Fungal ribotoxins: Natural protein-based weapons against insects

Bdelloid rotifers deploy horizontally acquired biosynthetic genes against a fungal pathogen

Coevolutionary antagonism generates relentless selection that can favour genetic exchange, including transfer of antibiotic synthesis and resistance genes among bacteria, and sexual recombination of disease resistance alleles in eukaryotes. We report an unusual link between biological conflict and DNA transfer in bdelloid rotifers, microscopic animals whose genomes show elevated levels of horizontal gene transfer from non-metazoan taxa. When rotifers were challenged with a fungal pathogen, horizontally acquired genes were over twice as likely to be upregulated as other genes - a stronger enrichment than observed for abiotic stressors. Among hundreds of upregulated genes, the most markedly overrepresented were clusters resembling bacterial polyketide and nonribosomal peptide synthetases that produce antibiotics. Upregulation of these clusters in a pathogen-resistant rotifer species was nearly ten times stronger than in a susceptible species. By acquiring, domesticating, and expressing non-metazoan biosynthetic pathways, bdelloids may have evolved to resist natural enemies using antimicrobial mechanisms absent from other animals.

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.

The interaction of the ribotoxin α-sarcin with complex model lipid vesicles

Fungal ribotoxins are extracellular RNases that inactivate ribosomes by cleaving a single phosphodiester bond at the universally conserved sarcin-ricin loop of the large rRNA. However, to reach the ribosomes, they need to cross the plasma membrane. It is there where these toxins show their cellular specificity, being especially active against tumoral or virus-infected cells. Previous studies have shown that fungal ribotoxins interact with negatively charged membranes, typically containing phosphatidylserine or phosphatidylglycerol. This ability is rooted on their long, non-structured, positively charged loops, and its N-terminal β-hairpin. However, its effect on complex lipid mixtures, including sphingophospholipids or cholesterol, remains poorly studied. Here, wild-type α-sarcin was used to evaluate its interaction with a variety of membranes not assayed before, which resemble much more closely mammalian cell membranes. The results confirm that α-sarcin is particularly sensitive to charge density on the vesicle surface. Its ability to induce vesicle aggregation is strongly influenced by both the lipid headgroup and the degree of saturation of the fatty acid chains. Acyl chain length is indeed particularly important for lipid mixing. Finally, cholesterol plays an important role in diluting the concentration of available negative charges and modulates the ability of α-sarcin to cross the membrane.

Antifungal Activity of Ageritin, a Ribotoxin-like Protein from Cyclocybe aegerita Edible Mushroom, against Phytopathogenic Fungi

Ageritin from poplar mushrooms is a specific endonuclease that hydrolyzes a single phosphodiester bond located in the sarcin-ricin loop (SRL) of the large rRNA, thereby blocking protein synthesis. Considering the possible biotechnological use of this enzyme, here we report its antifungal activity against virulent fungi affecting crops of economic interest. Our results show that ageritin (200 µg/plug; ~13.5 nmole) inhibits the growth of Botrytis cinerea (57%), Colletotrichum truncatum (42%), and Alternaria alternata (57%), when tested on potato dextrose agar plates. At the same time, no effect was observed against Trichoderma harzianum (a fungus promoting beneficial effects in plants). To verify whether the antifungal action of ageritin against B. cinerea and T. harzianum was due to ribosome damage, we tested ageritin in vitro on partially isolated B. cinerea and T. harzianum ribosomes. Interestingly, ageritin was able to release the Endo's fragment from both tested fungal ribosomes. We therefore decided to test the antifungal effect of ageritin on B. cinerea and T. harzianum using a different growth condition (liquid medium). Differently from the result in solid medium, ageritin can inhibit both B. cinerea and T. harzianum fungal growth in liquid medium in a concentration-dependent manner up to 35.7% and 38.7%, respectively, at the highest concentration tested (~200 µg/mL; 12 µM), and the analysis of RNA isolated from ageritin-treated cells revealed the presence of Endo's fragment, highlighting its ability to cross the fungal cell wall and reach the ribosomes. Overall, these data highlight that the efficacy of antifungal treatment to prevent or treat a potential fungal disease may depend not only on the fungal species but also on the conditions of toxin application.

