Component of the coat protein complex II (COPII) which promotes the formation of transport vesicles from the endoplasmic reticulum (ER). The coat has two main functions, the physical deformation of the endoplasmic reticulum membrane into vesicles and the selection of cargo molecules (By similarity). The COPII coat is composed of at least 5 proteins: the sec23/24 complex, the sec13/31 complex, and the protein sar1A or sar1B. Belongs to the small GTPase superfamily. SAR1 family.