Lecture 11

Question 1

What is the function of the cytoskeleton?

Answer 1

The cytoskeleton maintains cell shape, organization, and provides support for internal and external movement.

Question 2

Where are cytoskeletal proteins located?

Answer 2

The cytoskeleton is composed of cytoplasmic proteins.

Question 3

How do the functions of the cytoskeleton differ?

Answer 3

The proteins that make up the cytoskeleton have non-overlapping functions, meaning that they each have their own distinct functions in the cytoskeleton.

Question 4

What are the three classes of cytoskeletal filaments?

Answer 4

The three classes of cytoskeletal filaments are microfilaments, microtubules, and intermediate filaments.

Question 5

what is the thickness of microfilaments and what is an example?

Answer 5

Microfilaments are normally around 7-9nm in thickness.

• An example of a microfilament is Actin.

Question 6

What is the thickness of microtubules and what is an example?

Answer 6

Microtubules are around 25nm in diameter and they are also typically hollow on the inside.

• An example of a microtubule is Tubulin

Question 7

What is the thickness of intermediate filaments and what is an example?

Answer 7

Intermediate filaments are normally around 10nm in diameter (between microfilaments and microtubules)

• There are various intermediate filament proteins: Desmin, Latin, Keratin, etc.

Question 8

What are microtubules polymers of?

Answer 8

Microtubules are polymers of the protein tubulin.

Question 9

What is the structure of the tubulin that makes up microtubules?

Answer 9

The tubulin subunit of microtubules are heterodimers. These heterodimers are formed from two closely related globular proteins called a-tubulin and B-tubulin.

Question 10

Microtubules are what conformation on the inside?

Answer 10

Mirotubules are hollow on the inside, forming hollow protein tubes.

Question 11

What is an important feature of microtubules (hint - they have two ends)?

Answer 11

Microtubules have an important feature - their polarity. They have a minus (a) end and a plus (B) end.

Question 12

What is the main difference between the alpha and beta subunits of tubulin?

Answer 12

The alpha subunit contains bound GTP whilst the beta subunit contains bound GDP.

Question 13

What are alternative names for the plus and minus ends?

Answer 13

The plus end is also known as the B-tubulin end.

The minus end is also known as the a-tubulin end.

Question 14

What are microtubules build up of (they have 13)?

Answer 14

A microtubule is built up from 13 parallel protofilaments, each of which are composed of aB-tubulin heterodimers stacked head to tail and then folded into a tube.

Question 15

What are protofilaments?

Answer 15

Protofilaments are composed of aB-tubulin heterodimers stacked head to tail.

• They are then folded into a tube containing 13 protofilaments.
• remember that the B end is the plus end and the a end is the minus end.

Question 16

How is the structure of the microtubule slightly helical?

Answer 16

Each protofilament that joins bonds to the opposite subunit that it is, making the structure ever so slightly helical.
- B tends to bind to a on the next strand.

Question 17

What does their helical lattice lend to their structure?

Answer 17

The helical microtubules lattice makes the stiff and hard to bend.

Question 18

The helix forms from what arrangement in protofilaments?

Answer 18

The helix forms from a slight staggering in the protofilament lateral contacts (horizontal bonds between protofilaments)

Question 19

What gives microtubules their polarity?

Answer 19

The orientation of the microtubule subunits gives them their polarity.

Question 20

Which end of the microtubule grows and shrinks more rapidly?

Answer 20

The microtubule plus end grows and shrinks much more rapidly than its minus end.

Question 21

How does microtubule growth occur?

Answer 21

Rapid microtubule growth occurs by the addition of tubulin dimers at the ends - mostly occurring at the plus (B) end.

Question 22

What are the three phases of microtubule elongation?

Answer 22

• Lag phase
• Elongation phase
• Plateau phase

Question 23

What is microtubule nucleation?

Answer 23

Microtubule nucleation is the process in which several tubulin molecules interact to form a microtubule seed.
- This is also called the lag phase.

