Confocal microscopy of fluorescently labeled membranes. (a) Sar1p-GDP control: GUVs remained mostly unaffected. A minority of µm-sized pearling vesicles was observed (inset) [2µM Sar1p, 1µM Sec12ΔCp, 1mM GDP]. (b–d) GUV tubulation by Sar1p-GTP [1µM Sar1p, 0.5µM Sec12ΔCp, 1mM GTPr]. (b) Tubulation starts (arrow). (c) The GUV has exploded into a ‘ball' of thin, wiggling membrane tubules. The inset shows tubules at increased contrast. (d) Timeseries of the explosion. See also Movie S1. (e) Straight, rigid lipid tubules after prolonged incubation with Sar1p-GTP.