COP-coated vesicles

COP-coated vesicles (COP = coat protein) are intracellular vesicles whose coat does not consist of clathrin. The coat consists of other proteins that surround the vesicle membrane in a less structured manner. Vesicles coated with COP proteins mediate transport processes between the endoplasmic reticulum (ER) and the Golgi apparatus, between the individual Golgi cisternae and from the Golgi apparatus to the plasma membrane.

Two forms of vesicle coatings are distinguished:

• COPI is composed of the small GTP-binding protein (G-protein) ARF and the so-called coatomer, a complex consisting of 7 subunits with the relative molecular mass of 700 000. COPI mediates not only transport from the endoplasmic reticulum (ER) to the Golgi apparatus, but also retrograde transport from the Golgi apparatus to the ER. This is important for the recycling of proteins that have escaped from the ER during transport processes; corresponding proteins are characterized by the amino acid sequence KDEL, among others.
• COPII consists of the GTP-binding protein Sar1p, a protein complex with the subunits Sec13p and Sec31p, and a second complex with the subunits Sec23p and Sec24p. In contrast to COPI, COPII is exclusively involved in ER vesicle formation.

Both COPI and COPII recognize specific sequence motifs of the proteins transported in the vesicles they enclose, but overall transport appears to be less specific than is the case for transport events involving clathrin-coated vesicles. Prior to docking at the target membrane, dissolution of the COP shell occurs, triggered by hydrolysis of GTP on the associated G-proteins. This makes the donor membrane-derived protein v-SNARE accessible, which can then interact with the acceptor complex known as t-SNARE in the target membrane.