Lecture 4. The Cytoskeleton II - Tubulin Flashcards
What are microtubules ?
A network of filaments that generally extend from the centrosome, near the centre of the cell out to the periphery of the cytoplasm
What are the role of microtubules ?
To organise the cytoplasm
What are microtubules made up of ?
Repeating units of tubulin dimers.
What is a dimer ?
Composed of alpha and beta tubulin which form a non-covalently bound protofilament structure
Do protofilaments have structural polarity ?
Yes
Explain protofilaments structural polarity ?
The beta tubulin is exposed at the plus end and the alpha tubulin is exposed at the minus end
How many protofilaments stack against each other to form a lumen
13 parallel protofilaments
Are microtubules more rigid and straighter than actin filaments ?
Yes
How does microtubule nucleation occur ?
Via the gamma tubulin ring complex (Y TuRC)
Where does microtubule nucleation occur ?
At the microtubule organising centre (MTOC)
Does the nucleation step of polymerisation occur spontaneously ?
No
Why does the nucleation step of polymerisation not occur spontaneously ?
It involves the association of 13 protofilaments
What is the gamma tubulin ring composed of ?
Gamma tubulin
What does the gamma tubulin ring serve as ?
An initiation or nucleation site
What is the function of the tubulin dimers in microtubule nucleation ?
They associate with the gamma tubulin ring complex in a specific orientation, which results in the minus end of the microtubule being bound to this gamma tubulin ring complex, with the plus end growing outwards into the cell
What is the centrosome ?
The microtubule organising centre (MTOC) located near the centre of the animal cell
What is the centrosome composed of ?
A pair of centrioles surrounded by a fibrous matrix and contains up to hundreds of gamma tubulin ring complexes on its surface.
What is the function of the hundreds of gamma tubulin ring complexes on the surface of the centrosome ?
To allow the growth of hundreds of microtubules from the centrosome structure.
What is the function of the microtubules extending out to the cell periphery ?
Creates a series of tracks upon which organelles, vesicles and other cell components can be transported
What is dynamic instability ?
The distinctive growth pattern microtubules display after they have been nucleated
What does the term catastrophe mean ?
Rapid shrinkage phase
What is the distinctive growth pattern ?
A repeated sequence of rapid microtubule growth out from the centrosome followed by a rapid shrinkage phase
What is responsible for the distinctive growth pattern ?
The hydrolysis of GTP bound tubulin
Where initially is GTP bound tubulin incorporated ?
Into the growing microtubule
When is the GTP tubulin hydrolysed and by what ?
GTP tubulin is hydrolysed by GDP tubulin shortly after its incorporation into the microtubule.
What does the GDP bound tubulin have less of an affinity for ?
The other tubulin dimers inside the microtubule
How is the GTP cap formed ?
When polymerisation is rapidly proceeding the GTP bound tubulin dimers are being incorporated into the microtubule at a faster rate than that of the hydrolysis of GTP to GDP forming a GTP cap
What happens when the rate of microtubule growth slows to below the rate of GTP hydrolysis ?
The GTP tubulin subunits within the GTP cap will be hydrolysed to GDP tubulin, which dissociates from the microtubule.
Why does depolymerisation continue at a catastrophic rate ?
The GTP cap is hydrolysed to GDP tubulin and the rest of the microtubule is composed of GDP tubulin
What happens when GDP tubulin dimers are released from the microtubule into the cytosol ?
They exchange their bound GDP for GTP favouring the rescue phase and the reformation of the GTP cap
What is dynamic instability the main pathway of ?
Microtubule turnover in cells
What is an advantage of dynamic instability ?
The microtubules are very adaptable cytoskeletal filaments that can rapidly reorganise to different locations within the cell
What does GTP hydrolysis do ?
Controls the growth of microtubules
The dynamic properties of microtubules are essential for what functions ?
Allowing them to search intracellular space and quickly find their targets, regardless of location
What can the growing or shrinking of microtubules generate ?
They can generate force and be used to move vesicles and other subcellular organelles around the cell
What does the EB1 protein do ?
Binds to the GTP tubulin cap at the growing ends of the microtubules
What is the function of microtubules during mitosis ?
Form spindle
What do differentiated cells do to microtubules ?
They suppress microtubule dynamic instability
How are microtubule dynamic instability suppressed in differentiated cells ?
They use microtubule binding proteins to stabilise filaments.
What is the function of stabilised filaments in differentiated cells ?
Maintain organisation of cell
How do microtubules form spindle during mitosis ?
Microtubules become more dynamic enabling them to disassemble rapidly and reassemble into mitotic spindle
What organises the interior of the cell ?
Microtubules and their motor protein
What facilitates the transport of proteins, organelles and vesicles along microtubules ?
Specialised microtubule binding motor proteins
How do specialised microtubule binding proteins travel and carry their cargo ?
They use energy from ATP hydrolysis
What are the two motor protein families that move along the cytoplasmic microtubules ?
- Kinesins
2. Dyneins
What are kinesins ?
These move towards the plus end of the microtubule and away from the centrosome. Carry binding site in tail region
What are dyneins ?
Move towards the minus end or towards the centrosome
What are the two types of dyneins and their functions ?
- Cytoplasmic dyneins - vesicle trafficking and golgi localisation.
- Axonemal dyneins/cilliary dyneins - sliding movements of microtubules that drive ciliary and flagella beating
What are kinesins structurally similar to ?
Myosin
What do kinesins have roles in ?
Cell division
What are flagella and cilia built from ?
Axonemal dyneins and microtubules
Where are flagella found ?
Sperm and many protozoa
What enables flagella movement ?
The wavelike motion
Where would you find cilia
They line the respiratory tract and the oviduct
What type of motion do cilia have ?
Whiplike motion
What happens in isolated doublet microtubules ?
Dynein produced microtubule sliding
In normal flagellum what causes microtubule bending ?
Dynein
What are basal bodies ?
They are the root of cilia and flagella. Modified centriole
What are basal bodies composed of ?
9 fused triplet microtubules in a cartwheel
