The enemy within: ricin and plant cells

Ricin, a ribosome-inactivating protein from the seeds of the castor oil plant (Ricinus communis L.) is one of the most potent cell poisons known. It is able to bind and enter most mammalian cells where it exploits their fully reversible secretory pathway to reach the endoplasmic reticulum. Ricin is then able to exit the endoplasmic reticulum to access the cytosol where it inhibits protein synthesis, thus killing the cells. Castor bean ribosomes are sensitive to ricin, but the plant has developed strategies to protect its own cells from suicide. The intracellular routing of ricin has been traditionally studied by exogenously adding toxin to mammalian cells and by following its path through the cell. However, the extreme potency of this protein has prevented the final membrane transport step from being studied in detail. Now, the expression of ricin in heterologous plant cells is proving helpful in elucidating details of both toxin biosynthesis and vacuolar targeting, and in studying membrane translocation of the catalytic subunit from the endoplasmic reticulum to the cytosol.