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Published on February 23, 2007; 10.1104/pp.106.094110

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Received December 4, 2006
Accepted February 20, 2007

De Novo Formation of Plant Endoplasmic Reticulum Export Sites Is Membrane Cargo-induced and Signal-mediated

Sally L Hanton , Laurent Chatre , Luciana Renna , Loren A Matheson , and Federica Brandizzi *

Department of Biology, 112 Science Place, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2, Canada; Department of Energy, Plant Research Laboratory, Michigan State University, East Lansing, MI 48824, USA

* Corresponding author; email: brandizz{at}msu.edu.

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 leaf epidermis as a model system and tested if 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 fusion of the Sec24 COPII coat component (YFP-Sec24) which, differently from the previously described ERES marker, Sar1-YFP (daSilva et al., 2004), is visibly recruited at ERES in both the presence and absence of over-expressed 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 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 a signal-mediated recruitment of COPII machinery onto existing ERES, accompanied by the de novo generation of new ERES.




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