Deck 16: Cell Organization and Movement I: Microfilaments

Question 1

Actin-binding proteins that generate actin filament bundles:

A)are long and flexible.
B)bind only to the ends of actin filaments.
C)can also bundle microtubules.
D)are short and inflexible.

Answer 1

D

Question 2

All of the following statements about actin assembly are correct EXCEPT:

A)ATP-actin can assemble into filaments.
B)actin subunits can treadmill through an actin filament.
C)actin assembly can produce force for movement.
D)actin (−)ends assemble more rapidly than actin (+)ends.

Answer 2

D

Question 3

Which of the following proteins promotes actin assembly and is involved in signaling pathways controlling actin assembly at the plasma membrane?

A)myosin
B)profilin
C)thymosin β₄
D)filamin

Answer 3

B

Question 4

A well-coordinated mechanism involving cell-surface receptors and the actin cytoskeleton allows leukocytes to bind and engulf invading bacteria.Briefly describe how bacteria are phagocytosed.

Question 5

Within an actin filament,each actin subunit is surrounded by _____ neighboring actin subunits.

A)one
B)two
C)four
D)eight

Question 6

What cellular components cause some actin filaments to form bundles and others to form networks?

Question 7

Many actin cross-linking proteins contain:

A)an ATP-binding cleft.
B)a head domain.
C)a CH domain.
D)only one actin-binding site.

Question 8

Which of the following is a G-actin sequestering protein that allows cells to maintain a relatively high G-actin concentration in cells?

A)thymosin β₄
B)FH2 domain of formin
C)profilin
D)cofilin

Question 9

How do actin filaments appear when viewed by negative stain electron microscopy?

Question 10

If the activity of thymosin β4 was inhibited in fibroblasts,the overall effect would be:

A)a decrease in cell locomotion because less actin would be recruited to the leading edge.
B)an increase in cell locomotion because there would be a higher concentration of actin available.
C)no change in cell locomotion because thymosin β4 doesn't interact with Rho,Rac,or Cdc42.
D)none of the above

Question 11

At ATP-G-actin concentrations that are intermediate between the Cc for the (+)end and the Cc for the (-)end,_____ will be observed.The Cc needed for elongation at the (+)end is _____ than that at the (-)end of actin microfilaments.

A)growth;lower
B)shrinking,higher
C)treadmilling,lower
D)none of the above

Question 12

Gelsolin is activated by:

A)ATP binding.
B)phosphorylation.
C)Ca²⁺ binding.
D)dephosphorylation.

Question 13

What actin-binding protein mediates gel-sol transitions,and how is this protein's activity regulated?

Question 14

Decoration of actin filaments with myosin S1 is commonly used to:

A)attach actin filaments to cell membranes.
B)disassemble actin filaments.
C)reveal the polarity of actin filaments.
D)reveal the polarity of myosin filaments.

Question 15

The plasma membrane of eukaryotic cells is supported by:

A)actin filaments.
B)microtubules.
C)lamins.
D)intermediate filaments.

Question 16

Three major groups of filament systems comprising the cytoskeleton are all composed of polymers of assembled subunits,which vary in thickness when assembled.Which is the correct order,from smallest to largest,of the filament systems?

A)microtubules,microfilaments,intermediate filaments
B)intermediate filaments,microtubules,microfilaments,
C)microfilaments,microtubules,intermediate filaments
D)none of the above

Question 17

Listeria is a bacterial parasite that has evolved unique ways to enter animal cells and then use actin polymerization for its intracellular movement.Which of the following statements is true in regards to Listeria and actin polymerization?

A)Listeria ActA protein binds and inactivates the Arp2/3 complex.
B)Listeria ActA binds VASP,which enhances ATP-actin assembly.
C)Cofilin is necessary to accelerate the assembly of the (−)end of the filament.
D)CapZ promotes the elongation of the filament.

Question 18

Cofilin:

A)cleaves actin by binding the ATP-actin in the filament.
B)cleaves actin by twisting adjacent F-actin monomers in the filament.
C)recruits G-actin monomers to the elongating F-actin microfilament.
D)promotes exchange of ADP for ATP on G actin.

Question 19

During treadmilling,actin subunits add:

A)predominantly to filament (+)ends.
B)predominantly to filament (−)ends.
C)equally to both filament ends.
D)along the length of filaments.

Question 20

What is the function of thymosin β₄?

Question 21

Lamellipodia are located:

A)at a moving cell's trailing edge.
B)at a moving cell's leading edge.
C)around the entire periphery of a nonmotile cell.
D)throughout the cytosol of a moving cell.

Question 22

In the operational model for movement of myosin along an actin filament,the power stroke occurs during:

A)binding of ATP.
B)hydrolysis of ATP.
C)release of phosphate (Pi).
D)release of ADP.

Question 23

Membrane extension during cell locomotion is driven by:

A)myosin II.
B)actin depolymerization.
C)contraction.
D)actin polymerization.

Question 24

Which of the following is NOT a function of myosin-powered movements?

A)skeletal muscle contraction
B)cytoplasmic streaming
C)cytokinesis
D)flagellum-mediated cell motility

Question 25

What would occur if CapZ was inhibited in cells?

