The organization of endoplasmic reticulum export complexes

The overall average density of total ER buds based on the cross-sectional volume of the cytoplasm was found to be 0.5 buds per pom 3, or 0.14 buds per ixm 2 of total ER surface. However, the average density of ER-budding profiles found within the pericentrosomal area containing the Golgi apparatus (referred to as the Golgi exclusion zone) was five-to sevenfold higher (3.3 buds per txm 3 of cytoplasm or 0.8 buds per ixm 2 of ER surface) than the average value found on the total ER membrane. Outside of the Golgi region, the overall mean density was 1.5-3-fold lower (0.22 buds per ixm 3 cytoplasm or 0.04 buds per ixm 2 of ER surface) than the average values found for the cell. The markedly increased budding density around the Golgi apparatus is consistent with the highly focused export activity observed from ER transitional elements present in the Golgi region of pancreatic acinar cells (Palade, 1975) . Figure 1 . Export sites adjacent to the nuclear envelope of RBL cells. Six consecutive serial sections through a VTC adjacent to the nuclear envelope. ER buds (arrowheads) emerging from the nuclear envelope (section 1) and parallel ER membrane (stars in section 2--6) are facing VTCs. (Inset) Higher magnification view of two ER buds. The slice of the section presented in the inset encompasses either only the coat (right), or both the coat and the membrane and the luminal part of a bud (left). Individual 4-5-nm electron-dense particles arranged in a semiregular pattern (arrows.) Notice the same appearance of the coat under lower magnification of serial sections that contain a honeycombed appearance consisting of a semiregular array of electron-dense particles (arrows). Adjacent to buds, we observe a typical pleomorphic element (large asterisk in section 5) within a VTC that has numerous tubular projections (small asterisk) in adjacent thin-sections (small asterisks in sections 4 and 6), indicative of its fenestrated structure. Tubules in the fenestrated elements of the VTC possess a dark dense coat (arrows in sections 4 and 6) typical of those found on Golgi compartments and are readily distinguishable from the alveolate coat associated with COPII buds emanating from the ER. Bar, 0.1 Ixm. Figure 2 . Export complexes adjacent to the pericentrosomal Golgi region of RBL cells. 15 consecutive serial sections through three (a, b, and c) Golgi-adjacent export complexes (encircled by dotted lines) are shown with ER buds (arrowheads). Export complex a contains 13 ER-derived buds, export complex b contains 11 buds, and export complex c contains seven buds. Note the characteristic cup-shaped appearance of ER bud-bearing zones especially evident for export complex b.