Transcript Tutorial 8 – Cytoskeleton

INTRODUCTION
Unit 8 - Cytoskeleton
3 types of fibers make up the cytoskeleton
Intermediate
Actin
Microtubules
Filaments
Microfilaments
- Microtubules interact with motor systems: dyneins
and kinesins
- Microfilaments interact with myosins
- Intermediate filaments do not interact with motor
proteins
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
Intermediate Filaments - STRUCTURE
monomer
dimer
tetramer
2 tetramers coiled together
8 tetramers twisted into a ropelike filament
10nm
Have great
tensile
strength!
Strengthens
cells against
mechanical
stress!
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;
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
Intermediate Filaments
• Why can you use intermediate filaments to
tell whether a cancer has spread in the
body?
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
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
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
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
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
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
Motor Proteins
• Dyneins
– Travel towards
MINUS END
• Kinesins
– Travel towards
PLUS END
Dyneins
Kinesins
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
Bundles of Actin filaments in cells
Actin in RED:
Microvilli
Contractile
bundles in
cytoplasm
Sheet-like and
finger-like
protrusions
from the
leading edge of
a moving cell
Contractile
ring during
cell division
Actin - STRUCTURE
• Actin microfilaments are comprised of
actin monomers that polymerize to form
long, thin fibers. These are about 7nm in
diameter.
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
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.
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
Practice Questions:
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?
What cytoskeletal element is indicated by the arrow?
What are TWO other functions of this element?
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