A model for Arf1-mediated membrane separation. The process of COPI budding and fission is depicted. Arf1 is recruited to the neck of a growing bud and stabilized in this location by coatomer (Arf1-Y35A in red; Arf1-wt in blue). As the growing bud becomes completed, the local membrane curvature creates an unfavorable situation for the insertion of the myristoylated amphipathic helix of Arf1. If Arf1 is enforced in this area by its multiple interactions with the covering network of polymerized coatomer, this will lead to local strain in the membrane (in red). The resulting metastable intermediate can then be relaxed in an irreversible manner by membrane separation. The resulting shell of polymerized coatomer is drawn with a gap because any possible contribution to membrane fission by closing this gap is not addressed here.