Deck 16: The Cytoskeleton

Question 1

According to the following graph, which shows the elongation rate at the plus and minus ends of actin filaments as a function of actin subunit concentration, at what concentration (A to E) does the total length of the filament remain more or less constant with time (i.e. steady-state treadmilling occurs)?

Answer 1

B

Question 2

In the polymerization in vitro of actin filaments and microtubules from their subunits, what does the "lag phase" correspond to?

A) Nucleation
B) Reaching steady state
C) Nucleotide exchange
D) ATP or GTP hydrolysis
E) Treadmilling

Answer 2

A
Explanation: The lag phase corresponds to the rate-limiting nucleation step. If this step is bypassed by introduction of preformed filament seeds, the lag phase disappears.

Question 3

Consider the ATP-bound and ADP-bound forms of actin and the polarized nature of the actin filaments. In the following diagram that shows the various actin polymerization rate constants (k?n and k?ff values), which monomer corresponds to an ADP-bound actin incorporated at the plus end?

Answer 3

D

Question 4

The actin-nucleating protein formin has flexible "whiskers" containing binding sites that help recruit actin subunits in order to enhance polymerization by this protein. What protein would you expect to bind to these sites?

A) Thymosin
B) Profilin
C) Cofilin
D) Gelsolin
E) Tropomodulin

Question 5

Fill in the blank: Each microtubule is typically made of thirteen parallel …. that associate laterally to form a hollow tube.

Question 6

Which of the following cytoskeletal filaments are abundant in an animal cell nucleus?

A) Microfilaments
B) Microtubules
C) Septins
D) Intermediate filaments
E) Spectrin filaments

Question 7

If the concentration of free subunits is C, under which condition does the growth of a cytoskeletal filament proceed spontaneously?

A) C > 1/kₒn
B) C < 1/kₒff
C) C > kₒn/kₒff
D) C > kₒff/kₒn
E) C > Cc×kₒn/kₒff

Question 8

Indicate true (T) and false (F) statements below regarding the cell cytoskeleton. Your answer would be a four-letter string composed of letters T and F only; e.g. TTFF.
( ) The three major building blocks of cytoskeletal filaments can bind to and hydrolyze nucleotides.
( ) The building blocks of microfilaments and microtubules are globular proteins, whereas those of intermediate filaments are themselves filamentous proteins.
( ) Intermediate filaments are typically thicker than actin filaments but thinner than microtubules.
( ) Plant cells lack microtubules.

Question 9

In the following graph of actin elongation rates under different subunit concentrations, which of the following corresponds to the slope of the line?

A) kₒn
B) kₒff
C) kₒn/kₒff
D) kₒff/kₒn
E) kₒn - kₒff

Question 10

Which of the following is an actin homolog?

A) Arp2
B) Arp3
C) MreB
D) ParM
E) All of the above

Question 11

F-actin is not a straight polymer but can be considered a double-helical assembly. The two protofilaments twist around each other and cross over approximately every 13 actin subunits, and the subunits in each protofilament are repeated every 2.8 nm along the helix. F-actin is almost 8 nm wide. Different myosin motors have different "step lengths" on actin depending on their function. Which of the following step lengths would you expect to belong to a processive myosin motor that carries large cellular cargoes such as endocytic vesicles?

A) About 2.8 nm
B) About 8 nm
C) About 13 nm
D) About 27 nm
E) About 36 nm

Question 12

Which of the following drugs is toxic for our cells?

A) Cytochalasin B, which caps the plus end of actin filaments and prevents actin polymerization.
B) Phalloidin, which binds along actin filaments and stabilizes them.
C) Nocodazole, which binds to tubulin subunits and prevents microtubule polymerization.
D) Colchicine, which caps microtubule ends and leads to their depolymerization.
E) All of the above.

Question 13

Indicate if each of the following changes occurring during mitosis in a fibroblast is the result of the reorganization of actin filaments (A), microtubules (M), or intermediate filaments (I). Your answer would be a five-letter string composed of letters A, M, and I only; e.g. AIAAM.
( ) The cell rounds up.
( ) The endoplasmic reticulum collapses.
( ) The Golgi apparatus fragments.
( ) The primary cilium is resorbed (disappears).
( ) The contractile ring forms and constricts.

