribotoxin-like proteins (RL-Ps) are specific ribonucleases found in mushrooms that are able to cleave a single phosphodiester bond located in the sarcin-ricin loop (SRL) of the large rRNA. The cleaved SRL interacts differently with some ribosomal proteins (P-stalk). This action blocks protein synthesis because the damaged ribosomes are unable to interact with elongation factors. Here, the amino acid sequences of eryngitin 3 and 4, RL-Ps isolated from Pleurotus eryngii fruiting bodies, were determined to (i) obtain structural information on this specific ribonuclease family from edible mushrooms and (ii) explore the structural determinants which justify their different biological and antipathogenic activities. Indeed, eryngitin 3 exhibited higher toxicity with respect to eryngitin 4 against tumoral cell lines and model fungi. Structurally, eryngitin 3 and 4 consist of 132 amino acids, most of them identical and exhibiting a single free cysteinyl residue. The amino acidic differences between the two toxins are (i) an additional phenylalanyl residue at the N-terminus of eryngitin 3, not retrieved in eryngitin 4, and (ii) an additional arginyl residue at the C-terminus of eryngitin 4, not retrieved in eryngitin 3. The 3D models of eryngitins show slight differences at the N- and C-terminal regions. In particular, the positive electrostatic surface at the C-terminal of eryngitin 4 is due to the additional arginyl residue not retrieved in eryngitin 3. This additional positive charge could interfere with the binding to the SRL (substrate) or with some ribosomal proteins (P-stalk structure) during substrate recognition.

Deciphering Molecular Determinants Underlying Penicillium digitatum's Response to Biological and Chemical Antifungal Agents by Tandem Mass Tag (TMT)-Based High-Resolution LC-MS/MS

Penicillium digitatum is a widespread pathogen responsible for the postharvest decay of citrus, one of the most economically important crops worldwide. Currently, chemical fungicides are still the main strategy to control the green mould disease caused by the fungus. However, the increasing selection and proliferation of fungicide-resistant strains require more efforts to explore new alternatives acting via new or unexplored mechanisms for postharvest disease management. To date, several non-chemical compounds have been investigated for the control of fungal pathogens. In this scenario, understanding the molecular determinants underlying P. digitatum's response to biological and chemical antifungals may help in the development of safer and more effective non-chemical control methods. In this work, a proteomic approach based on isobaric labelling and a nanoLC tandem mass spectrometry approach was used to investigate molecular changes associated with P. digitatum's response to treatments with α-sarcin and beetin 27 (BE27), two proteins endowed with antifungal activity. The outcomes of treatments with these biological agents were then compared with those triggered by the commonly used chemical fungicide thiabendazole (TBZ). Our results showed that differentially expressed proteins mainly include cell wall-degrading enzymes, proteins involved in stress response, antioxidant and detoxification mechanisms and metabolic processes such as thiamine biosynthesis. Interestingly, specific modulations in response to protein toxins treatments were observed for a subset of proteins. Deciphering the inhibitory mechanisms of biofungicides and chemical compounds, together with understanding their effects on the fungal physiology, will provide a new direction for improving the efficacy of novel antifungal formulations and developing new control strategies.

Antifungal activity of the ribosome-inactivating protein BE27 from sugar beet (Beta vulgaris L.) against the green mould Penicillium digitatum

The ribosome-inactivating protein BE27 from sugar beet (Beta vulgaris L.) leaves is an apoplastic protein induced by signalling compounds, such as hydrogen peroxide and salicylic acid, which has been reported to be involved in defence against viruses. Here, we report that, at a concentration much lower than that present in the apoplast, BE27 displays antifungal activity against the green mould Penicillium digitatum, a necrotrophic fungus that colonizes wounds and grows in the inter- and intracellular spaces of the tissues of several edible plants. BE27 is able to enter into the cytosol and kill fungal cells, thus arresting the growth of the fungus. The mechanism of action seems to involve ribosomal RNA (rRNA) N-glycosylase activity on the sarcin-ricin loop of the major rRNA which inactivates irreversibly the fungal ribosomes, thus inhibiting protein synthesis. We compared the C-terminus of the BE27 structure with antifungal plant defensins and hypothesize that a structural motif composed of an α-helix and a β-hairpin, similar to the γ-core motif of defensins, might contribute to the specific interaction with the fungal plasma membranes, allowing the protein to enter into the cell.

Sequence comparison and phylogenetic analysis by the Maximum Likelihood method of ribosome-inactivating proteins from angiosperms

Ribosome-inactivating proteins (RIPs) from angiosperms are rRNA N-glycosidases that have been proposed as defence proteins against virus and fungi. They have been classified as type 1 RIPs, consisting of single-chain proteins, and type 2 RIPs, consisting of an A chain with RIP properties covalently linked to a B chain with lectin properties. In this work we have carried out a broad search of RIP sequence data banks from angiosperms in order to study their main structural characteristics and phylogenetic evolution. The comparison of the sequences revealed the presence, outside of the active site, of a novel structure that might be involved in the internal protein dynamics linked to enzyme catalysis. Also the B-chains presented another conserved structure that might function either supporting the beta-trefoil structure or in the communication between both sugar-binding sites. A systematic phylogenetic analysis of RIP sequences revealed that the most primitive type 1 RIPs were similar to that of the actual monocots (Poaceae and Asparagaceae). The primitive RIPs evolved to the dicot type 1 related RIPs (like those from Caryophyllales, Lamiales and Euphorbiales). The gene of a type 1 RIP related with the actual Euphorbiaceae type 1 RIPs fused with a double beta trefoil lectin gene similar to the actual Cucurbitaceae lectins to generate the type 2 RIPs and finally this gene underwent deletions rendering either type 1 RIPs (like those from Cucurbitaceae, Rosaceae and Iridaceae) or lectins without A chain (like those from Adoxaceae).

