The cytoskeleton is composed of a network of filaments
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Concepts in Molecular Biology
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University Of Vienna
The actine is concentrated in the cell periphery, in this case the cells on the right are differentiated into neural cells.
The functions of the cytoskeleton is:
The three types of filaments:
- Actine helical polimers of a single protein called actine, they concentrate in the cell cortex...
in this case the cells on the right are differentiated into neural cells the functions of the cytoskeleton is the three types of filaments acti
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14 LC Concepts in Molecular Biology 28.10.2022
➢ The cytoskeleton is composed of a network of filaments
The actine is concentrated in the cell periphery, in this case the
cells on the right are differentiated into neural cells.
The functions of the cytoskeleton is:
The three types of filaments:
- Actine → helical polimers of a single protein called
actine, they concentrate in the cell cortex and are
important determining the cell shape and movement
- Microtubules → are cylindrical polimers of alpha-beta tubuline dimers. They are larger and more
rigid, involved in positioning cell organells and directing cellular traffic and organizing centrosome.
- Intermediate filaments→ such as keratin, important for conferring mechanical strength
Microtubules
Microtubules and actine are both nucleotide binding proteins; we have tubuline dimers that are gtp
binding and in case of beta tubuline there is hydrolysis happening in the microtuble office. Actine is a
monomer with atp binding and hydrolysis that occurs as a polymerases.
,14 LC Concepts in Molecular Biology 28.10.2022
Hydrolysis incorporation into the lattex is what
drives this phenomenon known as dynamic instability:
- Hydrolysis occurs upon incorporation, and this occurs with a slight
delay on that incorporation such that the microtubule is capped at
its end with a gtp cap; further down in the latex this hydrolysis has
occurred and you have gdp. The result of that hydrolysis is
changing the conformation of the molecule which gives it a
propensity to bend. Rad destebilazes the latex, and the result is
that microtubules (and also actine) is prone to spontaneous
disassembly which is called cathastrophy; very occasionally that
can also revert back with the rescue. This is a stochastic process
and occurs as long as there is energy input in the system, because
you need to replenish this supply of gtp.
The curious concequences of dynamic instability
One of the effects of this hydrolysis sycle is that you have a tendency of existing filaments to grow, but also
this cathastrophy that results in de-polamerizatiopn and that can go to completion. The result is that if you
take a solution of tubulin that is at the stedy stead concentration, over time the amount of polimer does
not change but the number of filaments keep going down. → over time you effectively would end up a
single enormously long filament if it wont break. The only way to maintain a sensible cytoskeleton is to
have nucleators and these are the sites where new filaments will emerge. This is the way that the cells
shapes the cytoskeleton.
, 14 LC Concepts in Molecular Biology 28.10.2022
Gamma tubuline complexes nucleate and cap microtubules
So, in the case of microtubules the nucleate in question is gamma tubulin that forms the
gamma tubulin ring complexis (or gamma turks). This gamma tubuline is arranged into a
ring, and the gamma tubulin molecules are in contact with alpha tubuline in the
microtubule lattex and they effectively template or provide this pre form C for the
nucleation of a new filament.
This gamma tubulin will remain associated with the microtubules that it forms; this is a
way now the cell can position those microtubules in the cells.
The place where gamma tubuline is concentrated in animal cells, is the centrosome and
this gamma tork complexes are embedded in this pericentriolar
material that surrounds this pair of centrioles.
In this cells (to the right) in which centrosomes have been
removed, you do still have microtubules, because you do remove
the catalyst; nucleation can still occur in the absence of MTSOCs.
Upstairs can be seen what nucleation looks like in vitrio, are isolated centrosomes which have been
supplied with tubuline and gtp; and you can see the fenomenon pof dynamic instability with growing and
shrinking microtubules occurring at fixed concentrations of tubuline. This is the kind of behavior that you
would see with both actine and microtubules but not with intermediates.
EUKARYOTIC COMPLEXITY
Prokaryots harbor homologs of actin and tubulin
Actine and microtubules is something that you see across eukaryots, in fact there are homologs in both
(also in prokaryots, where protein called ParM -related to actin- is involved in the segregation of plasmids -
effectively performing the role of microtubules- segregating them into the two dougher cells).
One thing that prokaryotes do not have is motors.
➢ Motors
Mortos come in three superfamilies:
- Myosins → walk on actine
- Kinesins → walk on microtubules
- Dynein → walk on microtubules what this things do is: they use the ATP hydrolysis to move various
cargo on those filaments in a directed manner and thereby they do all of these things that the
cytoskepleton is associated with, they position organellls, generate movement or contractility and
are totally exclusive to eukaryots.
The existence of motors explain why eukaryotic cells are on average 10 times larger then prokaryotic cells;
so things like motor neurons have neurites that can be up to 1 meter in length and if you didn’t have
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