The Cytoskeleton Flashcards
Cytoskeleton
An interconnected network of filamentous polymers and regulatory
proteins
Controls
Ø Shape of the cell
Ø Mechanical strength of the cell
Ø Movement of the cell
Ø Position of organelles
Ø Intracellular transport
Ø Cell division
Ø Chromosome segregation during
cell division
Types
Actin microfilaments
Microtubules
Intermediate filaments
Structure of AMFs
polymers of the protein Globular
actin - G-actin.
G-actin molecule is polar and have structurally different regions.
Bound to ATP
How do they polymerise?
G-actin subunits polymerise head-to-tail to form the filamentous
actin, F-actin
Because the G-actin molecule has polarity, the F-actin polymer also has polarity
and have structurally different ends.
What molecules are bound to ATP
Free G-actin molecules are bound to ATP,
F-actin molecules are
bound to ADP
ATP binding/hydrolysis regulate actin filament
polymerisation & disassembly.
How are F actin filaments arranged
Actin filaments are around 8 nm wide.
Ø Actin filaments have right-handed helix conformation.
Ø F-actin filaments are arranged in a double-helix,
forming the actin filaments
According to the needs of the cell
Actin microfilaments get longer and shorter (polymerise
and depolymerise)
minus) end (pointed end) is more
commonly associated with
depolymerization (disassembly
If there is need, polymerization can
take place but at much slower rate.
(plus) end (barbed end) is more commonly
associated with polymerization
(assembly).
It is the the faster-growing end.
(plus) end (barbed end) is more commonly
associated with polymerization
(assembly)
It is the the faster-growing end.
Formation of AMFs
Step 1: Nucleation = G-actins form an initial aggregate (also
known as the ‘nucleus’)
Step 2: Elongation = G-actins polymerise at both sides of the
nucleus to form the F-actin molecule.
Step 3: Steady state = Through polymerization/ depolymerization
F-actin structure is maintained.
actin treadmilling?
Ø The barbed-end (plus-end)
of the actin filament grows
in length.
Ø The pointed-end (minus
end) shrinks in length.
Ø Total length of the
filament does not change!
Function of AMFs
Help a cell or
parts of a cell
to move.
Ø Determine cell
shape.
structure to its
function
treadmilling
branching off
cross-linking
Lamellipodia
Filopodia
Lamellipodia are broad, flat, sheet-like projections of the cell membrane that extend from the leading edge of a migrating cell. They are rich in actin filaments and help the cell move by adhering to the surface and pulling the cell forward.
Filopodia are thin, finger-like projections from the cell surface made of bundled actin filaments. They function as sensory structures, allowing the cell to probe its environment and guide movement or signaling.
Actin Binding Proteins
1- Actin monomer (G-actin) binding proteins
Function: control actin filament assembly
Examples:
v Thymosin (inhibitor) = When bound to G-actin, G-actin stays
in a locked state = cannot associate with either the plus or
the minus-end of the filament.
v Profilin = When bound to G-actin, G-actin can be associated
with the plus-end of the filament.
(Profilin and thymosin compete to bind to G-actin!)
2- Actin-nucleating proteins - their structure resembles actin
structure
Function: Accelerate polymerisation to generate branched or
straight filaments
Actin-filament binding proteins
Two main classes:
a) Those that bind to the side of the filament.
Function: Stabilise and stiffen the actin filament.
Example: Tropomyosin
b) Those that bind to the ends of a filament – called capping-proteins.
Function: Stabilise the actin filament.
Filament severing proteins – cut
the actin filaments.
Function: Cut the actin filaments
Filament destabilising proteins
Function: Control actin
filament disassembly
Cross-linking proteins
organise actin filaments into bundles
and networks.
Three forms of crosslinked actin filaments created by different
crosslinking proteins.
Myosin Superfamily
Myosin superfamily members are actin binding motor proteins that
regulate cell movement.
Example: Myosin II of the muscle cells.
Ø 2 copies of each of two light chains – form the
myosin head
Ø 2 heavy chains – form the myosin tail
Ø Tails bundle up with the tails of other myosin
molecules to form thick myosin filaments
MT Structure
Ø Microtubules are polymers of the protein tubulin.
GTP
Ø The protein tubulin is a heterodimer made of two subunits
a-tubulin
Ø Microtubules are polymers of the protein tubulin.
GTP
Ø The protein tubulin is a heterodimer
made of two subunits.
b-tubulin
GTP/GDP
a tubulin GTP
Tubulin dimers form protofilaments.
Ø Protofilaments form microtubules
This ‘hollow tube’
structure makes
microtubules stiff and
difficult to bend.
(Microtubules are among the stiffest
structural elements found in animal
cells.)
Ø Microtubules have
structural polarity.
Ø 13 protofilaments come together to form microtubules
How wide?
25 nm wide.
Where are the microtubules made in the cell?
.Microtubule nucleation requires help from other factors, g-tubulin and
accessory proteins.
* g-tubulin and the
accessory proteins
are enriched at the
MTOC.
single, well-defined MTOC, called
‘centrosome
The MTOC initiates the assembly (nucleates) of microtubules.
After nucleation, microtubules can grow and shrink
Ø The change from growth
to shrinkage is called
‘catastrophe’.
Ø The change from
shrinkage to growth is
called ‘rescue’.
Ø The rapid interconversion
between a growing and
shrinking state is called
‘dynamic instability’.
