Unit 8 - Cytoskeleton

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INTRODUCTION Unit 8 - Cytoskeleton
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3 types of fibers make up the cytoskeleton
                                                                                                                                  
Intermediate Filaments   Microtubules Actin Microfilaments

• Microtubules interact with motor systems
  dyneins and kinesins
• Microfilaments interact with myosins
• Intermediate filaments do not interact with motor
  proteins

 
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Intermediate Filaments
FUNCTION provide a supporting framework within
the cell
These supportive fibers provide for mechanical
reinforcement of tissues and range in diameter
from 8-10 nm
 
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Intermediate Filaments - STRUCTURE
monomer
dimer
Have great tensile strength! Strengthens cells
against mechanical stress!
tetramer
2 tetramers coiled together
8 tetramers twisted into a ropelike filament
10nm
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Types of intermediate filaments

• nuclear lamins form a meshwork that stabilizes
  the inner membrane of the nuclear envelope
• keratins junctions in epithelial cells
  (desmosomes) and also form hair and nails
• neurofilaments - strengthen the long axons of
  neurons

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What cytoskeletal element is stained green in
this immunofluorescence experiment? Why?
Intermediate filaments. They form a network that
links to other cells at junctions on the plasma
membrane
Cell boundaries stained in blue
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Intermediate Filaments

• Why can you use intermediate filaments to tell
  whether a cancer has spread in the body?

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Microtubules
FUNCTION These macromolecular assemblies are
involved mainly in the movement and positioning
of cell organelles.
- Minus end is attached to centrosome (or
Microtubule organization Center) - Plus end is
free
 
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Microtubules - STRUCTURE
Long, hollow cylinders made of 13 protofilaments
and built by the assembly of dimers of alpha
tubulin and beta tubulin.
Right. Electron Microscope Image of microtubules
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MICROTUBULES
Rapid Growth plus end
- The growing end of the microtubule has subunits
arranged with the beta-tubulin on the outside.
The subunits in the microtubule all show a
uniform polarity
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Microtubule polymerization

• Tubulin dimers GTP (red) bind more tightly to
  one another than tubulin dimers GDP (dark
  green) creating a GTP cap.

• microtubules keep growing with freshly added
  tubulin dimers and GTP

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Microtubule depolymerization
- Slow microtubule growth causes the "GTP cap to
hydrolyze GTP to GDP The GTP cap is lost

• the GDP-carrying subunits are less tightly bound
  in the polymer and readily released from the free
  end

The microtubule shrinks
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Centrosomes

• Organized array of microtubules that radiate
  outward from it through the cytoplasm.

• Contains ring-shaped structures of g-tubulin.
• Serves as starting point for growth of new
  microtubule

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Motor Proteins

• Dyneins
• Travel towards MINUS END
• Kinesins
• Travel towards PLUS END

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-


Kinesins
Dyneins
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Actin microfilaments
FUNCTION support components of the cell and
provide for the movement of cytoplasm and the
cell surface
Microfilaments Interact with many types of
molecules including its own class of motor
proteins, the myosins
 
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Bundles of Actin filaments in cells
Actin in RED
Sheet-like and finger-like protrusions from the
leading edge of a moving cell
Contractile ring during cell division
Microvilli
Contractile bundles in cytoplasm
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Actin - STRUCTURE

• Actin microfilaments are comprised of actin
  monomers that polymerize to form long, thin
  fibers. These are about 7nm in diameter.

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Actin Polymerization

• Can occur at either end
• Faster rate at PLUS end

• Triphosphate (ie ATP) hydrolyzed to diphosphate
  (ie ADP) soon after incorporation into actin
  filament.
• Nucleotide hydrolysis promotes depolymerization

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ATP Hydrolysis

• Actin monomers in the cytosol carry ATP, which is
  hydrolyzed to ADP soon after assembly into
  growing filament.
• ADP molecules remain trapped within actin
  filament, until the actin monomer that carries
  them dissociates from filament to form a monomer
  again.

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Drug Treatments

• Colchicine Binds free tubulin and inhibits
  formation of microtubules by preventing
  polymerization
• Taxol Stabilizes microtubules by preventing
  depolymerization
• Cytochalasin inhibits formation of actin by
  preventing polymerization
• Phalloidin stabilizes actin filaments preventing
  depolymerization

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Practice Questions
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Here we see Phalloidin linked to a green
fluorescent dye. What cytoskeletal element is
being stained green? How do you know?
Nucleus stained blue Golgi stained red
What technique was used?
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What cytoskeletal element is indicated by the
arrow? What are TWO other functions of this
element?
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