Cytoskeletal filaments

nucleation phase

Actin polymerization

when 3 subunits come together

formin helps this phase

elongation phase

actin polymerization

fast addition of actin monomers to both ends of the short oligomer

steady-state phase

actin polymerization

G-actin monomers exchange with no net growth (treadmilling)

ATP-bound actin have higher affinity for plus end

ATP hydrolysis decreases affinity for other actin proteins

Actin filament stabilization

• tropomyosin

  • bind lengthwise along filament groove

• Capping proteins

  • bind to plus of minus ends to prevent addition + removal of G-actin monomers

actin filament destabilization

• cofilin

  • binds to ADP-bound F-actin + twists the filament to destabilize it

  • new ends for growth

  • skips the nucleation phase

F-actin

• found in the cell cortex

• forms a mesh network

• used to make cell projections

Polymerization of actin

1. Nucleation phase

2. elongation phase

3. steady-state phase

Actin MFs parts

G-actin (globular)

F-actin (filamentuos)

G-actin

• Binds to ATP

• doesn’t provide energy

• higher affinity for binding other actins

F-actin

• Directionality/polarity

• slight twist in strand

Microfilaments (MFs)

• Smallest

• Actin MFs

• Profilin generates ATP-actin

• Myosin MFs

Actin MFs

• Most abundant

• thin + flexible

• Arranged in bundles + networks attached to the plasma membrane

• provide structure

• function in cell motility, mvmt., + contraction

• essential for cytokinesis

• Highly dynamic states

• Branching

• Actin binding proteins determine the function

Dynamic states of Actin

• Polymerization

• stabilization

• dissociation/destabilization

Actin Branching

• Arp2/3 promote formation of a web of branched actin filaments

Actin-based motor protein

• myosin

• walks along actin

• plus end motors

• used to contract muscles

what does myosin consist of

• 2 heavy chains; globular head + long coiled tails

• 2 light chains; help bend head region

• sometimes 2 regulatory light chains

structure of muscle fibers

• repeating contractile units = sarcomeres

components of sarcomeres

• actin filaments bound to Z-line (+ end)

• myosin thick filaments

tropomyosin

1. stabilizes actin filaments but blocks myosin binding site

troponin

1. 3 protein complex that modulates tropomyosin structure

Microtubule motor proteins

kinesins + dyneins

kinesin

• plus end directed

• powered through hydrolysis of ATP and ADP

• movement

  • trailing head = bound to ADP; leading head = unbound

  • Leading head binds to ATP + causing conformational change = swings rear head forward

  • New leading head releases ADP + bind to MT

  • New trailing head hydrolyzes ATP to ADP; weakens interaction w/ MT

dyneins

• minus end directed

• powered through hydrolysis of ATP and ADP

• Walks like a drunken sailor

• found in motile cilia + flagella

• projections for them cell

• Used for moving fluids