ReviewER export: public transportation by the COPII coach
Introduction
Public transport systems have been used as a metaphor for vesicular traffic within eukaryotic cells. Transport vehicles operate 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 diverse passengers. 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?
Section snippets
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 intracellular transport 2., 3., 4.. In mammalian cells, a relay between the two coats takes place at the level of ERGIC (ER–Golgi intermediate compartment), a dynamic
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 a similar process to that observed with the ER membrane ([7]; Fig. 1). Thus, the coat has the intrinsic ability to deform a lipid bilayer. The absence of a membrane protein
Different passengers
Fig. 2 schematizes some proteins that have been used as models for ER export in yeast. These cargo proteins are anything but similar. The COPII coat must accommodate cargo molecules of diverse size and membrane topology. Nevertheless, in vitro, the addition of purified COPII components and GTP to yeast microsomes promotes the incorporation of these proteins into COPII vesicles. In a few cases, the presence of different cargos in the same vesicle has been demonstrated [11]. However, the apparent
Privileged customers
A large number of proteins found in COPII vesicles are not in transit to other destinations but are recycling continuously between the ER and the Golgi. This includes proteins that operate in tethering and fusion mechanisms but also many proteins whose functions are unknown 16., 17., 18., 19.. As frequent flyers of COPII vesicles, they seem to have some privileges. Thus, the SNARE protein Bet1p interacts directly through its amino-terminal cytosolic domain with Sar1p and Sec23/24p [16]. Such
Special COPII vesicles for special parcels?
Pma1p, the plasma membrane ATPase, is a much more cumbersome vesicle passenger than Bet1p. Pma1p displays a large cytoplasmic domain and can form oligomers. The accommodation of such a structure may require novel coat subunits. Incorporation of Pma1p into COPII vesicles is favored by the concomitant action of Sec23/24p and Sec23/Lst1p, a complex including one of the two Sec24p homologues found in yeast 27., 28•.. Vesicles formed with this mixed coat are larger than standard COPII vesicles,
Protein assistance and metabolic control
In addition to the intervention of protein specific ‘outfitter’ gene products that guide membrane proteins into COPII vesicles 37., 38., recent evidence suggests an important role for metabolic regulation of the packaging of certain cargo proteins [39••]. In mammalian cells, two membrane proteins, SCAP (SREBP cleavage-activating protein) and SREBP, act in tandem to trigger the response to a low level of cholesterol. SCAP contains a sterol-sensing domain, whereas SREBP contains a cytosolic
Spatial and temporal control of COPII coat assembly and disassembly
At first glance, the connection between the GTPase cycle of Sar1p and the assembly of the COPII coat is simple. Upon GDP to GTP exchange, Sar1p binds to the ER and promotes COPII coat assembly. Subsequent GTP hydrolysis by Sar1p causes it to dissociate and the coat to disassemble 2., 3.. However, two factors complicate the issue: these are the polymeric nature of the coat and the fact that the GTPase activating protein (GAP) of Sar1p, Sec23p, is a subunit of the coat. Nucleotide exchange and
Conclusions
We now know that the COPII coat is responsible for direct capture of membrane cargo proteins and for the physical deformation of the ER membrane that accompanies the formation of a sharply curved transport vesicle. However, our picture of the COPII coat remains incomplete because none of the COPII components have been crystallized. Although this goal will almost certainly be achieved in the near future, we are for the time being restricted to models from which testable hypotheses about
Update
Recent work has further addressed the molecular basis of ER export of some important classes of membrane proteins. For inwardly rectifying potassium channels, the ER export motif identified by Ma et al. [24•] has been also identified by another group [47•]. For G-protein-coupled receptors, a recent study highlights the importance of a cytosolic motif adjacent to the last transmembrane segment [48•]. This motif forms an amphipathic helix, which lies at the membrane surface. Mutagenesis of
Acknowledgements
The authors acknowledge the Howard Hughes Medical Institute and the CNRS for support.
References and recommended reading
Papers of particular interest, published within the annual period of review,have been highlighted as:
•of special interest
••of outstanding interest
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