Transient injury-dependent up-regulation of CD105 and its specific targeting with an anti-vascular anti-mouse endoglin-nigrin b immunotoxin

Endoglin (CD105), a cell-surface co-receptor for transforming growth factor-beta (TGF-β) superfamily members, is over-expressed in tumor neovasculature and can be targeted with anti-endoglin antibodies, thus becoming an important tool for anti-tumoral therapy. Injury of the mouse tail induced the transient expression of endoglin, this peaking at three days after injury and disappearing six days later. An immunotoxin containing the anti-mouse endoglin rat monoclonal antibody MJ7/18 and the non-toxic ribosome-inactivating protein nigrin b (Ngb) was found to be very active in targeting mouse endoglin in the L929 fibroblast cell line (IC(50) of 4 x 10(-11) M). At that concentration, the immunotoxin lacked unspecific activity. Upon induction of endoglin after injury, the MJ7-Ngb immunotoxin strongly attacked and deranged the injured tail, inducing tissue damage. Such effects were dependent on the age of the animals and were evident in six-week-old mice, but not in eight-month-old mice. Our results indicate that endoglin is up-regulated in newly formed vessels upon injury and can be targeted by the MJ7-Ngb immunotoxin; thus, it could be a useful tool for tumor ablation research.

In vitro and in vivo effects of an anti-mouse endoglin (CD105)-immunotoxin on the early stages of mouse B16MEL4A5 melanoma tumours

TGF-beta superfamily co-receptors are emerging as targets for cancer therapy, acting both directly on cells and indirectly on the tumour neovasculature. Endoglin (CD105), an accessory component of the TGF-beta receptor complex, is expressed in certain melanoma cell lines and the endothelial cells of tumour neovessels. Targeting endoglin with immunotoxins is an attractive approach for actively suppressing the blood supply to tumours. Here, we report evidence indicating that endoglin is expressed in mouse melanoma B16MEL4A5 and mouse fibroblast L929 cell lines. We prepared an immunotoxin to target endoglin by coupling the rat anti-mouse MJ7/18 (IgG2a) monoclonal antibody (mAb) to the non-toxic type 2 ribosome-inactivating protein nigrin b (Ngb) with N-succinimidyl 3-(2-pyridyldithio)-propionate (SPDP) as a linker with a molar nigrin b at a MJ7/18 stoichiometry of 2:1. The MJ7-Ngb immunotoxin generated killed both cell lines, with IC50 values of 4.2 × 10(-9) M for B16MEL4A5 and 7.7 × 10(-11) M for L929 cells. For in vivo assays of the immunotoxin, B16MEL4A5 cells were injected subcutaneously into the right flanks of 6-week-old C57BL/6 J mice. When the animals developed palpable solid tumours, they were subjected to treatment with the immunotoxin. While treatment with either MJ7/18 mAb or Ngb did not affect tumour development, treatment with the immunotoxin completely and steadily blocked tumour growth up to 7 days, after which some tumours re-grew. Thus, vascular-targeting therapy with this anti-vascular immunotoxin could promote the destruction of newly created tumour vessels at early stages of B16MEL4A5 tumour development and readily accessible CD105+ B16MEL4A5 melanoma cells.

Effect of continued exposition to ethanol on activity of the ammonium and fructose transport systems in Saccharomyces cerevisiae var. ellipsoideus

Ethanol and cycloheximide inhibited the function of the ammonium transport system in growing cultures of Saccharomyces cerevisiae var. ellipsoideus measured as methylamine uptake. The effect was reversible with ethanol and irreversible with the antibiotic. The kinetic data are consistent with a reduction of the number of active carrier molecules located in the plasma membrane. In contrast, neither ethanol nor cycloheximide affected the specific rate of fructose uptake.