Question 14

The time courses of seeded actin polymerization under two different conditions are compared in the following graph. If the overall kₒn for polymerization is known to be the same under both of these conditions, which curve-(1) or (2)-corresponds to the condition with a higher kₒff rate constant? Which one corresponds to a higher Cc for polymerization?

A) Curve (1); curve (1)
B) Curve (1); curve (2)
C) Curve (2); curve (1)
D) Curve (2); curve (2)
E) Curve (2); both curves have the same Cc.

Question 15

Persistence length for a cytoskeletal filament is the minimum filament length at which random thermal fluctuations are likely to cause it to bend. Which of the following comparisons are true, considering the persistence lengths of (a) an actin filament, (b) a bundle of cross-linked actin filaments, and (c) a microtubule?

A) (a > b) and (a > c)
B) (a > b) and (a < c)
C) (a < b) and (a > c)
D) (a < b) and (a < c)
E) (a > b) and (b > c)

Question 16

From left to right, indicate if each of the following schematic drawings represents the typical overall organization of actin filaments (A), microtubules (M), or intermediate filaments (I) in an animal cell. Your answer would be a three-letter string composed of letters A, M, and I only; e.g. AMI.

Question 17

You have prepared actin filament seeds in a microfluidic chamber through which you then pass either unlabeled or fluorescently labeled actin subunits in alternation several times, keeping the total monomer concentration constant. You then observe the resulting filaments under a fluorescence microscope. If the filaments appear as drawn schematically in the example below (in which fluorescence is indicated by black color), which end do you think is the plus end-(a) or (b)? Was the concentration of free actin subunits below or above the critical concentration (Cc) for polymerization at the barbed end?

A) (a); below
B) (b); below
C) (a); above
D) (b); above

Question 18

Indicate if each of the following descriptions matches actin filaments (A), microtubules (M), or intermediate filaments (I). Your answer would be a four-letter string composed of letters A, M, and I only; e.g. AAMM.
( ) They form hollow structures with multiple lateral interactions.
( ) They form strong structures that are more resilient than the other two cytoskeletal filaments.
( ) Their subunits bind GTP and hydrolyze it.
( ) They form coiled-coil interactions between the subunits.

Question 19

Bacteria contain homologs of cytoskeletal filament subunits …

A) except those of the intermediate filaments.
B) but these homologs are incapable of nucleotide binding and hydrolysis.
C) that are less diverse in their function relative to their eukaryotic counterparts.
D) that can have different functions to those of their eukaryotic counterparts.
E) that are dispensable for cell growth and proliferation.

Question 20

Indicate if each of the following structures is based on actin filaments (A), microtubules (M), or intermediate filaments (I). Your answer would be a five-letter string composed of letters A, M, and I only; e.g. AAAMM.
( ) The cell cortex
( ) The mitotic spindle
( ) The nuclear lamina
( ) Cilia
( ) Filopodia

Question 21

If myosin II heads are attached to a glass slide and actin filaments are allowed to bind to them, the filaments will glide on the surface …

A) toward their plus end in the presence of ATP.
B) toward their plus end in the presence of GTP.
C) toward their minus end in the presence of ATP.
D) toward their minus end in the presence of GTP.
E) toward their plus end in the presence of ADP.

Question 22

Heart conditions known as cardiomyopathies include two main subtypes, namely the hypertrophic and dilated cardiomyopathies, in which some portions of the cardiac muscle become stiff or dilated, respectively. Drugs such as the calcium channel blocker verapamil are used to treat the former, while the Na?-K? ATPase inhibitor digoxin (which elevates intracellular calcium concentration in cardiac muscle cells) is sometimes used for the latter. Mutations in the subunits of the troponin complex are among the genetic causes of these diseases, and can be grouped into mutations that result in either decreased (D) or increased (I) calcium-ion sensitivity of actin-myosin contraction. Which of these mutations would you expect to be associated with hypertrophic cardiomyopathy? Write down D or I as your answer.

