1.4 Vesicular Transport
Endocytosis, Exocytosis, and Membrane Trafficking
Learning Objectives
- •Distinguish between exocytosis, phagocytosis, pinocytosis, and receptor-mediated endocytosis
- •Explain the role of clathrin, caveolin, and SNARE proteins in vesicle formation and fusion
- •Describe the endosomal pathway and lysosomal degradation
- •Understand transcytosis and its importance in epithelial cells
Overview of Vesicular Transport
While small molecules cross the membrane through channels and transporters, large molecules (proteins, polysaccharides, lipoproteins) and even entire cells require vesicular transport. This involves the formation of membrane-bound vesicles that either bud from or fuse with the plasma membrane.
Key Distinction
Exocytosis: Vesicles fuse with plasma membrane, releasing contents outward
Endocytosis: Plasma membrane invaginates to internalize material
Exocytosis
Exocytosis is the process by which intracellular vesicles fuse with the plasma membrane to release their contents into the extracellular space.
Constitutive Exocytosis
- • Continuous, unregulated process
- • Delivers membrane proteins and lipids
- • Secretes extracellular matrix components
- • Present in all cells
- • No external signal required
Regulated Exocytosis
- • Signal-dependent (often Ca²⁺)
- • Secretory vesicles stored until triggered
- • Examples: neurotransmitter release, hormone secretion
- • Specialized secretory cells
- • Rapid, precise timing
SNARE-Mediated Membrane Fusion
Vesicle fusion is mediated by SNARE (Soluble NSF Attachment protein REceptor) proteins, which form a four-helix bundle that brings membranes together.
v-SNAREs
Vesicle membrane
VAMP/Synaptobrevin
t-SNAREs
Target membrane
Syntaxin, SNAP-25
Regulatory
Ca²⁺ sensor
Synaptotagmin
Clinical Note: Botulinum toxin cleaves SNARE proteins, blocking neurotransmitter release and causing paralysis. Tetanus toxin similarly affects inhibitory interneurons in the spinal cord.
Endocytosis
Endocytosis internalizes extracellular material by plasma membrane invagination. Three major types exist based on mechanism and cargo size.
Phagocytosis ("Cell Eating")
Receptor-triggered engulfment of large particles (>0.5 μm) including bacteria, dead cells, and debris.
- • Requires actin polymerization (pseudopod extension)
- • Forms phagosome → phagolysosome
- • Professional phagocytes: macrophages, neutrophils, dendritic cells
- • Fc receptors recognize antibody-coated particles (opsonization)
Receptors Involved
- FcγR: IgG-coated particles
- CR3: Complement-coated particles
- Mannose receptor: Pathogen carbohydrates
- Scavenger receptors: Modified LDL, apoptotic cells
Pinocytosis ("Cell Drinking")
Non-selective uptake of extracellular fluid and small solutes through small vesicles (<150 nm).
Macropinocytosis
- • Actin-driven membrane ruffling
- • Forms large vesicles (0.5-5 μm)
- • Important for antigen sampling
- • Some tumor cells use for nutrient uptake
Caveolae-Mediated
- • Small flask-shaped invaginations (50-80 nm)
- • Caveolin-1 coat protein
- • Rich in cholesterol and sphingolipids
- • Important in endothelial transcytosis
Receptor-Mediated Endocytosis (RME)
Highly selective, high-affinity uptake of specific ligands via clathrin-coated pits.
Clathrin-Coated Vesicle Formation
- 1. Cargo selection: Adaptor proteins (AP2) recognize sorting signals in receptor cytoplasmic tails
- 2. Coat assembly: Clathrin triskelions polymerize into lattice, curving membrane
- 3. Scission: Dynamin GTPase wraps around neck, pinches off vesicle
- 4. Uncoating: Hsc70 + auxilin remove clathrin coat
- 5. Fusion: Vesicle fuses with early endosome
Classic Examples of RME
| Ligand | Receptor | Function |
|---|---|---|
| LDL | LDLR | Cholesterol uptake |
| Transferrin-Fe³⁺ | TfR | Iron delivery |
| Insulin | IR | Hormone signaling, glucose uptake |
| EGF | EGFR | Growth signaling, receptor downregulation |
| IgG (neonatal) | FcRn | Maternal antibody transfer |
The Endosomal Pathway
After internalization, endocytic vesicles fuse with early endosomes, where cargo is sorted for recycling, degradation, or transcytosis.
pH ~6.0-6.5. Primary sorting station. Contains Rab5 GTPase. Receives incoming vesicles from plasma membrane.
Rab11-positive. Returns receptors (TfR, LDLR) to plasma membrane. Located perinuclearly.
pH ~5.5. Contains Rab7. Also called MVB (multivesicular body). Forms intraluminal vesicles via ESCRT machinery.
pH ~4.5-5.0. Terminal degradative compartment. Contains ~60 hydrolases (proteases, lipases, nucleases, glycosidases).
Case Study: LDL Receptor Pathway
- 1. LDL binds LDLR in clathrin-coated pit
- 2. Vesicle internalizes, uncoats
- 3. Fuses with early endosome (pH 6.0)
- 4. Low pH releases LDL from receptor
- 5. LDLR recycles to plasma membrane
- 6. LDL goes to lysosome
- 7. Cholesterol esters hydrolyzed
Familial Hypercholesterolemia
Mutations in LDLR (or its adaptor ARH, or PCSK9) cause defective LDL uptake. Heterozygotes: 2-3× elevated LDL, early CAD. Homozygotes: 6-8× elevated LDL, MI in childhood without treatment.
Transcytosis
Transcytosis moves cargo across polarized epithelial or endothelial cells—endocytosis at one surface followed by exocytosis at the opposite surface.
Maternal IgG Transfer
- • FcRn binds IgG at pH 6 (endosome)
- • Releases IgG at pH 7.4 (plasma)
- • Syncytiotrophoblast → fetal circulation
- • Provides passive immunity to neonate
- • Also extends IgG half-life in adults
Intestinal IgA Transport
- • Polymeric Ig receptor (pIgR) binds dimeric IgA
- • Basolateral → apical transcytosis
- • pIgR cleaved, releasing secretory IgA
- • Protects mucosal surfaces
- • Similar mechanism in breast milk
Key Proteins in Vesicular Transport
| Protein | Function | Location/Process |
|---|---|---|
| Clathrin | Coat protein, membrane curvature | RME, TGN to endosome |
| AP2 | Adaptor, links cargo to clathrin | Plasma membrane RME |
| Dynamin | GTPase, vesicle scission | Clathrin and caveolar endocytosis |
| Caveolin | Coat protein for caveolae | Lipid raft endocytosis |
| Rab GTPases | Vesicle identity and trafficking | Rab5 (early), Rab7 (late), Rab11 (recycling) |
| SNAREs | Membrane fusion machinery | All vesicle fusion events |
| ESCRT | Intraluminal vesicle formation | MVB biogenesis, autophagy |
Clinical Relevance
Lysosomal Storage Diseases
- Gaucher disease: β-glucocerebrosidase deficiency
- Tay-Sachs: Hexosaminidase A deficiency
- Niemann-Pick: Sphingomyelinase deficiency
- Pompe disease: α-glucosidase deficiency
- Treatment: Enzyme replacement therapy for some
Pathogen Entry
- Viruses: HIV, influenza use RME for entry
- Bacteria: Listeria, Salmonella exploit phagocytosis
- Toxins: Diphtheria, anthrax require endosomal acidification
- Therapeutic target: Chloroquine blocks endosomal pH drop