CYTOSKELETON
A highly dynamic
structure that is
continuously reorganized
as a cell changes shape,
divides, and responds to its
environment.
The bones and muscles of
the cell.
Intracellular transport of
organelles, segregation of
chromosomes at mitosis.
10 nm static
25nm dynamic
*
*
7nm
*
dynamic
INTERMEDIATE FILAMENTS enable the cell to withstand mechanical
stress. These are the toughest, most durable and are found in the
cytoplasm of most animal cells forming a network found in the nucleus
(nuclear lamina) and throughout the cytoplasm.
Epidermal cells - network indirectly connected to neighbors through desmosomes
Amino terminal
globular head
Carboxyl-terminal
globular tail tail
Form stable dimers
Globular head and
tail are exposed on
the surface,
interact with other
components of the
cytoplasm, and
vary greatly in size
and amino acid
sequence.
The central rod domains are all similar in size and amino acid
sequence and pack together to form filaments of similar diameter
Diverse - gut, skin, hair,
feathers, claws
formed from a mixture of
different keratin subunits
directly connect through cell-cell
junctions like desmosomes
epidermolysis bullosa simplex mutation in keratine
Intermediate filaments strengthen animal cells and are
prominent in cells subject to mechanical stress, like
nerve cell axons, muscle cells, epithelial cells.
Nuclear lamina - lines the inner face of the nucleus and provides
attachment sites for DNA-containing chromatin. Composed of lamins.
MICROTUBULES
DYNAMIC
usually
growing out
of a
Microtubule
Organizing
Center
Extending out into the cell periphery,
they create a system of tracks along
which vesicles, organelles etc can be
moved.
STRUCTURE OF MICROTUBULE
Tubulin is a dimer
cylinder made of 13 parallel
protofilaments
has structural polarity with
the alpha subunit exposed at
one end the minus end and
the beta subunit exposed at
the other end, the plus end.
Grows by adding individual
tubulin dimers
Microtubules are maintained by a balance of assembly and disassembly.
Cell contains half microbubules (which are continually graowing and
shrinking) and half free tubulin
Colchicine binds tightly to free tubulin and prevents its polymerization.
Taxol binds tightly to microtubules and prevents them from losing
subunits. Same overall result - arrests dividing cells in mitosis and
eventually kills dividing cells.
Polymerization of tubulin on a centrosome, the major
microtubule-organizing center in animal cells. Centrosomes
contain hundreds of rin-shaped structures formed of gamma
tubulin. This serves as the starting point - nucleation site. It
adds alph/beta tubulin in a specific orientation, with the minus
end embedded in the centrosome - growth occurs at the plus
end.
Not
centrioles function
unknown
GTP hydrolysis is thought to control the growth of microtubules:
leading to dynamic instability: each free tubulin dimer contains one
tightly bound GTP molecule that is hydrolyzed to GDP shortly after it is
added to the growing chain. When polymerization occurs more rapidly
(lots of tubulin) it grows, when GTP is hydrolyzed more quickly, it
shrinks
Once started, it will continue
GTP cap prevents
depolymerization
What is the function of this continual shooting out of new microbubules
and retracting them? “Reaching” out. Once the microtubule establishes
contact with another rmolecule or cell structure it will establish a
relatively stable link - a road that links this structure to the centrosome.
The result is a dynamic, but highly organized system of microtubules
linking selected parts of the cell. This is used to position organelles and
move vesicles.
This allows cells to modify the dynamic instability for particular
purposes - mitosis. And to fix a structure in a differentiated cell - as in
the axons of neurons, in which proteins bind to the ends and along their
length to stabilize them - maintaining the organization of the cell
Differentiated cells are
polorized - ex. dendrites,cell
body, axon
Movement along the microtubule is faster
and directed.
