Movement of cargo through the secretory pathway of eukaryotic cells occurs as a discontinuous process involving the activity of 50–80-nm vesicles (28). Vesicles forming on the endoplasmic reticulum (ER) by the COPII coat machinery (35) were first detected as coated elevations on transitional elements directly facing the Golgi apparatus. Subsequently, intermediates harboring ER-derived cargo were found in regions distant from the Golgi stack (16, 33). These structures contained a different coat complex, COPI or coatomer, recognized to be involved in vesicle-mediated recycling (20). It is now apparent that ER export and recycling are tightly coupled. We review recent evidence regarding the organization and role of ER to Golgi intermediates in these events.
Stereological analysis of serial thin sections revealed the general morphological organization and distribution of ER export/recycling sites (4). Export from the ER was found to be...
