Cell Biology Chapter 17 v2.00

Identify the cytoskeletal structures (black lines) depicted in the epithelial cells shown in Figure Q17-1.

(A) microtubules (B) intermediate filaments (C) actin

Which of the situations below will enhance microtubule shrinkage? (a) addition of a drug that inhibits GTP exchange on free tubulin dimers (b) addition of a drug that inhibits hydrolysis of the GTP carried by tubulin dimers (c) addition of a drug that increases the affinity of tubulin molecules carrying GDP for other tubulin molecules (d) addition of a drug that blocks the ability of a tubulin dimer to bind to γ-tubulin

(a) A drug that inhibits GTP exchange on free tubulin dimers will effectively decrease the available pool of GTP-bound tubulin dimers available for addition to microtubule ends, thus tipping the balance in favor of microtubule disassembly. A drug that inhibits hydrolysis of GTP carried by tubulin dimers [choice (b)] or that increases the affinity of GDP-bound tubulin dimers for each other [choice (c)] will stabilize growing microtubules. Blocking the ability of a tubulin dimer to bind to γ-tubulin will decrease the rate of new microtubule formation but should not enhance microtubule shrinkage [choice (d)].

Compared to the normal situation, in which actin monomers carry ATP, what do you predict would happen if actin monomers that bind a nonhydrolyzable form of ATP were incorporated into actin filaments? (a) Actin filaments would grow longer. (b) Actin filaments would grow shorter because depolymerization would be enhanced. (c) Actin filaments would grow shorter because new monomers could not be added to the filaments. (d) No change, as addition of monomers binding nonhydrolyzable ATP would not affect actin filament length.

(a) Addition of monomers carrying a nonhydrolyzable form of ATP would stabilize the interactions between the monomers of a filament, stabilizing the filament and inhibiting depolymerization, resulting in longer actin filaments.

Which of the following statements about the function of the centrosome is false? (a) Microtubules emanating from the centrosome have alternating polarity such that some have their plus end attached to the centrosome while others have their minus end attached to the centrosome. (b) Centrosomes contain hundreds of copies of the γ-tubulin ring complex important for microtubule nucleation. (c) Centrosomes typically contain a pair of centrioles, which is made up of a cylindrical array of short microtubules. (d) Centrosomes are the major microtubule-organizing center in animal cells.

(a) Microtubules emanating from the centrosome are all arranged with their minus ends at the centrosomes and the plus ends extending into the cytoplasm.

The graph in Figure Q17-31 shows the time course of the polymerization of pure tubulin in vitro. Assume that the starting concentration of free tubulin is higher than it is in cells. Three parts of the curve are labeled above it as A, B, and C. You conduct a similar in vitro tubulin-polymerization experiment, only you include purified centrosomes in your preparation. When you plot your data, which part of your graph should be most dissimilar to the curve shown in Figure Q17-31? (a) A (b) B (c) C (d) None. The shape of my graph should be identical to the graph produced when tubulin is polymerized in the absence of purified centrosomes.

(a) Purified centrosomes should enhance the nucleation of microtubules, and thus decrease the lag time (seen in part A of the graph) for microtubule polymerization that occurs when microtubules are polymerized from only pure tubulin.

Which of the following items is not important for flagellar movement? (a) sarcoplasmic reticulum (b) ATP (c) dynein (d) microtubules

(a) The sarcoplasmic reticulum is important for muscle contraction. All other items are important for flagellar movement.

Intermediate filaments are made from elongated fibrous proteins that are assembled into a ropelike structure. Figure Q17-10 shows the structure of an intermediate filament subunit. You are interested in how intermediate filaments are formed, and you create an intermediate filament subunit whose α-helical region is twice as long as that of a normal intermediate filament by duplicating the normal α-helical region while keeping a globular head at the N-terminus and a globular tail at the C-terminus; you call this subunit IFαd. If you were to assemble intermediate filaments using IFαd as the subunit, which of the following predictions describes the most likely outcome? Figure Q17-10 (a) Filaments assembled using IFαd will interact with different cytoskeletal components. (b) Filaments assembled using IFαd will form dimers that are twice as long as dimers assembled from normal intermediate filaments. (c) Sixteen tetramers assembled from IFαd will be needed for a ropelike structure to form. (d) Dimers of IFαd will form by interactions with the N-terminal globular head and the C-terminal globular tail.

(b) Because the α-helical region is twice as long, you would predict that a coiled-coil dimer made up of two IFαd subunits would be about twice as long as a dimer assembled from a normal intermediate filament subunit. Because the globular head and tail regions usually interact with other cellular components, doubling the size of the α-helical region without changes in the globular regions is unlikely to cause changes in protein interactions [choice (a)]. Eight tetramers are usually needed to form a ropelike filament, and it is unlikely that 16 will be needed with IFαd, because it is the length of the coiled- coil region that is most affected by a doubling in size of the α-helical region [choice (c)]. Interactions in the α-helical region are important for dimerization, and thus choice [d] is untrue.

Which of the following conditions is likely to decrease the likelihood of skeletal muscle contraction? (a) partial depolarization of the T-tubule membrane, such that the resting potential is closer to zero (b) addition of a drug that blocks Ca2+ binding to troponin (c) an increase in the amount of ATP in the cell (d) a mutation in tropomyosin that decreases its affinity for the actin filament

(b) Ca2+ binding to troponin leads to a conformational change that causes a movement in tropomyosin so that myosin can bind to actin to initiate contraction. Thus, if troponin cannot bind Ca2+, the likelihood of contraction decreases. Partial depolarization of the T-tubule membrane will make it easier to depolarize the membrane, increasing the likelihood of muscle contraction [choice (a)]. ATP is required for myosin movement, so increasing the amount of ATP in the cell will not decrease contraction [choice (c)]. Tropomyosin normally binds to actin and blocks myosin binding, so a mutation in tropomyosin that decreases its affinity for actin should not decrease the likelihood of muscle contraction [choice (d)].

