Microtubules

Question 1

Explain the structure and function of microtubules

Answer 1

1. Microtubules are tubulin dimers composed of globular a-tubulin and b-tubulin, with one end attached to a centrosome
2. Microtubules grow or shrink via tubulin loss/addition
3. Grow out of the centrosome forming tracks for vesicles and organelles to move
4. Mitotic microtubules assemble at the mitotic spindle

Question 2

What are microtubule organising centres

Answer 2

The locus where microtubules are assembled, can be the centrosome (centre of cell), spindle poles (ends of the cells) or basal body (cell body in neuron/cilia)

Question 3

How are tubulin dimers lost and gained

Answer 3

1. Microtubules have a (-) and (+) end
2. A-tubulin exposed on the (-) end and b-tubulin on the (+) end
3. the (-) end is anchored to the MTOC
4. (-) end has a higher Cc than (+) end (higher conc of free tubulin needed to grow)
5. Tubulin dimers preferentially added to the (+) end and lost from the (-) end

Question 4

Explain the structure of a centrisme

Answer 4

1. Formed of triplet microtubules in a circular like cartwheel structure
2. Its role is to stabilise and nucleate microtubules through y-TuRC activity, located on the y-tubulin ring

Question 5

Explain how and why dynamic instability occurs, giving the 4 stages of microtubule development

Answer 5

1. B-tubulin binds GTP more stably than it does GDP
2. GTP tubulin caps the growing end
3. GTP gets gradually hydrolysed to GDP by B-tubulin
4. GTP-GDP hydrolysis leads to such instability since it induces a curve in protofilaments, tension increases and there’s a fall in dynamic stability
5. If GTP B-tubulin is added faster that it is hydrolysed, the tubule grows
6. If vice versa the GTP tubulin cap is lost and shrinkage occurs

• Assembly, catastrophe, disassembly, rescue - in a zigzag format

Question 6

Give post-translational modifications of tubulin affecting MT stability and function

Answer 6

1. Acetylation has a stabilising effect
2. MAP2 and Tau help bundle dimers together
3. Katanin severs microtubules exposing more unstable GDP tubulin
4. Stathmins binds to MT preventing assembly
5. TIPs bind to + end and links microtubules to membranes

Question 7

Explain what a protofilament is

Answer 7

Protofilament in MT is the vertical single chain of a-b tubulin dimers

Question 8

Explain how molecules are transported along microtubules

Answer 8

1. Kinesin and Dynein are motor proteins which drive movement towards the + end and - end respectively
2. Kinesin and Dynein bind molecules like vesicles
3. Kinesin uses ATP hydrolysis to coordinate walking with its Kinesin head domains
4. Dynein undergoes a power stroke upon ATP hydrolysis and requires other proteins to facilitate movement

Question 9

Explain how keratin IFs are assembled

Answer 9

1. A dimer is formed composed of a head, rod and tail region with the Head on the n-terminus and tail the C-terminus
2. Two dimers coalesce to make the tetramer (anti parallel filaments)
3. Tetramer aggregate into unit length filaments to form the protofilament
4. Protofibrils are formed from multiple protofilaments

Question 10

Difference between actin/microtubule and IF assembly

Answer 10

No polarity or involvement of nucleotide hydrolysis in the assembly of IFs

Question 11

Intermediate filaments function:

Microtubules function:

Answer 11

1. Stabilise cell and tissue by forming rope bundles
2. Help cells resist stress
3. Form tracks for organelle transport
4. Dynamic and therefore location and function is very different from IFs
5. Microtubules generate cell polarity