1. Regulation of Cytoskeletal Filaments
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2. Most cytoskeletal regulation is performed by accessory proteins that bind to either the filaments of their free subunits.
Microtubule-organizing center (MTOC) – a specific intracellular location where microtubule nucleation occurs

3. Nucleation of Microtubules by gamma-tubulin
- end end

4. The Centrosome, a MTOC
50 gamma-TuRC

5. Cross-section of a Centrosome
The centrioles organize the centrosome matrix ensuring its duplication during each cell cycle
Centrioles are composed of a short cylinder of modified microtubules and a large number of accessory proteins
Neither fungi nor most plants have centrioles
Centriole

6. Center-Seeking Behavior of a Centrosome

7. Reorganization of Microtubules

8. Nucleation by the ARP Complex
ARP – actin-related protein, each ARP
is about 45% identical to actin

9. Structures of Actin, Arp2, and Arp3

10. Actin Web Formation

11. Binding of Profilin and Thymosin
About 50% of actin in nonmuscle cells is in filaments and 50% as soluble monomers

12. Profin bound to Actin Monomer-
Profilin binds to the opposite side of the ATP-binding site, blocking the side of the monomer that would associate with the filament minus end, allowing this complex to add onto a free plus end +

13. Effects of Thymosin and Protilin on Actin Polymerization

14. Effects of Stathmin on Microtubules
Stathmin’s binding to tubulin is inhibited by the phosphorylation of stathmin

15. Organization of Microtubule Bundles
MAP – Microtubule-associated protein
MAPs have at least 1 domain that binds to the microtubule and another that projects outward

16. Localization of MAPS in a Neuron
-MAP2 protein stained orange in the cell body and dendrites
-tau stained green in the axon

17. Actin Filament Twisting Induced by Cofilin
Cofilin – is a small protein that binds actin in a 1:1 ratio and destabilizes actin filaments
Tropomyosin – an elongated protein that bind simultaneously to 7 actin monomers and stabilizes actin filaments

18. Filament Capping Changes Filament Dynamics
CapZ – Capping protein
Capping is regulated by intracellular signals, PIP2 (Phosphatidylinositol 4,5-bisphosphate) uncaps + ends

19. Proteins Binding Microtubule Ends
Proteins that bind to the ends of microtubules can control microtubule positioning

20. Cross-linking Cyoskeletal Elements
Red – Microtubules
Blue – Intermediate Filaments
Green – Cross-linking protein, Plectin
Plectin also links IF to actin filaments and microtubules
Filaggrin bundles keratin filaments in the epidermis of the skin to give it its toughness

21. Various Actin Arrays

22. Actin Cross-linking Proteins

23. Formation of 2 Types of Bundles

24. A Micorvillus

25. Filamin Cross-links Actin into a 3-Dimensional Network

26. Loss of Filamin Causes Abnormal Cell Motility
Actin formed by filamin is required for cells to extend the thin sheet-like membrane projections call lamellipodia

27. Filament Severing Changes Filament Dynamics

28. Microtubule Severing
Red - Microtubules

29. Actin Filament Severing by Gelsolin
-Activated by high levels of cytoplasmic calcium
-No energy needed
-Gelsolin is removed by PIP2
Severing of microtubules by Katanin
-made up of 2 subunits, one for severing and the other for targeting it
-the process requires ATP

30. Platelet Activation

31. Platelet Activation

32. Focal Contacts in Fibroblasts
Focal contacts –highly specialized type of attachment between actin filaments and the extracellular matrix

33. Effects of Extracellular Signals