Module 4: Figure COPII-coated vesicles
Cargo transport from the ER to the Golgi using COPII-coated vesicles.
The small GTPase Sar-1 initiates the transport process by bringing together the COPII coat made up of the Sec23/Sec24 and Sec13/Sec31 complexes that bind to the cargo. The COPII, perhaps helped by the Ca2+-binding proteins apoptosis-linked gene 2 (ALG-2) and annexin-11 (ANX11), then deforms the membrane to form a bud that is cut off and is transported along microtubules towards the ER–Golgi intermediate compartment (ERGIC) using the dynein motor. The COPII coat is removed allowing the vesicle to be captured by tethers such as trafficking protein particle 1 (TRAPP1), which enables the SNARE proteins to engage with each other and to fuse with the early Golgi compartment. Note that there is a change in scale between steps 1 and 4 and the subsequent steps in the sequence. See the text for further details.