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BIOL 2321 - Cell Biology > Cytoskeleton > Flashcards

Cytoskeleton Flashcards

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1
Q

What are the 3 components of the cytoskeleton?

A

microtubules

microfilaments

intermediate filaments

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2
Q

Briefly describe the structure and components of microtubules

A

structure: hollow, thick, rigid, unbranched
components: tubulin polymers (dimers of alpha and beta tubulin)

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3
Q

Briefly describe the structure and components of microfilaments

A

structure: solid, thin, flexible, branched, helical
components: actin polymers

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4
Q

Briefly describe the structure and components of intermediate filaments

A

structure: tough, rope-like
components: 70+ different proteins

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5
Q

What cell types are microtubules in?

A

all eukaryotes

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6
Q

What cell types are microfilaments in?

A

all eukaryotes

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7
Q

What cell types are intermediate filaments in?

A

animals only

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8
Q

Where are microtubules located?

A

cytoplasm

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9
Q

Where are microfilaments located?

A

cytoplasm

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10
Q

Where are intermediate filaments located?

A

cytoplasm and nucleus

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11
Q

Compare the diameter of microtubules, microfilaments, and intermediate filaments

A

microtubules: 25 nm
microfilaments: 8 nm

intermediate filaments: 10-12 nm

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12
Q

Which of the 3 cytoskeleton is the thinnest? which is the thickest?

A

thinnest = microfilaments

thickest = microtubules

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13
Q

What monomers make up microtubules?

A

alpha tubulin

beta tubulin

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14
Q

What monomers make up microfilaments?

A

actin

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15
Q

What monomers make up intermediate filaments?

A

varies

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16
Q

What is the enzyme activity in microtubules?

A

GTPase

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17
Q

What is the enzyme activity in microfilaments?

A

ATPase

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18
Q

What is the enzyme activity in intermediate filaments?

A

trick question! there’s no enzyme activity

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19
Q

Is there structural polarity in microtubules?

A

yes

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20
Q

Is there structural polarity in microfilaments?

A

yes

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21
Q

Is there structural polarity in intermediate filaments?

A

no

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22
Q

What motor proteins function in microtubules?

A

kinesins and dyneins

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23
Q

What motor proteins function in microfilaments?

A

myosin

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24
Q

What motor proteins function in intermediate filaments?

A

none!

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25
Q

Where does growth occur on microtubules?

A

+ end

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26
Q

Where does growth occur on microfilaments?

A

+ end

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27
Q

Where does growth occur on intermediate filaments?

A

internal

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28
Q

What are 5 functions of the cytoskeleton?

A

strength and structural support

internal organization of cellular components

allows cells to interact with each other and the environment

allows some cells to change shape and move

allows cells to rearrange their internal components as they grow, divide, and respond to external signals

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29
Q

What are the main functions of microtubules?

A

structural support

intracellular transport

spatial organization of organelles

cell motility

cell division

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30
Q

What are the main functions of microfilaments?

A

intracellular transport

cell motility

cell contractility

cell division

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31
Q

What are the main functions of intermediate filaments?

A

structural support

mechanical strength

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32
Q

What 5 general similarities do microtubules and microfilaments have?

A

polarity

nucleation

elongation

NTP hydrolysis

+ end caps

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33
Q

Briefly describe why microtubules and microfilaments have polarity in common

A

they both have + and - ends

growth is more common on the + end

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34
Q

Briefly describe why microtubules and microfilaments have nucleation in common

A

Both have a ‘lag’ phase - a slow process to form the initial aggregate

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35
Q

Briefly describe why microtubules and microfilaments have elongation in common

A

they both see rapid growth after the aggregate forms

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36
Q

Briefly describe why microtubules and microfilaments have NTP hydrolysis in common

A

Both hydrolyze an NTP (ATP for actin, GTP for tubulin) into NDP + Pi to incorporate incoming units at the + end

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37
Q

Briefly describe why microtubules and microfilaments have + end caps in common

A

Both have + caps that indicate elongation (they are bound to NTP and added at the + end)

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38
Q

Describe the polymer polarity. Which cytoskeleton elements have this?

A

unique + and - ends on microtubules and microfilaments

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39
Q

What occurs at the + end of microtubules and microfilaments?

A

addition (growth) of new units (tubulin dimers on microtubules and actin monomers on microfilaments)

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40
Q

What is added to the + end of microtubules?

A

tubulin dimers

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41
Q

What is added to the + end of microfilaments?