Question 24

What occurs during the process of microtubule nucleation?

Answer 24

• Tubulin dimers assemble into protofilaments
• Laterally associated linear protofilaments join filament

Question 25

How fast is nucleation of microtubules?

Answer 25

Microtubule nucleation is slow and takes a lot of energy.

Question 26

When are tubulin dimers favoured and when is filament growth favoured?

Answer 26

Tubulin dimers ae favoured when we have a very low amount of tubulin dimers present and they havent made a ton of bonds. Filament growth is favoured when the protofilaments begin to get longer and wider connections form

Question 27

What is dynamic instability?

Answer 27

Microtubules undergo a process called dynamic instability, in which individual microtubules alternate between cycles of growth and shrinkage. - Microtubules themselves are very dynamic but also unstable - they change length very rapidly.

Question 28

Why is the growth of a microtubule in a linear conformation?

Answer 28

The addition of GTP-tubulin to the plus end of a protofilament causes the end to grow in a linear conformation, which then assembles into the cylindrical wall of the microtubule.

Question 29

What is the GTP additon called?

Answer 29

The addition of several GTP tubulin units to the protofilament is called the GTP cap, which is stabilizing.

Question 30

What is characteristic of the less stable portion of the tubulin protofilament?

Answer 30

It isn't stable in the GDP state when there is not a GTP cap. It cannot exist without the GTP cap and will simply fall apart.

Question 31

How can we track microtubule dynamics?

Answer 31

We can track microtubule dynamics and how they grow with fluorescence microscopy.

Question 32

What is a catastrophe?

Answer 32

The change from growth to shrinkage is called a catastrophe. - This would be like having the loss of a GTP cap

Question 33

What is a rescue?

Answer 33

The change from shrinkage to growth is called a rescue. - This would be like regaining the GTP cap and allowing it to grow

Question 34

What does the hydrolysis of GTP cause in the conformation of tubulin subunits?

Answer 34

The hydrolysis of GTP after assembly changes the conformation of the subunits and tends to force the protofilament into a curved shape that is less able to pack into the microtubule wall.

Question 35

What can nucleation be dependent on?

Answer 35

Nucleation in many cases can be dependent on the y-tubulin ring complex. - y-tubulins provide support for growing mechanisms, anchoring them

Question 36

Where are microtubules generally nucleated from?

Answer 36

Microtubules are generally nucleated from the microtubule-organizing centre (MTOC) where y-tubulin is most enriched

Question 37

What can nucleation cause?

Answer 37

Nucleation and growth of microtubules can result in membranes being pushed out! - This mechanism can be observed in neuronal cells and fibroblasts.

Question 38

What is an MTOC that most animal cells possess?

Answer 38

Many animal cells possess a single well-defined MTOC called the centrosome - It is typically located at the centre - All growing microtubules are anchored here.

Question 39

What are centrosomes composed of?

Answer 39

Centrosomes are composed of two centrioles

Question 40

what are centrosomes surrounded by?

Answer 40

Centrosomes are surrounded by a dense mass of protein termed the pericentriolar material.

Question 41

What is the pericentriolar material?

Answer 41

The pericentriolar material is the y-tubulin and it anchors the minus end.

Question 42

What is the y-tubulin when the cell isn't dividing?

Answer 42

Unless the cell is dividing, y-tubulin is in a single spot - the centromere

Question 43

What are MAPs?

Answer 43

Microtubule associated proteins (MAPs) bind and stabilize microtubules.

Question 44

Examples of MAPs?

Answer 44

Tau (important), DCX, NUF2, EB, NDC80, WHAMM, PRC1, CAMSAP, TPX2, and more!

Question 45

What is the function of Map2 and Tau?

Answer 45

The function of Map2 and Tau is to set the spacing of the microtubule bundles. - Map2 sets them farther apart - Tau sets them closer together

Question 46

What do Tau mutations cause?

Answer 46

Tau mutations cause neurodegenerative diseases (like Alzheimer's disease)