Ricin: the endoplasmic reticulum connection.

Ricin is a potent, plant-derived, ribosome inactivating protein. To target ribosomes in the mammalian cytosol, ricin must firstly negotiate the endomembrane system of the cell to reach the endoplasmic reticulum. Here, the toxin is reduced and the catalytic A chain is recognised by ER components that facilitate its membrane translocation to the cytosol. To be toxic, ricin A chain must then avoid degradation, a conundrum made more tricky in that ubiquitination and proteasomal degradation are normally tightly coupled to the translocation process. This mini-review summarises current understanding of these events.