Specific dose-dependent damage of Lieberkühn crypts promoted by large doses of type 2 ribosome-inactivating protein nigrin b intravenous injection to mice

Nigrin b is a non-toxic type 2 ribosome-inactivating protein as active as ricin at ribosomal level but 10(5) and 5 x 10(3) times less toxic for animal cell cultures and mice, respectively, than ricin. The purpose of the present study was to analyze the effects of intravenous injection of large amounts of nigrin b to the mouse. Injection through the tail vein of 16 mg/kg body weight killed all mice studied before 2 days. Analysis of several major tissues by light microscopy did not reveal gross nigrin b-promoted changes, except in the intestines which appeared highly damaged. As a consequence of the injury, the villi and crypt structures of the small intestine disappeared, leading to profuse bleeding and death. In contrast, intravenous injection of 5 mg/kg body weight was not lethal to mice but did trigger reversible toxic effects. In both cases, lethal and sub-lethal doses, the target of nigrin b appeared to be the highly proliferating stem cells of the intestinal crypts, which had undergone apoptotic changes. In contrast to nigrin b, the injection of 3 mug/kg of ricin kills all mice in 5 days but does not trigger apoptosis in the crypts. Therefore, the effect seen with sub-lethal nigrin b concentrations seems to be specific. Nigrin b killed COLO 320 human colon adenocarcinoma cells with an IC(50) of 3.1 x 10(-8) M and the effect was parallel to the extent of DNA fragmentation of these cells. Accordingly, despite the low general toxicity exerted by nigrin b as compared with ricin, intravenous injection of large amounts of nigrin b is able to kill mouse intestinal stem cells without threatening the lives of the animals, thereby opening a door for its use for the targeting of intestinal stem cells.

Description, Distribution, Activity and Phylogenetic Relationship of Ribosome-Inactivating Proteins in Plants, Fungi and Bacteria

Ribosome-Inactivating Proteins (RIPs) are enzymes that trigger the catalytic inactivation of ribosomes and other substrates. They are present in a large number of plants and have been found also in fungi, algae and bacteria. RIPs are currently classified as type 1, those formed by a single polypeptide chain with the enzymatic activity, and type 2, those formed by 2 types of chains, i.e. A chains equivalent to a type 1 RIPs and B chains with lectin activity. Type 2 RIPs usually contain the formulae A-B, (A-B)2 and less frequent (A-B)4 and polymeric forms of type 2 RIPs lectins. RIPs are broadly distributed in plants, and are present also in fungi, bacteria, at least in one alga; recently RIP-type activity has been described in mammalian tissues. The highest number of RIPs has been found in Caryophyllaceae, Sambucaceae, Cucurbitaceae, Euphorbiaceae, Phytolaccaceae and Poaceae. However there are no systematic screening studies to allow generalisations about occurrence. The most known activity of RIPs is the translational inhibitory activity, which seems a consequence of a N-glycosidase on the 28 S rRNA of the eukaryotic ribosome that triggers the split of the A(4324) (or an equivalent base in other ribosomes), which is key for translation. This activity seems to be part of a general adenine polynucleotide glycosylase able to act on several substrates other than ribosomes, such as tRNA, mRNA, viral RNA and DNA. Other enzymatic activities found in RIPs are lipase, chitinase and superoxide dismutase. RIPs are phylogenetically related. In general RIPs from close families share good amino acid homologies. Type 1 RIPs and the A chains of type 2 RIPs from Magnoliopsida (dicotyledons) are closely related. RIPs from Liliopsida (monocotyledons) are at the same time closely related and distant from Magnoliopsida. Concerning the biological roles played by RIPs there are several hypotheses, but the current belief is that they could play significant roles in the antipathogenic (viruses and fungi), stress and senescence responses. In addition, roles as antifeedant and storage proteins have been also proposed. Future research will approach the potential biological roles played by RIPs and their use as toxic effectors in the construction of immunotoxins and conjugates for target therapy.

Non-toxic type 2 ribosome-inactivating proteins (RIPs) from Sambucus: occurrence, cellular and molecular activities and potential uses

Ribosome-inactivating proteins (RIPs) are a family of enzymes that trigger the catalytic inactivation of ribosomes. The most known member of the family is the highly poisonous two-chain ricin isolated from Ricinus communis L. Sambucus species contain a number of two-chain RIPs structurally and enzymatically related to ricin which have the noteworthy feature that, having an enzymatic activity on ribosomes, leading to the inhibition of protein synthesis, higher than ricin, they are lacking of the tremendous unspecific toxicity of ricin. Therefore, they have been called non-toxic type 2 RIPs. The most representative and studied members are nigrin b present in the bark of the common (black) elder Sambucus nigra L. and ebulin 1 present in the leaves of the dwarf elder Sambucus ebulus L. The molecular basis for the low unspecific activities of nigrin b and ebulin 1 as compared with ricin seems to be related with single changes of amino acids in the high affinity sugar binding sites of the B chains. These changes determine the intracellular traffic of these proteins and thus the cellular toxicity. Conjugation ofnigrin b or ebulin 1 to either transferrin or monoclonal antibodies provided highly active conjugates targeting cancer. Thus these non-toxic type 2 RIPs are promising tools for cancer therapy.

Evidence for distinct cellular internalization pathways of ricin and nigrin b

Nigrin b and ricin are type 2 (two chain) ribosome-inactivating proteins that exhibited nearly the same strong inhibitory activity on cell-free protein synthesis. Incubation of HeLa cells for 6 hr with ricin at 37 degrees C promoted protein synthesis inhibition with an IC50 of 0.2 ng/ml. Incubation of the cells for 6 hr at 18 degrees C abolished completely the inhibition. Incubation of HeLa cells with nigrin b for 6 hr at 37 degrees C was nearly 10(5) times less inhibitory than ricin. In contrast to the effects observed with ricin, incubation of HeLa cells with nigrin b at 18 degrees C slightly increased the inhibitory action on protein synthesis as compared with incubation at 37 degrees C. These results strongly support the hypothesis that the internalization of ricin and nigrin b could involve different receptors and therefore they could follow different intracellular pathways.

