The structure of the COPI coat determined within the cell.

Abstract

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 <i>in vitro</i> reconstitution systems using purified components. Previously we have determined a complete structural model of the <i>in vitro</i> 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 <i>Chlamydomonas reinhardtii</i> cells. The native algal structure resembles the <i>in vitro</i> 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 <i>in situ</i>, maintaining Golgi topology, shows that vesicles change their size, membrane thickness, and cargo content as they progress from <i>cis</i> to <i>trans</i>, but the structure of the coat machinery remains constant.