2.  Serve as cytoskeleton to maintain cell shape
 Involved in changes in cell shape, & serve as a
"temporary scaffolding" for other organelles.
 They function as diffusion channels for water and
metabolites and even macromolecules, thus aiding
intracellular transport.
 In mitosis, microtubules form the mitotic spindle along
which chromosomes move.
 Some cellular proteins act to disassemble microtubules,
either by severing microtubules or by increasing the rate
of tubulin depolymerization from microtubule ends.
 Other proteins (called microtubule-associated proteins
or MAPs) bind to microtubules and increase their
stability.
 Such interactions allow the cell to stabilize microtubules
in particular locations and provide an important
mechanism for determining cell shape and polarity.

3. MICROTUBULES
 Microtubules, like actin microfilaments, exhibit both
structural and functional polarity.
 Dimeric αβ–tubulin subunits interact end-to-end to form
protofilaments, which associate laterally into
microtubules.
 The wall is composed of 13 protofilaments of the
protein tubulin, and later bind to microtubule-
associated proteins (MAPs).

4. A stabilizing MAP consists of a basic microtubule-binding domain and an acidic
projection domain. The projection domain can bind to membranes, intermediate
filaments, or other microtubules, and its length controls how far apart
microtubules are spaced. The microtubule-binding domain contains repeats of a
conserved, positively charged 4-residue amino acid sequence that binds the
negatively charged C-terminal part of tubulin, thereby stabilizing the polymer.

5. The structure of a microtubule and its subunit. (A) The subunit of each
protofilament is a tubulin heterodimer, formed from a very tightly linked
pair of α- and β-tubulin monomers. The GTP molecule in the β-tubulin
monomer is less tightly bound and has an important role in filament
dynamics. Both nucleotides are shown in red. (B) One tubulin subunit (α-β
heterodimer) and one protofilament consist of many adjacent subunits
with the same orientation. (C) The microtubule is a stiff hollow tube formed
from 13 protofilaments aligned in parallel. (D) A short segment of a
microtubule viewed in an EM. (E) EM of a cross section of a microtubule.

6.  Unlike microfilaments, micro-
tubules are non-contractile
polarized structures with a (-)
end anchored to the centro-
some, and a free (+) end at
which tubulin monomers are
added or removed.
 They exhibit : (1) treadmilling,
the addition of subunits at one
end and their loss at the other
end, and (2) dynamic
instability, the oscillation
between growth and
shrinkage.
 Balance depends on whether
the exchangeable GTP bound
to β-tubulin is present on the
(+) end or whether it has been
hydrolyzed to GDP.

7. Stages in assembly of
microtubules.
Free tubulin dimers form
short, unstable
protofilaments (1), which
then form more stable
curved sheets (2). A
sheet wraps around into
a microtubule with 13
protofilaments. The
microtubule then grows
by the addition of
subunits (3).The free
tubulin dimers have GTP
(red dot) bound to the
exchangeable nucleotide-
binding site on the β-
tubulin monomer. After If the rate of polymerization is faster than the
incorporation of a rate of GTP hydrolysis, then a cap of GTP-
dimeric subunit into a bound subunits is generated at the (+) end,
microtubule, the GTP on although the bulk of β-tubulin in a microtubule
the β-tubulin is will contain GDP. The rate of polymerization is
hydrolyzed to GDP. twice as fast at the (+) end as at the (-) end.

8. Dynamic instability model
of microtubule growth and shrinkage.
Only microtubules
whose (+) ends are
associated with GTP-
tubulin (those with a GTP
cap) are stable and can
serve as primers for the
polymerization of
tubulin. Microtubules
with GDP-tubulin (blue)
at the (+) end, or those
with a GDP cap are
rapidly depolymerized
and may disappear
within 1 min. Because
the GDP cap is unstable,
the microtubule end
peels apart to release
tubulin subunits.

9. Microtubules (blue)
organized around
the MTOC and spindle
poles (1) establish an
internal polarity to
movements and
structures in the
interphase cell (left)
and the mitotic cell
(right). Assembly and
disassembly (2) cause
microtubules to probe
the cell cytoplasm and
are harnessed at
mitosis to move
chromosomes. Long-
distance movement of
vesicles are powered
by kinesin and dynein
motors. Both motors
are critical in the
assembly of the
spindle and the
separation of
chromosomes in
mitosis.

10. MOLECULAR MOTORS
 MOTOR PROTEINS- specialized motility structures in
eukaryotic cells consisting of highly ordered arrays of
motor proteins that move on stabilized filament tracks.
 They use the energy of ATP hydrolysis to move along
microtubules or actin filaments.
 They mediate the sliding of filaments relative to one
another and the transport of membrane-enclosed
organelles along filament tracks.

11. Motor
proteins pull
components
of the cyto-
skeleton
past each
other.
Motor molecules
also carry vesicles
or organelles to
various
destinations along
“monorails’
provided by the
cytoskeleton.

