Microtubules are filamentous structures in cells that serve as tracks for transporting organelles and vesicles. They are composed of tubulin subunits that can assemble and disassemble dynamically. This allows microtubules to remodel rapidly during cell division and change structure. Motor proteins move along microtubules to transport cargo within cells. Microtubules interact with other cellular components through microtubule associated proteins (MAPs) that can stabilize microtubule structure.

1. MICROTUBULES
 Microtubules are the track system of the cell.
 They form a railway for transporting organelles and vesicles and other
cellular organelles.
 Their growing center is the centrosome.
 When a cell enters mitosis, the cytoplasmic microtubules disassemble and
then reassemble into an intricate structure called the mitotic spindle.
 Microtubules can also form stable structures, such as rhythmically beating
cilia and flagella.
 A microtubule is made up of 13 protofilaments.
 The direction of synthesis is adding always (-) to (+).
 Each protofilament has a structural polarity.
 One end of the microtubule, thought to be the -tubulin end, is called its
plus end, and the other, the α-tubulin end, its minus end.
 The centrosome matrix includes hundreds of ring-shaped structures
formed from a special type of tubulin, called ȣ-tubulin, and each ȣ-tubulin
ring complex serves as the starting point, or nucleation site, for the growth
of one microtubule.
 The α-tubulin dimers add to each ȣ-tubulin ring complex in a specific
orientation, with the result that the minus end of each microtubule is
embedded in the centrosome, and growth occurs only at the plus end that
extends into the cytoplasm.
 Cells contain two populations of microtubules: stable, long-lived
microtubules and unstable, short-lived microtubules.

2.  Stable microtubules are generally found in non-replicating cells. They
include a central bundle of microtubules in cilia and flagella, extensions of
the plasma membrane that beat rhythmically to propel materials across
epithelial surfaces, to enable sperm to swim, or to push an egg through the
oviduct.
 A marginal band of stable microtubules present in some erythrocytes and
platelets enables these cells to pass through small blood vessels.
 Another example exists in nerve cells (neurons), which must maintain long
processes called axons.
 An internal core of stable microtubules in axons not only supports their
structure but also provides tracks along which vesicles move through the
axonal cytoplasm.
 The disassembly of such stable structures would have catastrophic
consequences—sperm would be unable to swim, a red blood cell would
lose its spring-like pliability, and axons would retract.
 In contrast with these permanent, stable structures, unstable microtubules
are found in cells that need to assemble and disassemble microtubule-
based structures quickly.
 For example, in mitosis, the cytosolic microtubule network characteristic of
interphase cells disassembles, and the tubulin from it is used to form the
spindle-shaped apparatus that partitions chromosomes equally to the
daughter cells.
 When mitosis is complete, the spindle disassembles and the interphase
microtubule network reforms.
 The building block of a microtubule is the tubulin subunit, a heterodimer of
α- and β-tubulin. Both of these 55,000- MW monomers are found in all
eukaryotes, and their sequences are highly conserved.

3.  Although a third tubulin, ȣ-tubulin, is not part of the tubulin subunit, it
probably nucleates the polymerization of subunits to form αβ–micro-
tubules.
 Encoded by separate genes, the three tubulins exhibit homology with a
40,000-MW bacterial GTPase, called FtsZ.
 Like tubulin, this bacterial protein has the ability to polymerize and
participates in cell division. Perhaps the protein carrying out these
ancestral functions in bacteria was modified in the course of evolution to
fulfill the diverse roles of microtubules in eukaryotes.
 Each tubulin subunit binds two molecules of GTP. One GTP-binding site,
located in α-tubulin, binds GTP irreversibly and does not hydrolyze it. The
second site, located on β- tubulin, binds GTP reversibly and hydrolyzes it to
GDP.
 Thus, tubulin is a GTPase like bacterial FtsZ protein.
 In the atomic structure of the tubulin subunit, the GTP bound to α-tubulin
is trapped at the interface between the α- and β-tubulin monomers and is
thus nonexchangeable. The second GTP lies at the surface of the β-tubulin
monomer; this GTP is freely exchangeable with GDP.
 Guanine bound to β-tubulin modulates the addition of tubulin subunits at
the ends of a microtubule.
 In a microtubule, lateral and longitudinal interactions between the tubulin
subunits are responsible for maintaining the tubular form. Longitudinal
contacts between the ends of adjacent subunits link the subunits head to
tail into a linear protofilament. Within each protofilament, the dimeric
subunits repeat every 8 nm. Through lateral inter- actions, protofilaments
associate side by side into a sheet or cylinder—a microtubule.

