Cytoskeleton 2 Flashcards
muscle contraction
filaments don’t shorten, myosin heads ‘walk’ towards the plus ends of the actin filament, pulling z discs closer
microtubules
long, hollow cylinders 25nm diameter, made up of tubulin Monomers
Rigid and straight
Grow from microtubule organising centre
Have polarity- minus and plus end. One end has alpha subunit exposed and the other a beta subunit exposed
Assembly of microtubules
13 protofilaments form a cylinder
heterodimer (see picture) of alpha and beta tubulin
subunits stack to form hollow cylindrical microtubule
With GTP bound, which is never hydrolysed
What does it mean to say microtubules are dynamic (/dynamic instability)
balance between assembly and disassembly
Cell contains a mixture of microtubules and free tubulin
Microtubules unstable allowing them to undergo rapid remodelling
Where do microtubules grow from
centrosome- microtubule organising centre
Centrosome
in cytoplasm. just outside nuclear membrane
centriole
each centrosome consists of 2 centrioles at right angles to one another
Centrioles built from cylindrical array of 9 microtubules each with 2 partial microtubules attached
gamma-tubulin
rings of y-tubulin surround pair of centrioles, act as nucleating sites which microtubules grow out of
Where are actin filaments nucleated vs microtubules
actin at pM
microtubules nucelated from centre of the cell
what controls growth of microtubules (dynamic instability)
GTP hydrolysis
If new subunits (which have GTP bound beta tubulin) are added quicker than beta tubulin hydrolyses GTP, a GTP cap will form.
When a GTP cap is present the microtubule end is stable so continues to grow.
If GTP is hydrolysed faster than new subunits are added the GTP cap is lost.
GDP bound beta tubulin at the end of a microtubule has a different conformation to GTP bound beta tubulin, weaker interactions with neighbouring subunits in the filament, so ends start to disassemble and peel away causing rapid shrinkage of the microtubule
how can microtubules be stabilised
interacting proteins on surface of organelles/PM
Functions of microtubules
cellular organisation, movement of organelles (travel along microtubules), cell polarity (apical and baselateral membrane different)
Cell division incl mitosis and meiosis
Cilia and flagella
Motor proteins associated with microtubules
Dyenins and Kinesins move along microtubules
Kinesins towards plus end and Dyneins towards the minus
Both have 2 binding heads and a tail, and ATPase activity
How does movement of motor proteins along microtubules work
ATP hydrolysis provides energy for conformation changes in the head that allows it to move along the microtubule in a cycle of release and rebinding
Tail regions of kinesins and dyneins is where ATP hydrolysis occurs
Also, tails interact with cargo that the motor proteins carry
Kinesins and ER
ER pulled outwards away from centre of cell along microtubules by kinesins
Dyneins and Golgi
Pull golgi in opposite direction, towards the centrosome, which is why golgi is found near centre of the cell
Mitosis
Centrosome duplicates, move apart until at opposite sides of nucleus, then nuclear envelope breaks down.
As mitosis proceeds, microtubules grow out from each centrosome with plus ends growing towards equatorial plate (midway between poles) forming spindle fibres.
Chromosomes pulled apart by spindles
Colchicine drug
binds free tubulin and prevents polymerisation into microtubules
Taxol drug
binds tightly to microtubules and prevents them from losing subunits
dynamics important so non-functional
Cilia and flagella
Grow out of basal bodies ( different type of microtubule organising centre)
Very stable microtubules
Same internal structure but cilia shorter than flagella
cilia extend from surfaces of cells and move fluid over a cell/move a cell through fluid
Arrangement of microtubules in cilia and flagella
Motor proteins associated with microtubules- dyenin
Walk along an adjacent microtubule doublet, this drives bending of cilia and flagella.
Stable connections between pair of microtubules - nexin
If stable connections (nexin) between microtubules were not present..
walking of the dynein along (towards minus end) neighbouring microtubule would cause sliding rather than bending (see useful diagram)
