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Molecules to Cells - Biochemistry > Lecture 3: The cytoskeleton I - Introduction to Actin/Treadmilling > Flashcards

Lecture 3: The cytoskeleton I - Introduction to Actin/Treadmilling Flashcards

1
Q

What is the cytoskeleton?

A

The cytoskeleton provides structural framework for cells and is responsible for giving cells their shape and structure

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2
Q

What is the cytoskeleton made up of?

A

The cytoskeleton is composed of a network of filaments

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3
Q

What do filaments do?

A

They facilitate cellular movement and control the movement of organelles, proteins and nucleic acid within the cell

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4
Q

What are filaments composed of ?

A

Protein polymers

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5
Q

What are the three types of cytoskeletal filaments?

A

actin filaments, microtubules and intermediate filaments

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6
Q

What processes do the cytoskeletal filaments carry out ?

A

cell division, cell motility, segregation of chromosomes during mitosis, shape changes in developing embryo, muscle contraction, transport of organelles in cells, intracellular movements

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7
Q

What is the most abundant protein in many cell types?

A

actin filaments

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8
Q

What are the three classes of the actin filaments

A

alpha, beta and gamma

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9
Q

What form do actin filaments exist in?

A
  1. monomeric form (G-actin)

2. 7nm helical filaments of uniformly oriented globular actin molecules (F-action)

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10
Q

What do actin filaments look like?

A

They are thin and flexible, normally cross linked for extra strength (cortex)

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11
Q

Where is alpha actin expressed?

A

In muscles cells

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12
Q

Where is beta and gamma actin expressed ?

A

Non muscle/ most other cell types

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13
Q

What can cause actin filaments to assemble or disassemble ?

A

The concentration of actin monomers, also termed G-actin or globular actin

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14
Q

What makes up the cortex?

A

Networks of actin filaments and other contractile proteins

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15
Q

What type of structure does G-actin have ?

A

A globular structure with exposed protein domains creating a barbed and a pointed end, separated by a cleft

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16
Q

What is a globular protein ?

A

A spherical protein

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17
Q

What is the purpose of the cleft on the G-actin ?

A

This is where ATP is bound

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18
Q

What is polymerisation ?

A

When monomers combine chemically to form polymer chains

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19
Q

What happens to the G-actin during polymerisation ?

A

The G-actin assembles into the filament in the same orientation, resulting in a filament that has a plus or barbed end of G-actin exposed at one end and the minus or pointed end exposed at the other.

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20
Q

Do actin filaments have structural polarity ?

A

Yes

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21
Q

Why do actin filaments have structural polarity ?

A

G-actin assembles into the filament in the same orientation, resulting in a filaments that has the plus or barbed end of G-actin exposed at one end and the minus or pointed end exposed at the other.

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22
Q

Is the assembly of actin monomers faster to the barbed ends or the pointed ends ?

A

The actin monomers assemble more readily to the barbed ends which are the fast growing ends, than the pointed ends which are slower growing.

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23
Q

What do monomeric actin (globular) polymerise into?

A

Filamentous actin

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24
Q

What percentage of the cell type is monomeric and in filaments ?

A

Depending on the cell type 50% is monomeric and 50% is in filaments

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25
Q

What causes there to be actin in filaments and in monomers ?

A

The presence of actin monomer binding proteins

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26
Q

Name 3 actin monomer binding proteins

A
  1. Profilin
  2. Thymosin
  3. ADF
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27
Q

What does the actin binding protein ADF stand for ?

A

Actin depolymerizing factor

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28
Q

What is the purpose of actin binding proteins ?

A

To bind actin monomers in the cytosol and prevent them from being incorporated into actin filaments, resulting in a reserve of actin monomers

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29
Q

Why is is there a reserve of actin monomers retained in the cell at all times ?

A

So that rapid polymerisation can occur when needed

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30
Q

What is the basis for cell movement ?

A

The ability of the cytoskeleton, particularly actin filaments to rapidly disassemble in a specific direction, generating the force for cell movement.

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31
Q

What is the process known as ruffling ?

A

The formation of actin rich membrane protrusions

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32
Q

How do cells migrate ?

A

Using a crawling motion

33
Q

What is the leading edge ?

A

The extension of protrusions in one direction

34
Q

What are the two types of protrusions ?

A
  1. Lamellipodia

2. Filopodia

35
Q

What is lamellipodia?

A

A protrusion of a meshwork of actin filaments with their plus ends close to the plasma membrane

36
Q

What is a filopodia ?

A

Thin, stiff bundles of actin filaments with their plus ends close to the plasma membrane, which push the cell forward to form new regions of actin cortex

37
Q

Where do the protrusions anchor themselves ?

A

To the surface on which the cell is crawling, making focal contacts.

