GTP-binding protein involved in transport from the endoplasmic reticulum to the Golgi apparatus. Activated by the guanine nucleotide exchange factor PREB. Involved in the selection of the protein cargo and the assembly of the COPII coat complex (By similarity). Synergizes with the cargo receptor SURF4 to mediate the export of lipoproteins from the endoplasmic reticulum, thereby regulating lipoprotein delivery and the maintenance of lipid homeostasis (By similarity). Homodimer (By similarity). Binds PREB (PubMed:11422940). Part of the COPII coat complex. Binds to the cytoplasmic tails of target proteins in the endoplasmic reticulum (By similarity). Interacts with SURF4 (By similarity). Associated with the endoplasmic reticulum and Golgi stacks, in particular in the juxta-nuclear Golgi region. Embryonic lethality during late-gestation (PubMed:33964306). Mice display gastrointestinal abnormalities associated with chylomicron retention disease: they show lower plasma levels of triglycerides, total cholesterol, and HDL-cholesterol, along with reduced chylomicron secretion following gastric lipid gavage (PubMed:33964306). Conditional deletion in the liver depletes plasma lipids (PubMed:33186557). Belongs to the small GTPase superfamily. SAR1 family.