Lecture 7 - Microtubules Flashcards

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1
Q

What are microtubules composed of?

A

alpha-beta tubulin heterodimers

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2
Q

Shape of microtubules

A

Hollow, tube like cylinder

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3
Q

What do microtubules take part in

A

Cell polarity, cell motility, structural support, and intracellular transport

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4
Q

Diameter of tube like cylinder

A

25nm

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5
Q

GTP binding sites in ab tubulin heterodimer

A

Both have GTP binding sites

GTP never hydrolysed by alpha tubulin

GTP hydrolysed by beta tubulin when heterodimer incorporated in microtubule

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6
Q

How do microtubule-binding drugs work?

A
  • Prevent microtubule assembly (colchicine) or disassembly (cancer drug taxol)
  • Inhibits cellular processes that depend on microtubules and polymer rearrangement
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7
Q

GTP on alpha-tubulin

A
  • Never hydrolysed or exchanged with free nucleotide
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8
Q

GTP on beta-tubulin

A
  • Hydrolyses bound GTP to GDP
  • Exchanges GDP for GTP after subunit disassembles
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9
Q

Size of ab-tubulin dimer

A

8nm

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10
Q

Microtubule singlets, doublets, triplets

A
  • Singlet
  • Doublet e.g. cilia, flagella (9+2 arrangement)
  • Triplet e.g. basal bodies, centrioles
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11
Q

Microtubule organzing centres (MTOCs)

A
  • Nucleate and organise microtubules
  • Almost all microtubules originate from MTOCs
  • (-) ends associate with MTOC
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12
Q

Main MTOC in animal cells?

A

Centrosomes

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13
Q

Are spindle poles a MTOC?

A

Yes

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14
Q

Where is MTOC located in nerve cells?

A

Base of axon

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15
Q

What are basal bodies?

A
  • MTOC that incorporates into ‘mother centriole’ - the centriole inherited from previous division
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16
Q

Microtubule polarity

A
  • head-to-tail assembly
  • A crown of alpha-tubulin at (-) end
  • A crown of beta-tubulin at (+) end
  • Nonpolarized animal cells, (-) ends associated with MTOCs
  • (+) ends extend towards cell periphery
  • Motor proteins dynein towards -
  • Kinesin (towards+) move in aspecific direction along the microtubule
17
Q

Centriole structure

A

EM of a centriole – heart of MTOC

Barrel like structures at right angles to each other

9 triplet microtubules arranged in a ring like structure

Surrounded by pericentriolar material

18
Q

What is the third type of tubulin?

A

Third type of tubulin - gamma

Found in the pericentriolar region

Binds alpha/beta dimers

Template for nucleation

19
Q

Tubulin ring complex

A

Gamma-TuRC

Gamma tubulin ring complex

Gamma is in red - nucleates the microtubule – starting with a –End at the MTOC

20
Q

Kinetics of microtubule assembly

A

Initially have dimers - at a certain concentration microtubules can form

At that point the concentration of dimers plateaus as they are being constantly incorporated

21
Q

Microtubules critical concentrations requirements

A

Microtubule growth dependent on dimer concentration

High growth

Low disassembly

Critical concentration higher at –End then +End

22
Q

Cc Values

A
  • Tubulin dimers assemble into microtubules above the Cc concentration
  • Cc different for the + and - ends
  • Steady-stae Cc tubulin dimers add to + end at same rate as leave - end - Stays same length, microtubules move down
23
Q

Treadmilling

A

It can happen, but unsure of concentrations required

24
Q

Dynamic instability

A

Model is one of dynamic instability

Because they can alternate quickly between growth and disassembly

Centred on the GDP bound to the beta-tubulin

If there’s lots of free dimers – added rapidly – GTP not hydrolysed quickly – exposed beta tubulin is bound to GTP

If supply dries up the exposed beta tubulin will hydrolyse its GTP to GDP – microtubule become unstable - disassembly

25
Q

Microtubule associated proteins (MAPs)

A

Stabilising MAPs - Promote assembly, increase microtubule assemly, crosslink microtubules

MAP2

Tau tangles on Alzheimers kills brain cells

Cognitive decline

26
Q

Examples of MAPs

A

MAP2 - Long projection

Tau - Similar binding - Shorter projection

MAP2 KO nerve cell – spacing is disrupted – nerve function changed

27
Q

Tau tangles

A

Alzheimers disease

Tau tangles

Progression of disease results in an increase in Tau tangles and we know they kill nerve cells

There is another protein involved – beta-amyloid

Beta-amyloid outside the nerve cells

Tau tangles inside the nerve cells

For many years the pharmaceutical companies have tried to target Beta-amyloid (easier target) - billions RD spent – Abs that clear plaques developed

But no cognitive improvement observed
Beta-amyloid not a good indicator of cognitive function
But Tau tangles well correlated
Tau much more important than beta-amyloid

28
Q

Drugs targeting Tau tangles

A

Salbutamol – asthmatics – relaxes smooth muscle

Being repurposed for Alzeimers – low ethical barriers as well tolerated

Inhibits Tau agreggation

29
Q

End binding microtubule proteins

A

+TIPs (plus end tracking proteins influence assembly and disassembly) eg EB1 (end binding 1)
Kinesin 13 helps disassembly
Stathmin binds to curved protofilaments and enhances their disassembly

30
Q

Ending binding 1 (EB1)

A

EB1 – attaches to end of microtubules

Importantly also associates with other end tracking proteins (TIPS)

31
Q

Role of EB!

A
  • Stimulates spontaneous nucleation and growth of microtubules
  • Promotes both catastrophes and rescues
32
Q

Kinesin 13 and Stathmin

A

Roles of Kinesin 13 and Stathmin much clearer

Both involved in disassembly of microtubules from the plus end

Even though they are structurally very different

33
Q
A