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Molecular Biology - Unit 2 - Zach H. > Cytoskeleton I - Structure > Flashcards

Cytoskeleton I - Structure Flashcards

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1
Q

What does the cytoskeleton maintain and insure in cells?

A
  • Maintains correctly shaped cells.

- Insures cells are properly structured internally.

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2
Q

List the functions of the cytoskeleton.

A
  • Changes shape of the cell (e.g., RBCs)
  • Moves the cell.
  • Re-arranges the cellular compartment.
  • Supports the plasma membrane.
  • Provide the mechanical strength.
  • Pulls chromosomes apart during cell division.
  • Splits dividing cells during cell division.
  • Guides intracellular traffic of organelles.
  • Vesicles move around by using cytoskeleton as a sidewalk.
  • Cells like sperm need to swim.
  • WBCs and fibroblasts need to crawl.
  • Muscle cell contraction.
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3
Q

What are the three families of cytoskeletal protein?

A

> Actin filaments (like Mardi-Gras beads)
Microtubules (slinky of life)
Intermediate filaments (like girders in building)

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4
Q

What is the function of actin filaments?

A

Determine the shape of cell’s surface and are necessary for whole-cell locomotion, secretion, and endocytosis.

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5
Q

What are actin filaments necessary for the cell to do?

A
  • Whole-cell locomotion
  • Secretion
  • Endocytosis
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6
Q

List functions of microtubules within the cell.

A

> Form tube like structure.
Determine the positions of membrane-enclosed organelles and direct the intracellular transport.
Make up centrioles and mitotic spindle.
Cilia and flagella.
Sperm swim and cilia moves eggs in Fallopian tubes.

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7
Q

What are the functions of intermediate filaments?

A

> Provide mechanical strength.
Strong filament.
Resist mechanical stress.

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8
Q

Besides actin filaments, microtubules, and intermediate filaments, what other proteins are essential for assembly of the cytoskeletal filaments?

A

Accessory Proteins

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9
Q

True of False:

Accessory proteins include motor proteins which are machines for converting the evergy of ATP hydrolysis into mechaniscal movement (move organelles along filaments or move filaments themselves).

A

True

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10
Q

Give an example of accessory proteins interacting to assemble cytoskeletal filaments.

A

Hundreds of actin binding proteins that modify the dynamics and organization of filaments.

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11
Q

What is actin a bundle of?

A

Two proto-filaments.

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12
Q

What are actin filaments also known as?

A

microfilaments

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13
Q

What are the characteristics of actin filaments (microfilaments)?

A
  • Are two-stranded helical polymers of the protein actin.
  • Actin subunits are compact and globular (G-actin vs. F-actin).
    > Actin is always G-actin in our cells.
  • Flexible structures 5-9 nm in diameter.
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14
Q

What are the characteristics of microtubules?

A

> long hollow cylinder
made of tubulin subunits that are compact and globular
long and straight
outer diameter 25 nm
more rigid than actin filaments
have one end attached to a single microtubule-organizing center called a centrosome

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15
Q

Are actin filaments or microtubules more rigid?

A

Microtubules are more rigid than actin filaments.

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16
Q

What are the characteristics of intermediate filaments?

A
  • are rope-like fiber, diameter 10 nm
  • large heterogeneous family
  • are made of smaller subunits that are themselves elongated and fibrous
  • extend across the cytoplasm to provide mechanical strength
  • span from on cell-cell juction to another to strengthen the epithelial sheet
  • do not readily polymerize
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17
Q

True or False:

The cytoskeleton is dynamic and adaptable.

A

True

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18
Q

Of the three cytoskeleton proteins, which two are polarized?

A
  • actin filaments

- microtubules

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19
Q

Which cytoskeleton rearranges to form a bi-polar mitotic spindle and separate chromosomes?

A

microtubules

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20
Q

Although the cytoskeleton is dynamic and adaptable, give 2 examples of the cytoskeleton forming stable structures.

A

> Intestinal cells with microvilli must last a few days.
Actin bundles in hair cells of inner ear must last a lifetime.

However, actin filaments replaced every 48 hours

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21
Q

What maintains polarity for intestinal cells - apical surface vs. basolateral surface?

A

Cytoskeleton

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22
Q

What does the apical surface of epithelial cells do?

A

Absorbs nutrients.

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23
Q

What are the purpose of microvilli on epithelial cells?

A

Quadruple surface area and increase absorption rate.

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24
Q

What happens at the basolateral surface of epithelial cells?

A

Where cells transfer nutrients to bloodsteam.

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25
Q

What is the polarity of intestinal cells maintained by?

A

> microtubule
actin filaments
intermediate filaments

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26
Q

In an intestinal cell, what are intermediate filaments attached to?

A

Attached to desmosomes (adhesive structures) and to desmosomes (cell to cell contact) and hemidesmosomes (cell matrix contact).

