Microtubules

Question 0

What are the components of MTs?

Answer 0

Tubulin heterodimer: alpha (can bind gtp) and beta tubulin (exchange and hydrolyze GTP)

Protofilament= tubulin heterodimers stacked end to end (plus end at one end and minus at another). Alpha doesn’t bind to alpha.

MT is composed of 13 linear protofilaments.

MTs are polar; plus end is faster growing than minus.

Polymerization kinetics: critical concentration, nucleation, elongation, steady state.

Question 1

What’s the purpose of MTs?

Answer 1

Organize mvt of chromosomes in mitosis and cell division, organelle transport, beating of cilia and flagella.

Question 2

What is a protofilament?

Answer 2

MT heterodimers assembled end to end in a vertical fashion.

Question 3

What is a MT composed of?

Answer 3

Tubulin Hererodimers arranged into protofilaments. 13 protofilaments make a microtubule. Makes a lumen because the protofilaments arranged into a circular fashion.

Question 4

How do MTs grow? From where? How do the polarized ends differ?

Answer 4

MTs nucleate at centrosome and grow outward. Minus ends are capped by gamma TURC (nucleator and prevents depolymerization) and are located at centrosome.
Plus ends are extended towards periphery

Question 5

What experiment proved tht MTs are nucleated at centrosomes?

Answer 5

Add MT depolymerization drug to MTs. Wash out drug. And see that MTs grow from minus end over time.

Question 6

How do you purify MTs?

Answer 6

From cow brains. Just need gtp to nucleate

Question 7

What is the significance of a GtP cap in MT synthesis?

Answer 7

GTP tubulin adds to + ends and GTP is hydrolyzed to GDP. The initial gtp at plus end is a gtp cap. The rest is a GDP lattice.

If tubulin conc is greater than Cc then you have more tubulin on than off which means you have a gtp cap.

Question 8

What is MT catastrophe?

Answer 8

When tubulin heterodimers disassemble from + end

Question 9

How are MTs dynamic?

Answer 9

Assembly, catastrophe, rescue, repeat. This can happen when at steady state or critical concentration.

In other words, alternate between growing and shrinking states in a single MT

Question 10

What is gamma TURC’s role in MT function?

Answer 10

Nucleates assembly and remains associated with the minus end. Exists at centrosome.

Question 11

What is stathmin’s role in MT function?

Answer 11

It binds subunits prevents assembly

Question 12

What is kinesin 13’s role in MT function?

Answer 12

Enhances catastrophic disassembly at + end

Question 13

What is katanin’s role in MT function?

Answer 13

It severs microtubules right in the middle

Question 14

What is MAPs’ role in MT function?

Answer 14

Stabilizes tubules by binding along sides

Question 15

What is xmap215’s role in MT function?

Answer 15

Stabilizes plus and and accelerates assembly. it also protects GDP lattice

Question 16

What are microtubule motor proteins?

Answer 16

Enzymes that use ATP hydrolysis to drive conformational changes and mvt along MTs

Eg: kinesins and dyneins: organelle positioning.

Question 17

What are kinesins?

Answer 17

Motor proteins that are guided by MTs in + direction: kinesin 1
Or - direction: kinesin 14
Or depolymerize MTs: kinesin 13

Moves cargo toward or away from centrosome.

Question 18

What does kinesin 13 do?

Answer 18

Depolymerizes MTs

Question 19

What are dyneins?

Answer 19

• end directed motor proteins guided by MTs. Cytoplasmic moves cargo thru ATP hydrolysis.

Question 20

What does gamma TURC do?

Answer 20

Gamma tubulin ring complex is the nucleator of MTs. It lies at the minus end at the centrosomes.

It also prevents Depolymerization