Fungal ribotoxins: a family of naturally engineered targeted toxins?

Abstract

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.

Use of Recombinant Mitogillin for Improved Serodiagnosis ofAspergillus fumigatus-Associated Diseases

“…Mitogillin is a small basic protein of approximately 18 kDa with cytotoxic activity released by A. fumigatus (16,17,31). Together with the related toxins A. fumigatus allergen I (AspfI), restrictocin from Aspergillus restrictus (20), and ␣-sarcin from Aspergillus giganteus (26), mitogillin is a member of a family of conserved RNases that cleave a single phosphodiester bond of the 29S rRNA of eukaryotic ribosomes (11,16,17).…”

The Therapeutic Potential of Fungal Ribotoxins

“…These mutational studies have revealed the involvement of several residues, which are conserved among the different microbial extracellular RNases, in catalysis. Thus, it is well known that His137 and Glu96 are the only residues that are essential for the cleavage reaction performed by α-sarcin [35,45,[70][71][72][73][74][75], whereas His-50, Tyr-48, Arg-121, and Leu-145 mostly contribute to the stabilization of the transition state [45][46][47][48], as stated above. Most of these residues are located in the central β-sheet and their side-chains point towards the concave face of the protein structure where the substrate is supposed to dock [29].…”

“…α-Sarcin is able to inactivate the ribosomes of a great variety of organisms in cell-free systems [4,35], but it displays marked selectivity when intact cells are used as targets. This specificity seems to be determined by its ability to penetrate cells, as explained above.…”

“…In summary, all the features outlined above suggest that the study of the evolution and the mechanism of action of ribotoxins is of particular interest, as they appear to be naturally engineered targeted toxins evolved from the other microbial nontoxic RNases to enter cells and specifically inactivate the 5 ribosomes [34,35]. Identification of the structural features that have allowed these proteins to become such efficient natural killers would be a major step towards their utilization, native or modified, as weapons against different human pathologies.…”

Immune Responses of Human Immature Dendritic Cells Can Be Modulated by the Recombinant Aspergillus fumigatus Antigen Aspf1

“…Among these are two proteins that have been used in this study: the 18-kDa immunoglobulin E (IgE)-binding protein Aspf1 and the glycosylphosphatidylinositol-anchored extracellular cell wall glucanase Crf1 (2). Aspf1 is a member of a family of conserved RNases that cleave defined phosphodiester bonds of the 28S rRNA of eukaryotic ribosomes (3,16). In this study, we analyzed different aspects of the response of human monocyte-derived DCs to Crf1 and, especially, Aspf1.…”