Question 23

You have used "optical tweezers" to study the mechanics of myosin movement in a newly discovered member of the myosin II subfamily. Your results indicate that each myosin exerts a force of about 3 pN along the length of the actin filament, and displaces the filament by about 10 nm in each cycle of ATP hydrolysis. Assuming that the free-energy change for ATP hydrolysis is -50 kJ/mol under your experimental conditions, what is the efficiency of the myosin motor in converting the free energy to mechanical work? Remember that under a constant force F (in newtons), work (W; in joules) is calculated as W = F × d, where d is displacement (in meters) in the direction of the force. Avogadro's number is approximately 6 × 10²³ molecules/mole. Write down your answer as a percentage with no decimals, e.g. 99%.

Question 24

Sort the following events to reflect the sequence during smooth muscle contraction upon delivery of muscle stimulation. Your answer would be a five-digit number composed of digits 1 to 5 only, e.g. 13452.
(A) Activation of the myosin light-chain kinase
(B) Interaction of myosin head with actin
(C) Phosphorylation of myosin
(D) Calcium release into the cytoplasm
(E) Activation of calmodulin

Question 25

In which of the following structures are actin-myosin II bundles regulated by the troponin complex?

A) Stress fiber
B) Circumferential belt
C) Contractile ring
D) Adherens junction
E) None of the above

Question 26

In the dendritic networks of actin filaments in lamellipodia, nucleation of actin polymerization is mostly performed by …

A) ERM proteins
B) WASp proteins
C) Formin
D) Arp 2/3 complex
E) γ-TuRC

Question 27

In the presence of an ATP analog that can bind myosin normally but cannot be hydrolyzed, …

A) a myosin cannot bind tightly to an actin filament and cannot move on it.
B) a myosin cannot be released from an actin filament and cannot move on it.
C) a myosin performs the power stroke but cannot be released from an actin filament.
D) a myosin performs the power stroke but cannot bind tightly to an actin filament.
E) a myosin is able to perform normally.

Question 28

Which of the following actin-binding proteins cannot bind to the same actin filament simultaneously?

A) Gelsolin and tropomodulin
B) Tropomyosin and tropomodulin
C) Profilin and tropomodulin
D) Cofilin and CapZ
E) Formin and CapZ

Question 29

Which of the actin-binding proteins (1 to 5) in the following schematic drawing represents tropomyosin, ?-actinin, Arp 2/3 complex, CapZ, and myosin, respectively? Your answer would be a five-digit number composed of digits 1 to 5 only; e.g. 15324.

Question 30

Skeletal muscle cells consume enormous amounts of ATP. From the following list, choose the two proteins that directly hydrolyze most of this ATP. Your answer would be a two-letter string composed of letters A to F only, in alphabetical order, e.g. AF.
A. Troponin complex
B. Ca²⁺-release channel
C. Myosin light chains
D. Myosin heavy chain
E. Myosin light-chain kinase
F. Sarcoplasmic reticulum Ca²⁺-pump

Question 31

Consider the structure of a sarcomere. Which of its features does NOT shorten during skeletal muscle contraction?

A) The dark band
B) The light band
C) The distance from the M line to the Z disc
D) The distance between two consecutive Z discs
E) The extension of the titin molecules

Question 32

Sort the following events to show the sequence in the mechanochemical cycle of myosin II, following ATP binding by the head domain. Your answer would be a four-digit number composed of digits 1 to 4 only, e.g. 1342.
(A) Myosin head binds tightly to actin.
(B) Power stroke is triggered.
(C) Cocking occurs and the head is displaced along the actin filament.
(D) The binding affinity of myosin for actin is reduced.

Question 33

The proteins of the ERM family such as moesin …

A) bind to and organize the cortical actin cytoskeleton.
B) interact with transmembrane proteins.
C) affect cortical stiffness and cell shape.
D) affect the localization and activity of cell signaling molecules.
E) All of the above.

Question 34

Indicate true (T) and false (F) statements below regarding cytoskeletal motor proteins. Your answer would be a five-letter string composed of letters T and F only, e.g. TTTFF.
( ) All myosin motors move toward the plus end of actin filaments.
( ) All myosin motors move toward the minus end of actin filaments.
( ) All kinesin motors move toward the plus end of microtubules.
( ) All kinesin motors move toward the minus end of microtubules.
( ) All dynein motors move toward the minus end of microtubules.

