Er export: public transportation by the copii coach

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ER export: public transportation by the COPII coach Bruno Antonny* and Randy Schekman†
The COPII coat produces ER-derived transport vesicles. Recent findings suggest that the COPII coat is a highly dynamic polymer and that efficient capture of cargo molecules into COPII vesicles depends on several parameters, including export signals, membrane environment, metabolic control and thepresence of a repertoire of COPII subunit homologues.
Addresses *Institut de Pharmacologie Moléculaire et Cellulaire, CNRS, 660 route des Lucioles, 06560 Valbonne, France; e-mail: antonny@ipmc.cnrs.fr † Department of Molecular and Cellular Biology, Howard Hughes Medical Institute, 535 Stanley Hall, University of California, Berkeley, California 94720, USA; e-mail: schekman@uclink4.berkeley.edu CurrentOpinion in Cell Biology 2001, 13:438–443 0955-0674/01/$ — see front matter © 2001 Elsevier Science Ltd. All rights reserved. Abbreviations ER endoplasmic reticulum SCAP SREBP cleavage-activating protein SREBP sterol regulatory element binding protein Figure 1

Introduction
Public transport systems have been used as a metaphor for vesicular traffic within eukaryotic cells. Transport vehiclesoperate along fixed pathways carrying varying numbers of passengers, each of whom selects a conveyance based on his destination. Protein passengers deposited in the endoplasmic reticulum (ER) may remain there or be shuttled to the Golgi apparatus to be presented with further transportation options. In this review, we discuss how COPII vesicles, the ER-specific shuttle coach, capture diversepassengers. Recent studies raise the following questions: are there different subtypes of COPII vesicles? Are there privileged passengers and do they have a choice in the sorting decision, and do they exert some control on vesicle formation?
COPII budding profiles on anionic liposomes incubated with purified COPII proteins and GMP-PNP at room temperature. The arrows point to budding regions decorated bythe coat. The bar indicates 100 nm. Micrograph courtesy of Lelio Orci.

A novel inhibitor of COPI movement is the viral glycoprotein NSP4, which is smuggled in COPII vesicles to the ERGIC, where it binds to microtubules through its cytosolic domain and impedes the movement of ERGIC [6•]. The spatial organization of the ER to Golgi pathway and the importance of the ERGIC and of the cytoskeletonvary between organisms [4].

COPII is number one
Like its distant cousin COPI, the COPII coat is a polymer formed by the ordered assembly of cytosolic proteins, which shape lipid membranes to produce transport vesicles [1]. Vesicles coated by the COPII coat mediate the export of newly synthesized proteins from the ER, whereas COPI-coated vesicles are involved at later stages of intracellulartransport [2–4]. In mammalian cells, a relay between the two coats takes place at the level of ERGIC (ER–Golgi intermediate compartment), a dynamic compartment at the crossroads between the ER and the Golgi [4,5•]. This relay has been visualized by time-lapse microscopy in living cells [5•]. COPII-labeled areas remain close to the ER, whereas COPI-labeled areas, after an initial overlap with COPII,travel on a microtubule network toward the Golgi [4,5•].

A blind coat?
The base of the COPII coat is formed by the small G-protein Sar1p-GTP, which binds directly to the lipid surface. Two large complexes, Sec23/24p and Sec13/31p, then bind sequentially [1]. This ordered assembly has been observed on liposomes [7]. Remarkably, this assembly leads to the budding of coated vesicles, which is asimilar process to that observed with the ER membrane ([7]; Figure 1). Thus, the coat has the intrinsic ability to deform a lipid bilayer. The absence of a membrane protein requirement in this reaction raises the possibility that cargo packaging and vesicle budding may not be coordinated. Indeed, biosynthetic cargos can be purged from ER membranes with no effect on COPII vesicle formation [8]....