Proteopedia entry: Ricin

This article describes a Proteopedia page that illustrates the structure of ricin, its enzymatic mechanism, and the site of depurination of the ribosome. (Figure 1; Link: http://proteopedia.org/wiki/index.php/Ricin). Ricin seems to be the preferred toxin for hate mail 1, but has not succeeded despite the high toxicity of ricin. The only confirmed assassination with ricin was of Bulgarian dissident Georgi Markov in 1978 2, who was injected with a ricin-coated pellet. Although ricin is toxic at a microgram level 3, it must gain entry into the circulation and then into the cells to have an effect. Although ricin was considered for development as a bioweapon, difficulties in forming aerosols 4 and the shortened environmental lifespan of ricin have limited its implementation 5. However, a larger risk may be the castor bean, the source material of ricin; consumption of just a few castor beans by a child or livestock can be toxic 6. Ricin is a type II ribosomal inhibitory protein (RIP). As its name suggests, the primary mechanism of ricin toxicity is inhibition of ribosomal action via depurination of A-4324 of the 28S rRNA subunit of the eukaryotic ribosome 7. This is the site of binding for elongation factors EF-2 (in eukaryotes) and EF-G (in prokaryotes). Type I RIPs have only the catalytic subunit, whereas Type II RIPs contain a lectin subunit which facilitates entry into the cell. In ricin, the two subunits are linked via a disulfide bond which is cleaved in the endoplasmic reticulum after endocytosis 8.