A New Optimized Version of a Colorectal Cancer-Targeted Immunotoxin Based on a Non-Immunogenic Variant of the Ribotoxin α-Sarcin

Due to its incidence and mortality, cancer remains one of the main risks to human health and lifespans. In order to overcome this worldwide disease, immunotherapy and the therapeutic use of immunotoxins have arisen as promising approaches. However, the immunogenicity of foreign proteins limits the dose of immunotoxins administered, thereby leading to a decrease in its therapeutic benefit. In this study, we designed two different variants of non-immunogenic immunotoxins (IMTXA33αSDI and IMTXA33furαSDI) based on a deimmunized variant of the ribotoxin α-sarcin. The inclusion of a furin cleavage site in IMTXA33furαSDI would allow a more efficient release of the toxic domain to the cytosol. Both immunotoxins were produced and purified in the yeast Pichia pastoris and later functionally characterized (both in vitro and in vivo), and immunogenicity assays were carried out. The results showed that both immunotoxins were functionally active and less immunogenic than the wild-type immunotoxin. In addition, IMTXA33furαSDI showed a more efficient antitumor effect (both in vitro and in vivo) due to the inclusion of the furin linker. These results constituted a step forward in the optimization of immunotoxins with low immunogenicity and enhanced antitumor activity, which can lead to potential better outcomes in cancer treatment.

Unveiling the Secretome of the Fungal Plant Pathogen Neofusicoccum parvum Induced by In Vitro Host Mimicry

Neofusicoccum parvum is a fungal plant pathogen of a wide range of hosts but knowledge about the virulence factors of N. parvum and host-pathogen interactions is rather limited. The molecules involved in the interaction between N. parvum and Eucalyptus are mostly unknown, so we used a multi-omics approach to understand pathogen-host interactions. We present the first comprehensive characterization of the in vitro secretome of N. parvum and a prediction of protein-protein interactions using a dry-lab non-targeted interactomics strategy. We used LC-MS to identify N. parvum protein profiles, resulting in the identification of over 400 proteins, from which 117 had a different abundance in the presence of the Eucalyptus stem. Most of the more abundant proteins under host mimicry are involved in plant cell wall degradation (targeting pectin and hemicellulose) consistent with pathogen growth on a plant host. Other proteins identified are involved in adhesion to host tissues, penetration, pathogenesis, or reactive oxygen species generation, involving ribonuclease/ribotoxin domains, putative ricin B lectins, and necrosis elicitors. The overexpression of chitosan synthesis proteins during interaction with the Eucalyptus stem reinforces the hypothesis of an infection strategy involving pathogen masking to avoid host defenses. Neofusicoccum parvum has the molecular apparatus to colonize the host but also actively feed on its living cells and induce necrosis suggesting that this species has a hemibiotrophic lifestyle.

Characterization and cytotoxic activity of ribotoxin-like proteins from the edible mushroom Pleurotus eryngii

Ribotoxin-like proteins (RL-Ps) represent a novel specific ribonuclease family found in edible mushrooms and are able to inhibit protein synthesis. Here, we report the characterization and cytotoxic effects of four novel RL-Ps, named eryngitins, isolated from fruiting bodies of the king oyster mushroom (Pleurotus eryngii). These proteins induced formation of α-fragment from rabbit ribosomes, characteristic of their enzymatic action. The two 15 kDa eryngitins (3 and 4) are considerably more thermostable than the 21 kDa ones (1 and 2), however their overall structural features, as determined by far-UV CD spectrometry, are similar. Complete in vitro digestibility by pepsin-trypsin, and lack of cytotoxicity towards human HUVEC cells suggest low toxicity of eryngitins, if ingested. However, eryngitins exhibit cytotoxic action against insect Sf9 cells, suggesting their possible use in biotechnological applications as bioinsecticides. This cytotoxicity was not enhanced in the presence of cytolytic protein complexes based on aegerolysin proteins from Pleurotus mushrooms.

