GTP-binding protein involved in transport from the endoplasmic reticulum to the Golgi apparatus (PubMed:11149932, PubMed:8138575). Activated by the guanine nucleotide exchange factor PREB (PubMed:11149932, PubMed:8138575). Involved in the selection of the protein cargo and the assembly of the COPII coat complex (PubMed:11149932, PubMed:8138575). 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 (PubMed:11739406). Binds PREB (PubMed:11739406). Part of the COPII coat complex (PubMed:11739406). Binds to the cytoplasmic tails of target proteins in the endoplasmic reticulum (PubMed:11739406). 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.