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27
Q

In intestinal cells, what is the function of the microtubules?

A

Form tracks to get newly syntehsized proteins to proper locations.

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28
Q

Give an example of a cellular function that would cause the reassembly of the cytoskeletal subunits to occur to reshape the cytoskeleton in a cell response to an external signal.

A

Neutrophil chasing a bacterium.

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29
Q

What type of bond holds cytoskeletal filaments (or polyers) together?

A

Weak noncovalent interactions, which means that their assembly and disassembly can occur rapidly.

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30
Q

What kind of subunits are used to assemble actin filaments?

A

actin subunits

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31
Q

What kind of subunits are used for formation of microtubuels?

A

tubulin subunits

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32
Q

How do actin subunits and tubulin subunits self-associate?

A

By using a combination of end-to-end and side-to-side protein contacts.

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33
Q

What are protofilaments?

A

Long linear strings of protein subunits joined end to end.

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34
Q

Are single protofilaments thermally unstabe?

A

Yes

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35
Q

You know that a single protofilament is thermally unstable, but what about cytoskeletal filaments from multiple protofilaments bound side to side?

A

Requires breakage of several bonds in middle of stable filaments - resists breakage by ambient thermal conditions.

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36
Q

Are mutiple protofilaments thermally stable?

A

Yes

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37
Q

What does staggered side to side binding of filaments allow filaments to tolerate?

A

Bending and stretching forming rope-like structures (intermediate filaments).

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38
Q

What is polymerization?

A

Assembly of actin or tubulin subunits (monomers) into a linear polymer.

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39
Q

What is depolymerization?

A

Removal of monomers at the ends of the polymer.

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40
Q

How must subunits assemble for new large filaments to form?

A

Assemble into initial aggregate or nucleus.

41
Q

For new large filaments to form, subunits must assemble into initial aggregate or nucleus. What is this initial process called?

A

Nucleation

42
Q

What is the formation of an actin nucleus caused by?

A

Random collision of 3 subunits.

43
Q

What can the time span for nucleation be tested in?

A

A test tube which examines the time course of actin polymerization.

44
Q

In the formation of an actin filament, what is the rate-limiting step?

A

> Filament nucleation - lag phase:

  • a process of formation of initial aggregate or nucleus
  • the rate-limiting step
45
Q

In which phase of actin filament formation are actin subunits quickly added onto the ends of nucleated filaments?

A

Filament elongation - growth phase.

46
Q

In the formation of cytoskeletal filaments, what is the critical concentration?

A

Steady state in which the rate of monomer addition equals the rate of monomer loss.

47
Q

What are the 3 courses of formation in cytoskeletal filaments?

A

1) Filament nucleation - lag phase
2) Filament elongation - growth phase
3) Steady state - equilibrium phase

48
Q

What is tubulin?

A

A hetero-dimer of alpha-tubulin and beta-tubulin with non-covalent bonds.

49
Q

What do both alpha-tubulin and beta-tubulin both have a binding site for?

A

GTP

  • GTP in the alpha-tubulin is never hydrolyzed *
50
Q

How many proto-filaments does 1 microtubule consist of?

A

13 - aligned in parallel.

51
Q

What does arrangements of alpha and beta tubulins create?

A

Structural polarity.

52
Q

Is beta-tubulin or alpha-tubulin considered the + end?

A

beta-tubulin

53
Q

In a microbule, what subunits are in longitudinal contact?

A

alpha-beta

54
Q

In a microtubule, what subunits are in lateral contact?

A

alpha-alpha or beta-beta

55
Q

How are actin filaments arranged to generate structural polarity?

A

Head-to-Tail to generate structural polarity.

56
Q

What binding site does an actin monomer contain?

A

Binding site for ATP or ADP.

57
Q

What makes up an actin filament?

A

Consists of 2 protofilaents, held by lateral contacts.

58
Q

Are actin filaments flexible and easily bent?

A

Yes

59
Q

What are the two distinct and dynamic ends that actin filaments or microtubules have?

A

plus and minus end

60
Q

How are such different ends (plus and minus end) made possible in actin filaments or microtubules?

A

by changes in the conformation of each subunit as it enters the polymer.

61
Q

What are the characteristics of the plus end?

A

> Fast-growing or shrinking end.

> Has beta-tubulin or referred as barbed end of actin filament.

62
Q

What are the characteristics of the minus end?

A

> Slow-growing or shrinking end.
Has alpha-tubulin or the GTP binding cleft on the actin monomer point towards to minus end, also referred as pointed end.

63
Q

True of False:

Elongation proceeds spontaneously when deltaG for addition of the monomer is less than zero, due to the [monomer] exceeds the critical concentration.

A

True

64
Q

What do actin and tubulin enzymes catalyze?

A

ATP (actin) or GTP (tubulin)

65
Q

What is bound to tubulin or actin in the T form?

