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

Antifungal Activity of Ribosome-Inactivating Proteins

The control of crop diseases caused by fungi remains a major problem and there is a need to find effective fungicides that are environmentally friendly. Plants are an excellent source for this purpose because they have developed defense mechanisms to cope with fungal infections. Among the plant proteins that play a role in defense are ribosome-inactivating proteins (RIPs), enzymes obtained mainly from angiosperms that, in addition to inactivating ribosomes, have been studied as antiviral, fungicidal, and insecticidal proteins. In this review, we summarize and discuss the potential use of RIPs (and other proteins with similar activity) as antifungal agents, with special emphasis on RIP/fungus specificity, possible mechanisms of antifungal action, and the use of RIP genes to obtain fungus-resistant transgenic plants. It also highlights the fact that these proteins also have antiviral and insecticidal activity, which makes them very versatile tools for crop protection.

Edodin: A New Type of Toxin from Shiitake Mushroom (Lentinula edodes) That Inactivates Mammalian Ribosomes

Ribosome-inactivating proteins (RIPs) are a group of proteins with rRNA N-glycosylase activity that irreversibly inhibit protein synthesis and consequently cause cell death. Recently, an RIP called ledodin has been found in shiitake; it is cytotoxic, strongly inhibits protein synthesis, and shows rRNA N-glycosylase activity. In this work, we isolated and characterized a 50 kDa cytotoxic protein from shiitake that we named edodin. Edodin inhibits protein synthesis in a mammalian cell-free system, but not in insect-, yeast-, and bacteria-derived systems. It exhibits rRNA N-glycosylase and DNA-nicking activities, which relate it to plant RIPs. It was also shown to be toxic to HeLa and COLO 320 cells. Its structure is not related to other RIPs found in plants, bacteria, or fungi, but, instead, it presents the characteristic structure of the fold type I of pyridoxal phosphate-dependent enzymes. Homologous sequences have been found in other fungi of the class Agaricomycetes; thus, edodin could be a new type of toxin present in many fungi, some of them edible, which makes them of great interest in health, both for their involvement in food safety and for their potential biomedical and biotechnological applications.

Hortensins, Type 1 Ribosome-Inactivating Proteins from Seeds of Red Mountain Spinach: Isolation, Characterization, and Their Effect on Glioblastoma Cells

Ribosome inactivating proteins (RIPs) are specific N-β-glycosylases that are well-characterized in plants. Their enzymatic action is to damage ribosomes, thereby blocking protein translation. Recently, several research groups have been working on the screening for these toxins in edible plants to facilitate the use of RIPs as biotechnological tools and biopesticides and to overcome public prejudice. Here, four novel monomeric (type 1) RIPs have been isolated from the seeds of Atriplex hortensis L. var. rubra, which is commonly known as edible red mountain spinach. These enzymes, named hortensins 1, 2, 4, and 5, are able to release the β-fragment and, like many other RIPs, adenines from salmon sperm DNA, thus, acting as polynucleotide:adenosine glycosidases. Structurally, hortensins have a different molecular weight and are purified with different yields (hortensin 1, ~29.5 kDa, 0.28 mg per 100 g; hortensin 2, ~29 kDa, 0.29 mg per 100 g; hortensin 4, ~28.5 kDa, 0.71 mg per 100 g; and hortensin 5, ~30 kDa, 0.65 mg per 100 g); only hortensins 2 and 4 are glycosylated. Furthermore, the major isoforms (hortensins 4 and 5) are cytotoxic toward human continuous glioblastoma U87MG cell line. In addition, the morphological change in U87MG cells in the presence of these toxins is indicative of cell death triggered by the apoptotic pathway, as revealed by nuclear DNA fragmentation (TUNEL assay).

Heterophyllin: A New Adenia Toxic Lectin with Peculiar Biological Properties

Ribosome-inactivating proteins (RIPs) are plant toxins that were identified for their ability to irreversibly damage ribosomes, thereby causing arrest of protein synthesis and induction of cell death. The RIPs purified from Adenia plants are the most potent ones. Here, we describe a novel toxic lectin from Adenia heterophylla caudex, which has been named heterophyllin. Heterophyllin shows the enzymatic and lectin properties of type 2 RIPs. Interestingly, in immunoreactivity experiments, heterophyllin poorly cross-reacts with sera against all other tested RIPs. The cytotoxic effects and death pathways triggered by heterophyllin were investigated in three human-derived cell lines: NB100, T24, and MCF7, and compared to ricin, the most known and studied type 2 RIP. Heterophyllin was able to completely abolish cell viability at nM concentration. A strong induction of apoptosis, but not necrosis, and the involvement of oxidative stress and necroptosis were observed in all the tested cell lines. Therefore, the enzymatic, immunological, and biological activities of heterophyllin make it an interesting molecule, worthy of further in-depth analysis to verify its possible pharmacological application.

The ribosome-inactivating proteins MAP30 and Momordin inhibit SARS-CoV-2

The continuing emergence of SARS-CoV-2 variants has highlighted the need to identify additional points for viral inhibition. Ribosome inactivating proteins (RIPs), such as MAP30 and Momordin which are derived from bitter melon (Momordica charantia), have been found to inhibit a broad range of viruses. MAP30 has been shown to potently inhibit HIV-1 with minimal cytotoxicity. Here we show that MAP30 and Momordin potently inhibit SARS-CoV-2 replication in A549 human lung cells (IC50 ~ 0.2 μM) with little concomitant cytotoxicity (CC50 ~ 2 μM). Both viral inhibition and cytotoxicity remain unaltered by appending a C-terminal Tat cell-penetration peptide to either protein. Mutation of tyrosine 70, a key residue in the active site of MAP30, to alanine completely abrogates both viral inhibition and cytotoxicity, indicating the involvement of its RNA N-glycosylase activity. Mutation of lysine 171 and lysine 215, residues corresponding to those in Ricin which when mutated prevented ribosome binding and inactivation, to alanine in MAP30 decreased cytotoxicity (CC50 ~ 10 μM) but also the viral inhibition (IC50 ~ 1 μM). Unlike with HIV-1, neither Dexamethasone nor Indomethacin exhibited synergy with MAP30 in the inhibition of SARS-CoV-2. From a structural comparison of the two proteins, one can explain their similar activities despite differences in both their active-sites and ribosome-binding regions. We also note points on the viral genome for potential inhibition by these proteins.

New Insights on Saporin Resistance to Chemical Derivatization with Heterobifunctional Reagents

Saporin is a type 1 ribosome-inactivating protein widely used as toxic payload in the construction of targeted toxins, chimeric molecules formed by a toxic portion linked to a carrier moiety. Among the most used carriers, there are large molecules (mainly antibodies) and small molecules (such as neurotransmitters, growth factors and peptides). Some saporin-containing targeted toxins have been used for the experimental treatment of several diseases, giving very promising results. In this context, one of the reasons for the successful use of saporin lies in its resistance to proteolytic enzymes and to conjugation procedures. In this paper, we evaluated the influence of derivatization on saporin using three heterobifunctional reagents, namely 2-iminothiolane (2-IT), N-succinimidyl 3-(2-pyridyldithio)propionate (SPDP) and 4-succinimidyloxycarbonyl-α-methyl-α-[2-pyridyldithio]toluene (SMPT). In order to obtain the highest number of inserted -SH groups with the lowest reduction of saporin biological activities, we assessed the residual ability of saporin to inhibit protein synthesis, to depurinate DNA and to induce cytotoxicity after derivatization. Our results demonstrate that saporin maintains an excellent resistance to derivatization processes, especially with SPDP, and permit us to define reaction conditions, in which saporin biological properties may not be altered. Therefore, these findings provide useful information for the construction of saporin-based targeted toxins, especially with small carriers.

