Lecture 7 - Microtubules

Question 1

What do microtubules consist of?

Answer 1

Microtubules are composed of α and β tubulin heterodimers assembled into a hollow, tube like cylinder. Microtubules provide structural support, are involved in cell division, intracellular transport, cell motility and help generate cell polarity.
Both α and β tubulin bind GTP and this form of the dimer can form micro tubulin. However, only β-tubulin is able to hydrolyse the bond GTP to GDP (after microtubule assembly) and exchange GDP for GTP after disassembly. The GTP bound to α tubulin is never hydrolysed or exchanged with a free nucleotide

Question 2

What is the medical importance of microtubule binding drugs?

Answer 2

Microtubule/tubulin-binding drugs are used to treat a variety of disease. They act by preventing microtubule assembly (e.g. colchicine) or disassembly (e.g. the cancer drug taxol)
Although the effects are opposite, the results are the same: inhibition of cellular processes that depend on microtubules and their dynamic rearrangement

Question 3

How are microtubules organised?

Answer 3

Microtubules can be found in singlets, doublets (Cilia and flagella) and triplets (basal bodies, centrioles)

Microtubule organising centres (MTOCs)
* Serve o nucleate and organize microtubules
* Almost all microtubules originate form the MTOC and the (-) ends of these microtubules remain associated with the MTOC.
E.g.
Centrosome
Spindle poles
Basel body
Microtubule organising centre in cell body of a nerve cell

Question 4

What is microtubule polarity?

Answer 4

• The basis of microtubule polarity is the head to tail assembly of αβ tubulin heterodimers, which results in a crown of a-tubulin at the (-) end and a crown of B-tubulin at the (+) end.
  
  • In non-polarised animal cells, (-) ends are typically associated with MTOCs and (+) ends may extend toward the cell periphery
    Microtubule motors can “read” the polarity of microtubule, and a specific motor protein will transport its cargo towards wither the (+) or (-) end of the microtubule

Question 5

What is ment by Critical concentration (Cc)

Answer 5

• Tubulin dimers assemble into microtubules above the critical concentration (of tubulin)
  
  • The Cc is often different for the + and - ends. Normally the Cc is lower at the positive end meaning that it assebles faster.
  • At steady state Cc tubulin dimers add to the + end at the same rate they leave at the - end meaning the microtubule length doesn’t change but the microtubules move down.
  • This treadmilling is similar to what happens in actin filaments
  • Microtubules grow more quickly from the + end in comparison to the - end

Question 5

What is Y-tubulin?

Answer 5

Microtubule organising centres (MTOCs ) are responsible for determining the arrangement of microtubules within a cell. A typica cell contains a single MTOC although mitotic cells contain two known as the spindle poles, and certain types of cells may contain several hundred e.g. in the form of basal bodies of cilia. Most microtubules are nucleated by y-tubulin ring complexes which are located in the pericentriolar material of an MTOC.
The y-tubulin provides a binding siter for aB-tubulin dimers, and the ring complex structure appears to provide a template for nucleating microtubule formation.

Question 6

What is dynamic instablity in relation to microtubules?

Answer 6

During dynamic instability, microtubules alternate between growth and shortening. The current model to account for dynamic instability is the GTP cap model.
According to this model, GTP-tubulin (subunits with GTP bound to β-tubulin) can add to the end of a growing microtubule, but at some time after assembly the GTP will be hydrolyzed to GDP, leaving GDP-tubulin, which makes up the bulk of the microtubule. Thus GTP-tubulin is present only at the microtubule end, and as long as this situation holds, the microtubule will continue to grow since the cap stabilizes the entire microtubule.
However, if GDP-tubulin becomes exposed at the end then the stabilizing cap is lost and the microtubule will begin to shorten.
The microtubule will then continue to shorten until it disappears or until GTP-tubulin returns to the end and a new GTP cap is formed.
* 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 hydrolyzed quickly – exposed beta tubulin is bound to GTP
* If supply dries up the exposed beta tubulin will hydrolyze its GTP to GDP – microtubule become unstable - disassembly

Question 7

What are the function of microtubule associated proteins?

Answer 7

• The best understood proteins involved in regulating microtubule assembly are the stabilizing MAPs. These proteins bind to microtubules and promote assembly,
  
  • increase microtubule stability, and, in some cases, crosslink microtubules into bundles.
  • MAP2 and Tau are examples of microtubule stabilizing
  • proteins.
    In Alzheimer’s disease something goes wrong with tau and it forms Tau Tangles which eventually kills the brain cell.
    Drugs targeting tau tangles - Salbutamol reduces the yield and rate of filament formation and additionally inhibits tau’s structural change into B-sheet rich aggregates.

End binding Microtubule proteins
* +TIPs (+ end tracking proteins influence assembly and disassembly) e.g. EB1 (End Binding 1)
* Kinesin 13 helps disassembly
* Stathmin binds to curved protofilaments and enhances their disassembly
EB1 stimulates spontaneous nucleation and growth of microtubule, and promotes both catastrophes (transitions from growth to shrinkage and rescues (reverse events)