Module 4: Figure COPI-coated vesicle
Retrograde cargo transport from the Golgi to the ER using COPI-coated vesicles.
The small GTPase Rab1b initiates the transport process by activating Arf1 that then acts to bring together the p23/p24 dimer, the COPI coat made up of multiple subunits and the cargo protein that is to be returned to the ER. The COPI then helps to deform the membrane to form a bud that is then cut off and transported along actin filaments towards the ER using kinesin-2 motors. The COPI coat components are removed allowing the vesicle to be captured by tethers such as syntaxin 18, which then enables the SNARE proteins to engage with each other and to fuse with the ER. 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.