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Vol. 19, Issue 10, 4352-4365, October 2008

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Drosophila Sec16 Mediates the Biogenesis of tER Sites Upstream of Sar1 through an Arginine-Rich Motif

Viorica Ivan^*, Gert de Voer^*, Despina Xanthakis, Kirsten M. Spoorendonk, Vangelis Kondylis, and Catherine Rabouille

The Cell Microscopy Centre, Department of Cell Biology and Institute of Biomembrane, University Medical Centre Utrecht, 3584CX Utrecht, The Netherlands

Submitted March 7, 2008; Revised June 11, 2008; Accepted June 25, 2008
Monitoring Editor: Benjamin S. Glick

tER sites are specialized cup-shaped ER subdomains characterized by the focused budding of COPII vesicles. Sec16 has been proposed to be involved in the biogenesis of tER sites by binding to COPII coat components and clustering nascent-coated vesicles. Here, we show that Drosophila Sec16 (dSec16) acts instead as a tER scaffold upstream of the COPII machinery, including Sar1. We show that dSec16 is required for Sar1-GTP concentration to the tER sites where it recruits in turn the components of the COPII machinery to initiate coat assembly. Last, we show that the dSec16 domain required for its localization maps to an arginine-rich motif located in a nonconserved region. We propose a model in which dSec16 binds ER cups via its arginine-rich domain, interacts with Sar1-GTP that is generated on ER membrane by Sec12 and concentrates it in the ER cups where it initiates the formation of COPII vesicles, thus acting as a tER scaffold.

* These authors contributed equally to this work.

Present address: Hubrecht Institute, Uppsalalaan 8, 3584 CT Utrecht, The Netherlands.

Address correspondence to: Catherine Rabouille (C.Rabouille{at}umcutrecht.nl) or Vangelis Kondylis (kondevans{at}hotmail.com)