Gene Organization, Expression, and Localization of Ribotoxin-Like Protein Ageritin in Fruiting Body and Mycelium of Agrocybe aegerita

The edible mushroom Agrocybe aegerita produces a ribotoxin-like protein known as Ageritin. In this work, the gene encoding Ageritin was characterized by sequence analysis. It contains several typical features of fungal genes such as three short introns (60, 55 and 69 bp) located at the 5' region of the coding sequence and typical splice junctions. This sequence codes for a precursor of 156 amino acids (~17-kDa) containing an additional N-terminal peptide of 21 amino acid residues, absent in the purified toxin (135 amino acid residues; ~15-kDa). The presence of 17-kDa and 15-kDa forms was investigated by Western blot in specific parts of fruiting body and in mycelia of A. aegerita. Data show that the 15-kDa Ageritin is the only form retrieved in the fruiting body and the principal form in mycelium. The immunolocalization by confocal laser scanning microscopy and transmission electron microscopy proves that Ageritin has vacuolar localization in hyphae. Coupling these data with a bioinformatics approach, we suggest that the N-terminal peptide of Ageritin (not found in the purified toxin) is a new signal peptide in fungi involved in intracellular routing from endoplasmic reticulum to vacuole, necessary for self-defense of A. aegerita ribosomes from Ageritin toxicity.

The ribotoxin α-sarcin can cleave the sarcin/ricin loop on late 60S pre-ribosomes

The ribotoxin α-sarcin belongs to a family of ribonucleases that cleave the sarcin/ricin loop (SRL), a critical functional rRNA element within the large ribosomal subunit (60S), thereby abolishing translation. Whether α-sarcin targets the SRL only in mature 60S subunits remains unresolved. Here, we show that, in yeast, α-sarcin can cleave SRLs within late 60S pre-ribosomes containing mature 25S rRNA but not nucleolar/nuclear 60S pre-ribosomes containing 27S pre-rRNA in vivo. Conditional expression of α-sarcin is lethal, but does not impede early pre-rRNA processing, nuclear export and the cytoplasmic maturation of 60S pre-ribosomes. Thus, SRL-cleaved containing late 60S pre-ribosomes seem to escape cytoplasmic proofreading steps. Polysome analyses revealed that SRL-cleaved 60S ribosomal subunits form 80S initiation complexes, but fail to progress to the step of translation elongation. We suggest that the functional integrity of a α-sarcin cleaved SRL might be assessed only during translation.

Beauveria bassiana ribotoxin inhibits insect immunity responses to facilitate infection via host translational blockage

Entomopathogenic fungi are promising bio-pesticides. To facilitate infection, fungi recruit multiple virulence factors and deploy different molecular strategies to evade host immunity. Fungal ribotoxins are extracellular secreted ribonucleases (RNases) with ribotoxic cytotoxicity and insecticidal activity. However, it remains unclear whether they have further biological functions. Here we show that the entomopathogenic fungus Beauveria bassiana ribotoxin (Rib) contributes to fungal virulence by inhibiting insect host immunity. Gene deletion of Rib (ΔRib) resulted in attenuated fungal virulence during infection. Pathogenesis analysis demonstrated that Rib mainly inhibits insect immunity through modulating the reactive oxygen species (ROS) response, suppressing antimicrobial peptides (AMPs) production and retarding hyphae penetration from insect cuticles. To further confirm this immunosuppressive function, recombinant ribotoxin (rRib) protein was purified and co-injected with living or heat-killed bacteria, bacteria-derived peptidoglycan (PGN) and lipopolysaccharide (LPS) separately, which also significantly inhibited the AMPs production in Drosophila fat bodies. Furthermore, co-injection of rRib with Escherichia coli or Staphylococcus aureus significantly enhanced bacterial pathogenicity and facilitated infection. In addition, rRib injection resulted in a global inhibition of protein expression in different tissues of Drosophila adults. This work identified B. bassiana ribotoxin as a key virulence factor that inhibits insect immunity.

