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BIOL 331 CMMB
Cytoskeleton
Microtubules
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Microtubules
are hollow, unbranched, tubular structures made of
tubulin
Microtubules have roles in cell support and movement of materials within a cell
They can
extend
across the length or breadth of a cell
The microtubule is composed of
13
protofilaments aligned side by side to form a
tube
through weak
non-covalent
interactions
Protofilaments are assembled from dimers of one
α-tubulin
and one
β-tubulin.
Protofilament is
asymmetric
; the microtubule itself has polarity:
α-tubulin end:
negative
end
β-tubulin end:
positive
end
The polarity helps direct
motor
proteins and the direction of microtubule
growth
Centrosome
: a type of microtubule-organizing centre which initiates microtubule formation.
Composed of two
centrioles
surrounded by pericentriolar material (
PCM)
Centrioles are cylinders made of
microtubules
PCM:
loosely
organized fibrous lattice
When centrosomes replicate, centrioles recruit
PCM
to form a new
centrosome.
Centrosomes usually remain at the
centre
of the cell's
microtubular
network.
Centrosome diagram: (for the tubules, think of what they're made of)
A)
PCM
B)
Centrosome
C)
Centriole
D)
microtubules
4
Centrosomes dictate:
number of
microtubules
their (microtubule)
polarity
the
number
of protofilaments
the time and
location
of microtubule
assembly
Centrosomes do NOT dictate the rate of
assembly
nor the
stability
New microtubules do not make direct contact with
centrioles
, instead they are initiated in the
PCM
PCM contains ϒ-TURC (ϒ-
tubulin ring complex
)
ϒ-TURC contains:
ϒ-tubulin
Non-tubulin proteins in a
ring
αβ-tubulin dimers assemble on the ϒ-tubulin, but only
α-tubulin
can bind to the ring of ϒ-tubulin
Where the microtubules begin to form:
A)
Centrosome
1
What is the polarity of each end of the microtubule and to which end does the microtubule grow?
The end with the α-tubulin attached is
negative
while the end with the β-tubulin is
positive.
Microtubule synthesis occurs at the PCM and grows outwards towards the
positive
end.
A)
Negative end
B)
Positive end
2
At which end of the microtubule does growth/disassembly occur?
Positive end
ϒ-TURC complex also acts a
cap
to prevent disassembly and assembly at the
negative
end
Stability of microtubules are determined by:
microtubule interacting proteins (
MAPs
)
+TIPS
: proteins that bind at the + end of growing microtubules
Temperature
: cold promotes disassembly
MAPs:
increase
stability and promotes assembly by
linking
tubulin dimers together
Activity of some MAPs is controlled by the presence of
phosphate
groups
*Certain MAPs can decrease
stability
and promote
disassembly
GTP = guanosine triphosphate
Energy source that is analogous to
ATP
β-
tubulin
is a G-protein: Hydrolyzes
GTP
to
GDP
after the dimer is added to the microtubule. Dimers to be added to the microtubule are bound to GTP
GTP bound to the β-tubulin subunit is required for microtubule
assembly
GTP hydrolysis affects microtubule
structure
Is α-tubulin a G-protein?
No
What type of tubulin is shown in each?
Alpha or beta?
A)
beta
B)
alpha
2
Why would adding ϒ-TURC help ensure that newly formed/elongated microtubule contains exactly 13 protofilaments in an in vitro experiment?
Because the α and
β
tubulins are built off of the 13
ϒ-tubulins
, there would be
13
protofilaments as well.
Label the microtubule
A)
+
B)
-
C)
protofilament
3
Microtubule growth/closure:
In a growing microtubule, the tip consists of tubulin-
GTP
dimers in an open
sheet
Tube closure is associated with the hydrolysis of
GTP
GDP-tubulin has a different
conformation
(it has a weaker affinity for its neighbours) introducing mechanical
strain.
Thus, it needs
MAPs
to stabilize the microtubule
In the absence of MAPs, protofilaments curl
outward
and undergo catastrophic
shrinkage
+Tips:
Bind to the + end of microtubule and regulate the rate of growth or shrinkage
Meditate the attachment to subcellular structures (eg: kinetochore of the mitotic chromosome)
Microtubule
polymerization
(growth)/
disassembly
(shrinkage) can effectively ' push ' and ' pull ' material within a cell
Label the 'stage' of β-tubulin in the diagram w/ 1,2,3, or 4
In a growing microtubule, the tip consists of tubulin- GTP dimers in an open sheet
Tube closure is associated with the hydrolysis of GTP
GDP-tubulin has a different conformation (it has a weaker affinity for its neighbours) introducing mechanical strain.
