412
Cell Nucleus Biogenesis, Structure and Function
upon which the motor proteins can move.
In eukaryotes, the cytoskeleton provides
the network that coordinates the move-
ment and location of cytoplasmic vesicles
and organelles.
Stated briefly for the sake of clarity,
the cytoskeleton of higher eukaryotes is
composed of three Flamentous networks:
Microtubules (MTs) – are long hollow
tubes that measure about 25 nm in di-
ameter. They are composed of multimeric
structures that are assembled from het-
erodimers of
α
-and
β
-tubulin. The tubules
form with polarity, polymerizing and de-
ploymerizing more rapidly at their
+
ends.
The ends of MTs are usually embedded in a
microtubule-organizing center. Typically,
in higher cells, this will be the centrosome.
MTs are dynamic, with a polymerization
rate of
1
µ
mm
in
1
. However, MTs are
stabilized through interactions with a large
number of other proteins – the micro-
tubule associate proteins (MAPs). During
mitosis of the cell cycle, phosphorylation
of MAPs by the cell cycle kinases has a dra-
matic impact on MT behavior; particularly
with respect to their dynamic properties,
which increase 5 to 10 fold. Under these
circumstances, the MTs play a vital role in
the process of chromosome segregation.
The MTs act as tracks for cytoplasmic
motor proteins that fall into two broad
classes. Generally speaking, kinesins are
plus-end directed MT motor proteins and
dyneins are minus-end directed MT motor
proteins. Small families of each of these
motors are known and these serve speciFc
purposes and ensure that deFned cargoes
a
r
ea
b
l
et
om
o
v
et
oa
p
p
r
o
p
r
i
a
t
es
i
t
e
s
throughout the cell.
MicroFlaments (M±s) – are much nar-
rower, typically about 8 nm across, helical
polymers of a single protein, actin. Like
the MTs a complex Flamentous network
of actin Fbers pervades the cytoplasm of
most eukaryotic cells. Like MTs, M±s are
dynamic and serve as tracks upon which
myosin motors can move organelles and
vesicles around the cell. Actin Flaments
play particularly important roles in cell
motility and cell architecture.
Intermediate
Flaments
(I±s) – are
a
much more complex family of proteins
that play particular roles in tissue architec-
ture. As this role implies, the I± networks
a
r
em
u
c
hm
o
r
eim
p
o
r
t
an
tinm
u
l
t
i
c
e
l
-
lular eukaryotes than in their primitive
ancestors; simple eukaryotes do not have
intermediate Flament proteins. I±s are
10 nm across and intermediate in size
between M±s and MTs, hence the name.
Members of the protein family have a
distinct elongated central domain that in-
teracts as a coiled-coil structure to generate
Frst dimers and then complex multimers
that form the Flaments. The family in
mammals has about 50 members, though
different members are expressed in a cell
lineage–dependent manner. Keratins, for
example, are expressed in the epidermis of
the skin and play a vital role in maintaining
the mechanical properties of this organ.
Cytoskeleton – is the structure formed
from these three networks. The networks
are closely associated and cross-linked,
through the behavior of associated proteins
so that the function of each is dependent on
the structure of the others. The cytoskele-
tonshou
ldbeconsideredasanintegrated
network with cooperative functions.
Even a superFcial appraisal of cytoplas-
mic architecture begs the question: do
the essential functions performed by the
cytoskeleton have counterparts in the nu-
cleus and if so do any analogous structures
exist? This question has been a focus of in-
tense debate over the past 25 years. We
have already discussed the function and
properties of the nuclear lamina. This
network of type V intermediate Flament
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