Targeting the Inside of Cells with Biologicals: Toxin Routes in a Therapeutic Context

Numerous toxins translocate to the cytosol in order to fulfil their function. This demonstrates the existence of routes for proteins from the extracellular space to the cytosol. Understanding these routes is relevant to multiple aspects related to therapeutic applications. These include the development of anti-toxin treatments, the potential use of toxins as shuttles for delivering macromolecular cargo to the cytosol or the use of drugs based on toxins. Compared with other strategies for delivery, such as chemicals as carriers for macromolecular delivery or physical methods like electroporation, toxin routes present paths into the cell that potentially cause less damage and can be specifically targeted. The efficiency of delivery via toxin routes is limited. However, low-delivery efficiencies can be entirely sufficient, if delivered cargoes possess an amplification effect or if very few molecules are sufficient for inducing the desired effects. This is known for example from RNA-based vaccines that have been developed during the coronavirus disease 2019 pandemic as well as for other approved RNA-based drugs, which elicited the desired effect despite their typically low delivery efficiencies. The different mechanisms by which toxins enter cells may have implications for their technological utility. We review the mechanistic principles of the translocation pathway of toxins from the extracellular space to the cytosol, the delivery efficiencies, and therapeutic strategies or applications that exploit toxin routes for intracellular delivery.

Seed Storage Protein, Functional Diversity and Association with Allergy

Plants are essential for humans as they serve as a source of food, fuel, medicine, oils, and more. The major elements that are utilized for our needs exist in storage organs, such as seeds. These seeds are rich in proteins, show a broad spectrum of physiological roles, and are classified based on their sequence, structure, and conserved motifs. With the improvements to our knowledge of the basic sequence and our structural understanding, we have acquired better insights into seed proteins and their role. However, we still lack a systematic analysis towards understanding the functional diversity associated within each family and their associations with allergy. This review puts together the information about seed proteins, their classification, and diverse functional roles along with their associations with allergy.

Phylogeny and domain architecture of plant ribosome inactivating proteins

Ribosome inactivating proteins (RIPs) are rRNA N-glycosylases (EC 3.2.2.22) best known for hydrolyzing an adenine base from the conserved sarcin/ricin loop of ribosomal RNA. Protein translation is inhibited by ribosome depurination; therefore, RIPs are generally considered toxic to cells. The expression of some RIPs is upregulated by biotic and abiotic stress, though the connection between RNA depurination and defense response is not well understood. Despite their prevalence in approximately one-third of flowering plant orders, our knowledge of RIPs stems primarily from biochemical analyses of individuals or genomics-scale analyses of small datasets from a limited number of species. Here, we performed an unbiased search for proteins with RIP domains and identified several-fold more RIPs than previously known - more than 800 from 120 species, many with novel associated domains and physicochemical characteristics. Based on protein domain configuration, we established 15 distinct groups, suggesting diverse functionality. Surprisingly, most of these RIPs lacked a signal peptide, indicating they may be localized to the nucleocytoplasm of cells, raising questions regarding their toxicity against conspecific ribosomes. Our phylogenetic analysis significantly extends previous models for RIP evolution in plants, predicting an original single-domain RIP that later evolved to acquire a signal peptide and different protein domains. We show that RIPs are distributed throughout 21 plant orders with many species maintaining genes for more than one RIP group. Our analyses provide the foundation for further characterization of these new RIP types, to understand how these enzymes function in plants.

Differential effects of the recombinant type 1 ribosome-inactivating protein, OsRIP1, on growth of PSB-D and BY-2 cells

Plant suspension cells were treated with recombinant OsRIP1, a type 1 ribosome-inactivating protein (RIP) from rice (Oryza sativa L.). OsRIP1 triggered cell death in tobacco BY-2 cells but not in Arabidopsis PSB-D cells. Phenotypic changes in BY-2 cells exposed to OsRIP1, included loss of growth capacity, loss of integrity of the plasma membrane and vacuolar collapse. These effects were also accompanied by RNA degradation and DNA fragmentation. Targeting of exogenous OsRIP1 to plant vacuoles and OsRIP1-induced accumulation of transcripts for vacuolar processing enzymes (VPEs) indicated that OsRIP1 provoked plant cell death in tobacco BY-2 cells through the activation of VPEs and subsequent vacuolar disruption, which was probably independent of its N-glycosylase activity on cytosolic ribosomes. Necrosis with limited production of H₂O₂ was observed after infiltration of high concentrations of OsRIP1 in epidermal cells of Nicotiana tabacum cv. Samsun NN plants. Our study provides the first evidence that OsRIP1 exerts differential effects on the growth of PSB-D and BY-2 cells. The vacuole-dependent cell death pathway is associated with the lethal effect of the exogenously applied OsRIP1 on BY-2 cells.

Analysis of the Sequence Preference of Saporin by Deep Sequencing

Ribosome-inactivating proteins (RIPs) are RNA:adenosine glycosidases that inactivate eukaryotic ribosomes by depurinating the sarcin-ricin loop (SRL) in 28S rRNA. The GAGA sequence at the top of the SRL or at the top of a hairpin loop is assumed to be their target motif. Saporin is a RIP widely used to develop immunotoxins for research and medical applications, but its sequence specificity has not been investigated. Here, we combine the conventional aniline cleavage assay for depurinated nucleic acids with high-throughput sequencing to study sequence-specific depurination of oligonucleotides caused by saporin. Our data reveal the sequence preference of saporin for different substrates and show that the GAGA motif is not efficiently targeted by this protein, neither in RNA nor in DNA. Instead, a preference of saporin for certain hairpin DNAs was observed. The observed sequence-specific activity of saporin may be relevant to antiviral or apoptosis-inducing effects of RIPs. The developed method could also be useful for studying the sequence specificity of depurination by other RIPs or enzymes.

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

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

Isolation, Characterization and Biological Action of Type-1 Ribosome-Inactivating Proteins from Tissues of Salsola soda L

Ribosome-inactivating proteins (RIPs) are known as RNA N-glycosylases. They depurinate the major rRNA, damaging ribosomes and inhibiting protein synthesis. Here, new single-chain (type-1) RIPs named sodins were isolated from the seeds (five proteins), edible leaves (one protein) and roots (one protein) of Salsola soda L. Sodins are able to release Endo's fragment when incubated with rabbit and yeast ribosomes and inhibit protein synthesis in cell-free systems (IC₅₀ = 4.83-79.31 pM). In addition, sodin 5, the major form isolated from seeds, as well as sodin eL and sodin R, isolated from edible leaves and roots, respectively, display polynucleotide:adenosine glycosylase activity and are cytotoxic towards the Hela and COLO 320 cell lines (IC₅₀ = 0.41-1200 nM), inducing apoptosis. The further characterization of sodin 5 reveals that this enzyme shows a secondary structure similar to other type-1 RIPs and a higher melting temperature (Tm = 76.03 ± 0.30 °C) and is non-glycosylated, as other sodins are. Finally, we proved that sodin 5 possesses antifungal activity against Penicillium digitatum.

