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BIOL 331 Midterm > Cytoskeleton (Ch. 9) > Flashcards

Cytoskeleton (Ch. 9) Flashcards

1
Q

What three structures make up the cytoskeleton?

A

Microtubules, microfilaments (actin filaments), and intermediate filaments

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2
Q

What are the general functions of each of the three cytoskeleton elements?

A

Microtubules: mechanical support for cell shape and organelle organization, assembly, and a pathway for motor proteins
Microfilaments: contraction, cell motility, and a pathway for myosins
Intermediate filaments: structure and cellular organization

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3
Q

List the 5 general functions of the cytoskeleton

A
  • structural support
  • transport of materials and organelles within a cell
  • contraction and motility
  • spacial organization
  • cell division
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4
Q

Tubulin makes up microtubules. Describe the physical properties of a microtubule

A
  • the subunits are held together by weak noncovalent bonds, which allows for rapid assembly and disassembly
  • they are hollow and unbranched
  • composed of 13 protofilaments, which are stacks of alternating alpha and beta tubulin subunits
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5
Q

A microtubule has polarity. Which end is positive and which end is negative? What happens at the positive end?

A

The alpha-tubulin end is negative and beta is positive. Microtubules are built at the positive end

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6
Q

What is a centrosome? What are its 2 main components? Where does the centrosome usually “dwell”?

A

It’s a type of microtubule-organizing centre which initiates microtubule formation. It is composed of 2 centrioles surrounded by pericentriolar material (PCM). It is usually found at the center of the cell’s microtubular network

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7
Q

Centrosomes dictate:

A
  • the number of microtubules
  • their polarity
  • the number of protofilaments
  • the time and location of microtubule assembly
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8
Q

Where does gamma-tubulin reside? What does gamma-TuRC stand for? What can we infer based on this?

A

Gamma-tubulin resides in the PCM. Gamma-TuRC: gamma-tubulin ring complex. We can infer that microtubule assembly is initiated in the PCM and that the synthesizing microtubules don’t actually make contact with the centrioles

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9
Q

True/False? Both beta and alpha tubulin can bind to gamma-tubulin

A

False. Only alpha-tubulin can interact with gamma-tubulin, giving the microtubule its positive and negative ends

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10
Q

Which of the following structures have microtubules that are sensitive to disassembly? Which ones are more stable?
a) neurons
b) mitotic spindle
c) cilia
d) flagella

A

The mitotic spindle is sensitive to degradation, as it is involved with cell division and is only needed during that process. Neurons, cilia, and flagella are all more stable

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11
Q

What determines microtubule stability?

A

MAPs, +TIPS, and temperature (cold = disassembly)

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12
Q

Describe the function of MAPs

A

They increase/decrease microtubule stability and promote assembly by linking tubulin dimers together. Their activity is regulated by phosphorylation

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13
Q

What is GTP?

A

Guanosine triphosphate. It acts as an energy source and is analogous to ATP. It is not hydrolyzed by alpha-tubulin, as it is not a G protein

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14
Q

Beta-tubulin is a G protein. What is the significance of this statement?

A

This means that beta-tubulin hydrolyzes GTP to GDP after the dimer is added to the microtubule. GTP bound to the beta-tubulin subunit is required for microtubule assembly

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15
Q

To form or elongate microtubules in vitro, adding which of the following could ensure that the newly formed/elongated microtubule contains exactly 13 protofilaments? Select all that apply.
a) MAPs
b) centrioles
c) gamma-TuRC complex
d) other microtubules with 13 protofilaments
e) +TIPS

A

C and D

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16
Q

Explain the steps of microtubule growth

A
  1. In a growing microtubule, the tip consists of tubulin-GTP dimers in an open sheet
  2. Tube closure is associated with the hydrolysis of GTP into GDP
  3. GDP-tubulin has a different conformation, introducing mechanical strain. MAPs are used to stabilize the molecules
  4. In the absence of stabilization (MAPs), protofilaments curl outwards and undergo catastrophic shrinkage
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17
Q

Describe the purposes of +TIPs

A
  • binds to positive end of the microtubule and regulates the rate of growth or shrinkage
  • mediates the attachment to subcellular structures (ex. kinetochore of mitotic chromosome)
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18
Q

Microtubule polymerization and disassembly has what effect on subcellular structures?

