Ribotoxin

Ribotoxin is a class of ribosome-inactivating proteins (RIPs) that inactivate eukaryotic ribosomes by enzymatically modifying ribosomal RNA. They typically act on the large subunit and target a conserved region called the sarcin-ricin loop in the 28S rRNA, a critical site for binding elongation factors. The enzymatic action either depurinates a specific adenine residue or cleaves the rRNA, leading to disruption of protein synthesis and irreversible inhibition of translation.

Ribotoxins are produced by several filamentous fungi and other microorganisms. Notable examples include alpha-sarcin and restrictocin, both of which have been studied for their potent ribosome-targeting activity. These proteins are generally single-chain polypeptides of around 20–30 kDa, and they can be extremely potent toxins on exposure.

Because of their high potency and specificity for ribosomes, ribotoxins are used as tools in molecular biology to study ribosome structure and function. They have also been explored as components of targeted cytotoxins (immunotoxins) for cancer therapy, where they are delivered to cancer cells by antibody or ligand targeting. In research, safer, inactivated forms or controlled delivery systems are used to minimize risk.

Due to extreme toxicity, ribotoxins require stringent biosafety handling and regulatory oversight in laboratories and are not suitable for casual exposure.