Multi-scale Molecular Imaging of Human Cells reveals COPI and COPII Vesicles at ER Exit Sites

Trafficking from the endoplasmic reticulum to the Golgi apparatus comprises the first steps toward the correct localization of 30% of eukaryotic proteins. Coat protein complexes COPII and COPI are involved in forward and retrograde transport of cargo and cargo receptors between the ER and the Golgi. Although COPII forms coated vesicles in vitro, the biogenesis, morphology and organization of transport carriers in mammalian cells is debated. We use in situ cryo-electron tomography and super-resolution fluorescence microscopy to reveal the molecular architecture of ER exit sites in human cells. We visualise ribosome-exclusion zones enriched with COPII and COPI-coated vesicles and thus resolve the debate regarding the existence of COPII coated vesicles. COPII vesicles derive from ER membranes, whereas COPI vesicles originate from the ER-Golgi intermediate compartment. We quantify coated vesicle morphology and positioning with respect to other ER exit site components, providing a molecular description of the mammalian early secretory pathway.