Cytoskeleton Flashcards
1
Q
3 core filament proteins
A
- intermediate filaments: mechanical strength, less dynamism, non polar
- actin filaments: dynamic, strong, cell shape/movement, polar
- thick microtubule filaments: dynamic, strong, ‘traffic highways’ in cell
- filaments are stacked units bound with non covalent forces
2
Q
Why are subunit based filaments used in the cell?
A
Subunit based filaments are used for their ability to rapidly diffuse in the cell and their modular nature gives strength/adaptability
3
Q
Intermediate filaments
A
- 8 tetramers twisted into a rope like filament
- no polarity (ie. directionaltiy)
- lateral hydrophobic interactoins
- flexible and hard to break but less dynamic
- example: keratin filaments help developing cells remember where they came from
4
Q
Actin Filaments
A
- square molecule with four lobes and a +/- end
- cleft of - end binds to ATP
- binds and hydrolyzes ATP to change its properties
- helical filament
- adaptable subunit
- polymerization requires energy
- cell movement/cell surface shape
5
Q
Tubulin
A
- a/B subunits
- a tubulin locked with GTP
- long chains that laterally interact and form large units
- polymerization requires energy
- hollow lumen interior
- end near membrane, - end near center
- organelle positioning and cargo transport
6
Q
Dynamic Filaments
A
- filaments have fast (+) and slow (-) growing ends
- as we add on one end we lose on the other so the filament stays the same length but moves along
7
Q
Phases of Dynamic Filament
A
- for a new filament to form, subunits must initially assemble into a nucleus that then elongates
- this nucleation is the RDS for growth
- critical concentration is the concentration of free subunits left in solution at the steady state (equilibrium) point
- Each filament end has its own critical concentration
- At the critical concentration the growth rate = loss rate of subunits
8
Q
Microtubule Treadmilling
A
- GDP depolymerizes 100 times faster than GTP
- GTP cap favors growth of filament but if lost rapid depolymerization (catastrophe event)
- microtubules often undergo this catastrophe = dynamic instability
9
Q
Why be dynamic?
A
- cells continually test their environment and need to recognise where structure is needed
- dynamism allows rapid change and adaptation
10
Q
Stoichastic
A
Not predictable when something will happen but it will happen
- this is the case with ATP hydrolysis by actin
- conformational change in actin monomer changes its affinity for other monomers
- ADP actin is preferentially lost from both ends
- ADP accumulation at - end where it is lost
- loss at - end anyway, this is just accelerated loss
11
Q
Phases of Actin Filament Formation
A
- nucleation (lag phase)
- elongation (growth phase)
- steady state (equilibrium phase)
Spontaneous nucleation of new filaments from monomers is too slow to rely on so nucleators facilitate nucleation
12
Q
Nucleators
A
- facilitate localization and timing of filament formation
- actin and microtubules have their own specific class of nucleators
- ARP2/3 complex = branched filaments
- Formins = elongated filaments
13
Q
Arp 2/3 Actin Nucleation
A
- branched
- Arp 2/3 complex is a stable multisubunit assembly of 2 actin related proteins and 5 novel proteins
- binds to the side of actin filaments creating branches at the + ends (70 degree angle)
- these are NPF (nuclear promoting factors)
- (-) end nucleation?
14
Q
Formins
A
- unbranched
- FH2 are donut shaped ring around barbed end, recruits 2 actin monomers and grows filaments by adding subunits to the barbed end
- FH1 are proline rich regions enhancing filament elongation by recruiting profiliin actin complexes to the FH2 domain
15
Q
Microtubule Nucleation
A
- nucleated from a specialised complex called a microtubule organising center (MTOC)
- MTOCs nucleate filaments from their - ends
- end nucleation complex is composed of a y tubulin ring complex (y-TuRC)
- y-TuRC is a template for 13 protofilaments
16
Q
Centrosomes/centrioles
A
- centrosomes are single MTOC containing >50 y-TuRC copies
- embedded in the centrosome is a cylindrical L shaped dimer structure comprising 2 centrioles
- fungi/plants have the spindle pole body instead
- centrosome acts as a cellular GPS
17
Q
3 classes of motor proteins
A
- all depend on ATP energy
- motor proteins depend on ATP hydrolysis to cause conformational changes in protein shape
- change in shape causes change in force
- myosins = actin filament motors
- kinesins = microtubule motors
- dyneins = microtubule motors
18
Q
Motor Principles
A
- ATP hydrolysis energy is used to stretch an elastic element in the motor, leading to a conformational change
- if the motor is anchored and the elastic force exceeds the resistance the track moves
- if the track is anchored and the elastic force exceeds the resistance the motor and any attached cargo will more (trafficking)
19
Q
Myosin
A
- globular head coiled-coil tail domain
- head generates power stroke and tail dimerization
- light chains associate with neck to stabilize head for power stroke
20
Q
Myosin in Muscle
A
- in muscle cells the tails link to form bipolar thick filaments
- 100s of myosin heads lined up opposing actin filaments
- created organised structure called a sarcomere = muscle contraction
- individual myosin motor heads hydrolyse ATP driving muscle contraction in response to rise in intracellular calcium ion
21
Q
How is force produced?
