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Vol. 17, Issue 4, 1664-1675, April 2006
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* Department of Biochemistry, Institute for Cancer Research, Faculty Division The Norwegian Radium Hospital, University of Oslo, Montebello, N-0310 Oslo, Norway;
Department of Molecular Biosciences, University of Oslo, N-0316 Oslo, Norway
Submitted October 18, 2005;
Revised January 17, 2006;
Accepted January 19, 2006
Monitoring Editor: Jeffrey Brodsky
The plant toxin ricin is transported retrogradely from the cell surface to the endoplasmic reticulum (ER) from where the enzymatically active part is retrotranslocated to the cytosol, presumably by the same mechanism as used by misfolded proteins. The ER degradation enhancing 
-mannosidase I-like protein, EDEM, is responsible for directing aberrant proteins for ER-associated protein degradation. In this study, we have investigated whether EDEM is involved in ricin retrotranslocation. Overexpression of EDEM strongly protects against ricin. However, when the interaction between EDEM and misfolded proteins is inhibited by kifunensin, EDEM promotes retrotranslocation of ricin from the ER to the cytosol. Furthermore, puromycin, which inhibits synthesis and thereby transport of proteins into the ER, counteracted the protection seen in EDEM-transfected cells. Coimmunoprecipitation studies revealed that ricin can interact with EDEM and with Sec61, and both kifunensin and puromycin increase these interactions. Importantly, vector-based RNA interference against EDEM, which leads to reduction of the cellular level of EDEM, decreased retrotranslocation of ricin A-chain to the cytosol. In conclusion, our results indicate that EDEM is involved in retrotranslocation of ricin from the ER to the cytosol.
These authors contributed equally to this work.
Address correspondence to: Kirsten Sandvig (kirsten.sandvig{at}imbv.uio.no).
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