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J. Biol. Chem., Vol. 283, Issue 23, 15869-15877, June 6, 2008

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The Role of CDC48 in the Retro-translocation of Non-ubiquitinated Toxin Substrates in Plant Cells^*

Richard S. Marshall¹, Nicholas A. Jolliffe, Aldo Ceriotti, Christopher J. Snowden, J. Michael Lord, Lorenzo Frigerio, and Lynne M. Roberts²

From the Department of Biological Sciences, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, United Kingdom and the Istituto di Biologia e Biotecnologia Agraria, Consiglio Nazionale delle Ricerche, Via Bassini 15, Milano, Italy

When the catalytic A subunits of the castor bean toxins ricin and Ricinus communis agglutinin (denoted as RTA and RCA A, respectively) are delivered into the endoplasmic reticulum (ER) of tobacco protoplasts, they become substrates for ER-associated protein degradation (ERAD). As such, these orphan polypeptides are retro-translocated to the cytosol, where a significant proportion of each protein is degraded by proteasomes. Here we begin to characterize the ERAD pathway in plant cells, showing that retro-translocation of these lysine-deficient glycoproteins requires the ATPase activity of cytosolic CDC48. Lysine polyubiquitination is not obligatory for this step. We also show that although RCA A is found in a mannose-untrimmed form prior to its retro-translocation, a significant proportion of newly synthesized RTA cycles via the Golgi and becomes modified by downstream glycosylation enzymes. Despite these differences, both proteins are similarly retro-translocated.

Received for publication, November 13, 2007 , and in revised form, March 19, 2008.

^* This work was supported in part by grants from The Leverhulme Trust (to L. M. R. and L. F.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ Supported by a UK Biotechnology and Biological Sciences Research Council studentship.

² To whom correspondence should be addressed. Tel.: 44-2476-523558; Fax: 44-2476-523568; E-mail: lynne.roberts{at}warwick.ac.uk.

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				K. L. Chamberlain, R. S. Marshall, N. A. Jolliffe, L. Frigerio, A. Ceriotti, J. M. Lord, and L. M. Roberts Ricin B Chain Targeted to the Endoplasmic Reticulum of Tobacco Protoplasts Is Degraded by a CDC48- and Vacuole-independent Mechanism J. Biol. Chem., November 28, 2008; 283(48): 33276 - 33286. [Abstract] [Full Text] [PDF]