Molecular action of tricholin, a ribosome-inactivating protein isolated from Trichoderma viride

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.

Peptides and proteins from fungi

The peptides and proteins secreted by fungi are reviewed in this article. They include ribosome inactivating peptides and proteins, antifungal peptides and proteins, lectins, ubiquitin-like peptides and proteins, peptides and proteins with nucleolytic activity, proteases, xylanases, cellulases, sugar oxidoreductases, laccases, invertases, trehalose phosphorylases, and various enzymes with applications in food industry, chemical production and the medical sector.

Mycotoxin production by indoor molds

Fungal growth in buildings starts at a water activity (a(w)) near 0.8, but significant quantities of mycotoxins are not produced unless a(w) reaches 0.95. Stachybotrys generates particularly high quantities of many chemically distinct metabolites in water-damaged buildings. These metabolites are carried by spores, and can be detected in air samples at high spore concentrations. Very little attention has been paid to major metabolites of Stachybotrys called spirocyclic drimanes, and the precise structures of the most abundant of these compounds are unknown. Species of Aspergillus and Penicillium prevalent in the indoor environment produce relatively low concentrations of mycotoxins, with the exception of sterigmatocystins that can represent up to 1% of the biomass of A. versicolor at a(w)'s close to 1. The worst-case scenario for homeowners is produced by consecutive episodes of water damage that promote fungal growth and mycotoxin synthesis, followed by drier conditions that facilitate the liberation of spores and hyphal fragments.

Isolation and characterization of viridin, a new 65 kDa antifungal protein from the mould Trichoderma viride

A new extracellular antifungal protein with a yield of 10 mg per liter was isolated from the culture medium of the mould Trichoderma viride. The protein, which we named viridin, was purified by carboxymethyl-cellulose cation-exchange chromatography and Superose 12 HR 10/30 high-performance liquid chromatography. Viridin, a basic protein of approximately 65 kDa as determined by SDS-PAGE, inhibits the growth of the cotton pathogen Verticillum dahliae, the IC50 being 6 microM.

Characterization of a new ribotoxin gene (c-sar) from Aspergillus clavatus

A new ribotoxin, c-sarcin, was isolated from a culture of Aspergillus clavatus. A full-length genomic DNA (c-sar) coding for c-sarcin was cloned and sequenced. The deduced amino acid sequence showed a high homology to restrictocin and alpha-sarcin. The native toxin as well as the recombinant protein hydrolysed ribosomes and naked RNA. The genomic structure of the c-sar gene had an intron located between the coding sequences for secretory signal peptide and the mature protein. The intron contained a stretch of 38 adenines. The intron sequence of c-sar was different from that of restrictocin but resembled that of alpha-sarcin. There was 34% identity between the intron of c-sarcin and alpha-sarcin, and this similarity was further increased to 83% if the stretch of polyadenine was omitted.

Insights into specificity of cleavage and mechanism of cell entry from the crystal structure of the highly specific Aspergillus ribotoxin, restrictocin

BACKGROUND

Restriction, a highly specific ribotoxin made by the fungus Aspergillus restrictus, cleaves a single phosphodiester bond in the 28S RNA of eukaryotic ribosomes, inhibiting protein synthesis. The sequence around this cleavage site is a binding site for elongation factors, and is conserved in all cytoplasmic ribosomes. The catalytic mechanism of restrictocin and the reasons for its high substrate specificity are unknown. No structure has been determined for any other member of the Aspergillus ribotoxin family.

RESULTS

The crystal structure of restrictocin was determined at 2.1 A resolution by single isomorphous replacement and anomalous scattering techniques, and refined to 1.7 A resolution using synchrotron Laue data. The structural core of the protein, in which a three-turn alpha helix is packed against a five-stranded antiparallel beta sheet, can be well aligned with that of ribonuclease T1. Large positively charged peripheral loops near the active site construct a platform with a concave surface for RNA binding.

