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. Binds PREB. 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. Belongs to the small GTPase superfamily. SAR1 family.