Final answer:
Ricin inhibits protein synthesis by cleaving a specific adenine in the 28S rRNA of the 60S ribosomal subunit, leading to cell death. It gained attention in 2003 when it was detected in a letter to the White House but caused no harm.
Step-by-step explanation:
The mechanism of action (MOA) of ricin, a toxin from the castor oil plant or castor beans, is that it acts as a potent inhibitor of protein synthesis. Ricin achieves this by inactivating ribosomes in the cell, which are the molecular machines responsible for making proteins. Specifically, ricin A chain enzymatically cleaves a specific adenine base from the 28S rRNA within the 60S ribosomal subunit of eukaryotic ribosomes. This cleavage event depurinates the ribosome, rendering it unable to bind elongation factors and thus halting protein synthesis, leading to cell death. In 2003, ricin was notably found in a letter addressed to the White House; however, thankfully, it did not result in any illnesses or fatalities.