Final answer:
COPII vesicles transport materials from the endoplasmic reticulum to the Golgi apparatus. It is initiated by the GTPase Sar1 and adaptor protein complexes such as Sec23/Sec24 and Sec13/Sec31.
Step-by-step explanation:
COPII is a type of vesicle that travels from the endoplasmic reticulum (ER) to the Golgi apparatus. The primary role of COPII is to transport proteins and lipids from the ER, where they are synthesized, to the Golgi apparatus, where they are further modified, sorted, and shipped to their final destination. The initiator or trigger for COPII vesicle formation is the Sar1 protein, which is a GTPase. When Sar1 binds to GTP, it undergoes a conformational change which allows it to insert into the ER membrane. This event, in turn, recruits the adaptor protein complexes Sec23/Sec24, which are part of the coat protein complex. Following this, the coat is completed by the assembly of the Sec13/Sec31 complex. These complexes help select cargo and form the vesicle that buds off from the ER.