A

Can push or pull material within a cell

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19
Q

Which of the following is/are accurate description(s) of the cytoskeleton?
a) a dynamic scaffold that provides structural support and helps to determine cell shape
b) an external framework that positions various organelles in the cell exterior
c) a network of tracks on the outside of the cell surface that directs the movement of materials
d) all of these

A

A. B and C are wrong because the cytoskeleton is located within the cell

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20
Q

Microtubules are agents of cellular motility:

A
  • transport of membranous vesicles from one membrane compartment to another
  • transport of nonmembrane bound cargo (RNA, ribosomes, cytoskeletal elements)
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21
Q

Which end of a neuron are the positive and negative ends, respectively, in relation to microtubules? What are the names of each direction of movement? (retrograde vs anterograde)

A

The positive end is where the axon terminals are located, and the negative end is where the dendrites are located. Towards the minus end is retrograde, and towards the plus end is anterograde

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22
Q

True/False? Motor proteins require ATP hydrolysis to generate their movement

A

True

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23
Q

What are five examples of cargo that may be carried by motor proteins?

A
  • membranous vesicles
  • nonmembranous (RNA, ribosomes)
  • organelles (lysosomes, mitochondria)
  • chromosomes
  • other cytoskeletal filaments
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24
Q

Name the three types of motor proteins and where they operate

A
  • kinesin (microtubule)
  • dynein (microtubule)
  • myosin (actin)
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25
Q

True/False? Motor proteins can move any direction necessary (according to cytoskeleton polarity)

A

False. They can only move in one direction: ex. kinesin moves towards the positive end and dynein moves towards the negative end

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26
Q

How many peptides make up a kinesin-1?

A

4 (tetramer) with 2 heavy chains and 2 light chains

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27
Q

Describe the structure of the globular head of kinesin-1

A
  • binds microtubules
  • ATP hydrolysis releases one “foot” from the microtubule so that it can swing around and re-attach
  • similar kinesins have conserved amino acid sequences in this region
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28
Q

Describe the structure of the tail of kinesin-1

A
  • binds to cargo
  • diverse amino acid sequences between other kinesins meant for carrying different cargo
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29
Q

Where does ATP hydrolysis occur during the “power stroke” of a kinesin globular head?

A

Hydrolysis occurs at the leading head which causes the trailing head to be swung forward

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30
Q

What is the length of one tubulin dimer? How does this relate to kinesin movement?

A

8nm. The globular heads of kinesin can only attach to the beta-tubulin, and each beta-tubulin is 8nm apart, so kinesin can move 8nm with one power stroke

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31
Q

What does it mean that kinesin movement is highly processive?

A

It is capable of moving considerable distances without falling off. At least one head is attached at all times

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32
Q

What is the relationship between ATP concentration and kinesin movement?

A

Kinesin speed is proportional to ATP concentration. Speed can reach up to 1micrometer/sec

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33
Q

Dynein heads are 10x larger than kinesin heads. What effect does this have on its speed?

A

Dynein is therefore faster than kinesin

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34
Q

Describe the function of dynein’s globular heads

A
  • force generation
  • ATP binding and hydrolysis
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35
Q

The heavy chains of dynein are found on its tail. What do they bind to?

A

The cargo via an adaptor protein (dynactin)

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36
Q

Dynein has one more “significant” part than kinesin does. What is it?

A

The microtubule binding site located on its stalks

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37
Q

True/False? Dynein is a tetramer, like kinesin

A

False. It is made up of 2 heavy chains and many intermediate and light chains

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38
Q

____-coated vesicles travel from the Golgi complex (-) to the ER (+) on microtubules pulled by the motor protein _______.
a) COPI/kinesin
b) COPII/kinesin
c) clathrin/kinesin
d) COPI/dynein
e) COPII/dynein

A

A. The vesicle is moving in the retrograde direction (COPI) towards the plus end (kinesin)

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39
Q

True/False? Both dynein and myosin can be attached to a vesicle at the same time

A

True. Dynein, myosin, and kinesins can be attached to the same vesicle

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40
Q

Give an example for where cilia can be located in the human body

A

Respiratory tract lining

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41
Q

Where is an axoneme located? What is it?

