MCB Lecture 34 Cytoskeleton I Flashcards
What are the three classes of filament?
Microfilaments / actin
Microtubules
Intermediate filaments
What are some general functions of the cytoskeleton?
Cell shape
Cell locomotion
Resistance to compressive forces
Movement of organelles within a cell
What is an example of intermediate filaments?
Keratin, rope like, very strong
What are the general features of intermediate filaments?
Very strong and rope like
Describe intermediate filament formation
Alpha helix synthesised
Two alpha helices form a coiled coil: dimer
Two dimers interact
Protofilament: four dimers interacting
Many protofilaments line up to form the rope like filament
Describe the polarity of intermediate protofilaments
They are non polar, since the protofilaments line up anti parallel
What are some general features of actin filaments?
They are flexible
Describe the formation of actin filaments
G-actin monomers bind head to tail to form a string
Two strings intertwine to form a protofilament
Many protofilaments are connected by cross-linking proteins
Differentiate between F and G-actin
G-actin are the individual subunits
F-actin is the filament formed
Describe the polarisation of actin filaments
Since the monomers line up head to tail, the filament is polarised, and has a plus and a minus end
What is the function of cross linking proteins inaction filaments?
Forms the assemblies of actin filaments
Describe the function of contractile belts, and which type of cytoskeleton filaments is involved
The contractile belts carry out epithelial folding
Actin and Adherens junctions are involved
Describe the function of migratory processes, and which cytoskeletal filaments are involved
Migratory processes extend out so that the cell can move
Actin is involved
What is the function of the cell cortex?
What are some of the proteins involved?
The cell cortex forms the broad protrusions during cell locomotion (lamellipodium)
Filamin
Spectrin
What is Filamin?
This is a protein that binds to actin filaments in the cell cortex.
It allows lamellipodium formation
What is Spectrin?
This is a protein that binds to actin filaments in RBC, regulating mesh work
What is a lamellipodium? Compare with a Filopodium
This is the broad protrusion, actin is more disordered
Filopodia are smaller protrusions with ordered bundles of actin
Under what conditions will actin polymerisation occur?
Kon > Cc
The concentration of subunits is greater than the critical concentration
Under what conditions will actin depolarisation occur?
Koff < Cc
The concentration of subunits falls below the critical concentration.
The whole filament then disassembles
Describe the function of the plus end
Monomers are added and removed more rapidly from this end
Which two proteins regulate the polymerisation and depolymerisation of F-actin?
Thymosin
Profilin
What is the function of thymosin?
It binds to G-actin, stopping it from binding to the plus and minus ends of the F-actin
It effectively reduces the substrate concentration
What is the function of profilin?
It binds to G-actin subunits, allowing for their rapid addition to the plus end of F-actin
It effectively increases the concentration of actin subunits, increasing the rate of polymerisation
What is nucleation?
This is the formation of a stable nucleus, so that polymerisation can occur
What is the rate limiting step in F-actin polymerisation?
Nucleation
Describe how the rate limiting step of actin polymerisation may be overcome
By adding a preformed nucleus of actin monomers, we don’t wait for it to spontaneously happen, which is very rare and thus slow
Describe the function of the ARP complex
The ARP complex, when activated, binds actin monomers and allows for rapid poly erasion of F-actin
Describe broadly how remodelling of F-actin occurs
There are 100s of proteins that stabilise, promote or inhibit polymerisation, promote or inhibit depolymerisation etc.
What is the classification of motor proteins? How do they relate to the cytoskeleton?
They are proteins, not cytoskeletal elements.
They are relevant because they interact with actin filaments and microtubule tracts
Describe the myosin family
There are many, many types
What is the function of myosin?
It moves actin filaments (in muscle contraction)
Describe how myosin interacts with actin in muscle contraction
- ATP binds, and the head dissociates from actin
- ATP is hydrolysed, head cocks and it binds to actin
- Dissociation of the phosphate triggers the power stroke
- ADP dissociates during the power stroke
Describe the presence of actin within a motile cell (3)
1. Filopodia Very ordered, tight parallel bundle 2. Cell cortex Looser arrangement, gel like network with Filamin protein 3. Stress fibres Contractile bundle
Differentiate between the ways that actin and microtubules area polymerised and stabilised
Actin:
- Arp Allows for network of actin filaments by attaching to filaments
- thymosin
- profilin
Microtubules
- GTP cap: binds to the beta (+) and allows for rapid polymerisation, loss of which causes a catastrophe
- MAP: binds to GTP cap on beta end and stabilises the microtubule
Differentiate between the nucleation of actin and microtubules
Actin:
- ARP complex nucleates actin
Microtubules:
- Gamma-Tubulin- ring complex: nucleation of microtubules, found in controsome
What are the three type of actin assemblies?
Contractile, mesh work, bundle
Where are actin bundle seen?
Filopodia extensions
Contractile ring
Which motor protein is highly involved in cell locomotion?
Specifically, which phase of cell locomotion?
Myosin
In the traction phase
Which protein complex is really important in protrusion during cell locomotion?
The arp complex
Desmin Lamin Vilmentin Glial fibrillary acidic protein and Neurofilament are all examples of what?
Intermediate filament
Profilin competes with … For binding to …, promoting …
Thymosin
Actin monomers (G actin)
Promoting actin filament polymerisation
What is the structure of the monomer that makes up microtubule protofilaments?
It is a Tubulin heterodimer: the two bits making up the dimer are different
What is the role of microtubules in the secretory pathway?
It provides a track along which the vesicles move from the ER to the Golgi, then to the plasma membrane