A

GTP/ATP

66
Q

What is bound to tubulin or actin in the D form?

A

GDP/ADP

67
Q

True or False:

Each monomer carries a tightly bound ATP or GTP molecule that is hydrolyzed to ADP or GDP soon after the monomer assembles into the polymer.

A

True

68
Q

What are the two types of subunit structures?

A

T form and D form.

69
Q

What form does the tip of the polymer remain in if the rate of polymerization is faster than the rate of hydrolysis of the bound nucleotide?

A

T form - as an ATP cap or GTP cap.

70
Q

What is treadmilling?

A

The plus end grows while the minus end shrinks.

71
Q

In treadmilling, what form does the plus end remain in?

A

Plus end remains in T formation.

72
Q

In treadmilling, what form does the minus end remain in?

A

Minus end adopts D formation.

73
Q

In treadmilling, at which ends are subunits added and removed?

A

Subunits added at plus end and removed at minus end.

74
Q

In treadmilling, does the polymer maintain a constant length?

A

Yes

75
Q

Does treadmilling predominate in actin filaments or microtubules?

A

actin filaments

76
Q

What is dynamic instability?

A

The rapid inter-conversion between a growing and shrinking state at a constant concentration of free subunits.

77
Q

What causes catastrophe?

A

If nucleotide hydrolysis proceeds more rapidly than subunit addition, the cap is lost and the microtubule begins to shrink.

78
Q

What causes rescue?

A

GTP-containing subunits may still add to the shrinking end, and if enough add to form a cap, then microtubule growth resumes.

79
Q

What are the two different structures that microtubules can take during the addition or hydrolysis of GTP?

A

> Adding tubulin-GTP produces straight proto-filaments.

> Hydrolysis of GTP after assembly changes the conformation of subunits, making the proto-filaments curved.

80
Q

What makes the proto-filaments curved?

A

Hydrolysis of GTP after assembly changes the conformation of subunits.

81
Q

What makes the proto-filaments straight?

A

Adding tubulin-GTP produces straight proto-filaments.

82
Q

Is dynamic instability predominate in microtubules or actin filaments?

A

Microtubules

83
Q

What allows the GDP-protofilaments to relax into the more curved conformation?

A

loss of GTP cap

84
Q

On rapidly growing microtubules, protofilaments containing GDP-subunits are forced into a linear conformation by what?

A

Many lateral bonds, given a stable cap of GTP-subunits.

85
Q

What is each monomer of an intermediate filament?

A

Each monomer is an elongated molecule with an extended central alpha-helical domain.

86
Q

How do monomers connect to each other in constructing an intermediate filament?

A

Monomer forms a parallel coiled-coil dimer with another monomer.

87
Q

How do dimers associate in constructing an intermediate filament?

A

A pair of dimers associate in an antiparallel manner (N to C; C to N) to form a staggered tetramer (C).

88
Q

True or False:

Intermediate filaments have no nucleotide binding sites and no structural polarity.

A

True

  • only polarity is the N- and C-terminus *
89
Q

In the construction of intermediate filaments, how is each tetramer structured to allow association with another tetramer?

A

Within each tetramer, the two dimers are offset, allowing it to associate with another tetramer.

Each parallel tetramers (proto-filaments) pack together lateraly to form the filament

90
Q

How many parallel tetramers (proto-filaments) pack together laterally to form the filament?

A

8 parallel tetramers (proto-filaments) pack together laterally to form the filament.

91
Q

What is the most diverse group of intermediate filaments?

A

Keratins

92
Q

In the human genome, how many distinct keratin genes are there?

A

50 distinct keratin genes in the human genome.

93
Q

How many of the keratin genes are found in human epithelial cells?

A

about 20 found in human epthelial cells

94
Q

How many of the keratins genes are specific to hair and nails?

A

10 specific to hair and nails.

95
Q

How do keratins impart mechanical strength?

A

By anchoring intermediate filaments at site of cell-cell contacts, desmosomes, or cell-matrix contacts, called hemidesmosomes.

96
Q

What type of protein imparts mechanical strength by anchoring intermediate filaments at site of cell-cell contacts, desmosomes, or cell-matrix contacts, called hemidesmosomes?

A

Keratin

97
Q

What are the 3 types of cytoskeleton proteins?

A
  • actin filaments
  • microtubules
  • intermediate filaments
98
Q

List the features of the cytoskeleton.

A
  • Cytoskeleton are built from protein subunits.
  • Cytoskeletal filaments are formed from proto-filaments.
  • Nucleation is the rate-limiting step in the formation of cytoskeletal filament.
  • Polymerization of protein subunits creates structural polarity.
  • Nucleotide hydrolysis by tubulin and actin results is filament treadmilling and dynamic instability.

Molecular Biology - Unit 2 - Zach H. (11 decks)

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