You discover a protein, MtA, and find that it binds to the plus ends of microtubules in cells. The hypothesis that best explains this localization is________________. (a) MtA is involved in stabilizing microtubules. (b) MtA binds to GTP-bound tubulin on microtubules. (c) MtA is important for the interaction of microtubules with the centrosome. (d) MtA will not bind to purified microtubules in a test tube.

(b) GTP-bound tubulin molecules are found at the growing end of a microtubule, which is its plus end. A function of MtA in stabilizing microtubules cannot be inferred simply from its localization, as protein factors that stabilize and destabilize microtubules can bind to plus ends [choice (a)]. The localization of MtA at the plus end of the microtubule means that MtA is at the end of the microtubule furthest from the centrosome [choice (c)]. Whether MtA will bind to purified microtubules in a test tube cannot be inferred from the discovery of its localization at the plus end of microtubules in cells [choice (d)].

Which of the following statements is false? (a) Cytochalasins prevent actin polymerization. (b) Actin filaments are usually excluded from the cell cortex. (c) Integrins are transmembrane proteins that can bind to the extracellular matrix. (d) ARPs can promote the formation of branched actin filaments.

(b) Much of the actin in the cell is concentrated in the cell cortex, the region of the cell just beneath the plasma membrane.

Figure Q17-57shows an electron micrograph of a skeletal muscle fiber, where various points along a fiber and various regions have been labeled. Figure Q17-57 Which of the following statements is true about muscle contraction? (a) Point A will move closer to point B. (b) Point B will move closer to point C. (c) Region D will become smaller. (d) Region E will shrink in size.

(b) The dark region in the center of the micrograph corresponds to the thick filaments of a myofibril and is composed of many myosin molecules. The light regions (labeled A, B, and C) correspond to actin filaments, which are attached to the Z discs (also see Figure A17-57). During muscle contraction, the myosin filaments will travel along the actin filaments, bringing points B and C closer together. Points A and B will not move relative to each other, as contraction occurs within each sarcomere [choice (a)]. The absolute size of region D will not change during contraction [choice (c)], and the width of the myofibril will not shrink [choice (d)]. Figure A17-57

Consider the mechanism by which actin and tubulin polymerize. Which of the items below does not describe something similar about the polymerization mechanisms of actin and microtubules? (a) Although both filaments can grow from both ends, the growth rate is faster at the plus ends. (b) Depolymerization initiates at the plus ends of filaments. (c) Nucleotide hydrolysis promotes depolymerization of filaments. (d) Free subunits (actin and tubulin) carry nucleoside triphosphates.

(b) The shrinkage of microtubules that occurs involves a switch from growth to shrinkage only at the plus end of microtubules. However, actin loses subunits from its minus end during actin treadmilling.

You are examining a cell line in which activation of the Rho family member Rac promotes lamellipodia formation. Which of the following statements is most likely to be true? (a) Cells carrying a Rac mutation that makes Rac act as if it is always bound to GTP will polymerize more unbranched actin filaments than normal cells. (b) Cells carrying a Rac mutation that makes Rac unable to exchange GDP for GTP will polymerize more unbranched actin filaments than normal cells. (c) Cells carrying a Rac mutation that makes Rac act as if it is always bound to GTP will polymerize more branched actin filaments than normal cells. (d) Cells carrying a Rac mutation that makes Rac unable to exchange GDP for GTP will polymerize more branched actin filaments than normal cells.

(c) Activation of Rac promotes lamellipodia formation by enhancing actin nucleation using the ARP complex, which promotes the formation of branched actin filaments. Because lamellipodia formation involves branched actin filaments, a mutation that creates a constitutively active form of Rac (a GTP-bound form of Rac) will promote the formation of a greater number of branched actin filaments. Rac that is mutated and unable to exchange GDP for GTP will not be active.

Which of the following statements is correct? Kinesins and dyneins ___________________. (a) have tails that bind to the filaments. (b) move along both microtubules and actin filaments. (c) often move in opposite directions to each other. (d) derive their energy from GTP hydrolysis.

(c) All other answers are false. The motor heads bind to the filaments [choice (a)]. Both motors move along microtubules [choice (b)] and use ATP hydrolysis for energy [choice (d)].

Figure Q17-40A shows how the movement of dynein causes the flagellum to bend. If instead of the normal situation, the polarity of the adjacent doublet of microtubules were to be reversed (see Figure Q17-40B), what do you predict would happen? (a) No bending would occur. (b) Bending would occur exactly as diagrammed in Figure Q17-40A. (c) Bending would occur, except that the right microtubule doublet would move down relative to the left one. (d) The two microtubule doublets would slide away from each other.

(c) Because the polarity of the microtubule bundle is reversed, the dynein motors should walk in the opposite direction from the normal situation diagrammed in Figure Q17-40A. Microtubule sliding would occur if the linking proteins were absent [choice (d)], which is not true here.

Figure Q17-52 shows the leading edge of a lamellipodium. Which of the following statements is false? Figure Q17-52 (a) Nucleation of new filaments near the leading edge pushes the plasma membrane forward. (b) ARP proteins nucleate the branched actin filaments in the lamellipodium. (c) Capping proteins bind to the minus end of actin filaments. (d) There is more ATP-bound actin at the leading edge than in the actin filaments away from the leading edge.

(c) Capping protein binds to the plus end of actin filaments, preventing further assembly or disassembly from the growing end.

The microtubules in a cell form a structural framework that can have all the following functions except which one? (a) holding internal organelles such as the Golgi apparatus in particular positions in the cell (b) creating long, thin cytoplasmic extensions that protrude from one side of the cell (c) strengthening the plasma membrane (d) moving materials from one place to another inside a cell

(c) One function of actin filaments, but not microtubules, is to provide a meshwork beneath the plasma membrane that helps to form and strengthen this membrane. Microtubules have all of the other functions that are listed.

