-cytoskeleton

Introduction to the Cytoskeleton

  • The cytoskeleton is a highly dynamic scaffold of protein filaments extending throughout the cytoplasm of eukaryotic cells.

  • Primary Functions:

    • Structural Support: Acts as the "bones" of the cell, maintaining morphology.

    • Motility: Acts as "muscles," facilitating both intracellular transport and whole-cell crawling.

    • Signal Transduction: Links the extracellular matrix to the nucleus, influencing gene expression.

    • Cell Division: Critical for chromosome segregation and cytokinesis.

Structure and Function of the Cytoskeleton

  • The cytoskeleton is primarily composed of three distinct filament types that differ in size, protein composition, and mechanical properties:

    1. Actin Filaments (AF): Microfilaments (~7 nm7\text{ nm} diameter).

    2. Microtubules (MTs): Large hollow tubes (~25 nm25\text{ nm} diameter).

    3. Intermediate Filaments (IF): Rope-like fibers (~8−12 nm8-12\text{ nm} diameter).

Actin Filaments (Microfilaments)

  • Molecular Composition:

    • Composed of globular actin (G-actin) subunits that polymerize into filamentous actin (F-actin).

    • The filament has a distinct polarity: a "plus end" (fast-growing) and a "minus end" (slow-growing).

  • Dynamics and Regulation:

    • Treadmilling: Occurs when the rate of assembly at the plus end matches the rate of disassembly at the minus end. This is fueled by ATP hydrolysis.

    • Actin-Binding Proteins (ABPs): Regulate the pool of available G-actin monomers and cross-link filaments.

  • Functional Roles:

    • Cell Movement: Formation of lamellipodia and filopodia via polymerization at the leading edge.

    • Contractility: Interaction with Myosin II motors drives muscle contraction and the formation of the contractile ring during cytokinesis.

Microtubules (MTs)

  • Structure:

    • Formed from heterodimers of α\alpha-tubulin and β\beta-tubulin.

    • These dimers align head-to-tail to form protofilaments; 13 protofilaments typically associate side-to-side to form a hollow tube of ~25 nm25\text{ nm}.

    • Like actin, MTs are polar: β\beta-tubulin is exposed at the plus end, and α\alpha-tubulin at the minus end.

  • Dynamic Instability:

    • Microtubules undergo rapid cycles of growth and shrinkage. Growth depends on a "GTP cap" at the plus end.

    • If GTP hydrolysis catches up to the rate of addition, the GTP cap is lost, leading to a "catastrophe" (rapid depolymerization).

  • Organizing Centers (MTOCs):

    • The Centrosome is the primary MTOC in animal cells, containing a pair of centrioles and γ\gamma-tubulin ring complexes that nucleate microtubule growth.

  • Motor Proteins:

    • Kinesins: Usually move cargo toward the plus end (cell periphery).

    • Dyneins: Move cargo toward the minus end (cell center/centrosome).

Intermediate Filaments (IFs)

  • Structure and Properties:

    • IFs are non-polar (unlike AF and MT) and are characterized by high tensile strength. They do not undergo treadmilling or dynamic instability.

  • Diversity and Tissue Specificity:

    • Cytoplasmic: Keratins (epithelia), Vimentin (connective tissue/muscle), Neurofilaments (neurons).

    • Nuclear: Nuclear Lamins (support the nuclear envelope in all animal cells). Disassembly is regulated by phosphorylation during cell division.

  • Cell Junctions:

    • IFs link to Desmosomes (cell-to-cell) and Hemidesmosomes (cell-to-matrix), distributing mechanical stress across a tissue sheet to prevent tearing.

Experimental Methodology and Pharmacological Tools

  • Specific Inhibitors:

    1. For Actin:

      • Cytochalasin D: Binds to the plus end, preventing polymerization.

      • Phalloidin: Binds and stabilizes filaments, preventing depolymerization.

    2. For Microtubules:

      • Colchicine/Colcemid: Binds tubulin dimers, preventing assembly.

      • Taxol (Paclitaxel): Stabilizes microtubules, preventing disassembly; used clinically as a chemotherapy drug to inhibit mitosis in cancer cells.