Cell Systems III: The Dynamic Cytoskeleton

Eukaryotic Cytoskeleton

• Consists of three major components:

  • Actin filaments (microfilaments)

  • Intermediate filaments

  • Microtubules

Actin Filaments

• Smallest diameter component of cytoskeleton, made of globular protein subunits (actin).

• Structure:

  • Two strands coiled; has distinct polarity (plus and minus ends).

• Function:

  • Provides structural support; involved in movements, especially through interaction with motor protein myosin.

  • Key roles include muscle contraction, cytokinesis, cytoplasmic streaming, and cell crawling.

Intermediate Filaments

• Vary in type (70 genes in humans), provide mechanical strength without polarity.

• Structure:

  • Composed of different proteins (e.g., keratins), form a flexible internal scaffolding.

• Function:

  • Secure cell shape, provide internal support, contribute to nuclear stability.

Microtubules

• Largest diameter; made from α- and β-tubulin dimers.

• Structure:

  • Polar, with distinct ends; originate from microtubule-organizing center (MTOC) like centrosomes.

• Function:

  • Provide stability, tracks for vesicle transport, separate chromosomes during cell division.

Cytoskeletal Motor Proteins

• Myosin: Interacts with actin for muscle contraction and movement.

• Kinesin: Moves vesicles along microtubules toward the plus end (MTOC).

• Dynein: Moves toward the minus end; essential for movement of cilia and flagella.

  • Required for axoneme bending and locomotion of eukaryotic cells.

Conclusion

• Cytoskeleton is dynamic and integral for maintaining cell shape, enabling movement, and facilitating intracellular transport.