Biological Action and Structural Characterization of Eryngitin 3 and 4, Ribotoxin-like Proteins from Pleurotus eryngii Fruiting Bodies

Ribotoxin-like proteins (RL-Ps) are specific ribonucleases found in mushrooms that are able to cleave a single phosphodiester bond located in the Sarcin-Ricin Loop (SRL) of the large rRNA. The cleaved SRL interacts differently with some ribosomal proteins (P-stalk). This action blocks protein synthesis because the damaged ribosomes are unable to interact with elongation factors. Here, the amino acid sequence of eryngitin 3 and 4, RL-Ps isolated from Pleurotus eryngii fruiting bodies, was determined to: i) obtain structural information on this specific ribonuclease family from edible mushrooms; and ii) explore the structural determinants which justify their different biological and antipathogenic activities. Indeed, eryngitin 3 (exhibited higher toxicity with respect to eryngitin 4 against tumoral cell lines and model fungi. Structurally, eryngitin 3 and 4 consist of 132 amino acids, most of them identical and exhibit a single free cysteinyl residue. The amino acidic differences between the two toxins are: i) an additional phenylalanyl residue at the N-terminus of eryngitin 3, not retrieved in eryngitin 4; and ii) an additional arginyl residue at C-terminus of eryngitin 4, not retrieved in eryngitin 3. The 3D-models of eryngitins show slight differences at the N- and C-terminal regions. In particular, the positive electrostatic surface at C-terminal of eryngitin 4 is due to the additional arginyl residue not retrieved in eryngitin 3. This additional positive charge could interfere with the binding to the SRL (substrate) or with some ribosomal proteins (P-stalk structure), during substrate recognition.