Chapter 9- the cytoskeleton

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Chapter 9- the cytoskeleton
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Where were going

• Framing the problem- cells arent just bags!
• Three types of cytoskeletal components
• Cilia movement
• Muscle movement
• Amoeboid movement

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Microtubules

• Microtubules
• I. Introduction Long, hollow cylinders, 25 nm
  in diameter, made of tubulin. The basic subunit
  is a heterodimer of a and ß tubulin (9.8) 13
  protofilaments in a typical cylinder. See below
  about GTP binding, treadmilling, growth and
  dynamic instability (9.26). There is a end,
  fast growing, w/ß tubulin at its end, and a
  end, slow growing, w/a tubulin at its end. The
  GTPs are important in assembly (9.8)
• A. They have MAPs, that influence their use-
  linking them together, stabilizing them, or
  destabilizing them.
• B. They form a network, coming from the
  microtubule organizing center, which is usually
  the centrosome or centriole, w/ the end
  anchored there. (9.10-13, 19)
• C. Also form cilia and flagella, and spindle
  fibers in mitosis.

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13 protofilaments
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Nucleation

• Gamma tubulin in MTOC/centriole- MTs grow from
  there

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Like MTOC/ cenriole!
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MTs are a highway- bringing things out and back
from the center of the cell.
http//www.mpasmb-hamburg.mpg.de/ktdock/
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• F. MICROTUBULE DYNAMICS 9.25
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• http//www.ncbi.nlm.nih.gov/books/bv.fcgi?ridmboc
  4.box.2966
• Key points- the cap means that subunits are added
  easily- loss of GTP harder to add subunits,
  need higher subunit conc. to add.
• Produces microtubule catastrophes!

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W/ cap- slowly growing w/o- rapidly shrinking-
MT catastrophe!
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Dynamic instability!
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Remodeling

• The fact that MTs arent fixed means that cells
  can remodel their shape- plant cells, our cells
  in mitosis- round up, as MTs used to make
  spindle fibers

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MT drugs

• Colchicine- prevents MT formation- arrests cells
  at metaphase
• Taxol-Stabilizes MTs
• Useful in determining role of MTs in a process

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Cilia action

• Cilia short, many
• Flagella long, few NOT the same as the
  bacterial flagellum!! (the bacterial flagellum is
  WAY cooler!)

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92 nexin, radial spokes, dynein
A,B
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Slide-to-bend model for movement.
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The different sides of the cilium may slide,
depending on the direction of sliding.
These sliding more
These sliding more
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Regulation

• Beats 10-40X /sec!
• They beat together!
• The central pair may rotate, which send the
  signal (and simply moves the problem to how these
  rotate)
• Weird paramecium surgery experiments

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Intermediate filaments

• 10 nm in diameter
• Only in animals! (??plant/fungal nucleus??)
• Variety of types- 60 genes!
• Seem to be involved in providing strength to
  cells.
• Able to interact with both MT's and
  microfilaments (actin filaments).

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Octamers of Tetramers make up the structure. No
polarity! Subunits are filamentous, rather
than globular.
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Some types to remember

• Keratin- epithelial cells, hair, nails
• Neurofilaments- in, well, nerves
• Lamins- lines the nucleus

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When they are mutant

• Smaller nerve fibers- a natural mutant quail!
• Fragile skin
• Sometimes muscle weakness
• Sometimes nothing!

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Microfilaments (Actin)

• Where were going
• Basic structure, polarity, treadmilling
• Its good buddy, myosin, w/ all its types
• Muscle contraction
• Amoeboid movement

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Minus end
ATP binding cleft
Domains 1-4
Subunits G actin-bound w/ATP F-actin
microfilaments
Looks like a double helix!
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S1 is a myosin fragment that binds to actin- the
points point to the minus end
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Treadmilling-its easier to add to the than
end at any concentration, and at some
concentrations its adding at the end at the
rate its coming off the end treadmilling.
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Poisons!

• CytochalasinB-depolymerizes
• Phalloidin- stabilizes. The Amanita mushroom has
  TWO nasty toxins that cell biologists like. Well
  meet alpha amanitin later.

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Heeelp! Ive been poisoned! w/ cytochcalasin!
These are sea urchin cells. The projections are
dissolving
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Myosin- Actins good buddy

• CONVENTIONAL This is the muscle actin- aka Type
  II
• UNCONVENTIONAL These are the rest- types I
  III- XV (named by the same people who named the
  Super Bowl)
• Conventional BIG molecule- ½ million MW! Looks
  like two golf clubs wrapped around each other.
  There are also two types of light chains (9.48)
  The tails are able to aggregate, producing a
  bipolar filament (9.50)
• The unconventional myosins do things in ordinary
  cells some are motors, hauling vesicles along
  actin filaments. If you believe 9.53, the MTs
  act as main highways, and the actin as side roads
  at the ends of the cell.

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Myosin I, hauling a vesicle
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Microtubules interstate actin side roads
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Muscle Contraction

• Three types of muscle fibers
• Skeletal, striated, voluntary
• Heart- more like skeletal, but not
  multinucleated. Its structure allows the
  propagation of an action potential (the heart
  beats by itself, w/o outside signals)
• Involuntary, smooth muscle- gut, uterus, etc.

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Multinucleated cell, arises from fusion great
big thing- 100mm X 100 um!
2.5 uM length
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These are myofibrils
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Light band
Dark band
Light band
http//www.youtube.com/watch?v0kFmbrRJq4w
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www.wiley.com/college/karp
Troponin binds Ca, moves the tropomyosin 1.5
nm- myosin binds
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Actin accessory proteins
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Filamin
(thymosins)
Fimbrin
Profilin
ARP
See note on what to learn
gelsolin
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Actin going bad. These bacteria are moved from
one cell to another by the actin tails. The
simple polymerization of actin provides force for
movement
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Now we go onto amoeboid action
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The amazing thing is- this cell has somewhere to
go!!!!
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Grow lamelipodia
Attach to new location
Contract the rear end
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Neutrophil abuse!
http//www.youtube.com/watch?vZUUfdP87Ssg
ARP2/3 breaks off, binds to the newly formed actin
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Things to know

• Three major cytoskeletal components
• Major stories- cilia movement, muscle
  contraction, amoeboid movement
• Major motor proteins for both MT and actin, major
  poisons
• Structure of the ends of MTs and actin
• Accessory proteins that weve mentioned.