Sapovaccarin-S1 and -S2, Two Type I RIP Isoforms from the Seeds of Saponaria vaccaria L

Type I ribosome-inactivating proteins (RIPs) are plant toxins that inhibit protein synthesis by exerting rRNA N-glycosylase activity (EC 3.2.2.22). Due to the lack of a cell-binding domain, type I RIPs are not target cell-specific. However once linked to antibodies, so called immunotoxins, they are promising candidates for targeted anti-cancer therapy. In this study, sapovaccarin-S1 and -S2, two newly identified type I RIP isoforms differing in only one amino acid, were isolated from the seeds of Saponaria vaccaria L. Sapovaccarin-S1 and -S2 were purified using ammonium sulfate precipitation and subsequent cation exchange chromatography. The determined molecular masses of 28,763 Da and 28,793 Da are in the mass range typical for type I RIPs and the identified amino acid sequences are homologous to known type I RIPs such as dianthin 30 and saporin-S6 (79% sequence identity each). Sapovaccarin-S1 and -S2 showed adenine-releasing activity and induced cell death in Huh-7 cells. In comparison to other type I RIPs, sapovaccarin-S1 and -S2 exhibited a higher thermostability as shown by nano-differential scanning calorimetry. These results suggest that sapovaccarin-S1 and -S2 would be optimal candidates for targeted anti-cancer therapy.

Ribosome inactivating proteins - An unfathomed biomolecule for developing multi-stress tolerant transgenic plants

Transgenic crops would serve as a tool to overcome the forthcoming crisis in food security and environmental safety posed by degrading land and changing global climate. Commercial transgenic crops developed so far focus on single stress; however, sustaining crop yield to ensure food security requires transgenics tolerant to multiple environmental stresses. Here we argue and demonstrate the untapped potential of ribosome inactivating proteins (RIPs), translation inhibitors, as potential transgenes in developing transgenics to combat multiple stresses in the environment. Plant RIPs target the fundamental processes of the cell with very high specificity to the infecting pests. While controlling pathogens, RIPs also cause ectopic expression of pathogenesis-related proteins and trigger systemic acquired resistance. On the other hand, during abiotic stress, RIPs show antioxidant activity and trigger both enzyme-dependent and enzyme-independent metabolic pathways, alleviating abiotic stress such as drought, salinity, temperature, etc. RIPs express in response to specific environmental signals; therefore, their expression obviates additional physiological load on the transgenic plants instead of the constitutive expression. Based on evidence from its biological significance, ecological roles, laboratory- and controlled-environment success of its transgenics, and ethical merits, we unravel the potential of RIPs in developing transgenic plants showing co-tolerance to multiple environmental stresses.

Ribotoxic Proteins, Known as Inhibitors of Protein Synthesis, from Mushrooms and Other Fungi According to Endo's Fragment Detection

rRNA N-glycosylases (EC 3.2.2.22) remove a specific adenine (A₄₃₂₄, rat 28S rRNA) in the sarcin ricin loop (SRL) involved into ribosome interaction with elongation factors, causing the inhibition of translation, for which they are known as plant 'ribosome inactivating proteins' (RIPs). However, protein synthesis inactivation could be the result of other enzymes, which often have rRNA as the target. In this scenario, Endo's assay is the most used method to detect the enzymes that are able to hydrolyze a phosphodiester bond or cleave a single N-glycosidic bond (rRNA N-glycosylases). Indeed, the detection of a diagnostic fragment from rRNA after enzymatic action, with or without acid aniline, allows one to discriminate between the N-glycosylases or hydrolases, which release the β-fragment after acid aniline treatment or α-fragment without acid aniline treatment, respectively. This assay is of great importance in the mushroom kingdom, considering the presence of enzymes that are able to hydrolyze phosphodiester bonds (e.g., ribonucleases, ribotoxins and ribotoxin-like proteins) or to remove a specific adenine (rRNA N-glycosylases). Thus, here we used the β-fragment experimentally detected by Endo's assay as a hallmark to revise the literature available on enzymes from mushrooms and other fungi, whose action consists of protein biosynthesis inhibition.

Highly sensitive MALDI-MS measurement of active ricin: insight from more potential deoxynucleobase-hybrid oligonucleotide substrates

Herein, we report an improved MALDI-MS method for active ricin to contribute toward countermeasures against its real threat to the public. Compared with commonly used DNA or RNA substrates, the deoxynucleobase-hybrid oligonucleotide (RNA_dA, Rd) substrate containing functional Gd[combining low line]A[combining low line]GA loop was revealed as a substrate with more potential and used for the first time in ricin measurement via MALDI-MS. The Rd sequence greatly prompted ricin to exhibit its catalytic activity as rRNA N-glycosylase in ex vitro condition, which was supported by molecular docking simulation and enzymatic parameters depicted in MALDI-MS. Furthermore, we discovered that a highly pure matrix was the most crucial parameter for enhancing the sensitivity, which addressed the major obstacle encountered in the oligo(deoxy)nucleotide measurement, i.e., the interfering alkali metal ion-adducted signals in MALDI-MS. After the optimization of pH and enzymatic reaction buffer composition in this ex vitro condition, this method can provide a wide linearity of up to three orders of magnitude, i.e., 1-5000 ng mL-1, and a high sensitivity of 1 ng mL-1 without any enrichment. Denatured and active ricin could be distinctly differentiated, and the application to practical samples from one international exercise and a soft drink proved the feasibility of this new method. We believe this MALDI-MS method can contribute to the first response to ricin occurrence events in public safety and security, as well as pave a new way for a deep understanding of ricin and other type II ribosome inactivating proteins involved toxicology.

A Simple, Fast and Portable Method for Electrochemical Detection of Adenine Released by Ricin Enzymatic Activity

International authorities classify ricin toxin present in castor seed as a potential agent for use in bioterrorism. Therefore, the detection, identification, and characterization of ricin in various sample matrices are considered necessary actions for risk assessment during a suspected exposure. This study reports a portable electrochemical assay for detecting active ricin based on the adenine electro-oxidation released from herring sperm DNA substrate by its catalytic action. Also, kinetic parameters were calculated, and the values were K_m of 3.14 µM and K_cat 2107 min⁻¹. A linear response was found in optimized experimental conditions for ricin concentrations ranging from 8 to 120 ng/mL, and with a detection limit of 5.14 ng/mL. This proposed detection strategy emphasizes the possibility of field detection of active ricin in food matrices and can be applied to other endonucleolytic activities.

