Effect of wild-type Sar1B on tubule density in the presence of 5 µM GDP, 5 µM GTP, and 5 µM GMP-PNP. In these experiments, 50 nM Sar1B was titrated onto an SLB containing approximately 100 pre-formed tubules in each field of view sampled. *:P < 0.05, **: P < 0.01, ***:P < 0.001, ****:P < 0.0001 relative to the absence of protein. Error bars ± SEM. To test for significance, data were compared to 0 nM condition using Student’s t-test. All samples used 3:1 DOPC/DOPS with 0.1% LRB-DOPE as the fluorescent tracer.