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BMS 242 - Advanced Cell and Molecular > The cytoskeleton and cell shape > Flashcards

The cytoskeleton and cell shape Flashcards

1
Q

What 3 things defines cell shape?

How?

A

1) Adjoining cells
- Physically boxes in cells

2) Cell adhesions
- To the ECM and other cells

3) Extra-cellular matrix
- Confines the shape of the cell

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2
Q

What is the shape of the cell critical for?

A

Function

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3
Q

What processes is cell shape involved in?

A
  • Migration
  • Phagocytosis
  • Transport
  • Cytoskeletal dynamics
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4
Q

What is cortical actin and where is is present?

What is the function?

A

Cortex of actin around the surface of the cell

Important in providing support to the cell and giving it shape

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5
Q

What are stress fibres?

A

Aligned polymerised actin the in cell

With crosslinking

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6
Q

What are podosomes?

Where are they present?

A

Foot-like structures on the ventral part of the cell

Are adhesive - anchoring the cell to the ECM

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7
Q

What are the actin structures in a migrating cell?

A

1) Microvilli
2) Filopodia
3) Lamellipodia
4) Podosome
5) Stress fibres
6) Cortex

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8
Q

What are filopodia used for?

What are they extensions of?

A

For the cell to ‘sense’ the environment

Extensions of the lamellipodia

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9
Q

What are lamellipodia important in?

A

Movement of the cell

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10
Q

What gives strength to the migrating cell?

A

Many stress fibres in strips

Actin cortex

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11
Q

What are the dynamic changes in actin structure in the cell regulated by?

A

Phosphorylation

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12
Q

What happens to actin when the cell is inactive?

A

Low amounts of polymerised actin

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13
Q

When does actin spontaneously polymerise?

A

When there is an appropriate concentration of actin and enough salt in the solution

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14
Q

What is the only ‘bottle neck’ in actin polymerisation?

Why?

A

Initial oligomerisation (first 2 monomers coming together)

Very energetically unfavourable and therfore slow to occur

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15
Q

What occurs at the end of actin polymerisation?

A

Polymerisation reaches saturation

Where the rate of new monomers being added is equal to the rate of monomers coming off

Steady state of fibres

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16
Q

Describe the process of actin treadmilling

A
  • Monomers are added to the barbed end (+)
  • Monomers are bound to ATP when they bind
  • Phosphate then released (ATP–>ADP)
  • Monomers leave the pointed end (-)
  • Monomers bound to ADP
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17
Q

What is treadmilling regulated by?

A
  • Phosphorylation

- Many accessory proteins

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18
Q

What does the accessory protein profillin do?How?

What kind of protein is it?

A

Monomer binding protein:

  • Increases the rate of monomer addition to the + end
  • Binds to actin monomers and enables the monomer to be phosphorylated
  • Then, delivers the monomers to the growing filament
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19
Q

What 2 things do the accessory proteins Arp2/3 do? How?

What kind of proteins are they?

A

Nucleators and branching proteins:

  • Form the starting base of actin polymerisation
  • By resembling actin and binding to the - end actin
  • Interact with existing filaments allowing other actin branches to nucleate from them
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20
Q

What does the branching of actin provide in the cell?

A

Strength

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21
Q

What does the accessory protein Gelosin do? How?

What kind of protein is it?

A

Capping and severing protein:

  • Severs filaments
  • Regulates actin assembly and disassembly
  • By binding to the + end of actin - preventing further growth
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22
Q

What do the accessory proteins apha-actin and filamin do? How?

What kind of protein is it?

A

Crosslinking proteins:

  • Bind filaments together to give strength
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23
Q

What are the 2 classes of G protein and how are they different?

A

1) Small GTPases
- Monomeric

2) Heterotrimeric G proteins
- Membrane bound complex

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24
Q

What do small GTPases do?

A

Bind and hydrolyse GTP into GDP

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25
Q

What size are small GTPases?

A

21kDa

26
Q

What occurs to GTPases after translation and why?

A

Post-translational lipid modifications

In order to target them to specific sites in the membrane

27
Q

What superfamily do GTPases belong to?

What are the members of the family sometimes called? What does this mean?

A

Ras superfamily

Family members sometimes called ras-type proteins (meaning small monomeric GTPase)

28
Q

How many different family members are there in the Ras family of GTPases?

A

36

29
Q

Why is the superfamily of small monomeric GTPases called Ras?

A

Ras family is the founding GTPase

30
Q

What are the proteins in the Ras branch important in?

A

Cell proliferation and cancer

31
Q

What is Rho?

A

A family of Ras-like GTPases

32
Q

What are Rho proteins involved in?

A

Cytoskeletal dynamics and migration

33
Q

What the 3 main branches within the Rho family?

A

1) Rac
2) Rho
3) Cdc42

34
Q

How is Rac activated?

A

By binding to GTP

35
Q

What are the downstream targets of Rac?

