Ricin: structure, synthesis, and mode of action

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Abstract

Ricin is a lectin produced by the seeds of the Ricinus communis plant. It is potently toxic to mammalian cells, where it acts to inhibit the essential process of protein synthesis. Structurally, ricin is a heterodimer comprised of an enzymatic polypeptide (the A chain) disulphide bonded to a cell-binding lectin (the B chain). After surface binding, the holotoxin is internalised to endosomes from where a small fraction can be transported by a retrograde route to the endoplasmic reticulum (ER). After reduction in the ER lumen, the A chain is rendered competent for translocation to the cytosol and, whilst most is apparently degraded there, a proportion evades degradation to refold and inactivate ribosomes. In this review we present our current understanding of the biosynthesis and mode of action of this highly cytotoxic plant protein.