1. Three systems of cytoskeletal filaments exist in most eukaryotic cells. Compare them in terms of composition, function, and structure.

2. Actin filaments have a defined polarity. What is filament polarity? How is it generated at the subunit level? How is filament polarity detectable?

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3. In cells, actin filaments form bundles or networks. How do cells form such structures, and what specifically determines whether actin filaments will form a bundle or a network?

4. Much of our understanding of actin assembly in the cell is derived from experiments using purified actin in vitro. What techniques can be used to study actin assembly in vitro? Explain how each of these techniques works. Which of these techniques would tell you whether the mass of actin filaments is made up of many short actin filaments or fewer longer filaments?

5. The predominant forms of actin inside a cell are ATP–G-actin and ADP–F-actin. Explain how the interconversion of the nucleotide state is coupled to the assembly and disassembly of actin subunits. What would be the consequence for actin filament assembly/disassembly if a mutation prevented actin’s ability to bind ATP? What would be the consequence if a mutation prevented actin’s ability to hydrolyze ATP?

6. Actin filaments at the leading edge of a crawling cell are believed to undergo treadmilling. What is treadmilling, and what accounts for this assembly behavior?

7. Although purified actin can assemble reversibly in vitro, various actin-binding proteins regulate the assembly of actin filaments in the cell. Predict the effect on a cell’s actin cytoskeleton if function-blocking antibodies against each of the following were independently microinjected into cells: profilin, thymosin-β₄, CapZ, and the Arp2/3 complex.

8. Predict how actin would polymerize on a myosin-decorated short actin filament (as shown in Figure 17-9) in the presence of CapZ, tropomodulin, or profilin-actin.

9. Compare and contrast the ways in which formin and WASp are activated and explain how each stimulates actin filament formation.

10. There are at least 20 different types of myosin. What properties do all types share, and what makes them different? Why is myosin II the only myosin capable of producing contractile force?

11. The ability of myosin to walk along an actin filament may be observed with the aid of an appropriately equipped microscope. Describe how such assays are typically performed. Why is ATP required in these assays? How can such assays be used to determine the direction of myosin movement or the force produced by myosin?

12. Contractile bundles occur in nonmuscle cells; these structures are less organized than the sarcomeres of muscle cells. What is the purpose of nonmuscle contractile bundles? Which type of myosin is found in contractile bundles?

13. How does myosin convert the chemical energy released by ATP hydrolysis into mechanical work?

14. Myosin II has a duty ratio of 10 percent, and its step size is 8 nm. In contrast, myosin V has a much higher duty ratio (about 70 percent) and takes 36-nm steps as it walks down an actin filament. What differences between myosin II and myosin V account for their different properties? How do the different structures and properties of myosin II and myosin V reflect their different functions in cells?

15. Contraction of both skeletal and smooth muscle is triggered by an increase in cytosolic Ca²⁺. Compare the mechanisms by which each type of muscle converts a rise in Ca²⁺ concentration into contraction.

16. Phosphorylation of myosin light-chain kinase (MLC kinase) by protein kinase A (PKA) inhibits MLC kinase activation by Ca²⁺/calmodulin. Drugs such as albuterol bind to the β-adrenergic receptor, which causes a rise in cAMP in cells and activation of PKA. Explain why albuterol is useful for treating the severe contraction of the smooth muscle cells surrounding airway passages involved in an asthma attack.

17. Several types of cells use the actin cytoskeleton to power their locomotion across surfaces. How are different assemblies of actin filaments involved in locomotion?

18. To move in a specific direction, a migrating cell must use extracellular cues to establish which portion of the cell will act as the front and which will act as the back. Describe how small GTPase proteins appear to be involved in the signaling pathways used by migrating cells to determine direction of movement.

19. Cell motility has been described as being like the motion of tank treads. At the leading edge, actin filaments form rapidly into bundles and networks that make protrusions and move the cell forward. At the rear, cell adhesions are broken and the tail end of the cell is brought forward. What provides the traction for moving cells? How does cell-body translocation happen? How are cell adhesions released as cells move forward?