Which of the following structures shorten during muscle contraction? (a) myosin filaments (b) flagella (c) sarcomeres (d) actin filaments

(c) Sarcomeres contain actin filaments and myosin filaments that slide past each other during muscle contraction, leading to shortening of the sarcomere; the actin filaments and myosin filaments do not change in length. Flagella are microtubule-based structures that are not present on muscle cells.

Which of the following statements about the cytoskeleton is false? (a) The cytoskeleton is made up of three types of protein filaments. (b) The cytoskeleton controls the location of organelles in eukaryotic cells. (c) Covalent bonds between protein monomers hold together cytoskeletal filaments. (d) The cytoskeleton of a cell can change in response to the environment.

(c) The protein monomers of the cytoskeleton are held together by noncovalent interactions between the protein monomers. All other statements are true.

For both actin and microtubule polymerization, nucleotide hydrolysis is important for ______. (a) stabilizing the filaments once they are formed. (b) increasing the rate at which subunits are added to the filaments. (c) promoting nucleation of filaments. (d) decreasing the binding strength between subunits on filaments.

(d) ATP hydrolysis in actin polymerization decreases the binding strength between monomers in the actin filaments; GTP hydrolysis during tubulin polymerization decreases the binding strength between the tubulin subunits in the microtubule.

Which of the following statements about actin is false? (a) ATP hydrolysis decreases actin filament stability. (b) Actin at the cell cortex helps govern the shape of the plasma membrane. (c) Actin filaments are nucleated at the side of existing actin filaments in lamellipodia. (d) The dynamic instability of actin filaments is important for cell movement.

(d) Dynamic instability is a phenomenon associated with microtubules and not actin. Actin disassembly and assembly are both important for cell movement. However, this differs from dynamic instability in that the growth of actin filaments occurs at the leading edge; this growth occurs in a directed fashion because of actin-binding proteins that promote the formation of new filaments at the leading edge. Actin-binding proteins that destabilize actin filaments promote actin disassembly away from the leading edge. The actin assembly and disassembly in moving cells differs from the stochastic growth and disassembly of the microtubules.

Your friend works in a biotech company that has just discovered a drug that seems to promote lamellipodia formation in cells. Which of the following molecules is unlikely to be involved in the pathway that this drug affects? (a) Rac (b) ARP (c) actin (d) myosin

(d) Myosins are not directly involved in lamellipodia formation. Lamellipodium formation involves branched actin structures that use ARP for their formation, and thus ARP and actin are likely to be involved [choices (b) and (c)]. The Rho family member Rac triggers lamellipodia formation when activated, and thus may be involved [choice (a)].

The hydrolysis of GTP to GDP carried out by tubulin molecules ________________. (a) provides the energy needed for tubulin to polymerize. (b) occurs because the pool of free GDP has run out. (c) tips the balance in favor of microtubule assembly. (d) allows the behavior of microtubules called dynamic instability.

(d) The hydrolysis of GTP to GDP occurs after a GTP-bound tubulin molecule is incorporated into a microtubule, and it makes the microtubule more susceptible to disassembly. It is the resulting switch in microtubule stability that gives rise to the phenomenon known as dynamic instability.

You are interested in understanding the regulation of nuclear lamina assembly. To create an in vitro system for studying this process you start with partly purified nuclear lamina subunits to which you will add back purified cellular components to drive nuclear lamina assembly. Before you start doing experiments, your instructor suggests that you consider what type of conditions would be most amenable to the assembly of the nuclear lamina from its individual subunits in vitro. Which of the following conditions do you predict would be most likely to enhance the assembly of the nuclear lamina? (a) addition of phosphatase inhibitors (b) addition of ATP (c) addition of a concentrated salt solution that is 10 times the concentration normally found in the nucleoplasm (d) addition of protein kinase inhibitors

(d) The phosphorylation of nuclear lamins by protein kinases induces conformational changes that weaken the binding between nuclear lamin tetramers; thus, inhibiting protein kinases may enhance assembly of the nuclear lamina. Adding phosphatase inhibitors [choice (a)] or ATP [choice (b)] will enhance the activity of any co-purifying protein kinases and enhance disassembly. Because noncovalent protein-protein interactions hold the nuclear lamina together, the addition of a very concentrated salt solution will inhibit proper nuclear lamina assembly [choice (c)].

Cell movement involves the coordination of many events in the cell. Which of the following phenomena is not required for cell motility? (a) Myosin-mediated contraction at the rear of the moving cell. (b) Integrin association with the extracellular environment. (c) Nucleation of new actin filaments. (d) Release of Ca2+ from the sarcoplasmic reticulum.

(d) The release of Ca2+ from the sarcoplasmic reticulum is important for muscle contraction, not cell motility.

Which of the following statements about the structure of microtubules is false? (a) Microtubules are built from protofilaments that come together to make a hollow structure. (b) The two ends of a protofilament are chemically distinct, with α-tubulin exposed at one end and β-tubulin exposed at the other end. (c) Within a microtubule, all protofilaments are arranged in the same orientation, giving the microtubule structural polarity. (d) α-Tubulin and β-tubulin are covalently bound to make the tubulin dimer that then assembles into protofilaments.

(d) α-Tubulin and β-tubulin bind with each other through noncovalent interactions.

The graph in Figure Q17-30 shows the time course of the polymerization of pure tubulin in vitro. You can assume that the starting concentration of free tubulin is much higher than it is in cells. A. Explain the reason for the initial lag in the rate of microtubule formation. B. Why does the curve level out after point C?

A. Before they can polymerize to form microtubules, tubulin molecules have to form small aggregates that act as nucleation centers. This aggregation step is slow because the molecules have to come together in the right configuration. This is why there is a lag phase before microtubules start to be formed. B. After point C, an equilibrium point has been reached, where the rates of polymerization and depolymerization are exactly balanced.