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.

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.

Characterization of a new non-toxic two-chain ribosome-inactivating protein and a structurally-related lectin from rhizomes of dwarf elder (Sambucus ebulus L.)

A new N-glycosidase ribosome-inactivating protein (RIP) belonging to the novel family of the nontoxic type 2 RIPs from Sambucaceae has been isolated from rhizomes of dwarf elder (Sambucus ebulus L.) and named ebulin r. Dwarf elder rhizomes also contain a novel monomeric N-Ac-galactosamine-binding lectin that we named SEAII. Ebulin r and SEAII have two isoforms each one, which were readily resolved by ion exchange. Both isoforms of ebulin (ebulins r1 and r2) strongly inhibited protein synthesis in mammalian but not in plant ribosomes by promoting depurination of sensitive ribosomes. Ebulin r and SEAII have apparent molecular masses of 56 and 33.5 kDa, respectively. Ebulins r1 and r2 are composed of two dissimilar subunits (types A-B) of apparent molecular masses of 26 and 30 kDa by disulphide bridges. The rhizome SEAII and the lectins SNA II and SNA III from elder (Sambucus nigra L.) share good amino acid sequence homology. This rhizome ebulin-A chain is more sequence-related to RIP members of cucurbitaceae than to any other plant family. The rhizome ebulin B chain shares a large homology in amino acid sequence with ebulin 1-B chain and SEAII. Anti-ebulin 1 polyclonal antibodies raised in rabbits reacted better with ebulin r1 than with ebulin r2, thus suggesting that both RIP isoforms could have some differences.

Toxicity and cytotoxicity of nigrin b, a two-chain ribosome-inactivating protein from Sambucus nigra: comparison with ricin

Nigrin b, a lectin isolated from the bark of elderberry (Sambucus nigra L.), has structure and enzymatic activity similar to that of ricin and other type 2 ribosome inactivating proteins (RIPs), and yet is much less toxic to cells and animals. In an attempt to explain this difference, we studied (1) the cytotoxicity of both lectins at 18 and 37 degrees C, and in the presence of substances interfering with intracellular routing, and (2) the binding of nigrin b to, and its uptake and degradation by HeLa cells, in parallel with ricin. As compared with the latter, (1) less nigrin b was bound and more was degraded by cells, with a resulting lower concentration remaining inside the cells, and (2) there is evidence for a different intracellular routing followed by the two lectins. These results may explain at least partly the different cytotoxicity and consequently the lower toxicity to mice of nigrin b compared with ricin.

Differential sensitivity of HELA cells to the type 2 ribosome-inactivating proteins ebulin l, nigrin b and nigrin f as compared with ricin

The new type 2 RIPs ebulin l, nigrin b and nigrin f present in Sambucus display toxicity to HELA cells several orders of magnitude lower than that displayed by ricin. Despite N-terminal amino acid homology between the three RIPs in both the A and the B chains, these compounds display very different degrees of toxicity to HELA cells that does not seem to be paralleled by immunologic correlations. It is suggested that small changes in the protein structure are most probably responsible for the different degrees of toxicity.

Recent advances in the uses and applications of ribosome-inactivating proteins from plants

Plant ribosome-inactivating proteins (RIPs) are inhibitors present in all parts of plants that irreversibly inactivate eukaryotic ribosomes, thus impairing protein synthesis. RIPs are enzymes with N-glycosidase activity on the large rRNA. Their powerful inhibitory activity has been made use of advantageously to construct conjugates with suitable carriers targeted to altered specific cells. RIPs may be used to inhibit replication of both animal and plant viruses. The introduction of genes coding for RIPs into the genome of plants leads to an increase in resistance towards fungal pathogens and viruses. RIPs are important tools for the treatment of cancer and AIDS and for the protection of crop production.

Sensitivity of ribosomes from Agrobacterium tumefaciens to the ribosome-inactivating protein crotin 2 depending on the translocational state

The GTP analog guanylylmethylene diphosphonate (GppCH2p) strongly inhibited polyuridylic acid-directed polypeptide synthesis in a cell-free translation system prepared from Agrobacterium tumefaciens. Fusidic acid increased even further the inhibitory action. The pre-translocational ribosomal complexes formed with the GppCH2p and the elongation factor G protected the ribosome against the depurinating action of crotin 2 assayed as the acid-dependent release of the RNA fragment whose terminal sequence is 5'-GAGGACCGGGAUGGAC-3'. The results allowed to conclude that the interaction of both crotin 2 and the elongation factor G with the A. tumefaciens ribosomes in the pre-translocational state must take place at overlapping, either sterically or allosterically, ribosomal sites which are equally accessible to the RIP.

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.