Question 35

In the following graph, the elongation rate of pure actin filaments as a function of actin subunit concentration is shown as a dashed line. Which one of the other five lines (A to E) would you think better shows what happens when a plus-end capping protein such as CapZ is present?

Question 36

Actin filaments that are held together by the cross-linking protein fimbrin are not contractile. This is probably because …

A) the very weak cross-linking by this protein cannot convert myosin II activity into a coherent contraction.
B) fimbrin arranges the actin filaments in parallel bundles in which all the plus ends point to the same direction.
C) the very tight packing of actin filaments by this small protein excludes myosin II filaments and other large proteins.
D) fimbrin arranges the actin filaments into gel-like networks in which myosin II activity does not produce contraction.
E) fimbrin is a large protein that binds to several actin filaments and resists contraction.

Question 37

In the structure of which of the following proteins are the actin-filament binding sites furthest apart in space?

A) Spectrin
B) Filamin
C) α-Actinin
D) Fimbrin
E) Myosin II

Question 38

Cofilin binds preferentially to ADP-containing actin filaments rather than to ATP-containing filaments. Consequently, this protein …

A) competes with profilin for actin binding.
B) binds to older actin filaments.
C) binds to the plus end of actin filaments.
D) stabilizes actin filaments.
E) All of the above.

Question 39

The pathogen Listeria monocytogenes can hijack the actin cytoskeleton in human cells and spread inside the host. Indicate true (T) and false (F) statements below about this phenomenon. Your answer would be a four-letter string composed of letters T and F only; e.g. TTFF.
( ) The movement can be reconstituted in vitro by placing the bacteria in a mixture of actin, formin, gelsolin, and capping protein.
( ) Cofilin counteracts the movement by depolymerizing actin filaments.
( ) The actin filaments grow with their minus ends pointed toward the bacterium.
( ) The movement depends on myosin activity to transport the bacteria on the actin filaments.

Question 40

After an animal dies, its muscles start to stiffen before the decomposition of tissues relaxes the muscles again. Which of the following would you expect to explain this muscle stiffening (i.e. rigor mortis)?

A) The myosin II heads in muscle fibers remain attached to actin filaments due to the absence of Ca²⁺ in these cells.
B) The ATPase activity of muscle myosin II is inhibited by the elevated Ca²⁺.
C) The myosin II heads in muscle cells remain attached to actin filaments due to the elevated Ca²⁺ in the muscle fibers.
D) Titin molecules unfold, preventing muscle relaxation.
E) The myosin II heads in muscle cells remain attached to actin filaments due to covalent cross-linking.

Question 41

Indicate if each of the following descriptions applies to (1) EB1, (2) kinesin-1, (3) kinesin-13, or (4) katanin. Your answer would be a four-digit number composed of digits 1 to 4 only, e.g. 1432.
( ) This is a microtubule-severing protein that can release microtubules from the microtubule-organizing centers.
( ) It increases the frequency of catastrophe by deforming microtubule protofilaments.
( ) This is a conventional motor protein that moves toward the plus end of a microtubule.
( ) It recognizes the structure of a growing microtubule end and binds to it, helping other proteins to also bind to the plus end.

Question 42

Using time-lapse fluorescence microscopy, you have recorded the transport of fluorescently labeled mitochondria in a region of an axon in a fruit-fly larva; you have plotted the results in the following graph, in which the vertical axis is time (from the top to the bottom), the horizontal axis is the position along the axon, and each trace represents one mitochondrion. Is the retrograde transport or the anterograde transport of mitochondria faster in these axons? Does the trace indicated by an asterisk correspond to a mitochondrion that was transported by a dynein or a kinesin?

A) Anterograde; kinesin
B) Anterograde; dynein
C) Retrograde; kinesin
D) Retrograde; dynein

Question 43

A microtubule appears as a left-handed helix due to an approximately 0.9-nm stagger in the lateral contacts between adjacent protofilaments. In the lateral contacts, α- and β-tubulins in one protofilament interact with α- and β-tubulins, respectively, in the neighboring protofilament, except for a longitudinal discontinuity along the microtubule called the seam. Along the seam, lateral contacts have to be made between different tubulins (i.e. α-tubulin with β-tubulin). Which of the following do you think is acceptable as the repeat distance of tubulin monomers along a protofilament?

