Retrograde transport of ricin.

The plant toxin ricin binds to terminal galactose-containing cell-surface receptors. The toxin is endocytosed and transported to the Golgi apparatus. Recent evidence suggests that ricin binds to galactosylated calreticulin, which may carry the toxin from the Golgi apparatus to the endoplasmic reticulum (ER). From the ER, the ricin A fragment is translocated to the cytosol. Ricin is perceived to be a candidate for ER-associated degradation (ERAD) and is translocated through the Sec61p translocon to the cytosol. Part of the toxin is degraded by the proteasome, but a fraction of the ricin avoids degradation and inhibits protein synthesis by inactivating ribosomes, ultimately leading to cell death.