17.2
COP Coated Vesicles
Membrane-enclosed structures called vesicles transport proteins and lipids across the cell. The vesicles derive their cargo from the plasma membrane,…
Coated vesicles are transport vesicles that bud off from specialized regions of the cell membrane. Specific coat proteins cover their cytosolic surface.
Based on the type of protein coats, these vesicles can be of three types: coat protein or COP-coated vesicles, COPI and COPII, and clathrin-coated vesicles.
COPI vesicles transport molecules between different parts of the Golgi body and from the Golgi back to the rough ER.
COPII vesicles are formed in the ER membranes and mediate transport from ER to the Golgi.
COPI and COPII vesicles are composed of similar coat protein complexes called coatomers. These bend the membrane to form a bud that is released from the donor membrane as a vesicle.
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Q1: What are COP-coated vesicles and how do they differ from other 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.
Q2: How do COPI and COPII vesicles differ in their transport functions?
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.
Q3: What role do coat proteins play in vesicle formation?
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.
Q4: What cargo do COPII-coated vesicles transport from the ER?
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.
Q5: What are the two proposed models for COPII vesicle transport?
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.
Q6: How do coated vesicles maintain protein distribution between the ER and Golgi?
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.
Q7: What structural features do COP coat proteins share with clathrin?
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.