Lecture 9

Question 1

Describe the structure / dynamics of microtubules

Answer 1

• unbranched cylinders of 25nm diameter assembled from tubulin heterodimers

Question 2

Are microtubules polar/nonpolar? Describe the two ends

Answer 2

Polar
plus ends: grow rapidly - beta tubulin is exposed
minus ends: grow slowly, if at all

Question 3

How is microtubule polymerization observed in vitro?

Answer 3

Light microscopy

Question 4

What is nucleation?

Answer 4

When cells use a template made of gamma-tubulin and other proteins to speed up polymerization (tubulin conc. is too low for polymerization to occur spontaneously)

Question 5

where do microtubules grow from?

Answer 5

at their plus ends from y-tubulin ring complexes of the centrosome

Question 6

Where are the two ends of the microtubule located?

Answer 6

plus ends : cell periphery
minus ends : cell centre, at the centrosome

Question 7

What is special about ciliated cells?

Answer 7

They have an extra set of microtubules in the cilia, which are nucleated by the basal body

Question 8

What is dynamic instability?

Answer 8

Each microtubule can switch between growing and shrinking (independently of its neighbours)

Question 9

Protein shape can alter depending on what?

Answer 9

The nucleotide bond

Question 10

A slow hydrolysis rate gives a what?

Answer 10

Switch activity (ATP and GTP)

Question 11

What is tubulin?

Answer 11

a GTPase

Question 12

Can GDP tubulin polymerize?

Answer 12

No

Question 13

In the microtubule, what happens to GTP?

Answer 13

gradually hydrolysed to GDP

Question 14

What does the protein EB1 do?

Answer 14

Binds preferentially to GTP tubulin, so it marks growing microtubules

Question 15

Does GTP or GDP tubulin bind more tightly?

Answer 15

GTP dimers bind more tightly to each other than GDP dimers because their shape is slightly different

Question 16

What will the MT do if the GTP cap is/is not present?

Answer 16

If it is present: MT will continue growing
If it is not present: MT will depolarize

Question 17

How can microtubules be depolymerized experimentally?

Answer 17

By putting cells on ice - can depolymerize, but can’t grow
Using drugs that bind free tubulin dimers - preventing new assembly

Question 18

What drugs are used to bind free tubulin dimers?

Answer 18

Nocodazole
Colcemid
Colchicine

Question 19

Describe the structure of actin filaments

Answer 19

Assembled from monomeric actin - thin, flexible, helical filaments - 7nm diameter

Question 20

What is actin?

Answer 20

An ATPase

Question 21

What does phalloidin do to alter actin polymerization?

Answer 21

Stabilizes actin filaments

Question 22

What does cytochalasin do to alter actin polymerization?

Answer 22

caps filament ends, preventing actin polymerization from existing ends

Question 23

What does lactrunculin do to alter actin polymerization?

Answer 23

binds to actin monomers, preventing actin polymerization

Question 24

How come not all actin is polymerized in the cell when it is in a test tube?

Answer 24

Because many proteins bind actin filaments and alter their organization and dynamics

Question 25

Nucleating protein

Answer 25

Promote polymerization - cell utilize them to control Where polymerization happenq