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BIOL2200 > MODULE 3: the cytoskeleton > Flashcards

MODULE 3: the cytoskeleton Flashcards

1
Q

microfilaments: actin

stages of actin polymerisation

polarity of actin polymerisation (treadmilling)

A

actin can exsist in two forms:

  • G actin (globular, monomeric)
  • F actin (filaments, polymeric)

stages of actin polymerisation:

1) nucleation
- slowest phase
- assembly of actin monomers to >3 monomer stage (up to 8)
- come together from motion - no assistance
2) elongation
- addition of actin monomer to both ends
- filament begins to form
3) steady state
- addition of monomers to positive end and removal from negative end
- no net change in filament length
- “treadmilling”

actin filaments constantly polymerising/depolymerising

ATP-G actin subunits added to positive end

after binding, ATP hydrolysised to ADP and subunits lost at negative end = treadmilling

treadmilling enhanced by other proteins

1) profillin
- promotes formation of ATP-G actin, providing greater supply for binding at positive end
- enhances assembly
2) cofilin
- destabilises ADP-actin in filaments
- enhancing disassembly at negative end
3) thymosin beta-4
- sequesters away ATP-G actin
- acts as buffer for supply of ATP-G actin at positive end
- prevents assembly

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2
Q

regulation of actin filament assembly

formins, arp2/3

A

nucleation is rate limiting step

regulated by different vlasses of actin nucleating proteins, formins and arp2/3 complex

formins:

  • assemble unbranched filaments
  • formins have FH2 domain that forms a dimer
  • dimer binds 2 actin subunits
  • by rocking back and forth, additional subunits added
  • “ladder”
  • regulated by profilin

arp2/3 complex:

  • nucleate branched filaments
  • binds to F actin
  • needs additional protein, WASp, results in nucleation of actin filaments
  • binds to side of actin filaments and generates branch
  • actin close to plasma membrane often branched to allow more connections
  • regulated by WASp
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3
Q

attachments of actin filaments to plasma membrane

A

in non-muscle cells, major actin filaments anchoring proteins are ezrin and EBP50

in muscle, dystrophin links actin to plamsa membrane (defective = muscle dystrophy)

in muscle cells, need two attachments for contraction

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4
Q

arp2/3 complex in endocytosis

A

WASp promotes arp 2/3 dependent assembly of branched actin filaments

branched actin filaments promote membrance changes and movement into cell

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5
Q

myosins (I, II, IV)

diagram of myosin

neck length and movement

A

all actin filament motors are members of myosin protein family

myosin 1:

  • found in cell periphery
  • involved in endocytosis
  • contains stress fibres, lamelipodium, filipodia (diagram)
  • single head
  • step size = 10-14nm

myosin 2:

  • found in muscle and non-muscle cells
  • required for cytokinesis and focal adhesion
  • involved in contraction
  • two heads
  • step size = 5-10nm

myosin 4:

  • required for organelle transport
  • walks hand over hand
  • two heads, long neck
  • step size = 36nm

myosin head: contains motor, binds ATP, catalytic activity
myosin tail: structural role

increased neck length gives increased velocity of movement

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6
Q

myosin and actin interaction (power stroke)

A

myosin uses ATP to pull on actin

1) myosin head binds to ATP, releases actin
2) head hydrolyses ATP to ADP + Pi, rotates to cocked state
3) cocked state binds actin
4) Pi released, myosin head moves along the filament = power stroke
5) head remains bound to actin while ADP is present. when ADP is exchanged for ATP, myosin head released (cycle restarts)

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7
Q

regulation of myosin and actin interaction

A

power stroke regulated by calcium

release of Ca2+ from SR triggers contraction

re-uptake of Ca2+ into SR relaxes muscle

tropomyosin (TM) and troponin (TN) are accessory molecules bound to actin thin filament

in absence of calcium, TM and TN molecules block interaction of myosin with F actin

in presence of calcium, TN induces TM to move to a new site, exposing myosin-binding sites on actin

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8
Q

myosin I: cell migration

A

1) lammelipodia extension
2) formation of new focal adhesions
3) bulk of cytoplasm flows forward due to contraction at rear of cell
4) detachment of rear

cell migration results form coordinated formationof actin filament bundles and network in cells

form loose attachments that are easily formed and broken to allow movement
too weak: not enough friction for movement
too strong: unable to move