A new procedure for the preparation of highly active melonin from the dry seeds of Cucumis melo L

Melon (Cucumis melo L.) dry seeds contain melonin, a protein that strongly inhibits ribosomes from different prokaryotic and eukaryotic sources including those from melon. The protein was purified by a new method to yield highly active and stable protein preparations that involves chromatography through S-Sepharose Fast Flow, CM-Sepharose, Superdex 75 and Mono-S. Melonin shows important functional properties: 1) its inhibitory effects on translation were irreversible; 2) it is a single unglycosylated polypeptide chain with an apparent M(r) of 22000; 3) it degrades RNA in a dose-dependent way without affecting DNA. In the light of present results melonin can be considered as a new plant RNase of unusual properties.

Effects and molecular action of ribosome-inactivating proteins on ribosomes from Streptomyces lividans

The effects of 29 type 1 and 2 type 2 ribosome-inactivating proteins (RIPs) from plants on polyuridylic acid-directed polyphenylalanine synthesis carried out by purified ribosomes from Streptomyces lividans were studied. Only dianthin 32, saporins R1 and R3, momordin I, trichokirin, Hura crepitans RIP 5 from latex, crotins 2 and 3, and PAPs C, R, and S, inhibited polyphenylalanine synthesis. Both the type 2 RIPs ricin and volkensin were ineffective on translation. The magnesium concentration affected the inhibition of translation to a considerable extent. Upon treatment with inhibitory RIPs, extraction of rRNA and further treatment with acid aniline, S. lividans ribosomes released an RNA fragment of about 130 nucleotides. The 5' terminal sequence of this rRNA fragment was 5'-GAGGACCGGGACGGACGAACCUCUGGUGUGCCAGUUGU-3', similar to the sequence obtained in Escherichia coli. This indicates that the most probable molecular action of these RIPs on S. lividans and E. coli ribosomes is the same: depurination of the rRNA at a site relevant to the translation mechanism and that has been highly conserved throughout evolution.

Sensitivity of translation by Brevibacterium lactofermentum ribosomes to type 1 and type 2 ribosome-inactivating proteins

An active cell-free translation system was prepared from Brevibacterium lactofermentum, a Gram-positive bacteria used in molecular cloning and protein expression. The system contained high speed postribosomal supernatant (S 370), purified ribosomes and a tRNA mixture from Escherichia coli, and synthesized polyuridylic acid-directed polyphenylalanine once optimized for mono and divalent ions, time, and temperature. The translation system was evaluated for sensitivity to several translational inhibitors including several N-glycosidase ribosome-inactivating proteins (RIPs) isolated from plants. The pattern of inhibition by RIPs resembled that observed recently for Gram-negative bacteria such as Escherichia coli and Agrobacterium tumefaciens [Girbés et al., J. Bacteriol., 175, 6721-6724 (1993)]. A typical inhibitory type 1 RIP such as crotin 2 promoted depurination of the rRNA, which upon treatment with acid aniline released a fragment of approximately 230 nucleotides. On these grounds, we propose that bacterial ribosome sensitivity to plant RIPs depends on the bacterial ribosome-specific presence of protein recognition domains in the RIP present only in some RIP but not in others.

Cusativin, a new cytidine-specific ribonuclease accumulated in seeds of Cucumis sativus L

Dry seeds of Cucumis sativus L. were found to contain a heat-sensitive endoribonuclease of a novel type which we have named cusativin. It was purified to apparent electrophoretic homogeneity by chromatography through S-Sepharose Fast Flow, Sephadex G-75, CM-Sepharose, Superdex 75-FPLC (fast protein liquid chromatography) and Mono S-FPLC. It is a single unglycosylated polypeptide chain with an apparent molecular mass (M(r)) of 22900. Polyclonal anti-cusativin antibodies raised in rabbits only reacted with melonin, the translation inhibitor from Cucumis melo L. Functional, Western blot and enzyme-linked immunosorbent assay (ELISA) analyses indicated that cusativin is present in the coat and cotyledons of dry seeds, but not in embryonic axes. Cusativin is accumulated in maturing seeds. By contrast, after seed germination there is degradation of the cusativin present in cotyledons but not that present in the seed coat. The preference of cusativin for polynucleotide cleavage was poly(C) >> poly(A) acids, poly(U) and poly(G) being unaffected by cusativin. Under the denaturing conditions used for RNA sequencing, cusativin acted only on poly(C). Cusativin proved to be useful for RNA sequencing, in particular, complementing the data obtained with RNase CL3. Cusativin represents a new class of plant RNase and, as far as we are aware, is the first plant enzyme that shows cleavage specificity for cytidine under the denaturing conditions of RNA sequencing.

