Guanine nucleotide-exchange factor (GEF) required for the formation or budding of transport vesicles from the ER. This function involves the cytoplasmic domain of the protein, which is thought to interact with the small GTP-binding protein SAR1. Required for autophagy. Cycling between endoplasmic reticulum and the early Golgi. In the process of transport, SEC12 itself may migrate to the Golgi apparatus and function in subsequent transport events. Present with 6160 molecules/cell in log phase SD medium. Belongs to the WD repeat SEC12 family.