Chapter 17 - Cytoskeleton

Question 1

Which is the toughest and most durable of the different types of cytoskeletal filaments?

• dynein
• actin filaments
• intermediate filaments
• myosin
• microtubules

Answer 1

Intermediate filaments

(Intermediate filaments are the toughest and most durable of the three types of cytoskeletal filaments and can even survive treatment with concentrated salt solutions and detergents.)

Question 2

Intermediate filaments are found in what structure?

• nuclear lamina
• centrosomes
• mitotic spindle
• cell cortex
• cilia

Answer 2

Nuclear lamina

(Intermediate filaments, specifically lamin proteins, are found in the nuclear lamina. This cytoskeletal meshwork underlies and strengthens the nuclear envelope.)

Question 3

Which term best describes the structure of intermediate filament monomers?

• tubelike
• springlike
• ropelike
• meshlike

Answer 3

Ropelike

(Intermediate filament monomers are best described as ropelike structures. Fibrous, intermediate filament proteins twist together to form these ropelike structures.)

Question 4

Which of the following types of intermediate filaments are found in all animal cells?

• nuclear lamins
• keratin filaments
• vimentin and vimentin-related filaments
• neurofilaments

Answer 4

Nuclear lamins

Nuclear lamins are intermediate filaments that are found in the nuclei of all animal cells as well as plant cells.

Question 5

GTP hydrolysis and whether GTP or GDP is bound to tubulin is an important mechanism to control the dynamic instability of microtubules. Certain aspects of dynamic instability can be viewed using GFP-EB1. Which process(es) is it useful for visualizing and why?

• growing and shrinking microtubules, because EB1 binds to the GDP-tubulin cap on microtubules
• shrinking microtubules, because EB1 binds to the GTP-tubulin cap on microtubules
• growing microtubules, because EB1 binds to the GTP-tubulin cap on microtubules
• growing and shrinking microtubules, because EB1 binds to the GTP-tubulin cap on microtubules

Answer 5

growing microtubules, because EB1 binds to the GTP-tubulin cap on microtubules

(GFP-EB1 binds the GTP-bound tubulin subunits in the microtubule, thereby labeling the growing microtubules.)

Question 6

How would the animation of microtubule dynamics change after adding a non-hydrolyzable analog of GTP to the cells expressing GFP tubulin?

• Microtubule dynamics would not change.
• Dynamic instability would increase as microtubules rapidly switch between growing and shrinking.
• Microtubules would shrink.
• Microtubules would grow longer.

Answer 6

Microtubules would grow longer.

The microtubules would have permanent GTP caps and would continue to grow and not shrink.

Question 7

Within this image, the _______ cargo is being transported by kinesin and the ______ cargo is being transported by dynein.

Answer 7

Red; Blue

(Within this image, the red cargo is being transported by kinesin. Kinesin uses the hydrolysis of ATP to migrate toward the plus end of a microtubule.

Correct. Within this image, the blue cargo is being transported by dynein. Dynein uses the hydrolysis of ATP to migrate toward the minus end of a microtubule.)

Question 8

What does the cellular motility of sperm depend on?

• actin and myosin
• actin and kinesin
• microtubules and dynein
• microtubules and kinesin

Answer 8

Microtubules and dynein

(Flagella propel a sperm cell through fluid using a repetitive wavelike motion that is dependent on flagellar microtubules and the motor protein dynein.)

Question 9

Which of the following cytoskeletal structures are the most common for providing tracks for guiding intracellular transport?

• kinesins
• dyneins
• intermediate filaments
• actin filaments
• microtubules

Answer 9

Microtubules

(Microtubules are cytoskeletal structures that provide tracks for guiding intracellular transport of vesicles, organelles, and other cell components in the cytosol.)

Question 10

What do microtubules resemble?

• meshlike networks
• ropelike strands
• hollow tubes
• interlocking chains

Answer 10

Hollow tubes

Microtubules are composed of 13 parallel protofilaments that, bundled together, resemble a hollow tube.

Question 11

Dynamic instability in microtubules stems from the intrinsic capacity of tubulin molecules to hydrolyze what?

• peptide bonds
• GTP
• ATP
• water
• tubulin dimers

Answer 11

GTP

(Dynamic instability in microtubules stems from the intrinsic capacity of tubulin molecules to hydrolyze GTP. β-tubulin hydrolyzes its bound GTP shortly after a dimer is added to a growing microtubule.)

Question 12

Which of the following describes the structure of an actin filament?

• It is a structure with a long tail and two globular heads.
• It is a twisted chain of actin molecules.
• It is a hollow cylinder made of actin molecules.

