the

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