The organization of endoplasmic reticulum export complexes.

TLDR

Cargo export from the ER relies on 60–70 nm COPII‑coated vesicular carriers. The authors used serial‑thin sectioning, stereology, and quick‑freeze deep‑etch electron microscopy to map the three‑dimensional organization of ER export sites and vesicular‑tubular clusters. ER budding occurs in specialized export complexes where COPII‑driven budding zones on individual cisternae cluster around a central COPI‑coated vesicular‑tubular core, concentrating cargo and restricting anterograde transport to regional foci that facilitate retrograde recycling.

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.