In the budding yeast, activation of the GTP-binding protein Cdc42 occurs on binding of an external signal (pheromone) to a G-protein-coupled receptor. Activation of Cdc42 promotes actin polymerization. Predict what would happen to actin polymerization, in comparison with pheromone-treated cells, in the following cases. A. You add pheromone to an inhibitor of G-protein-coupled receptors. B. You add pheromone to a nonhydrolyzable analog of GTP.

A. Less actin polymerization. Cdc42 will not be able to be activated by the G- protein-coupled receptor. B. More actin polymerization. Cdc42 will be more active, because it will bind the nonhydrolyzable form of GTP and will not be able to be turned off.

Figure Q17-41 shows two isolated outer-doublet microtubules from a eukaryotic flagellum with their associated dynein molecules. A. Sketch what will happen to this structure if it is supplied with ATP. B. Sketch what will happen to this structure if the linking proteins are removed and it is supplied with ATP. C. In a complete flagellum, what would happen if all the dynein molecules were active at the same time?

A. See Figure A17-41A. B. See Figure A17-41B. [Note to instructor: this question should be marked as correct only if the microtubule is shown bending in the correct direction (in A) and the correct microtubule is shown pushed forward (in B)]. C. The flagellum will not bend because there is no significant relative motion of one microtubule doublet to another: each is trying to push its neighbor forward at the same time. For the flagellum to bend, sets of dynein molecules on one side of the flagellum must be selectively activated.

You are curious about the dynamic instability of microtubules and decide to join a lab that works on microtubule polymerization. The people in the lab help you grow some microtubules in culture using conditions that allow you to watch individual microtubules under a microscope. You can see the microtubules growing and shrinking, as you expect. The professor who runs the lab gets in a new piece of equipment, a very fine laser beam that can be used to sever microtubules. She is very excited and wants to sever growing microtubules at their middle, using the laser beam. A. Do you predict that the newly exposed microtubule plus ends will grow or shrink? Explain your answer. B. What do you expect would happen to the newly exposed plus ends if you were to grow the microtubules in the presence of an analog of GTP that cannot be hydrolyzed, and you then severed the microtubules in the middle with a laser beam?

A. The newly exposed microtubule plus ends will most probably shrink if you sever the microtubules in the middle. This is because a microtubule grows by adding GTP-carrying subunits to the plus end. The GTP is hydrolyzed over time, leaving only a cap of GTP-carrying subunits at the plus end, with the remainder of the tubulin protofilament containing GDP-carrying subunits. Therefore, if you sever a growing microtubule in its middle, you will most probably create a plus end that contains GDP-carrying subunits. The GDP-carrying subunits are less tightly bound than the GTP-carrying subunits and will peel away from each other, causing depolymerization of the microtubule and shrinkage. B. If you were to polymerize the microtubules in the presence of a nonhydrolyzable analog of GTP and you then severed the microtubules with a laser, the newly exposed plus end would contain a GTP cap and so would probably continue to grow.

You are interested in studying kinesin movements. You therefore prepare silica beads and coat them with kinesin molecules so that each bead, on average, has only one kinesin molecule attached to it. You add these kinesin-coated beads to a preparation of microtubules you have polymerized. Using video microscopy, you watch the kinesin [labeled with green fluorescent protein (GFP)] move down the microtubules. A. Kinesin-GFP has been measured to move along microtubules at a rate of 0.3 μm/sec, and single-molecule studies have revealed that kinesin moves along microtubules progressively, with each step being 8 nm. How many steps can the kinesin molecule take in 4 seconds, assuming that the kinesin stays attached to the microtubule for the entire 4 seconds? B. Because each kinesin molecule is thought to take approximately 100 steps before falling off the microtubule, will you see your silica beads detach from the microtubule during your 4 seconds of observation? C. What would you predict would happen to the kinesin-coated silica beads if you were to add AMP-PNP (a nonhydrolyzable ATP analog)?

A. You would expect the kinesin molecule to travel 150 steps. The calculation is as follows: 0.3 μm = 300 nm. Therefore, in 4 seconds, the kinesin molecule could travel 1200 nm if it were to move at a rate of 0.3 μm/sec. Because the step size is 8 nm, 1200 nm/(8 nm per step) = 150 steps. B. Yes, you should see silica beads detach some time during the 4 seconds of observation, because each kinesin will take more than 100 steps in that 4-second time frame (see part A above). C. If you were to add AMP-PNP, you would no longer see the silica beads moving down the microtubule. It is thought that one molecule of ATP is hydrolyzed per step that kinesin takes; without ATP hydrolysis, translocation of the beads will be inhibited. However, you may still see the beads associated with the microtubules, because AMP-PNP does not inhibit the association of kinesin with the microtubule.

Indicate which of the three major classes of cytoskeletal elements each statement below refers to. A. monomer that binds ATP B. includes keratin and neurofilaments C. important for formation of the contractile ring during cytokinesis D. supports and strengthens the nuclear envelope E. their stability involves a GTP cap F. used in the eukaryotic flagellum G. a component of the mitotic spindle H. can be connected through desmosomes I. directly involved in muscle contraction J. abundant in filopodia

A. actin B. intermediate filaments C. actin D. intermediate filaments E. microtubules F. microtubules G. microtubules H. intermediate filaments I. actin J. actin

Indicate whether each of the following statements refers to a ciliary microtubule, a microtubule of the mitotic spindle, both types of microtubule, or neither type of microtubule. A. The basal body is the organizing center. B. The monomer is sequestered by profilin. C. It is arranged in a "9 + 2" array. D. It is nucleated at the centrosome. E. It uses dynein motors. F. It is involved in sperm motility. G. It is involved in moving fluid over the surface of cells.

A. ciliary microtubules B. neither C. ciliary microtubules D. microtubules of the mitotic spindle E. both F. neither (this involves flagellar microtubules) G. ciliary microtubules

Match the following labels to the numbered lines on Figure Q17-36. A. minus end of microtubule B. tail of motor protein C. cargo of motor protein D. head of motor protein Which of the two motors in Figure Q17-36 is most probably a kinesin? Explain your answer.

