homework 15
Kinesin and dynein motor proteins each use the energy of _____________ to power their movements in ___________ direction(s) along microtubules. First Blank: -ATP -ADP -GTP -GDP Second Blank: -single -multiple
First Blank: - ATP Second Blank: - a single Explanation: Kinesin and dynein motor proteins each use the energy of ATP to power their movements. The hydrolysis of ATP drives movement along the microtubule; the globular heads of kinesin and dynein are ATPases. Specifically, dynein can only move toward the minus end of a microtubule and kinesin can only move toward the plus end of a microtubule. Tubulin, when associated with GTP, will favor polymerization into a microtubule, but the GTP is not part of the motor protein.
The energy in ATP is used to fuel the movement of kinesin motor proteins along microtubules. Sort each of the following events into the proper category, indicating whether they occur on the leading or trailing head of kinesin. Categories: Leading Head Trailing Head Events: - ADP release -Pi release - ATP binding - Neck linker zippers to catalytic core of head group - ATP hydrolysis
Leading Head: - ADP release - ATP binding - Neck linker zippers to catalytic core of head group Trailing Head: - ATP hydrolysis - Pi release Explanation: The kinesin head groups are bound to ADP in solution. When the first head group encounters a microtubule, the head binds tightly and ADP is released. ATP can then bind to this leading head, which triggers the neck linker to bind tightly to the head group. The conformational change of the neck linker causes the trailing head to move forward and become the new leading head. The new trailing head on the microtubule is now bound to the ATP. The new trailing head hydrolyzes the bound ATP and releases a phosphate in preparation for moving forward as the leading head.
How would the animation of microtubule dynamics change after adding a non-hydrolyzable analog of GTP to the cells expressing GFP tubulin? A. Microtubules would grow longer. B. Microtubule dynamics would not change. C. Microtubules would shrink. D. Dynamic instability would increase as microtubules rapidly switch between growing and shrinking.
Microtubules would grow longer. Explanation: The animation shows microtubules rapidly switching between growth and shrinkage at the plus end of the microtubule. Growth occurs when the microtubule contains a GTP cap. Rapid shrinking occurs when all GTP has been hydrolyzed to GDP and the end becomes unstable. If a non-hydrolyzable analog of GTP were added, the growing microtubule would have a permanent stable GTP cap and would continue to grow without periods of shrinkage.
An experiment was performed to determine the role that ATP plays in kinesin movement along microtubules. Kinesin and microtubules were incubated together in a test tube, but instead of ATP, a non-hydrolyzable analog of ATP was added to the tube. What impact on kinesin function do you expect to observe in the presence of this ATP analog? A. The kinesin would bind tightly to microtubules and not release. B. The leading head would bind tightly, but the trailing head would remain free. C. The kinesin would be unable to bind to microtubules since the kinesin would remain inactive. D. The kinesin would walk faster along the microtubule since the kinesin would remain active while bound to ATP.
The kinesin would bind tightly to microtubules and not release. Explanation: ATP binds to the empty leading head, causing the neck linker to zipper next to the head and pull the trailing head forward. The ATP on the new trailing head must then be hydrolyzed in order for the trailing head to release and allow the new leading head to pull the trailing head forward. In the absence of ATP hydrolysis, the trailing head remains tightly bound to the microtubule. The use of non-hydrolyzable analogs of ATP helped identify the first kinesin protein as it was tightly bound to microtubules.
If GTP hydrolysis occurs on a tubulin molecule at the plus end of a microtubule protofilament before another tubulin molecule is added, what typically happens? A. The GDP is rapidly exchanged for a fresh molecule of GTP. B. The microtubule remains the same size. C. The microtubule polymerizes. D. The microtubule depolymerizes.
The microtubule depolymerizes. Explanation: If GTP hydrolysis occurs on a tubulin molecule at the plus end of a microtubule protofilament before another tubulin molecule is added, then the microtubule depolymerizes. GDP-bound tubulin subunits associate less tightly, so hydrolysis of GTP generally causes a microtubule to disassemble. Because the rest of the microtubule is composed of GDP-tubulin, once depolymerization has started, it tends to continue; the microtubule starts to shrink rapidly and continuously and may even disappear.
Which statement about intermediate filaments is not true? A. They are wider than actin filaments. B. They rupture under stress. C. They have the highest tensile strength of all the cytoskeletal filaments. D. Their disruption can lead to premature aging. E. They lack polarity.
