The binding of ricin to its receptor is not required for the expression of its toxicity

Ricin toxin is a toxic glycoprotein comprising two polypeptide chains, A and B, joined by a disulfide bond. The binding of its B-chain to the cell surface glycoconjugate having non-reducing terminal galactose (ricin receptors) has been assumed to allow the internalization of ricin into the cell, followed by the release of the free A-chain into cytosol, which then inhibits cellular protein synthesis in eukaryotic cells (cytotoxic effect). In order to investigate whether the binding of ricin to its receptors is essential to the expression of its toxicity, ricin was allowed to be incorporated into the cells using liposome encapsulated ricin (ricin-encapsulated liposomes). Protein synthesis in cultured Hela cells was inhibited by incubation not only with intact ricin but also with ricin-encapsulated liposomes, indicating that the binding of ricin to its receptor is not required for the expression of its toxicity.