Cell Cytoskeleton

Question 0

Which of the stages of microfilament formation does the cell need to overcome?

Answer 0

The nucleation stage- formation of the nuclei of the microtubules by the individual subunits
Overcome in Vigo using s microtubule organising centre (MTOC) imbedded in centrosomes, they grow form gamma tubulin ring complexes

Question 1

What are microtubules made from?

Answer 1

Tubulin
A diner made from alpha and beta subunits
Each bind s GTP molecule but only in the beta sub unit is this hydrolysed to GDP
The tubulin molecules bind to form a protofilament
13 protofilaments form a microtubule

Question 2

What are MAP proteins and how do they affect microtubule structure?

Answer 2

Bind to and stabilises microtubules
Speeds up polymerisation
Also connect to vesicles, actin or other microtubules
Often regulated by phosphorylation

Question 3

Describe actin?

Answer 3

Globular monomeric protein
Has three subunits; alpha beta and gamma
Each can bind a molecules of ATP or ADP
Subunits are polar and assemble head to tail

Question 4

What do actin binding proteins (ABP) do?

Answer 4

Can for cross links between fibres

Or from fibres to the plasma membrane

Question 5

What is the structure of intermediate filaments?

Answer 5

Alpha helix monomer
Coiled-coil dimer
The dimers then form a rope like structure by further polymerising

Question 6

Which filaments have a motor function?

Answer 6

Actin microfilaments- need ATP and myosin

Microtubules- need ATP and kinesins or dyesins

Question 7

What are the names and functions of the three domains in myosin?

Answer 7

Head domain- binds to F-actin and used ATP hydrolysis to generate force
Neck domain- acts a linker and as a lever to transducer the force generated by the head
Tail domain- mediates interaction with cargo molecules or other myosin tail molecules

Question 8

Describe muscle myosin?

Answer 8

Myosin 2, conventional myosin
Made from 6 polypeptides; two identical heavy chains
Two pairs of light chains
Moved toward the plus/barbed end of F-actin

Question 9

Summarise the myosin cross bridge cycle

Answer 9

Myosin tightly bound to actin & ATP site is empty
ATP binds to myosin head & causes s conformational change to unbind it from actin
ATP hydrolysis & neck undergoes conformational change to displace it along the filament
Weak binding to actin causes the release of Pi & then tight binding to actin
Release of Pi causes power stroke & ADP release & head region returns to original conformation

Question 10

What is the difference between processive and non processive motors?

Answer 10

Non processive- net movement results from uncoordinated myosin heads cycling independently eg. Myosin 2
Processive- net movement results from myosin heads cycling in a coordinated manner eg. Myosin 5

Question 11

How is (non muscle) myosin 2 regulated?

Answer 11

Myosin light chain kinase (MLCK)

Phosphorylation of MLCK causes myosin 2 to take on an extended active state exposing it’s actin binding sites

Question 12

Describe kinesins motor cycle

Answer 12

Lagging head has ATP bound & locked strongly to the MT
Leading head is randomly searching for the next binding site and weakly docks & the lagging strand hydrolyses the ATP to ADP
Release of Pi from the lagging head weakens it’s binding to MT & the leading head rebinds ATP and locks down onto the MT
The leading head locking down throws the lagging head forward and begins the cycle again

Question 13

What are the differences between myosin and kinesin cycling

Answer 13

Myosin uses ATP binding to release actin binding & kinesin uses ATP for strong binding
Myosin used ATP hydrolysis to reset the neck region & kinesin uses ATP hydrolysis to get Pi
Myosin uses Pi for strong binding & kinesin uses Pi to weaken MT binding
Myosin uses ADP release for the power stroke & kinesin uses the release of ADP to strengthen MT binding

Question 14

Describe the motor cycle of dyne ins

Answer 14

ATP hydrolysis causes stalk attachment to MT
Release of ADP and Pi causes power stroke
Cycle repeats with other dyne in head

Question 15

Name all the cell-cell junctions and their associated filaments

Answer 15

Tight junctions- actin
Adherents junctions- actin
Desmosomes- keratin intermediate filaments
Gap junctions

Question 16

Name the cell-matrix junctions and associated filaments

Answer 16

Hemidesmosomes- keratin intermediate filaments

Focal adhesions- actin

Question 17

What is pemphigus foliaceus?

Answer 17

Desmoglein 3 (upper epidermis des isomers) pathogenic antibodies
Epidermis mainly intact- good prognosis

Question 18

What is pemphigus vulgaris?

Answer 18

Desmoglein 3 (lower epidermis des isomers) pathogenic antibodies
Epidermis severely compromised- fatal if untreated

Question 19

What is epidermolysis bullosa simplex?

Answer 19

Separation in basal layer due to a mutation in the hemidesmosome causing blisters

Question 20

What is junctional epidermolysis bullosa?

Answer 20

Separation in the basement membrane due to a mutation in laminin/BPAG2/alpha6/beta4 subunits

Question 21

What is dystrophic epidermolysis bullosa?

Answer 21

Separation in upper dermis due to a mutation in collagen 7

Question 22

What are the minimum requirements for actin based movement?

Answer 22

Nucleation of new actin filaments
Capping of old filaments
Recycling of monomers from old filaments

Question 23

Name the complex used to accelerate actin nucleation?

Answer 23

Arp2/3 complex

Question 24

Describe actin based movement

Answer 24

Protrusion- filopodia form testing the environment then lamellipodia form commiting to that direction
Translocation- formation of beefcake adhesions via integrins to the ECM generating traction
Actin/myosin contractility
Detachment- protease degrade contacts

Question 25

What happens to a cell if there is no expression of E-cadherin?

Answer 25

No adherens junctions
Changes in cell morphology
Loss of polarity
Increased motility
Increased invasiveness 
Cancerous characteristics

Question 26

How are actins on focal adhesions switched from inactive to active conformations?

Answer 26

Outside-in signalling
When cells come into contact with basement membrane integrins bind to ECM proteins so chains switch to their active conformations
Inside-out signalling
Platelets express alpha11b-beta3 integrin
When platelets come into contact with thrombin takin is activated which activates the integrin do it has an increased affinity for fibrinogen and leads to the formation of a playlet plug at the site of the wound

Question 27

How are collagen fibres formed?

Answer 27

Assembly of alpha chain triple helix I. ER then a secretory vesicles transports to the ECM where the helicase into fibrils and then aggregation into fibres

Question 28

Describe the formation of an elastic fibre

Answer 28

Tropoelastin formed in the ER
Translocated to the plasma membrane, secreted and cross linked to elastin and then cross linking with fibrillin microfilaments to form elastic fibres

Question 29

Name some phospholipids

Answer 29

Phosphatidylethanolamine
Phosphatidylserine 
Phosphatidylcholine
Sphingomyelin
Sphingosine

Question 30

Name drugs inhibit microtubule polymerisation?

Answer 30

Colchicine
Vinblastine
Nocidazole

Question 31

Name a drug that favours microtubule polymerisation

Answer 31

Taxol

Question 32

Name drugs that inhibit actin polymerisation

Answer 32

Phalloidin
Cytochalasin D
Latranculin