Week 5: The cytoskeleton
The cytoskeleton: an overview
Microtubule: 25n, thick, polar occurs in animal, plant, and
fungal cells
Intermediate filament: 10nm thick, non-polar occurs only
in animal cells
Actin filament: 7 nm thick, polar occurs in animal, plant,
and fungal cells
Polarity: means that something (e.g. a molecule, cell or
organism) displays spatial differences in structure: one end
differs from the other end
Microtubules are hollow tubes consisting of alfa- and beta-
tubulin dimers
Microtubules are polar, with structurally distinct ends
(referred to as the plus and minus end)
The hollow tubes consist of 13 profilaments
Beta-tubulin is exposed at the plus end, and alfa-tubulin at
the minus end
In a test tube, microtubules can grow at both ends; growth
at the plus end is more rapid
Most assembly in cells takes place at plus ends
Microtubules grow from microtubule organizing centers (e.g.
centrosomes)
In animal cells, the centrosome is the major microtubule
organizing center
Plant and fungal cells lack centrosomes
Gama-tubulin: a special type of tubulin, which acts as a
nucleation site for the growth of one microtubule
Microtubules display dynamic instability
Switching back and forth between polymerization and
depolymerization is a process known as dynamic instability
Microtubules grow for a while (tubulin dimers are added to
the tip) and can then suddenly shrink (tubulin dimers
dissociate from the tip)
, Each microtubule grows and shrinks independently of his
neighbours
Depolymerization is faster than polymerization
The tips of growing and shrinking microtubules are
different
Dynamic instability of microtubules
Each tubulin dimer contains a GTP molecule tightly bound
to beta-tubulin … which hydrolyzes to GDP shortly after
incorporation in the microtubule
GTP-tubulin binds more tightly in the microtuble than GDP-
tubulin
Loss of the GTP-cap results in a catastrophe
Growing microtubules have GTP-bound tubulin at their
ends enabling rapid association of more GTP-tubulin
Shrinking microtubules also have GDP-bound tubulin at
their ends that lost sideways affinity. Rapid of subunits
Cytoskeleton visualization: GFP coupled to tubulin visualizes
microtubules in living cells
Reorganizing microtubules in a plant cell in which GFP: beta-
tubulin is expressed
Dynamic instability of microtubules: summary
Alternating phases of growth (polymerization),
catastrophes, shrinkage (depolymerization), and rescues
Since all alfa/beta dimers point into the same direction,
microtubules are polar (with two distinct ends: the plus
and minus end)
Growth at the plus end is faster than growth at the minus
end (test tube)
GTP-tubulin exposed at the plus end forms a GTP-cap, and
GTP-tubulin binds strongly to its neighbors
GTP hydrolyzes to GDP in the microtubule lattice
With GDP-tubulin exposed, microtubules shrink, since GDP-
dimers bind less tightly
Gamma-tubulin stabilizes the minus end and is often a
start point (nucleation site) for polymerization
The cytoskeleton: an overview
Microtubule: 25n, thick, polar occurs in animal, plant, and
fungal cells
Intermediate filament: 10nm thick, non-polar occurs only
in animal cells
Actin filament: 7 nm thick, polar occurs in animal, plant,
and fungal cells
Polarity: means that something (e.g. a molecule, cell or
organism) displays spatial differences in structure: one end
differs from the other end
Microtubules are hollow tubes consisting of alfa- and beta-
tubulin dimers
Microtubules are polar, with structurally distinct ends
(referred to as the plus and minus end)
The hollow tubes consist of 13 profilaments
Beta-tubulin is exposed at the plus end, and alfa-tubulin at
the minus end
In a test tube, microtubules can grow at both ends; growth
at the plus end is more rapid
Most assembly in cells takes place at plus ends
Microtubules grow from microtubule organizing centers (e.g.
centrosomes)
In animal cells, the centrosome is the major microtubule
organizing center
Plant and fungal cells lack centrosomes
Gama-tubulin: a special type of tubulin, which acts as a
nucleation site for the growth of one microtubule
Microtubules display dynamic instability
Switching back and forth between polymerization and
depolymerization is a process known as dynamic instability
Microtubules grow for a while (tubulin dimers are added to
the tip) and can then suddenly shrink (tubulin dimers
dissociate from the tip)
, Each microtubule grows and shrinks independently of his
neighbours
Depolymerization is faster than polymerization
The tips of growing and shrinking microtubules are
different
Dynamic instability of microtubules
Each tubulin dimer contains a GTP molecule tightly bound
to beta-tubulin … which hydrolyzes to GDP shortly after
incorporation in the microtubule
GTP-tubulin binds more tightly in the microtuble than GDP-
tubulin
Loss of the GTP-cap results in a catastrophe
Growing microtubules have GTP-bound tubulin at their
ends enabling rapid association of more GTP-tubulin
Shrinking microtubules also have GDP-bound tubulin at
their ends that lost sideways affinity. Rapid of subunits
Cytoskeleton visualization: GFP coupled to tubulin visualizes
microtubules in living cells
Reorganizing microtubules in a plant cell in which GFP: beta-
tubulin is expressed
Dynamic instability of microtubules: summary
Alternating phases of growth (polymerization),
catastrophes, shrinkage (depolymerization), and rescues
Since all alfa/beta dimers point into the same direction,
microtubules are polar (with two distinct ends: the plus
and minus end)
Growth at the plus end is faster than growth at the minus
end (test tube)
GTP-tubulin exposed at the plus end forms a GTP-cap, and
GTP-tubulin binds strongly to its neighbors
GTP hydrolyzes to GDP in the microtubule lattice
With GDP-tubulin exposed, microtubules shrink, since GDP-
dimers bind less tightly
Gamma-tubulin stabilizes the minus end and is often a
start point (nucleation site) for polymerization