12.  All known motor proteins that move on actin
filaments are members of the myosin superfamily.
 Motor proteins that move on microtubules are
members of either the kinesin superfamily or the
dynein family.
 The myosin and kinesin superfamilies are diverse,
with about 40 genes encoding each type of
protein in humans.
 The only structural element shared among all
members of each superfamily is the motor "head"
domain. These heads can be attached to a wide
variety of "tails," which attach to different types
of cargo and enable the various family members
to perform different functions in the cell.

14. Functions of myosin tail domains.
(a) Myosin I and myosin V are localized to cellular membranes by
undetermined sites in their tail domains. As a result, these
myosins are associated with intracellular membrane vesicles or
the cytoplasmic face of the plasma membrane. (b) In contrast,
the coiled-coil tail domains of myosin II molecules pack side by
side, forming a thick filament from which the heads project. In a
skeletal muscle, the thick filament is bipolar. Heads are at the
ends of the thick filament and are separated by a bare zone,
which consists of the side-by-side tails.

15. Cycle of
structural
changes
used by
myosin to
walk along
an actin
filament.

16. Summary of the coupling between ATP hydrolysis and conformational
changes for myosin II. Myosin begins its cycle tightly bound to the actin
filament, with no associated nucleotide, the so-called "rigor" state. ATP
binding releases the head from the filament. ATP hydrolysis occurs while
the myosin head is detached from the filament, causing the head to
assume a cocked conformation, although both ADP and inorganic
phosphate remain tightly bound to the head. When the head rebinds to the
filament, the release of phosphate, followed by the release of ADP, trigger
the power stroke that moves the filament relative to the motor protein. ATP
binding releases the head to allow the cycle to begin again. In the myosin
cycle, the head remains bound to the actin filament for only about 5% of
the entire cycle time, allowing many myosins to work together to move a
single actin filament.

18.  KINESINS move towards the (+) ends of tubules,
while dyneins move towards the (–) ends.
 Kinesin is responsible for movement of vesicles
and organelles in the cytoplasm, dynein regulates
2-way traffic and dynamin serves as a motor for
sliding movements during microtubule elongation.

19. Kinesin and kinesin-related proteins. (A) Structures of four kinesin
superfamily members. Conventional kinesin has the motor domain at
the N-terminus of the heavy chain. The middle domain forms a long
coiled coil, mediating dimerization. The C-terminal domain forms a
tail that attaches to cargo, such as a membrane-enclosed organelle.
These kinesins generally travel toward the minus end instead of the
plus end of a microtubule. (B) Freeze-etch EM of a kinesin molecule
with the head domains on the left.

20. Summary of the coupling between ATP hydrolysis and conformational
changes for kinesin. At the start of the cycle, one of the two kinesin
heads, the front or leading head (dark green) is bound to the microtubule,
with the rear or trailing head (light green) detached. Binding of ATP to the
front head causes the rear head to be thrown forward, past the binding
site of the attached head, to another binding site further toward the plus
end of the microtubule. Release of ADP from the second head (now in the
front) and hydrolysis of ATP on the first head (now in the rear) brings the
dimer back to the original state, but the two heads have switched their
relative positions, and the motor protein has moved one step along the
microtubule protofilament. In this cycle, each head spends about 50% of
its time attached to the microtubule and 50% of its time detached.

21. CENTROSOME
 Also called the centrosphere or cell center, which
refers to a specialized zone of cytoplasm containing
the centrioles and a variable number of small dense
bodies called centriolar satellites.
 Considered to be a center of activities associated
with cell division, usually adjacent to the nucleus.
 The Golgi apparatus often partially surrounds the
centrosome on the side away from the nucleus.
 It consists of an amorphous matrix of protein
containing the g-tubulin ring complexes that nucleate
microtubule growth.
 They serve as basal bodies and sites of anchor for
epithelial cilia.
 Plant and fungal cells have a structure equivalent to a
centrosome, although they do not contain centrioles .

22.  The matrix of the centrosome
is organized by a pair of
centrioles.
 An electron micrograph of a
thick section of a centrosome
showing an end-on view of a
centriole. The ring of modified
microtubules of the centriole
is visible, surrounded by the
fibrous centrosome matrix.

23.  Centrioles are self-duplicating organelles that exhibit
continuity from one cell generation to the next. They
double in number immediately before cell division but they
do not undergo transverse fission.
 Paired centrioles are called diplosome. The long axes of
the two centrioles are usually perpendicular to each other.
 Centrioles become prominent in mitosis. In prophase they
separate and a new procentriole develops adjacent to
each.
 Microtubule organizing centers (MTOCs) become
nucleation sites around each centriole to form the fibers of
the aster and the mitotic spindle.
 MTOCs determine cell polarity including the organization
of cell organelles, direction of membrane trafficking, and
orientation of microtubules.
 Because microtubule assembly is nucleated from MTOCs,
the (-) end of most microtubules is adjacent to the MTOC
and the (+) end is distal.

24. A centrosome with
attached
microtubules. The
minus end of each
microtubule is
embedded in the
centrosome, having
grown from a Ý-
tubulin ring
complex, whereas
the plus end of
each microtubule is
free in the
cytoplasm.

