Cytoskleton Flashcards
What is the cytoskeleton?
- a network of proteinaceous filaments found in cells
- important in intracellular transport, cells division, cell motility, cell organisation and polarity
What are the different cytoskeletal components?
microtubules (tubulin), intermediate filaments (IF) and microfilaments (actin filaments)
What is the actin protein?
- one of the most abundant proteins
- binds ATP/ADP
- makes up microfilaments
How does actin generate force?
Through polymerisation and protein/membrane interactions
- needs ATP/ADP and Mg
What are the 3 isoforms of actin and where can they be found?
α - striated muscle
β - found in all cells
γ - smooth muscle
What are the 2 structures actin can form?
G actin (globular) and F actin (filamentous)
Which actin structure has directionality/ polarity?
Actin Filaments
How do actin filaments grow?
- actin growth occurs on the barbed end - the plus end
- ATP hydrolysis means that the negative end shrinks, actin falls off
- Recycling purpose
- actin tread milling
- Stays the same length but moves to the right direction
- actin filament length remains the same even though there is a net flux of monomers through the filament
Why do actin filaments grow faster at the plus end than the minus end?
One end will have more ATP’s as ATP likes binding to the positive end
- ATP does not like to bind to the negative end
- degradation of ATP and ATP hydrolysis occurs in the negative end
- ATP hydrolysis results in a conformational change that reduces affinity for neighbouring monomers
- affinity lowers in the negative end
What regulates the polymerisation, length and organisation of microfilaments?
Actin-binding proteins
Name actin monomer binding proteins that regulate filament formations and what they do.
Thymosin:
- ensures a pool of free G-actin
- prevents binding so there is no plus-end growth
Profilin:
- increases affinity of free actin for the filament
- promotes growth and assembly an nucleotide exchange (ADP->ATP)
Name some nucleating proteins that promote growth of the actin filament.
ARP2/3: - nucleating proteins - needs promoting factor nWASP - creates branches Formin: - produces "fingers" that branch out and bind to G-actin (grabs it) and then binds it to the growing filament
How does actin interact with membranes and organelles?
Through anchoring protein Spectrin
- forms a lattice under the plasma membrane and can be stretched and compressed
- links to actin and membrane proteins
- allows cells to be flexible (useful in rbc)
Which actin binding proteins prevent growth and stabilisation of the filament?
CapZ:
- capping protein
- binds to plus end so addition of subunits is prevented
- slow growth will have to happen in minus end
Tropomodulin
- also capping protein
- binds to minus end preventing loss of subunits
- important in muscle cells
Which actin binding protein helps with depolymerisation and recycling of actin?
Cofilin
- binds to F-actin, promoting ATP to ADP hydrolysis
- binds to G-actin, prevents ADP to ATP exchange
- promotes de-assembling of filaments by chopping them up
What is Cofilin dependent on?
Concentration dependence
- low conc: severs F-actin, promotes depolymersation of minus end
- high conc: increases polymerisation by nucleating new filaments
What are motor proteins?
class of molecular motors that can move along the cytoplasm of animal cells
Name the 3 major classes of motor proteins?
Myosin, dynein, and kinesin
Which motor protein is responsible for vesicle transport?
Kinesin: vesicle transport walking from + to - and from - to +
Dyneins: vesicle transport walking from + to - only
- 7 times faster than Kinesins
What do microtubules do?
- maintenance of cell shape
- help separate chromatins during mitosis
- tracks for intracellular transport
- organelle contacts and organisation
- controls polarised cell migration
What are microtubules made of?
α and β tubulin dimers
- form together to make protofilaments
- laterally associate to form tubulins
- made up of 13 protofilaments
How do microtubules grow?
- similar to actin growth
- treadmilling towards positive side
- dimers reversible attach to both ends of microtubules but 2X faster at positive end
What causes microtubules filaments to curve?
- GTP hydrolysis changes subunit conformation and weakens bond in the polymer
- creating a curved protofilament
How does microtubule shrinking occur?
