BSCI Today; 4/12/01
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    In this lecture we reviewed Table 22-1 and Figures 22 - 1, 2, 3, 4, 5, 6, 7, 10, 15, 16, 17, 20, and 22.


    Cytoskeleton
    Chapter 22
  • Network of interconnected filaments and tubules throughout the cytoplasm
  • Involved in cell division
  • Positions and moves organelles
  • May be site of attachment and localization for some enzymes
  • Cell motility
  • Cell-cell adhesion


    • Microtubules
  • Hollow tube with 13 protofilaments
  • 25 nm outside diameter
  • Composed of a and b tubulin monomers
  • Functions:
    •     -Chromosome movements
        -Position and move organelles
        -Animal cell shape
        -Axonemal motility


    Microfilaments

  • 2 intertwined chains of F (filamentous) actin
  • 7nm diameter
  • Made of G (globular) actin monomers
  • Movement:
    •     -Muscle
        -Amoeboid movement
        -Cytoplasmic streaming
        -Cell division


    Intermediate Filaments

  • 8 protofilaments joined end to end
  • 8 - 12 nm
  • Made of many different proteins
    •     -Note structure - function relationships
  • Cell shape, structural support, keeps muscle fibers in alignment



    • Drugs Used to Study the Cytoskeleton
  • Colchicine
    •     -From the Crocus, inhibits mtubule assembly and promotes disassembly
  • Taxol
    •     -From the Yew tree, stabilizes mtubules and promotes formation of new ones
  • Cytochalasin D
    •     -Fungal product, inhibit polymerization of actin mfilaments (MFs)
  • Phaloidin
    •     -From the Death Cap mushroom Amanita phalloides
        -Blocks depolymerization of mfilaments (MFs)


    Microtubules
  • Axonemal - cellular movement
    •     -Cilia , flagella, basal bodies
        -Central shaft (axoneme)
        -Very stable
  • Cytoplasmicmtubules
    •     -Cell scaffolding


    Microtubular structure: Tubulins

  • 4 -5 nm in diameter
  • ~ 50,000 MW
  • Three domains:
    •     -GTP binding at N terminus
        -Middle domain (binds colchcine)
        -C terminus binds microtubule associated proteins (MAPs)
  • Isotypes are present


    • mtubule polymerization
  • Favored by:
    •     -High [GTP]
          -Warm temperatures (~ 37o C)
        -High [Mg++]
        -High [tubulin dimers]




    GTP/GDP & mtubules
  • Each heterodimer binds 2 GTP molecules
    •     -GTP may be hydrolyzed to GDP
  • Bound GTP necessary for polymerization
  • Dynamic Instability Model
    •     -Shrinking vs. growing populations
        -Depends upon GTP vs. GDP at + end



    MTOC mtubule organizing center
  • Site at which assembly begins and serves as an anchor for mtubules
    •     -Minus end is anchored
  • e.g., centrosome in mitosis


    • MAPs mtubule associate Proteins
  • Motor MAPs
    •     -Kinesisn and dynein
  • Non-Motor MAPs
  • Examples in neurites
    •     -Tau in axons --> tight bundles
    -MAP2 in dendrites --> looser


    Microfilaments
  • Made of G actin monomers
    •     -375 amino acids; MW = 42,000
        -U - shaped molecule with ATP binding site at center 
  • Polymerizes to form F actin
  • Muscle actins ( a - actins) and non-muscle actins (b and g -actins)



    • See Figure 22-15 for actin structure

    MFs and GTP-binding Proteins
  • G proteins induce phosphorylation of inositol phospholipids
    •     -Rac, Rho, Cdc42
  • A number of proteins that bind to + end of MFs also bind to membrane polyphospho-inositides
  • May act to modulate actin cytoskeleton


    • Capping Proteins

  • CapZ is bound to the + end of a MF
  • Stabilizes MF at that length
  • binds to polyphospho-inositides