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.
Keywords: GAGA tetraloop; SRL; basidiomycetes; entomotoxicity; gene expression; rRNA; riboendonuclease; ribosome; α-sarcin.
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