A

It is located within cilia and flagella. It is a structure containing microtubules oriented longitudinally with the plus end on the tip (distal end) and the minus end near the basal body (proximal)

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42
Q

What is the basal body?

A

The microtubule-organizing center of cilia and flagella

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43
Q

Explain the 9+2 array

A

Nine peripheral doublet microtubules situated around a central pair of single microtubules. Dynein tails are anchored to one of the tubules in each pair (the “A tubule”)

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44
Q

Where is the interdoublet nexin bridge found?

A

Between two doublet microtubules

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45
Q

Explain the steps of cilia/flagella movement

A
  1. Dynein tails attached to the A tubule and dynein stalks bind to B tubules
  2. Power stroke caused by ATP hydrolysis
  3. Dynein stalks detach
  4. Dynein stalks reattach
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46
Q

Tre/False? The A tubule acts as the cargo in cilia and flagella movement

A

True. The dynein tails are attached to the A tubule, so the tubule moves with the dynein

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47
Q

What is the function of nexin?

A

This link limits the extent of movement/sliding so the tubules cannot slide past each other

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48
Q

Growth of cilia occurs at the distal ends of the projections. Proteins synthesized in the body of the cell need to reach the distal ends of the cilia. Intracellular transport (IFT) trains carrying cargo are moved to the distal ends of the cilia
a) what is the key cargo?
b) what is moving the trains to the distal ends?

A

a) tubulin is cargo, as microtubules are being built
b) kinesin, as it moves towards the positive end of the microtubule

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49
Q

Actin filaments are composed of:

A

G-actin

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50
Q

What cellular motile processes is actin involved with?

A
  • movement of vesicles
  • phagocytosis
  • cytokinesis
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51
Q

In which ways does actin provide structural support?

A
  • shape of cells
  • support for cellular projections (microvilli)
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52
Q

Like microtubules, actin filaments have polarity. What are the physical features of the plus end and minus end?

A

The plus end is barbed while the minus end is pointed

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53
Q

Explain why the plus end of an actin filament is also referred to as “barbed”

A

The individual G-actin monomers have directionality and are added to the filament in a particular orientation within the parallel double helix, giving it a “barbed” look

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54
Q

True/False? The ends of an actin filament are based on binding of a fragment of the myosin motor protein Sar1

A

False. S1 is the relavant motor protein

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55
Q

True/False? ATP-actin is incorporated into the filament, and then hydrolyzed to ADP by actin

A

True

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56
Q

True/False? Actin can be added to either end of the filament with the same speed of addition at a low ATP-actin concentration

A

False. Addition at the barbed end is faster due to it having a lower critical concentration

57
Q

What is a critical concentration?

A

The concentration of available ATP-actin required to elongate the filament

58
Q

Explain the steps of actin filament assembly and disassembly

A
  1. Preformed actin filament (seed) in the presence of ATP-actin
  2. At high ATP-actin concentrations, it will be added to both ends
  3. Concentration reaches the critical concentration of the pointed end; addition stops
  4. Loss of subunits occurs at the pointed end because ADP-actin dissociates more readily than ATP-actin, but addition continues at the barbed end
  5. Relative position of subunits is continually moving
59
Q

What is the steady state?

A

The subunits leave the pointed end at the same rate they’re added to the barbed end, resulting in a filament with constant length

60
Q

What is treadmilling?

A

The relative position of subunits is continually moving. Does not have to happen at the steady state

61
Q

Assume we’re starting with a high available concentration of ATP-actin, which is decreasing as the subunits get incorporated into the filament.
Initial concentration: 100
Pointed end critical concentration: 70
Barbed end critical concentration: 30
a) as available ATP-actin decreases, the critical concentration of which end will be reached first?
b) treadmilling happens when the cell’s available ATP-actin is between _____ and _____

A

a) pointed end
b) 70, 30

62
Q

Which type of myosin is the “conventional type”?

A

Type II

63
Q

Which types of myosin are the “unconventional type”s?

A

Type I, type III-XVII (3-17)

64
Q

True/False? All types of myosin move towards the minus end

A

False. All types of myosin except type VI move towards the plus end

65
Q

What is the function of the motor (head), tail, and neck/lever arm of conventional myosin?

