Sequence_01_13


Overview of Cytoskeleton

  • Definition: The cytoskeleton is the interior architecture of the cell, contributing to its shape and structure.

  • Classes of Cytoskeletal Components:

    • Microtubules:

      • Size: 24 nanometers in diameter.

      • Made from polymers of a protein called tubulin.

    • Intermediate Filaments:

      • Size: 10 nanometers in diameter.

      • Composed of various proteins, forming a catch-all category.

    • Microfilaments:

      • Size: 6 nanometers in diameter.

      • Made from the protein actin.

Assembly and Disassembly of Cytoskeleton

  • Dynamic nature with ability to assemble/disassemble on demand for cellular shape changes.

  • All assembly and disassembly processes are governed by non-covalent interactions, allowing rapid changes.

  • Types of Pools:

    • Soluble pool (individual subunits)

    • Polymerized pool (long filaments)

Regulation of Assembly and Disassembly

  • Both microfilaments and microtubules function based on the binding of nucleotide triphosphates (NTPs): GDP and ADP which affects polymerization.

  • Filament Polarity:

    • Plus End: New monomers are more likely added, has higher affinity for NTPs.

    • Minus End: Monomers are less likely to be added, preferentially tends to lose units, contributing to depolymerization.

Actin as a Component of the Cytoskeleton

  • Structure: Flexible network; primarily found below the plasma membrane. Acts as the primary structural component defining cell shape.

  • Concentration in Cells:

    • Contractile rings for cell division

    • Strengthening microvilli for increased surface area in intestinal cells

    • Function in muscle contractions and intracellular transport.

Protein Types Affecting Actin Dynamics

  • Nucleation: Beginning process of filament assembly regulated by Arp 2/3 complex, encouraging polymerization.

  • Proteins that affect actin filament structure:

    • Thymosins: Inhibit polymerization by sequestering actin monomers.

    • Cofilin: Increases depolymerization rate by binding to ADP-bound monomers.

    • Fimbrin and others: Cross-link actin to create stiffer structures or bundles.

Dynamic Instability and Treadmilling

  • Dynamic Instability: State of constant turnover of subunits within a filament while maintaining the same overall length.

  • Treadmilling: The phenomenon where addition at the plus end matches loss at the minus end, causing the filament to appear to move through the cell.

  • Example: Traffic jams demonstrating migration of threads.

GTPases and Actin Regulation

  • Regulatory Roles: Small GTPases (like Rac and Rho) directly affect polymerization and structure. For example:

    • Rac promotes lamellipodia formation (sheet-like projections) for cell movement.

    • Rho regulates stress fiber formation (bundling) for structural support.

    • Cdc42 facilitates short projections known as filopodia.

Summary

  • Cytoskeletal dynamics are regulated by multiple levels, from nucleotide binding to GTPase control, influencing cell motility and structure. Understanding this regulation is crucial for grasping cellular behavior during processes such as division and movement.