Ricin toxin B chain (RTB) (RTB)

The Ricin toxin B chain (RTB) is a 262-amino-acid lectin from the castor bean plant (Ricinus communis) that forms one half of the heterodimeric ricin toxin, linked by a disulfide bond to the enzymatically active A chain. RTB exhibits a bilobal, barbell-like structure with two galactose-binding sites, enabling it to recognize and bind terminal β-D-galactose or N-acetylgalactosamine residues on eukaryotic cell surface glycoproteins and glycolipids, as well as mannose-type glycans on certain cells like macrophages. This binding promotes receptor-mediated endocytosis, intracellular trafficking through endosomes, and translocation of the A chain to the cytosol, where it depurinates 28S rRNA to halt protein synthesis and induce cell death. While ricin itself is a potent toxin causing fatal pulmonary edema upon inhalation, RTB's role is primarily facilitative for toxicity rather than catalytic. In therapeutic contexts, RTB has been explored in immunotoxins, where it enhances targeted delivery of the A chain to cancer cells, though challenges include off-target binding and vascular leak syndrome. No drugs directly target RTB as a therapeutic receptor or enzyme; instead, efforts focus on inhibitors to neutralize ricin exposure.

Other names

Ricin B chainRTBRCL III B chain

Competitive inhibition of galactose-binding sites to block cell surface attachment and endocytosis, steric hindrance in immunotoxins to reduce non-specific binding