Formation of the COPII vesicle on the surface of the endoplasmic reticulum. In yeasts, Sec12p recruits Sar1p to the ER membrane and converts Sar1p-GDP into Sar1p-GTP. Sar1p-GTP, through direct binding to Sec23p, recruits Sec23p-Sec24p heterodimers from the cytoplasm to the ER membrane. Sar1p, Sec23p, and Sec24p form the inner layer of the COPII coat and the prebudding complex that recruits Sec13p-Sec31p heterotetramers. Sec13p-Sec31p form the outer layer of the COPII coat and facilitate budding of the COPII-coated vesicles. Cargo selection occurs primarily through direct or indirect interaction with Sec24. In mammals, there are 2 paralogs for Sar1, 2 for Sec23, 4 for Sec24, and 2 for Sec31.