cell cytoskeleton

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  • cytoskeleton
    • Basics
      • cell's skeleton
      • filamentous structure found throughout the cell
      • protein monomers that assemble into repeat structures
      • dynamic - assembles and disassembles to suit cell
      • 3 types of filament
        • actin
        • microtubules
        • intermediate filament
    • Actin Filaments
      • globular protein actin
      • assembles into double stranded helical polymers
      • concentrated beneath cortex
      • cell shape and motility
    • microtubules
      • globular protein tubulin
      • dimerize and form hollow tubes
      • more rigid
      • attached to microtubule organising centre (centrosome)
        • other ends grow and shrink around this centre
      • positioning organelles, intracellular transport and mitosis
    • intermediate filaments
      • various intermediate proteins, filamentous themselves
      • extended a helical regions that wind together
        • form rope like structure
      • mechanical support of cell
      • lamins nuclear lamina
    • Assembly
      • monomers form end-to-end and side-to-side interactions
      • interactions aren't covalent
        • easier to disassemble/assemble
      • disassembles at a constant rate
      • assembly relative to monomer conc
      • critical concentration of monomers where they're equal
      • cytoskeltal elements will grow until equilibrium is reached
      • auxiliary proteins can affect these processes
      • Confirmational Change
        • actin and microtubules have fast and slow growing ends
        • subunits attach to fast end, confirmational change occurs, readying for next monomer
        • fast end has higher affinity for monomers than slow end
      • Nucleotide Hydrolysis
        • actin monomers carry ATP
        • Actin ATP has high affinity for actin polymer
        • after attachment, ATP forms ADP
        • tubulin carries GTP
    • Accessory Proteins
      • dynamic control of filaments
        • nucleation
          • auxiliary proteins may act as initiator for new filaments
        • assembly
          • proteins which bind monomers change concentration
        • disassembly
          • proteins bind, either stabilise or destabilise filaments
      • link other cellular components to cytoskeleton
      • motor proteins move organelles along filaments
    • Motor Proteins
      • myosins
        • actin based motor proteins
        • myosin II - muscle contraction
        • 2 myosin II chains wind together to form a long tail domain
        • tail domains can interact with other chains
        • form large bipolar thick filaments, hundred of myosin heads
        • Attached - myosin head binds actin filament
        • Release - myosin binds ATP, releases actin
        • Cocked - ATP hydrolysis and confirmational change
        • Grip - reattachment to actin and phosphate release
        • Pull - confirmational change and ADP release
      • Muscle Units
        • sarcomeres arranged in long repeating chains (myofibrils)
        • thin and thick filament areas
        • slide theory
        • don't lose length, sarcomere shortens though
      • Kinesins and Dynesins
        • microtubular motor proteins
        • kinesin slow to fast end
        • dynasin fast to slow
        • most have 2 globular heads that use ATP hydrolysis to function
        • Kinesins
          • Leading head bound to tubulin
          • trailing head bound to ADP
          • confirmational change as head 1 binds to ATP
            • throws head 2 forward
          • head 2 binds tubulin
          • head 2 release ADP; head 1 hydrolyses ATP
          • back to beginning
    • cell motility
      • cell crawling
        • macrophages/neutrophils
        • rearrangement of actin skeleton
        • protrusion
          • actin fibres fom at leading end
        • attachment
          • attaches to surface across cell membrane
        • traction
          • cytoplasm pulled
      • flagella and cilia
      • microvilli
  • concentrated beneath cortex

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