Question 1

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.

Accepted Answer

Kinesin-1, also called the "conventional kinesin", is a plus-end directed microtubule motor. Katanin, named after the Japanese word for "sword" is an ATP-dependent microtubule-severing protein. EB1 and kinesin-13 are +TIPs. The latter also acts as a catastrophe factor.

Question 2

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?

Accepted Answer

In this graph, the traces with a negative slope and indicated by thicker lines represent anterograde movement toward the axon terminal mediated by kinesins; those traces with a positive slope represent retrograde motion mediated by dynein. The speed is inversely related to the slope in this graph, and is higher for the retrograde movement.

Question 3

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?

Accepted Answer

Thirteen protofilaments have a total of thirteen staggers between them. Thus, a full turn of the helix has a rise of approximately 12 nm (= 13 staggers × 0.9 nm/stagger). Since there is a seam, this rise should correspond to an odd number of monomer distances, i.e. 1, 3, 5, etc. In reality, the number is 3, equivalent to an axial tubulin displacement of 4 nm (= 12 nm/3).

Question 4

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?

Accepted Answer

Cytoplasmic dyneins rely on the dynactin complex to associate with their cargoes, including the Golgi components, which are held near the center of the cell by the action of these motors.

Question 5

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

Accepted Answer

The centrosome is the major microtubule-organizing center (MTOC) in most animal cells. The γ-tubulin ring complexes associated with the centrosome nucleate microtubules that emanate from the center of the cell toward its periphery.

Question 6

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.

Accepted Answer

Dyneins are fast, minus-end directed, microtubule-based motors that have large structures unrelated to those of kinesins (also microtubule-based) and myosins (actin-based) and are also faster than these other two motor proteins.

Question 7

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?

Accepted Answer

Myosin V is a processive motor and, not surprisingly, its duty ratio is significantly higher than that of myosin II. This is consistent with ADP release or ATP binding being slow (and rate-limiting) in the myosin V cycle, making the motor stay longer in the attached post-stroke state. In myosin II, inorganic phosphate (Pᵢ) release is rate-limiting instead, keeping the motor in the released or cocked state for a longer fraction of the cycle.

Question 8

"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?

Accepted Answer

Kinesins normally carry out the outward transport of melanosomes toward the plus end of microtubules at the cell periphery, by winning a tug-of-war against dyneins that would otherwise transport the pigments back to the center of the cell. The headless mutant inhibits kinesin, resulting in inhibition of the outward movement.

Question 9

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

Accepted Answer

In the mechanochemical cycle of kinesins, an ATP-bound head binds tightly to its microtubule track. Myosins release their actin filament once bound to ATP.

Question 10

In contrast to growing microtubules, shrinking microtubules …

Accepted Answer

Without a GTP cap, shrinking microtubules have curved protofilaments at their plus ends with weak lateral interactions. Rescue can also happen randomly without the help of any associated protein.

Question 11

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

Accepted Answer

They both nucleate the filaments and remain bound to the minus end. They also both employ homologs of the filament subunits to catalyze nucleation.

Question 12

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.

Accepted Answer

The dynein tail domain is bound to the A microtubule of one doublet, while the motor domain interacts with the B microtubule of the neighboring doublet. The former doublet is pushed toward its minus end (and the base of the flagellum) by the minus-end directed movement of the motor domain on the latter doublet. Proteins like nexin prevent such sliding, directing the force into a bending motion.

Question 13

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.

Accepted Answer

The highly resilient intermediate filaments have smaller persistence length compared to other cytoskeletal filaments, and are not polar.

Question 14

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

Accepted Answer

XMAP215 behaves as a plus-end tracking protein (+TIP), and stabilizes the microtubules by binding near the plus end and lowering catastrophe frequency.

Question 15

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.

Accepted Answer

Cilia are found in large numbers on ciliated cells such as those lining our respiratory tract. They are relatively short and beat with a whiplike motion. Axoneme constitutes the structural core of both cilia and flagella.

Question 16

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.

Accepted Answer

The step frequency of kinesin-1 is about 125 steps per second [= (1000 nm/sec)/(8 nm/step)], while that of a runner is only about 3 steps per second [= (180 steps/min)/(60 sec/min)]. If the runner had the step frequency of the kinesin motor, they could run at 1750 km/h (= 42 km/h × 125/3). This is a supersonic speed, faster than bullets and regular passenger planes.

Question 17

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.

Accepted Answer

Primary cilia are nonmotile interphase structures with a very widespread distribution among various cells. They are thought to have sensory and signaling functions.

Question 18

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 …

Accepted Answer

The result shows that the unique insert is necessary for the minus-end directionality of myosin VI, but not necessary for its motor activity. The experiment does not address the sufficiency of the insert.

Question 19

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?

Accepted Answer

Catastrophe is the transition from growth to shrinkage, while rescue is the transition from shrinkage to growth. The transition frequencies are generally higher in more dynamic microtubules, which also tend to be shorter.

Question 20

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.

Accepted Answer

Essentially all microtubules in axons are oriented with their plus end pointed at the axon terminal. Anterograde movements on these microtubules are exclusive to kinesins. In dendrites, however, the microtubules are of mixed polarity. The microtubules are too short to span the entire length of axons or even dendrites.

Quiz 16: The Cytoskeleton