Mechanism of specific target recognition and RNA hydrolysis by ribonucleolytic toxin restrictocin

The Toll pathway mediates Drosophila resilience to Aspergillus mycotoxins through specific Bomanins

Host defense against infections encompasses both resistance, which targets microorganisms for neutralization or elimination, and resilience/disease tolerance, which allows the host to withstand/tolerate pathogens and repair damages. In Drosophila, the Toll signaling pathway is thought to mediate resistance against fungal infections by regulating the secretion of antimicrobial peptides, potentially including Bomanins. We find that Aspergillus fumigatus kills Drosophila Toll pathway mutants without invasion because its dissemination is blocked by melanization, suggesting a role for Toll in host defense distinct from resistance. We report that mutants affecting the Toll pathway or the 55C Bomanin locus are susceptible to the injection of two Aspergillus mycotoxins, restrictocin and verruculogen. The vulnerability of 55C deletion mutants to these mycotoxins is rescued by the overexpression of Bomanins specific to each challenge. Mechanistically, flies in which BomS6 is expressed in the nervous system exhibit an enhanced recovery from the tremors induced by injected verruculogen and display improved survival. Thus, innate immunity also protects the host against the action of microbial toxins through secreted peptides and thereby increases its resilience to infection.

Fungal proteinaceous compounds with multiple biological activities

Fungi comprise organisms like molds, yeasts and mushrooms. They have been used as food or medicine for a long time. A large number of fungal proteins or peptides with diverse biological activities are considered as antibacterial, antifungal, antiviral and anticancer agents. They encompass proteases, ribosome inactivating proteins, defensins, hemolysins, lectins, laccases, ribonucleases, immunomodulatory proteins, and polysaccharopeptides. The target of the present review is to update the status of the various bioactivities of these fungal proteins and peptides and discuss their therapeutic potential.

Ribotoxin restrictocin manifests anti-HIV-1 activity through its specific ribonuclease activity

Restrictocin, a highly specific ribonuclease produced by Aspergillus restrictus, cleaves a single phosphodiester bond in a universally conserved stem and loop structure termed sarcin/ricin loop within the large ribosomal RNA of all organisms. In the current study, we demonstrate restrictocin to manifest anti-HIV-1 activity in two model cell systems. Using two mutants of restrictocin, we further show that the anti-HIV-1 activity of restrictocin is due to its specific ribonucleolytic activity. The study suggests that restrictocin is able to recognize region(s) within HIV-1 genome as its target. Restrictocin appears to have potential as a therapeutic antiviral agent against HIV-1.

The ribotoxin restrictocin recognizes its RNA substrate by selective engagement of active site residues

Restrictocin and related fungal endoribonucleases from the α-sarcin family site-specifically cleave the sarcin/ricin loop (SRL) on the ribosome to inhibit translation and ultimately trigger cell death. Previous studies showed that the SRL folds into a bulged-G motif and tetraloop, with restrictocin achieving a specificity of ∼1000-fold by recognizing both motifs only after the initial binding step. Here, we identify contacts within the protein−RNA interface and determine the extent to which each one contributes to enzyme specificity by examining the effect of protein mutations on the cleavage of the SRL substrate compared to a variety of other RNA substrates. As with other biomolecular interfaces, only a subset of contacts contributes to specificity. One contact of this subset is critical, with the H49A mutation resulting in quantitative loss of specificity. Maximum catalytic activity occurs when both motifs of the SRL are present, with the major contribution involving the bulged-G motif recognized by three lysine residues located adjacent to the active site: K110, K111, and K113. Our findings support a kinetic proofreading mechanism in which the active site residues H49 and, to a lesser extent, Y47 make greater catalytic contributions to SRL cleavage than to suboptimal substrates. This systematic and quantitative analysis begins to elucidate the principles governing RNA recognition by a site-specific endonuclease and may thus serve as a mechanistic model for investigating other RNA modifying enzymes.

Fungal ribotoxins: molecular dissection of a family of natural killers

RNase T1 is the best known representative of a large family of ribonucleolytic proteins secreted by fungi, mostly Aspergillus and Penicillium species. Ribotoxins stand out among them by their cytotoxic character. They exert their toxic action by first entering the cells and then cleaving a single phosphodiester bond located within a universally conserved sequence of the large rRNA gene, known as the sarcin-ricin loop. This cleavage leads to inhibition of protein biosynthesis, followed by cellular death by apoptosis. Although no protein receptor has been found for ribotoxins, they preferentially kill cells showing altered membrane permeability, such as those that are infected with virus or transformed. Many steps of the cytotoxic process have been elucidated at the molecular level by means of a variety of methodological approaches and the construction and purification of different mutant versions of these ribotoxins. Ribotoxins have been used for the construction of immunotoxins, because of their cytotoxicity. Besides this activity, Aspf1, a ribotoxin produced by Aspergillus fumigatus, has been shown to be one of the major allergens involved in allergic aspergillosis-related pathologies. Protein engineering and peptide synthesis have been used in order to understand the basis of these pathogenic mechanisms as well as to produce hypoallergenic proteins with potential diagnostic and immunotherapeutic applications.