Classification of Aspergillus, Penicillium, Talaromyces and related genera (Eurotiales): An overview of families, genera, subgenera, sections, series and species

The Eurotiales is a relatively large order of Ascomycetes with members frequently having positive and negative impact on human activities. Species within this order gain attention from various research fields such as food, indoor and medical mycology and biotechnology. In this article we give an overview of families and genera present in the Eurotiales and introduce an updated subgeneric, sectional and series classification for Aspergillus and Penicillium. Finally, a comprehensive list of accepted species in the Eurotiales is given. The classification of the Eurotiales at family and genus level is traditionally based on phenotypic characters, and this classification has since been challenged using sequence-based approaches. Here, we re-evaluated the relationships between families and genera of the Eurotiales using a nine-gene sequence dataset. Based on this analysis, the new family Penicillaginaceae is introduced and four known families are accepted: Aspergillaceae, Elaphomycetaceae, Thermoascaceae and Trichocomaceae. The Eurotiales includes 28 genera: 15 genera are accommodated in the Aspergillaceae (Aspergillago, Aspergillus, Evansstolkia, Hamigera, Leiothecium, Monascus, Penicilliopsis, Penicillium, Phialomyces, Pseudohamigera, Pseudopenicillium, Sclerocleista, Warcupiella, Xerochrysium and Xeromyces), eight in the Trichocomaceae (Acidotalaromyces, Ascospirella, Dendrosphaera, Rasamsonia, Sagenomella, Talaromyces, Thermomyces, Trichocoma), two in the Thermoascaceae (Paecilomyces, Thermoascus) and one in the Penicillaginaceae (Penicillago). The classification of the Elaphomycetaceae was not part of this study, but according to literature two genera are present in this family (Elaphomyces and Pseudotulostoma). The use of an infrageneric classification system has a long tradition in Aspergillus and Penicillium. Most recent taxonomic studies focused on the sectional level, resulting in a well-established sectional classification in these genera. In contrast, a series classification in Aspergillus and Penicillium is often outdated or lacking, but is still relevant, e.g., the allocation of a species to a series can be highly predictive in what functional characters the species might have and might be useful when using a phenotype-based identification. The majority of the series in Aspergillus and Penicillium are invalidly described and here we introduce a new series classification. Using a phylogenetic approach, often supported by phenotypic, physiologic and/or extrolite data, Aspergillus is subdivided in six subgenera, 27 sections (five new) and 75 series (73 new, one new combination), and Penicillium in two subgenera, 32 sections (seven new) and 89 series (57 new, six new combinations). Correct identification of species belonging to the Eurotiales is difficult, but crucial, as the species name is the linking pin to information. Lists of accepted species are a helpful aid for researchers to obtain a correct identification using the current taxonomic schemes. In the most recent list from 2014, 339 Aspergillus, 354 Penicillium and 88 Talaromyces species were accepted. These numbers increased significantly, and the current list includes 446 Aspergillus (32 % increase), 483 Penicillium (36 % increase) and 171 Talaromyces (94 % increase) species, showing the large diversity and high interest in these genera. We expanded this list with all genera and species belonging to the Eurotiales (except those belonging to Elaphomycetaceae). The list includes 1 187 species, distributed over 27 genera, and contains MycoBank numbers, collection numbers of type and ex-type cultures, subgenus, section and series classification data, information on the mode of reproduction, and GenBank accession numbers of ITS, beta-tubulin (BenA), calmodulin (CaM) and RNA polymerase II second largest subunit (RPB2) gene sequences.