A

Motor: binds the actin filament, binds and hydrolyzes ATP
Tail: intertwining of the two heavy chains, allows for the formation of filaments of myosin
Neck: moves during the power stroke

66
Q

Which type of myosin is involved with muscle contraction?

A

Type II

67
Q

Unconventional myosin type V movement:

A
  • moves processively along actin filaments
  • moved in a hand-over-hand movement
  • long necks act as swinging arms
  • can take 36nm steps
  • can carry cargo
68
Q

Which types of myosin can associate with vesicles and organelles (like kinesins)?

A

Types I, V, and VI

69
Q

True/False? Vesicles can only contain either a microtubule motor or actin filament motor

A

False. They can have both kinds

70
Q

Motor protein movement on the microtubule is associated with ___________ distances

A

Long

71
Q

Motor protein movement on the actin filament is associated with ________ movement in the __________ of the cell

A

Local, outskirts

72
Q

Describe an example of transport by unconventional myosins

A

Melanosomes in fish are transported along both microtubules and actin filaments. These change the colour of the fish

73
Q

In S1-decorated microfilaments, the barbed end is the _____ and the pointed end is the ______
a) plus end/minus end
b) fast-growing end/slow-growing end
c) both a) and b) are correct
d) neither of a) nor b) are correct

A

C

74
Q

Individual actin monomers move down the length of the microfilament from the plus end to the minus end in a process known as __________
a) unconventional myosin trafficking
b) conventional myosin trafficking
c) treadmilling
d) processivity
e) contractility

A

C

75
Q

What is an S1 fragment?

A

The machinery required for motor activity: a single actin head and neck

76
Q

When myosin type II is attached to a glass plate so that the heads are located on the actin and the tails are located on the glass, which way will the actin move? (The glass is immobile)

A

Towards the minus end, as myosin type II moves towards the plus end

77
Q

Which part of myosin II helps to form filaments? How is the motor protein oriented?

A

The tails point towards the center and the heads point outwards. The tails are responsible for holding the filament together

78
Q

Describe a bipolar thick filament

A

A myosin filament (thick) which has a reversal of direction at the filaments center (bipolar)

79
Q

List the organization of muscle cells from the largest compartments to the smallest compartments

A

Largest
- muscle
- muscle cell/fiber
- myofibril
- sarcomere
Smallest

80
Q

What is the Z-line?

A

It contains proteins important for sarcomere structure stability and is found on the most outward ends of the sarcomere

81
Q

How are actin filaments oriented in sarcomeres?

A

The minus end facing the center and plus end facing the Z-line

82
Q

What is the M-line?

A

It is in the center of the sarcomere and contains anchoring proteins

83
Q

What are the I-bands?

A

Contain only thin (actin) filaments, most outward bands

84
Q

What is the H-zone?

A

It contains solely thick (myosin II) filaments

85
Q

What is the A-band?

A

It is an overlap of thick and thin filaments, and also includes the H-zone

86
Q

Which zones decrease in length?

A

I-bands and H-zone

87
Q

Which zone stays the same length?

A

A-band

88
Q

When a sarcomere is contracted, the thin filaments slide _________ the center of the sarcomere

A

Towards

89
Q

Myosin II heads in a thick filament binds to _____ surrounding actin filaments

A

Six

90
Q

Why is myosin II non-processive?

A

It is only in contact with actin for a fraction of the time

91
Q

A single myosin “power stroke” moves an actin filament ____

A

10nm

92
Q

Describe the steps of the actin-myosin contraction cycle

A
  1. ATP binds to myosin head and myosin dissociates from actin
  2. ATP hydrolysis, ADP and Pi remain bound to myosin
  3. Energized myosin binds actin
  4. Release of phosphate triggers conformational change: power stroke” actin moves towards center of the sarcomere
  5. ADP is released, but myosin remains attached
93
Q

Which neurotransmitter is involved in muscle contraction? What does it cause?

A

Acetylcholine causes an influx of Na+ ions into muscle cells, causing an action potential

94
Q

Muscle cells within a ___________ are stimulated _____________ by a single motor neuron

A

motor unit, simultaneously

95
Q

What is the neuro-muscular junction?