A—4; B—2; C—1; D—3 The top motor is more likely to be kinesin, because kinesins usually move toward the plus end of the microtubules.

Place the following in order of size, from the smallest to the largest. A. protofilament B. microtubule C. α-tubulin D. tubulin dimer E. mitotic spindle

C, D, A, B, E

Phosphorylation of nuclear lamins regulates their assembly and disassembly during mitosis. You add a drug to cells undergoing mitosis that inhibits the activity of an enzyme that dephosphorylates nuclear lamins. What do you predict will happen to these cells? Why?

Cells should become arrested in mitosis. Normally, the lamins are phosphorylated during mitosis, causing disassembly of the nuclear envelope. At the end of mitosis, the nuclear lamins are dephosphorylated, causing the lamins to reassemble. Inhibition of this last step should therefore prevent the nuclear lamins from reassembling after mitosis.

All intermediate filaments are of similar diameter because ____________. (a) the central rod domains are similar in size and amino acid sequence. (b) the globular domains are similar in size and amino acid sequence. (c) covalent bonds among tetramers allow them to pack together in a similar fashion. (d) there is only a single type of intermediate filament in every organism.

Choice (a) is correct. Globular domains vary among intermediate filaments in size and have different types of amino acids [choice (b)]. The interactions among all intermediate filament subunits involve noncovalent bonding [choice (c)]. There are several classes of intermediate filaments and an organism can have more than one class (and sometimes, more than one member of each class) [choice (d)].

Which of the following statements about microtubules is true? (a) Motor proteins move in a directional fashion along microtubules by using the inherent structural polarity of a protofilament. (b) The centromere nucleates the microtubules of the mitotic spindle. (c) Because microtubules are subject to dynamic instability, they are used only for transient structures in a cell. (d) ATP hydrolysis by a tubulin heterodimer is important for controlling the growth of a microtubule.

Choice (a) is true. Microtubules are nucleated by the centrosome [not the centromere, choice (b)]. Although microtubules are subject to dynamic instability, their interaction with microtubule-binding proteins can stabilize them so that they can be used to form stable structures such as cilia and flagella [choice (c)]. GTP (not ATP) hydrolysis is important for controlling the growth of a microtubule [choice (d)].

Intermediate filaments help protect animal cells from mechanical stress because ___________. (a) filaments directly extend from the interior of the cell to the extracellular space and into the next cell, linking one cell to the next, helping to distribute locally applied forces. (b) filaments in each cell are indirectly connected to the filaments of a neighboring cell through the desmosome, creating a continuous mechanical link between cells. (c) filaments remain independent of other cytoskeletal elements and keep the mechanical stress away from other cellular components. (d) filaments make up the desmosome junctions that connect cells; these junctions are more important than the internal network of filaments for protecting cells against mechanical stress.

Choice (b) is correct. Intermediate filaments do not directly extend from cell to cell [choice (a)]. The linking of intermediate filaments to other cytoskeletal elements (like actin) is thought to help protect cells from mechanical stress [choice (c)]. Desmosome junctions are made up of many different kinds of proteins including cadherins in the extracellular space (which mediate cell-cell adhesion) as well as proteins within the cytoplasm that mediate the attachment of desomosomes to intermediate filaments. These junctions alone are not sufficient for protection against mechanical stress and need the interaction with the intermediate filament network in the cell [choice (d)].

Which of the following statements about skeletal muscle contraction is false? (a) When a muscle cell receives a signal from the nervous system, voltage-gated channels open in the T-tubule membrane. (b) The changes in voltage across the plasma membrane that occur when a muscle cell receives a signal from the nervous system cause an influx of Ca2+ into the sarcoplasmic reticulum, triggering a muscle contraction. (c) A change in the conformation of troponin leads to changes in tropomyosin such that it no longer blocks the binding of myosin heads to the actin filament. (d) During muscle contraction, the Z discs move closer together as the myosin heads walk toward the plus ends of the actin filaments.

Choice (b) is false. Muscle contraction is triggered by an efflux of Ca2+ from the sarcoplasmic reticulum into the cytosol.

Which of the following statements about organellar movement in the cell is false? (a) Organelles undergo saltatory movement in the cell. (b) Only the microtubule cytoskeleton is involved in organellar movement. (c) Motor proteins involved in organellar movement use ATP hydrolysis for energy. (d) Organelles are attached to the tail domain of motor proteins.

Choice (b) is untrue; both the actin cytoskeleton and the microtubule cytoskeleton are involved in organellar movement.

You are studying nuclear lamins in yeast. Using recombinant DNA technology,you alter the coding sequence of a nuclear lamin gene such that the gene now codes for a nuclear lamin protein that can no longer be phosphorylated when the nuclear envelope is broken down during mitosis. What do you predict would happen if the yeast cell only had the altered nuclear lamin gene (and not the unaltered version)? (a) Mitosis should proceed as usual because the dephosphorylation of the lamin is what is important for nuclear lamina assembly during mitosis, so phosphorylation will not be necessary. (b) Disassembly of the nuclear lamins will occur prematurely because the lamins cannot be phosphorylated. (c) Nuclear lamins will no longer disassemble properly during mitosis. (d) Nuclear lamins will be unable to produce dimers, as coiled-coil formation will be disrupted.

Choice (c) is correct. Although it is true that dephosphorylation of the lamin is necessary for the reassembly of the nuclear lamins at the end of mitosis, the cycle of phosphorylation and dephosphorylation is important for mitosis [choice (a)]. Disassembly of the lamins occurs when they are phosphorylated, as this weakens the bonds between the lamin tetramers. Lamins should be more stable if they cannot be phosphorylated [choice (b)]. Dimer formation depends on the α-helical rods; the binding of the lamin tetramers is what is affected by phosphorylation [choice (d)].

