Cytoskeleton (J.B) Flashcards
What are the requirements for the cytoskeleton?
- Strong
- Very flexible/dynamic
What are the three core cytoskeletal filament proteins?
Why is a subunit based system?
- Allow for quick decisions to take place –> disassembly here and assembly on the other side.
Outline the structure of intermediate filaments.
Key feature –> fibrous/rope like property –> due to rope like units that wrap around each other.
These subunits are not polar –> no plus or minus end.
- They have a degree of dynamism but not as much as the others.
Where would you normally find a lot of intermediate filaments?
- Tissue that needs a lot of mechanical strength –> keratin filaments in the epithelial cells (skin).
- Nucleus –> find them in lamins of the nuclear envelope.
- Axonal –> neuronal strength
Outline the structure of actin filaments.
- Small monomer
- Four lobed square
- Plus end and minus end
- ATP binding site in the middle –> Binds to ATP hydrolyzes to ADP.
Outline the structure of tubulin microtubules.
- Dimer made of alpha and beta subunits
- Binds to GTP –> GTP hydrolysed to GDP.
- Forms more complicated microtubules –> structure with lumen in the middle –> tubule is thicker than actin filaments.
Where do actin filaments and alpha/beta-tubulin obtain energy for their dynamic activity?
- GTP is in the beta subunit –> alpha subunit is locked
Do they filaments and microtubules have polarity?
Yes, they have polarity –> plus (fast-growing) and a minus (slow growing) ends.
Outline the principle of polymerisation.
T form –> ATP/GTP bound –> charged state –> tendency for growth/polymerisation.
Hydrolysis from T to D
D form –> ADP/GDP bound –> tendency to shrink as it is less stable.
Allows for growth and disassembly.
Technically –> both ends can shrink or grow BUT the plus end has a greater affinity for growth and the minus end has a greater affinity for loss.
I.e –> if the filament needs to shrink –> minus end shrinks faster than the plus end.
What are the different phases of a dynamic filament?
- Nucleation phase (lag phase)
- Elongation (growth phase)
- Steady state (equilibrium phase)
What happens in the nucleation phase?
- Subunits Pool –> waiting for enough subunits to form a nucleus (platform for growth)
What happens in the elongation phase?
Growth –> subunits added
What happens in the steady-state phase?
The rate of gain equal rate of loss –> balance out –> occurs at the critical concentration.
Note!
There are two critical concentrations –> one for the plus end and the other for the minus end.
Minus end has a critical concentration where growth equals loss.
Plus end has a critical concentration where growth equals loss.
Explain the concept of treadmilling.
Note –> Actin filaments follow this process perfectly but microtubules are slightly different.
Remember both the plus and the minus ends have different critical concentrations.
Plus end has a much lower critical concentration (C.C) (i.e. 0.04 micromolar) –> has an affinity for growth –> when you go above the C.C –> plus end grows.
Minus end has a much higher C.C (i.e 0.18 micromolar) –> has an affinity for loss –> when you go below the C.C –> minus end shrinks.
So between the two C.C (0.04 and 0.18) –> the plus end will grow whereas the minus end will shrink –> this creates a treadmilling effect.
Note the growth/loss also depends on the state of the nucleotide (ATP/ADP or GTP/GDP).
What is dynamic instability in microtubules?
Remember hydrolysis if GTP to GDP (or ATP) is random —> stochastic process –> filaments age –> they more likely to have the hydrolysed form.
GTP cap present on growing end –> growth
However, when GDP is present at the tip –> increased instability –> rapid loss –> known as dynamic instability.
Why is it important for cells to be dynamic?
What are other names for the plus and minus end of the actin filament?
Plus –> barbed end
Minus –> pointed
When having filament what are the relative differences between ATP acting gain and loss on the plus and minus end.
Plus end –> Rate of addition –> 12 per second –> rate of loss 1.4 per second –> this means that the rate of addition is a lot higher than loss –> implying the plus end grows when in a pool of ATP Actin.
Minus end –> Rate of addition –> 1.3 per second –> rate of loss 0.8 per second –> this means that the rate of addition is slightly higher than loss –> implying the minus end grows when in a pool of ATP Actin.
This is counterintuitive as this would mean that both ends always grow –> This is because we haven’t taken into the consideration ATP hydrolysis to ADP.
When having actin filament what are the relative differences between ADP acting gain and loss on the plus and minus end.
Note –> the exact number aren’t important (numbers change) –> relative difference is.
Plus end –> rate of addition of ADP is 4 per second –> rate of loss of ADP is 8 per second –> this means that an ADP monomer is more likely to be lost from the end (2 times more likely).
Minus end – > rate of addition of ADP is 0.1 per second – > rate of loss of ADP is 0.3 per second –> this means that an ADP monomer is much more likely to be lost from the minus end than gained (3 times more likely)
What is the effect of ATP being preferentially gained and ADP being preferentially lost have on the actin filament?
- ATP is added to both ends
- However, over time the filament will hydrolyse ATP to ADP (random process)
- Since ATP is gained at the plus end continuously at a fast rate –> there will be an accumulation of ADP monomers present on the minus end –> ADP molecules are preferentially lost.
Results in this treadmilling effect –> preferential gain at plus end and preferential loss at the minus end.
Summarise the treadmilling effect.
Why is the critical concentration for the addition of ATP actin different between the plus and minus end?
This is due to the different rates of addition.
When the rate of addition is naturally higher –> need a lower critical concentration.
When the rate of addition is naturally lower –> need a lower critical concentration.
What is a problem that is caused by the lag phase?