[Highly toxic type Ⅱ ribosome-inactivating proteins ricin and abrin and their detection methods: a review]

Type Ⅱ ribosome-inactivating proteins (RIPs) are an important class of protein toxins that consist of A and B chains linked by an interchain disulfide bond. The B-chain with lectin-like activity is responsible for binding to the galactose-containing receptors on eukaryotic cell surfaces, which is essential for A-chain internalization by endocytosis. The A-chain has N-glycosidase activity that irreversibly depurinates a specific adenine from 28S ribosomal RNA (28S rRNA) and terminates protein synthesis. The synergistic effect of the A-B chain inactivates the ribosome, inhibits protein synthesis, and exhibits high cytotoxicity. Ricin and abrin that are expressed by the plants Ricinus communis and Abrus precatorius, respectively, are typical type Ⅱ RIPs. The toxicity of ricin and abrin are 385 times and 2885 times, respectively, more that of the nerve agent VX. Owing to their ease of preparation, wide availability, and potential use as a bioterrorism agent, type Ⅱ RIPs have garnered increasing attention in recent years. Ricin is listed as a prohibited substance under schedule 1A of the Chemical Weapons Convention (CWC). The occurrence of ricin-related bioterrorism incidents in recent years has promoted the development of accurate, sensitive, and rapid detection and identification technology for type Ⅱ RIPs. Significant progress has been made in the study of toxicity mechanisms and detection methods of type Ⅱ RIPs, which primarily involve qualitative and quantitative analysis methods including immunological assays, mass spectrometry analysis methods, and toxin activity detection methods based on depurination and cytotoxicity. Immunoassays generally involve the specific recognition of antigens and antibodies, which is based on oligonucleotide molecular recognition elements called aptamers. These methods are fast and highly sensitive, but for highly homologous proteins in complex samples, they provide false positive results. With the rapid development of biological mass spectrometry detection technology, techniques such as electrospray ionization (ESI) and matrix-assisted laser desorption ionization (MALDI) are widely used in the identification of proteins. These methods not only provide accurate information on molecular weight and structure of proteins, but also demonstrate accurate quantification. Enzyme digestion combined with mass spectrometry is the predominantly used detection method. Accurate identification of protein toxins can be achieved by fingerprint analysis of enzymatically digested peptides. For analysis of protein toxins in complex samples, abundant peptide markers are obtained using a multi-enzyme digestion strategy. Targeted mass spectrometry analysis of peptide markers is used to obtain accurate qualitative and quantitative information, which effectively improves the accuracy and sensitivity of the identification of type Ⅱ RIP toxins. Although immunoassay and mass spectrometry detection methods can provide accurate identification of type Ⅱ RIPs, they cannot determine whether the toxins will retain potency. The widely used detection methods for activity analysis of type Ⅱ RIPs include depurination assay based on N-glycosidase activity and cytotoxicity assay. Both the methods provide simple, rapid, and sensitive analysis of type Ⅱ RIP toxicity, and complement other detection methods. Owing to the importance of type Ⅱ RIP toxins, the Organization for the Prohibition of Chemical Weapons (OPCW) has proposed clear technical requirements for the identification and analysis of relevant samples. We herein reviewed the structural characteristics, mechanism of action, and the development and application of type Ⅱ RIP detection methods; nearly 70 studies on type Ⅱ RIP toxins and their detection methods have been cited. In addition to the technical requirements of OPCW for the unambiguous identification of biotoxins, the trend of future development of type Ⅱ RIP-based detection technology has been explored.

Kirkiin: A New Toxic Type 2 Ribosome-Inactivating Protein from the Caudex of Adenia kirkii

Ribosome-inactivating proteins (RIPs) are plant toxins that irreversibly damage ribosomes and other substrates, thus causing cell death. RIPs are classified in type 1 RIPs, single-chain enzymatic proteins, and type 2 RIPs, consisting of active A chains, similar to type 1 RIPs, linked to lectin B chains, which enable the rapid internalization of the toxin into the cell. For this reason, many type 2 RIPs are very cytotoxic, ricin, volkensin and stenodactylin being the most toxic ones. From the caudex of Adenia kirkii (Mast.) Engl., a new type 2 RIP, named kirkiin, was purified by affinity chromatography on acid-treated Sepharose CL-6B and gel filtration. The lectin, with molecular weight of about 58 kDa, agglutinated erythrocytes and inhibited protein synthesis in a cell-free system at very low concentrations. Moreover, kirkiin was able to depurinate mammalian and yeast ribosomes, but it showed little or no activity on other nucleotide substrates. In neuroblastoma cells, kirkiin inhibited protein synthesis and induced apoptosis at doses in the pM range. The biological characteristics of kirkiin make this protein a potential candidate for several experimental pharmacological applications both alone for local treatments and as component of immunoconjugates for systemic targeting in neurodegenerative studies and cancer therapy.

Antiviral Activity of Ribosome-Inactivating Proteins

Ribosome-inactivating proteins (RIPs) are rRNA N-glycosylases from plants (EC 3.2.2.22) that inactivate ribosomes thus inhibiting protein synthesis. The antiviral properties of RIPs have been investigated for more than four decades. However, interest in these proteins is rising due to the emergence of infectious diseases caused by new viruses and the difficulty in treating viral infections. On the other hand, there is a growing need to control crop diseases without resorting to the use of phytosanitary products which are very harmful to the environment and in this respect, RIPs have been shown as a promising tool that can be used to obtain transgenic plants resistant to viruses. The way in which RIPs exert their antiviral effect continues to be the subject of intense research and several mechanisms of action have been proposed. The purpose of this review is to examine the research studies that deal with this matter, placing special emphasis on the most recent findings.

The role of enzymatic activities of antiviral proteins from plants for action against plant pathogens

Antiviral proteins (AVPs) from plants possess multiple activities, such as N-glycosidase, RNase, DNase enzymatic activity, and induce pathogenesis-related proteins, salicylic acid, superoxide dismutase, peroxidase, and catalase. The N-glycosidase activity releases the adenine residues from sarcin/ricin (S/R) loop of large subunit of ribosomes and interfere the host protein synthesis process and this activity has been attributed for antiviral activity in plant. It has been shown that AVP binds directly to viral genome-linked protein of plant viruses and interfere with protein synthesis of virus. AVPs also possess the RNase and DNase like activity and may be targeting nucleic acid of viruses directly. Recently, the antifungal, antibacterial, and antiinsect properties of AVPs have also been demonstrated. Gene encoding for AVPs has been used for the development of transgenic resistant crops to a broad range of plant pathogens and insect pests. However, the cytotoxicity has been observed in transgenic crops using AVP gene in some cases which can be a limiting factor for its application in agriculture. In this review, we have reviewed various aspects of AVPs particularly their characteristics, possible mode of action and application.

Quinoa as source of type 1 ribosome inactivating proteins: A novel knowledge for a revision of its consumption

This study investigates on the presence of toxic proteins in quinoa seeds. To this aim, a plethora of biochemical approaches were adopted for the purification and characterization of quinoin, a type 1 ribosome-inactivating protein (RIP) contained in quinoa seeds. We determined its melting temperature (68.2 ± 0.6 °C) and thermostability (loss of activity after 10-min incubation at 70 °C). Considering that quinoa seeds are used as a food, we found that quinoin is cytotoxic against BJ-5ta (human fibroblasts) and HaCaT (human keratinocytes) in a dose- and time-dependent manner. Moreover, in an in vitro digestive pepsin-trypsin treatment, 30% of quinoin is resistant to enzymatic cleavage. This toxin was found in seeds (0.23 mg/g of seeds) and in sprouted seeds obtained after 24-h (0.12 mg/g of sprout) and 48-h (0.09 mg/g of sprout). We suggest a thermal treatment of quinoa seeds before consumption in order to inactivate the toxin, particularly in sprouts, generally consumed raw.