A) 4 nm
B) 5 nm
C) 6 nm
D) 7 nm
E) 10 nm

Question 44

Dynamitin is a subunit of the dynactin complex. Its overexpression leads to the disassembly of the complex, and it is therefore considered a dynactin inhibitor. Which of the following processes would happen if dynamitin is overexpressed in a cell?

A) The motile cilia (if any) would stop beating because the axonemal dyneins could not attach to their neighboring microtubules.
B) The Golgi apparatus would fragment and become dispersed because the cytoplasmic dyneins could not associate with the Golgi membranes.
C) The endoplasmic reticulum would collapse because it could not recruit enough kinesins.
D) The nuclear envelope would disintegrate because the nuclear lamins could not interact with the cytoplasmic cytoskeleton.
E) The cortex would lose its integrity because the Arp 2/3 complex would also be inhibited.

Question 45

What is the major microtubule-organizing center in animal cells?

A) The γ-tubulin ring complex
B) The centrosome
C) The cell cortex
D) The primary cilium
E) The spindle pole body

Question 46

Indicate whether each of the following descriptions applies to myosins (M), kinesins (K), or dyneins (D). Your answer would be a five-letter string composed of letters M, K, and D only, e.g. MMMDD.
( ) They have larger structures than the other two.
( ) They are generally faster than the other two.
( ) They are structurally unrelated to the other two.
( ) They walk on a different cytoskeletal filament than the other two.
( ) They are all minus-end directed.

Question 47

The duty ratio for a motor protein is defined as the fraction of time in each cycle of activity of the motor in which the head is bound with high affinity to its cytoskeletal track. Which of the following is reasonable given this definition?

A) Myosin V has a higher duty ratio than myosin II, because Pᵢ release is the rate-limiting step in its cycle.
B) Myosin II has a higher duty ratio than myosin V, because Pᵢ release is the rate-limiting step in its cycle.
C) Myosin V has a higher duty ratio than myosin II, because ADP release is the rate-limiting step in its cycle.
D) Myosin II has a higher duty ratio than myosin V, because ADP release is the rate-limiting step in its cycle.

Question 48

"Headless" kinesin mutants only contain the stalk (middle) and tail domains and can therefore dimerize with their wild-type kinesin partners. However, since they lack the motor (head) domain, the resulting dimers are unable to carry out processive transport of their cargoes and the mutation thus behaves as "dominant negative," meaning that the mutant not only is nonfunctional, but can also interfere with the function of its wild-type counterparts. If a headless mutant of a kinesin heavy chain involved in melanosome movement is overexpressed in fish melanocytes, what would you predict happens in these cells?

A) Pigment dispersion would be inhibited and there would be more tug-of-war between the motors.
B) Pigment aggregation would be inhibited and there would be more tug-of-war between the motors.
C) Pigment dispersion would be inhibited and there would be less tug-of-war between the motors.
D) Pigment aggregation would be inhibited and there would be less tug-of-war between the motors.

Question 49

Unlike a myosin head, a kinesin head is tightly bound to its cytoskeletal track when bound to ...

A) ATP.
B) ADP.
C) no nucleotide.
D) GTP.
E) GDP.

Question 50

In contrast to growing microtubules, shrinking microtubules …

A) have a GTP cap at their plus end.
B) have strong lateral interactions at their plus ends.
C) have curved protofilaments at their plus ends.
D) cannot be rescued unless microtubule-stabilizing proteins bind and inhibit depolymerization.
E) All of the above.

Question 51

The γ-tubulin ring complex is to microtubules what … is to actin filaments.

A) the Arp 2/3 complex
B) the dynactin complex
C) the troponin complex
D) formin
E) contractile ring

Question 52

In the presence of ATP in a flagellum, an axonemal dynein that is interacting through its tail with the A microtubule of a peripheral doublet can push this doublet toward the ...(1) of the flagellum, but due to the presence of linking proteins such as ...(2), this force is converted into a bending motion.

