Furthermore, coat completion may actively contribute to fission. In the absence of Sar1p and membranes, the coat has a propensity to assemble into spherical structures, which have positive curvature in both principal directions1223. This propensity may help to promote the transition from tubular membranes, which have positive curvature in only one direction, to vesicles, which have positive curvature in both directions (Fig. 3c).