Isolation and characterization of two new N-glycosidase type-1 ribosome-inactivating proteins, unrelated in amino-acid sequence, from Petrocoptis species

Two new N-glycosidase type-1 ribosome-inactivating proteins (RIPs), denoted petroglaucin 1 and petrograndin, respectively, were isolated from the plants Petrocoptis glaucifolia (Lag.) Boiss sp. viscosa (Rothm.) Lainz and Petrocoptis grandiflora Rothm. These new RIPs do not share H2N-terminal amino-acid sequence homology with petroglaucin (now denoted as petroglaucin 2), the only other type-1 RIP to be isolated from P. glaucifolia (Arias et al. (1992) Planta 186, 532-540). Petroglaucin 1 shares amino-acid sequence homology with RIPs from Cucurbitaceae while petroglaucin 2 and petrograndin do so with saporins and dianthin 30 (Caryophyllaceae). The new RIPs strongly inhibited protein synthesis at subnanomolar concentrations in rabbit reticulocyte lysates and other eukaryotic cell-free systems, but they were inactive on bacterial ribosomes.

Screening survey of several plant species in search for ribosome-inactivating protein-like activities

A screening survey of 41 plants revealed potent translational inhibitory activities in 19 plant species belonging to several different families, some of them hitherto unreported. Our results indicate that the active principles were ribosome-inactivating proteins. Additionally, human erythrocyte-agglutinating activity was found in 20 plants.

Distribution and properties of major ribosome-inactivating proteins (28 S rRNA N-glycosidases) of the plant Saponaria officinalis L. (Caryophyllaceae)

We have studied the distribution of the protein synthesis inhibitory activity in the tissues of Saponaria officinalis L. (Caryophyllaceae). Seven major saporins, ribosome-inactivating proteins, were purified to apparent homogeneity from leaves, roots and seeds using a new procedure of RIPs isolation including ion-exchange and hydrophobic chromatography. They all catalysed the depurination of rat liver ribosomes, which generate the Endo's diagnostic rRNA fragment upon treatment with acid aniline, thus indicating that A4324 from the 28S rRNA has been released (Endo et al. (1987) J. Biol. Chem. 262, 5908-5912). The molecular mass of saporins by SDS-PAGE ranged between 30.2 and 31.6 kDa and by gel-filtration between 27.5 and 30.1 kDa. Amino acid composition and amino-terminal amino acid sequence indicate that all saporins may be considered isoforms. Only two saporins present in roots were glycosylated (SO-R1 and SO-R3). All saporins are very active on cell-free translation systems derived from rabbit reticulocyte lysates, rat liver, Triticum aestivum L., Cucumis sativus L. and Vicia sativa L. However, they are poor inhibitors of an Escherichia coli translation system. They inhibit protein synthesis in HeLa, BeWo and NB 100 cells, HeLa cells being the most resistant. The enzymatic activity of at least one saporin isoform was dependent on magnesium concentration in the standard rat liver cell-free system.

Preparation, optimization and characterization of a polyphenylalanine synthetizing system from Agrobacterium tumefaciens

A very active cell-free translation system was prepared from Agrobacterium tumefaciens, a bacterium broadly used to transfect plant cells to introduce foreign genes and one that produces tumours in plants. Once optimized for Mg2+, NH4+, high speed supernatant S 370, purified ribosomes and time, the system translates polyuridylic acid very efficiently. A. tumefaciens purified ribosomes were inhibited in vitro by several well-known translational inhibitors including some ribosome-inactivating proteins. Treatment of A. tumefaciens purified ribosomes with type 1-RIP crotin 2 lead to the depurination of the 23S rRNA which, upon treatment with acid aniline, released a diagnostic RNA fragment of about 235 nucleotides.

Isolation and partial characterization of nigrin b, a non-toxic novel type 2 ribosome-inactivating protein from the bark of Sambucus nigra L

The bark of Sambucus nigra L. contains a non-toxic novel type 2 ribosome-inactivating protein that we named nigrin b. In vitro, nigrin b strongly inhibited mammalian protein synthesis but did not affect plant nor bacterial protein synthesis. The protein (M(r) 58,000) contains two subunits, A (M(r) 26,000) and B (M(r) 32,000); linked by disulphide bridge(s). Nigrin b was found to be an rRNA N-glycosidase of the rRNA of intact mammalian ribosomes and shares a very good N-terminal amino-acid sequence homology with the anti-HIV-1 proteins TAP 29 and trichosanthin.

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

A novel type 2 ribosome-inactivating protein (RIP) that we named ebulin 1 has been isolated from leaves of Sambucus ebulus L. (Caprifoliaceae). In vitro ebulin 1 strongly inhibited protein synthesis by rabbit reticulocyte lysates, rat brain, and rat liver cell-free systems but did not affect in vitro plant nor bacterial protein synthesis. Ebulin 1 is composed of two subunits, a catalytic A subunit (M(r) 26,000) and a D-galactose-binding lectin B subunit (M(r) 30,000). Amino-terminal amino acid sequence homology revealed the novelty that the ebulin 1 A-chain shares a high degree of homology not with the A-chain of other type 2 RIPs but rather with the Cucurbitaceae type 1 RIP briodin S and the anti-human immunodeficiency virus type I proteins trichosanthin and TAP 29. Upon treatment with acid aniline the rRNA from ebulin 1-treated rabbit reticulocyte ribosomes released the RNA fragment which is diagnostic of RIP catalytic action. Ebulin 1 was nontoxic to mice up to 2 mg/kg of body weight and did not inhibit protein synthesis in cultured NHC human epithelial cells which are highly sensitive to ricin.