- if filaments are growing faster than the rate of GTP hydrolysis, GTP cap stops it from growing and is stable
- GTP hydrolysis catches up with the GTP cap
- curving occurs and it peels back away from the microtubule
- shrinking
Where does microtubule shrinking usually occur?
the positive end
What does catastrophe and rescue mean in terms of dynamic instability of microtubules?
catastrophe: - filament is falling apart as there is no addition of monomers - not stable - shrinking rescue: - when the filaments are being rebuild - growth
What is “search and capture” in terms of dynamic instability of microtubules?
- what occurs in mitosis
- allows tubules to explore cell to find interacting partners
- constant growth and movement - catastrophe and rescue happening
Which type of tubulin nucleates filaments at the minus end?
γ tubulin
What are microtubule organising centres (MTOC)?
structure found in eukaryotic cells from which microtubules emerge
- pair of centrioles inside
- spherical centrosome matrix
- γ tubulin ring complexes are nucleating sites that microtubules grow from, anchoring from the minus end
What is the microtubule organising centre called for cilia and flagella?
basal bodies
Which proteins can promote rescue and catastrophe for microtubules?
TIPs - plus end tracking proteins
EB1:
- promotes rescue
- Binds to and interacts to the plus end
- Helps to promote the binding of new monomers
- Interacts surrounding proteins
- Can change the microtubules behaviour in many ways
MCAK:
- promotes catastrophe
- Uses ATP hydrolysis to speed up the depolymerisation of the microtubules
- Linked to kinesin complex
- Actual disassembly of the molecule helps it to pull
What doe microtubule associated proteins (MAPs) do?
- can stabilise microtubules
- Provide the “highway” for the movement of products
- If there is a defect it causes neurodegenerative diseases
- Provide overall structure
- e.g. Tau stabilised filaments in neurons
Name a severing protein and explain what they do.
Katanin
- cuts at specific locations
- uses ATP hydrolysis
- forms rings which stick to the side and destabilises structures between components
- promote tubule longevity
What is hyperglutamylation?
Adding too much glutamate to tubulin components
Why is hyperglutamylation dangerous?
- can contribute to neurodegenrative diseases in absense of deglutamylases
- neuron loss and astrogliosis
- axonal swellings
- reduced mitochondria traffic
- impairs movement
- triggers initiation of spastin causing falling apart of cell
How do microtubules contribute to cell migration?
- mictrotubule-driven intracellular transport towards the front of the migrating cell
- regulation of actin cytoskeleton assembly at the leading edge of a migrating cell
- delivery of factors which stabilise the rear of the cell to maintain directional migration
- targeting of focal adhesions for disassembly to allow rear retraction
What do integrins do?
allow the ability to grab onto components
- pick up the back and moved to the front so they can be engaged with other components
- can be endocytosed
- can move motor proteins to the location needed to speed up processes
- directionality
What specialised structures can microtubules form?
Cilia and flagella
- can form dimers
- interactions between dynein and MT often seen such as in airway moving mucus
What is the function of intermediate filaments?
6 different classes provide mechanical support for the plasma membrane - forms a stress network - forms specialised structures such as keratin - hair and ocular lens (class VI)
How are intermediate filaments formed?
- helical bundles that coil themselves into dimers
- laterally polymerise to form unit length filaments
- unit length is approx 60nm long
- average diameter (10 nm)
- non-polar
How are keratin proteins formed?
class 1 and class 2 intermediate filaments form hetreodimers with each other
What is keratin responsible for forming?
Specialised structures such as hair and nails
- associated to larger structures
- bundles are packaged and excreted from cells
“simple” keratins on the epidermis layer can resist force and absorb energy by forming a stress network
- interact with desmosomes and provide mechanical strength
What happens if there is a mutation in Keratin 14?
Keratin 14: mutation causes cells not being able to hold on to the interactions in the layers and the top layer of skin starts to come off
