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 th e cell surface glycoconjugate having non-reducing terminal galactose ( ricin receptors) has been assumed to allow the internalization of rici n into the cell, followed by the release of the free A-chain into cyst osol, which then inhibits cellular protein synthesis in eukaryotic cel ls (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 tells using liposome en capsulated ricin (ricin-encapsulated liposomes), Protein synthesis in cultured Hela cells was inhibited by incubation not only with intact r icin but also with ricin-encapsulated liposomes, indicating that the b inding of ricin to its receptor is net required for the expression of its toxicity.