Primary Sequence and 3D Structure Prediction of the Plant Toxin Stenodactylin

Stenodactylin is one of the most potent type 2 ribosome-inactivating proteins (RIPs); its high toxicity has been demonstrated in several models both in vitro and in vivo. Due to its peculiarities, stenodactylin could have several medical and biotechnological applications in neuroscience and cancer treatment. In this work, we report the complete amino acid sequence of stenodactylin and 3D structure prediction. The comparison between the primary sequence of stenodactylin and other RIPs allowed us to identify homologies/differences and the amino acids involved in RIP toxic activity. Stenodactylin RNA was isolated from plant caudex, reverse transcribed through PCR and the cDNA was amplificated and cloned into a plasmid vector and further analyzed by sequencing. Nucleotide sequence analysis showed that stenodactylin A and B chains contain 251 and 258 amino acids, respectively. The key amino acids of the active site described for ricin and most other RIPs are also conserved in the stenodactylin A chain. Stenodactylin amino acid sequence shows a high identity degree with volkensin (81.7% for A chain, 90.3% for B chain), whilst when compared with other type 2 RIPs the identity degree ranges from 27.7 to 33.0% for the A chain and from 42.1 to 47.7% for the B chain.

Early Response to the Plant Toxin Stenodactylin in Acute Myeloid Leukemia Cells Involves Inflammatory and Apoptotic Signaling

Stenodactylin, a highly toxic type 2 ribosome-inactivating protein purified from the caudex of Adenia stenodactyla Harms, is a potential anticancer drug candidate. Previous studies demonstrated that stenodactylin induces apoptosis and necroptosis in treated cells, involving the production of reactive oxygen species. We analyzed the effect of stenodactylin on Raji and Ramos (Human Burkitt’s lymphoma cells) and MOLM-13 (acute myeloid leukemia cells). Moreover, we focused on the early events in MOLM-13 cells that characterize the cellular response to the toxin by whole-genome microarray analysis of gene expression. Treatment with stenodactylin induced the depurination of 28S rRNA within 4 h and increased the phosphorylation of p38 and JNK. A time-dependent activation of caspase 1, 2, 8, 9, 3/7 was also observed. Genome-wide gene expression microarray analysis revealed early changes in the expression of genes involved in the regulation of cell death, inflammation and stress response. After 4 h, a significant increase of transcript level was detectable for ATF3, BTG2, DUSP1, EGR1, and JUN. Increased upstream JUN signaling was also confirmed at protein level. The early response to stenodactylin treatment involves inflammatory and apoptotic signaling compatible with the activation of multiple cell death pathways. Because of the above described properties toward acute myeloid leukemia cells, stenodactylin may be a promising candidate for the design of new immunoconjugates for experimental cancer treatment.

Mayahuelin, a Type I Ribosome Inactivating Protein: Characterization, Evolution, and Utilization in Phylogenetic Analyses of Agave

Agaves resist extreme heat and drought. In A. tequilana var. azul, the central spike of the rosette -containing the shoot apical meristem and folded leaves in early stages of development- is remarkably heat tolerant. We found that the most abundant protein in this organ is a 27 kDa protein. This protein was named mayahuelin to honor Mayáhuel, the agave goddess in the Aztec pantheon. LC-MS/MS analyses identified mayahuelin as a type I RIP (Ribosome Inactivating Protein). In addition to the spike, mayahuelin was expressed in the peduncle and in seeds, whereas in mature leaves, anthers, filaments, pistils, and tepals was absent. Anti-mayahuelin antibody raised against the A. tequilana var. azul protein revealed strong signals in spike leaves of A. angustifolia, A. bracteosa, A. rhodacantha, and A. vilmoriniana, and moderate signals in A. isthmensis, A. kerchovei, A. striata ssp. falcata, and A. titanota, indicating conservation at the protein level throughout the Agave genus. As in charybdin, a type I RIP characterized in Drimia maritima, mayahuelin from A. tequilana var. azul contains a natural aa substitution (Y76D) in one out of four aa comprising the active site. The RIP gene family in A. tequilana var. azul consists of at least 12 genes and Mayahuelin is the only member encoding active site substitutions. Unlike canonical plant RIPs, expression of Mayahuelin gene in S. cerevisiae did not compromise growth. The inhibitory activity of the purified protein on a wheat germ in vitro translation system was moderate. Mayahuelin orthologs from other Agave species displayed one of six alleles at Y76: (Y/Y, D/D, S/S, Y/D, Y/S, D/S) and proved to be useful markers for phylogenetic analysis. Homozygous alleles were more frequent in wild accessions whereas heterozygous alleles were more frequent in cultivars. Mayahuelin sequences from different wild populations of A. angustifolia and A. rhodacantha allowed the identification of accessions closely related to azul, manso, sigüín, mano larga, and bermejo varieties of A. tequilana and var. espadín of A. angustifolia. Four A. rhodacantha accessions and A. angustifolia var. espadín were closer relatives of A. tequilana var. azul than A. angustifolia wild accessions or other A. tequilana varieties.

Ribosome depurination by ricin leads to inhibition of endoplasmic reticulum stress-induced HAC1 mRNA splicing on the ribosome

Ricin undergoes retrograde transport to the endoplasmic reticulum (ER), and ricin toxin A chain (RTA) enters the cytosol from the ER. Previous reports indicated that RTA inhibits activation of the unfolded protein response (UPR) in yeast and in mammalian cells. Both precursor (preRTA) and mature form of RTA (mRTA) inhibited splicing of HAC1^u (u for uninduced) mRNA, suggesting that UPR inhibition occurred on the cytosolic face of the ER. Here, we examined the role of ribosome binding and depurination activity on inhibition of the UPR using mRTA mutants. An active-site mutant with very low depurination activity, which bound ribosomes as WT RTA, did not inhibit HAC1^u mRNA splicing. A ribosome-binding mutant, which showed reduced binding to ribosomes but retained depurination activity, inhibited HAC1^u mRNA splicing. This mutant allowed separation of the UPR inhibition by RTA from cytotoxicity because it reduced the rate of depurination. The ribosome-binding mutant inhibited the UPR without affecting IRE1 oligomerization or cleavage of HAC1^u mRNA at the splice site junctions. Inhibition of the UPR correlated with the depurination level, suggesting that ribosomes play a role in splicing of HAC1^u mRNA. We show that HAC1^u mRNA is associated with ribosomes and does not get processed on depurinated ribosomes, thereby inhibiting the UPR. These results demonstrate that RTA inhibits HAC1^u mRNA splicing through its depurination activity on the ribosome without directly affecting IRE1 oligomerization or the splicing reaction and provide evidence that IRE1 recognizes HAC1^u mRNA that is associated with ribosomes.

Intracellular Transport and Cytotoxicity of the Protein Toxin Ricin

Ricin can be isolated from the seeds of the castor bean plant (Ricinus communis). It belongs to the ribosome-inactivating protein (RIP) family of toxins classified as a bio-threat agent due to its high toxicity, stability and availability. Ricin is a typical A-B toxin consisting of a single enzymatic A subunit (RTA) and a binding B subunit (RTB) joined by a single disulfide bond. RTA possesses an RNA N-glycosidase activity; it cleaves ribosomal RNA leading to the inhibition of protein synthesis. However, the mechanism of ricin-mediated cell death is quite complex, as a growing number of studies demonstrate that the inhibition of protein synthesis is not always correlated with long term ricin toxicity. To exert its cytotoxic effect, ricin A-chain has to be transported to the cytosol of the host cell. This translocation is preceded by endocytic uptake of the toxin and retrograde traffic through the trans-Golgi network (TGN) and the endoplasmic reticulum (ER). In this article, we describe intracellular trafficking of ricin with particular emphasis on host cell factors that facilitate this transport and contribute to ricin cytotoxicity in mammalian and yeast cells. The current understanding of the mechanisms of ricin-mediated cell death is discussed as well. We also comment on recent reports presenting medical applications for ricin and progress associated with the development of vaccines against this toxin.

