Cytoskeleton Summary

Cytoskeleton Filaments

• Network of filaments providing structural support, organelle positioning, chromosome segregation, and motility.

Three Major Types

• Actin Filaments:

  • Two-stranded helical polymers of actin.

  • Diameter: 5-9 nm.

  • Form bundles, networks, and gels, concentrated in the cell cortex.

  • Determine cell shape and movement.

• Microtubules:

  • Hollow cylinders of tubulin.

  • Diameter: 25 nm, more rigid than actin.

  • Organized from MTOCs (centrosomes).

  • Position organelles and direct traffic.

• Intermediate Filaments:

  • Rope-like fibers, diameter ~10 nm.

  • Made of various proteins, found in different locations (e.g., nuclear lamina).

  • Provide mechanical strength.

Properties of Cytoskeletal Filaments

• Adaptability:

  • Small, soluble subunits allow rapid assembly/disassembly.

• Stability:

  • Intermediate filaments are very resistant to breakage.

Polymer Dynamics

• Subunits add/remove at polymer ends.

• $k_{on}$ (rate of addition), units of $M^{-1} sec^{-1}$.

• $k_{off}$ (rate of loss), units of $sec^{-1}$.

• Critical Concentration ($C_c$):

  • Equilibrium where subunit addition equals subunit loss.

  • $k<em>{on}C = k</em>{off}$, so $C<em>c = \frac{k</em>{off}}{k_{on}} = \frac{1}{K}$.

• Plus and Minus Ends:

  • Different polymerization rates.

  • Plus end: fast-growing.

  • Minus end: slow-growing.

Nucleotide Hydrolysis

• ATP/GTP Caps:

  • Subunit addition faster than nucleotide hydrolysis.

• Dynamic Instability:

  • Predominates in microtubules.

• Treadmilling:

  • May predominate in actin filaments.

  • Net assembly at the plus end and disassembly at the minus end.

Consequences of Nucleotide Hydrolysis

• Dynamic Instability:

  • Microtubules alternate between growth and rapid disassembly.

• Treadmilling:

  • $C<em>c$ (minus end) > $C</em>c$ (plus end).

Drugs Affecting Filament Dynamics

• Actin:

  • Latrunculin: depolymerizes, binds actin subunits.

  • Cytochalasin B: depolymerizes, caps plus ends.

  • Phalloidin: stabilizes, binds along filaments.

• Microtubules:

  • Taxol: stabilizes, binds along filaments.

  • Nocodazole/Colchicine: depolymerizes, bind tubulin subunits.

Regulation of Filament Dynamics by Binding Proteins

• Microtubules:

  • g-TuRC nucleates assembly at the minus end.

  • Kinesin 13 enhances catastrophic disassembly at the plus end.

  • MAPs stabilize tubules.

• Actin Filaments:

  • Formin nucleates assembly at the plus end.

  • ARP complex nucleates assembly to form a web at the minus end.

  • Capping protein prevents assembly/disassembly at the plus end.

Nucleation

• Rate-limiting step in polymer formation.

• y-Tubulin Complexes:

  • Nucleate and cap microtubules.

• Actin Nucleation:

  • Arp 2/3 complex.

  • Form branched networks.

  • Formins.

Microtubule Stabilizers & Bundlers

• MAPs.

End-Binding Proteins

• Plus-end trackers.

Actin Networks

• Stress fibers, filopodia, cell cortex, contractile bundles, gel-like networks.

Actin Bundlers

• a-Actinin, villin, fimbrin, filamin.

Connecting Membranes and the Cytoskeleton

• ERM-mediated cross-linking.