Cytoskeleton in Essential Cell Biology

The cytoskeleton is a complex network of protein filaments that provides support and shape to the cell.
It is highly dynamic, reorganizing itself as the cell changes shape, moves, or divides.

Actin Filaments:
- Diameter: 7 nm
- Structure: Helical polymers made of actin proteins.
- Function: Maintains cell shape, enables movement, and is concentrated in the cell cortex.
Microtubules:
- Diameter: 25 nm
- Structure: Hollow cylinders made of alpha and beta tubulin proteins.
- Function: Support cell shape, provide tracks for intracellular transport, and are involved in cell division.
Intermediate Filaments:
- Diameter: 10 nm
- Structure: Ropelike fibers made of fibrous intermediate filament proteins.
- Function: Provide mechanical strength to cells, support nuclear membrane structure, and maintain nuclear shape.

Compare and contrast microtubules, actin filaments, and intermediate filaments.
Describe their formation and functions in detail.
Understand the impact of anti-cancer drugs on microtubule dynamics, including treadmilling and dynamic instability.
Analyze the role of motor proteins in cytoskeleton mechanics, including muscle contraction and cell migration.

Cytoskeletal filaments are dynamic, rapidly assembling and disassembling.
Regulated by accessory proteins through noncovalent interactions between small subunits.
Small subunits facilitate quick localization, resulting in rapid cellular responses.

Assembled from tubulin heterodimers (α and β tubulin) and exhibit structural polarity (plus end and minus end).
GTP-bound tubulins stabilize the MT at the plus end; GDP-bound tubulins induce disassembly at the minus end.
Dynamic instability allows MT to grow and shrink rapidly, while treadmilling occurs when addition and loss rates of monomers are equal, maintaining constant length.
Microtubule Organizing Center (MTOC), primarily the centrosome, initiates MT assembly and orientation.

Taxol stabilizes microtubules, preventing depolymerization, while Colchicine inhibits polymerization of tubulin dimers, both showing toxicity to dividing cells due to disrupted mitotic processes.

Intracellular Transport: Facilitating movement of organelles via motor proteins (kinesins and dyneins).
Cell Movement: Flagella movement orchestrated by dynein interaction with MTs, causing bending and propulsion.

Actin filaments (F-actin) are dynamic structures crucial for maintaining cell shape and enabling cell movement.
Comprised of G-actin monomers, they exhibit polarity with a plus end exhibiting more dynamic behavior.

Treadmilling occurs when monomer addition at the plus end equals loss at the minus end, maintaining filament length but allowing dynamic turnover.
Actin is involved in muscle contraction (e.g., sarcomeres), cell migration, and the interaction with myosin motor proteins.
Myosin II directly mediates the movement of actin filaments, enabling contraction in muscle tissues.

Intermediate filaments are stable structures providing mechanical strength and organization to cellular architecture, notably in epithelial cells.
Comprised of helical proteins, lacking polarity, which distinguishes them from microtubules and actin filaments.
Key functions include supporting the nuclear lamina and regulating gene expression in response to cellular structure changes.

Microtubules, actin filaments, and intermediate filaments serve distinct but interconnected roles within the cytoskeleton.
Understanding their dynamics, interactions with drugs, and mechanical functions is crucial for comprehending cellular behavior and health.