Cytoskeleton Overview: Microtubules, Intermediate Filaments, and Microfilaments

Microtubules, Intermediate Filaments, and Microfilaments

• Cytoskeleton Components:
  • Microtubules (MTs)
  • Intermediate Filaments (IFs)
  • Microfilaments (MFs)

Microtubules (MTs)

• Structure:
  • Hollow tubes formed from α-tubulin and β-tubulin dimers.
  • Provides structural support and intracellular tracks for transport.
• Functions:
  • Cellular shape and mobility.
  • Chromosome segregation during cell division.
  • Intracellular transport (vesicles, organelles).
• Motor Proteins:
  • Kinesin: Moves towards the plus end of MTs, ATP-dependent mechanism.
  • Dynein: Moves towards the minus end of MTs.

Key Points on Kinesin Mechanism:

1. ATP binding to leading head induces conformational change, swinging the trailing head 180º.
2. The new leading head binds tubulin, releasing ADP to move cargo forward.
3. ATP hydrolysis in the trailing head causes detachment.
4. Repeat cycle with ATP binding.

Intermediate Filaments (IFs)

• Structure:
  • Composed of protein subunits; sizes range from 10-12 nm.
  • Stable structures that do not undergo dynamic remodeling like MTs/MFs.
  • Not polar; no distinct plus or minus ends.
• Functions:
  • Provide mechanical strength and structural support to cells.
  • Exclusive to multicellular animals.
• Examples of IFs:
  • Keratins (epithelial cells)
  • Neurofilaments (neurons)
  • Lamins (nucleus).

Microfilaments (MFs)

• Structure:
  • Double helix of actin monomers (globular actin or G-actin).
  • Polar with distinct plus and minus ends (growth occurs faster at the plus end).
  • Diameter ~8 nm, thinnest component.
• Functions:
  • Maintain cell shape and enable cell movement (crawling).
  • Vesicle transport and muscle contraction.
• Assembly Process:
  • G-actin → dimer → trimer → filament; involves ATP binding for stability.
• Actin-Binding Proteins:
  • Profilin: Enhances growth of filaments.
  • Cofilin: Disassembles filaments.
  • Capping proteins: Block subunit exchange at the plus end.

Actin Nucleation and Branching

• Arp2/3 Complex:
  • Nucleates new actin fibers from existing filaments to create branched networks, crucial for cell migration.
• Key Steps in Nucleation:
  1. Activation of NPF brings actin monomer to nucleation site.
  2. Arp2/3 binds forming a new filament at ~70° angle.

Interactions and Cooperation

• Motor Proteins and Transport:
  • Myosins (MFs) and kinesins (MTs) cooperate to facilitate intracellular transport (e.g., pigment granules).

Summary of Major Functions