Structure and Activity of a Cytosolic Ribosome-Inactivating Protein from Rice

Ribosome-inactivating proteins (RIPs) are cytotoxic enzymes that inhibit protein translation by depurinating ribosomal RNA. Although most plant RIPs are synthesized with leader sequences that sequester them away from the host ribosomes, several RIPs from cereals lack these signal peptides and therefore probably reside in the cytosol near the plant ribosomes. More than 30 RIP genes have been identified in the rice (Oryza sativa spp. japonica) genome, many of them lacking a signal peptide. This paper focuses on a presumed cytosolic type-1 RIP from rice, referred to as OsRIP1. Using 3D modeling it is shown that OsRIP1 structurally resembles other cereal RIPs and has an active site that meets the requirements for activity. Furthermore, localization studies indicate that OsRIP1-eGFP fusion proteins reside in the nucleocytoplasmic space when expressed in epidermal cells of Nicotiana benthamiana or Arabidopsis thaliana suspension cells. Finally, OsRIP1 was recombinantly produced in Escherichia coli and was demonstrated to possess catalytic activity. Interestingly, this recombinant RIP inactivates wheat ribosomes far less efficiently than rabbit ribosomes in an in vitro system. These findings raise some interesting questions concerning the mode of action and physiological role of OsRIP1. This is the first time a RIP from rice is investigated at protein level and is shown to possess biological activity.

Ricin: An Ancient Story for a Timeless Plant Toxin

The castor plant (Ricinus communis L.) has been known since time immemorial in traditional medicine in the pharmacopeia of Mediterranean and eastern ancient cultures. Moreover, it is still used in folk medicine worldwide. Castor bean has been mainly recommended as anti-inflammatory, anthelmintic, anti-bacterial, laxative, abortifacient, for wounds, ulcers, and many other indications. Many cases of human intoxication occurred accidentally or voluntarily with the ingestion of castor seeds or derivatives. Ricinus toxicity depends on several molecules, among them the most important is ricin, a protein belonging to the family of ribosome-inactivating proteins. Ricin is the most studied of this category of proteins and it is also known to the general public, having been used for several biocrimes. This manuscript intends to give the reader an overview of ricin, focusing on the historical path to the current knowledge on this protein. The main steps of ricin research are here reported, with particular regard to its enzymatic activity, structure, and cytotoxicity. Moreover, we discuss ricin toxicity for animals and humans, as well as the relation between bioterrorism and ricin and its impact on environmental toxicity. Ricin has also been used to develop immunotoxins for the elimination of unwanted cells, mainly cancer cells; some of these immunoconjugates gave promising results in clinical trials but also showed critical limitation.

A new type 1 ribosome-inactivating protein from the seeds of Gypsophila elegans M.Bieb

Ribosome-inactivating proteins (RIPs) are enzymes with N-glycosylase activity that remove adenine bases from the ribosomal RNA. In theory, one single RIP molecule internalized into a cell is sufficient to induce cell death. For this reason, RIPs are of high potential as toxic payload for anti-tumor therapy. A considerable number of RIPs are synthesized by plants that belong to the carnation family (Caryophyllaceae). Prominent examples are the RIPs saporin from Saponaria officinalis L. or dianthin from Dianthus caryophyllus L. In this study, we have isolated and characterized a novel RIP (termed gypsophilin-S) from the tiny seeds of Gypsophila elegans M. Bieb. (Caryophyllaceae). It is noteworthy that this is the first study presenting the complete amino acid sequence of a RIP from a Gypsophila species. Gypsophilin-S was isolated from the defatted seed material following ammonium sulphate precipitation and HPLC-based ion exchange chromatography. Gypsophilin-S-containing fractions were analysed by SDS-PAGE and mass spectrometry. The full amino acid sequence of gypsophilin-S was assembled by MALDI-TOF-MS-MS and PCR. Gypsophilin-S exhibited strong adenine releasing activity and its cytotoxicity in human glioblastoma cells was investigated using an impedance-based real-time assay in comparison with recombinant saporin and dianthin.

Ligand-Induced Conformational Changes near the Active Site Regulating Enzyme Activity of Momorcharins from Seeds of Bitter Gourd

It is reasonable to consider that Type I-ribosomal inactivation proteins (RIP) retain some specific affinity to harmful pathogens to complete the role as a bio-defense relating protein. In the present studies, it was shown that two Type I-RIPs, α- and β-momorcharins, maintained the abilities to bind with N-acetylglucosamine (NAG) to change the conformation around the active sites and to regulate their N-glycosidase activities. By the binding of NAG, the freedom of internal motion of Trp192 in α-momorcharin was increased 1.5 times near the active site and, on the other hand, the corresponding motion of Trp190 was limited 50% in β-momorcharin. The results in the fluorescence resonance excitation energy transfer experiments demonstrated that Trp-190 of β-momorcharin was kept away from Tyr-70 but Trp192 contrarily approached closer to the nearest neighboring Tyr residue consisting of the active center of α-momorcharin by the binding with NAG. These conformational changes near the active site close correlated with promotion and/or suppression of the N-glucosidase activities of β- and α-momorcharins.

Correlation of abrin-mediated inhibition of protein synthesis and apoptosis

The plant toxin, abrin, a type-II ribosome inactivating protein, is extremely lethal, the human fatal dose being ~1 μg/kg body weight. Abrin has been classified as an agent for bioterrorism, which is of concern. Conversely, the high toxic property of abrin has been employed in generating immunotoxins, whereas its toxin moiety is conjugated to cell surface marker-specific antibodies for cell-targeted killing. Different cell types exhibit variable levels of sensitivity to abrin toxicity; therefore, adequate knowledge of the molecular mechanism that governs the activity of the protein would be a safeguard. To gain insights into this, two cell lines requiring strikingly different concentrations of abrin for inactivating ribosomes were studied. Employing conjugates of the wild-type and active site mutant of abrin A chain with the ricin B chain, it was found that abrin-induced apoptosis was dependent on inhibition of protein synthesis (PSI) leading to ER-stress in Ovcar-3 cells, but not in KB cells. Abrin was also observed to cause direct DNA damage in KB cells, while in Ovcar-3 cells abrin-induced DNA damage was found to be dependent on caspases. Overall, the study demonstrates that the correlation of abrin-mediated PSI and apoptosis is cell-specific and abrin can induce more than one pathway to cause cell death. © 2018 IUBMB Life, 71(3):357-363, 2019.