Thus, it needs MAPs to stabilize the microtubule
In the absence of MAPs, protofilaments curl outward and undergo catastrophic shrinkage
A)
1
B)
GDP
C)
GTP
3
Label the 'stage' of β-tubulin in the diagram
In a growing microtubule, the tip consists of tubulin- GTP dimers in an open sheet
Tube closure is associated with the hydrolysis of GTP
GDP-tubulin has a different conformation (it has a weaker affinity for its neighbours) introducing mechanical strain.
Thus, it needs MAPs to stabilize the microtubule
In the absence of MAPs, protofilaments curl outward and undergo catastrophic shrinkage
A)
2
1
Label the 'stage' of β-tubulin in the diagram
In a growing microtubule, the tip consists of tubulin- GTP dimers in an open sheet
Tube closure is associated with the hydrolysis of GTP
GDP-tubulin has a different conformation (it has a weaker affinity for its neighbours) introducing mechanical strain.
Thus, it needs MAPs to stabilize the microtubule
In the absence of MAPs, protofilaments curl outward and undergo catastrophic shrinkage
A)
3
1
Label the 'stage' of β-tubulin in the diagram
In a growing microtubule, the tip consists of tubulin- GTP dimers in an open sheet
Tube closure is associated with the hydrolysis of GTP
GDP-tubulin has a different conformation (it has a weaker affinity for its neighbours) introducing mechanical strain.
Thus, it needs MAPs to stabilize the microtubule
In the absence of MAPs, protofilaments curl outward and undergo catastrophic shrinkage
A)
4
1
Microtubules as structural supports
microtubules help provide mechanical support as they are
stiff
enough to resist compression or
bending
forces
Help determine the
shape
of a cell
Maintains intracellular location of
organelles
makes animal cells spherical, while helps supports cellulose in plant cells
Microtubules as agents of intracellular motility
-Transports membranous
vesicles
from one membrane compartment to another.
-Transports non-membrane bound
cargo
(RNAs, ribosomes, cytoskeletal elements)
Microtubule motor proteins: how things actually get transported through the microtubule
Motor proteins utilize
ATP
hydrolysis to generate a mechanical force that move the motor protein and attached cargo along the cytoskeleton.
2 types of microtubule motor proteins:
Kinesins
Dyneins
*Each type of motor protein moves
unidirectional
and move in a step-wise manner
Kinesin-1 is a
tetramer
(2 heavy and 2 light chains) with a
globular
head and
tail.
Globular head:
binds to
microtubules
and does the actual walking along it
Binds to and
hydrolyzes
ATP
have highly
conserved
sequence
Tail:
Binds to
cargo
Diverse
sequences
Requires an
adaptor
to link cargo to tail
Label the kinesin
A)
Head
B)
Tail
2
Kinesin movement:
kinesin moves towards the
+
end of the microtubule (towards growth / shrinkage)
Leading head binds ONE
ATP
→
hydrolysis
and release of
ADP
and
phosphate
result in power stroke that swings leading head forwards
Moves the motor
8nm
(length of one tubulin dimer)
Kinesin heads primarily steps on the
β-tubulin
Kinesin walking!
A)
Leading head
B)
Cargo
C)
Adaptor
D)
+
4
The step-wise or hand-over-hand mechanism means that at least one head of the
motor
protein is attached to the
microtubule
at all times.
Kinesin movement is highly processive: capable of moving considerable distances
without
falling off.
Its speed is dependent on ATP concentration with a max speed of
1
micro meter/sec)
Dynein structure
Dynein is much
larger
than kinesin
Its head is also
faster
than kinesin
Made up of
2
heavy chains + multiple intermediate and
light
chains
Globular head:
-Binds to
ATP
and hydrolyzes it (generates force for movement)
Tail:
-Binds to cargo via the adaptor protein
dynactin
Stalks:
-Binds to
microtubule
Dynein structure diagram
A)
microtubule
B)
Tail
C)
Head
3
Dynein movement
Dynein moves towards the
-
end of the microtubule
Roles in:
Positioning the
spindle
and moving chromosomes during
mitosis
Positioning organelles and moving
vesicles
Kinesins and dynein move similar materials in
opposite
directions along the same
microtubule
Neuron directions...
Moving retrograde = moving towards
-
end of microtubule (towards
dendrites
)
Moving anterograde = moving towards
+
end of microtubule (towards
terminal knob
)
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