Heterologous Production and Functional Characterization of Ageritin, a Novel Type of Ribotoxin Highly Expressed during Fruiting of the Edible Mushroom Agrocybe aegerita

Fungi produce various defense proteins against antagonists, including ribotoxins. These toxins cleave a single phosphodiester bond within the universally conserved sarcin-ricin loop of ribosomes and inhibit protein biosynthesis. Here, we report on the structure and function of ageritin, a previously reported ribotoxin from the edible mushroom Agrocybe aegerita The amino acid sequence of ageritin was derived from cDNA isolated from the dikaryon A. aegerita AAE-3 and lacks, according to in silico prediction, a signal peptide for classical secretion, predicting a cytoplasmic localization of the protein. The calculated molecular weight of the protein is slightly higher than the one reported for native ageritin. The A. aegerita ageritin-encoding gene, AaeAGT1, is highly induced during fruiting, and toxicity assays with AaeAGT1 heterologously expressed in Escherichia coli showed a strong toxicity against Aedes aegypti larvae yet not against nematodes. The activity of recombinant A. aegerita ageritin toward rabbit ribosomes was confirmed in vitro Mutagenesis studies revealed a correlation between in vivo and in vitro activities, indicating that entomotoxicity is mediated by ribonucleolytic cleavage. The strong larvicidal activity of ageritin makes this protein a promising candidate for novel biopesticide development.IMPORTANCE Our results suggest a pronounced organismal specificity of a protein toxin with a very conserved intracellular molecular target. The molecular details of the toxin-target interaction will provide important insight into the mechanism of action of protein toxins and the ribosome. This insight might be exploited to develop novel bioinsecticides.

Ageritin, a Ribotoxin from Poplar Mushroom ( Agrocybe aegerita) with Defensive and Antiproliferative Activities

Ribotoxins make up a group of extracellular rRNA endoribonucleases produced by ascomycetes that display cytotoxicity toward animal cells, having been proposed as insecticidal agents. Recently, the ribotoxin Ageritin has been isolated from the basidiomycetes Agrocybe aegerita (poplar mushroom), suggesting that ribotoxins are widely distributed among fungi. To gain insights into the protective properties of Ageritin against pathogens and its putative biotechnological applications, we have tested several biological activities of Ageritin, comparing them with those of the well-known ribotoxin α-sarcin, and we found that Ageritin displayed, in addition to the already reported activities, (i) antibacterial activity against Micrococcus lysodeikticus, (ii) activity against the tobacco mosaic virus RNA, (iii) endonuclease activity against a supercoiled plasmid, (iv) nuclease activity against genomic DNA, (v) cytotoxicity to COLO 320, HeLa, and Raji cells by promoting apoptosis, and (vi) antifungal activity against the green mold Penicillium digitatum. Therefore, Ageritin and α-sarcin can induce resistance not only to insects but also to viruses, bacteria, and fungi. The multiple biological activities of Ageritin could be exploited to improve resistance to different pathogens by engineering transgenic plants. Furthermore, the induction of cell death by different mechanisms turns these ribotoxins into useful tools for cancer therapy.

Ageritin from poplar mushrooms: scale-up purification and cytotoxicity towards undifferentiated and differentiated SH-SY5Y cells

Ageritin from poplar mushroom is a valuable selective neurotoxin towards undifferentiated neuroblastoma SH-SY5Y cells.

High cell density fed-batch production of insecticidal recombinant ribotoxin hirsutellin A from Pichia pastoris

BACKGROUND

The fungal ribotoxin hirsutellin A (HtA) exhibits strong insecticidal activity; however, efficient systems for expressing recombinant HtA (rHtA) are lacking. Here, we established an efficient heterologous expression system to produce large amounts of rHtA.

RESULTS

Recombinant Pichia pastoris transformants with high levels of secretory rHtA were screened, and in a fed-batch reactor, rHtA was secreted at levels up to 80 mg/l following methanol induction, which was more than sixfold higher than that in shake flasks. Approximately 7 mg of highly pure rHtA was obtained from 300 ml of fed-batch culture supernatant by Ni+-nitriloacetic acid affinity chromatography and CM Sepharose ion-exchange chromatography. Mass spectrometry results revealed rHtA as a native N-terminal non-glycosylated monomeric protein with a molecular weight of 15.3 kDa. Purified rHtA exhibited excellent thermal and protease stability and dose-dependent cytotoxicity to Sf9 insect cells and insecticidal activity against Galleria mellonella larvae.