Intracellular organelle movement - taken with less than a minute between 1st and last frame
Both actin filaments and
microbubules are
involved in saltatory
movement - start/stop jerky
• Accessory proteins bind to microtubules
and serve various functions
– stabilize microtubules against disassembly
– coordinate and interact with other cytoskeleton
components
– distribute membranes
• Motor proteins use energy of ATP
hydrolysis to transport organelles, vesicles,
etc. along tracks provided by actin
filaments and microtubules
Motor protein
Microtubule motor proteins Kinesins and dyneins generally move in
opposite directions.
Heads are ATPhydrolyzing enzymes
Organelles move along microtubules in a directed fashion. Kinesins
move toward the plus end and dyneins move toward the minus end.
Both exist in many forms, each thought to transport different “cargo”.
The tail of the motor protein determines the cargo.
Both the endoplasmic reticulum and the golgi apparatus depend on
microtubules for their alignment and positioning. As the cell develops
and the ER grows, kinesins attached to the outside of the ER membrane
pull it outward along microtubules. Dyneins pull the golgi the other way.
When cells are treated with colchicine, microtubules disassemble and ER
collapses to the center of the cell, while the golgi fragments into small
vesicles.
Golgi yellow
ER - blue
Cilia of the human respiratory tract sweep layers of mucus with trapped dust etc
Cilia contain a core of
stable microtubules
arranged in a bundle
which grow from a
basal body. Are
covered in the plasma
membrane Figure 8C.
Move in a
whiplike motion
Flagella move in a repetitive wavelike
motion as in the flagella of sperm or this
green alga
“9 + 2” array of
cilia and flagella in
all eucaryotic cells.
Microtubule-associated proteins
serve as cross-links too hold the
bundle together and others
generate the force to bend.
Ciliary dynein resembles cytoplasmic dynein. Attached by its tail to one
microtubule, its heads interact with adjacent microtubules, sliding
between the two - generating the force needed to bend.
Because the two microtubules are
linked, they bend rather than slide.
ACTIN FILAMENTS: found in all eucaryotic cells. Depending on their
association with different proteins, actin filaments various permanent
structures such as (A) microvilli on brush-border cells lining the
intestine, (B) small contractile bundles in the cytoplasm that act like the
“muscles” of the cell, (C) temporary structures like the filopodia
(pseudopodia) of a crawling fibroblast, or (D) the contractile ring that
pinches the cytoplasm in two when an animal cell divides. Which
structures are formed is determined by the actin-binding proteins present
in the cell at this time.
Found as crosslinked bundles
and networks of
actin filaments
Actin filaments are
twisted chains of
identical globular
actin molecules, all
pointing the same
way - polar again.
They are thinner,
more flexible, and
usually shorter than
microtubules. There
are many more actin
filaments in a cell
than microtubules
Actin and tubulin polymerize by similar mechanisms. Actin filaments
grow from both ends, but plus end is faster. Actin filament is also
inherently unstable, disassembly at both ends. ATP is hydrolyzed instead
of GTP, and this promotes depolymerization. The ability to assemble
and disassemble is also required for actin filaments - toxins that prevent
polymerization or depolymerization freeze cell movements like crawling.
Regulate actin polymerization
Major classes o f actin-bindng proteins
control behavior of
filaments
Convert an actin gel into a
more fluid state
Hold actin filaments together
in a gel-like meshwork within
the cell cortex - just beneath
the plasma membrane
And microvilla
Form contractile bundles as in
muscle cells, serve as tracks along
which motor proteins transport
organells.
The plasma membrane must be strengthened by the cell cortex. This is a
framework of proteins attached to the membrane via transmembrane
proteins.The shape and mechanical properties of the plasma membrane is
determined by a meshwork of fibrous proteins - the cell cortex.
Red blood cells
are very simple,
allowing study
of the cell
cortex in simple
form.
Genetic
abnormalities in
spectrin structure
result in anemia,
spherical, fragile
rbcs
The spectrin-based cell cortex of human red blood cells. Much simpler
than other cells.
