The Cytoskeleton

Question 1

Cytoskeleton

Answer 1

An interconnected network of filamentous polymers and regulatory
proteins

Question 2

Controls

Answer 2

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

Question 3

Types

Answer 3

Actin microfilaments
Microtubules
Intermediate filaments

Question 4

Structure of AMFs

Answer 4

polymers of the protein Globular
actin - G-actin.
G-actin molecule is polar and have structurally different regions.
Bound to ATP

Question 5

How do they polymerise?

Answer 5

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.

Question 6

What molecules are bound to ATP

Answer 6

Free G-actin molecules are bound to ATP,
F-actin molecules are
bound to ADP
ATP binding/hydrolysis regulate actin filament
polymerisation & disassembly.

Question 7

How are F actin filaments arranged

Answer 7

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

Question 8

According to the needs of the cell

Answer 8

Actin microfilaments get longer and shorter (polymerise
and depolymerise)

Question 9

minus) end (pointed end) is more
commonly associated with
depolymerization (disassembly

Answer 9

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.

Question 10

(plus) end (barbed end) is more commonly
associated with polymerization
(assembly)

Answer 10

It is the the faster-growing end.

Question 11

Formation of AMFs

Answer 11

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.

Question 12

actin treadmilling?

Answer 12

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!

Question 13

Function of AMFs

Answer 13

Help a cell or
parts of a cell
to move.
Ø Determine cell
shape.

Question 14

structure to its
function

Answer 14

treadmilling
branching off
cross-linking

Question 15

Lamellipodia
Filopodia

Answer 15

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.

Question 16

Actin Binding Proteins
1- Actin monomer (G-actin) binding proteins

Answer 16

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!)

Question 17

2- Actin-nucleating proteins - their structure resembles actin
structure

Answer 17

Function: Accelerate polymerisation to generate branched or
straight filaments

Question 18

Actin-filament binding proteins

Answer 18

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.

Question 19

Filament severing proteins – cut
the actin filaments.

Answer 19

Function: Cut the actin filaments

Question 20

Filament destabilising proteins

Answer 20

Function: Control actin
filament disassembly

Question 21

Cross-linking proteins

Answer 21

organise actin filaments into bundles
and networks.
Three forms of crosslinked actin filaments created by different
crosslinking proteins.

Question 22

Myosin Superfamily

Answer 22

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

Question 23

MT Structure

Answer 23

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

Question 24

Tubulin dimers form protofilaments.
Ø Protofilaments form microtubules

Answer 24

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

Question 25

How wide?

Answer 25

25 nm wide.

Question 26

Where are the microtubules made in the cell?

Answer 26

.Microtubule nucleation requires help from other factors, g-tubulin and
accessory proteins.
* g-tubulin and the
accessory proteins
are enriched at the
MTOC.

Question 27

single, well-defined MTOC, called
‘centrosome

Answer 27

The MTOC initiates the assembly (nucleates) of microtubules.

Question 28

After nucleation, microtubules can grow and shrink

Answer 28

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’.

Question 29

1

Answer 29

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)

Question 30

2

Answer 30

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.

Question 31

Scenario 1- Rapid growth of the microtubule

Answer 31

Not enough time to hydrolyse GTP-tubulin
Ø A GTP-cap is formed

Question 32

Scenario 2- Slow growth of the microtubule

Answer 32

Ø Sufficient time to hydrolyse GTP-tubulin
Ø Catastrophe!

Question 33

MT tredmilling

Answer 33

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.

Question 34

Function of MT

Answer 34

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.

Question 35

How do microtubules control cell motility?

Answer 35

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.

Question 36

What controls the polymerization/organisation
of microtubules?
Microtubule-binding proteins
What controls the filament dynamics and
organisation?

Answer 36

Microtubule-binding proteins
Microtubule-associated proteins - MAPs

Question 37

1- Microtubule Plus-end binding proteins

Answer 37

Function: affect the frequency
of catastrophes or rescues.
Examples:
v Catastrophe factor:
increases the rate of
catastrophes
v XMAP215: decreases the
rate of catastrophes

Question 38

2- Tubulin-sequestering and microtubule-severing proteins

Answer 38

Function: Destabilise microfilaments by:
* sequestering tubulin molecules and inhibiting their
incorporation into the microtubule
* severing the microtubule

Question 39

3- Motor proteins

Answer 39

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.

Answer 40

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.

Question 41

Where are IFs found

Answer 41

IFs are
found only in some metazoans (vertebrates, nematodes
and molluscs)

Question 42

IFs are encoded by 70 different genes (in humans)

Answer 42

Ø Different IFs have different functions.
Ø Different IFs are expressed in different tissues.

Question 43

Structure

Answer 43

monomer with a-helical region
coiled-coil dimer
staggered tetramer
8 tetramers form the unit length filaments (ULFs)

Question 44

Structure of IFs

Answer 44

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.

Question 46

IFs are
considered as

Answer 46

strongest and
most stable
elements of the
cytoskeleton.

Question 47

Post-translational modifications control the
shape of IFs

Answer 47

Ø Chemical modification of IFs controls their shape and
function e.g.:
Ø Phosphorylation and dephosphorylation e.g., during
mitosis.

Question 48

Function

Answer 48

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.

Question 49

Ø Class I – Acidic keratins and
Class II – Basic keratins

Answer 49

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…

Question 50

Class III – Desmin, GFAP, vimentin, peripherin

Answer 50

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.

Question 51

Class IV – Neurofilaments

Answer 51

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

Question 52

Class V – Lamins

Answer 52

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.

Question 53

Class VI – Nestin

Answer 53

Ø Involved in axonal growth in
developing neurones.
Ø Abundant in neuronal
precursor cells.