Answer 12

It is a twisted chain of actin molecules.

(Each filament is a twisted chain of identical globular actin monomers, all of which “point” in the same direction along the axis of the chain.)

Question 13

What is the name of the thin, sheetlike structures that a fibroblast regularly extends during cell crawling?

• phagosomes
• filopodia
• lamellipodia
• pseudopods

Answer 13

Lamellipodia

(The lamellipodia are the thin, sheetlike structures that a fibroblast regularly extends during cell crawling. These structures contain a dense meshwork of actin filaments.)

Question 14

In what kind(s) of cells is myosin-I present?

• nerve cells only
• muscle cells only
• epithelial cells only
• all types of cells

Answer 14

All types of cells

(Myosin-I is present in all types of cells. A specialized form of myosin-II is present only in muscle cells, but myosin-I is needed in all cells.)

Question 15

Which actin-binding proteins would be most involved in the assembly of the contractile ring?

• formins
• actin-related proteins
• γ-tubulin
• thymosins

Answer 15

Formins

(Formins aid in the assembly of unbranched actin filaments, including those in the contractile ring that pinches a dividing animal cell in two.)

Question 16

An actin filament undergoing treadmilling at the leading edge of a lamellipodium can do what?

• remain the same size
• collapse and instantly disappear
• experience exponential growth
• add actin monomers to its minus end while losing them from its plus end

Answer 16

Remain the same size

(Treadmilling involves a simultaneous gain of monomers at the plus end of an actin filament and loss of monomers from the minus end; hence, actin filaments tend not to undergo drastic changes in length.)

Question 17

Actin filaments can undergo ______ where actin monomers in the cytosol that carry ATP can join the filament.

Answer 17

treadmilling

Question 18

Which does not contain both actin and myosin?

• the contractile ring that carries out cytokinesis
• the lamellipodium at the leading edge of a crawling cell
• a muscle cell sarcomere
• a contractile bundle in a nonmuscle cell

Answer 18

the lamellipodium at the leading edge of a crawling cell

The lamellipodia at the leading edge of a crawling cell contain actin but not myosin.

Question 19

Inside a muscle fiber, what triggers sarcomeres to contract?

• a sudden rise in cytosolic Ca2+
• an increased availability of ATP
• a sudden rise in Ca2+ inside an organelle lumen
• a sudden rise in cytosolic Na+
• polarization of the muscle fiber plasma membrane

Answer 19

A sudden rise in cytosolic Ca2+

Inside a muscle fiber, a sudden rise in cytosolic Ca2+ activates proteins that initiate contraction of the sarcomeres.

Question 20

Which organelle sequesters Ca2+ inside muscle fibers?

• lysosomes
• mitochondria
• Golgi apparatus
• sarcoplasmic reticulum

Answer 20

Sarcoplasmic reticulum

(The sarcoplasmic reticulum sequesters Ca2+ inside muscle fibers. This specialized region of the endoplasmic reticulum in muscle cells contains ATP-driven Ca2+ pumps that remove Ca2+ from the cytoplasm.)

Question 21

When a muscle is stimulated to contract, what does Ca2+ bind to, and what effect does that have?

• tropomyosin, which moves the troponin that otherwise blocks the interaction of actin and myosin
• troponin, which moves the tropomyosin that otherwise blocks the interaction of actin and myosin
• myosin, allowing it to associate with actin
• actin, allowing it to associate with myosin

Answer 21

Troponin, which moves the tropomyosin that otherwise blocks the interaction of actin and myosin

(When a muscle is stimulated to contract, Ca2+ binds to troponin, which moves the tropomyosin that otherwise blocks the interaction of actin and myosin. In the absence of Ca2+, tropomyosin binds to actin monomers, preventing their interaction with myosin.)

Question 22

Which of the following statements are consistent with the structure and function of intermediate filaments?
Choose one or more:

• Intermediate filaments are constructed of identical subunits found in all eukaryotic cells.
• Intermediate filaments can connect cells at cell–cell junctions called desmosomes.
• Intermediate filaments protect cells from mechanical stress because they have high tensile strength and resist stretching.
• Each filament is made of eight strands, and each strand is made from staggered tetramers linked end to end.

Answer 22

• Intermediate filaments can connect cells at cell–cell junctions called desmosomes.
• Intermediate filaments protect cells from mechanical stress because they have high tensile strength and resist stretching.
• Each filament is made of eight strands, and each strand is made from staggered tetramers linked end to end.

(Each filament with eight strands connects cells at desmosomes to protect from mechanical stress.)