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9
Q

control of actin organisation in cells

diagram

A

small GTP binding proteins cdc42, rac and rho control organisation of actin

rho:

  • stress fibre formation
  • activated at back of cell leading to myosin II activation

rac:

  • lamellipodia formation
  • activated at front leading to arp2/3 activation

cdc42:

  • filopodia formation
  • activation at front
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10
Q

chemotaxis

regulation of chemotaxis

A

chemotaxis = cells sense chemical gradients and move towards them

chemokines = secreted molecules that induce chemotaxis

binding of chemokine to its specific receptor results in intracellular activation of G-proteins, leading to production of its specific lipid PIP3

PIP3 activates rac to change actin configuration within cell

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11
Q

microtubule assembly: MTOCs

MTOC is dividing cells

A

MTOC = microtubule organising centre, specific point where microtubule is assembled

MTOC = centrosome in non-mitotic cell

MTOC anchors to negative end, allows addition to positive end of microtubule

during mitosis, cells reassemble microtubules to form bipolar spindle

spindle assembles form two MTOCs called spindle poles

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12
Q

microtubule dynamics (GDP/GTP)

A

alpha and beta tubulin dimers bind GTP and assemble onto positive ends. have high affinity for GTP –> tightly bound. dimers bound to GTP assemble at positive end.

after dimer is incorperated onto microtubule, GTP hydrolysed to GDP (beta subunit). have low affinity for GTP –> released. dimers bound to GDP fall apart.

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13
Q

microtubule motors: kinesin

kinesin diagram

kinesin cycle

A

kinesin is an ATPase (like myosin)

kinesin transports anterograde i.e. away from MTOC and towards positive end/periphery

kinesin head groups walk along microtubule

adaptor protein “kinectin” binds cargo to kinesin

kinesin cycle:

1) binding of leading head induces loss of ADP. this nucleotide-free head now binds strongly. trailing head binds weakly
2) forward head binds ATP
3) conformational change causes trailing head to move forward
4) new leading head binds tubulin, releases ADP. trailing head hydrolyses ATP to ADP

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14
Q

microtubule motors: dynein

dynein diagram

A

dynein is an ATPase (like myosin)

dynein transports retrograde i.e. away from positive end/periphery and towards MTOC

adaptor protein “dynactin” binds cargo to dynein

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15
Q

cilia and flagella

A

motile structures with microtubule motors

movement based on sliding of filaments

have “9+2” arrangement. 9 doublets of alpha + beta tubulin. two singlets in centre

nexin = attachment between doublets
dynein = inner and outer part of doublet
radial spoke proteins

motor walks towards negative end but is constrained by nexin links. this produces a kink —-> motion

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16
Q

intermediate filaments (IFs)

tissue specific expression of IFs

A

10nm diameter

stable and high tensile strength

no polarity, therefore they are not motors

dimers of subunits assemble into tetramers which further assemble into protofibrils (13 units)

epithelial cells = keratins
mesenchymal cells = vimentin
muscle cells = desmin
neurones = neurofilaments
nuclei = lamens
17
Q

cell interaction with ECM

A

cell-matrix (and cell-cell) adhesions mediated by cell adhesion molecules = CAMs

cell interacts with ECM via specific receptors

integrins = family of receptors including ECM components (fibronectin, collagen)

integrins occur as heterodimers of alpha and beta chains

chains interact to create binding site for ECM

adherens junctions = areas of adhesion between cell and ECM (otherwise called plaques or focal contacts)

mediated by vinculin and talin

18
Q

cell-cell adhesions

adherens junction

A

occluding/tight junctions = control flow of solutes between cells

channel forming junctions (gap junctions) = permit rapid diffusion of small solutes between cells

adherens junctions involve cadherin which bind to eachother in between adjascent cells. calcium dependent interaction

catenin = links cadherin to F-actin
vinculin = links cadherin to F-actin
VASP = binds vinculin to F-actin
alpha-actin = crosslinks actin filaments into bundles
ZO1 = links catenins to actin filaments
19
Q

adhesions in normal vs cancer cells

podosomes and invadopodia

A

podosomes = adhesion structure in cells that do not associate with stress fibres

abundant in cancer cells = invadopodia

incadopodia contain arp2/3m WASp and cortactin

podosomes and invadopodia can break down ECM. secrete metalloproteases which break down ECM

breaking down ECM allows invasion - locally or long distance (metastases)

BIOL2200 (7 decks)

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