the cytoskeleton
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The Cytoskeleton. Functions Structural scaffold creating and supporting cell shape Framework positioning organelles within cytoplasm Network of molecular “ roads ” for intracellular transport of materials Framework for whole cell movement Framework for cell division. The Cytoskeleton.

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the cytoskeleton
The Cytoskeleton
  • Functions
    • Structural scaffold creating and supporting cell shape
      • Framework positioning organelles within cytoplasm
    • Network of molecular “roads” for intracellular transport of materials
    • Framework for whole cell movement
    • Framework for cell division
the cytoskeleton1
The Cytoskeleton
  • Three major structural components
    • Microtubules
      • Major role: support, intracellular transport
    • Intermediate filaments
      • Major role: mechanical strength to resist physical stresses
    • Microfilaments
      • Major role: muscle contraction, motility
the cytoskeleton2
The Cytoskeleton
  • Microtubules (MTs)
    • Major role
      • Intracellular transport
        • Motor proteins drag cargo along them
      • Structural support
        • Resist compression forces
        • Resist shear (bending) forces
    • Hollow, rigid
    • 25nm diameter, 4nm wall thickness
    • Radiate outward toward plasma membrane

from near nucleus (MTOC)

the cytoskeleton3
The Cytoskeleton

plus-end

  • Microtubules (MTs)
    • Unit = alpha / beta tubulin heterodimer
      • alpha subunit + beta subunit
      • Heterodimer is asymmetric
      • Beta end is called “plus” end
      • Alpha end is called “minus” end
        • Not referring to a charge difference

minus-end

the cytoskeleton4
The Cytoskeleton
  • Microtubules (MTs)
    • alpha / beta (a/b)-tubulin heterodimer
    • Beta subunit is a GTPase
  • Assembly
    • Polymer grows by addition of units at the “plus” end
    • GTP-bound tubulin can add
    • GTP form hydrolyzes to GDP form
    • GDP-bound tubulin cannot add
    • GDP-bound tubulin can release only from “plus” end
    • GDP-bound tubulin cannot release from “minus” end or from central region
the cytoskeleton5
The Cytoskeleton
  • Dynamic instability
    • MTs can assemble/disassemble at different rates in different locations within a single cell
    • Various proteins can bind and stabilize MTs
the cytoskeleton6
The Cytoskeleton
  • Microtubule-associated proteins (MAPs)
    • Form bridges crosslinking adjacent MTs for parallel alignment
    • Increase MT stability
    • Promote assembly
    • Regulated by phosphorylation state

Anti-tubulin antibody stain

the cytoskeleton7
The Cytoskeleton
  • Microtubule organizing centers (MTOCs)
    • GTP-bound a/b-tubulin spontaneously assembles into MTs very slowly
    • GTP-bound a/b-tubulin add to an existing MT very rapidly
    • MTOCs are the nucleation points for MT assembly
      • Centrosome
      • Basal body
the cytoskeleton8
The Cytoskeleton
  • Microtubule organizing centers (MTOCs)
    • Centrosome
      • 2 centrioles at right angles to each other near nucleus
        • Contain gamma-tubulin subunit
        • Nucleate “minus” end of a/b-tubulin
          • Plus-end is oriented outward toward plasma membrane
the cytoskeleton9
The Cytoskeleton
  • Microtubule organizing centers (MTOCs)
    • Basal body
      • Single centriole at the base of cilia and flagella
slide12
Eukaryotic cilia and flagella
    • Hair-like motile organelle projecting from cell surface
    • Covered by plasma membrane
slide13
Eukaryotic cilia and flagella
    • Central protein core is called an “axoneme”
slide14
Eukaryotic cilia and flagella
    • Central protein core is called an “axoneme”
      • Composed of 11 MTs arranged in a “9+2” array
        • 9 outer MTs
        • 2 central MTs
        • Connected by various MAPs
        • Locomotion caused by sliding outer tubules past each other
          • Action of motor proteins (dynein)
the cytoskeleton10
The Cytoskeleton
  • Motor proteins that “walk” on MTs
    • Kinesin gene family
      • Plus-end directed
        • Outward or “anterograde”

transport

    • Dynein gene family
      • Minus-end directed
        • Inward or

“retrograde”

transport

the cytoskeleton11
The Cytoskeleton
  • Kinesins are composed of 2 heavy and 2 light polypeptides
    • Cargo-interaction domain “tail”
      • Different kinesins have different specificities
    • ATPase “head”
      • Binds to MT
      • ATP hydrolysis propels heads forward
      • Highly processive
the cytoskeleton12
The Cytoskeleton
  • Kinesins are composed of 2 heavy and 2 light polypeptides
    • ATPase “head”
      • Binds to MT
      • ATP hydrolysis propels heads forward
      • Highly processive
the cytoskeleton13
The Cytoskeleton
  • Motor proteins that “walk” on MTs
    • Dynein gene family
      • Minus-end directed
        • Inward or “retrograde” transport
      • Very large (1.5MDa)
        • Involved in cilia/flagella movement
the cytoskeleton14
The Cytoskeleton
  • Three major structural components
    • Intermediate filaments (~65 genes)
      • Major role: mechanical strength to resist physical stresses
        • Hemidesmosomes and desmosomes
slide21
Intermediate filaments (IFs)
    • Animal specific
    • Strong, rope-like
slide22
Intermediate filaments (IFs)
    • Animal specific
    • Strong, rope-like
    • Bridged together with other cytoskeletal elements
      • (e.g. plectin crosslinks MTs and IFs)
the cytoskeleton15
The Cytoskeleton
  • Intermediate filaments
    • Composition and assembly
      • Monomers form dimers
      • Dimers form tetramers lacking polarity
      • Tetramers form larger fibers
      • Incorporation into existing filaments not limited to end regions
the cytoskeleton16
The Cytoskeleton
  • Three major structural components
    • Microfilaments (MFs)
      • Major role: muscle contraction, motility
      • Solid, branched
      • 8nm diameter
      • Molecular unit= actin
the cytoskeleton17
The Cytoskeleton
  • Microfilaments (MFs)
    • Actin molecule is asymmetric
      • “plus”-end versus “minus”-end
    • Actin is an ATPase
    • ATP-bound actin can be incorporated into growing MFs
    • plus-end of MFs grows 10x faster than minus-end
    • Higher dissociation rate from minus-end leads to treadmilling
the cytoskeleton18
The Cytoskeleton
  • Microfilaments (MFs)
    • Drugs
      • Cytochalasin D blocks plus-end addition leading to complete MF depolymerization
      • Phalloidin blocks turn-over locking MFs into polymerized state

+ cytochalasin D

the cytoskeleton19
The Cytoskeleton
  • Actin binding proteins

+ cytochalasin D

the cytoskeleton20
The Cytoskeleton
  • Motors that walk on Microfilaments (MFs)
    • Myosin gene family
      • ATPase “head” domain
      • Cargo-interacting “tail” domain
the cytoskeleton21
The Cytoskeleton
  • Motors that walk on Microfilaments (MFs)
    • Myosin gene family
      • Type V can walk on actin filaments carrying a bound cargo
      • Type II forms bipolar filaments via tail - tail interactions
the cytoskeleton22
The Cytoskeleton
  • Myosin type II in muscle contraction
    • Muscle fiber
      • Large cell, 100mm long, 10-100 microns thick
      • Contain >100 nuclei
      • Derived from the fusion of many myoblast cells
    • Myofibrils
      • thin protein strands composed of repeating units called “sarcomeres” that give muscle its “striated” appearance
    • Sarcomere
      • Z, I, A, H and M regions
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