COP2 (or more accurately COPII) is a coat protein complex that facilitates the formation of transport vesicles from the endoplasmic reticulum (ER) to the Golgi apparatus. It plays a crucial role in anterograde transport, which means moving newly synthesized proteins forward from the ER to the Golgi for further processing and sorting. Specifically, COPII assembles on the ER membrane, helping to package cargo proteins into vesicles that bud off from the ER and transport their cargo to the Golgi apparatus or the ER-Golgi intermediate compartment. This complex is made up of two layers: an inner layer formed by Sar1, Sec23, and Sec24 proteins, and an outer lattice composed of Sec13 and Sec31 proteins. The activated Sar1 initiates coat assembly by inserting into the ER membrane and recruiting other coat proteins to form the vesicle coat. In summary:
- COPII coats vesicles for transport from the rough ER to the cis-Golgi (forward direction).
- This process is essential for protein secretion and membrane trafficking.
- It is distinct from COPI, which mediates retrograde transport (from Golgi back to ER).
A useful mnemonic is: "Two steps forward (COPII) and one step back (COPI)" to remember their direction of transport in the cell's secretory pathway.
