Microfilaments and Cytoskeleton Quiz

Cytoskeleton and Its Composition

• The cytoskeleton is a protein filament network within the cell's cytoplasm, crucial for structure and function.
• Microfilaments are primarily composed of actin, providing flexibility and a dynamic structure.
• Intermediate filaments consist of proteins like vimentin and lamin, offering mechanical strength without flexibility.
• Microtubules are large, tubulin-based structures that provide support and shape to the cell.

Actin Filaments and Their Characteristics

• Microfilaments, made of actin and myosin, facilitate various cellular processes including plasma membrane movement.
• F-actin is filamentous actin, while G-actin refers to monomeric actin units; two G-actin molecules form F-actin.
• Actin filaments exhibit polarity, with a slow-growing minus end and a fast-growing plus end.

Actin Dynamics

• Treadmilling describes the process where G-actin is added to the plus end of a filament and simultaneously removed from the minus end, essential for maintaining filament length.
• ATP-bound G-actin is necessary for polymerization into actin filaments.

Regulation by Actin-Binding Proteins

• Various proteins regulate actin dynamics:
  • Thymosin prevents G-actin polymerization, maintaining a pool of monomers.
  • Profilin facilitates ATP-ADP exchange on actin, promoting filament formation.
  • Gelsolin caps the plus end of filaments and can sever them, playing a role in cellular motility.
  • Cytochalasins inhibit actin polymerization, halting cell movement.
  • Phalloidins bind tightly to actin filaments, preventing depolymerization.

Spectrin and Cell Structure Maintenance

• Spectrin anchors actin filaments to the cell membrane, essential for maintaining the shape of red blood cells.
• Junctional complexes involve spectrin tetramers linked with short actin filaments.
• Ankyrin connects spectrin to membrane proteins, such as band 3, and helps stabilize the connection between membrane and cytoskeleton.

Disorders Related to Cytoskeleton Mutations

• Hereditary spherocytosis results from mutations in ankyrin, causing spherical red blood cells.
• Hereditary elliptocytosis is linked to spectrin mutations that lead to elliptical red blood cells.
• Sickle cell anemia is associated with issues in actin modifications, causing aggregated forms that disrupt normal cell function.

Structural Components of Epithelial Cells

• Microvilli, found on epithelial cells' apical surfaces, enhance surface area and consist of actin filaments bundled by fimbrin and villin.
• The terminal web is a network of actin and associated proteins anchored to the plasma membrane, contributing to cell stability.

Dystrophin and Muscle Function

• Dystrophin is a crucial protein in skeletal muscles, anchoring actin filaments to cell membranes and associated with muscular dystrophy.