38
Q

What are focal contacts ?

A

Adhesions by which cells attach to the underlying substrates

39
Q

How does the cell drag itself forward ?

A

It retraces its trailing edge by using the traction from the new anchorage points/focal contact.

40
Q

What are the 3 stages of actin polymerisation ?

A
  1. Nucleation
  2. Elongation
  3. Steady state
41
Q

What is the slowest stage of actin polymerisation ?

A

Nucleation stage

42
Q

What does the nucleation stage involve ?

A

The association of three actin monomers to initiate the filament structure

43
Q

What does the elongation stage involve ?

A

The rapid growth of the actin filament at both ends.

44
Q

When does the growth of the filament slow ?

A

When the free monomer concentration diminishes.

45
Q

What does the steady state involve ?

A

There is no net growth or shrinkage of the filament and there is a constant exchange of monomers between the filament and the free monomer pool

46
Q

How can actin polymerisation be studied ?

A

In vitro using physiological salt concentrations

47
Q

What is the critical concentration ?

A

The concentration of free actin monomers during the steady state stage or the concentration of actin monomers required to form filaments

48
Q

When do filaments form ?

A

When the concentration of G-actin is above the critical concentration

49
Q

When do G-actin remain as monomers?

A

When the G-actin concentration is below the critical concentration.

50
Q

When can the critical concentration be influenced ?

A

By the presence of actin binding proteins

51
Q

Where does the critical concentration differ ?

A

The critical concentration differs at the plus and minus ends of the filaments

52
Q

When does the hydrolysis to ADP and the inorganic phosphate begin in treadmilling of actin filaments ?

A

When the monomer which is ATP bound is incorporated into the filaments

53
Q

In actin treadmilling, what is ADP bound actins role?

A

It has an altered conformation and is less strongly bound within the filament.

54
Q

What is actin treadmilling ?

A

Occurs when one end of the filament grows in length while the other shrinks.

55
Q

What results in the difference of growth rates at the two ends of the filament when monomer concentrations become limiting ?

A

ATP and ADP being bound to the filaments.

56
Q

Where does treadmilling occur ?

A

At monomer concentrations between the two critical concentration ranges

57
Q

When does treadmilling occur ?

A

When ATP actin adds to the plus end of an actin filament at the same time that ADP is lost from the minus end.

58
Q

Does unregulated treadmilling occur ?

A

No

59
Q

What happens when the rate of addition and loss are equal ?

A

The filament stays the same length, although individual actin monomers move through the filament from the plus to the minus end.

60
Q

What nucleates actin filaments ?

A

The leading edge of migrating cells

61
Q

What is the purpose of nucleating proteins during cell migration ?

A

Serve to skip the rate limiting nucleation phase of actin filament formation, allowing intermediate filament elongation to occur.

62
Q

What does ARP complex stand for ?

A

Actin related protein complex

63
Q

What does the ARP complex consist of ?

A

ARP2, ARP3 and other accessory proteins

64
Q

What do ARP2, ARP3 and the other accessory proteins do ?

A

Attach to the sides of pre-existing filaments and allow branched actin filaments to polymerise from the complex

65
Q

What does the ARP complex do ?

A

It enhances the rate of actin filament assembly at the leading edge.

66
Q

What does the resulting branching structure do after polymerisation ?

A

It pushes the plasma membrane forward, facilitating cell movement

67
Q

What are the three nucleator systems ?

A
  1. ARP2/ARP3
  2. Spire
  3. Formin
68
Q

What is the spire nucleator system ?

A

Slows the rate limiting step of polymerisation by binding 4 actin monomers, generating a tetramer which acts as the nucleating seed allowing rapid formation of new actin filaments

69
Q

What is the formins nucleating system ?

A

A progressive elongation machine, associating with the barbed end of existing filaments to generate longer filaments

70
Q

What are some of the multiple signals that lead to actin nucleation and cell motility ?

A

Growth factors, receptor tyrosine kinase, G coupled receptors, integrins and Rho family GTPases

71
Q

What is the basis of muscle contraction ?

A

Myosins bind actin filaments and use hydrolysis to move along the filament

72
Q

What is myosin II

A

Muscle myosin

73
Q

What is myosin I and other myosins ?

A

Non muscle

74
Q

What is the function of myosin filaments ?

A

Driving membrane furrowing during cell division and muscle contraction

75
Q

What is the structure of myosin II

A

Two globular heads and a coiled-coil tail

76
Q

What do the globular heads of myosin II have ?

A

ATPase and motor activity

77
Q

What is the structure of the myosin II tail ?

A

Heavy chains form coiled-coil rod structure

78
Q

What do the tails of myosin II form when they associate with each other ?

A

A bipolar myosin filament

Molecules to Cells - Biochemistry (17 decks)

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