Question 23

Mutation of the muscle-specific intermediate filament desmin leads to the rare disease desmin-related myopathy. This disorder starts with weakness of the lower limbs when patients are in their 20s or 30s. As symptoms worsen, weakness in respiratory and cardiac muscles occurs, which can lead to serious problems including sudden cardiac arrest.

Which of the following mutations would disrupt desmin intermediate filament structure or function and could explain the symptoms of desmin-related myopathy? Choose all of the possible mutations.
Choose one or more:

• Alteration in the twist or coiling of the dimers, blocking formation of staggered tetramers.
• Disruption of the polarity of the final desmin strands.
• Alteration in head groups, so tetramers are unable to link end to end.
• Mutation such that formation of dimers is blocked.

Answer 23

• Alteration in the twist or coiling of the dimers, blocking formation of staggered tetramers.
• Alteration in head groups, so tetramers are unable to link end to end.
• Mutation such that formation of dimers is blocked.

(Intermediate filament function depends on their proper assembly. Assembly begins with monomers forming coiled-coil dimers. The dimers then associate to form a staggered tetramer. The tetramers form such that both ends of the tetramer are identical, so intermediate filaments do not have polarity. The staggered tetramers join end to end to make a long strand. Eight strands associate to make the final ropelike intermediate filament. Disruption of any of these stages of assembly would block proper filament assembly and function and could lead to desmin-related myopathy.)

Question 24

Which statement about intermediate filaments is not true?

• They rupture under stress.
• They have the highest tensile strength of all the cytoskeletal filaments.
• Their disruption can lead to premature aging.
• They are wider than actin filaments.
• They lack polarity.

Answer 24

They rupture under stress.

(Intermediate filaments deform under stress, but they do not rupture. The tensile strength that maintains cell integrity, especially that of epithelial cells, is due to keratin—a strong intermediate filament polymer.)

Question 25

Which of the following would be a consequence of mutations that disrupt the interaction between cross-linking, stabilizing proteins and keratin filaments?

• neurodegeneration caused by an interference with normal axonal transport
• corneal damage caused by cell rupture from mechanical trauma
• defects in muscle development caused by improper organization of sarcomeres
• skeletal and cardiac abnormalities caused by a weakened nuclear envelope
• developmental defects caused by abnormal assembly of cilia

Answer 25

corneal damage caused by cell rupture from mechanical trauma

(Defects in keratin make cells vulnerable to rupture by mechanical stress, particularly in the skin and the cornea. Thus, skin and cornea damage could be a consequence of mutations that disrupt the interaction between cross-linking, stabilizing proteins and keratin filaments.)

Question 26

Epidermolysis bullosa simplex is a genetic skin condition in which skin is especially fragile and prone to blistering.

With the identification of skin stem cells, it has become possible to add the normal copy of a mutated gene into skin stem cells in vitro, grow the skin, and then transplant onto the patient. This approach was used to successfully treat a patient with a different form of epidermolysis bullosa known as junctional epidermolysis bullosa. (see Nature 2017 Nov 16;551(7680):327-332 ). What gene would need to be introduced into the skin stem cells of a patient with epidermolysis bullosa simplex in order to regain proper epithelial function?

• myosin
• keratin
• tubulin
• nuclear lamins

Answer 26

Keratin

(Keratins are intermediate filaments that span epithelial cells and indirectly connect to keratins in other epithelial cells via desmosomes. If the keratin filaments are not working properly, cells do not have mechanical strength, and can easily shear off from the basal lamina, as depicted below.)

Question 27

The energy in ATP is used to fuel the movement of kinesin motor proteins along microtubules. What events occur on the leading head?

Answer 27

• ATP binding
• ADP release
• Neck linker zippers to catalytic core of head group

(ADP release and subsequent ATP binding on the leading head contribute to the conformational changes of the neck linker region.)

Question 28

The energy in ATP is used to fuel the movement of kinesin motor proteins along microtubules. What events occur on the trailing head?

Answer 28

• Pi release
• ATP hydrolysis

(The trailing head hydrolyzes ATP and releases a Pi.)

Question 29

An experiment was performed to determine the role that ATP plays in kinesin movement along microtubules. Kinesin and microtubules were incubated together in a test tube, but instead of ATP, a non-hydrolyzable analog of ATP was added to the tube. What impact on kinesin function do you expect to observe in the presence of this ATP analog?

• The leading head would bind tightly, but the trailing head would remain free.
• The kinesin would walk faster along the microtubule since the kinesin would remain active while bound to ATP.
• The kinesin would bind tightly to microtubules and not release.
• The kinesin would be unable to bind to microtubules since the kinesin would remain inactive.

