Th1L06 Actin

Th1L06 Actin

1. part of the cytoskeleton along with IF and MT

   • Th1L06 Intermediate Filaments
   • Th1L06 Microtubules
   • network of protein filaments that extend throughout the cell that determine:
     • cell shape and polarity
     • tissue structure
     • adhesion
     • cell movement
     • intracellular movement (of vesicles and of chromosomes)
     • essential to anchor cells to each other
     • essential to anchor cells to anchor extracellular matrix at cell junctions

2. structure

   • microfilament
   • composed of actin-binding proteins
     • called F-actin
       • filamentous actin or microfilament
       • polymers of individual actin proteins called G-actin (globular actin)
   • polarised double helix
     • 13 actin subunits for every complete turn
     • 7nm diameter

3. 5% total protein

4. Growth

   • (1) requires ATP to be bound to the actin monomer (G-actin)
     • Not very stable over time
     • ATP eventually hydrolyses to ADP and will depolarise and come off at the negative end
   • very dyanmic
   • monomers can be added and removed from both ends of the polymer
   • G-proteins add more rapidly to (+) end of the filament
     • once incorporated, ATP is hydrolysed to ADP
   • G-protein is removed more rapidly from the (-) end of the filament

5. Function

   • provide support
     • Maintains the shape of cells
       • erythrocytes
     • absorption in the gut by forming an adhesion belt
       • microvilli in gut
     • detect vibration in the cochlea
       • In sterocillia
         • cells are depolarised or hyperpolarised by deflections caused by sound; actin filaments keep them rigid
     • Cell migration
     • modulate polymerisation dynamics and function
       • e.g. myosin, capping proteins, severin, etc.
     • required to hold synaptic vesicles close to the presynaptic membrane

6. Cell motility

   • e.g. migration of neutrophils (WBC) to sites of infection for phagocytosis
   • four events
     • (1) cell pushes out protrusions at the front (leading edge) ○ actin filament polymerisation provides to force of membrane protrusion
     • (2) ○ protrusions adhere to the surface on which the cell is moving through contact junctions; F-actin connects to the focal adhesions to provide a contractile force for the cell
     • (3) the rest of the cell pulls against the anchorage points to drag itself forward
     • (4) actin depolymerises at the rear
   • Lamellipodia
     • Th1L06 Lamellipodia (actin based movement)
     • cytoskeletal protein actin projection on the leading edge of the cell
       • extend and withdraw
       • generated by rapid growth of actin filaments at the cell membrane
       • the (+) end of actin filaments are oriented towards the periphery

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