CONCLUSIONS

This is the first report of rHtA expression in P. pastoris. The rHtA was expressed at a high level under high-cell-density fed-batch fermentation and was efficiently purified using a two-step purification method. Purified rHtA exhibited thermal and protease stability, as well as appropriate bioactivities. Our results indicate that fed-batch production by P. pastoris is an efficient method to produce functional rHtA.

Antifungal Activity of α-Sarcin against Penicillium digitatum: Proposal of a New Role for Fungal Ribotoxins

Among the putative defense proteins that occur in fungi, one of the best studied is α-sarcin, produced by the mold Aspergillus giganteus. This protein is the most significant member of the ribotoxin family, which consists of extracellular rRNA ribonucleases that display cytotoxic activity toward animal cells. Ribotoxins are rRNA endonucleases that catalyze the hydrolysis of the phosphodiester bond between G4325 and A4326 from the rat 28S rRNA. The results of several experimental approaches have led to propose ribotoxins as insecticidal agents. In this work, we report that α-sarcin displays a strong antifungal activity against Penicillium digitatum, being able to enter into the cytosol where it inactivates the ribosomes, thus killing the cells and arresting the growth of the fungus. This is the first time that a ribotoxin has been found to display antifungal activity. Therefore, this protein could play, besides the already proposed insecticidal function, a role in nature as an antifungal agent.

Fungal Ribotoxins: A Review of Potential Biotechnological Applications

Fungi establish a complex network of biological interactions with other organisms in nature. In many cases, these involve the production of toxins for survival or colonization purposes. Among these toxins, ribotoxins stand out as promising candidates for their use in biotechnological applications. They constitute a group of highly specific extracellular ribonucleases that target a universally conserved sequence of RNA in the ribosome, the sarcin-ricin loop. The detailed molecular study of this family of toxic proteins over the past decades has highlighted their potential in applied research. Remarkable examples would be the recent studies in the field of cancer research with promising results involving ribotoxin-based immunotoxins. On the other hand, some ribotoxin-producer fungi have already been studied in the control of insect pests. The recent role of ribotoxins as insecticides could allow their employment in formulas and even as baculovirus-based biopesticides. Moreover, considering the important role of their target in the ribosome, they can be used as tools to study how ribosome biogenesis is regulated and, eventually, may contribute to a better understanding of some ribosomopathies.

Characterization of a new toxin from the entomopathogenic fungus Metarhizium anisopliae: the ribotoxin anisoplin

Metarhizium anisopliae is an entomopathogenic fungus relevant in biotechnology with applications like malaria vector control. Studies of its virulence factors are therefore of great interest. Fungal ribotoxins are toxic ribonucleases with extraordinary efficiency against ribosomes and suggested as potential insecticides. Here we describe this ribotoxin characteristic activity in M. anisopliae cultures. Anisoplin has been obtained as a recombinant protein and further characterized. It is structurally similar to hirsutellin A, the ribotoxin from the entomopathogen Hirsutella thompsonii. Moreover, anisoplin shows the ribonucleolytic activity typical of ribotoxins and cytotoxicity against insect cells. How Metarhizium uses this toxin and possible applications are of interest.

RNA damage in biological conflicts and the diversity of responding RNA repair systems

RNA is targeted in biological conflicts by enzymatic toxins or effectors. A vast diversity of systems which repair or ‘heal’ this damage has only recently become apparent. Here, we summarize the known effectors, their modes of action, and RNA targets before surveying the diverse systems which counter this damage from a comparative genomics viewpoint. RNA-repair systems show a modular organization with extensive shuffling and displacement of the constituent domains; however, a general ‘syntax’ is strongly maintained whereby systems typically contain: a RNA ligase (either ATP-grasp or RtcB superfamilies), nucleotidyltransferases, enzymes modifying RNA-termini for ligation (phosphatases and kinases) or protection (methylases), and scaffold or cofactor proteins. We highlight poorly-understood or previously-uncharacterized repair systems and components, e.g. potential scaffolding cofactors (Rot/TROVE and SPFH/Band-7 modules) with their respective cognate non-coding RNAs (YRNAs and a novel tRNA-like molecule) and a novel nucleotidyltransferase associating with diverse ligases. These systems have been extensively disseminated by lateral transfer between distant prokaryotic and microbial eukaryotic lineages consistent with intense inter-organismal conflict. Components have also often been ‘institutionalized’ for non-conflict roles, e.g. in RNA-splicing and in RNAi systems (e.g. in kinetoplastids) which combine a distinct family of RNA-acting prim-pol domains with DICER-like proteins.

