Topic 4 - The cytoskeleton
Structural support: cytoskeleton allows cell to take on different shapes (1)
also important in allowing things around within the cell (molecule, vesicles, organelles) By movinf things
around it becomes important in organizing the contents within the cell.(2)=intracellular transport
Without the cytoskeleton your cuts would never heal because they rely on cell movements. Muscle cells
would be useless without one
(4) the positioning of organelles within the cell
-largest of cytoskleletal elements. Hollow tube like structure. (avg 25nm size), found in cytoplasm, very
dynamic and constantly changing shape, and also in cylia and flagella (where very stable)
Microtubules made up of tubulins. Important are alpha and beta tubulins (slightly different proteins) they form
a hetero dimer. 2-units, each is different. they are the building blocks. Both are bound together by non-
covalent bond, but they have a stable association. each of the alpha and beta is able to bind GTP, and its
presence on the beta subunit increases the changes that tubulin will form microtubules they stack up
in a protofilament, and because of this shape it has a polarity. Positive end has BETA tubulin exposed, and
at minus end an ALPHA tubulin is exposed. 13 protofilaments in a circle give you a microtubule.
Process of forming a microtubulin is much more likely when the GTP is bound to the beta than when it is
hydrolysed to GDP.
=> as time goes on beta is likely to hydrolyze some of the GTP, and you see them start to form towards the
minus end of the molecule.
*NEW GTP DIMERS ADDED AT PLUS END & HYDROLYSIS AT MINUS END*
process is called TREADMILLING and as a result you have a microtubule moving around in the cell
As soon as rate of addition of new GTP dimers falls off, then the plus end can become hydrolysed and as
this occurs the plus end begins to break down, and then the whole microtubule collapses in process called
catastrophe. drugs can be used to manipulate microtubules dynamics.
1.colchicine: binds to tubulin dimers and prevents the from joining microtubules. this greatly increases the
likely hood of the breaking down the microtubules.
2.Taxol: Comes from U tree, best know as anti cancer drug, and stabilizes microtubules in the form they
have in the cell by preventing them from growing or shrinking
To increase likelihood of microtubule formation, there are structures called microtubule organizing centres
(MTOC) they increase likelihood of microtubule formation. One of the best know MTOC is the centrosomes,
containing 2 centrioles at 90 degrees to each other, and surrounding that theres the diffuse pericentriolar
material, and within this material you often find gama tubulin, which is generally used as a template for
microtubule to grow on. Gama tubulin binds ALPHA tubulin. Meaning as it grows, the minus end is at the
centrosome, and the plus end is out. Helps in organizing. Theres also the basal bodies, which are also used
as a template
Microtubules and microfilaments function as highways within the cell. For highways to be usefull you need a
car. In the case of the cell they are motor proteins. Motor proteins are proteins that are capable of hydrolyzing ATP, and as a result of that change shape, and physically move. They have a hatch usually
because there point is to move things around. They bassicly "walk" around. Much more efficient than a car.
Kinesin and dyenins => 2 families of motor proteins.
they are families, and function in pairs. they have an attached point so they can attach cargo and move it
along. The main difference between the two is that kinesins always move towards the plus end and dyenins
always move towards the minus end. *specific directions. Minus ends are at the centrosome, and the plus
ends extend out towards the perifery of the cell, and because of that kinesins typically move "out-bound"
cargo, and dyenins move cargo back in towards centrosome.
Example of how theses proteins are used:
Axonal transport (axon can be up to 1m long)
Anything that happens along the axon requires transport otherwise the transport would be very slow.
there are cell in the skin of amphibians, fish and reptiles called chromatophores, they contain pigment
granules, and they are used to change the frogs colour. When pigment granules is aggregated in center of
cell, then anima