Fungal ribotoxin
Thus, in order to penetrate the cells, they must take advantage of changes in permeability and the biophysical properties of the membranes, produced by phenomena such as tumour transformation or a viral infection.
One of the determining factors in this process of entry into cells appears to be their ability to interact with phospholipids whose polar headgroup shows a net negative electrical charge.
[7] Today it is known that ribotoxins constitute a broad family, produced by many types of fungi, with common characteristics that make them optimal candidates to be used for biotechnological purposes, such as pest control, and for the development of anti-cancer drugs in the form of immunotoxins.
[1][8][9] Ribotoxins have been detected in many different fungi,[10] including entomopathogenic[11][12] and edible species,[13] but the three-dimensional structure has only been resolved for three of them: α-sarcin,[14] restrictocin,[15] and hirsutellin A (HtA).
[12] All known ribotoxins are proteins of between 130 and 150 amino acids that share at least two different elements of ordered secondary structure: a β-sheet, where the active center is located, and a short α-helix.
These ribotoxin bonds are responsible for recognition of both the negatively charged acid phospholipids that facilitate their entry into cells, and the ribosome-specific features that allow them to cause inactivation.
Using dinucleosides, such as GpA, it has been demonstrated that the breakage of the phosphodiester bond 3′-5′ of the substrate takes place through the formation of a cyclic intermediate that becomes the corresponding derivative 3′-monophosphate, the final product of the reaction.
In both cases, both ribotoxins and RIPs produce complete inactivation of the ribosome by causing the SRL loop to be unable to interact with the elongation factors of the translation.
[26] The positively charged ribotoxin surface allows them to establish favourable electrostatic interactions between the residues of their active site and the rRNA, explaining why they can carry out this highly specific recognition of the SRL.
[30] The discovery that the entomopathogenic fungus Hirsutella thompsonii synthesized HtA,[11] followed by the recent characterization of anisopline,[12] suggests the possibility that ribotoxins behave as insecticidal proteins.
The additional benefit of not showing any detectable undesirable side effects, most likely due to the highly specific recognition of the antigen by the antibody used,[1][33][34] makes them attractive for the therapeutic treatment of certain solid tumors.
This approach has recently been improved with the incorporation of different artificial variants of ribotoxins, such as one that cannot cross the membranes on its own, but retains the ribosome inactivating activity,[35] or a de-immunized version of α-sarcin which, in vitro, has been proven incapable of triggering a T-lymphocyte response.
