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J. Biol. Chem., Vol. 281, Issue 33, 23377-23385, August 18, 2006

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The N-terminal Ricin Propeptide Influences the Fate of Ricin A-chain in Tobacco Protoplasts^*

Nicholas A. Jolliffe¹, Alessandra Di Cola¹, Catherine J. Marsden, J. Michael Lord, Aldo Ceriotti, Lorenzo Frigerio, and Lynne M. Roberts²

From the Department of Biological Sciences, University of Warwick, Coventry CV4 7AL, United Kingdom and the Istituto di Biologia e Biotecnologia Agraria, Consiglio Nazionale delle Ricerche, Via Bassini 15, 20133 Milano, Italy

The plant toxin ricin is synthesized in castor bean seeds as an endoplasmic reticulum (ER)-targeted precursor. Removal of the signal peptide generates proricin in which the mature A- and B-chains are joined by an intervening propeptide and a 9-residue propeptide persists at the N terminus. The two propeptides are ultimately removed in protein storage vacuoles, where ricin accumulates. Here we have demonstrated that the N-terminal propeptide of proricin acts as a nonspecific spacer to ensure efficient ER import and glycosylation. Indeed, when absent from the N terminus of ricin A-chain, the non-imported material remained tethered to the cytosolic face of the ER membrane, presumably by the signal peptide. This species appeared toxic to ribosomes. The propeptide does not, however, influence catalytic activity per se or the vacuolar targeting of proricin or the rate of retrotranslocation/degradation of A-chain in the cytosol. The likely implications of these findings to the survival of the toxin-producing tissue are discussed.

Received for publication, March 22, 2006 , and in revised form, June 13, 2006.

^* This work was supported by Biotechnology and Biological Sciences Research Council Grant C17404 [GenBank] (to L. F. and L. M. R.) and a WT program grant (to L. M. R. and J. M. L.). 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 Both authors contributed equally to this work.

² To whom correspondence should be addressed. Tel.: 44-2476-523558; Fax: 44-2476-523701; E-mail: lynne.roberts{at}warwick.ac.uk.

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