cytoskeleton

Question 1

what are the 3 filament types (smallest to largest)

Answer 1

1. actin (microfilaments) - actin-binding proteins
2. intermediate filaments
3. microtubules: microtubule-associated proteins (MAPs)

Question 2

describe actin structure

Answer 2

• F-actin
• polymers of actin proteins called G-proteins
• polarised double helix

Question 3

how many acts subunits for each complete turn? diameter?

Answer 3

13

7nm diameter

Question 4

explain growth of actin filament

Answer 4

• ATP bound to actin monomer (G-actin)
• monomers added/removed from both ends of polymer
• add more rapidly to +end
• once incorporated, ATP –> ADP

Question 5

3 main functions of actin

Answer 5

1. mechanical support
2. cell shape changes and maintenance
3. cell motility

Question 6

9 functions/roles of actin proteins

Answer 6

1. G-actin monoers
2. actin-sequestering proteins (eg. profilin, thymosin) prevent G-actin polymerising
3. actin-bundling proteins (a-actinin in muscle))
4. motor proteins (muscle myosin)
5. side-binding proteins (interact w other proteins)
6. capping proteins (prevent growth)
7. cross linking proteins (eg. transgelin)
8. severing (gelsolin, severin)
9. membrane attachment proteins (spectrin)

Question 7

diameter intermediate filaments

Answer 7

10nm

visible by e.m.

Question 8

functions of intermediate filaments

Answer 8

• dense around nucleus
• anchor cells at some cell junctions
• support nuclear structure

Question 9

examples of intermediate filaments

Answer 9

Named vary by cell types:

eg. keratin, vimentin, glial fibriallary acidic protein, neurofilamin

Question 10

how is an intermediate filament polymer formed

Answer 10

1. monomer
2. helical dimer (2 monomers)
3. 2 dimers combine = tetramer (fundamental unit)
4. tetramers link - staggered formation
5. Subunit exchange is slow but occurs throughout the length of the filament.

Question 11

example of modulation of IF

Answer 11

plectin molecules link IFs to actin filaments and microtubules

Question 12

diameter microtubules

Answer 12

25nm

visible by e.m. or light microscopy

Question 13

describe structure of microtubules

Answer 13

• polymer built from TUBULIN monomer which consist of one molecule of a-tubulin and beta-tubulin
• asymmetric monomer: polymer has polarity (one end stops with a, one with b)

Question 14

describe arrangement of columns of tubulin

Answer 14

13 columns of tulip polymer arranged in hollow cylinder

Question 15

assembly and disassembly of microtubule

Answer 15

• (negative) end: GPD bound monomers dissociate rapidly
• GTP bound monomers assemble onto + end

GTP–>GDP, cycle starts again

Question 16

where are microtubules polymerised

Answer 16

centrosomes

minus end remains close to centrosome, plus end points out towards cell periphery

Question 17

how does cytoskeleton help cell shape and orientation

Answer 17

• actin: support. maintain shape of cell e.g. erythrocytes
• IF: stabilise axon shape
  microtubules: stabilise shape of platelets and axons

Question 18

how do stereocilia detect vibration in cochlea

Answer 18

The cells are depolarised or hyperpolarised by deflections caused by sound.
Actin filaments keep the stereocilia rigid.

Question 19

how does cytoskeleton help anchoring organelles

Answer 19

• actin: hold synaptic vessels close to presynaptic membrane
• IF: hold cell nucleus
• microtubules: organise ER of a cell
• cytoskeleton holds cell next to each other and to extracellular matrix at cell junctions

Question 20

describe 4 steps of actin-based cell movement

Answer 20

1. leading edge - cell push forward. actin polymerises
2. focal contact junctions allow adherence of projections. F-actin connects focal adhesions
3. backside of cell pulls against anchorag epounts
4. actin depolymerises at rear

Question 21

what are lamellipodia and how do they function

Answer 21

• sample environment - extend and withdraw
• plus end of actin orientated to cell periphery
• When lamellipodia or filopodia touch down: attach to the extracellular matrix through the formation of focal adhesions (focal contacts)
• Actin connect the focal adhesion to the rest of the cytoskeleton

Question 22

how does myosin help actin

Answer 22

myosin II: motor protein

1. head region interacts w actin and binds ATP. Energy release from hydrolysis = myosin tail moves
2. ADP released from head and replaced by ATP = detach from actin
3. head binds further down filament

Question 23

describe microtubule based movement

Answer 23

• cilia
• 9+2 structure
• DYNEIN (a MAP): initiate movement. minus end-directed motor protein
• microtubules slide along each other = bend cilium

Question 24

how does cytoskeleton help movement of intracellular contents

Answer 24

microtubules

e. g. move synaptic vessels along axons to synapse
- motor proteins kinesin (moves to + end) and dynein (moves to - end) move vesicles

Question 25

what's the difference in the action of kinesis and myosin II

Answer 25

- kinesin stays attached to microtubule during ATP hydrolysis = processive motor protein. moves large distances - myosin II: detaches from actin at end of the cycle, travel short distances

Question 26

e..g. when cytoskeleton is used tome cell contents

Answer 26

separation of chromosomes during cell division - microtubules

Question 27

3 anti-cancer therapeutics and their mechanism

Answer 27

colchicine, vinblastine, taxol - inhibit function of mitotic spindle and thus cell division - C&V: destabilise microtubules, T stabilises

Question 28

actin abnormalities leading to disease

Answer 28

- mutation in dystrophin = Duchenne and Becker Muscular Dystrophy - mutation in myosin VII = Usher's syndrome - heriditary deafness and blindness

Question 29

intermediate filament abnormalities leading to disease

Answer 29

- Epidermolysis bullosa symplex | - Amyotrophic Lateral Sclerosis (ALS, Lou Gehrig’s Disease)

Question 30

microtubule abnormalities leading to disease

Answer 30

- Alzheimer’s Disease | - Hereditary Spastic Paraplegia

Question 31

how does Listeria bacteria affect actin cytoskeleton

Answer 31

- engulfed by host cell - escapes - F-actin polymerises at back of bacterium = motility - drives bacterium into neighbouring cell

Question 32

what is Epidermolysis bullosa symplex.

Answer 32

IF - Mutations in keratin genes= failure to form proper keratin filaments in epidermis - skin sensitive to mechanical injury - Blistering in adults, sloughing of epidermis in newborns - Plectin mutations also cause EBS with muscular dystropy

Question 33

Amyotrophic Lateral Sclerosis

Answer 33

IF (ALS, Lou Gehrig’s Disease). -Some hereditary forms are caused by mutations in neurofilamin genes -causes of motor neuron degeneration are unclear.

Question 34

Alzheimer’s Disease.

Answer 34

microtubules - Alongside amyloid plaques, AD brains display neurofibrillary tangles comprising the MAP, Tau. - Tau is hyperphosphorylated in tangles and cannot bind MTs.

Question 35

Hereditary Spastic Paraplegia

Answer 35

microtubules | The most common form is caused by mutations in spastin, a microtubule severing protein.