Endoplasmic reticulum-associated degradation of ricin A chain has unique and plant-specific features.

Proteins that fail to fold in the endoplasmic reticulum (ER), or that cannot find a pattern for assembly, are often disposed by a process named ER-associated degradation (ERAD) and that involves transport of the substrate protein across the ER membrane (dislocation) followed by rapid proteasome-mediated proteolysis. Different ERAD substrates have been shown to be ubiquitinated during or soon after dislocation, and an active ubiquitination machinery has been found to be required for the dislocation of certain defective proteins. We have previously shown that, when expressed in tobacco protoplasts, the A chain of the heterodimeric toxin ricin is degraded by a pathway that closely resembles ERAD, but that is characterised by an unusual uncoupling between the dislocation and the degradation steps. Since lysine residues are a major target for ubiquitination, we have investigated the effects of changing the lysine content on the retrotranslocation and degradation of ricin A chain in tobacco protoplasts. Here we show that modulating the number of lysine residues does not affect recognition events within the ER lumen, nor the transport of the protein from this compartment to the cytosol. Rather, the introduced modifications have a clear impact on the degradation of the dislocated protein. While the substitution of the two lysine residues present in ricin A chain with arginine slowed down degradation, the introduction of four extra lysyl residues had an opposite effect, and converted ricin A chain to a standard ERAD substrate that is disposed via a process in which dislocation and degradation steps are tightly coupled.