Bouganin, an Attractive Weapon for Immunotoxins

Bougainvillea (Bougainvillea spectabilis Willd.) is a plant widely used in folk medicine and many extracts from different tissues of this plant have been employed against several pathologies. The observation that leaf extracts of Bougainvillea possess antiviral properties led to the purification and characterization of a protein, named bouganin, which exhibits typical characteristics of type 1 ribosome-inactivating proteins (RIPs). Beyond that, bouganin has some peculiarities, such as a higher activity on DNA with respect to ribosomal RNA, low systemic toxicity, and immunological properties quite different than other RIPs. The sequencing of bouganin and the knowledge of its three-dimensional structure allowed to obtain a not immunogenic mutant of bouganin. These features make bouganin a very attractive tool as a component of immunotoxins (ITs), chimeric proteins obtained by linking a toxin to a carrier molecule. Bouganin-containing ITs showed very promising results in the experimental treatment of both hematological and solid tumors, and one bouganin-containing IT has entered Phase I clinical trial. In this review, we summarize the milestones of the research on bouganin such as bouganin chemico-physical characteristics, the structural properties and de-immunization studies. In addition, the in vitro and in vivo results obtained with bouganin-containing ITs are summarized.

Review: Potential biotechnological assets related to plant immunity modulation applicable in engineering disease-resistant crops

This review emphasizes the biotechnological potential of molecules implicated in the different layers of plant immunity, including, pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI), effector-triggered susceptibility (ETS), and effector-triggered immunity (ETI) that can be applied in the development of disease-resistant genetically modified (GM) plants. These biomolecules are produced by pathogens (viruses, bacteria, fungi, oomycetes) or plants during their mutual interactions. Biomolecules involved in the first layers of plant immunity, PTI and ETS, include inhibitors of pathogen cell-wall-degrading enzymes (CWDEs), plant pattern recognition receptors (PRRs) and susceptibility (S) proteins, while the ETI-related biomolecules include plant resistance (R) proteins. The biomolecules involved in plant defense PTI/ETI responses described herein also include antimicrobial peptides (AMPs), pathogenesis-related (PR) proteins and ribosome-inhibiting proteins (RIPs), as well as enzymes involved in plant defensive secondary metabolite biosynthesis (phytoanticipins and phytoalexins). Moreover, the regulation of immunity by RNA interference (RNAi) in GM disease-resistant plants is also considered. Therefore, the present review does not cover all the classes of biomolecules involved in plant innate immunity that may be applied in the development of disease-resistant GM crops but instead highlights the most common strategies in the literature, as well as their advantages and disadvantages.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

GENERAL SIGNIFICANCE

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

Which Plant Proteins Are Involved in Antiviral Defense? Review on In Vivo and In Vitro Activities of Selected Plant Proteins against Viruses

Plants have evolved a variety of defense mechanisms to tackle virus attack. Endogenous plant proteins can function as virus suppressors. Different types of proteins mediate defense responses against plant viruses. Pathogenesis-related (PR) proteins are activated upon pathogen infections or in different stress situations and their production is one of many components in plant defense. Ribosome-inactivating proteins (RIPs) suppress translation by enzymatically damaging ribosomes and they have been found to have antiviral activity. RNA-binding proteins (RBPs) bind to target RNAs via specialized RNA-binding domain and can directly or indirectly function in plant defense system against RNA viruses. Proteins involved in silencing machinery, namely Dicer-like (DCL) proteins, Argonaute (AGO) proteins, and RNA-dependent RNA polymerases (RDRs) confer innate antiviral defense in plants as they are able to degrade foreign RNA of viral origin. This review aims to provide a comprehensive and up-to-date picture of plant proteins participating in antiviral defense. As a result we discuss proteins conferring plant antiviral resistance and their potential future applications in different fields of life including agriculture and medicine.

The Use of Plant-Derived Ribosome Inactivating Proteins in Immunotoxin Development: Past, Present and Future Generations

Ribosome inactivating proteins (RIPs) form a class of toxins that was identified over a century ago. They continue to fascinate scientists and the public due to their very high activity and long-term stability which might find useful applications in the therapeutic killing of unwanted cells but can also be used in acts of terror. We will focus our review on the canonical plant-derived RIPs which display ribosomal RNA N-glycosidase activity and irreversibly inhibit protein synthesis by cleaving the 28S ribosomal RNA of the large 60S subunit of eukaryotic ribosomes. We will place particular emphasis on therapeutic applications and the generation of immunotoxins by coupling antibodies to RIPs in an attempt to target specific cells. Several generations of immunotoxins have been developed and we will review their optimisation as well as their use and limitations in pre-clinical and clinical trials. Finally, we endeavour to provide a perspective on potential future developments for the therapeutic use of immunotoxins.

Two Saporin-Containing Immunotoxins Specific for CD20 and CD22 Show Different Behavior in Killing Lymphoma Cells

Immunotoxins (ITs) are hybrid proteins combining the binding specificity of antibodies with the cytocidal properties of toxins. They represent a promising approach to lymphoma therapy. The cytotoxicity of two immunotoxins obtained by chemical conjugation of the plant toxin saporin-S6 with the anti-CD20 chimeric antibody rituximab and the anti-CD22 murine antibody OM124 were evaluated on the CD20-/CD22-positive cell line Raji. Both ITs showed strong cytotoxicity for Raji cells, but the anti-CD22 IT was two logs more efficient in killing, probably because of its faster internalization. The anti-CD22 IT gave slower but greater caspase activation than the anti-CD20 IT. The cytotoxic effect of both immunotoxins can be partially prevented by either the pan-caspase inhibitor Z-VAD or the necroptosis inhibitor necrostatin-1. Oxidative stress seems to be involved in the cell killing activity of anti-CD20 IT, as demonstrated by the protective role of the H₂O₂ scavenger catalase, but not in that of anti-CD22 IT. Moreover, the IT toxicity can be augmented by the contemporary administration of other chemotherapeutic drugs, such as PS-341, MG-132, and fludarabine. These results contribute to the understanding of the immunotoxin mechanism of action that is required for their clinical use, either alone or in combination with other drugs.

Transcriptomic and proteomic insight into the effects of a defined European mistletoe extract in Ewing sarcoma cells reveals cellular stress responses

BACKGROUND

The hydrophobic triterpenes, oleanolic and betulinic acid as well as the hydrophilic mistletoe lectins and viscotoxins possess anticancer properties. They do all occur in combination in European mistletoe (Viscum album L.). Commercial Viscum album L. extracts are aqueous, excluding the insoluble triterpenes. We have previously shown that mistletoe lectins and triterpene acids are effective against Ewing sarcoma in vitro, ex vivo and in vivo.

METHODS

We recreated a total mistletoe effect (viscumTT) by combining an aqueous extract (viscum) and a triterpene extract (TT) solubilised with cyclodextrins and analysed the effects of viscumTT and the single extracts on TC-71 Ewing sarcoma cells in vitro by transcriptomic and proteomic profiling.

RESULTS

Treatment with the extracts strongly impacted Ewing sarcoma cell gene and protein expression. Apoptosis-associated and stress-activated genes were upregulated, proteasomal protein abundance enhanced and ribosomal and spliceosomal proteins downregulated. The mechanism of action of viscum, TT and viscumTT in TC-71 and MHH-ES-1 cells suggests the involvement of the unfolded protein response. While viscum and viscumTT extract treatment indicate response to oxidative stress and activation of stress-mediated MAPK signalling, TT extract treatment suggests the involvement of TLR signalling and autophagy.

CONCLUSIONS

Since the combinatory extract viscumTT exerts highly effective pro-apoptotic effects on Ewing sarcoma cells in vitro, this phytopolychemotherapy could be a promising adjuvant therapeutic option for paediatric patients with Ewing sarcoma.

