Because of their compartmentalized structure, eukaryotic cells require pathways for trafficking cargo between lipid-bilayer-enclosed organelles. The most prominent pathways involve coat proteins that help recruit cargo and shape the lipid bilayer into small, highly curved vesicles. Assembly of the COPII coat, involved in ER-to-Golgi traffic, requires the Arf-like GTPase Sar1. When activated by Sec12, the guanine-nucleotide exchange factor, Sar1 exposes an N-terminal amphipathic helix (AH), which embeds in the membrane, and recruits the Sec23/Sec24 and Sec13/Sec31 complexes. In addition to coat recruitment, it was shown that Sar1 has a role in vesicle fission12. The mechanical pathway of COPII vesicle formation and separation from ER membranes, however, has been difficult to pin down.