They rupture under stress. Explanation: Intermediate filaments deform under stress, but they do not rupture. These nonpolar filaments distribute the effects of locally applied forces, allowing membranes to retain their structural integrity. Defects in a particular intermediate filament are associated with premature aging.
The drug Taxol binds tightly to microtubules and prevents them from depolymerizing. The drug colchicine binds tightly to free tubulin and prevents its polymerization into microtubules. Which of these drugs arrests cell division? A. Taxol only B. colchicine only C. both colchicine and Taxol
both colchicine and Taxol Explanation: Any drug that inhibits the function of microtubules will have profound effects on the cell. Colchicine and Taxol are drugs that both arrest cell division because of their effects on microtubules, even though they have different mechanisms of action. Colchicine binds to tubulin dimers and prevents their polymerization, causing the mitotic spindle to disappear, which prevents the separation of chromosome during mitosis. In contrast, Taxol binds to assembled microtubules and prevents their disassembly, meaning that microtubules can't depolymerize. The end result of Taxol treatment is the same as colchicine: preventing the separation of chromosomes during mitosis. Therefore, even though colchicine and Taxol have different mechanisms, their effect on a dividing cell is the same.
GTP hydrolysis and whether GTP or GDP is bound to tubulin is an important mechanism to control the dynamic instability of microtubules. Certain aspects of dynamic instability can be viewed using GFP-EB1. Which process(es) is it useful for visualizing and why? A. growing and shrinking microtubules, because EB1 binds to the GDP-tubulin cap on microtubules B. shrinking microtubules, because EB1 binds to the GTP-tubulin cap on microtubules C. growing microtubules, because EB1 binds to the GTP-tubulin cap on microtubules D. growing and shrinking microtubules, because EB1 binds to the GTP-tubulin cap on microtubules
growing microtubules, because EB1 binds to the GTP-tubulin cap on microtubules Explanation: EB1 binds to the GTP-tubulin cap on the plus end of microtubules. The plus end of microtubules experiences dynamic instability, the rapid switch between growth and shrinkage of the microtubule. When the tubulin subunits are bound to GTP, the end is stable and continues to grow. If all of the GTP is hydrolyzed to GDP, the end becomes unstable and rapidly shrinks (See Figure 17.16). EB1 labels the GTP-bound tubulin subunits in the microtubule, thereby labeling the growing microtubules. Shrinking microtubules are not visualized by GFP-EB1.
Which is the toughest and most durable of the different types of cytoskeletal filaments? A. intermediate filaments B. microtubules C. dynein D. myosin
intermediate filaments Explanation: Intermediate filaments are the toughest and most durable of the three types of cytoskeletal filaments and can survive treatment with concentrated salt solutions and detergents. The other two types of cytoskeletal filaments, actin and microtubules, can break or rupture under stress.
Which of the following cytoskeletal structures are the most common for providing tracks for guiding intracellular transport? A. actin filaments B. intermediate filaments C. microtubules D. dyneins E. kinesins
microtubules Explanation: Microtubules are cytoskeletal structures that provide tracks for guiding intracellular transport of vesicles, organelles, and other cell components in the cytosol. Microtubules have structural polarity: the end with β-tubulin showing is called its plus end, and the opposite end, which contains exposed α-tubulin, is called the minus end. This polarity allows kinesins and dyneins to exhibit directional motion.
In the eukaryotic flagellum, what drives the bending of microtubules? A. the motor protein ciliary dynein B. the basal body C. ciliary actin and myosin D. the fluid that surrounds the flagellum
the motor protein ciliary dynein Explanation: In the eukaryotic flagellum, ciliary dynein drives the bending of microtubules. Ciliary dynein generates a sliding force between two parallel microtubules. Because these microtubules are connected by linking proteins, the sliding force generates a bending motion. This mechanism is illustrated in the figure below. In isolated doublet microtubules (left panel), dynein will cause the microtubules to slide past each other. However, in an intact flagellum (right panel), linking proteins prevent the doublets from sliding, and instead result in bending of the flagellum.
Within this image, the _________ cargo is being transported by kinesin and the __________ cargo is being transported by dynein. - blue: red - red: blue
- red: blue Explanation: Microtubules guide the transport of organelles, vesicles, and macromolecules in both directions along a nerve cell axon. In the image, the red cargo is being transported by kinesin and the blue cargo is being transported by dynein. Kinesin uses the hydrolysis of ATP to migrate toward the plus end of a microtubule and dynein uses the hydrolysis of ATP to migrate toward the minus end of a microtubule.