Involvement of loop 5 lysine residues and the N-terminal β-hairpin of the ribotoxin hirsutellin A on its insecticidal activity

Ribotoxins are cytotoxic members of the family of fungal extracellular ribonucleases best represented by RNase T1. They share a high degree of sequence identity and a common structural fold, including the geometric arrangement of their active sites. However, ribotoxins are larger, with a well-defined N-terminal β-hairpin, and display longer and positively charged unstructured loops. These structural differences account for their cytotoxic properties. Unexpectedly, the discovery of hirsutellin A (HtA), a ribotoxin produced by the invertebrate pathogen Hirsutella thompsonii, showed how it was possible to accommodate these features into a shorter amino acid sequence. Examination of HtA N-terminal β-hairpin reveals differences in terms of length, charge, and spatial distribution. Consequently, four different HtA mutants were prepared and characterized. One of them was the result of deleting this hairpin [Δ(8-15)] while the other three affected single Lys residues in its close spatial proximity (K115E, K118E, and K123E). The results obtained support the general conclusion that HtA active site would show a high degree of plasticity, being able to accommodate electrostatic and structural changes not suitable for the other previously known larger ribotoxins, as the variants described here only presented small differences in terms of ribonucleolytic activity and cytotoxicity against cultured insect cells.

Efficient in vivo antitumor effect of an immunotoxin based on ribotoxin α-sarcin in nude mice bearing human colorectal cancer xenografts

Tagging of RNases, such as the ribotoxin α-sarcin, with the variable domains of antibodies directed to surface antigens that are selectively expressed on tumor cells endows cellular specificity to their cytotoxic action. A recombinant single-chain immunotoxin based on the ribotoxin α-sarcin (IMTXA33αS), produced in the generally regarded as safe (GRAS) yeast Pichia pastoris, has been recently described as a promising candidate for the treatment of colorectal cancer cells expressing the glycoprotein A33 (GPA33) antigen, due to its high specific and effective cytotoxic effect on in vitro assays against targeted cells. Here we report the in vivo antitumor effectiveness of this immunotoxin on nude mice bearing GPA33-positive human colon cancer xenografts. Two sets of independent assays were performed, including three experimental groups: control (PBS) and treatment with two different doses of immunotoxin (50 or 100 μg/ injection) (n = 8). Intraperitoneal administration of IMTXA33αS resulted in significant dose-dependent tumor growth inhibition. In addition, the remaining tumors excised from immunotoxin-treated mice showed absence of the GPA33 antigen and a clear inhibition of angiogenesis and proliferative capacity. No signs of immunotoxin-induced pathological changes were observed from specimens tissues. Overall these results show efficient and selective cytotoxic action on tumor xenografts, combined with the lack of severe side effects, suggesting that IMTXA33αS is a potential therapeutic agent against colorectal cancer.

Preparation of an engineered safer immunotoxin against colon carcinoma based on the ribotoxin hirsutellin A

