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COP II is a vesicle coating protein involved in trafficking between the Endoplasmic reticulum and the Golgi apparatus. It is involved in the retrieval/ anterograde pathway within the cell.



COPII is comprised of five key Protein subunits; Sec13, Sec 31, Sec 23, Sec 24 and Sar1. Sec 13 and Sec 31 bind tightly together to form a heterodimer whereas Sec 23 and Sec 24 form a heterotetramer with two of each subunit[1]. Sar1 is a GTPase that recruits the other two subunit complexes.


GTP binds to Sar1, causing a conformational change, which reveals an amphipathic N-terminal tail to insert into the membrane[2]. The anchoring of the N-terminal induces membrane curvature and Vesicle formation. Once Sar1 has anchored itself to the membrane it recruits the Sec 23/Sec 24 dimer, together these form the inner COP II coat, known as the ‘pre-budding’ complex[3]. This complex can now engage the Sec 13/Sec 31 tetramer to form the outer coat. Once the coat is complete, GTP is hydrolysed leading to another conformational change which releases the coating complex along with the new vesicle. The proteins can then be recycled to coat another vesicle[4].

Role in the Cell

COP II vesicles bud primarily from areas of the endoplasmic reticulum that lack Ribosomes, known as ER exit sites[5]. From here they direct the transport of specific cargo to the Golgi apparatus.

There is also thought that COP II may be involved in the quality control systems within the cell. When proteins are mistranslated or incorrectly modified they are taken from the endoplasmic reticulum to the cytoplasm where they undergo ER-associated degradation (ERAD). COP II playing a role in the transport of the misfolded proteins has been implied[6].


  1. Jensen D, Schekman R. COPII-mediated vesicle formation at a glance. Journal of Cell Science. 2010;124(1):1-4.
  2. Hughes H, Stephens D. Assembly, organization, and function of the COPII coat. Histochemistry and Cell Biology. 2007;129(2):129-151.
  3. Lee M, Miller E. Molecular mechanisms of COPII vesicle formation. Seminars in Cell & Developmental Biology. 2007;18(4):424-434.
  4. Hardin J, Bertoni G, Becker W. Becker's world of the cell. Harlow: Pearson; 2017.
  5. Sato K, Nakano A. Mechanisms of COPII vesicle formation and protein sorting. FEBS Letters. 2007;581(11):2076-2082.
  6. Sato K, Nakano A. Mechanisms of COPII vesicle formation and protein sorting. FEBS Letters. 2007;581(11):2076-2082.
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