A

Point of contact between motor neuron and muscle fiber; site of transmission of the nerve impulse

96
Q

True/False? A motor neuron can stimulate multiple muscle fibers

A

True

97
Q

What are transverse (T) tubules?

A

Membrane folds that propagate an impulse to the interior of a muscle cell

98
Q

What is the sarcoplasmic reticulum (SR)?

A

A special smooth ER in muscle cells which stores Ca2+ in lumen (pumped in from the cytosol)

99
Q

How does an action potential effect a muscle cell?

A

The action potential travels down to the SR, which opens its Ca2+ channels into the cytoplasm

100
Q

What three proteins are included in thin filaments?

A

Actin, tropomyosin (rod-shaped), and troponin (globular)

101
Q

In the absence of Ca2+:

A

Troponin controls the position of tropomyosin which blocks myosin-binding sites on actin

102
Q

In the presence of Ca2+:

A

Ca2+ binds troponin which moves tropomyosin, exposing the myosin-binding site on actin

103
Q

What blocks the myosin-binding sites on actin thin filaments in a resting sarcomere?
a) troponin
b) Ca2+
c) myosin inactivating peptide
d) tropomyosin
e) nothing

A

D

104
Q

What blocks the myosin-binding sites on actin thin filaments in a stimulated sarcomere?
a) troponin
b) Ca2+
c) myosin inactivating peptide
d) tropomyosin
e) nothing

A

E. Myosin is in contact with actin when the sarcomere is stimulated

105
Q

What happens when a nerve impulse (action potential) reaches the neuromuscular junction?

A

Calcium diffuses into the presynaptic cell, causes
synaptic vesicles to fuse and release
neurotransmitters into the synaptic cleft.
Neurotransmitters bind to receptors on the muscle
cell (postsynaptic cell) and positive ions (like
sodium) enter. This causes membrane
depolarization, and the action potential gets
propagated through the muscle cell via Transverse
tubules

106
Q

What happens when a nerve impulse (action potential) reaches the SR?

A

Nerve impulse arrives at the sarcoplasmic
reticulum (SR) via T tubules. Once it arrives, calcium
channels in the SR membrane open and calcium
diffuses out of the SR into the cytoplasm, bathing
the myofibrils in calcium. Calcium binds to troponin,
exposing the myosin-binding site on actin.

107
Q

Cell cortex:

A

Actin network on the inner face of the plasma membrane; capable of dynamic remodeling

108
Q

What are the functions of the cell cortex?

A
  • enables cells to crawl/move
  • enables phagocytosis
  • cellular constriction during cell division
109
Q

Actin-binding proteins:

A

Regulate the assemble, disassembly, and rearrangements of actin networks

110
Q

What is a filament nucleating protein? Provide two examples

A

Proteins that enhance the rate at which actin filaments are formed. They are the slowest step in action filament formation (ex. Arp2/3 complex and formins)

111
Q

What is the Arp2/3 complex?

A
  • binds to the side of an existing actin filament (creates branches)
  • remains at the pointed end of the new branch
  • similar structure to actin monomers
112
Q

What are formins?

A
  • generate unbranched filaments
  • stay associated with the barbed end
  • promote rapid elongation of filaments
113
Q

What is a monomer-sequestering protein?

A
  • bind to actin-ATP monomers to prevent them from being added to the elongating filament
  • able to modulate the available monomer pool in certain regions at certain times
114
Q

What are end-blocking/capping proteins?

A
  • regulate length of actin filaments
  • bind at either end
115
Q

What is a monomer polymerizing protein? Provide an example

A
  • binds to actin monomers to promote growth of actin filaments
  • promotes replacement of ADP with ATP on the actin monomers
    ex. profilin
116
Q

Cofilin is an example of:
a) a depolarizing protein
b) a monomer-sequestering protein
c) a filament-severing protein
d) none of the above
e) a) and c) are both correct

A

E

117
Q

What is a depolarizing protein? Provide an example

A
  • bind to actin-ADP at the pointed end to promote depolymerization
    ex. cofilin
118
Q

What is a cross-linking and bundling protein?

A
  • multiple actin-binding sites, allowing them to alter the 3D organization
119
Q

What is a filament-severing protein? Provide one example

A
  • break an existing filament in two
    ex. cofilin
120
Q

How are actin filaments oriented in cross-linking versus bundling?