4.  In addition to the simple singlet structure, doublet or triplet micro- tubules
are found in specialized structures such as cilia and flagella (doublet
microtubules) and centrioles and basal bodies (triplet microtubules). Each
doublet or triplet contains one complete 13-protofilament microtubule (A
tubule) and one or two additional tubules (B and C) consisting of 10
protofilaments.
 The remarkable behavior of microtubules—switching back and forth
between polymerization and depolymerization—is known as dynamic
instability. It allows microtubules to undergo rapid remodeling, and is
crucial for their function.
 A microtubule growing out from the centrosome can, however, be
prevented from disassembling if its plus end is stabilized by attachment to
another molecule or cell structure so as to prevent its depolymerization.
 The end of a rapidly growing microtubule is composed entirely of GTP-
tubulin dimers, which form a “GTP cap.” GTP-associated dimers bind more
strongly to their neighbors in the microtubule than do dimers that bear
GDP, and they pack together more efficiently. Thus the microtubule will
continue to grow.
 The selective stabilization of microtubules enables the centrosome and
other nucleating centers to set up a highly organized system of
microtubules in selected parts of the cell. The same strategy is used to
position organelles relative to one another.

5.  Colchicine binds tightly to free tubulin dimers and prevents their
polymerization into microtubules, the mitotic spindle rapidly disappears,
and the cell stalls in the middle of mitosis, unable to partition the
chromosomes into two groups.
 Taxol binds tightly to microtubules and prevents them from losing
subunits. Because new subunits can still be added, the microtubules can
grow but cannot shrink. However, despite this difference in their
mechanism of action, Taxol has the same overall effect as colchicine—
arresting dividing cells in mitosis.
 Microtubule-stabilizing or microtubule-destabilizing antimitotic drugs
include colchicine, Taxol, vincristine, and vinblastine— are used in the
treatment of human cancer.
 Microtubules serve their function by whether assembling or disassembling
depending on the cellular state.
 As shown in the figure above, the microtubules run in one specific
direction, reflecting the cell polarity.

6.  Motor proteins use the energy derived from repeated cycles of ATP
hydrolysis to travel steadily along the microtubule or actin filament in a
single direction.
 There are dozens of different motor proteins; they differ in the type of
filament they bind to, the direction in which they move along the filament,
and the cargo they carry.
 The motor proteins that move along cytoplasmic microtubules, such as
those in the axon of a nerve cell, belong to two families: the kinesins
generally move toward the plus end of a microtubule.
 The dyneins move toward the minus end
 Both kinesins and dyneins are generally dimers that have two globular
ATP-binding heads and a single tail. The heads interact with microtubules
in a stereospecific manner, so that the motor protein will attach to a
microtubule in only one direction. The tail of a motor protein generally
binds stably to some cell component, such as a vesicle or an organelle, and
thereby determines the type of cargo that the motor protein can transport.
 The globular heads of kinesin and dynein are enzymes with ATP-
hydrolyzing (ATPase) activity. This reaction provides the energy for driving
a directed series of conformational changes in the head that enable it to
move along the microtubule by a cycle of binding, release, and rebinding to
the microtubule.
 Microtubules and motor proteins play an important part in positioning
organelles within a eukaryotic cell.
 As a cell grows, kinesins attached to the outside of the ER membrane (via
receptor proteins) pull the ER outward along microtubules, stretching it
like a net.

7.  Cytoplasmic dyneins attached to the Golgi membranes pull the Golgi
apparatus along microtubules in the opposite direction, inward toward the
nucleus.
 MAPs are classified into two groups on the basis of their function. One
group stabilizes microtubules. The structure of a stabilizing MAP consists of
two domains—a basic microtubule-binding domain and an acidic
projection domain.
 In the electron microscope, the projection domain appears as a
filamentous arm that extends from the wall of the microtubule. This arm
can bind to membranes, intermediate filaments, or other microtubules,
and its length controls how far apart microtubules are spaced.
 The microtubule-binding domain contains several repeats of a conserved,
positively charged four-residue amino acid sequence that binds the
negatively charged C- terminal part of tubulin. This binding is postulated to
neutralize the charge repulsion between tubulin subunits within a
microtubule, thereby stabilizing the polymer.
 MAP1A and MAP1B are large, filamentous molecules found in axons and
dendrites of neurons as well as in non- neuronal cells. Each of these MAPs
is derived from a single precursor polypeptide, which is proteolytically
processed in a cell to generate one light chain and one heavy chain.
 Other stabilizing MAPs include MAP2, MAP4, Tau, and CLIP170.
 MAP4, the most widespread of all the MAPs, is found in neuronal and non-
neuronal cells.
 In mitosis, MAP4 regulates microtubule stability, and CLIP170 cross-links
microtubules to chromosomes.
 MAP2 is found only in dendrites, where it forms fibrous cross-bridges
between microtubules and links microtubules to intermediate filaments.