Consider the in vitro motility assay using purified kinesin and purified polymerized microtubules shown in Figure Q17-63. The three panels are images taken at 1-second intervals. In this figure, three microtubules have been numbered to make it easy to identify them. Which of the following statements about this assay is false? Figure Q17-63 (a) Kinesin molecules are attached by their tails to a glass slide. (b) The microtubules used in this assay must be polymerized using conditions that stabilize tubule formation or else they would undergo dynamic instability. (c) ATP must be added for this assay to work. (d) Addition of the nonhydrolyzable ATP analog (AMP-PNP) would cause the microtubules to move faster.

Choice (d) is false. Addition of AMP-PNP would block movement, because ATP hydrolysis is required for the kinesin to step along a microtubule. Addition of AMP-PNP would cause the microtubules to attach to the kinesin heads without being released. Kinesin molecules are attached to the slide by their tails (the cargo-binding domain) so that the heads are available to move the microtubules along the slides. If they were in solution with the microtubules, there would be no force and thus no movement [choice (a)]. The microtubules used in this assay are stabilized with a nonhydrolyzable form of GTP, because otherwise they might shrink during the course of the assay [choice (b)]. ATP is required for kinesin movement, and thus must be added for this assay to work [choice (c)].

Which of the following statements about the cytoskeleton is true? (a) All eukaryotic cells have actin, microtubules, and intermediate filaments in their cytoplasm. (b) The cytoskeleton provides a rigid and unchangeable structure important for the shape of the cell. (c) The three cytoskeletal filaments perform distinct tasks in the cell and act completely independently of one another. (d) Actin filaments and microtubules have an inherent polarity, with a plus end that grows more quickly than the minus end.

Choice (d) is true. Not all eukaryotic cells have cytoplasmic intermediate filaments [choice (a)]. The cytoskeleton is not rigid and unchangeable; in fact, it can be quite dynamic [choice (b)]. Each of the three cytoskeletal systems is not completely independent. For example, proteins such as plectin are known to link intermediate filaments to the actin and microtubule cytoskeleton [choice (c)].

Microtubules are important for transporting cargo in nerve cell axons, as diagrammed in Figure Q17-33. Notice that the two types of cargo are traveling in opposite directions. Which of the following statements is likely to be false? (a) The gray cargo is attached to dynein. (b) The black cargo and the gray cargo require ATP hydrolysis for their motion. (c) The black cargo moving toward the axon terminal contains a domain that specifically interacts with the tail domain of a particular kind of motor. (d) The black cargo and the gray cargo are moving along microtubules of opposite polarity.

Choice (d) is unlikely to be the case. Microtubules in nerve cell axons are generally organized such that their plus ends are facing the axon terminal while the minus ends reside in the cell body. Thus, the gray cargo is likely to be attached to a dynein motor because it is moving toward the cell body [choice (a)]. Because cargo attaches to the tail domains of both dynein and kinesin motors, the attachment of a cargo to either tail is unlikely to affect directionality [choice (c)]. Both dynein and kinesin require ATP hydrolysis for their movement [choice (b)].

Which of the following statements regarding dynamic instability is false? (a) Each microtubule filament grows and shrinks independently of its neighbors. (b) The GTP cap helps protect a growing microtubule from depolymerization. (c) GTP hydrolysis by the tubulin dimer promotes microtubule shrinking. (d) The newly freed tubulin dimers from a shrinking microtubule can be immediately captured by growing microtubules and added to their plus end.

Choice (d) is untrue. A newly dissociated tubulin dimer will be bound to GDP; this GDP will need to be exchanged for GTP before it can be added to a newly growing microtubule.

Which of the statements below about intermediate filaments is false? (a) They can stay intact in cells treated with concentrated salt solutions. (b) They can be found in the cytoplasm and the nucleus. (c) They can be anchored to the plasma membrane at a cell-cell junction. (d) Each filament is about 10 μm in diameter.

Choice (d) is untrue. Intermediate filaments are about 10 nm (not μm) in diameter. All the other statements are true.

Keratins, neurofilaments, and vimentins are all categories of intermediate filaments. Which of the following properties is not true of these types of intermediate filaments? (a) They strengthen cells against mechanical stress. (b) Dimers associate by noncovalent bonding to form a tetramer. (c) They are found in the cytoplasm. (d) Phosphorylation causes disassembly during every mitotic cycle.

Choice (d) is untrue. Keratins, neurofilaments, and vimentins are cytoplasmic intermediate filaments [choice (c)], which tend to be very stable once formed. The nuclear intermediate filaments are disassembled and reformed during mitosis; this process is regulated by phosphorylation.

Do you agree or disagree with the following statement? Explain your answer. When skeletal muscle receives a signal from the nervous system to contract, the signal from the motor neuron triggers the opening of a voltage-sensitive Ca2+ channel in the muscle cells' plasma membrane, allowing Ca2+ to flow into the cell.

Disagree. The increase in intracellular Ca2+ during muscle contraction comes from an intracellular source. The Ca2+ is released from the lumen of the sarcoplasmic reticulum, which is a specialized region of endoplasmic reticulum inside a muscle cell. The signal from the nerve terminal triggers an action potential in the muscle cell plasma membrane, which causes a voltage-sensitive transmembrane protein in the membranous transverse tubules to open a Ca2+-release channel in the membrane of the sarcoplasmic reticulum.

Do you agree or disagree with this statement? Explain your answer. Minus-end directed microtubule motors (like dyneins) deliver their cargo to the periphery of the cell, whereas plus-end directed microtubule motors (like kinesins) deliver their cargo to the interior of the cell.

Disagree. The plus ends of microtubules usually point toward the cell periphery, whereas the minus ends point toward the cell center. This is because the γ-tubulin in the centrosome serves to nucleate microtubule growth. Because the centrosomes are near the center of the cell, the minus ends of microtubules are located there. Therefore, a minus- end directed microtubule motor would direct its cargo toward the center of the cell, and a plus-end directed microtubule motor would direct its cargo toward the cell periphery.

