Microtubules

Question 1

Microtubule structure:

Answer 1

Composed of alpha and beta tubulin dimers.
Dimers form protofilaments, which arrange in circles to form:
- Singlets
- Doublets (Cilia, flagella)
- Triplets (Basal bodies, centrioles)

Microtubules are polarised (like actin filaments), they have a plus-end and a minus-end:

• Plus-end = beta-tubulin end, due to exposed GTP
• Minus-end = alpha-tubulin end, no exposed GTP

Question 2

Dynamic instability of Microtubules:

Answer 2

1. Microtubule Assembly
2. Catastrophe —-> Disassembly
3. Rescue —-> Assembly
   etc.

Question 3

Tubulin Polymerisation:

Answer 3

1. Nucleation: Nucleation of individual dimers into oligomers.
2. Elongation: Growth of the microtubule through addition of subunits .
3. Steady State: Microtubules has subunits both coming on and off.

Note: Microtubules grow preferentially at the plus-end.

Question 4

What are the main components of the centrosome/MTOC?

Answer 4

2 Centrioles and Peri-Centriolar Material (PCM)

Question 5

Where Do Microtubules Grow From In The Cell? Where Do They Grow Towards?

Answer 5

The Centrosome/MTOC.

The Periphery of the Cell (In Most Cases)

Question 6

Microtubules are nucleated at the plus end by alpha-tubulin ring complexes.
TRUE OR FALSE

Answer 6

FALSE

Microtubules are nucleated at the MINUS end by GAMMA-tubulin ring complexes.

Question 7

What Controls Microtubule Stability?

Answer 7

GTP Hydrolysis:
Changes subunits conformation (GTP ->GDP) and weakens the bond in the polymer, causing a straight protofilament to curve. Dimers fall off and protofilaments start to “peel” off.

A GDP-bound tubulin subunit at the tip of a microtubule will tend to fall off. So a GTP cap prevents disassembly.

Essentially:
GTP-Cap = Stable
Hydrolysis of GTP-Cap = GDP-Cap = Falls off = Protofilaments peel off

Question 8

Microtubule Plus-End Binding Proteins:

Answer 8

Play an important role in regulating microtubule dynamics: e.g.

• Myosins
• Actin-Microtubule Crosslinking Proteins
• Rho and Formin Regulators
• Membrane Vesicles

Question 9

Motor Proteins Share Similar Domain Structures, Such As:

Answer 9

• Motor Domain ATPase
• Linker Region
• Cargo Domain

e.g. Kinesin, Dynein, Myosin

Question 10

Kinesin-1-catalyzed vesicle transport:

Answer 10

• Vesicle: Bound To Kinesin via a Kinesin Receptor
• Kinesin-1 uses ATP hydrolysis to “walk” the vesicle along a microtubule (Kinesin is a plus end directed motor).

2 Motor Heads: Leading head binds ATP, inducing a conformational changes that causes the trailing head to become the leading head. The new leading head finds another binding site on the microtubule. The new leading head releases ADP and the new trailing head hydrolyses ATP to ADP + Pi.

Question 11

Dynein-Dynactin Complex:

Answer 11

Dynein is a minus end directed motor:

• Vesicle bound to ankyrin via membrane glycoproteins. - Ankyrin is bound to the “Dynactin Complex”
• “Dynactin Complex” is bound to Dynein, which interacts with the microtubule.