02.11.09
Lecture 11 - The microtubule
cytoskeleton
The cytoskeleton
• Gives the cell its shape
• Allows the cell to
organize its components
• Produces large-scale
movements (I.e. muscle
contraction, cell
crawling, propulsion via
cilia and flagella)
The cytoskeleton is composed of
networks of 3 different filaments
Cytoskeletal filaments exhibit
different physical properties
The cytoskeleton is dynamic
Microtubules are organized to perform
specific functions
What do microtubules do?
• Establish an internal polarity to movements
and structures in the interphase cell
• Participate in chromosome segregation
during cell division
• Establish cell polarity during cellular
movement
• Produce extracellular movement via beating
of cilia and flagella
Microtubule structure
Microtubules exhibit a behavior termed
dynamic instability
• Total mass of polymerized tubulin
remains constant, but individual
microtubules are dynamic
• Growth: assembly of microtubule
• Shrinkage: disassembly of
microtubule
• Catastrophe: switching from growth
to shrinking
• Rescue: switching from shrinking
to growth
QuickTime™ and a
Graphics dec ompres sor
are needed to s ee this pic ture.
Tubulin subunit addition takes place
predominantly at the plus end
Growing microtubules have a “cap”
of GTP at the plus end
Microtubule-associated proteins
• MAPs can function
as cross-bridges
connecting
microtubules.
• They can affect
microtubule rigidity
and assembly rate.
The centrosome is the primary microtubule
nucleation site in most cells
Centrosomes act to polarize the
microtubule network
• Plus end - fast growing, usually in the cytoplasm
• Minus end - slow growing, anchored at the
centrosome in most cells
Centrosome duplication occurs once
per cell cycle
Centrosomes are often abnormal in
cancer cells
Why are microtubules dynamic?
• Microtubule dynamics allow the cell to quickly
reorganize the network when building a
mitotic spindle
• Dynamics also allow microtubules to probe
the cytoplasm for specific objects and sites
on the plasma membrane - search and
capture
Search and capture model
Search & capture during cell polarization
Search & capture during mitosis
Motor proteins
• Enzymes that convert ATP hydrolysis directly
into movement along cytoskeletal filaments
• Some motors move towards the plus end,
others move to the minus end
• Carry cargo (organelles, protein complexes,
RNA) and mediate microtubule/microtubule
sliding
First evidence of microtubule motors
came from study of axonal transport
Extruded axoplasm assays Cytosol
is squeezed from the axon with a
roller onto a glass coverslip.
Addition of ATP shows movement
by videomicroscopy
Vesicle movement in this system
is about 1-2um/s similar to fast
axonal
transport.
Motor proteins
QuickTime™ and a
MPEG-4 Video decompressor
are needed to see this picture.
There are two families of microtubule
motors
• Kinesins
• Dyneins
– Move cargo to the plus
end
– In mitosis, participate in
mitotic spindle dynamics
– Usually dimers of 2
heavy chains and 2 light
chains
– Move cargo to the minus
end
– In mitosis, participate in
mitotic spindle dynamics
– Power beating of cilia
and flagella
– Large protein complex
with many subunits
Structure of kinesin
• 2 heavy chains + 2
light chains
• Microtubule and
ATP binding sites in
the head
• Cargo-binding site
in the tail and light
chains
Kinesin “walks” along microtubules
Kinesin “walks” along microtubules
QuickTime™ and a
Photo - JPEG decompressor
are needed to see this picture.
Dynein is a large complex of many
proteins
There are two classes of dyneins
• Cytoplasmic dynein
– Carries cargo in the
cytoplasm
– Involved in mitotic
spindle dynamics
• Axonemal dyneins
– Localized exclusively in
cilia and flagella
– The motors that power
cilliary and flagellar
beating
General model for kinesin- and
dynein-mediated transport
Flagella and cilia are specialized
microtubule-based cellular structures
Cilia and flagella
• Cilia line the epithelial tissue of the respiratory
tract to sweep particulate matter out of the
airways
• Cilia line the oviduct to push the egg
• Non-motile cilia detect signals
• Flagella allow sperm to swim
• Flagella are essential for left-right asymmetry
during development (Kartagener syndrome:
situs inversus, sinusitis, brochiectasis)
Cilia in the respiratory tract
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
Structure of a motile axoneme
Dynein movement causes flagella to
bend
Mutations that disrupt cilia cause
multiple diseases
• Fertility (sperm motility,
ectopic pregnancy)
• Polycystic kidney
disease
• Respiratory infection
• Retinal degeneration
• Hearing/balance loss
(Usher syndrome)
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
Sinus invertus: left-right
body asymmetry
• Defects affecting placement
of lungs, heart, liver stomach
and spleen
• Morphogens secreted on the
right side of the embryo
aretransported to the left
side by ciliary beating
• Immotile cilia fail to establish
proper morphogen gradients