A) (1) tip; (2) nexin
B) (1) base; (2) nexin
C) (1) tip; (2) nebulin
D) (1) base; (2) nebulin
E) (1) tip; (2) tau

Question 53

Indicate whether each of the following descriptions matches actin filaments (A), microtubules (M), or intermediate filaments (I). Your answer would be a three-letter string composed of letters A, M, and I only, e.g. MMA.
( ) They have the smallest persistence length.
( ) They lack polarity.
( ) They have the highest tensile strength.

Question 54

Which of the following proteins do you expect to be enriched near the plus end of microtubules?

A) Dynein
B) XMAP215
C) γ-Tubulin
D) Katanin
E) All of the above

Question 55

Indicate whether each of the following descriptions applies to cilia (C), flagella (F), or both (B). Your answer would be a four-letter string composed of letters C, F, and B only, e.g. CCFB.
( ) They are short and present at high numbers per cell.
( ) They have a whiplike motion that resembles breaststroke in swimming.
( ) They are based on the axoneme structure.
( ) They are found in the epithelial cells of the human respiratory tract.

Question 56

A dimeric kinesin-1 molecule has 8-nm steps and can move at rates of about 1 µm/sec. Olympic 100-meter sprinters typically run at about 180 steps per minute and can reach speeds of about 42 km/h. With the same step size, if the Olympic runner had a step frequency of a kinesin-1 molecule, how fast could she run? Write down your answer in km/h, e.g. 52 km/h.

Question 57

Indicate true (T) and false (F) statements below regarding the primary cilia. Your answer would be a four-letter string composed of letters T and F only, e.g. TFFF.
( ) Primary cilia are found on the surface of almost all cell types in our bodies.
( ) Primary cilia are motile.
( ) Primary cilia are made in interphase at basal bodies.
( ) Primary cilia are thought to function in sensing and responding to external signals.

Question 58

A small unique insert found near the end of the motor domain in myosin VI has been linked to the singular ability of this myosin to move toward the pointed end of actin filaments. If the unique insert is removed by genetic engineering, the resulting mutant myosin VI is a plus-end directed motor. This observation indicates that …

A) the unique insert is necessary and sufficient for minus-end directionality.
B) the unique insert is necessary for motor activity.
C) the unique insert is necessary for plus-end directed directionality.
D) the unique insert is not sufficient for minus-end directionality.
E) None of the above.

Question 59

In the following graph that shows changes in the lengths of two microtubules over time, which time point corresponds to a catastrophe for both microtubules? Which trace corresponds to a microtubule with greater dynamic instability?

A) t₁; trace (a)
B) t₁; trace (b)
C) t₂; trace (a)
D) t₂; trace (b)

Question 60

Indicate true (T) and false (F) statements below regarding the microtubule cytoskeleton in neurons. Your answer would be a five-letter string composed of letters T and F only, e.g. TTFFF.
( ) All microtubules in an axon are normally oriented in the same direction.
( ) All microtubules in a dendrite are normally oriented in the same direction.
( ) Each microtubule in an axon has one end at the cell body and its other end at the axon terminal.
( ) Anterograde transport in axons is exclusively carried out by kinesins.
( ) Anterograde transport (toward the tip) in dendrites is exclusively carried out by kinesins.

Question 61

How is membrane protrusion by blebbing different from that by lamellipodia?

A) Blebbing usually occurs on a rigid substratum such as glass, whereas lamellipodia form on pliable substrata.
B) Blebbing is mostly observed in vitro, whereas lamellipodia are observed both in vitro and in vivo.
C) Blebbing does not involve myosin II activity, but formation of lamellipodia does.
D) Blebbing requires loss of membrane interaction with actin filaments, whereas lamellipodia require a close interaction between the two.
E) All of the above.

Question 62

Which of the following is NOT an example of a crawling cell?

A) Macrophage
B) Osteoclast
C) Keratocyte
D) Neural crest cell
E) Sperm

Question 63

In lamellipodia, actin polymerization is nucleated by ...(1), while depolymerization is catalyzed by ...(2).

A) (1) formin; (2) gelsolin
B) (1) Arp 2/3 complex; (2) gelsolin
C) (1) formin; (2) cofilin
D) (1) Arp 2/3 complex; (2) cofilin
E) (1) formin; (2) capping protein

Question 64

Considering the diagram below, which summarizes two signaling pathways initiated by activated GTPases of the Rho family, what letter (A to D) corresponds to each of the following? Your answer would be a four-letter string composed of letters A to D only, e.g. ABCD.