A
- myosin ATP hydrolysis causes a structural change
- change causes swing of myosin lever arm which is stabilised by 2 light chains in the locked position
- cocked head binds actin and released phosphate, swinging heads
- displacement of attached actin filament (power stroke)
- changes in head conformation coupled to changes in actin binding affinity
- ADP loss resets cycle (rigor state)
- full cycle of myosin structural change is the ‘duty cycle’
22
Q
Kinesins
A
- ‘superhighway’ transporters
- 2 heavy chains with 2 heads and an a helical coiled coil tail with a light chain bound to the ends of each heavy chain
- cargo binds to tail but mechanism is poorly understood
- core of catalytic domain is folded but small
- kinesins are also ATPases but bind microtubules
- ATPase cycle is similar to muscle myosin but all intermediates have a reasonable affinity for microtubules
- 2 heads ‘step’ along
- at least one head is bound at all times, kinesins moves processing towards the + end
- movement is produced by folding/unfolding of a neck linker segment of the heavy chain connecting the head to the tail
- binding of one head influences binding of another
- lagging head is firmly bound to ATP and forward head is loosely bound
- ADP in forward head changes to ATP and displaces near head
- near head hydrolyses ATP and freely moves to a new step in ADP bound state to restart
23
Q
Dyneins
A
- 2/3 large heavy chains and several tail associated chains
- cargo binds to the tail in cooperation with the dynactin complex
- catalytic domain is barrel shaped ATPase of 6 domains
- coiled coil stalk protrudes from the catalytic domain and binds a microtubule
- force production not fully understood (drunken sailor walk)
- dynactin complex links cytoplasmic dynein isoforms to membrane cargo for transport to - end of microtubules
24
Q
Two Types of Kinesins
A
Kinesin 1 = homodimer with N terminal catalytic domain transports organelles to the + end
Kinesin 14 = homodimer with C terminal catalytic domain moves towards the - end of microtubules
25
Titin
- massive protein spanning Z to M line in sarcomere
- can increase in length under force and recoil to original length when force is removed
- molecular spring responsible for passive elasticity of muscle
26
Myosin Muscle Motors
- cross bridge cycle regulated by calcium ions released from the sarcoplasmic reticulum
- in resting muscle, myosin binding sites an actin monomers are blocked by tropomyosin
- tropomyosin held in place by troponin (3 protein complex, one of which binds calcium ions)
- ion binding to troponin induces a conformational change transmitted to tropomyosin
- tropomyosin shifts and exposes myosin binding sites
- number of active cross bridges is a function of calcium ion concentration
- calcium ion drops after release as it is pumped out by calcium ATPases
- falling concentration = fewer myosin binding sites = fewere cross bridges = relaxation of muscle
27
Comet Tails
- viral and bacterial pathogens have adapted mechanisms that tap into actin nucleation and branching to move within and long cells
eg. listeria actively induces the polymerization actin filaments to drive cytoplasmic movement of bacteria
28
Microtubules and Cancer
- Taxol prevents microtubule disassembly by stabilizing them so stops cell division
29
Monomer Binding Proteins
- soluble concentration of monomeric actin in a cell is often much higher than the critical concentration
- monomer binding proteins sequester actin from forming new filaments
eg. profilin delivers actin monomers to growing filaments via formins
filament binding proteins link together filament in bundles of gel like networks to give the cell different properties in terms of mechanical properties of cytoplasm/plasma membrane
30
Filament Cross Linking
Fimbrin: small proteins with 2 actin binding domains. bundles filaments tightly in microvilli (parallel bundle with actin and myosin can't enter)
a-actinin: homodimeric rods with an actin binding domain at each end (contractile bundle with actin and myosin can enter)
31
Filament Severing Proteins
- ADF/Cofilin actin binding proteins
- multifunctional so can bind or sever filaments
- phosphorylation regulation allows cell control
32
Microtubule Binding Proteins
- microtubule stabilizing MAPs like Tau/MAP related proteins have multiple tubulin binding sites and bind along protofilaments to stabilise them
- reduce catastrophe frequency so microtubules are longer and less dynamic
33
Actin
- each subunit carries ATP/ADP and there are 3 isoforms : a/B/y
- subunits assembly head to tail to form a tight right handed helix of filamentous actin
- persistance length: minimum length at which random thermal fluctuations are likely to cause it to bend
- kinetic rate constants for actin addition and dissociation are greater on the + end but the affinity for actin monors is the same as the - end
34
Actin Critical Concentration
When the nucleotide is hydrolyzed the energy released is stored in the polymer so the free energy change for dissociation of a subunit is more negative for an ADP polymer
- critical concentration of ADP actin is greater than Cc of ATP actin so at certain concentrations of free subunits the D polymers shrink and T polymers grow
35
Cell Polarization/Migration
cell migration depends on membrane protrusion and attachment/traction
- Actin polymerization drives protrustion
- Blebbing = plasma membrane detaches locally from the underlying actin cortex so cytoplasmic flow pushes membrane
- Lamellipodia/Fillopodia are different protrusive structures
