The cytoskeleton

Question 1

What is the cytoskeleton

Answer 1

A network of protein filaments extending throughout the cell determine the cell shape and polarity

Question 2

What are the three types of cytoskeletons- from smallest to largest ?

Answer 2

Actin
Intermediate Filaments
Microtubule

Question 3

Describe the structure of actin

Answer 3

The filaments are called F-actin and they are composed of G-actin ( monomers- individuals actin proteins)

It is a polarised double helix- hence why it will be striped in pictures

Diameter=7nm

13 actin subunits for a complete turn

Question 4

Actin filaments are described to be dynamic- what is meant by this?

Answer 4

It has the ability to change shape

The G actin can be added and removed from both plus and minus ends of the polymer.

The monomer is more readily added to the plus end of the filament

G actin needs to be bound ATP- hydrolysed to ADP

Question 5

What are the major functions of actin?

Answer 5

Mechanical support
Cell Shape changes and maintenance- RBC and adherent junction
Cell Motility

Question 6

Summarise and list the actin binding proteins

Answer 6

Motor Proteins eg myosin in muscle
Membrane attachment proteins eg Spectrin
Cross linking proteins eg transgelin
Capping proteins (prevents filament growth)
Side binding proteins (allows the interaction with other proteins)
Severing proteins (severin,gelesdom)
Actin-Bundling protein- Alpha actinin in muscle

Question 7

What are actin sequestering proteins

Answer 7

Bind to G actin preventing it from polymerising

eg. profilin, thymosin

Question 8

Describe the structure of intermediate filaments

Answer 8

Polymer of individual intermediate filament proteins

10nm in diameter

Networks typically most dense around the nucleus- extends to the periphery

Intermediate filament proteins form cross

Question 9

What are the major functions of the intermediate filaments

Answer 9

Anchor cell at the some junctions

Support nuclear structure

Question 10

Describe the formation of the intermediate polymer

Answer 10

1) intermediate filament protein- monomer
2) Helical dimer
3) 2 dimers combine to form a tetramer- the fundamental unit of IF
4) Tetramers link in a staggered formation end to end to form the filament
5) Subunit exchange os slow but occurs along the length of the whole filament

NB proteins bind to the IF to modulate the function
Plectinin molecules link IFs to actin filaments

Question 11

Describe the structure of microtubules

Answer 11

Long relatively stiff hollow tubes (approx 25nm diameter)
Visible by EM/light microscope

Can be rapidly assembled and deassembled

The polymer is built from monomers of tubulin- one monomer which has alpha tubulin and beta tubulin.

Therefore the molecule is asymmetric and so polymer has polarity- one end start with alpha, the other beta

Question 12

How are microtubules assemble (and deassembled)

Answer 12

Monomers bind to GTP
And are added to plus end

The monomers gradually lose the GTP as it is dephosphorylated- LOSES AFFINITY

Microtubule polymerises in centrosome ( minus stays close to the centrosome and plus toward the periphery)

Question 13

What are the uses for cytoskeleton

Answer 13

Cell Shape and orientation
Anchoring the organelle 
Anchoring the cells
Cell Motility 
Movement

Question 14

Describe the “cell shape and orientation” aspect

Answer 14

Actin filament bundles provide support
Actin filament sheets also provide support-
1. dense sheets of actin- found in cortex of cell
2. Maintains the shape of cells eg erythrocytes
eg Stereocillia in the inner ear-
cells are depolarised/hyperpolarised by deflections caused by sound. Here, actin filaments keep stereocillia rigid despite the vibrations of sound

Intermediate filaments stabilise the shape of axons (soma - synapse)

Microtubules stabilise the shape of platelets (formed instantly when stimulus applied)

Question 15

Describe the “anchoring cells” aspect

Answer 15

The cytoskeleton is essential to anchor the cells to each other and extracellular matrix at cell junctions
eg at adherens junctions- forming adhesion belt due to the cadherin dimer being held by actin

Question 16

Describe the “anchoring the organelles” aspect

Answer 16

Microtubules organise the ER of a cell

IF form a meshwork around the nucleus of a cell nucleus (LAMINA) to hold in position the transcription machinery- keeping it ordered (anchored)

Actin is also required to hold the synaptic vesicles close to the presynaptic membrane

Question 17

Describe the “ Cell Motility “ aspect (actin based movement)

Answer 17

Actin Based Movement-

1) Cell pushes pushes out protrusions at the front- actin filament polymerisation provides the force for membrane protrusion
2) Protrusions adhere to the surface to the surface on which the cell is moving through focal contact junctions

F actin connect to focal adhesions to provide a contractile force for the cell

3) The rest of the cell pulls against the anchorage point to drag itself forward
4) Actin depolarises at the rear

Question 18

Explain the example of Lamelipodia

Answer 18

Lamellipodia- samples the environment, extends and withdraw

generated by rapid growth of actin filaments at the cell membrane. The plus end of the actin filaments are orientated towards the periphery of the cell

When the lameilpodia touches down on the surface, they attach onto the ECM through the formation of the focal adhesion (integrins).
The actin filaments are crucial in the sense that they connect the focal adhesion to the rest of the cytoskeleton

The actin filaments are then pulled upon by myosin to drag the cell forward.
The myosin head interacts with actin having been bound to ATP
energy released from the ATP hydrolysis forces myosin head to move. ADP released from the head and the head detaches from the actin filament.

Question 19

Describe cell motility aspect ( microtubule)

Answer 19

eg. Cilia lining respiratory epithelium-

have a structure of: 9 double microtubules periphery and 2 in the middle

The movement is initiated by dynein- a minus end directed motor proteins
microtubules slide along each other which causes the cilia to bend
They are anchored to the CSM which allows for the swaying movement

Question 20

Describe the “movement of intracellular contents and organelles

Answer 20

Microtubules- allows for movement of organelles eg. synaptic vesicles from axons- synaptic cleft.
2 Motor protein- kinesin and dynein
Kinesin moves towards the positive end (cell periphery)
dynein moves towards negative end ( nucleus)

Question 21

Contrast the function of kinesin and myosin

Answer 21

Kinesin Stays attached to the microtubule throughout ATP hydrolysis cycle- processive motor protein- significant movements

Myosin II detaches completely from actin filaments at the end of the cycle- not processive

Question 22

Describe the action of chemotherapeutic agents

Answer 22

Colchine, vinbalstine (destabilise microtubules)
taxol (stabilises microtubules)

Both inhibit function of mitotic spindle- therefore cell division

Question 23

Describe how cytoskeleton abnormalities lead to disease

Answer 23

With actin- mutations in dystrophin can cause Duchenne Muscular Dystrophy
With IF- epidermis bullosa symplex- mutations in keratin genes= failure to form proper keratin filaments- skin is highly sensitive to mechanical injury- blistering in adults and sloughing of epidermis in newborns.
- ALS- hereditary forms of mutations in neurofilamin genes

With Microtubules- Alzheimers disease- amyloid plaques- AD brains display neurofibrillary tangles

Question 24

Describe the hijaking of actin cytoskeleton by listeria bacteria

Answer 24

Listeria engulfed by host cell

The bacterium escapes from the phagocytic vesicle

F-actin is polymerised at the back of the bacterium providing motility

The actin comet drives the bacterium into the neighbouring cell