Intermediate filaments are elongated fibrous proteins with an N-terminal globular _________________ region and a C-terminal globular _________________ region; these regions flank the elongated rod domain. The α-helical region of the rod interacts with the α-helical region of another monomer in a ____________________ configuration to form a dimer. ______________ dimers will line up to form a staggered tetramer. ______________ strands of tetramers come together and twist together to form the _________________ nm filament. The ___________________ domains are exposed on the surface of the intermediate filament, allowing for interaction with cytoplasmic components. antiparallel four tail β barrel globular ten coiled-coil head trimeric covalent rod twenty-five eight seven two

Intermediate filaments are elongated fibrous proteins with an N-terminal globular head region and a C-terminal globular tail region; these regions flank the elongated rod domain. The α-helical region of the rod interacts with the α-helical region of another monomer in a coiled-coil configuration to form a dimer. Two dimers will line up to form a staggered tetramer. Eight strands of tetramers come together and twist together to form the ten nm filament. The globular domains are exposed on the surface of the intermediate filament, allowing for interaction with cytoplasmic components.

Intermediate filaments are found mainly in cells that are subject to mechanical stress. Gene mutations that disrupt intermediate filaments cause some rare human genetic diseases. For example, the skin of people with epidermolysis bullosa simplex is very susceptible to mechanical injury; people with this disorder have mutations in their __________________ genes, which code for the intermediate filament found in epithelial cells. These filaments are usually connected from cell to cell through junctions called __________________s. The main filaments found in muscle cells belong to the __________________ family; people with disruptions in these intermediate filaments can have muscular dystrophy. In the nervous system, __________________s help strengthen the extremely long extensions often present in nerve cell axons; disruptions in these intermediate filaments can lead to neurodegeneration. People who carry mutations in the gene for __________________, an important protein for cross-linking intermediate filaments, have a disease that combines symptoms of epidermolysis bullosa simplex, muscular dystrophy, and neurodegeneration. Humans with progeria, a disease that causes premature aging, carry mutations in a nuclear ____________. desmosome lamin synapse keratin neurofilament vimentin kinase plectin

Intermediate filaments are found mainly in cells that are subject to mechanical stress. Gene mutations that disrupt intermediate filaments cause some rare human genetic diseases. For example, the skin of people with epidermolysis bullosa simplex is very susceptible to mechanical injury; people with this disorder have mutations in their keratin genes, the intermediate filament found in epithelial cells. These filaments are usually connected from cell to cell through junctions called desmosomes. The main filaments found in muscle cells belong to the vimentin family; people with disruptions in these intermediate filaments can have muscular dystrophy. In the nervous system, neurofilaments help strengthen the extremely long extensions often present in nerve cell axons; disruptions in these intermediate filaments can lead to neurodegeneration. People who carry mutations in the gene for plectin, an important protein for cross-linking intermediate filaments, have a disease that combines symptoms of epidermolysis bullosa simplex, muscular dystrophy, and neurodegeneration. Humans with progeria, a disease that causes premature aging, carry mutations in a nuclear lamin.

Microtubules are formed from the tubulin heterodimer, which is composed of the nucleotide-binding __________________ protein and the __________________ protein. Tubulin dimers are stacked together into protofilaments; __________________ parallel protofilaments form the tubelike structure of a microtubule. __________________ rings are important for microtubule nucleation and are found in the __________________ , which is usually found near the cell's nucleus in cells that are not undergoing mitosis. A microtubule that is quickly growing will have a __________________ cap that helps prevent the loss of subunits from its growing end. Stable microtubules are used in cilia and flagella; these microtubules are nucleated from a __________________ and involve a "__________________ plus two" array of microtubules. The motor protein __________________ generates the bending motion in cilia; the lack of this protein can cause Kartagener's syndrome in humans. α-tubulin dynein nine ATP four thirteen basal body γ-tubulin twenty-one β-tubulin GTP UTP centrosome kinesintwo vimentin δ-tubulin myosin

Microtubules are formed from the tubulin heterodimer, which is composed of the nucleotide-binding β-tubulin protein and the α-tubulin protein. Tubulin dimers are stacked together into protofilaments; thirteen parallel protofilaments form the tubelike structure of a microtubule. γ-Tubulin rings are important for microtubule nucleation and are found in the centrosome, which is usually found near the cell's nucleus in cells that are not undergoing mitosis. A microtubule that is quickly growing will have a GTP cap that helps prevent the loss of subunits from its growing end. Stable microtubules are used in cilia and flagella; these microtubules are nucleated from a basal body and involve a "nine plus two" array of microtubules. The motor protein dynein generates the bending motion in cilia; the lack of this protein can cause Kartagener's syndrome in humans.

Kinesins were purified by adding the nonhydrolyzable analog AMP-PNP to cytoplasmic extracts containing microtubules, purifying the microtubules, and then releasing the kinesin proteins, which were still attached to the microtubules, by adding ATP. Would this trick have worked to purify myosin motors attached to actin filaments? Explain.

No, addition of a nonhydrolyzable form of ATP would not increase the affinity of myosin for actin. This is because when the myosin motor binds to ATP, the myosin head undergoes a conformational change that reduces its affinity for actin.

Your friend discovers a protein that she names EBP. EBP binds to microtubule plus ends, and she hypothesizes a role for EBP in increasing dynamic instability. To determine the function of EBP, she examines its effect on microtubules. She polymerizes microtubules from purified centrosomes in a Petri plate and determines the number of shrinking microtubules over a three-minute time interval for different concentrations of EBP. The data she obtained are shown in Figure Q17-25. Is this result consistent with her hypothesis? Explain.

