Fig. 1.

COPII-dependent packaging. (1) At ERES Sec16 binds the membrane protein Sec12, which acts as a GEF for Sar1–GDP promoting GDP–GTP exchange. Active Sar1–GTP then binds the ER membrane. (2) The inner COPII components Sec23/24 are recruited and Sec24 binds cargo proteins from the ER lumen. (3) TFG and Sec16 help organize the ERES to which Sec13/Sec31 are recruited by binding to the inner coat complex. (4) A COPII vesicle begins to form as the membrane deforms, Sar1–GTP accumulates at the base of this structure and undergoes conformational changes through GTP-hydrolysis induced by Sec23–Sec24 together with Sec13–Sec31. (5) The COPII vesicle containing the cargo proteins is released from the ER and traffics to the ERGIC. (6) Fusion with the ERGIC membrane takes place after uncoating of the vesicle. (7) Subsequently, cargo proteins are transported from the ERGIC to the Golgi