1. Institut de Pharmacologie Moléculaire et Cellulaire, CNRS, 660 route des Lucioles, 06560 Valbonne-Sophia-Antipolis, France
2. Centre Commun de Microscopie Appliquée, Université de Nice, Parc Valrose, 06103 Nice cedex 2, France

Correspondence to: Bruno Antonny¹ Email: antonny@ipmc.cnrs.fr

Protein coats deform flat lipid membranes into buds and capture membrane proteins to form transport vesicles^{1, 2, 3}. The assembly/disassembly cycle of the COPI coat on Golgi membranes is coupled to the GTP/GDP cycle of the small G protein Arf1. At the heart of this coupling is the specific interaction of membrane-bound Arf1–GTP with coatomer, a complex of seven proteins that forms the building unit of the COPI coat^{4, 5, 6, 7}. Although COPI coat disassembly requires the catalysis of GTP hydrolysis in Arf1 by a specific GTPase-activating protein (ArfGAP1)^{8, 9, 10}, the precise timing of this reaction during COPI vesicle formation is not known. Using time-resolved assays for COPI dynamics on liposomes of controlled size, we show that the rate of ArfGAP1-catalysed GTP hydrolysis in Arf1 and the rate of COPI disassembly increase over two orders of magnitude as the curvature of the lipid bilayer increases and approaches that of a typical transport vesicle. This leads to a model for COPI dynamics in which GTP hydrolysis in Arf1 is organized temporally and spatially according to the changes in lipid packing induced by the coat.