Cryo-electron tomography reveals how COPII assembles on cargo-containing membranes

Abstract

AbstractProteins traverse the eukaryotic secretory pathway via membrane trafficking between organelles. The COPII coat mediates the anterograde transport of newly synthesised proteins from the endoplasmic reticulum, engaging cargoes with wide ranges of sizes and biophysical properties. The native architecture of the COPII coat and the cargo-dependent regulation of its assembly remain poorly understood. Here, we have reconstituted COPII-coated membrane carriers using purifiedS. cerevisiaeproteins and cell-derived microsomes as a native membrane source. Using cryo-electron tomography with subtomogram averaging, we demonstrate that the COPII coat binds cargo and forms largely spherical vesicles from native membranes. We reveal the architecture of the inner and outer coat layers and shed light on how spherical carriers are formed. Our results provide novel insights into the architecture and regulation of the COPII coat and challenge our current understanding of how membrane curvature is generated.