The organization of endoplasmic reticulum export complexes

To develop a detailed understanding of the topological organization of E R export throughout the cytoplasm, we carried out a morphological reconstruction of those sites that were not adjacent to Golgi elements, referred to as peripheral sites. A n analysis of peripheral sites allows us to discriminate the structure of ER-derived intermediates from that of the fenestrated cis-Golgi network (CGN), which is always associated with ER buds near the Golgi. ER buds on peripheral sites (Fig. 3) typically emanated from short stretches of ER membrane. These regions were separated by long distances from other similar budding foci. Frequently, ER buds found on different cisternae were closely juxtaposed and faced each other. These features are more evident in Fig. 4 (A and B) , which presents an overlapping reconstruction of serial sections of the peripheral site shown in Fig. 3 . Budding profiles (blue) pro-trude from the ER (green) into a central region containing a collection of vesicles and tubular elements comprising VTCs (red). This typical organization of peripheral sites is also characteristic of budding sites associated with the nuclear envelope (Fig. 1) and budding sites adjacent to the Golgi stack (Fig. 2) . The close topological relationship between ER buds and distinct VTCs suggests that these structures function as a compact morphological unit that we now refer to in its entirety as an export complex.