COP Coated Vesicles

COP-coated vesicles are transport carriers with protein coats that bud from cell membranes. Two types exist: COPI and COPII, composed of similar coatomer protein complexes. Unlike clathrin-coated vesicles, COP vesicles mediate bidirectional transport between the ER and Golgi, with COPI handling intra-Golgi and retrograde transport, while COPII mediates anterograde ER-to-Golgi transport.

COPII vesicles form at ER membranes and transport cargo from the ER to the Golgi, carrying enzymes and membrane proteins involved in biosynthetic pathways. COPI vesicles transport molecules between Golgi cisternae and mediate retrograde transport from the Golgi back to the rough ER, maintaining dynamic protein distribution between these compartments.

Coat proteins assemble on donor membranes to initiate vesicle formation and help sort cargo. COPI and COPII coats contain seven core coatomer units including α-COP, β-COP, and γ-COP. These proteins bend the membrane to form a bud that is released as a vesicle, with coat assembly and gtpases driving the process.

COPII-coated vesicles select and transport enzymes involved in biosynthetic pathways, membrane proteins required for docking and fusion with target compartments, and membrane proteins that bind soluble cargo. These vesicles concentrate and package proteins with specific exit signals for export from the ER.

The bulk flow transport model proposes that ER-resident proteins use C-terminal signal sequences for retention and recycling. The selective transport model suggests proteins with specific exit signals are concentrated and packaged for export. Evidence indicates COPII transport likely involves a combination of both mechanisms rather than one exclusively.

Different classes of coat proteins, including COPI and COPII, mediate bidirectional transport that maintains dynamic protein distribution. COPII vesicles transport proteins forward from ER to Golgi, while COPI vesicles recycle proteins backward from Golgi to ER, creating a balanced system of anterograde and retrograde transport.

COPI and COPII coat proteins share similar structural features with clathrin, though they differ in cargo sorting and vesicle formation mechanisms. All three coat protein classes form spherical, protein-coated carriers with 50-100 nm diameter that mediate transport across the cell, but employ distinct molecular strategies for cargo selection.