Cytoskeleton Flashcards
What are actin filaments?
Helical polymers of the protein actin called f actin
Monomers are g actin
Diameter of 8nm
Organise into 2d networks, 3d gels.
Concentrated in cortex (under plasma membrane)
Dynamic
Microtubules?
2x isoforms –> alpha and beta tubulin - formed in cell and uses dimeric structure to build micro tubules.
Hollow cylinders, 25nm diameter
Centrosome in animals
Growing and shrinking ends
What are intermediate filaments?
Eg kerotin
Not found in all kingdoms
Diameter 10nm
Made of intermediate filament proteins
Can for, mesh work called nuclear lamina underneath nuclear membrane
Can extend across cytoplasm giving mechanical strength
What is nucleation?
Helical polymer is stabilised by being in contact with multiple other monomers.
Dimers (2 monomers) bind weakly and go back to being monomers
Need 3 or more to be him forming a nucleus
How does ATP affect the formation of f-actin?
ATP bound g actin have higher affinity to other monomers, so are more stably integrated into the polymer
However, overtime actin monomers can hydrolyse ATP–> ADP. (-Ve end) and this makes it less stable and f actin begins to fall apart
Stages of formation of actin micro filaments?
Lag - time taken for nucleation
Growth phase - when monomers add to exposed ends of growing filaments causing elongation
Equilibrium phase - reached when growth and disassembly balances
What does arp 2+3 do?
Nucleation protein
Arp 2+3 complex is activated, and binds to monomer (minus end) and brings g actin together to form nucleating complex
Also aids branching, by binding to existing actin micro filaments…. At 70 degrees
What does profilin do?
High affinity for ATP bound g actin.
Can prevent nucleation by binding to all the g actin with ATP, but once initiation happens, quickly nucleatex
Adds g actin to plus end
What’s cofilin/ADF?
It induces additional twist in the actin filament which makes it more easy to break
Attracted to ADP bound parts of f action
Promotes subunit loss at minus end or shorter actin fragments
What are gamma tubulins?
It forms complex for initiation of nucleation for micro tubulins
Gamma tubulin ring complex
Part of centrosome in animal and yeast cells
Joins with association proteins to form structure
Centrosomes?
Form in animal and yeast cells
Diff in plant cells - small dispersed nucleus
What is dynamic instability in micro tubule?
Micro tubules depolymerise 100 times faster from an end containing GDP than GYP.
GTP cal favours growth, and if it’s lost the tubulin falls apart
What does katanin do?
Binds to ATP, pray itself into micro tubules structure and breaks micro tubules into 2
By doing so, it can promote growth in a certain direction
Eg. Plant growth, micro tubules start growing longitudinally once exposed to light. Thanks to katanin
What is cytoskeleton made of?
Actin and micro tubules and intermediate filaments
What is kinesin?
A motor protein.
Motor subunits - KHC - heavy chains - interact with micro tubules - n terminal
Interaction with cargo - c terminal - two light chains KLC
How does Kinesin work?
ATP hydrolysis changes the conformation of the motor domains, so that there is an apparent shift in location
Might step over each other, or trailing… Dunno
Move towards PLUS END
What are kinesin light chains?
They are connected to the ends of the ‘necks’ of the kinesin………………..
They can interact directly or indirectly with cargo
Light chain has TPR and variable c terminal Domain. This domain can specifically interact with proteins on vesicles
How do we look at kinesin behaviour in vitro?
Add kinesin Fluro protein, and ATP.
Working as cargo
USE gold cargo for better res to see steps
What do kinesins do when there’s a obstacle?
Tries to side step
Some fall off
What is dye in
Motor protein
Moves to minus end
Uses ATP to fuel
12 subunits
Donut shaped head - where ATPase happens
Stalk binds to microtubule
What is a dyenin receptor
Dynactin
Protein complex that binds to end of dyenin which enhances interact film between dyenin and cargo
What happens when both kinesin and dyenin bound to a cargo?
Usually kinesin is stronger (5 fold)
But when cAMP level is low, dyenin will win
(Cyclic amp)
What is myosin?
Actin based motor protein
Moves really fast 7micrometers per sec
Light chains bound to motor proteins
Work by hydrolysing ATP
P
How does myosin move?
Takes long steps, head domain binds actin, uses ATP hydrolysis to walk along filament
Walks to plus end
Tail mediates interaction with cargo
How does myosin work in muscles?
Thick filament - myosin
Thin - actin
Head domains stick out from sides of filaments ready to walk along adjacent actin based filaments when muscle is contracted
Myosin head hydrolysis atp, then binds to actin, then then bind ATP, detach from actin
How is cell migration fuelled by actin polymerisation ?
Protein called integrin links the cell to the surface(substrate) and let’s go at the back
Actin polymerisation, and through myosin, it pushes the actin forward
What is the cellulose synthase complex in plant cells?
Cellulose micro fibrils - tensile strength equal to steel
How you’ve laid these down determines how the plant grows… Can only expand in those directions
Cellulose is synthesised by a large protein complex that moves along micro tubules (joined by CS1)
Catalytic activity of this complex moves along micro tubules
Are dyenin in plant cells?
Nah