1
In the free tubulin dimers, b-tubulin
is (mostly) bound to GTP – we will call
this dimer: GTP-tubulin.
But…
GTP-tubulin wants to polymerise.
So, it joins the chain.
(= incorporated into growing
microtubule)
2
When incorporated in a
microtubule, GTP is hydrolysed to
GDP.
and… GDP-tubulin wants to
depolymerise!
(because the shape of the tubulin
dimers changes from straight to
bendy.
Scenario 1- Rapid growth of the microtubule
Not enough time to hydrolyse GTP-tubulin
Ø A GTP-cap is formed
Scenario 2- Slow growth of the microtubule
Ø Sufficient time to hydrolyse GTP-tubulin
Ø Catastrophe!
MT tredmilling
Ø Some microtubules exhibit
treadmilling.
Ø In cases where neither end of
microtubule is stabilised, tubulin
dimers are added to the (+) end
and lost from the (-) end.
Ø Overall length of these
microtubules remains fairly
constant, but the dimers are
always in flux.
Function of MT
ü To form an architectural framework
ü To form an internal transport network for the trafficking of
vesicles.
ü To organise movement of chromosomes during mitosis
ü To generate force and movement in motile structures such as cilia
and flagella.
How do microtubules control cell motility?
Ø Flagella and cilia are highly specialized motility structures built from
microtubules and dynein.
Ø Flagella are found on many protozoa and on sperm. They enable the cells
to which they are attached to swim through liquid media.
Ø Cilia beat with a whiplike motion which can either propel single cells
through a fluid or can move fluid over the surface of a group of cells in a
tissue.
What controls the polymerization/organisation
of microtubules?
Microtubule-binding proteins
What controls the filament dynamics and
organisation?
Microtubule-binding proteins
Microtubule-associated proteins - MAPs
1- Microtubule Plus-end binding proteins
Function: affect the frequency
of catastrophes or rescues.
Examples:
v Catastrophe factor:
increases the rate of
catastrophes
v XMAP215: decreases the
rate of catastrophes
2- Tubulin-sequestering and microtubule-severing proteins
Function: Destabilise microfilaments by:
* sequestering tubulin molecules and inhibiting their
incorporation into the microtubule
* severing the microtubule
3- Motor proteins
Dynein binds
to cargo and
“walks”
towards the
minus-end of
the
microtubule.
Kinesin binds
to cargo and
“walks”
towards the
plus-end of
the
microtubule.
Motor proteins require energy from ATP hydrolysis
Ø Structural polarity (minus and
plus ends of microtubules)
determines the direction of the
molecular transport microtubules
support.
Ø ATP hydrolysis results in
reversible conformational changes
in motor proteins.
Where are IFs found
IFs are
found only in some metazoans (vertebrates, nematodes
and molluscs)
IFs are encoded by 70 different genes (in humans)
Ø Different IFs have different functions.
Ø Different IFs are expressed in different tissues.
Structure
monomer with a-helical region
coiled-coil dimer
staggered tetramer
8 tetramers form the unit length filaments (ULFs)
Structure of IFs
Ø IFs are highly stable polymers that have great mechanical
strength.
Ø Intermediate filament assembly and disassembly are
controlled by post-translational modification of individual
IF proteins.
IFs are
considered as
strongest and
most stable
elements of the
cytoskeleton.
Post-translational modifications control the
shape of IFs
Ø Chemical modification of IFs controls their shape and
function e.g.:
Ø Phosphorylation and dephosphorylation e.g., during
mitosis.
Function
ü Provide mechanical strength to cells.
ü Stabilise cell structure and resist tension: have
tensile strength and elasticity.
ü Support cell shape.
ü Hold organelles in position.
ü Anchor the cell in place.
Ø Class I – Acidic keratins and
Class II – Basic keratins
Ø Synthesised by epithelial
cells.
Ø Protect epithelial cells from
damage and stress.
Ø Main component of outer
hardened tissues e.g. horns,
hair, feathers, nails,
hooves…
Class III – Desmin, GFAP, vimentin, peripherin
Desmin:
Ø found in heart (cardiac) muscle
and muscles used for
movement (skeletal muscle).
Ø Desmin helps maintain the
structure of sarcomeres.
GFAP (Glial fibrillary acidic protein):
Ø Gives structure to astrocytes in
the brain.
Vimentin:
Ø Main IF of mesenchymal cells.
What are mesenchymal cells? Multipotent stromal cells that can
differentiate into a variety of cell types, including osteoblasts (bone
cells), chondrocytes (cartilage cells), myocytes (muscle cells) and
Peripherin:
Ø Expressed in peripheral
neurones.
Ø has important roles in neurite
outgrowth and stability, axonal
transport, and axonal
myelination.
Class IV – Neurofilaments
Ø Expressed in neurones,
Ø Involved in the regulation of
axon diameter.
Ø Biomarkers of brain damage in
cerebrospinal fluid.
Ø Different neurofilaments have
different sizes:
NF-light
NF-medium
NF-heavy
Class V – Lamins
Forms the nuclear lamina,
which lines the interior of
the nuclear envelope.
Ø Provides nucleus with
tensile strength and its
shape.
Ø Have roles in chromatin
organization and gene
regulation.
Class VI – Nestin
Ø Involved in axonal growth in
developing neurones.
Ø Abundant in neuronal
precursor cells.
