The addition of Sar1BGTP to STuBs alters the morphology of lipid tubules. (A) A flexible tubule. (B) Same tubule as in (A) following addition of Sar1BGTP, exhibiting a rigid morphology. (C) A pseudo-vesiculated tubule. (D) A rigid, bifurcated tubule and a tubule exhibiting a beads-on-a-string morphology. (E) As Sar1BGTP concentration increases, the number of flexible tubules per field of view significantly decreases. (F) The prevalence of Sar1BGTP-induced structures changes as a function of concentration. Statistics by 1-way ANOVA. * indicates comparison to 0-nM Sar condition, † indicates comparison to 50-nM Sar1B condition. Adjusted P Values; *, †: P < 0.05, **, †† P: < 0.01, ***, †††: P < 0.001. All are in Buffer A + 0.5 mM MgCl2 + 5 μM GTP. Scale bars 10 µm. Error bars ± SEM.