L17- the cytoskeleton 2 Flashcards
What does spontaneous polymerisation in microtubules need?
A high concentration of tubulin, more than is usually present in the cytoplasm. So cells nucleate microtubule assembly.
How do cells nucleate microtubule assembly?
Using special structures:
centrosomes and basal bodies
How do microtubules grow from centrosomes?
they grow at their plus ends. Their - ends anchored at the gamma-tubulin ring complexes of the centrosome.
Which filament has dynamic instability and what does this mean?
Microtubules. Each microtubule can switch between growing and shrinking, independently of their neighbours
What causes dynamic instability?
The GTP cap on mictrotubules. GDP tubulin can not polymerise. GTP tubulin CAN polymerise. GTP in the microtubule is gradually hydrolysed to GDP, then stops growing.
Why does the microtubule shrink?
GTP-tubulin dimers bind more tightly to each other than GDP-tubulin dimers. When GTP is hydrolysed faster than microtubule grows, the GTP cap gets smaller until only the GDP-tubulin is left.
This is weak so disassembles, shrinking.
What will happen if you cut the microtubule half way?
The + end will shrink because it’s GDP-bound rather than GTP.
How can microtubules be stabilised?
By binding microtubule-associated proteinss (MAPs) all along the microtubule. e.g. Tau and MAP2.
And by binding the drug Taxol.
What is a less understood way to stabilise microtubules?
By “capture” of the plus ends by proteins at the cell cortex. Capping
How can microtubules be depolymerised (shrunk) experimentally?
By putting cells on ice
Using drugs that free tubulin dimers.
e.g. nocodazole, colcemid, colchicine
Where are actin filaments found?
- Muscle
- Contractile ring in dividing cells
- Stress fibres
- microvilli
- Filopodia and lamellipodia
Which is the thinnest filament?
Actin filament - 7nm diameter
What are actin filaments made up of?
Monomeric actin
Describe the structure of the actin filament and what it binds to?
Has a plus and minus end. Monomers add on tot he plus end and disassembly happens at the minus end. Only actin monomers bound to ATP can join on.
Hydrolysis ATP-> ADP, then monomer leaves on minus end.
What is the difference between the growth and disassembly in actin filaments and microtubules?
In actin, monomers join on the plus end and detach on the minus end once the ATP is hydrolysed.
In microtubules growth and shrinking (almost) all happens on the plus end.
