What statement best describes the basis for how/why microtubules are “tubes”?

BICD 110 Fall 2020, Dr. Kiger

Problem Set 8 Lectures 7A-7B




1. What statement best describes the basis for how/why microtubules are “tubes”?


___A. tubulin and tubulin assemble into small filament rings that stack into a tube

___B. tubulin dimers assemble into filaments that spiral into a tube

_X_C. tubulin dimers assemble into parallel protafilaments that fold into a tube

___D. MAPs bind and curve the tubulin dimers so that filament assembly forms a tube

___E. ATPase activity of kinesin motor proteins bends a sheet of protafilaments into a tube


2. What is a shared property of both actin and tubulin subunits with respect to microfilament and microtubule dynamics, respectively?


___A. predominantly added to filament/protofilament (+) ends.

___B. predominantly added to filament/protofilament (−) ends.

___C. equally efficient at being added to both ends of filament/protofilament.

___D. added along the length within an assembled filament/protofilament.


3. During dynamic instability of microtubules, within the tubule…


(i)…the -tubulin subunits: (ii)….the -tubulin subunits:


___A. undergo ATP hydrolysis ___A. undergo ATP hydrolysis

___B. undergo GTP hydrolysis ___B. undergo GTP hydrolysis

___C. remain locked in GDP bound state ___C. remain locked in GDP bound state

___D. remain locked in ADP bound state ___D. remain locked in ADP bound state

___E. remain locked in GTP bound state ___E. remain locked in GTP bound state


(iii) Compare and contrast the above properties of tubulin subunits in microtubule ‘dynamic instability’ to those of actin subunits with microfilament ‘treadmilling’, providing key details. What is similar? What is distinct?








4. Define ‘critical concentration’ (Cc) as it relates to microfilament and microtubule formation, as well as to the different ends of the polymers. Define steady state.








5. Fill in the blanks.


Microtubules are typically not static structures. _____Dynamic instability_____ is the phrase used to describe how a microtubule undergoes alternating periods of rapid growth and shrinkage, called _____rescue_______ and ______catastrophy_________, respectively. These dynamics occur with growth happening at the microtubule ____positive (+)_____ ends, since the ____negative (-)_____ ends are typically inaccessible while stabilized at the ______MTOC_______. At the microtubule minus-ends, you will invariably find the specific microtubule subunit, __________________, which directly interacts with another tubulin subunit, __________________ in -TuRC. Growing microtubule ends are normally stabilized by __________________ ‘caps,’ while ___GTP____ hydrolysis can lead to rapid disassembly.

6. Compare and contrast the proteins, -tubulin and formin (what do they do? how do they do it? where do they do what they do?).













7. Name and describe the organization and roles for the three different major classes of microtubules that contribute to mitosis.


Microtubules and Motor proteins


8. Motor proteins are what kinds of enzymes?




9. Draw and label a simple cartoon of the general protein domains found in common between the structures for different types of motor proteins. Indicate the ‘motor’ region and what specific types of proteins interact with the different protein domains.










10. Which of the following properties is not shared by all myosins? May be one or more than one answer.


___A. the ability to bind ATP

___B. the formation of homodimers

___C. the ability to bind F-actin

___D. the presence of a head domain

___E. the ability to do work

___F. the ability to bind G-actin


11. In the model for myosin movement on microfilaments, the power stroke occurs during:


___A. binding of ATP.

___B. hydrolysis of ATP.

___C. release of phosphate (Pi).

___D. release of ADP.

___E. the assembly of a myosin thick filament


12. Match the cell functions on the right with the specific motor (A-F) most likely involved. You may use an answer more than once or not at all.


A. Myosin I ________ Cilia movement

B. Myosin II ________ Cell contraction

C. Myosin V ________ Organelle and vesicle transport (>1 correct!)

D. Kinesin I ________ Microtuble plus-end directed sliding

E. Kinesin 5 ________ Microfilament to membrane tethering

F. Dynein ________ Microfilament plus-end directed vesicle transport

13. All of the following statements describe Kinesin I except:


___A. Kinesin I is a (−) end-directed motor.

___B. Kinesin I transports vesicles along microtubules.

___C. Kinesin I binds and hydrolyzes ATP to produce movement.

___D. Kinesin I is composed of two heavy chains and two light chains.

___E. Kinesin is a (+) end-directed motor.


14. With respect to motor protein function, specifically what effect would the addition of AMP-PNP (a non-hydrolyzable analog of ATP) have on axonal transport? Why?









15. You purify what appears (by protein sequence homology) to be an ATPase protein complex that is required in a cell free assay for endosome intracellular transport. You call it Endomytin. You want to determine if Endomytin acts as a motor protein, and if so, to characterize its motor properties. Name three basic criteria (properties or predictions about protein function) that you expect if Endomytin is a motor protein, AND how you would test Endomytin for each of these properties.





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