Celbiologie & Immunologie Samenvatting ‘Essential Cell Biology’ en Hoorcolleges DT II
ESSENTIAL CELL BIOLOGY: CHAPTER 17 – CYTOSKELETON
The cytoskeleton is an intricate network of protein filaments throughout the cytoplasm that helps to support the large volume of cytoplasm. The
cytoskeleton is most prominent in the eukaryotic cell. It is responsible for large-scale movements, including the crawling of cells along a surface, the
contraction of muscle cells, and the changes in cell shape that take place as an embryo develops. The cytoskeleton is built on a framework of three types
of protein filaments:
۰ Intermediate filaments formed by a family of fibrous proteins
۰ Microtubules formed by globular tubulin subunits
۰ Actin filaments formed by globular actin subunits
INTERMEDIATE FILAMENTS
Intermediate filaments have great strength, and their main function is to enable cells to withstand
the mechanical stress that occurs when cells are stretched. The filaments are called “intermediate”
because their diameter (about 10 nm) is between that of the thinner actin filaments and the thicker
myosin filaments. Intermediate filaments are found in the cytoplasm of most animal cells. They form
a network throughout the cytoplasm, surrounding the nucleus and extending out to the cell
periphery. There, they are often anchored to the plasma membrane at cell–cell junctions called
desmosomes, where the plasma membrane is connected to that of another cell.
An intermediate filament is like a rope in which many long strands are twisted together to provide
tensile strength, an ability to withstand tension without breaking. The strands of this cable are made
of intermediate filament proteins, fibrous subunits each containing a central elongated rod domain
with distinct unstructured domains at either end (A). The rod domain consists of an extended α-
helical region that enables pairs of intermediate filament proteins to form stable dimers by wrapping
around each other in a coiled-coil configuration (B). Two of these coiled-coil dimers, running in
opposite directions, associate to form a staggered tetramer (C). The tetramers associate with each
other side-by-side (D) and then assemble to generate the final rope like intermediate filament (E).
Almost all of the interactions between the intermediate filament proteins depend on noncovalent bonding; it is the combined strength of the
overlapping lateral interactions along the length of the proteins that gives intermediate filaments their great tensile strength. The central rod domains of
different intermediate filament proteins are all similar in size and amino acid sequence, so that when they pack together they always form filaments of
similar diameter and internal structure. By contrast, the terminal head and tail domains vary greatly in both size and amino acid sequence from one type
of intermediate filament protein to another. These unstructured domains are exposed on the surface of the filament, where they allow it to interact with
specific components in the cytoplasm.
Intermediate filaments are particularly prominent in the cytoplasm of cells that are
subject to mechanical stress. In all these cells, intermediate filaments distribute the
effects of locally applied forces, thereby keeping cells and their membranes from
tearing in response to mechanical shear.
Intermediate filaments can be grouped into four classes:
1. Keratin filaments in epithelial cells;
2. Vimentin and vimentin-related filaments in connective-tissue cells, muscle cells,
and supporting cells of the nervous system;
3. Neurofilaments in nerve cells;
4. Nuclear lamins, which strengthen the nuclear envelope.
The keratin filaments are the most diverse class of intermediate filament. Every kind of epithelium in the vertebrate body has its own distinctive mixture
of keratin proteins. The keratin filaments are formed from a mixture of different keratin subunits. Keratin filaments typically span the interiors of
epithelial cells from one side of the cell to the other, and filaments in adjacent epithelial cells are indirectly connected through desmosomes. The ends of
the keratin filaments are anchored to the desmosomes, and the filaments associate laterally with other cell components through the globular head and
tail domains that project from their surface. These strong cables, formed by the filaments throughout the epithelial sheet, distribute the stress that
occurs when the skin is stretched. A mutation in the keratin genes can cause epidermolysis bullosa simplex, in which the formation of keratin filaments
in the epidermis is interfered.