CONCLUSIONS

Restriction appears to combine the catalytic components of T1 ribonucleases with the base recognition components of Sa ribonucleases. Modeling studies using an NMR structure of an RNA substrate analog suggest that the tertiary structure of the substrate RNA is important in protein-RNA recognition, fitting closely into the concavity of the presumed binding site. We speculate that the large 39-residue loop L3, which has similarities to loops found in lectin sugar-binding domains, may be responsible for restrictocin's ability to cross cell membranes.

Substrate specificity of monomeric and dimeric alpha-sarcin

The substrate specificity of monomeric and dimeric forms of alpha-sarcin was investigated by membrane blotting procedures. Dimeric alpha-sarcin fails to inactivate ribosomes as well as to hydrolyze mini-stem-loop RNA, whereas monomeric alpha-sarcin catalyzes both substrates. Both monomeric and dimeric alpha-sarcin are effective ribonucleases that are displayed by in situ RNA-impregnated gel electrophoresis. The same purine base specificity was detected for both dimeric and monomeric forms. alpha-Sarcin is also an effective deoxyribonuclease to supercoiled DNA. The action of alpha-sarcin as deoxyribonuclease and ribonuclease is inhibited by the presence of SDS (3.5 x 10(-6) M); the inhibition on ribonuclease, but not on deoxyribonuclease, is reversible if the proteins are renatured.

Fungal ribotoxins: a family of naturally engineered targeted toxins?

alpha-Sarcin, mitogillin, and restrictocin are small (approximately 17 kDa) basic robosome-inactivating proteins (RIPs) produced by the Aspergilli that catalytically inactivate the large ribosomal subunits of all organisms tested to date. These three fungal ribotoxins act as specific ribonucleases by hydrolyzing one single phosphodiester bond in the universally conserved alpha-sarcin domain of 23-28S rRNAs and are among the most potent inhibitors of protein synthesis known. Previous molecular studies of ribotoxins indicated that they belong to the superfamily of ribonucleases and analysis of the mitogillin gene employing PCR-mediated site-specific mutagenesis suggests that certain domains in ribotoxins, which share homologies with motifs in ribosome-related proteins, may be responsible for the targeting of ribotoxins to the ribosome. The applications of the ribotoxins as tools in research and their uses as therapeutic and diagnostic agents are also reviewed in this paper.

Determination of base specificity of multiple ribonucleases from crude samples

Ribonucleases are widely found on the tissues of living organisms, but the functions of individual ribonucleases are not clear. To facilitate characterization of individual ribonucleases, I have developed a rapid method to separate and identify each ribonuclease from a crude sample by gel electrophoresis instead of by time-consuming purification steps. The ribonucleases in a crude sample are first separated by RNA-cast SDS-polyacrylamide gel electrophoresis and then eluted from the gel after ethidium bromide staining. To determine the base specificity of each ribonuclease, a 5' labelled oligonucleotide with known sequence is added to the enzyme eluate and the digested products are analyzed by denaturing gel electrophoresis. The base specificity of bovine pancreatic ribonuclease (RNase A), bullfrog oocyte-specific ribonuclease (RC-RNase), human serum ribonucleases and sweet potato leaf ribonucleases were determined by this method. Other properties of individual ribonucleases, e.g. substrate preference, may also be determined from crude samples by this method without further purification steps.

Production of type II ribotoxins by Aspergillus species and related fungi in Taiwan

A molecular investigation was conducted on the production of type II ribotoxin of the species Aspergillus and related fungi in Taiwan. Species that carried ribotoxin were confirmed by (1) cross-reactivity to anti-alpha-sarcin serum; (2) Southern dot hybridization; (3) PCR amplification of genomic DNA with specific primers; and (4) analysis of ribotoxic activity. Five new strains, A. clavatus, A. oryzae var. effusus, A. ostianus, A. tamarii, and Neosartorya fischeri var. spinosa, were identified to contain an alpha-sarcin-like ribotoxin. These positive strains exhibit ribotoxic activity by cleaving ribosomes and generating an alpha-fragment.