JA

Cytoskeleton, Actin, and Listeria Infection

Cytoskeleton: Overview and Actin

Introduction to Cytoskeleton

  • Definition: The cytoskeleton is a dynamic network of protein filaments that provides structural support, facilitates cell motility, and plays roles in intracellular transport and cell division.
  • Main Cytoskeletal Elements: Most human cells contain three primary types, differentiated by diameter:
    • Microfilaments (Actin Filaments): The smallest, with a diameter of approximately 8 \, \text{nm}. Key for cell shape, motility, and resisting mechanical stress.
    • Microtubules: The largest, with a diameter of approximately 25 \, \text{nm}. Hollow, water-filled tubes. Essential for cell shape, motility, and intracellular transport.
    • Intermediate Filaments: Intermediate in size, with a diameter of approximately 10 \, \text{nm}. Primarily provide mechanical strength and resistance to stress.
  • Other Cytoskeletal Elements: Briefly mentioned are Septins, which also form cytoskeletal elements.
  • Previously Covered: Spectrin-based cytoskeleton is an important membrane-associated cytoskeleton linked to the erythrocyte membrane, covered in other lectures.
  • Visualization: Historically difficult to identify using traditional light or electron microscopy stains. Immunofluorescence or fluorescent protein fusions are preferred methods for visualization in fixed or living cells, respectively.
    • Example Micrographs: Immunofluorescence images show actin in red, microtubules in green, and chromatin/chromosomes in blue.
    • Interphase vs. Mitosis: Compared configurations in an interphase cell (actin at periphery, microtubules extending) versus a mitotic cell (rounded actin cortex, mitotic spindle formed by microtubules attaching to chromosomes).

Core Functions of the Cytoskeleton

  • Cell Division and Cytokinesis: Microtubules form the mitotic spindle (attaching to chromosomes), while actin forms the contractile ring during cytokinesis.
  • Cell Shape and Motility: Regulates cell shape and enables migration (e.g., neutrophils).
  • Mechanical Stress Resistance: Provides structural integrity and strength to cells and tissues.
  • Specialized Structures:
    • Neutrophil Chemotaxis: Neutrophils, professional phagocytes, utilize actin to form a lamellipodium for directional movement (chemotaxis) towards bacteria. Phagocytosis itself also relies on the actin cytoskeleton.
    • Epithelial Support: In simple columnar epithelia with apical microvilli:
      • Actin bundles support non-motile microvilli, increasing apical plasma membrane surface area for nutrient absorption. These bundles terminate in a terminal web anchored to zonulae adherens.
      • Intermediate filaments are attached to desmosomes and hemidesmosomes, transmitting forces between cells to ensure the epithelium functions as a single unit.
      • Microtubules have a polarized distribution, though not associated with junctions.

Comparing Actin Filaments and Microtubules

  • Polarity: Both actin filaments and microtubules are polarized polymers of asymmetric subunits, having a distinct plus (+) and minus (-) end.
    • Intermediate filaments are not polarized; they are assembled from anti-parallel units and have similar ends.
  • Subunit Composition and Strength:
    • Actin Filaments: Composed of a single actin polypeptide chain monomer (G-actin) that polymerizes. They are single-stranded and relatively easier to break.
    • Microtubules: Composed of tubulin dimers (alpha-beta tubulin) as the polymerizing unit. They contain 13 protofilaments with strong side-to-side and end-to-end interactions, making them physically stronger. Microtubules are generally not severed in cells, unlike actin filaments.

Actin Filaments (F-actin)

  • Monomer (G-actin):
    • A single polypeptide chain.
    • Binds ATP.
    • Possesses intrinsic ATP hydrolytic activity (ATP to ADP + Pi).
    • Asymmetric, with a plus and minus end, dictating filament polarity.
  • Polymer (F-actin):
    • Forms a thin filament, approximately 8 \, \text{nm} in diameter.
    • Helical repeating structure.
    • Plus (Barbed) End: Characterized by rapid growth/polymerization. Appears