Neurofilaments are intermediate filaments that are found along the axons of vertebrate neurons, where they provide strength and stability to the long
axons that nerve cells use to transmit information. A mutation in the neurofilament genes can cause amyotrophic lateral sclerosis (ALS) where an
abnormal accumulation of neurofilaments in the cell bodies and axons of motor neurons occurs. This accretion explains the axon degeneration and
muscle weakness seen in these patients.
,Whereas cytoplasmic intermediate filaments form ropelike structures, the intermediate filaments lining
and strengthening the inside surface of the inner nuclear membrane are organized as a two-dimensional
meshwork. As mentioned earlier, the intermediate filaments that form this tough nuclear lamina are
constructed from a class of intermediate filament proteins called lamins. The collapse and reassembly of
the nuclear lamina during the cell cycle is controlled by the phosphorylation and dephosphorylation of
the lamins. Phosphorylation of lamins by protein kinases weakens the interactions between the lamin
tetramers and causes the filaments to fall apart. Defects in a particular nuclear lamin are associated with
certain types of the disorder progeria. Children with progeria have wrinkled skin, lose their teeth and
hair, and often develop severe cardiovascular disease by the time they reach their teens.
Many intermediate filaments are further stabilized and reinforced
by accessory proteins, such as plectin, that cross-link the
filaments into bundles and connect them to microtubules, to
actin filaments, and to adhesive structures in desmosomes.
Mutations in the gene for plectin cause a disease that combines
features of epidermolysis bullosa simplex (caused by disruption
of skin keratin), muscular dystrophy (caused by disruption of
intermediate filaments in muscle), and neurodegeneration
(caused by disruption of neurofilaments). Plectin and other
proteins also interact with protein complexes that link the
cytoplasmic cytoskeleton to structures in the nuclear interior,
including chromosomes and
the nuclear lamina. These bridges mechanically couple the
nucleus to the cytoskeleton, and they are involved in many
processes, including the movement and positioning of the nucleus within the cell interior and the overall organization of the cytoskeleton.
MICROTUBULES
Microtubules have a crucial organizing role in all eukaryotic cells. These long and relatively stiff, hollow tubes of protein can rapidly disassemble in one
location and reassemble in another. In a typical animal cell, microtubules grow out from a small structure near the center of the cell called the
centrosome. Extending out toward the cell periphery, they create a system of tracks within the cell, along which vesicles, organelles, and other cell
components can be transported. 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. These hairlike structures extend
from the surface of many eukaryotic cells, which use them either to swim or to sweep fluid over their surface.
Microtubules are built from subunits—molecules of tubulin—each of which is a dimer composed of two
very similar globular proteins called α-tubulin and β-tubulin, bound tightly together by noncovalent
interactions. The tubulin dimers stack together to form the wall of the hollow, cylindrical microtubule.
This tube like structure is made of 13 parallel protofilaments, each a linear chain of tubulin dimers with α-
and β-tubulin alternating along its length. Each protofilament has a structural polarity, with α-tubulin
exposed at one end and β-tubulin at the other. The end with β-tubulin showing is called its plus end, and
the opposite end, which contains exposed α-tubulin, is called the minus end. The polarity of the
microtubule is crucial, both for the assembly of microtubules and for their role once they are formed.
Inside cells, microtubules grow from specialized organizing centers that control the location, number, and
orientation of the microtubules. The centrosome consists of a pair of centrioles, surrounded by a matrix
of proteins. The centrosome matrix includes hundreds of ringshaped
structures formed from a special type of tubulin called γ-tubulin, and each
γ-tubulin ring complex serves as the starting point, or nucleation site, for
the growth of one microtubule. The αβ-tubulin dimers add to each γ-
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.
The paired centrioles at the center of an animal cell centrosome are
curious structures. Each centriole, sitting perpendicular to its partner, is
made of a cylindrical array of short microtubules. Yet centrioles have no
role in the nucleation of microtubules from the centrosome: the γ-tubulin
ring complex alone is sufficient. Centrioles do, however, act as the
organizing centers for the microtubules in cilia and flagella, where they are
called basal bodies.