Answer 29

The kinesin would bind tightly to microtubules and not release.

(The kinesin would remain bound to the microtubules as ATP hydrolysis and phosphate release prepares the trailing head for being released and moved to the front as the new leading head.)

Question 30

If GTP hydrolysis occurs on a tubulin molecule at the plus end of a microtubule protofilament before another tubulin molecule is added, what typically happens?

• The microtubule remains the same size.
• The microtubule polymerizes.
• The microtubule depolymerizes.
• The GDP is rapidly exchanged for a fresh molecule of GTP.

Answer 30

The microtubule depolymerizes.

(Correct; because GDP-bound tubulin subunits associate less tightly, hydrolysis of GTP generally causes a microtubule to disassemble.)

Question 31

As a cell grows, which microtubule-associated protein pulls the ER membrane outward, stretching it like a net?

• dynein
• kinesin
• cohesin
• actin

Answer 31

Kinesin

(As a cell grows, kinesins attached to the outside of the endoplasmic reticulum membrane pull the ER outward, stretching it like a net. This movement of kinesin is toward the plus end of the microtubules.)

Question 32

Motor neuron degeneration occurs in several diseases and leads to loss of muscle control. One form of motor neuron degeneration was found to have defects in retrograde transport (backward transport to cell body, in blue below) that were caused by mutations in a gene that codes for a particular protein.

What protein, when mutated, would inhibit backward, but not outward, transport along axon microtubules?

• myosin
• tubulin
• dynein
• kinesin

Answer 32

Dynein

(Cytoplasmic dyneins transport cargo toward the minus ends of microtubules and are responsible for backward transport in axon microtubules. In contrast, kinesins move toward the plus end of microtubules.)

Question 33

An actin-binding protein called cofilin binds preferentially to ADP-containing actin filaments rather than ATP-containing actin filaments. Based on this preference, which is true?

• Cofilin binds to older actin filaments.
• Cofilin competes with profilin for binding to actin.
• Cofilin binds to the plus ends of treadmilling actin filaments.
• Cofilin binds to the plus ends of actin filaments.

Answer 33

Cofilin binds to older actin filaments.

(Because cofilin binds preferentially to ADP-containing actin filaments rather than ATP-containing actin filaments, it can be assumed that cofilin binds to older actin filaments. Older actin filaments are more likely to contain ADP-actin.)

Question 34

What are the steps of muscle contraction?

Answer 34

1. Neuron stimulates a muscle cell
2. Action potential triggers opening of Ca2+ release channels
3. Ca2+ is released from the sarcoplasmic reticulum
4. Troponin moves tropomyosin protein
5. Myosin interacts with actin

(You have placed the events between neuron signaling and muscle contraction in the correct order. Neuronal stimulation triggers an action potential, which leads to the release of Ca2+ from the sarcoplasmic reticulum. Ca2+ binding to troponin triggers the movement of tropomyosin away and myosin interaction with actin, leading to muscle contraction.)

Question 35

Which of the following would increase the level of muscle contraction?
Choose one or more:

• mutation in troponin such that it no longer binds tropomyosin
• addition of a molecule to bind free Ca2+
• blockage of the Ca2+ pump
• addition of a leaky Ca2+ channel to the sarcoplasmic reticulum

Answer 35

• blockage of the Ca2+ pump
• addition of a leaky Ca2+ channel to the sarcoplasmic reticulum

(Both addition of a leaky Ca2+ channel and blocking reuptake of Ca2+ by a Ca2+ pump would lead to increased cytosolic Ca2+, and therefore muscle contraction.)

Question 36

In a motility assay, investigators attach myosin motor proteins to a glass slide and then add actin filaments. Once the filaments have bound to myosin, what can be expected to occur?

• In the presence of ATP, the filaments will glide toward their minus ends.
• In the presence of ATP, the filaments will glide toward their plus ends.
• In the presence of ADP, the filaments will glide toward their plus ends.
• In the presence of GTP, the filaments will glide toward their minus ends.
• In the presence of GTP, the filaments will glide toward their plus ends.

Answer 36

In the presence of ATP, the filaments will glide toward their minus ends.

(Myosin walks toward the plus end of the actin filament; because the myosin is anchored to a glass slide, the filament will then move—toward its minus end.)

Question 37

When a muscle is stimulated to contract, myosin heads walk along actin filaments in repeated cycles of attachment and detachment. Which of the following represents a correct description of the events in this cycle?

