A Primer on Vesicle Budding

How do vesicular coats form on the donor membrane? Sebastian Springer,* Anne Spang,* and Randy Schekman*†‡ Do cargo proteins influence this process? Recent work *Department of Molecular and Cell Biology and on the exchange of proteins between the endoplasmic †Howard Hughes Medical Institute reticulum (ER) and the Golgi apparatus in SaccharoUniversity of California myces cerevisiae and higher eukaryotes suggests that Berkeley, California 94720 all three requirements for functional vesicle budding could be regulated in a single mechanistic step, the formation of a “priming complex” of a small GTPase, a Transport vesicles carry lumenal and membrane cargo membrane protein, and a coat subunit (Figure 1). proteins between secretory organelles in eukaryotic To Begin: Recruit a GTPase cells. To make such a vesicle, cytosolic coat proteins to the Donor Membrane assemble on a donor membrane and deform it into a COPII (coat protein complex II)–coated vesicles transbud. Cargo proteins are sorted into the originating vesiport proteins from the ER to the Golgi. The COPII coat cle, which then separates from the donor membrane, consists of the small GTPase Sar1p and the heteroditravels a certain distance, and finally fuses with the acmeric protein complexes Sec23/24p and Sec13/31p ceptor organelle. (Barlowe, 1998). These five proteins are necessary and At the time of vesicle budding, three specific requiresufficient to produce COPII vesicles from ER microments have to be fulfilled in order to produce a functional somes (Barlowe et al., 1994) and from chemically defined vesicle. First, different vesicular transport processes are liposomes (Matsuoka et al., 1998). mediated by different coat protein complexes, and Proteins can also be recycled to the ER from the cistherefore a donor membrane must attract the correct Golgi via a backward, or retrograde, route. Such proteins species of cytosolic coat proteins. Second, a transport include ER residents that have escaped ER retention, vesicle needs to be equipped with certain membrane and functional components of COPII (“anterograde”) proteins that have essential tasks at a later stage. vesicles that return to participate in another round of v-SNAREs, for example, are required for fusion with the COPII vesicle formation. Retrograde vesicles are coated acceptor membrane (reviewed by Nichols and Pelham, with the COPI coat (Cosson and Letourneur, 1997; 1998). These proteins must be included into the vesicles Gaynor et al., 1998), which consists of the small GTPase with high fidelity. And third, cargo proteins have to be recognized and included into the originating vesicles. ARF (in S. cerevisiae, Arf1p and/or Arf2p) and coatomer