A)increased organization of actin in stereocilia due to stabilized actin dynamics
B)increased rates of polymerization due to reduced availability for ADP-G-actin
C)increased steady-state treadmilling of F-actin
D)faster movement of cells toward a chemical signal

Question 26

Describe the functional properties of the head,neck,and tail domains of myosin.

Question 27

Which region of myosin interacts with actin filaments?

A)the head domain
B)the rod domain
C)the light chains
D)the tail domain

Question 28

What are the functions of the myosin head domain and tail domain,respectively?

Question 29

In the budding yeast S.cerevisiae,which of the following is NOT transported into the bud by myosin V?

A)peroxisome
B)vacuole
C)mRNA
D)nucleus

Question 30

What contractile structure is essential for cell division?

Question 31

Small G proteins,including Rho,Rac,and Cdc42,contribute to the coordinated movement and overall polarity of a migrating cell.Assuming that the cell is migrating in a left-to-right fashion,which of the following is correct?

A)active Cdc42 at the leading edge of the cell and active Rho at the back of the cell
B)active Rac at the back of the cell and active Cdc42 at the front of the cell
C)active Cdc42 at the back of the cell and active Rac at the leading edge of the cell
D)active Rac at the leading edge of the cell and active Rho at the back of the cell

Question 32

The elastic Brownian ratchet model has been proposed to explain:

A)membrane extension.
B)focal adhesion formation.
C)cell-body translocation.
D)gel-sol transitions.

Question 33

Which of the following proteins is involved in formation of actin bundles in microvilli by providing crosslinks between actin filaments?

A)α actinin
B)cofilin
C)fimbrin
D)profilin

Question 34

Human erythrocytes depend on microfilament networks to provide strength and flexibility.Describe this network and explain how microfilaments are attached to the plasma membrane in human erythrocytes.

Question 35

What is the function of CapZ and tropomodulin in the sarcomere?

A)to center myosin thick filaments
B)to attach actin thin filaments to the Z disk
C)to maintain a constant actin thin filament length
D)to make contraction sensitive to Ca²⁺

Question 36

How do the different types of actin-binding proteins relate to the ability of actin to form bundles and networks?

A)Proteins promoting bundle formation have one actin-binding site,whereas proteins promoting network formation have two actin-binding sites.
B)Different types of actin bind different actin-binding proteins to form bundles or networks.
C)Proteins that promote bundle formation bind ADP-actin,whereas proteins that promote network formation bind ATP-actin.
D)Proteins promoting bundle formation tend to be short and inflexible,whereas proteins that promote network formation tend to be longer and more flexible.

Question 37

The two proteins that play the most important role in actin microfilament elongation are:

A)thymosin β4 and cofilin.
B)cofilin and profilin.
C)profilin and thymosin β4.
D)CapZ and cofilin.

Question 38

Multinucleated cells may result from a defect in:

A)myosin V.
B)myosin I.
C)stress fiber formation.
D)myosin II.

Question 39

All myosins move toward the (+)end of actin filaments EXCEPT:

A)myosin I.
B)myosin II.
C)myosin V.
D)myosin VI.

Question 40

Which of the following properties is not shared by all myosins?

A)the ability to bind ATP
B)the ability to form dimers
C)the ability to bind actin
D)the presence of a head domain

Question 41

How do cells grip the substrate to generate locomotion?

Question 42

How are actin-binding proteins involved in gel-sol transitions?

Question 43

The correct order of events in cell locomotion is:

A)rear focal adhesion,membrane protrusion,front focal adhesion,cell body translocation,de-adhesion.
B)membrane protrusion,front focal adhesion,cell body translocation,de-adhesion,rear focal adhesion.
C)front focal adhesion,cell body translocation,de-adhesion,rear focal adhesion,membrane protrusion.
D)cell body translocation,de-adhesion,rear focal adhesion,membrane protrusion,front focal adhesion.

Question 44

In response to a chemotactic signal,a cell forms structures to aid in locomotion.In lamellipodia,active Rac will stimulate F-actin polymerization at the leading edge via _____,whereas actin that will form stress fibers will be recruited by formin downstream of activation of this small GTPase _____.

A)Arp 2/3;Rho
B)Arp 2/3;Cdc42
C)WASP;Rho
D)Rho kinase;Cdc42

Question 45

Like Listeria,other bacterial pathogens have also evolved to take advantage of actin-based cell motility systems in their hosts.For example,some pathogenic strains of E.coli make a cytotoxic factor (CNF1)that converts a specific glutamine residue on RhoGTPases to glutamate.This change blocks both the intrinsic and GAP-stimulated GTP hydrolysis activity of the Rho protein.Predict the effects of CNF1 on human epithelial cells in culture.

Question 46

In a scratch wound assay,cells are treated with inhibitors of Rho kinase.What would be observed?

A)The cells completely close the scratch,and only lamellipodia and filapodia are seen with actin staining.
B)The distance the cells migrate into the scratch is small,and actin staining reveals stress fibers,lamellipodia,and filapodia.
C)The cells are stimulated to undergo mitosis.
D)The cells migrate into the scratch,but no stress fibers can be observed with actin staining.