Structural and functional studies of RegB, a new member of a family of sequence-specific ribonucleases involved in mRNA inactivation on the ribosome

The RegB endoribonuclease participates in the bacteriophage T4 life cycle by favoring early messenger RNA breakdown. RegB specifically cleaves GGAG sequences found in intergenic regions, mainly in translation initiation sites. Its activity is very low but can be enhanced up to 100-fold by the ribosomal 30 S subunit or by ribosomal protein S1. RegB has no significant sequence homology to any known protein. Here we used NMR to solve the structure of RegB and map its interactions with two RNA substrates. We also generated a collection of mutants affected in RegB function. Our results show that, despite the absence of any sequence homology, RegB has structural similarities with two Escherichia coli ribonucleases involved in mRNA inactivation on translating ribosomes: YoeB and RelE. Although these ribonucleases have different catalytic sites, we propose that RegB is a new member of the RelE/YoeB structural and functional family of ribonucleases specialized in mRNA inactivation within the ribosome.

NMR structure of the noncytotoxic alpha-sarcin mutant Delta(7-22): the importance of the native conformation of peripheral loops for activity

The deletion mutant Delta(7-22) of alpha-sarcin, unlike its wild-type protein counterpart, lacks the specific ability to degrade rRNA in intact ribosomes and exhibits an increased unspecific ribonuclease activity and decreased interaction with lipid vesicles. In trying to shed light on these differences, we report here on the three-dimensional structure of the Delta(7-22) alpha-sarcin mutant using NMR methods. We also evaluated its dynamic properties on the basis of theoretical models and measured its correlation time (6.2 nsec) by time-resolved fluorescence anisotropy. The global fold characteristic of ribotoxins is preserved in the mutant. The most significant differences with respect to the alpha-sarcin structure are concentrated in (1) loop 2, (2) loop 3, which adopts a new orientation, and (3) loop 5, which shows multiple conformations and an altered dynamics. The interactions between loop 5 and the N-terminal hairpin are lost in the mutant, producing increased solvent accessibility of the active-site residues. The degree of solvent exposure of the catalytic His 137 is similar to that shown by His 92 in RNase T1. Additionally, the calculated order parameters of residues belonging to loop 5 in the mutant correspond to an internal dynamic behavior more similar to RNase T1 than alpha-sarcin. On the other hand, changes in the relative orientation of loop 3 move the lysine-rich region 111-114, crucial for substrate recognition, away from the active site. All of the structural and dynamic data presented here reveal that the mutant is a hybrid of ribotoxins and noncytotoxic ribonucleases, consistent with its biological properties.

Role of cis prolines 112 and 126 in the functional activity of ribonucleolytic toxin restrictocin

Restrictocin is a 149 amino acid ribonucleolytic toxin produced by the fungus Aspergillus, which specifically cleaves a single phosphodiester bond within 28S rRNA resulting in a potent inhibition of protein synthesis in eukaryotic cells. Restrictocin has 12 prolines out of which three at positions 48, 112, and 126 are cis. Prolines at position 112, 118, and 126 were individually mutated to alanine to investigate their role in the catalytic and membrane interaction activity of restrictocin. All mutants were expressed in Escherichia coli, and recombinant proteins purified to homogeneity. Mutation of P112 resulted in a remarkable 50- and 100-fold reduction, respectively, in the ribonucleolytic and cytotoxic activities of restrictocin, whereas the interaction of P112A with phospholipid membranes increased. Mutants P118A and P126A exhibited 3-5-fold decreased ribonucleolytic and cytotoxic activities, however, their membrane interaction activity was marginally reduced compared to restrictocin. The study demonstrates that P112 is absolutely essential to maintain the functionally active conformation of restrictocin. Also, prolines 112, 118, and 126 do not appear to be directly involved in the membrane interaction activity of restrictocin.

Analysis by systematic deletion of amino acids of the action of the ribotoxin restrictocin

A series of contiguous deletions were made in a cDNA encoding the ribonuclease restrictocin with the purpose of identifying the amino acids that are essential for the cleavage of the phosphodiester bond on the 3' side of G4325 in the alpha-sarcin/ricin domain of mammalian (rat) 28S rRNA. In all 93 of 149 amino acids, 62% of the residues in restrictocin, were not essential for the action of the toxin. Of the five residues that have been proposed to constitute the active site, three could be deleted without loss of activity if they were part of a deletion of three or five amino acids but not if they were removed singly. It is likely that the loss of these three residues is compensated for by a neighboring residue that occupies the structural space created by the larger amino acid deletions. This was demonstrated to be the case for the active site residue Glu95 which in the deletion mutant Delta91-95 is replaced by Asp90. Systematic deletion of amino acids is a rapid, cost effective method for identifying the residues in a protein likely to contribute directly to function and, hence, deserving of closer scrutiny. Moreover, a semiquantitative estimate of the contribution of the residue to function can be made. For this reason the method may be useful for functional proteomics.