Ribosome depurination by ricin leads to inhibition of endoplasmic reticulum stress-induced mRNA splicing on the ribosome.

Ricin undergoes retrograde transport to the endoplasmic reticulum (ER), and ricin toxin A chain (RTA) enters the cytosol from the ER. Previous reports indicated that RTA inhibits activation of the unfolded protein response (UPR) in yeast and in mammalian cells. Both precursor (preRTA) and mature form of RTA (mRTA) inhibited splicing of ( for uninduced) mRNA, suggesting that UPR inhibition occurred on the cytosolic face of the ER.