( ) Rac-GTP
( ) Rho-GTP
( ) Lamellipodia formation
( ) Stress-fiber formation

Question 65

In the following schematic drawing, in which direction (1 or 2) is the keratocyte migrating? What is the approximate length of the cell indicated by the bar on the left?

A) Direction 1; about 20 µm
B) Direction 1; about 2 µm
C) Direction 2; about 20 µm
D) Direction 2; about 2 µm
E) Either direction; about 2 µm

Question 66

These proteins are found in the budding yeast Saccharomyces cerevisiae; they form "neck filaments" between a mother cell and its growing bud, and help polarize protein distribution between the two. These proteins ...

A) can polymerize to form filaments and sheets.
B) bind GTP.
C) are also involved in contractile ring formation during cytokinesis in animal cells.
D) form filaments that are thought to be nonpolar.
E) All of the above.

Question 67

Consider an actin subunit that has just been incorporated into an actin filament at the leading edge of a lamellipodium. Before its ATP is hydrolyzed, how does its distance from the leading front edge of the plasma membrane change over time? How does its distance from the F-actin minus end change over time?

A) Decreases; decreases
B) Decreases; remains constant
C) Decreases; increases
D) Increases; decreases
E) Increases; remains constant

Question 68

In a cross section of a vertebrate axon, longitudinally organized cytoskeletal proteins can be seen as dots, as shown in the schematic drawing below. What type of filaments do these dots represent?

A) Microtubules and microfilaments
B) Microtubules and vimentins
C) Microtubules and neurofilaments
D) Microtubules and keratins
E) Microtubules and septins

Question 69

What are the typical consequences of mutations in keratins and their associated proteins?

A) Neurodegeneration as a result of interference with normal axonal transport.
B) Muscle development defects as a result of sarcomere disorganization.
C) Skeletal and cardiac abnormalities as a result of a weakened nuclear envelope.
D) Cornea disorders as a result of cell rupture from mechanical trauma.
E) Developmental defects as a result of abnormal signaling related to the primary cilia.

Question 70

In the following schematic drawing of a polarized neutrophil engaged in chemotaxis, in what region (a or b) does Rac activity dominate? What other member of the Rho family dominates at the other region?

A) Region (a); Rho
B) Region (a); Cdc42
C) Region (b); Rho
D) Region (b); Cdc42

Question 71

Indicate whether each of the following structures has actin organized mostly in a one- (1), two- (2), or three- (3) dimensional arrangement. Your answer would be a four-digit number composed of digits 1 to 3 only, e.g. 1333.
( ) Stress fibers
( ) Invadopodia
( ) Lamellipodia
( ) Filopodia

Question 72

If a certain isoform of myosin II is depleted from a cell, stress fibers are lost and focal adhesions disappear. If these cells are placed on a surface coated with an array of flexible pillars to measure traction forces, would you expect the traction to increase (I) or decrease (D) in these cells compared to wild-type cells? Write down I or D as your answer.

Question 73

SUN and KASH proteins embedded in the nuclear envelope provide connections between the organization of the nucleus and the cytoplasm. Which of the following pairs of proteins DO NOT bind directly to each other in these connections?

A) Nuclear lamina and SUN proteins
B) Plectins and KASH proteins
C) KASH proteins and microfilaments
D) KASH proteins and SUN proteins
E) SUN proteins and cytoplasmic intermediate filaments.

Question 74

Indicate whether each of the following descriptions matches Cdc42 (C), Rac (R), or Rho (H) from the Rho family of monomeric GTPases. Your answer would be a three-letter string composed of letters C, R, and H only, e.g. HRR.
( ) It activates formin.
( ) It inhibits myosin II activity.
( ) When constitutively active, it induces the formation of many prominent stress fibers.

Question 75

Keratins are intermediate filaments that ...

A) are localized inside the nucleus.
B) are composed of 50% type I and 50% type II keratin proteins.
C) are bundled in the epidermis to make cells more flexible.
D) anchor the intermediate filament network at the adherens junctions.
E) All of the above.