THE ORGANIZATION OF ENDOPLASMIC-RETICULUM EXPORT COMPLEXES

Export of cargo from the ER occurs through the formation of 60-70-nm C OPII-coated vesicular carriers. We have applied serial-thin sectioning and stereology to quantitatively characterize the three-dimensional o rganization of ER export sites in vivo and in vitro. We find that ER b uds in vivo are nonrandomly distributed, being concentrated in regiona l foci we refer to as export complexes. The basic organization of an e xport complex can be divided into an active COPII-containing budding z one on a single ER cisterna, which is adjacent to budding zones found on distantly connected ER cisternae. These budding foci surround and f ace a central cluster of morphologically independent vesicular-tubular elements that contain COPI coats involved in retrograde transport. Ve sicles within these export complexes contain concentrated cargo molecu les. The structure of vesicular-tubular clusters in export complexes i s particularly striking in replicas generated using a quick-freeze, de ep-etch approach to visualize for the first time their three-dimension al organization and cargo composition. We conclude that budding from t he ER through recruitment of COPII is confined to highly specialized e xport complexes that topologically restrict anterograde transport to r egional foci to facilitate efficient coupling to retrograde recycling by COPI.