In vitro reconstitution experiments with artificial lipid bilayers, COPII proteins and non-hydrolysable GTP analog can be performed in different ways, involving different degrees of mechanical processing and separation: (i) Reactions of GUVs with COPII and GMP-PNP without any centrifugation step yielded very long vesicle strings, shorter strings and some single vesicles (this work, Fig. 3a–h). (ii) Reactions of LUVs with COPII and GMP-PNP visualized by EM after chemical fixation and sedimentation showed buds, single vesicles and short strings of vesicles2. (iii) Reactions of LUVs with COPII and GMP-PNP that were ultracentrifuged on a gradient contained a homogeneous fraction of small coated vesicles8. Together, these observations suggest that the vesicle strings formed with non-hydrolysable GTP analog may be precursors to single vesicles. Moreover, in vitro scission appears to be promoted by mechanical force.