Ricin A chain utilises the endoplasmic reticulum-associated protein degradation pathway to enter the cytosol of yeast.

Cytotoxic proteins such as ricin A chain (RTA) have target substrates in the cytosol and therefore have to reach this cellular compartment in order to act. RTA is thought to translocate into the cytosol from the lumen of the endoplasmic reticulum (ER), although how it traverses the ER membrane has not been established. Using yeast mutants defective in various aspects of the ER-associated protein degradation (ERAD) pathway, we show that RTA introduced into the yeast ER subverts this pathway to enter the cytosol via the Sec61p translocon. A significant proportion of the exported RTA avoided proteasomal degradation. These data are consistent with the contention that the RTA component from ricin endocytosed by mammalian cells may likewise exploit ERAD to translocate into the cytosol.