A

In cross-linking, filaments branch outwards, in bundling, filaments are parallel

121
Q

What are membrane-binding proteins? Provide one example

A
  • actin filaments linked to the plasma membrane
  • enables the plasma membrane to protrude outwards (cell locomotion) or inwards (phagocytosis)
    ex. spectrin family
122
Q

A shift in the concentration or activity of which type(s) of actin binding proteins can cause a shift in the equilibrium between available actin monomers and filamentous actin (select all that apply)?
a) monomer-sequestering proteins
b) cross-linking proteins
c) depolarizing proteins
d) membrane-binding proteins
e) nucleating proteins

A

ACE

123
Q

Describe the steps of cell motility

A
  1. Movement is initiated by a protrusion of the cell in the direction of movement (lamellipodium)
  2. A portion of the protrusion anchors to the surface below
  3. The bulk of the cell is pulled to the front, over the adhesive contacts
  4. Adhesive contacts break, causing retraction of the trailing edge (tail)
124
Q

Describe the steps of lamellipodium formation

A
  1. A stimulus is received at the cell surface
  2. Arp2/3 complex at the site of stimulation gets activated
  3. Arp2.3 binds the side of an existing filament
  4. ATP-actin monomers bind to the Arp2/3 complex, forming a new actin branch. Profilin is involved with polymerization
  5. Additional Arp2/3 complexes can bind to the sides of the new filaments, forming additional branches. Older filaments are capped at their barbed ends so they stay short and stable
  6. Newer filaments continue to grow at the barbed end, pushing the membrane of the lamellipodium outward. Older capped filaments undergo disassembly promoted by cofilin
125
Q

Describe traction forces (step 2 of cell motility)

A

When the cell grips the surface (at adhesion points called focal adhesions)

126
Q

What are focal adhesions (step 2 of cell motility)?

A

Structures in the cell membrane where integrin proteins connect to actin

127
Q

What are integrin proteins (step 2 of cell motility)?

A

Transmembrane proteins that mediate the interaction between actin and extracellular components

128
Q

What are contraction forces (step 3 of cell motility)?

A

Pull the bulk of the cell forward. Myosins are found near the rear of the lamellipodium

129
Q

Order the following steps of cell locomotion:
1. Integrin adhesion to the surface is broken at the trailing edge of the cell
2. Microfilaments grow and push the cell membrane forward
3. Myosin contracts along the actin filaments to pull the cell body
4. Activation of the Arp2/3 complex
5. Lamellipodia adhere to the surface via focal adhesions

A

4, 2, 5, 3, 1

130
Q

True/False? Intermediate filaments are strong, flexible, unbranched, and have no polarity

A

True

131
Q

True/False? Intermediate filaments are only found in plant cells

A

False. They are only found in animal cells

132
Q

What does it mean that intermediate filaments are chemically heterogenous?

A

They consist of multiple different types of subunits which are all coded by different genes

133
Q

How are monomers oriented into a dimer in intermediate filaments?

A

Parallel. The N-terminals are paired together and the C-terminals are paired together

134
Q

How are dimers oriented into tetramers in intermediate filaments?

A

Antiparallel and staggered

135
Q

How many tetramers are associated together to make 60nm of intermediate filament?

A

8

136
Q

Explain dynamic remodeling of intermediate filaments

A

Insertion of unit lengths of filament into the middle of existing filaments (and dissociation) controlled by phosphorylation status. Can be added or dissociated from the main filament at any time. Does not require ATP or GTP

137
Q

What role does plectin play in intermediate filaments?

A

Bridging to intermediate filaments stabilizes other cytoskeletal elements, increasing cell stability

138
Q

Neurofilaments:

A
  • three distinct proteins
  • have sidearms that help to maintain proper spacing
  • important for determining the axon’s diameter (which has a role in action potential speed)
  • aggregation of neurofilaments seen in ALS and Parkinson’s
139
Q

You inject fluorescently-labelled keratin subunits into cultured skin cells. What will you see shortly after injection?
a) filaments become labelled at sites scattered throughout their length
b) filaments become labelled at both ends simultaneously
c) filaments become labelled at one end exclusively

A

A. Dynamic remodeling of intermediate filaments

BIOL 331 Midterm (3 decks)

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