The structure of the COPI coat determined within the cell.

COPI-coated vesicles mediate trafficking within the Golgi apparatus and from the Golgi to the endoplasmic reticulum. The structures of membrane protein coats, including COPI, have been extensively studied with reconstitution systems using purified components. Previously we have determined a complete structural model of the reconstituted COPI coat (Dodonova et al., 2017). Here, we applied cryo-focused ion beam milling, cryo-electron tomography and subtomogram averaging to determine the native structure of the COPI coat within vitrified cells. The native algal structure resembles the mammalian structure, but additionally reveals cargo bound beneath β'-COP. We find that all coat components disassemble simultaneously and relatively rapidly after budding. Structural analysis , maintaining Golgi topology, shows that vesicles change their size, membrane thickness, and cargo content as they progress from to , but the structure of the coat machinery remains constant.