Membrane-enclosed structures called vesicles transport proteins and lipids across the cell. The vesicles derive their cargo from the plasma membrane, Golgi, ER, or endosome. Coated vesicles are spherical, protein-coated carriers with a 50–100 nm diameter that mediate bidirectional transport between the ER and the Golgi. The distribution of proteins between the ER and Golgi complex is dynamic and is maintained by different coated vesicles. Their formation is driven by the assembly of different classes of coat proteins, namely COPI, COPII, clathrin, or retromer complex. Coat proteins can help sort the cargo and assemble on the donor membrane to initiate vesicle formation.
The COPI and COPII coat proteins share similar structural features with Clathrin but differ in cargo sorting and vesicle formation mechanisms. Experiments have shown that seven core customer units are involved in COPI coats: α-COP, β-COP, β′-COP, γ-COP, δ-COP, ε-COP, and ζ-COP. Proteins selected by COPII-coated vesicles include enzymes that act in the biosynthetic pathways, membrane proteins involved in the docking and fusion of the vesicle with the target compartment, and membrane proteins that bind to soluble cargo.
COPI-coated vesicles mediate protein transport among the Golgi cisternae and play an active part in the disassembly and reassembly of the Golgi stack during cell division. They also function as crucial transport vesicles between the cis-Golgi network and the rough ER. In contrast, COPII vesicles mediate transport between ER sites and from ER to the Golgi. Two models, namely, the bulk flow transport and selective transport, were proposed to describe this transport. According to the bulk transport model, short amino acid sequences at the C terminal of ER-resident proteins called signal sequences are necessary for the retention and recycling of cargo.
In contrast, the selective transport model proposes that proteins with specific exit signals are concentrated and packaged for export from the ER. Barring a few examples, neither of the models has been fully established. They may not necessarily be mutually exclusive, and COPII vesicle transport may involve a combination of both models.
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