Lecture 9 Flashcards
Describe the structure / dynamics of microtubules
- unbranched cylinders of 25nm diameter assembled from tubulin heterodimers
Are microtubules polar/nonpolar? Describe the two ends
Polar
plus ends: grow rapidly - beta tubulin is exposed
minus ends: grow slowly, if at all
How is microtubule polymerization observed in vitro?
Light microscopy
What is nucleation?
When cells use a template made of gamma-tubulin and other proteins to speed up polymerization (tubulin conc. is too low for polymerization to occur spontaneously)
where do microtubules grow from?
at their plus ends from y-tubulin ring complexes of the centrosome
Where are the two ends of the microtubule located?
plus ends : cell periphery
minus ends : cell centre, at the centrosome
What is special about ciliated cells?
They have an extra set of microtubules in the cilia, which are nucleated by the basal body
What is dynamic instability?
Each microtubule can switch between growing and shrinking (independently of its neighbours)
Protein shape can alter depending on what?
The nucleotide bond
A slow hydrolysis rate gives a what?
Switch activity (ATP and GTP)
What is tubulin?
a GTPase
Can GDP tubulin polymerize?
No
In the microtubule, what happens to GTP?
gradually hydrolysed to GDP
What does the protein EB1 do?
Binds preferentially to GTP tubulin, so it marks growing microtubules
Does GTP or GDP tubulin bind more tightly?
GTP dimers bind more tightly to each other than GDP dimers because their shape is slightly different
What will the MT do if the GTP cap is/is not present?
If it is present: MT will continue growing
If it is not present: MT will depolarize
How can microtubules be depolymerized experimentally?
By putting cells on ice - can depolymerize, but can’t grow
Using drugs that bind free tubulin dimers - preventing new assembly
What drugs are used to bind free tubulin dimers?
Nocodazole
Colcemid
Colchicine
Describe the structure of actin filaments
Assembled from monomeric actin - thin, flexible, helical filaments - 7nm diameter
What is actin?
An ATPase
What does phalloidin do to alter actin polymerization?
Stabilizes actin filaments
What does cytochalasin do to alter actin polymerization?
caps filament ends, preventing actin polymerization from existing ends
What does lactrunculin do to alter actin polymerization?
binds to actin monomers, preventing actin polymerization
How come not all actin is polymerized in the cell when it is in a test tube?
Because many proteins bind actin filaments and alter their organization and dynamics
Nucleating protein
Promote polymerization - cell utilize them to control Where polymerization happenq
