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The Cytoskeleton Flashcards

1
Q

Cytoskeleton

A

An interconnected network of filamentous polymers and regulatory
proteins

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2
Q

Controls

A

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

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3
Q

Types

A

Actin microfilaments
Microtubules
Intermediate filaments

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4
Q

Structure of AMFs

A

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

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5
Q

How do they polymerise?

A

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.

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6
Q

What molecules are bound to ATP

A

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

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7
Q

How are F actin filaments arranged

A

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

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8
Q

According to the needs of the cell

A

Actin microfilaments get longer and shorter (polymerise
and depolymerise)

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9
Q

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

A

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.

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10
Q

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

A

It is the the faster-growing end.

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11
Q

Formation of AMFs

A

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.

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12
Q

actin treadmilling?

A

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!

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13
Q

Function of AMFs

A

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

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14
Q

structure to its
function

A

treadmilling
branching off
cross-linking

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15
Q

Lamellipodia
Filopodia

A

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.

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16
Q

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

A

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!)

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17
Q

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

A

Function: Accelerate polymerisation to generate branched or
straight filaments

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18
Q

Actin-filament binding proteins

A

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.

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19
Q

Filament severing proteins – cut
the actin filaments.

A

Function: Cut the actin filaments

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20
Q

Filament destabilising proteins

A

Function: Control actin
filament disassembly

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21
Q

Cross-linking proteins

A

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

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22
Q

Myosin Superfamily

A

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

23
Q

MT Structure

A

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

24
Q

Tubulin dimers form protofilaments.
Ø Protofilaments form microtubules

A

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

25
How wide?
25 nm wide.
26
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.
27
single, well-defined MTOC, called ‘centrosome
The MTOC initiates the assembly (nucleates) of microtubules.
28
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’.
29
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)
30
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.
31
Scenario 1- Rapid growth of the microtubule
Not enough time to hydrolyse GTP-tubulin Ø A GTP-cap is formed
32
Scenario 2- Slow growth of the microtubule
Ø Sufficient time to hydrolyse GTP-tubulin Ø Catastrophe!
33
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.
34
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.
35
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.
36
What controls the polymerization/organisation of microtubules? Microtubule-binding proteins What controls the filament dynamics and organisation?
Microtubule-binding proteins Microtubule-associated proteins - MAPs
37
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
38
2- Tubulin-sequestering and microtubule-severing proteins
Function: Destabilise microfilaments by: * sequestering tubulin molecules and inhibiting their incorporation into the microtubule * severing the microtubule
39
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.
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.
41
Where are IFs found
IFs are found only in some metazoans (vertebrates, nematodes and molluscs)
42
IFs are encoded by 70 different genes (in humans)
Ø Different IFs have different functions. Ø Different IFs are expressed in different tissues.
43
Structure
monomer with a-helical region coiled-coil dimer staggered tetramer 8 tetramers form the unit length filaments (ULFs)
44
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.
45
46
IFs are considered as
strongest and most stable elements of the cytoskeleton.
47
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.
48
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.
49
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…
50
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.
51
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
52
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.
53
Class VI – Nestin
Ø Involved in axonal growth in developing neurones. Ø Abundant in neuronal precursor cells.

Cell Biology (10 decks)

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