Where is the cytoskeleton located
THE CYTOSKELETON - MICROTUBULES, INTERMEDIATE FILAMENTS, MICROFILAMENTS
the cytoskeleton of a eukaryote
specifically of an animal cell has three
kinds of cytoskeletal filaments which
provide the structure aid in movement
and help with transport within the cell
microtubules organize the positions of
organelles and direct intracellular
transport intermediate filaments are
rope like fibers found along the
interface of the nuclear envelope and
they also build a network of cables
which connect the cells of epithelial
sheets these filaments provide
mechanical strength microfilaments or
actin filaments are most concentrated
right beneath the plasma membrane at the
so called cortex of the cell and they
control the outer shape of the cell and
are important in locomotion actin can
form several kinds of cell surface
projections including micro villi
lamellipodia and philip Odia these help
move cells over solid substrates the
three kinds of cytoskeleton filaments
work in concert with countless accessory
proteins which attach the filaments to
each other and to other cell components
and direct their assembly distribution
and disassembly the cytoskeleton is not
a static structure but rather is dynamic
able to change or persist to suit the
cell's needs this is because the
cytoskeletal structures are composed of
tiny polar subunits that can rapidly
assemble and disassemble thanks to weak
non covalent linkages into polymers
these macro molecular components of the
cytoskeleton filaments are constantly in
flux and nucleation of a cytoskeletal
polymer is the rate-limiting step since
the small initial aggregate is less
stable and more likely to fall apart
it is important to note that
cytoskeletal filaments do not build by
the addition of a single subunit at a
time
multiple proto filaments are being built
constantly which then associate
laterally through non covalent or
hydrophobic interactions this composite
structure is much harder to break some
cells in your body require rapid
rearrangement of your cytoskeleton while
others require the maintenance of stable
structures a white blood cell pursuing a
bacterium is able to rapidly move thanks
to a perpetually shifting leading edge
of actin meanwhile and established
neuron or epithelial cell requires a
more stable cytoskeletal structure in
the case of epithelial tissues the
cytoskeleton also maintains polarity
over the course of the cell's lifetime
specialized cell surface protrusion face
the lumen from the apical surface
providing more surface area for the
transfer of nutrients while the basal
lateral surface is much flatter
let's discuss each type of cytoskeleton
filament in more detail in this video we
will mostly focus on microfilaments and
microtubules microfilaments composed of
the globular protein actin are a double
helix six nanometers in diameter they
are organized into linear bundles 2d
networks and 3d networks loosely
orthogonal e cross linked with
properties of semi solid gels actin
molecules are tightly bound to an ATP
molecule when they are added to a
growing polymer the ATP is soon
hydrolyzed to ADP this hydrolysis makes
it more likely to dissociate from the
end of the filament
if actin molecules are added quickly
enough the actin filament can acquire
what's called an ATP cap tred milling is
observed in actin filaments and
microtubules mostly in actin filaments
in this process the filament appears to
be moving but really one end is growing
while the other is shrinking growth of
the polymer proceeds until the
concentration of free monomer is such
that the growth at the positive end
equals disassembly at the negative end
actin filaments are typically nucleated
at the plasma membrane and external
signals can trigger this nucleation
nucleation is promoted by the ARP
complex ARP stands for actin related
proteins because they are 45% identical
to actin the ARP complex is also known
as the ARP 2 3 complex it nucleates
actin filament growth from the minus end
so the plus end can grow rapidly ARP
nucleates most efficiently when it is
attached to the side of another actin
filament the result is a gel-like tree
like web with 70 degree angles at the
leading edge actin filaments become
capped by capping protein which means
they can neither grow nor shrink
meanwhile cofilin causes d
polymerization getting rid of older
filaments this causes the actin filament
network as a whole to move forward
despite individual filaments remaining
stationary
actin filament nucleation can also be
triggered by Foreman's however the actin
filaments nucleated by this accessory
protein are not branched instead the
filaments form parallel bundles these
parallel bundles formed for instance
during cell division they form the
cleavage furrow that helps to daughter
cells pinch off actin has many many
accessory proteins these are just a few
of the most important ones
intermediate filaments are ten
nanometers in diameter and are more
stable than actin filaments they are
composed of intermediate filament
proteins which are a diverse family of
proteins that are elongated in fibrous
you can think of them as cables they
help the cell maintain its shape by
bearing tension they anchor organelles
and structure the nuclear lamina they
are important in epithelial tissues
where together with proteins and
desmosomes they form cell to cell
connections check out my video on
junctions to learn more while
intermediate filaments resist tension
microtubules mostly resist compression
microtubules are hollow tubes composed
of tubulin dimers which are made of two
globular proteins alpha and beta tubulin
these heterodimers spontaneously bind
together forming a proto filament
thirteen such protofilaments arranged
together into a cylinder forming a
microtubule a microtubule is 23
nanometers in diameter and the inside
diameter is 15 nanometers while
microfilaments have some bend to them
microtubules are much more rigid hence
they are long and straight microtubules
are polar molecules the positively
charged end with beta subunits exposed
grows relatively quickly while the
negatively charged end with alpha
subunits exposed grows relatively slowly
as with actin different kinds of
proteins alter properties of growing
microtubule ends remember how we
mentioned that actin molecules are bound
to ATP and then this ATP gets hydrolyzed
to ADP soon after the actin molecule
gets added on to a growing filament well
microtubules are similar except they
have gtp tightly bound to the tubulin
heterodimer again once the dimer binds
into a growing microtubule the gtp soon
hydrolyzes to GDP this reduces the
affinity of the subunit to lateral proto
filaments as well as the subunits in
front and behind it hence increasing the
odds of it dissociating
dynamic instability occurs due to
differences between the two polar ends
of the microtubule if the rate of
addition of subunits exceeds the rate of
hydrolysis of gtp to GDP the microtubule
acquires a GTP cap a microtubule without
a GTP cap d polymerizes around 100 times
faster than one with a GTP cap hence a
microtubule with a GTP cap grows rapidly
if the cap is lost because nucleotide
hydrolysis occurs more quickly than
addition
then catastrophe occurs and the
microtubule begins to shrink an event
called a rescue occurs if gtp bound
subunits are added to the shrinking end
fast enough to form a new cap
microtubule nucleation occurs primarily
near the nucleus
since microtubules extend from the
center of the cell they establish a
general coordinate system then the cell
can use various measuring mechanisms to
organize itself microtubules are
nucleated and organized by microtubule
organizing centers or MTO C's
centrosomes containing a pair of
centrioles at right angles to one
another are the primary MTO C's they are
centrally located organelles that act as
the spindle pole during mitosis and
meiosis which separates the chromosomes
rapid reorganization of the cytoskeleton
occurs during cell division after
chromosomal replication we can see the
bipolar mitotic spindle as mentioned
previously actin is responsible for the
contractile ring that pinches the cell
into
basal bodies are also MTO sees and are
found in cilia and flagella cilia and
flagella have the same cross-section
featuring a 9+2 structure meaning nine
doublet microtubules and two single
microtubules the nine doublets are in a
ring and their position relative to one
another is maintained thanks to necks in
between them there is also an inner and
an outer Dinan arm motor proteins that
allow one doublet to move along the
other I will go into more detail on the
structure of cilia and flagella in a
later video
in fact this video is just an
introduction to the cytoskeleton and
I'll be making several more in-depth
videos on the topic please subscribe to
see them when they're uploaded