Once a microtubule has been nucleated, it typically grows outward from the organizing center for many minutes by the addition of αβ-tubulin dimers to
its free plus end. Then, the microtubule can suddenly undergo a transition that causes it to shrink rapidly by losing tubulin dimers from its plus end. The
, microtubule may shrink partially and then start growing again, or it may disappear completely, to be replaced by a new microtubule that grows from the
same γ-tubulin ring complex. This switching back and forth between polymerization and depolymerization, is known as dynamic instability.
A microtubule growing out from the centrosome can be prevented from disassembling if its plus end is stabilized by attachment to another molecule or
cell structure so as to prevent its depolymerization. If stabilized by attachment to a structure in a more distant region of the cell, the microtubule will
establish a relatively stable link between that structure and the centrosome.
The dynamic instability of microtubules stems from the intrinsic capacity of
tubulin dimers to hydrolyze GTP to GDP and phosphate. Each free tubulin
dimer contains one GTP molecule tightly bound to β-tubulin, which
hydrolyzes the GTP to GDP shortly after the dimer is added to a growing
microtubule. The GDP produced by this hydrolysis remains tightly bound to
the β-tubulin. When polymerization is proceeding rapidly, tubulin dimers
add to the end of the microtubule faster than the GTP they carry is
hydrolyzed. As a result, the end of a rapidly growing microtubule is
composed entirely of GTP-tubulin dimers, which form a “GTP cap”.
Because of the randomness of chemical processes, however, it will
occasionally happen that the tubulin dimers at the free end of the
microtubule will hydrolyze their GTP before the next dimers are added, so
that the free ends of protofilaments are now composed of GDP-tubulin.
These GDP-bearing dimers associate less tightly, tipping the balance in favor
of disassembly. The GDP-tubulin that is freed as the microtubule
depolymerizes joins the pool of unpolymerized tubulin already in the
cytosol.
Drugs that prevent the polymerization or depolymerization of tubulin dimers can have a rapid and profound effect on the organization of microtubules
and thereby on the behavior of the cell. If a cell in mitosis is exposed to the drug colchicine, which 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. The drug Taxol has the opposite effect on microtubule growth. It binds tightly to microtubules and prevents them from losing subunits.
Despite this difference in the mechanism of action, Taxol and colchicine both arrest dividing cells in mitosis. For the mitotic spindle to function,
microtubules must be able to assemble and disassemble. Because cancer cells divide in a less controlled way than do normal cells of the body, they can
sometimes be destroyed by drugs that stabilize or destabilize microtubules. Such antimitotic drugs, which include colchicine and Taxol, are used to treat
cancers.
When a cell has differentiated into a specialized cell type, the dynamic instability
of its microtubules is often suppressed by proteins. The stabilized microtubules
then serve to maintain the organization of the differentiated cell. Most
differentiated animal cells are polarized; one end of the cell is structurally or
functionally different from the other. This helps to position organelles in their
required location within the cell and to guide the streams of vesicular and
macromolecular traffic moving between one part of the cell and another.
Movement guided by microtubules is immeasurably faster and more efficient
than movement driven by free diffusion. The microtubules in living cells do not
act alone. Their activity depends on a large variety of accessory proteins that
bind to them. Some of these microtubule-associated proteins stabilize
microtubules against disassembly, for example, while others link microtubules to
other cell components, including the other types of cytoskeletal filaments.
If a living cell is observed in a light microscope, its cytoplasm is seen to be in
continual motion. This saltatory movement is much more sustained and
directional than the continual, small, Brownian movements caused by random thermal motions. Saltatory movements can occur along either
microtubules or actin filaments. In both cases, the movements are driven by motor proteins, which use the energy derived from repeated cycles of ATP
hydrolysis to travel steadily along the microtubule or actin filament in a single direction. Because the motor proteins also attach to other cell
components, they can transport this cargo along the filaments. The motor proteins that move along cytoplasmic microtubules belong to two families:
۰ The kinesins generally move toward the plus end of a microtubule (outward from the cell body)
۰ The dyneins move toward the minus end (toward the cell body)
Kinesins and cytoplasmic dyneins are generally dimers that have two globular ATP-binding heads
and a single tail. The heads of kinesin and cytoplasmic dynein 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)
