Ricin Trafficking in Cells

Abstract

The heterodimeric plant toxin ricin binds exposed galactosyls at the cell surface of target mammalian cells, and, following endocytosis, is transported in vesicular carriers to the endoplasmic reticulum (ER). Subsequently, the cell-binding B chain (RTB) and the catalytic A chain (RTA) are separated reductively, RTA embeds in the ER membrane and then retrotranslocates (or dislocates) across this membrane. The protein conducting channels used by RTA are usually regarded as part of the ER-associated protein degradation system (ERAD) that removes misfolded proteins from the ER for destruction by the cytosolic proteasomes. However, unlike ERAD substrates, cytosolic RTA avoids destruction and folds into a catalytic conformation that inactivates its target ribosomes. Protein synthesis ceases, and subsequently the cells die apoptotically. This raises questions about how this protein avoids the pathways that are normally sanctioned for ER-dislocating substrates. In this review we focus on the molecular events that occur with non-tagged ricin and its isolated subunits at the ER–cytosol interface. This focus reveals that intra-membrane interactions of RTA may control its fate, an area that warrants further investigation.

Toxicity of ricin A chain is reduced in mammalian cells by inhibiting its interaction with the ribosome

“…This is interesting in view of our in vitro data showing that depurination by R193A/R235A was dramatically reduced relative to G212E. A possible explanation is that in vivo , R193A/R235A may have better access to the ribosome because chaperone proteins help fold RTA (Spooner and Lord, 2015), while in vitro the interaction of RTA with the ribosome would be solely dependent on electrostatic interactions. In yeast, the expression of RTA is controlled by the galactose inducible GAL1 promoter and it is therefore possible to see early differences in depurination (at zero or 1 h after induction).…”

Pathogenic Hijacking of ER-Associated Degradation: Is ERAD Flexible?

“…ERAD exploitation by bacterial CTx and plant ricin is well characterized (Spooner and Lord, 2015;Wernick et al, 2010). CTx consists of A1 and A2 chains, which are derived from a single A chain (27 kDa) by limited digestion in the host intestine, and five B chains (each 11.5 kDa).…”

Yeast Reporter Assay to Identify Cellular Components of Ricin Toxin A Chain Trafficking

“…Over the years, intensive analyses of ricin trafficking in mammalian cells identified a complex network of pathways that is parasitized by the toxin [4,5,6,7,8,9]. Although intoxication initiates by RTB binding to terminal galactose and/or N -acetylglucosamine residues in cell surface proteins or lipids, followed by ricin uptake through clathrin-dependent as well as -independent endocytosis and vesicular transport to early endosomes, RTA without RTB is likewise capable to kill yeast and mammalian cells (IC 50 of 50–100 µg/mL), though cell killing is much more efficient for ricin holotoxin (IC 50 of 2 ng/mL) [10,11,12,13,14,15,16,17].…”