No, the data graphed in Figure Q17-25 are not consistent with her hypothesis. If EBP were to increase the dynamic instability of microtubules, you would expect an increase in both the number of shrinking microtubules and the number of growing microtubules. Since the graph shows a reduction in the number of shrinking microtubules, dynamic instability has evidently decreased. Further experiments would be needed to determine the net effect of these changes on microtubules and whether they become longer or shorter on average following EBP treatment.

In the three cell outlines in Figure Q17-18, indicate the arrangement of the microtubules, showing clearly their free and attached ends. On each figure, indicate the plus end and the minus end for one of the microtubules.

See Figure A17-18.

You isolate some muscle fibers to examine what regulates muscle contraction. When you bathe the muscle fibers in a solution containing ATP and Ca2+, you see muscle contraction (experiment 3 in Table Q17-61). Ca2+ is necessary, as solutions containing ATP alone or nothing do not stimulate contraction and thus the muscle remains in a relaxed state (experiments 1 and 2 in Table Q17-61). From what you know about the mechanism of muscle contraction, fill in your predictions of whether the muscle will be contracted or relaxed for experiments 4, 5, and 6. Explain your answers.

See Table A17-61. Table A17-61 In experiment 4, the muscle will be relaxed because troponin will not be able to bind Ca2+. By preventing troponin from binding to Ca2+, troponin will not be able to undergo the conformational change that causes tropomyosin to alter its association with actin. This altered association is normally required for myosin to bind actin. In the absence of troponin regulation by Ca2+, myosin cannot bind actin and the muscle cannot contract. In experiment 5, the muscle will contract because tropomyosin cannot bind to actin. If tropomyosin cannot bind actin, myosin can. The presence of ATP means that ATP will be available for myosin to hydrolyze, causing muscle contraction. In experiment 6, the muscle will remain relaxed. The presence of Ca2+ will induce a conformational change in troponin that causes tropomyosin to shift, exposing actin for myosin to bind. However, when myosin binds, the myosin molecule will attach and then release because the myosin will bind the nonhydrolyzable analog of ATP. Because no ATP hydrolysis can occur, the muscle will remain in the relaxed state.

Some lower vertebrates such as fish and amphibians can control their color by regulating specialized pigment cells called melanophores. These cells contain small, pigmented organelles, termed melanosomes, that can be dispersed throughout the cell, making the cell darker, or aggregated in the center of the cell to make the cell lighter. You purify the melanosomes from melanophores that have either aggregated or dispersed melanosomes and find that: 1. aggregated melanosomes co-purify with dynein; 2. dispersed melanosomes co-purify with kinesin. Given this set of data, propose a mechanism for how the aggregation and dispersal of melanosomes occur.

The melanosomes are transported in the cell on microtubules. When it is advantageous for the animal to become lighter, a signal is sent to the pigment cell that causes the melanosomes to associate with dynein. Because dynein is a minus-end directed motor, it will transport the melanosomes toward the center of the cell, causing the melanosomes to aggregate in the center and the cell to take on a lighter appearance. When the animal wants to become darker, a signal is sent to the pigment cell that causes the melanosomes to associate with kinesin. Kinesin is usually a plus-end directed motor and will move the melanosomes away from the center of the cell so that they are more dispersed, making the cell look darker.

Cytochalasin is a drug that caps actin filament plus ends, thus preventing actin polymerization. Phalloidin is a drug that binds to and stabilizes actin filaments, preventing actin depolymerization. Even though these drugs have opposite effects on actin polymerization, the addition of either of these drugs instantaneously freezes the cell movements that depends on actin filaments. Explain why drugs that have opposite effects on actin filaments can have a similar effect on cell movements.

These drugs both stop cell movements because actin polymerization and depolymerization are both required for this process. As cells move forward, the growth of actin filaments near the plasma membrane helps push out the membrane. As this occurs, continuous depolymerization of actin filaments occurs at the actin filaments away from the plasma membrane.

Actin-binding proteins bind to actin and can modify its properties. You purify a protein, Cap1, that seems to bind and cap one end of an actin filament, although you do not know whether it binds the plus end or the minus end. To determine which end of the actin filament your protein binds to, you decide to examine the effect of Cap1 on actin polymerization by measuring the kinetics of actin filament formation in the presence and the absence of Cap1 protein. You obtain the following results (see Figure Q17-48). Do you think Cap1 binds the plus end or the minus end of actin? Explain your reasoning.

You would predict that Cap1 binds the plus end of actin, because it seems to inhibit actin polymerization. Actin filaments grow through the addition of monomers to the plus end of the actin filament. A capping protein that binds the plus end of actin can block monomer addition to the actin filament. Thus, less actin polymerization will be seen in the presence of the Cap1 protein.

Rank the following cytoskeletal filaments from smallest to largest in diameter (1 = smallest in diameter, 4 = largest) ______ intermediate filaments ______ microtubules ______ actin filament ______ myofibril

__2___ intermediate filaments (10 nm diameter) __3___ microtubules (25 nm) __1___ actin filament (5-9 nm) __4___ myofibril (1-2 μm)

Actin can adopt a variety of shapes. Match the name of the actin form with the type of actin structure depicted as black lines within the cells in Q17-43. _____ lamellipodia _____ contractile bundles _____ contractile ring _____ microvilli

__C__ lamellipodia __B__ contractile bundles __D__ contractile ring __A__ microvilli

Match the type of intermediate filament with its appropriate location. lamins _________ A. nerve cells neurofilaments _________ B. epithelia vimentins _________ C. nucleus keratins _________ D. connective tissue

lamins ____C____ neurofilaments____A____ vimentins ____D____ keratins ____B____

The following proteins are important for cell movement. Match the following proteins with their function. myosin _____ A. nucleation of new actin filaments at the side of an existing filament ARP proteins _____ B. regulation of the availability of actin monomers profilin _____ C. important for the growth of straight, unbranched actin filaments integrins _____ D. contracting the rear of the cell formins _____ E. involvement in focal contacts

myosin ___D_____ ARP proteins ___A_____ profilin ___B_____ integrins ___E_____ formins ___C_____