Question

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?

(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

Answer 1

The false statement in the motility assay is that the nonhydrolyzable ATP analog AMP-PNP would cause the microtubules to move faster. In reality, this analog would inhibit movement by preventing ATP hydrolysis which is necessary for kinesin's motor function.

Step-by-step explanation:

The direct answer is D:

Addition of the nonhydrolyzable ATP analog (AMP-PNP) would cause the microtubules to move faster, is the false statement about the in vitro motility assay utilizing purified kinesin and polymerized microtubules. Kinesin motor proteins indeed require ATP to function, but a nonhydrolyzable ATP analog like AMP-PNP would not support the movement of microtubules; in fact, it would inhibit it.

Motor proteins such as kinesin are ATPases, which means they use the energy derived from the hydrolysis of ATP to power movement along microtubules. If ATP is replaced with AMP-PNP, a nonhydrolyzable analog that cannot be broken down, this will lead to the motor proteins binding to the microtubule without detaching, essentially 'freezing' them in place. Hence, no movement would be observed. Kinesin molecules are indeed attached by their tails to a glass slide in these assays. It is also true that microtubules must be polymerized under conditions that prevent dynamic instability, ensuring stable structures necessary for the motility assays. Furthermore, ATP is essential for the assay, as it is the source of energy for kinesin's movement.