Molecular mechanism of inhibition of mammalian protein synthesis by some four-chain agglutinins. Proposal of an extended classification of plant ribosome-inactivating proteins (rRNA N-glycosidases)

The four chain agglutinins from Abrus precatorius, Viscum album and Ricinus communis promote depurination of the 28 S rRNA from rabbit reticulocyte ribosomes characteristic of the common ribosome-inactivating proteins (RIPs). These agglutinins inhibited mammalian protein synthesis at nanomolar concentrations but they do not affect plant protein synthesis under the same conditions. Therefore, they should also be considered as true RIPs but of a new class, the four-chain RIPs. An extended classification of RIPs is presented based on the former one from Stirpe et al. [Bio/technology 10 (1992) 405-412].

A Cucumis sativus cell-free translation system: preparation, optimization and sensitivity to some antibiotics and ribosome inactivating proteins

A cell-free translation system was prepared from 3- to 5-day-old embryonic axes of gherkin (Cucumis sativus L.). The system was optimized for Mg²⁺ , K⁺ , NH⁺₄ , high speed supernatants, tRNA mixture from wheat germ, time and temperature. The system translates efficiently both endogenous mRNA (using a 30000 g supernatant) and polyuridylic acid (using either a 30000 g supernatant or a 100000 g supernatant supplemented with purified ribosomes). Translation by gherkin ribosomes was inhibited by several well-known eukaryotic inhibitors, antibiotics and ribosome-inactivating proteins. A translational inhibitory activity found in Cucumis sativus L. dry seeds acted on polypeptide synthesis carried out by cell-free systems from several mammals and plants, including gherkin embryonic axes. Our results indicate that the inhibitor is located in the seed bark and cotyledons, and is either blocked or absent in the embryonic axes, thus allowing the isolation of active gherkin ribosomes. The presence of the putative inhibitor appeared to be unevenly distributed in developing plants.

Molecular action of the type 1 ribosome-inactivating protein saporin 5 on Vicia sativa ribosomes

The type 1 ribosome-inactivating protein (RIP) saporin 5 isolated from seeds of Saponaria officinalis L. strongly inhibited translation carried out by Vicia sativa L. purified ribosomes. The toxin multidepurinated V. sativa rRNA, which upon treatment with acid aniline releases several RNA fragments including an RNA fragment of approximately 370 nucleotides the 5'-end sequence of which was 5'-GAGGAACG-3'.

Messenger-dependent action of the pokeweed antiviral protein and fusidic acid on in vitro Vicia sativa L. translation

Plant ribosome sensitivity to either pokeweed antiviral protein (PAP) or fusidic acid has been investigated using a highly coupled translation system obtained from Vicia sativa L., a plant that lacks ribosome-inactivating protein activity. Protein synthesis directed by endogenous messengers was more sensitive to PAP than synthesis of polyphenylalanine directed by polyuridylic acid. Conversely, the effects of fusidic acid were the highest in polyphenylalanine synthesis. Additionally, the concentration of magnesium ions during translation also played a determinant role in the extent of inhibition by both PAP and fusidic acid. The effects seem to be exerted on the interaction of the translational elongation factor 2 with the ribosome.

Fusidic acid-dependent ribosomal complexes protect Escherichia coli ribosomes from the action of the type 1 ribosome-inactivating protein crotin 2

The type 1 ribosome-inactivating protein crotin 2 depurinated Escherichia coli ribosomes which, upon treatment of the isolated rRNA with acid aniline, released a fragment of around 240 nucleotides whose 5'-end sequence was 5'-GAGGACCGGAGUGGAC-3'. The formation of fusidic acid-dependent ribosomal complexes completely prevented release of the fragment. Ribosomes from crotin 2-pretreated fusidic acid complexes were insensitive to acid aniline. They released the RNA fragment only after a second treatment with crotin 2 and acid aniline whereas unprotected ribosomes released the fragment directly after acid aniline.

Presence of single-chain ribosome-inactivating protein-like activities in several plant species

The present screening work was devoted to the search for new ribosome-inactivating proteins (RIPs) in 21 plant species. Eight plants proved to be very active, inhibiting protein synthesis in eukaryotic in vitro systems (rat liver, Vicia sativa and wheat germ). They fulfil the major requirements for consideration as type 1 RIPs. Also, eight plants were found to contain haemagglutinating activity of human red cells but this was not related to the simultaneous presence of RIPs.

Some ribosome-inactivating proteins depurinate ribosomal RNA at multiple sites

Saporin-S6, a ribosome-inactivating protein (RIP) from Saponaria officinalis released more than 1 mol of adenine/mol of ribosomes from house fly (Musca domestica) larvae and from rat liver. The release of adenine from rat liver ribosomes by several RIPs (plant enzymes with RNA N-glycosidase activity) was examined. Saporins, pokeweed antiviral protein from roots of Phytolacca americana (PAP-R), and trichokirin from Trichosanthes kirilowii seeds depurinated rat liver ribosomes at more than one site. Up to 33 mol of adenine were released from 1 mol of ribosomes. This property is not common to all RIPS.