25.  In EM, each centriole
is found to be a hollow
cylinder closed at one
end and open at the
other.
 The central cavity is
occupied by small
dense granules.
 In transverse section,
its wall is composed of
9 evenly spaced triplet
microtubules (9 x 3).
 Each triplet (A, B and C) is set at an angle of about
400o to its respective tangent.
 Subunit A is nearest to the centriole axis; short fibers
connect it to subunit C of the adjacent triplet.

26. Orientation of cellular
microtubules.
(a) In interphase animal cells, the
(-) ends of most microtubules are
proximal to the MTOC. Similarly, the
microtubules in flagella and cilia
have their (-) ends continuous with
the basal body, which acts as the
MTOC for these structures.
(b) As cells enter mitosis, the
microtubule network rearranges,
forming a mitotic spindle. The (-)
ends of all spindle microtubules
point toward one of the two
MTOCs, or poles, as they are called
in mitotic cells. (c) In nerve cells,
the (-) ends of all axonal
microtubules are oriented toward
the base of the axon, but dendritic
microtubules have mixed polarities.

27.  The restructuring of the microtubule cytoskeleton is
directed by duplication of the centrosome to form two
separate MTOCs at opposite poles of the mitotic spindle.
 The centrioles and other components of the centrosome
are duplicated in interphase cells, but they remain together
on one side of the nucleus until the beginning of mitosis.
 The two centrosomes then separate and move to opposite
sides of the nucleus, forming the two poles of the mitotic
spindle.
 As the cell enters mitosis, the dynamics of microtubule
assembly and disassembly also change dramatically.
 First, the rate of microtubule disassembly increases about
tenfold, resulting in overall depolymerization and shrinkage
of microtubules.
 At the same time, the number of microtubules emanating
from the centrosome increases by five- to tenfold.

28.  In combination, these changes result in disassembly of
the interphase microtubules and the outgrowth of large
numbers of short microtubules from the centrosomes.
 As mitosis proceeds, the two chromatids of each
chromosome are then pulled to opposite poles of the
spindle. This chromosome movement is mediated by
motor proteins associated with the spindle microtubules.
 In the final stage of mitosis, nuclear envelopes reform, the
chromosomes decondense, and cytokinesis takes place.
 After cell division, each cell acquires 2 centrioles, one
from the parent cell, and one which arose as a
procentriole.
 If mitotic cells are exposed to drugs like colchicine (binds
to monomeric tubulin and prevent polymerization),
vinblastine and taxol (disrupt microtubule dynamics),
microtubules disappear and mitosis is arrested because
of inadequate formation of the mitotic spindle. These
drugs are useful in the treatment of certain cancers.

29. Effect of the drug taxol on microtubule organization.
(A) Molecular structure of taxol. Recently, organic chemists have
succeeded in synthesizing this complex molecule, which is widely used for
cancer treatment. (B) Immunofluorescence micrograph showing the
microtubule organization in a liver epithelial cell before the addition of
taxol. (C) Microtubule organization in the same type of cell after taxol
treatment. Note the thick circumferential bundles of microtubules around
the periphery of the cell. (D) A Pacific yew tree, the natural source of taxol.

30. CILIA & FLAGELLA
 Characteristic 9 x 2
arrangement of
microtubules
 Tubulin forms doublets
composed of subunit A, a
complete microtubule
with 13 protofilaments,
joined to a C-shaped
subunit B with only 10.
 Lateral arms composed of
the MAP axonemal dynein
project from subunit A to
subunit B of the next.
 Major motor portion of the
flagellum is called the
axoneme.

32. Ciliary dynein is a large motor protein assembly composed of 9-12
polypeptide chains (A) The heavy chains form the major portion of the
globular head & stem domains, & many of the smaller chains are
clustered around the base of the stem. The base of the molecule binds
tightly to an A microtubule in an ATP-independent manner, while the
large globular heads have an ATP-dependent binding site for a B
microtubule. When the heads hydrolyze their bound ATP, they move
toward the (-) end of the B microtubule, thereby producing a sliding
force between the adjacent microtubule doublets in a cilium or
flagellum. (B) Freeze-etch EM of a cilium showing the dynein arms
projecting at regular intervals from the doublet microtubules

33. The bending of an axoneme.
(A) When axonemes are exposed to the proteolytic enzyme trypsin, the
linkages holding neighboring doublet microtubules together are broken.
Addition of ATP allows the motor action of the dynein heads to slide one
pair of doublet microtubules against the other pair. (B) In an intact
axoneme (such as in a sperm), sliding of the doublet microtubules is
prevented by flexible protein links. The motor action therefore causes a
bending motion, creating waves or beating motions

34. The contrasting motions of flagella
and cilia. (A) The wavelike motion of
the flagellum of a sperm cell. Waves
of constant amplitude move
continuously from the base to the tip
of the flagellum. (B) The beat of a
cilium, which resembles the breast
stroke in swimming.