A

PAK/WAVE2

36
Q

What are GEFs?

What do they do?

A

Guanine nucleotide exchange factors:

  • Catalyse the exchange of GDP –> GTP, to regulate the activity of the GTPase
37
Q

When is the only time a GTPase can activate its downstream targets?

A

When bound to GTP

38
Q

What happens quickly after the GTPase is bound to GTP?

What is this mechanism?

A

GTP is broken down into GDP, causing the GTPase to go into an inactive state

Timed mechanism

39
Q

What are GAPs and what do they do?

A

GTPase activating protein:

  • Increase the enzymatic activity of a small GTPase by increasing the inactive form of the GTPase
  • Causes GTP –> GDP
40
Q

What do guanine nucleotide dissociation inhibitors do?

A

Binds to the GDP-bound form an prevents exchange, maintaining the small GTPase in an inactive state

41
Q

How do GTPases signal?

A

By activating downstream pathways when they are in the active state (bound to GTP)

42
Q

What function does the hydrolysis of GTP by a GTPase serve?

A

Timing function - to bring the enzyme back into its inactive state

43
Q

What does the binding of GTP to Rac1 cause?

What does this allow?

A

A structural change in the ‘switch region’

Allowing the GTPase to interact with their downstream targets

44
Q

What are the switch regions in a GTPase?

A

2 loops: switch 1 and switch 2, which bind either side of the nucleotide

45
Q

What are the principle Rho family GTPases and what do they do?

What do these features do?

A

RhoA
Rac1
Cdc42

Coordinate actin cytoskeletal organisation, which ultimately controls:

  • Cell morphology
  • Cell movement
  • Cell motility

These features organise cells to form multicellular organisms

46
Q

How does Cdc42 control actin cytoskeletal organisation?

A

Controls the polymerisation of actin filaments and the formation of actin spikes/fillopodia

47
Q

How does Rac1 control actin cytoskeletal organisation?

A

Controls the organisation of new actin filaments into dynamic ruffling/lamellipodia

48
Q

How does RhoA control actin cytoskeletal organisation?

A

Stabilises and consolidates actin filaments into stress fibres (more rigid skeletal framework)

49
Q

How can the function of GTPases be studied?

Where are these mutations present and what do they result in?

A

By identifying mutations that change their ability to be turned on/off

Point mutations in the nucleotide-binding sites, can make the protein:

  • Constitutively active
  • Dominant negative
50
Q

Describe the constitutively active GTPase mutant

A

Substitution of the catalytic glutamine in the switch, stopping GTP hydrolysis, by inactivating the enzymatic activity

51
Q

What is special about the enzymatic regions of GTPases?

A

The enzymatic region in ALL small monomeric GTPases is conserved

52
Q

Describe the dominant negative GTPase mutant

A

Substitution of the P-loop stops nucleotide binding - nucleotide free

Still capable of binding to GEFs - to mop them up and prevent them from activating the wild type GTPase

53
Q

What does constitutively active Rho mutants lead to?

What does this cause in the cell?

A

Stress fibre formation

Causes the cell to become very rigid

54
Q

What does constitutively active Rac mutants lead to?

What does this cause in the cell?

A

Many membrane ruffles (lamellipodia in all directions)

Cell is immobile, as tries to move in all directions

55
Q

What does constitutively active Cdc42 mutants lead to?

What does this cause in the cell?

A

Lots of filopodia formation

56
Q

What is the downstream pathway of Rac?

A

Activation of WAVE proteins:

  • Leading to the activation of Arp2/3
  • Leading to the polymerisation of actin filaments
57
Q

What is the downstream pathway of Cdc42?

How is this different to Rac?

A

Activation of WASP proteins:
- Leading to the activation of Arp2/3

  • Leading to the polymerisation of actin filaments in a DIFFERENT WAY to Rac - causing a different response in the cytoskeleton
58
Q

What is the downstream pathway of Rho?

A

Activation of Rho kinase:

  • Leading to Myosin activation
  • Leading to increases myosin contractility and stress fibre formation
59
Q

What do activated Rho family proteins bind? What is this?

A

A CRIB motif in effector proteins:

  • A specific 16 amino acid sequence
  • Cdc42/Rac1 Interactive Binding
60
Q

What things activate GEFs and GAPs?

Where do these occur?

What outputs do these ultimately lead to how?

A
  • Integrin interaction with the ECM
  • RTK
  • GPCRs
  • Cadherins

Ultimately leads to:

  • Changes in cell adhesion
  • Actin polymerisation
  • Focal adhesions
  • Cell migration

By controlling the small GTPases that organise the cytoskeleton (Rac, Rho, Cdc42)

61
Q

What are the accessory proteins which aid treadmilling?

A
  • Profillin
  • Arp2/3
  • Gelosin
  • Alpha-actin
  • Filamin

BMS 242 - Advanced Cell and Molecular (24 decks)

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