Extensive Evolution of Cereal Ribosome-Inactivating Proteins Translates into Unique Structural Features, Activation Mechanisms, and Physiological Roles

Ribosome-inactivating proteins (RIPs) are a class of cytotoxic enzymes that can depurinate rRNAs thereby inhibiting protein translation. Although these proteins have also been detected in bacteria, fungi, and even some insects, they are especially prevalent in the plant kingdom. This review focuses on the RIPs from cereals. Studies on the taxonomical distribution and evolution of plant RIPs suggest that cereal RIPs have evolved at an enhanced rate giving rise to a large and heterogeneous RIP gene family. Furthermore, several cereal RIP genes are characterized by a unique domain architecture and the lack of a signal peptide. This advanced evolution of cereal RIPs translates into distinct structures, activation mechanisms, and physiological roles. Several cereal RIPs are characterized by activation mechanisms that include the proteolytic removal of internal peptides from the N-glycosidase domain, a feature not documented for non-cereal RIPs. Besides their role in defense against pathogenic fungi or herbivorous insects, cereal RIPs are also involved in endogenous functions such as adaptation to abiotic stress, storage, induction of senescence, and reprogramming of the translational machinery. The unique properties of cereal RIPs are discussed in this review paper.

Ribosome-Inactivating Proteins from Plants: A Historical Overview

This review provides a historical overview of the research on plant ribosome-inactivating proteins (RIPs), starting from the first studies at the end of eighteenth century involving the purification of abrin and ricin, as well as the immunological experiments of Paul Erlich. Interest in these plant toxins was revived in 1970 by the observation of their anticancer activity, which has given rise to a large amount of research contributing to the development of various scientific fields. Biochemistry analyses succeeded in identifying the enzymatic activity of RIPs and allowed for a better understanding of the ribosomal machinery. Studies on RIP/cell interactions were able to detail the endocytosis and intracellular routing of ricin, thus increasing our knowledge of how cells handle exogenous proteins. The identification of new RIPs and the finding that most RIPs are single-chain polypeptides, together with their genetic sequencing, has aided in the development of new phylogenetic theories. Overall, the biological properties of these proteins, including their abortifacient, anticancer, antiviral and neurotoxic activities, suggest that RIPs could be utilized in agriculture and in many biomedical fields, including clinical drug development.

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

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

Functional Analysis of a Type-I Ribosome Inactivating Protein Balsamin from Momordica balsamina with Anti-Microbial and DNase Activity

Ribosome inactivating proteins (RIPs) have received considerable attention in biomedical research because of their unique activities towards tumor and virus-infected cells. We extracted balsamin, a type-I RIP, from Momordica balsamina. In the present study, a detailed investigation on DNase activity, antioxidant capacity and antibacterial activity was conducted using purified balsamin. DNase-like activity of balsamin towards plasmid DNA was pH, incubation time and temperature dependent. Moreover, the presence of Mg(2+) (10-50 mM) influenced the DNA cleavage activity. Balsamin also demonstrated reducing power and a capacity to scavenge free radicals in a dose dependent manner. Furthermore, the protein exhibited antibacterial activity against Staphylococcus aureus, Salmonella enterica, Staphylococcus epidermidis and Escherichia coli, which suggests potential utility of balsamin as a nutraceutical.

High in Vitro Anti-Tumor Efficacy of Dimeric Rituximab/Saporin-S6 Immunotoxin

The anti-CD20 mAb Rituximab has revolutionized lymphoma therapy, in spite of a number of unresponsive or relapsing patients. Immunotoxins, consisting of toxins coupled to antibodies, are being investigated for their potential ability to augment Rituximab efficacy. Here, we compare the anti-tumor effect of high- and low-molecular-weight Rituximab/saporin-S6 immunotoxins, named HMW-IT and LMW-IT, respectively. Saporin-S6 is a potent and stable plant enzyme belonging to ribosome-inactivating proteins that causes protein synthesis arrest and consequent cell death. Saporin-S6 was conjugated to Rituximab through an artificial disulfide bond. The inhibitory activity of HMW-IT and LMW-IT was evaluated on cell-free protein synthesis and in two CD20⁺ lymphoma cell lines, Raji and D430B. Two different conjugates were separated on the basis of their molecular weight and further characterized. Both HMW-IT (dimeric) and LMW-IT (monomeric) maintained a high level of enzymatic activity in a cell-free system. HMW-IT, thanks to a higher toxin payload and more efficient antigen capping, showed stronger in vitro anti-tumor efficacy than LMW-IT against lymphoma cells. Dimeric HMW-IT can be used for lymphoma therapy at least for ex vivo treatments. The possibility of using HMW-IT augments the yield in immunotoxin preparation and allows the targeting of antigens with low internalization rates.

Transition State Structure of RNA Depurination by Saporin L3

Saporin L3 from the leaves of the common soapwort is a catalyst for hydrolytic depurination of adenine from RNA. Saporin L3 is a type 1 ribosome inactivating protein (RIP) composed only of a catalytic domain. Other RIPs have been used in immunotoxin cancer therapy, but off-target effects have limited their development. In the current study, we use transition state theory to understand the chemical mechanism and transition state structure of saporin L3. In favorable cases, transition state structures guide the design of transition state analogues as inhibitors. Kinetic isotope effects (KIEs) were determined for an A14C mutant of saporin L3. To permit KIE measurements, small stem-loop RNAs that contain an AGGG tetraloop structure were enzymatically synthesized with the single adenylate bearing specific isotopic substitutions. KIEs were measured and corrected for forward commitment to obtain intrinsic values. A model of the transition state structure for depurination of stem-loop RNA (5'-GGGAGGGCCC-3') by saporin L3 was determined by matching KIE values predicted via quantum chemical calculations to a family of intrinsic KIEs. This model indicates saporin L3 displays a late transition state with the N-ribosidic bond to the adenine nearly cleaved, and the attacking water nucleophile weakly bonded to the ribosyl anomeric carbon. The transition state retains partial ribocation character, a feature common to most N-ribosyl transferases. However, the transition state geometry for saporin L3 is distinct from ricin A-chain, the only other RIP whose transition state is known.

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.

New Surface-Enhanced Raman Sensing Chip Designed for On-Site Detection of Active Ricin in Complex Matrices Based on Specific Depurination

Quick and accurate on-site detection of active ricin has very important realistic significance in view of national security and defense. In this paper, optimized single-stranded oligodeoxynucleotides named poly(21dA), which function as a depurination substrate of active ricin, were screened and chemically attached on gold nanoparticles (AuNPs, ∼100 nm) via the Au-S bond [poly(21dA)-AuNPs]. Subsequently, poly(21dA)-AuNPs were assembled on a dihydrogen lipoic-acid-modified Si wafer (SH-Si), thus forming the specific surface-enhanced Raman spectroscopy (SERS) chip [poly(21dA)-AuNPs@SH-Si] for depurination of active ricin. Under optimized conditions, active ricin could specifically hydrolyze multiple adenines from poly(21dA) on the chip. This depurination-induced composition change could be conveniently monitored by measuring the distinct attenuation of the SERS signature corresponding to adenine. To improve sensitivity of this method, a silver nanoshell was deposited on post-reacted poly(21dA)-AuNPs, which lowered the limit of detection to 8.9 ng mL(-1). The utility of this well-controlled SERS chip was successfully demonstrated in food and biological matrices spiked with different concentrations of active ricin, thus showing to be very promising assay for reliable and rapid on-site detection of active ricin.