title: The organization of endoplasmic reticulum export complexes.
abstract: Export of cargo from the ER occurs through the formation of 60-70nm COPII-coated vesicular carriers. We have applied serial-thin sectioning and stereology to quantitatively characterize the three-dimensional organization of ER export sites in vivo and in vitro. We find that ER buds in vivo are nonrandomly distributed, being concentrated in regional foci we refer to as export complexes. The basic organization of an export complex can be divided into an active COPII-containing budding zone on a single ER cisterna, which is adjacent to budding zones found on distantly connected ER cisternae. These budding foci surround and face a central cluster of morphologically independent vesicular-tubular elements that contain COPI coats involved in retrograde transport. Vesicles within these export complexes contain concentrated cargo molecules. The structure of vesicular-tubular clusters in export complexes is particularly striking in replicas generated using a quick-freeze, deep-etch approach to visualize for the first time their three-dimensional organization and cargo composition. We conclude that budding from the ER through recruitment of COPII is confined to highly specialized export complexes that topologically restrict anterograde transport to regional foci to facilitate efficient coupling to retrograde recycling by COPI.
authors.affiliation: Scripps Research Institute, Department of Cell Biology, La Jolla, California 92130, USA.
authors.forename: S I
authors.initials: SI
authors.lastname: Bannykh
authors.forename: T
authors.initials: T
authors.lastname: Rowe
authors.forename: W E
authors.initials: WE
authors.lastname: Balch
mesh_terms: D000818:Animals; D001692:Biological Transport; D002460:Cell Line; D018699:Coated Vesicles / Q000737:chemistry / Q000378:metabolism / Q000648:ultrastructure*; D004721:Endoplasmic Reticulum / Q000737:chemistry / Q000378:metabolism / Q000648:ultrastructure*; D005613:Freeze Etching / Q000379:methods; D019204:GTP-Binding Proteins / Q000235:genetics / Q000502:physiology; D006056:Golgi Apparatus / Q000648:ultrastructure; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D007668:Kidney; D015471:Leukemia, Basophilic, Acute; D008562:Membrane Glycoproteins; D008565:Membrane Proteins / Q000032:analysis; D020559:Monomeric GTP-Binding Proteins; D009154:Mutation; D009685:Nuclear Envelope / Q000648:ultrastructure; D051381:Rats; D029701:Saccharomyces cerevisiae Proteins; D014407:Tumor Cells, Cultured; D033921:Vesicular Transport Proteins; D014721:Vesicular stomatitis Indiana virus; D014759:Viral Envelope Proteins / Q000378:metabolism
chemical_list: C028391:G protein, vesicular stomatitis virus; D008562:Membrane Glycoproteins; D008565:Membrane Proteins; D029701:Saccharomyces cerevisiae Proteins; D033921:Vesicular Transport Proteins; D014759:Viral Envelope Proteins; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D019204:GTP-Binding Proteins; D020559:Monomeric GTP-Binding Proteins; C077395:SAR1 protein, S cerevisiae