Plant Physiology Preview
Published on December 23, 2004; 10.1104/pp.104.055434
Received October 22, 2004
Returned for revision November 9, 2004
Accepted November 9, 2004
Endoplasmic Reticulum-Associated Degradation of Ricin A Chain Has Unique and Plant-Specific Features
Alessandra Di Cola , Lorenzo Frigerio , J. Michael Lord , Lynne M. Roberts , and Aldo Ceriotti *
Department of Biological Sciences, University of Warwick, Coventry CV4 7AL, United Kingdom
Istituto di Biologia e Biotecnologia Agraria, Consiglio Nazionale delle Ricerche, 20133 Milan, Italy
* Corresponding author; email: ceriotti{at}ibba.cnr.it.
Proteins that fail to fold in the endoplasmic reticulum (ER) or cannot find a pattern for assembly are often disposed of by a process named ER-associated degradation (ERAD), which 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 (Nicotiana tabacum) protoplasts, the A chain of the heterodimeric toxin ricin is degraded by a pathway that closely resembles ERAD but is characterized by an unusual uncoupling between the dislocation and the degradation steps. Since lysine (Lys) residues are a major target for ubiquitination, we have investigated the effects of changing the Lys content on the retrotranslocation and degradation of ricin A chain in tobacco protoplasts. Here we show that modulating the number of Lys 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 Lys residues present in ricin A chain with arginine slowed down degradation, the introduction of four extra lysyl residues had an opposite effect and converted the ricin A chain to a standard ERAD substrate that is disposed via a process in which dislocation and degradation steps are tightly coupled.
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