8.  Tau, which is much smaller than most other MAPs, is present in both axons
and dendrites. This protein exists in several isoforms derived from
alternative splicing of a tau mRNA. The ability of Tau to cross-link
microtubules into thick bundles may contribute to the stability of axonal
microtubules.
 Findings from gene transfection experiments implicate Tau in axonal
elongation. Deletion of the genes encoding Tau and MAPIB leads to more
severe phenotypes of axonal dysgenesis and lethality.
 Furthermore, aberrant polymerization of Tau into filaments is linked to
neurodegenerative diseases such as human dementia in Alzheimer’s
patients.
 Although bound MAPs generally dampen the rate of microtubule
disassembly, the assembly of microtubules is affected to varying degrees:
some MAPs, such as Tau and MAP4, stabilize microtubules, whereas other
MAPs do not. Because of the effect of assembly MAPs on microtubule
dynamics, modulating the binding of MAPs can control the length of
microtubules.
 Phosphorylated MAPs are unable to bind to microtubules; thus they
promote micro- tubule disassembly. MAP kinase, a key enzyme for
phosphorylating MAPs, is a participant in many signal-transduction
pathways, indicating that MAPs are targets of many extracellular signals.
MAPs, especially MAP4, are also phosphorylated by a cyclin-dependent
kinase (CDK) that plays a major role in controlling the activities of various
proteins in the course of the cell cycle.

9.  A second group of MAPs directly destabilizes microtubules in many cell
types. One of this group, called katanin, severs in- tact cytosolic
microtubules by an ATP-dependent process. Internal bonds between
tubulin subunits in the microtubule wall are broken, causing microtubules
to fragment. This activity may release microtubules at the MTOC. Another
protein, called Op18 or stathmin, increases the frequency of rapid dis-
assembly of microtubules in the mitotic spindle. This protein may act by
binding tubulin dimers, thereby reducing the pool of dimers available for
polymerization. Phosphorylation inactivates Op18 and inhibits its
destabilizing effect.
 Cell polarity including the organization of cell organelles, direction of
membrane trafficking, and orientation of microtubules is determined by
microtubule- organizing centers (MTOCs). Most interphase animal cells
contain a single, perinuclear MTOC from which cytosolic microtubules
radiate.
 A -tubulin–containing complex is a major component of the pericentriolar
material and is able to nucleate the polymerization of tubulin subunits to
form microtubules in vitro.

10.  Two families of motor proteins, kinesin and dynein, transport membrane-
limited vesicles, proteins, and organelles along microtubules.
 Nearly all kinesins move cargo toward the (+) end of microtubules
(anterograde transport), whereas dyneins transport cargo toward the (-)
end (retrograde transport).
 Most kinesins are dimers with a head domain that binds microtubules and
ATP and a tail domain that binds vesicles or other cargo. The flexible neck
region determines the direction of kinesin movement, and the tail domain
determines cargo specificity.
 Cytosolic dyneins are linked to their cargoes (vesicles and chromosomes)
by dynactin, a large multiprotein complex.
 Dynactin also binds to microtubules, thereby increasing the processivity of
dynein-mediated transport.
 In microtubule-poor regions of the cell, vesicles are probably transported
along microfilaments powered by a myosin motor.
 Flagellar beating propels cells forward, and ciliary beating sweeps materials
across tissues.
 The axoneme in both flagella and cilia contains nine outer doublet
microtubules arranged in a circle around two central singlet microtubules.
 Axonemal dyneins, which are larger and more complex than cytosolic
dyneins, are permanently attached to doublet microtubules in axonemes.
The dynein arms with their small globular heads project toward the
adjacent doublet.

11.  Walking of dynein arms extending from one doublet to- ward the (-) end of
a neighboring doublet generates a sliding force in the axoneme. This linear
force is converted into a bend by regions that resist sliding.