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CELL BIOLOGY AND SIGNAL TRANSDUCTION

De Novo Formation of Plant Endoplasmic Reticulum Export Sites Is Membrane Cargo Induced and Signal Mediated^1,[W],[OA]

Department of Biology, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 5E2 (S.L.H., L.C., L.R., L.A.M., F.B.); and Department of Energy, Plant Research Laboratory, Michigan State University, East Lansing, Michigan 48824 (F.B.)

The plant endoplasmic reticulum (ER) contains functionally distinct subdomains at which cargo molecules are packed into transport carriers. To study these ER export sites (ERES), we used tobacco (Nicotiana tabacum) leaf epidermis as a model system and tested whether increased cargo dosage leads to their de novo formation. We have followed the subcellular distribution of the known ERES marker based on a yellow fluorescent protein (YFP) fusion of the Sec24 COPII coat component (YFP-Sec24), which, differently from the previously described ERES marker, tobacco Sar1-YFP, is visibly recruited at ERES in both the presence and absence of overexpressed membrane cargo. This allowed us to quantify variation in the ERES number and in the recruitment of Sec24 to ERES upon expression of cargo. We show that increased synthesis of membrane cargo leads to an increase in the number of ERES and induces the recruitment of Sec24 to these ER subdomains. Soluble proteins that are passively secreted were found to leave the ER with no apparent up-regulation of either the ERES number or the COPII marker, showing that bulk flow transport has spare capacity in vivo. However, de novo ERES formation, as well as increased recruitment of Sec24 to ERES, was found to be dependent on the presence of the diacidic ER export motif in the cytosolic domain of the membrane cargo. Our data suggest that the plant ER can adapt to a sudden increase in membrane cargo-stimulated secretory activity by signal-mediated recruitment of COPII machinery onto existing ERES, accompanied by de novo generation of new ERES.

² These authors have contributed equally to the paper.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Federica Brandizzi (brandizz{at}msu.edu).

^[W] The online version of this article contains Web-only data.

^[OA] Open Access articles can be viewed online without a subscription.

www.plantphysiol.org/cgi/doi/10.1104/pp.106.094110

^* Corresponding author; e-mail brandizz{at}msu.edu; fax 517–353–9168.

Received December 4, 2006; accepted February 20, 2007; published February 23, 2007.

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