• ATP binding enhances the affinity of myosin for actin; myosin attaches to actin; myosin is “cocked” as its head is displaced along the actin filament; the power stroke puts myosin in a “rigor” configuration.
• The power stroke puts myosin in a “rigor” configuration; myosin attaches to actin; myosin is “cocked” as its head is displaced along the actin filament; ATP hydrolysis enhances the affinity of myosin for actin.
• Myosin attaches to actin; ATP binding reduces the affinity of myosin for actin; myosin is “cocked” as its head is displaced along the actin filament; the power stroke puts myosin in a “rigor” configuration.
• ATP binding enhances the affinity of myosin for actin; myosin attaches to actin; the power stroke puts myosin in a “rigor” configuration; myosin is “cocked” as its head is displaced along the actin filament.
• ATP hydrolysis enhances the affinity of myosin for actin; myosin is “cocked” as its head is displaced along the actin filament; myosin attaches to actin; the power stroke puts myosin in a “rigor” configuration.

Answer 37

Myosin attaches to actin; ATP binding reduces the affinity of myosin for actin; myosin is “cocked” as its head is displaced along the actin filament; the power stroke puts myosin in a “rigor” configuration.

(Myosin attaches to actin; ATP binding reduces affinity of myosin for actin; myosin is “cocked” as its head is displaced along the actin filament; the power stroke puts myosin in a “rigor” configuration. At the end of this cycle, the myosin head has moved to a new position on the actin filament and is prepared for the next cycle.)

Question 38

Investigators incubate myosin with an ATP analog that can bind to myosin but cannot be hydrolyzed. What effect will this treatment have on the activity of myosin?

• The myosin will initiate a power stroke but will not be able to bind to the actin filament again at a new position.
• No effect—the myosin will behave normally.
• The myosin will bind to an actin filament and initiate a power stroke, but will not be subsequently released.
• The myosin will be unable to bind to an actin filament.
• The myosin will bind to an actin filament but will not be released; its movement along actin will be inhibited.

Answer 38

The myosin will be unable to bind to an actin filament.

(Incubation of myosin with an ATP analog that can bind to myosin but cannot be hydrolyzed will mean that the myosin will be unable to bind to an actin filament. The binding of ATP weakens myosin’s affinity for actin, allowing movement along the filament. An ATP analog will perform the same function, so myosin will be unable to bind to an actin filament.)

Question 39

Heart conditions known as cardiomyopathies include two main subtypes. In hypertrophic cardiomyopathies, the heart muscle becomes stiff. Such disorders can be treated by drugs like verapamil, which block calcium channels. In a second class of cardiomyopathies, the heart muscle becomes dilated. These disorders are sometimes treated with the drug digoxin, which blocks Na+ pumps and elevates intracellular Ca2+ in cardiac muscle cells. Both types of disorder can be caused by mutations in the genes encoding troponin. Some of these mutations increase troponin’s sensitivity to Ca2+; other mutations reduce it. Based on the treatment protocols outlined, what type of mutation is responsible for hypertrophic cardiomyopathy?

• mutations that decrease troponin’s sensitivity to verapamil
• mutations that either increase or decrease troponin’s sensitivity to Ca2+
• mutations that increase troponin’s sensitivity to Ca2+
• mutations that decrease troponin’s sensitivity to digoxin
• mutations that decrease troponin’s sensitivity to Ca2+

Answer 39

mutations that increase troponin’s sensitivity to Ca2+

(Mutations that increase troponin’s sensitivity to Ca2+ are most likely responsible for hypertrophic cardiomyopathy. An increased sensitivity to Ca2+ can cause hypertrophy by overstimulating muscle contraction; this condition can be counteracted by drugs that block Ca2+ channels, thus reducing the intracellular Ca2+ concentration.)

Question 40

Rigor mortis is a muscle stiffening that sets in a few hours after death. It is due to muscles being in a contracted state. It dissipates after several days as muscle proteins degenerate and cells break down. Rigor mortis is one sign that coroners take into account when estimating a time of death. At a cellular level, what is the mechanism behind rigor mortis?

• In the absence of ATP, kinesin and dynein no longer move along microtubules and remain locked in place.
• After death, ATP production ceases and ATP is needed for myosin release from actin.
• After death, membranes become leaky, and sodium flowing into motor neurons trigger action potentials that contract muscles.
• After death, calcium ions are no longer available to bind troponin.

Answer 40

After death, ATP production ceases and ATP is needed for myosin release from actin.

(ATP binding to myosin releases myosin from actin, allowing muscle relaxation. In the absence of ATP, muscles remain in a locked, contracted state.)