Cytoskeleton

Introduction

  • Overview of the lecture content and objective.

    • Focus on the functions of the cytoskeleton.

    • Understanding the major building blocks of the cytoskeleton (rails and engines).

    • Exam preparation: emphasize differences between components.

Major Functions of the Cytoskeleton

  • Maintaining Cell Shape

    • Importance: Keeps the cell from collapsing and maintains structural integrity.

    • Interaction with the cell membrane.

  • Cell Movement

    • Role in cell motility (physical movement of the cell).

    • Examples of movement mechanisms (amoeboid movement, cilia, flagella).

  • Cellular Transport

    • Internal movement of cargo within the cell.

    • Utilization of cytoskeleton components to move vesicles and organelles.

  • Cell Division

    • Functioning as a major player in the process of cytokinesis (cell division).

Structure of the Cytoskeleton

  • Rails: Components that provide structure and serve as tracks for movement.

    • Microfilaments (Actin Filaments)

    • Structure: Smallest filament components.

    • Composition: Comprised of actin monomers (G-actin) that polymerize to form filaments (F-actin).

    • Dynamic behavior: Treadmilling; growth at plus end and disassembly at minus end.

    • Microtubules

    • Structure: Largest components of the cytoskeleton.

    • Composition: Made of tubulin heterodimers (alpha and beta tubulin).

    • Formation: Aligns into protofilaments, forming hollow tubes.

  • Intermediate Filaments

    • Characteristics: Intermediate size, do not serve as pathways for transport.

    • Composition: Made of varied proteins (e.g., keratin, vimentin).

    • Role: Primarily provide structural support rather than transport.

Prokaryotes and Cytoskeleton

  • Prokaryotes do possess a form of cytoskeleton.

  • Functions similarly but is simpler than in eukaryotes.

  • Evolutionary perspective: Eukaryotic cytoskeleton is more similar to that of archaea than bacteria.

Specific Components of the Cytoskeleton

  • Microfilaments

    • Actin forms are involved in processes like amoeboid movement.

    • Polymerization is ATP-dependent; G-actin is the monomeric subunit.

    • Dynamics: Plus end grows faster than the minus end, where ADP-bound actin is more prevalent.

    • Process of treadmilling: coordinated addition and removal of actin units.

  • Microtubules

    • Comprised of alpha and beta tubulin, forming heterodimers and protofilaments.

    • GTP is essential for assembly; operates under a dynamic instability model.

    • Treadmilling also occurs, but a distinct feature is the process of dynamic instability due to varying concentrations of tubulin.

Motors of the Cytoskeleton

  • Engines: Kinesin, Dynein, Myosin

    • Movement directionality:

    • Kinesin: Moves vesicles from minus to plus end on microtubules.

    • Dynein: Moves from plus to minus end on microtubules.

    • Myosin: Generally moves along microfilaments and can be directional depending on the type.

  • Mechanisms of Action:

    • All engines perform their function using ATP to power movement.

    • Each has specific binding sites for cargo (vesicles) and conformational changes allow for locomotion along the cytoskeletal rails.

Mechanisms of Kinesin Movement

  • Kinesin, as an example of motor proteins:

    • Composed of two motor heads and a cargo-binding region.

    • Movement process:

    1. Binding of one head to the microtubule (ADP-bound).

    2. ATP binding to the head leads to conformational change, allowing movement of the other head forward.

    3. Hydrolysis of ATP causes one foot to release from the track.

    4. Process repeats, resulting in stepwise movement of the cargo.

Cilia and Flagella

  • Structure:

    • Composed of microtubules arranged in a specific pattern (9+2 for motile; 9+0 for non-motile).

    • Function in cell motility and movement of fluids across cell surfaces.

Conclusion and Implications

  • The cytoskeleton's role in cellular processes is critical for maintaining shape, facilitating movement, and enabling transport.

  • The mechanisms described, along with the structure-function relationships of the cytoskeletal components, illustrate the complex interplay necessary for cellular function and behavior on both micro and macro scales.

  • Future implications include understanding how cytoskeletal functions can be disrupted in pathological conditions and potential therapeutic approaches to target these systems.