Immunotoxins are chimeric proteins composed of an antibody domain that specifically directs the action of the toxic domain, resulting in the death of the targeted cells. Over recent years, immunotoxins have been widely studied and the number of different constructions has increased exponentially. Protein engineering has allowed the design of optimized versions of immunotoxins with an improved tumor binding affinity, stability or cytotoxic efficacy, although sometimes this has compromised the safety of the patient in terms of undesirable adverse secondary reactions. A triple mutant at three Trp residues (HtA3ΔW) of the ribotoxin hirsutellin A retains its specific ribonucleolytic activity, although cell internalization capacity is lacking. This toxin variant has been fused to the single chain variable fragment A33 (scFvA33). This immunoconjugate (IMTXA33HtA3ΔW) was produced in the methylotrophic yeast Pichia pastoris and purified using nickel-nitrilotriacetic acid affinity chromatography. Both target and toxic domains were characterized. The immunotoxin showed an exquisite specific binding against GPA33-positive culture cells, which results in the death of the targeted cells because of specific ribonucleolytic activity against ribosomes of the engineered hirsutellin A variant. IMTXA33HtA3ΔW represents a promising structure in the search for an improved immunotoxin without compromising the safety of patients.

Involvement of loops 2 and 3 of α-sarcin on its ribotoxic activity

Ribotoxins are a family of fungal ribosome-inactivating proteins displaying highly specific ribonucleolytic activity against the sarcin/ricin loop (SRL) of the larger rRNA, with α-sarcin as its best-characterized member. Their toxicity arises from the combination of this activity with their ability to cross cell membranes. The involvement of α-sarcin's loops 2 and 3 in SRL and ribosomal proteins recognition, as well as in the ribotoxin-lipid interactions involving cell penetration, has been suggested some time ago. In the work presented now different mutants have been prepared in order to study the role of these loops in their ribonucleolytic and lipid-interacting properties. The results obtained confirm that loop 3 residues Lys 111, 112, and 114 are key actors of the specific recognition of the SRL. In addition, it is also shown that Lys 114 and Tyr 48 conform a network of interactions which is essential for the catalysis. Lipid-interaction studies show that this Lys-rich region is indeed involved in the phospholipids recognition needed to cross cell membranes. Loop 2 is shown to be responsible for the conformational change which exposes the region establishing hydrophobic interactions with the membrane inner leaflets and eases penetration of ribotoxins target cells.

Comparative transcriptomics of the model mushroom Coprinopsis cinerea reveals tissue-specific armories and a conserved circuitry for sexual development

Background

It is well known that mushrooms produce defense proteins and secondary metabolites against predators and competitors; however, less is known about the correlation between the tissue-specific expression and the target organism (antagonist) specificity of these molecules. In addition, conserved transcriptional circuitries involved in developing sexual organs in fungi are not characterized, despite the growing number of gene expression datasets available from reproductive and vegetative tissue. The aims of this study were: first, to evaluate the tissue specificity of defense gene expression in the model mushroom Coprinopsis cinerea and, second, to assess the degree of conservation in transcriptional regulation during sexual development in basidiomycetes.

Results

In order to characterize the regulation in the expression of defense loci and the transcriptional circuitries controlling sexual reproduction in basidiomycetes, we sequenced the poly (A)-positive transcriptome of stage 1 primordia and vegetative mycelium of C. cinerea A43mutB43mut. Our data show that many genes encoding predicted and already characterized defense proteins are differentially expressed in these tissues. The predicted specificity of these proteins with regard to target organisms suggests that their expression pattern correlates with the type of antagonists these tissues are confronted with. Accordingly, we show that the stage 1 primordium-specific protein CC1G_11805 is toxic to insects and nematodes. Comparison of our data to analogous data from Laccaria bicolor and Schizophyllum commune revealed that the transcriptional regulation of nearly 70 loci is conserved and probably subjected to stabilizing selection. A Velvet domain-containing protein was found to be up-regulated in all three fungi, providing preliminary evidence of a possible role of the Velvet protein family in sexual development of basidiomycetes. The PBS-soluble proteome of C. cinerea primordia and mycelium was analyzed by shotgun LC-MS. This proteome data confirmed the presence of intracellular defense proteins in primordia.

Conclusions

This study shows that the exposure of different tissues in fungi to different types of antagonists shapes the expression pattern of defense loci in a tissue-specific manner. Furthermore, we identify a transcriptional circuitry conserved among basidiomycetes during fruiting body formation that involves, amongst other transcription factors, the up-regulation of a Velvet domain-containing protein.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-492) contains supplementary material, which is available to authorized users.