Lecture 4. The Cytoskeleton II - Tubulin

Question 1

What are microtubules ?

Answer 1

A network of filaments that generally extend from the centrosome, near the centre of the cell out to the periphery of the cytoplasm

Question 2

What are the role of microtubules ?

Answer 2

To organise the cytoplasm

Question 3

What are microtubules made up of ?

Answer 3

Repeating units of tubulin dimers.

Question 4

What is a dimer ?

Answer 4

Composed of alpha and beta tubulin which form a non-covalently bound protofilament structure

Question 5

Do protofilaments have structural polarity ?

Answer 5

Yes

Question 6

Explain protofilaments structural polarity ?

Answer 6

The beta tubulin is exposed at the plus end and the alpha tubulin is exposed at the minus end

Question 7

How many protofilaments stack against each other to form a lumen

Answer 7

13 parallel protofilaments

Question 8

Are microtubules more rigid and straighter than actin filaments ?

Answer 8

Yes

Question 9

How does microtubule nucleation occur ?

Answer 9

Via the gamma tubulin ring complex (Y TuRC)

Question 10

Where does microtubule nucleation occur ?

Answer 10

At the microtubule organising centre (MTOC)

Question 11

Does the nucleation step of polymerisation occur spontaneously ?

Answer 11

No

Question 12

Why does the nucleation step of polymerisation not occur spontaneously ?

Answer 12

It involves the association of 13 protofilaments

Question 13

What is the gamma tubulin ring composed of ?

Answer 13

Gamma tubulin

Question 14

What does the gamma tubulin ring serve as ?

Answer 14

An initiation or nucleation site

Question 15

What is the function of the tubulin dimers in microtubule nucleation ?

Answer 15

They associate with the gamma tubulin ring complex in a specific orientation, which results in the minus end of the microtubule being bound to this gamma tubulin ring complex, with the plus end growing outwards into the cell

Question 16

What is the centrosome ?

Answer 16

The microtubule organising centre (MTOC) located near the centre of the animal cell

Question 17

What is the centrosome composed of ?

Answer 17

A pair of centrioles surrounded by a fibrous matrix and contains up to hundreds of gamma tubulin ring complexes on its surface.

Question 18

What is the function of the hundreds of gamma tubulin ring complexes on the surface of the centrosome ?

Answer 18

To allow the growth of hundreds of microtubules from the centrosome structure.

Question 19

What is the function of the microtubules extending out to the cell periphery ?

Answer 19

Creates a series of tracks upon which organelles, vesicles and other cell components can be transported

Question 20

What is dynamic instability ?

Answer 20

The distinctive growth pattern microtubules display after they have been nucleated

Question 21

What does the term catastrophe mean ?

Answer 21

Rapid shrinkage phase

Question 22

What is the distinctive growth pattern ?

Answer 22

A repeated sequence of rapid microtubule growth out from the centrosome followed by a rapid shrinkage phase

Question 23

What is responsible for the distinctive growth pattern ?

Answer 23

The hydrolysis of GTP bound tubulin

Question 24

Where initially is GTP bound tubulin incorporated ?

Answer 24

Into the growing microtubule

Question 25

When is the GTP tubulin hydrolysed and by what ?

Answer 25

GTP tubulin is hydrolysed by GDP tubulin shortly after its incorporation into the microtubule.

Question 26

What does the GDP bound tubulin have less of an affinity for ?

Answer 26

The other tubulin dimers inside the microtubule

Question 27

How is the GTP cap formed ?

Answer 27

When polymerisation is rapidly proceeding the GTP bound tubulin dimers are being incorporated into the microtubule at a faster rate than that of the hydrolysis of GTP to GDP forming a GTP cap

Question 28

What happens when the rate of microtubule growth slows to below the rate of GTP hydrolysis ?

Answer 28

The GTP tubulin subunits within the GTP cap will be hydrolysed to GDP tubulin, which dissociates from the microtubule.

Question 29

Why does depolymerisation continue at a catastrophic rate ?

Answer 29

The GTP cap is hydrolysed to GDP tubulin and the rest of the microtubule is composed of GDP tubulin

Question 30

What happens when GDP tubulin dimers are released from the microtubule into the cytosol ?

Answer 30

They exchange their bound GDP for GTP favouring the rescue phase and the reformation of the GTP cap

Question 31

What is dynamic instability the main pathway of ?

Answer 31

Microtubule turnover in cells

Question 32

What is an advantage of dynamic instability ?

Answer 32

The microtubules are very adaptable cytoskeletal filaments that can rapidly reorganise to different locations within the cell

Question 33

What does GTP hydrolysis do ?

Answer 33

Controls the growth of microtubules

Question 34

The dynamic properties of microtubules are essential for what functions ?

Answer 34

Allowing them to search intracellular space and quickly find their targets, regardless of location

Question 35

What can the growing or shrinking of microtubules generate ?

Answer 35

They can generate force and be used to move vesicles and other subcellular organelles around the cell

Question 36

What does the EB1 protein do ?

Answer 36

Binds to the GTP tubulin cap at the growing ends of the microtubules

Question 37

What is the function of microtubules during mitosis ?

Answer 37

Form spindle

Question 38

What do differentiated cells do to microtubules ?

Answer 38

They suppress microtubule dynamic instability

Question 39

How are microtubule dynamic instability suppressed in differentiated cells ?

Answer 39

They use microtubule binding proteins to stabilise filaments.

Question 40

What is the function of stabilised filaments in differentiated cells ?

Answer 40

Maintain organisation of cell

Question 41

How do microtubules form spindle during mitosis ?

Answer 41

Microtubules become more dynamic enabling them to disassemble rapidly and reassemble into mitotic spindle

Question 42

What organises the interior of the cell ?

Answer 42

Microtubules and their motor protein

Question 43

What facilitates the transport of proteins, organelles and vesicles along microtubules ?

Answer 43

Specialised microtubule binding motor proteins

Question 44

How do specialised microtubule binding proteins travel and carry their cargo ?

Answer 44

They use energy from ATP hydrolysis

Question 45

What are the two motor protein families that move along the cytoplasmic microtubules ?

Answer 45

1. Kinesins | 2. Dyneins

Question 46

What are kinesins ?

Answer 46

These move towards the plus end of the microtubule and away from the centrosome. Carry binding site in tail region

Question 47

What are dyneins ?

Answer 47

Move towards the minus end or towards the centrosome

Question 48

What are the two types of dyneins and their functions ?

Answer 48

1. Cytoplasmic dyneins - vesicle trafficking and golgi localisation. 2. Axonemal dyneins/cilliary dyneins - sliding movements of microtubules that drive ciliary and flagella beating

Question 49

What are kinesins structurally similar to ?

Answer 49

Myosin

Question 50

What do kinesins have roles in ?

Answer 50

Cell division

Question 51

What are flagella and cilia built from ?

Answer 51

Axonemal dyneins and microtubules

Question 52

Where are flagella found ?

Answer 52

Sperm and many protozoa

Question 53

What enables flagella movement ?

Answer 53

The wavelike motion

Question 54

Where would you find cilia

Answer 54

They line the respiratory tract and the oviduct

Question 55

What type of motion do cilia have ?

Answer 55

Whiplike motion

Question 56

What happens in isolated doublet microtubules ?

Answer 56

Dynein produced microtubule sliding

Question 57

In normal flagellum what causes microtubule bending ?

Answer 57

Dynein

Question 58

What are basal bodies ?

Answer 58

They are the root of cilia and flagella. Modified centriole

Question 59

What are basal bodies composed of ?

Answer 59

9 fused triplet microtubules in a cartwheel