17.2 Vesicle formation, targeting, and fusion

Vesicle Formation and Cargo Selection

Cells constantly shuttle materials between compartments using membrane-bound vesicles. Each vesicle has to form at the right place, pick up the right cargo, travel to the right destination, and fuse with the correct target membrane. The specificity of this system depends on coat proteins, SNAREs, and Rab GTPases working together.

Role of Coat Proteins

Three major coat protein families drive vesicle formation, and each one operates on a specific transport route:

• Clathrin coats vesicles involved in endocytosis at the plasma membrane and in transport from the trans-Golgi network (TGN) to endosomes. Clathrin assembles into a lattice of triskelions (three-legged structures) that molds the membrane into a coated pit.
• COPII coats vesicles that move cargo in the anterograde direction, from the ER to the Golgi. If a newly synthesized protein needs to leave the ER, COPII is the coat that gets it there.
• COPI coats vesicles that move cargo in the retrograde direction, from the Golgi back to the ER, and also between Golgi cisternae. This retrieval pathway recycles ER-resident proteins that escaped forward and returns transport machinery for reuse.

A common point of confusion: coat proteins don't grab cargo directly. Instead, they rely on adaptor proteins that serve as intermediaries. Adaptor proteins recognize sorting signals on the cytoplasmic tails of transmembrane cargo molecules (receptors, enzymes, etc.) and simultaneously bind the coat proteins. For clathrin-coated vesicles, the key adaptors are the AP complexes (AP-1 at the TGN, AP-2 at the plasma membrane). For COPII vesicles, the Sec23/24 complex acts as the cargo-binding adaptor.

Vesicle Budding, Targeting, and Fusion

Mechanisms of Vesicle Transport

Vesicle budding follows a general sequence:

1. Coat assembly and membrane curvature. Coat proteins are recruited to the donor membrane (often by a small GTPase like Sar1 for COPII or ARF1 for COPI and clathrin). As coat subunits polymerize, they deform the membrane into a bud.
2. Cargo selection. Adaptor proteins capture transmembrane cargo and soluble lumenal cargo (which binds to cargo receptors) into the forming bud.
3. Scission. The neck of the bud is severed to release a free vesicle. For clathrin-coated vesicles, the GTPase dynamin wraps around the neck and uses GTP hydrolysis to pinch it off. COPI and COPII vesicles use different scission mechanisms that are less well characterized.
4. Uncoating. The coat is disassembled shortly after budding so the vesicle can interact with targeting and fusion machinery. For clathrin, the chaperone Hsc70 and its cofactor auxilin drive uncoating.

Vesicle targeting ensures each vesicle reaches the correct compartment:

• Rab GTPases on the vesicle surface recruit tethering factors, which are long, extended proteins or multi-subunit complexes that catch the vesicle and hold it near the target membrane before SNAREs engage. Think of tethering as a long-range capture step.
• Different Rab proteins mark different compartments: Rab5 marks early endosomes, Rab7 marks late endosomes, Rab11 marks recycling endosomes. This compartment-specific Rab identity is a major source of targeting specificity.

Vesicle fusion delivers the cargo:

1. After tethering brings the vesicle close, v-SNAREs (on the vesicle) and t-SNAREs (on the target membrane) zipper together from their N-termini toward the membrane to form a tight four-helix bundle called the trans-SNARE complex.
2. This zippering pulls the two lipid bilayers into direct contact, overcoming the energy barrier to membrane fusion.
3. The membranes merge, releasing the vesicle's lumenal contents into the target compartment (or inserting transmembrane cargo into the target membrane).
4. After fusion, NSF (an AAA+ ATPase) and α-SNAP disassemble the cis-SNARE complex so the individual SNAREs can be recycled for another round.

Proteins for Vesicle Targeting

SNARE proteins (Soluble NSF Attachment protein REceptors) are the core fusion machinery:

• v-SNAREs (also called R-SNAREs because they contribute an arginine residue to the complex) sit on the vesicle.
• t-SNAREs (also called Q-SNAREs because they contribute glutamine residues) sit on the target membrane. A functional SNARE complex typically contains one R-SNARE and three Q-SNAREs.
• Specific v-SNARE/t-SNARE pairings add another layer of targeting specificity. For example, the vesicle SNARE VAMP2/synaptobrevin pairs with syntaxin-1 and SNAP-25 at the presynaptic membrane.

Rab GTPases act as molecular switches:

• In the GTP-bound (active) state, Rabs associate with the vesicle membrane and recruit effectors (tethering factors, motor proteins).
• In the GDP-bound (inactive) state, Rabs are extracted from the membrane by GDI (GDP dissociation inhibitor) and held in the cytosol.
• GEFs (guanine nucleotide exchange factors) activate Rabs; GAPs (GTPase-activating proteins) inactivate them. This cycling is what makes Rab-mediated targeting regulatable.

Tethering factors bridge the gap between vesicle and target before SNAREs can engage:

• Coiled-coil tethers (e.g., p115, GM130, EEA1) are long, rod-shaped proteins that can reach across distances of up to 200 nm.
• Multi-subunit tethering complexes (e.g., the exocyst, HOPS complex, COG complex) coordinate with Rabs and SNAREs to ensure docking fidelity.

Importance of Vesicle Fusion

These molecular mechanisms underlie some of the most critical secretory events in the body:

• Neurotransmitter release. Synaptic vesicles loaded with neurotransmitters (glutamate, GABA, acetylcholine) dock at the active zone of the presynaptic terminal. When an action potential triggers $Ca^{2+}$ influx through voltage-gated channels, the calcium sensor synaptotagmin on the vesicle binds $Ca^{2+}$ and accelerates SNARE-mediated fusion. Neurotransmitters are released into the synaptic cleft within a millisecond, enabling rapid signal transmission between neurons.
• Insulin secretion. Pancreatic beta cells store insulin in dense-core secretory granules. When blood glucose rises, increased glucose metabolism raises intracellular ATP, closing $K_{ATP}$ channels, depolarizing the cell, and opening voltage-gated $Ca^{2+}$ channels. The resulting $Ca^{2+}$ influx triggers SNARE-dependent fusion of insulin granules with the plasma membrane. Disruption of this pathway contributes to diabetes.
• Exocytosis of digestive enzymes. Pancreatic acinar cells package digestive enzymes (amylase, lipase, trypsinogen) into zymogen granules. Upon stimulation (e.g., by cholecystokinin), these granules fuse with the apical plasma membrane, releasing their contents into the pancreatic duct and ultimately the small intestine. The enzymes are secreted as inactive precursors (zymogens) to prevent self-digestion of the pancreas.