Isolation and partial characterization of a new ribosome-inactivating protein from Petrocoptis glaucifolia (Lag.) Boiss

Petrocoptis glaucifolia, a paleoendemic member of the Caryophyllaceae from the North of Spain, was found to contain at least five proteins that inhibit protein synthesis in a rabbit reticulocyte lysate. One of them, for which the name petroglaucin is proposed, was purified to apparent electrophoretic homogeneity by chromatography through S-Sepharose Fast Flow, Sephadex G-75 and CM-Sepharose Fast Flow. The apparent Mr of the preparation was 27500. This protein does not contain appreciable glycan chains and displays 45.8% of NH2-terminal amino-acid sequence homology with some ribosome-inactivating proteins from Saponaria officinalis, another member of the Caryophyllaceae. Petroglaucin shows the following functional properties: (i) it strongly inhibits the rabbit-reticulocyte-lysate system and Vicia sativa cell-free extracts, both coded by endogenous messengers, and also inhibits poly(U)-directed polyphenylalanine synthesis by Vicia sativa cell-free extracts and purified rat-liver ribosomes; (ii) it shows much less inhibitory capacity in wheat-germ, Cucumis sativus and rat-liver cell-free systems coded by endogenous messengers; (iii) the inhibitory effects on purified rat-liver ribosomes were irreversible; (vi) it promotes the release of adenine from purified rat-liver ribosomes. The total activity of this translational inhibitor has been found to increase up to 11-fold during its purification, indicating that some regulatory factor that normally blocks the translational inhibitory activity of the ribosome-inactivating protein in crude extracts of the plant is removed during purification.

Changes in sensitivity of in vitro rat brain protein synthesis to the acute action of ethanol and isopropanol as a consequence of the long-term ingestion of isopropanol

Long-term treatment of rats with isopropanol in the drinking water results in a change or process of adaptation occurring in in vitro brain protein synthesis which increases the resistance of the ribosomal machinery to the acute effect of either ethanol or isopropanol. Such an increase was observed both in the system coded by endogenous messenger and in the system coded by polyuridylic acid. In both translation systems, the adaptation seems to affect the ribosomal step of polypeptide chain elongation. The increase in resistance to the alkanols apparently did not affect the inhibitory action of puromycin, fusidic acid and cycloheximide on the ribosome.

Effect of L-azetidine 2-carboxilic acid on the activity of the general amino-acid permease from Saccharomyces cerevisiae var. ellipsoideus

Addition of the L-proline analogue L-azetidine 2-carboxylic acid to growing cultures of Saccharomyces cerevisiae var. ellipsoideus promoted fast deactivation of the general aminoacid permease, measured as L-valine uptake, without an immediate decrease in the growth rate. Cells preincubated with the analogue for 3 h were unable to restore either growth ability or general aminoacid permease activity in analogue-free medium. Eadie-Hofstee plots of L-valine uptake in the presence of the analogue are consistent with a strong reduction in the number of active molecules of the general amino-acid permease located in the plasma membrane. Inhibitory effects on protein synthesis were seen after preincubations of the yeast with the analogue for 3 h although a 30 min preincubation had no effect.

Adaptation of in vitro rat brain protein synthesis to long-term ingestion of n-butanol

Long-term treatment of rats with n-butanol leads to a change in in vitro brain protein synthesis which increases the resistance of this process to either ethanol or isopropanol. The change seems to be related to ribosomal events since the synthesis of aminoacyl-tRNA was not affected in the same conditions.

Effect of the chronic ethanol action on the activity of the general amino-acid permease from Saccharomyces cerevisiae var. ellipsoideus

The presence of ethanol and cycloheximide during growth were found to inhibit the function of the general amino-acid permease of Saccharomyces cerevisiae var. ellipsoideus. Contrary to cycloheximide, the effect of ethanol upon growth in alcohol-free medium was reversible. The effect of both inhibitors could be explained in terms of reduction of the number of active carrier molecules located in the plasma membrane.

Effect of acute ethanol administration and nutritional status on secretory protein synthesis in isolated rat liver cells

Treatment of isolated rat liver cells with 20 mm-ethanol inhibited basal secretory protein synthesis by 30% only when the donors were starved for 48 hr, immediately before they were killed. This inhibition was unaffected by the presence of ethanol in the diet of the donor animals. Independently, d-glucose and l-proline enhanced rates of secretory protein synthesis in a dose-dependent manner but only in cells from 48-hr-fasted donors. This latter stimulation was prevented by the presence of 20 mm-ethanol in the incubation medium. By contrast, up to 100 mm-ethanol did not alter polypeptide synthesis by a post-mitochondrial supernatant from rat liver.

Polyvalent inhibitory action of fusidic acid in isolated rat liver cells

The steroidic antibiotic fusidic acid showed a polyvalent action on isolated rat liver cells. It displayed a strong inhibitory capability on protein synthesis in intact cells even stronger than that previously reported in cell-free extracts. Also, it inhibited basal gluconeogenesis and promoted an increase of the membrane permeability to trypan blue. However, the effects on both protein synthesis and basal gluconeogenesis were observed at doses smaller than those required to reduce the cell viability.