Chapter 17B

Overview of the Cytoskeleton

  • Components:
    • Actin
    • Tubulin
    • Chromatin

Microtubules

  • Structure: 25 nm in diameter
  • Dynamic: Changes in length rapidly

Intermediate Filaments

  • Structure: 10 nm in diameter
  • Stable: Maintain structural integrity

Actin Filaments

  • Smallest component: 7 nm in diameter
  • Dynamic: Key role in cell motility and signaling
  • Length: Can reach up to 25 μm
  • Characteristics: Form the dynamic cytoskeleton, essential in various cell processes.
    • Most dynamic cytoskeletal component: Supports movement and adapts to signals.

Actin-Rich Structures

  • Primarily composed of actin
  • Types of Structures Include:
    • (A) Microvilli
    • (B) Lamellipodia
    • (C) Contractile ring
    • (D) Stress fibers

Actin as a Protein

  • Function: ATP binding protein with "slow" ATPase activity
    • Structure: 42 kD in size
    • Pathways:
    • ATP enters the system
    • Products of ATP hydrolysis exit the system

Tubulin and Microtubules

  • Tubulin Dimer:
    • Composed of two subunits:
    • a-tubulin
    • β-tubulin
  • Function: GTPases, vital for microtubule dynamics
    • Proper functioning relies on the hydrolysis of GTP.

Nucleation of Actin Filaments

  • Formin Dimer:
    • Role: Nucleates formation of new actin filaments
    • Structure:
    • Has a plus and minus end orientation
    • Formin binds to monomers, initiating filament growth

Listeria monocytogenes Outbreak

  • Associated with melons grown on the ground
  • Advisories on how to wash produce to avoid Listeria infection.

Dynamics of Actin

  • Actin-Specific Drugs:
    • Phalloidin: Binds and stabilizes actin filaments; caps filament plus ends to stop growth
    • Cytochalasin: Severing drug that prevents polymerization and stops growth
    • Swinholide: Similar function as Cytochalasin
    • Latrunculin: Binds subunits and prevents their polymerization

Actin Binding Proteins (over 250 identified)

  • Categories and Functions:
    • Severing Proteins: Cofilin, Gelsolin
    • Nucleating Proteins: Formin, ARPs
    • Cross-Linking Proteins: Filamin
    • Monomer-Sequestering Proteins: Thymosin-4
    • Bundling Proteins: Fimbrin, important in filopodia
    • Motor Proteins: Myosin
    • Side-Binding Proteins: Tropomyosin
    • Capping Proteins: Prevent growth and stabilize filaments

Structure of Actin Filaments in Red Blood Cells (RBCs)

  • Spectrin-Rich Cortex:
    • Acts as a support network for RBCs and contributes to durability and flexibility
    • The structure is crucial for sustaining the functions of RBCs.

Motor Proteins and Their Functions

  • Myosin Family:
    • Defined by the presence of a motor domain
    • Different types include: Myosin II, Myosin I, Myosin V, etc.
  • Roles:
    • Myosin motors are primarily plus-end directed and involved in muscle contraction.
    • Myosin II motors tend to form bundles, pulling actin filaments against each other to facilitate contraction.
  • Skeletal Muscle:
    • Myosin II motors aggregate into large bundles with approximately 300 motor heads available for contraction.

Conclusion: Actin Dynamics and Cell Movement

  • Important in motile cells (e.g., keratocytes)
  • Structures like the lamellipodium rely on the dynamic properties of actin for movement across surfaces.