Intracellular Membrane Traffic and Vesicular Transport

Intracellular Membrane Traffic

  • Mechanisms of Protein Trafficking:

    1. Gated transport
      • Example: Nuclear pores.
    2. Transmembrane (translocation)
      • Process: Unfolded proteins pass through translocators from cytosol to different topological spaces.
    3. Vesicular transport
      • Moves between topologically equivalent spaces.

  • Topology of the Cell:

    • Refers to continuity and ability to exchange without crossing the membrane.
    • Major components include:
      • Rough ER
      • Inner and outer nuclear membranes
      • Plasma membrane
      • Golgi apparatus
      • Lysosomes
      • Endosomes

Pathways of Vesicular Transport

  1. Biosynthetic Pathway:
    • Delivers newly synthesized proteins and membranes from ER to other organelles like Golgi, plasma membrane, endosomes, lysosomes.
  2. Exocytosis Pathway:
    • Sends newly synthesized or stored proteins into the extracellular space.
    • Examples include neurotransmitters and insulin.
  3. Endocytosis Pathway:
    • Uptake of external and plasma membrane-bound proteins, ligands, nutrients, and cholesterol.
      • Important for cellular uptake and surface property alteration.

Components of Vesicular Transport

  • Secretory Pathway:
    • Sequence: ER → vesicles → Golgi → vesicles → plasma membrane.
  • Endocytic Pathway:
    • Sequence: Plasma membrane → vesicles → early endosomes → late endosomes → endolysosomes → lysosomes.
  • Retrieval Pathway:
    • Compartment returning to its origin post-delivery.

Cargo Transport and Membrane Identity

  • Maintaining Distinct Organelles:
    • Proteins specific to each organelle dictate what to import and export.
    • Importance of organelle identity markers, which prevent homogenization of organelles.

Vesicular Transport Processes

  1. Membrane Fission:
    • Budding of vesicles from donor compartment and fusion with target compartment.
    • Transfer of cargo (membrane proteins, lipids, soluble molecules).
  2. Budding Mechanism:
    • Mediated by vesicle coats; outer coat shapes membrane, inner coat determines cargo.
  3. Coated Vesicles Types:
    • Clathrin-coated, COPI-coated, and COPII-coated vesicles each serve different transport functions.

Regulation of Vesicle Formation

  • Coat Assembly Regulation:
    • Mediated by adaptor proteins and membrane components.
    • Cargo binding, coat assembly, and disassembly is critical for vesicle construction.
  • Role of GTPases:
    • Monomeric GTPases (e.g., Sar1) regulate coat assembly, while phosphoinositides facilitate selective cargo recruitment and coat assembly.

Membrane Fusion Processes

  • Targeting and Fusion Mechanisms:
    1. Rab GTPases:
      • Direct transport targeting by binding to Rab effectors on target membranes.
    2. SNARE Proteins:
      • v-SNARE (vesicle) and t-SNARE (target) proteins form a trans-SNARE complex to promote membrane fusion.
      • Membrane fusion involves steps: tethering (initial association), docking (specific contact) and fusion (merging of membranes).

Summary of Membrane Traffic

  • Membrane traffic requires:
    1. Formation of transport vesicles carrying membrane, proteins, and luminal content.
    2. Sorting of cargo, membrane fission, targeting by Rab GTPases, and SNARE-mediated fusion.
    3. Specific pathways for ER to Golgi transport and subsequent delivery to lysosomes or extracellular spaces via secretion or recycling pathways.