Actin filaments in most cells is more concentrated, linked by actin- binding
proteins into a meshwork gel that gives the membrane shape and
mechanical strength. The cortex provides the molecular basis for shape
changes and locomotion
• Cells must be able to move toward food,
along a tract (axon of a new neuron), toward
an infection (neutrophiles). This requires
actin, and entails coordinated changes of
many molecules in different regions of the
cell. Three interelated processes (all which
require actin) known to be essential include.
– 1. The cell pushes out protrusions at its leading
edge
– 2. these protrusions adhere to the surface the
cell is crawling over
– 3. the rest of the cell drags itself forward on
molecules which anchor it.
First step, pushing the cytoplasm forward, is driven by actin
polymerization.
A human fibroblast
Both filopodia and the larger
lamellipodia are exploratory, motile
structures that form and retract with
great speed. Both are generated by
rapid local growth of actin
filaments which are nucleated at the
plasm membrane and push it out
without tearing it
Growth of filopodia:
A nucleation complex at the plasma membrane (like the centrosome)
organizes growth of actin filaments. Difference between actin and
microtubules = nucleation complex is at the + or growing end.
Step 2. Transmembrane proteins including integrins which bind to the extracellular
matrix or other cells capture these areas of nucleation. Dark staining shows areas of
close contact between the glass and the fibroblast. Here, by staining actin, it is shown
that actin filaments terminate at or close to sites of contact. Therefore, external
adhesion molecules and internal actin filaments are working together in close contact.
Step 3. Cell
moves forward.
Exact mechanism
is unknown. But
release of
adhesion
molecules
(integrins) from
the substratum
(surface the cell is
moving over) and
contraction of the
cell cortex may
Step 3
do the trick.
Step 1
Step 2
Adhesion molecules
associated with actin
filaments.
MYOSINS:
All actin-dependent motor
proteins belong to the mysoin
family, provide energy by
hydrolyzing ATP, and move along
actin from the minus end to the
plus end. Studied in skeletal
muscle but found in other cells contractile bundles, contractile
ring of the dividing cell.
The heads hydrolyze ATP and change
shape to bind, detach and rebind.
Moving along the actin filament.
The tails of Myosin II can associate with
each other to form mysoin II filaments.
The tail varies and
determines which cell
components will be
dragged along
The head interacts with actin filaments and
contains the ATP hydrolyzing activity
dimer
Coiled-coil tail
ACTIN FILAMENTS: found in all eucaryotic cells. Depending on their
association with different proteins, actin filaments various permanent
structures such as (A) microvilli on brush-border cells lining the
intestine, (B) small contractile bundles in the cytoplasm that act like the
“muscles” of the cell, (C) temporary structures like the filopodia
(pseudopodia) of a crawling fibroblast, or (D) the contractile ring that
pinches the cytoplasm in two when an animal cell divides. Which
structures are formed is determined by the actin-binding proteins present
in the cell at this time.
Found as crosslinked bundles
and networks of
actin filaments
Myosin II has
two heads
pointing in the
opposite
direction. One
binds to actin
filaments in one
orientation and
moves them that
way. The other
binds an actin
filament in the
opposite
direction.
SOME ROLES OF MYOSIN-1 AND MYOSIN-2
Skeletal muscle cell: huge multinucleated cells also called muscle fibers
contain numerous myofibrils.
During muscle contraction actin
filaments slide against myosin
filaments.
Cardiac muscle and smooth muscle
(gut etc) have a different structure,
but use actin and myosin in a
similar way to contract.
A) What properties of intermediate filament monomers
distinguish them from the monomers
that make up actin filaments or microtubules?
A. They bind to each other covalently.
B. They are fibrous rather than globular proteins.
C. They do not bind and hydrolyze nucleotides.
D. There are numerous different types in different cell
types.
E. They are glycosylated.
(B) Which of the above differences makes intermediate
filaments less dynamic structures than
actin filaments or microtubules?
(