1. The Cytoskeleton Assembly and Dynamic Structure
of Cytoskeletal Filaments
Pages

2. The Cytoskeleton Green = Microtubules Red = Actin Blue = Coomassie
Blue stain

3. Functions of the Cytoskeleton
-Pull chromosomes apart at mitosis and then splits the dividing cell into two
-intracellular trafficing of organelles and other proteins
-support of the plasma membrane
-allows the cell to bear stresses and strains
-allows cells to swim (sperm) or crawl (fibroblasts)
-provides machinery for muscle contraction
-allows neurons to extend axons and dendrites

4. Types of Cytoskeletal Filaments
1. Actin Filaments
-determine the shape of the cell’s surface
-necessary for whole-cell locomotion
-diameter: 5-9 nm
2. Intermediate Filaments
-provide mechanical strength and resistance shear stress
-diameter: 10 nm
3. Microtubules
-determine the positions of membrane-enclosed organelles
-directs intracellular transport
-diameter: 25 nm

5. Changes in the Cytoskeleton
Microtubules
Actin
Red = Actin
Green = Microtubules

6. Actin and Microtubules in Drosophila Embryos
Actin – Red
Microtubules – Green
Division every 10 min

7. Neutrophil Movement
Assembly and disassembly of the actin filaments allows the cell to change directions

8. Similarities between the Types of Filaments
1. They form as helical assemblies of subunits
2. They all self-associate using a combination of end-
to-end and side-to-side protein contacts
3. Assembly and disassembly can occur rapidly
4. Accessory proteins regulate the spatial distribution
and dynamic behavior of the filaments

9. Cytoskeletal Structure
-Assymetric
-Characteristic Shape

10. Polarity of Actin in Yeast

11. Rapid Reorganization of the Cytoskeleton

12. Protofilament Formation
Protofilament – long linear strings of subunits joined end-to-end
Composed of globular subunits that make a similar number
of longitudinal and lateral bonds

13. Actin and Microtubule Formation

14. Intermediate Filaments
Composed of fibrous subunits that make more lateral bonds than longitudinal bonds

15. 3 Stages of Polymerization
Nucleation – Subunits must assemble into an initial aggregate that is stabilized by many subunit-subunit interactions
-This is the rate-limiting step in polymerization
-Special proteins catalyze nucleation at specific sites

16. Microtubule Structure
GTP/GDP
Tightly
bound
13 protofilaments

17. Actin Filament Structure
2 protofilaments

18. Preferential Growth of Microtubules
-Polarity of Microtubules and Actin Filaments is created by the parallel orientation of its subunits
-Alpha units are exposed at + end and beta subunits at the – end
-The + end is more dynamic, growing and shrinking faster, while the – end is slower

19. Treadmilling in a Living Cell
Microtubules

20. Treadmilling of Actin T form contains ATP D form contains ADP
Treadmilling – Subunits are added at the + end while being removed at the - end

21. Dynamic Instability of Microtubules
Cc(D) > Cc(T)
-The D form leans more readily toward disassembly
-The T form leans more readily towards assembly
Dynamic Instability – the rapid interconversion between a growing and shrinking state at a uniform free subunit concentration

22. GTP Hydrolysis Causes Structural Changes
GTP- straight filaments
GDP – curved filaments

23. Structural Changes in Microtubules
Curved filaments (GDP) doesn’t allow for as many interactions between protofilaments

24. Dynamic Instability in a Living Cell
Treadmilling and dynamic instability large amounts of energy are used, but it gives the cell spatial and temporal flexibility in response to its environment

25. FtsZ, a Tubulin Homologue in Procaryotes
-Essential in cell division, a band of FtsZ protein forms at the site of separation, where the new cell wall is to form
-Constriction and disassembly of the FtsZ band through GTP hydrolysis helps to pinch the two daughter cells apart

26. Actin Interactions -Actin is found in all eucaryotic cells
-humans have 6 different actin genes, alpha actin is expressed in only muscle cells while beta and gamma are in almost all nonmuscle cells
-Tubulin is also found in all eucaryotic cells and there are also multiple forms of the subunits

27. Intermediate Filament Subunits
-no overall polarity

28. Intermediate Filament Construction
-They are easily bent but extremely difficult to break
-Protein phosphorylation probably regulates their disassembly

30. Mechanical Properties of Cytoskeletal Filaments

31. Keratin Filaments in Epithelial Cells
-About 20 different types found in human epithelial cells and about 10 more that are specific to hair and nails
-Keratin filaments are made of equal numbers of type I (acidic) and type II (neutral/basic) chains
-Usually they are crosslinked by disulfide bonds

32. Blistering of the Skin due to Mutated Keratin
Epidermolysis bullosa simplex – the skin blisters in response to even slight mechanical stress, which ruptures the basal cells