1. 02.15.10 Lecture 12 - The actin cytoskeleton

2. Actin filaments allow cells to adopt different shapes and perform different functions
Villi
Contractile
bundles
Sheet-like &
Finger-like
protrusions
Contractile
ring

3. Actin filaments are thin and flexible
7 nm in diameter
Less rigid than microtubules
Plus end - fast growing
Minus end - slow growing
Monomers polymerize into a helical chain

4. Actin and microtubules polymerize using similar mechanisms
Monomeric actin binds to ATP
Upon polymerization, actin ATPase activity cleaves ATP to ADP
ATP hydrolysis acts as a molecular “clock”
Older actin filaments with ADP are unstable and disassemble

5. Actin architecture and function is governed by actin-binding proteins

6. Example: actin in microvilli

7. Example: actin in the cell cortex

8. Actin polymerization can produce “pushing” forces
Polymerization at the front of a cell pushes the leading edge forward
Phagocytosis - formation of pseudopods
Intracellular movement and cell-to-cell spreading of pathogens

9. Actin polymerization drives protrusion of the cell membrane
Filopodia
Lamellipodia

10. Model for actin polymerization at membranes

11. Actin polymerization powers engulfment during phagocytosis

12. Movement of Listeria monocytogenes
Pathogenic bacterium that colonizes the epithelial cells lining the gut
Found in contaminated dairy products
Infection can be lethal to newborns and immunocompromised individuals

13. Listeria move on an actin-based “comet-tail”

14. Myosins are actin-based motor proteins
Myosins convert ATP hydrolysis into movement along actin filaments
Many different classes of myosins (>30 in humans)
Some myosins move cargoes, other myosins slide actin (as in muscles)
Actin & ATP binding sites in N-terminal head domain

15. Myosins “walk” along actin filaments

16. Myosin I can carry organelles or slide actin filaments along the membrane

17. Myosin II slides actin filaments to produce contractile forces

18. Myosin-based contraction drives cytokinesis

19. Skeletal muscle cells are packed with myofibrils, each of which contains repeating chains of sarcomeres

20. Sarcomeres are contractile units of actin and myosin II

21. In muscle cells, myosin II is a filament of many motors

22. Muscle contraction is driven by myosin II

23. The myosin cycle in muscle

24. Contraction is activated by calcium release from the sarcoplasmic reticulum

25. Calcium release channels are opened by a voltage-sensitive transmembrane protein in the T-tubule

26. Contraction is regulated by a Ca+2-mediated change in the conformation of troponin

27. Muscle contraction