The Cytoskeleton Flashcards
Name 4 functions of the cytoskeleton
- allows cells to take the stress and strain needed to be flexible
- used for the division of chromosomes and mitosis
- drives organelle movement
- provides support for the plasma membrane
What are the 3 components of the cytoskeleton?
- Actin
- Microtubules
- Intermediate filaments
What characteristics are shared by all 3 components of the cytoskeleton?
- repetitive subunits of a helical arrangement
- held together by non-covalent bonds
What are microtubules composed of?
- Alpha and beta tubulin dimers
- Each binds to one molecule of GTP, however for beta tubulin this can also be GDP
What are the properties of microtubules?
- Long, straight and rigid
- Made from parallel protofilaments with a hollow centre
How do the two ends of a microtubule differ?
- One end is positively charged, the other negatively charged
- The + end grows and shrinks however the - end is always unstable and needs additional proteins to be added
What is the MTOC (microtubule organising point)? Give an example
The central point from which microtubules are nucleated
e.g centrosoles, + end grows out to form the mitotic spindle
Describe the process of dynamic instability
- Microtubule has rapid growth when capped with GTP
- When cap is lost the structure of GDP bound dimers breaks down
- When cap is regained, can start rapid growth again
What behaviour is shown by microtubules in migrating cells?
- Constantly extend towards the leading edge of the cell
- Via motor proteins
- Pattern of dynamic instability known as a growth cone
Can microtubules be modified post translation?
Yes, this affects the binding of proteins and does not stabilise with age
What are the two types of microtubule associated proteins?
Stabilisers (MAD1B, MAD2, tau (alzh), EB1) and motors (dynein, kinesins)
Can link microtubules to actin binding proteins
What is the role of stabiliser protein EB1?
Binds the GTP-tubulin cap in microtubules
What are the 3 types of motor associated proteins, how do they work and how do they differ?
- Myosin, kinesin and dynein
- Bind to polarised filaments and use ATP hydrolysis to move
- Kinesin move towards positive end and dynein moves towards the negative end, also differ in cargo and filaments they bind to
What is the structure of kinesin?
- 2 helical heavy chains which dimerize to form a ‘stalk’
- 2 light chains form the flexible ‘neck’
- tail interacts with cargo
- two globular heads interact with the microtubule and ATP
How is dyenin different to kinesin?
- can have 2-3 heads
- large macromolecular assembly
- needs associated proteins to attach to cargo
- used for beating of cilia and flagella
Describe the structure of cilia/flagella
- 9 doublet microtubules, one complete and one half which are all fused
- dyenin bridges
How do cilia/flagella move?
- Dyenin uses ATP to move
- As sliding of microtubules is prevented by protein barriers this results in bending
What is actin formed from?
Helical polymers of monomers of globular actin (G-actin)
When does G-actin form F-actin (filamentous)?
When is polymarizes
What 3 ways can actin exist?
ATP-actin ADP-pi-actin and ADP-actin
How is an actin filament formed?
When a linear chain of actin forms a protofilament which then wraps around another actin protofilament
What are the 2 ends of an actin filament?
‘pointed’ and ‘barbed’ end with addition at the later and slow dissociation at the prior
What change happens as actin ages?
ATP-actin hydrolyses to ADP-pi-actin from which phosphate is released to form ADP-actin
What are the overall properties of actin?
Flexible forming linear bundles and meshworks
How is cell movement coordinated?
- adhesion of protrusion
- traction via motors
- dissasembly of old cell-substratum contracts
What are the 3 forms of F-actin?
- Meshwork
- Tight parallel bundle
- Contractile bundle
What are the 6 possible roles of actin binding proteins?
- Bind/sequester actin monomers
- Nucleate actin filaments
- Cap/uncap barbed/pointed ends
- Sever F-actin
- Bundle, crosslink, branch or stabilise F-actin
- Anchor F-actin to membrane
Where are intermediate filaments found?
In vertebrates, nematodes and molluscs
What is the structure of intermediate filaments?
Elongated with central alpha helical domain that forms a coil-coil with another molecule in the same direction. Form a staggered tetramer with a rope-like structure
What are the 4 different isoforms of intermediate filaments and their subunits?
- Nuclear - lamins
- Vimentin-like: vimentin, desmin, peripehrin, glial fibrillary acidic protein
- Epithelial - keratins
- Axonal - neurofilaments
These can form further parallel bundles
