Schematic representation of Sar1-mediated COPII vesicle formation. Activation of Sar1 by the GEF Sec12 leads to its enhanced binding to the ER membrane. The activated Sar1, along with the inner coat proteins Sec23/24 and cargo, forms the prebudding complex. Binding of the outer coat proteins Sec13/31 to Sec23/24 increases the GAP activity of Sec23, resulting in the inactivation of Sar1 and the subsequent formation of the COPII vesicle.