1. Filaments Of The Cytoskeleton
Actin Filaments
Helical polymers of protein actin
Microtubules
Hollow cylinders of protein tubulin
Intermediate filaments
Ropelike fibers of inter-mediate filament proteins

2. Microtubule Structure
Tubulin subunits are heterodimers of a-tubulin and b-tubulin
GTP bound to tubulin
Hollow cylinder of 13 parallel protofilaments
Tubulin subunits in same orientation

3. Actin Filament Structure
Monomeric actin subunits
ATP bound to actin subunits
Two helical protofilaments
Actin subunits in same orientation

4. Plus And Minus Ends Of Actin Filaments And Microtubules
Polarity from regular orientation of subunits
During elongation, subunits added preferentially to plus end

5. Nucleotide Hydrolysis
Free actin and tubulin subunits are triphosphate form
Association with filament stimulates nucleotide hydrolysis
Diphosphate form more likely to dissociate from end
ATP/GTP caps dependent upon rate of addition

6. Treadmilling
Subunits assembled at plus end and disassembled at minus end at same rate; triphosphate cap at plus end
Subunits travel through filament

7. Dynamic Instability
Alternating states of elongation and depolymerization
Dependent on presence or hydrolysis of triphosphate cap

8. Drugs Affecting Actin And Microtubules
Toxins from plants specifically affect polymerization or depolymerization
Taxol stabilizes microtubules, used in cancer therapy

9. Intermediate Filament Assembly
Dimer has central regions wound into coiled-coil
Staggered side-by-side arrangement of two dimers forms tetramer
Tetramer is basic subunit for assembly of filaments

10. Types Of Intermediate Filaments
Type Component Polypeptides Cellular Location
Epithelial type I keratins (acidic) epithelial cells and type II keratins (basic) their derivatives
Axonal neurofilament proteins neurons
Nuclear lamins A, B, and C inside surface of nuclear membrane
Vimentin-like vimentin and related various
proteins }

11. Keratin Genetic Diseases
Epidermolysis bullosa simplex
Defective keratin in basal layer of epidermis
Rupture of cells and blistering from mechanical stress

12. Polymerization Of Actin And Microtubules In Vitro
Nucleation: slow formation of stable oligomers
Elongation: rapid addition to filament
Steady-state: same rates of polymerization and depolymerization

13. Nucleation Of Microtubules
Nucleate from g-tubulin ring complex
Centrosome: site of nucleation
Multiple g-tubulin ring complexes
Pair of centrioles

14. Nucleation Of Actin Filaments
Nucleate from actin-related protein complex

15. Regulating Filament Elongation
Thymosin: sequesters actin monomers
Profilin: competes with thymosin, promotes actin assembly
Other proteins bind free tubulin

16. Cross-Linking Actin Filaments
Organization into bundles or gel-like networks
Actin-binding bundling and gel-forming proteins

17. Myosin: Actin Based Motor Proteins
Head domain uses ATP hydrolysis to move toward plus end
Myosin II: two heavy chains each with head domain
Myosin I: one head domain

18. Microtubule Based Motor Proteins
Kinesin: moves toward plus end
Dynein: moves toward minus end; cytoplasmic and axonemal (ciliary) types

19. Mechanism Of Myosin Movement
Cycle of ATP binding, ATP hydrolysis, and phosphate release

20. Microtubules In Cilia And Flagella
9 + 2 arrangement of microtubules; outer doublets
Cross-linking proteins
Ciliary dynein

21. Movement Of Cilia And Flagella
Motor force of dynein converted to bending motion

22. Basal Bodies Organizing center for microtubules in cilia and flagella
Nine triplet microtubules

23. Cell Migration Polymerization of actin causes protrusion at front
New contacts form with solid surface
Back of cell contracts