Vesicle Coat Map (COPII, COPI, Clathrin)

Core Concept

Intracellular transport between organelles is mediated by coated vesicles. A small GTPase (Sar1 for COPII, Arf1 for COPI and clathrin at the Golgi) is activated by a GEF, inserts into the donor membrane, and recruits coat proteins that bend the membrane and concentrate cargo into a bud; the vesicle pinches off and then sheds its coat to expose fusion machinery. COPII moves cargo forward (ER → Golgi, anterograde); COPI moves cargo backward (Golgi → ER, retrograde, retrieving escaped ER-resident proteins); clathrin mediates traffic from the Golgi to endosomes/lysosomes and from the plasma membrane inward (endocytosis). Each coat thus encodes a direction and a cargo set, powered by GTP hydrolysis.

Key Components

Mechanism

1. Activation. A GEF (e.g., Sec12 for Sar1) exchanges GDP for GTP on the small GTPase, exposing an amphipathic helix that inserts into the membrane.
2. Coat assembly. Membrane-bound GTPase recruits coat subunits (Sec23/24, coatomer, or AP + clathrin); adaptors simultaneously bind cargo and coat, concentrating cargo into the bud.
3. Budding and scission. The coat deforms the membrane into a vesicle; at the plasma membrane dynamin (a GTPase) constricts and pinches the neck; COPII/COPI buds are thought to self-assemble and pinch off.
4. Uncoating. A GAP stimulates GTP hydrolysis, releasing the coat so the vesicle can tether and fuse (coat removal is required for fusion).
5. Fusion. The uncoated vesicle fuses with its target via SNAREs and Rab GTPases.

Energy and Directionality

Coat assembly is self-assembly driven by GTP: the GTP-bound form of Sar1/Arf1 is the "on" state that recruits coat; GTP hydrolysis (accelerated by GAPs) drives coat disassembly, making the cycle irreversible and directional. Dynamin consumes GTP to perform membrane scission. Directionality is thus written twice — in which GTPase/adaptor is used (defining the donor and destination) and in the GTP on/off switch that prevents reverse assembly. Additional directionality comes from cargo-receptor and SNARE pairing at the target membrane.

Experimental Evidence

• Temperature-sensitive mutants (Schekman, yeast): sec mutants block ER→Golgi transport and identified COPII components; accumulation of vesicles in mutants revealed coat requirements.
• Cell-free budding assays: adding GTPγS (non-hydrolyzable) or blocking GAP activity causes coat accumulation and blocks uncoating/fusion — proving GTP hydrolysis is needed for disassembly.
• KDEL/KDEL-receptor studies: ER-resident proteins (e.g., BiP) carry a KDEL sequence that the KDEL receptor retrieves from the Golgi back to the ER via COPI — evidence for retrograde salvage.
• Dominant-negative dynamin (GTPase-dead): deep invaginations with collared necks accumulate at the plasma membrane, proving dynamin's scission role in endocytosis.
• Electron microscopy: distinct coat morphologies (COPII cages, clathrin triskelion lattices) directly visualized the coat architectures.

Common confusions

• "COPII and COPI go the same direction" — Opposite: COPII is anterograde (ER→Golgi), COPI is retrograde (Golgi→ER).
• "Coat proteins are permanent vesicle parts" — Coats are removed before fusion; a coated vesicle cannot fuse (coat hides SNAREs).
• "GTP hydrolysis builds the coat" — GTP binding builds it; hydrolysis takes it apart.
• "Dynamin coats all vesicles" — Dynamin (scission GTPase) is key at the plasma membrane for clathrin; COPII/COPI buds pinch off without dynamin.
• "Clathrin itself selects cargo" — Adaptors (AP complexes) select cargo and link it to the clathrin cage.

Quick review

• COPII (Sar1, Sec23/24+13/31) ER→Golgi; COPI (Arf1, coatomer) Golgi→ER; clathrin (AP1/AP2) Golgi→lysosome and PM→endosome (dynamin).
• GTP-bound GTPase recruits coat; GAP-driven hydrolysis uncoats; fusion requires uncoating.
• KDEL receptor drives retrograde retrieval of ER residents.