A

actin monomers

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42
Q

describe nucleation

A

the formation of an initial aggregate of individual units (tubulin dimers or actin monomers)

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43
Q

Is nucleation slow or fast initially? why?

A

slow because the first few units are not very stable (they lack lots of subunit-subunit interactions)

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44
Q

What happens once the initial aggregate has formed?

A

elongation (rapid growth) is much faster

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45
Q

What does polymerization eventually reach over time? when does it reach this?

A

an equilibrium phase or steady state when the growth of the polymer is equal to the shrinkage of the polymer

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46
Q

What does actin bind to?

A

ATP

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47
Q

What does tubulin bind to?

A

GTP

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48
Q

What happens to new units that join the + end of a polymer?

A

they bind to either ATP (actin) or GTP (tubulin)

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49
Q

What happens to the NTP that has bound the new unit at the + end of a polymer?

A

it is hydrolyzed

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50
Q

T or F: the older units of a polymer are bound to ATP/GTP

A

false! the NTP they are bound to is hydrolyzed to ADP/GDP

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51
Q

Describe the + end caps

A

when a polymer is extending, a ‘cap’ of newly added ATP/GTP-bound units are at the plus end

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52
Q

What does the presence of a + cap signify?

A

elongation of the polymer

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53
Q

What are microtubules composed of?

A

heterodimers of alpha and beta tubulin

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54
Q

In microtubules, what is alpha tubulin bound to?

A

GTP

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55
Q

T or F: the GTP that alpha tubulin binds to is hydrolyzed

A

FALSE it is never hydrolyzed

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56
Q

In microtubules, what is beta tubulin bound to?

A

either GDP or GTP

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57
Q

On a microtubule, where will alpha tubulin be located? What does this contribute to?

A

On the bottom or beneath a beta tubulin

contributes to structural polarity

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58
Q

On a microtubule, where will beta tubulin be located? What does this contribute to?

A

on the top or above alpha tubulin

contributes to structural polarity

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59
Q

How are alpha-beta tubulin heterodimers oriented in microtubules? What does this form?

A

head-to-tail in 13 staggered protofilaments to form a hollow tube

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60
Q

Describe protofilaments

A

the way alpha-beta tubulins are organized to form the hollow microtubule

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61
Q

How many protofilaments are required to make a microtubule?

A

13

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62
Q

Are microtubules hollow or solid?

A

hollow

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63
Q

In a microtubule, what forms the plus end?

A

exposed Beta tubulins

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64
Q

Where does shrinkage occur on microtubules?

A

at the plus end

old dimers can be lost here

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65
Q

T or F: only growth occurs at the + end of microtubules

A

false, both growth and shrinkage occurs here

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66
Q

T or F: shrinkage of microtubules occurs at the - end

A

false! most of the growth and shrinkage occurs at the + end

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67
Q

What is added to the microtubule to extend it?

A

new heterodimers at the plus end

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68
Q

Which subunit dictates assembly and disassembly of the microtubule?

A

beta subunit

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69
Q

When will microtubule assembly happen?

A

when the exposed B subunit at the plus end is bound to GTP

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70
Q

What causes disassembly at the + end of microtubules?

A

the GTP bound to the beta subunit is hydrolyzed and GDP + Pi cause the disassembly at the plus end

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71
Q

What prevents disassembly at the plus end of microtubules?

A

the presence of GTP-containing Beta subunits at the + end

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72
Q

What causes the microtubule to shrink?

A

if GTP hydrolysis is occurring faster than subunit addition, the GTP cap will be lost and the disassembly will outweigh the assembly

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73
Q

Describe a catastrophe and what causes it

A

microtubule disassembly from the plus end caused by GTP hydrolysis occurring more rapidly than the addition of heterodimers (the loss of the GTP cap)

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74
Q

Describe a rescue and how it occurs

A

A rescue is when enough GTP-bound tubulin dimers are added to the shrinking end of a microtubule (GTP cap) and growth can resume

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75
Q

Describe dynamic instability

A

a microtubule’s alternation between growth, catastrophe, rescue, catastrophe, etc.

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76
Q

What is the function of Microtubule Accessory Proteins (MAPs)?

A

they stabilize microtubules and promote growth/assembly

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77
Q

How do MAPs stabilize microtubules?

A

one of their domains binds to the microtubule and the other projects outwards

78
Q

What mediates the binding of MAPs to a microtubule?

A

phosphorylation and dephosphorylation

79
Q

What is an example of a MAP in humans? Describe its relationship to Alzheimer’s Disease

A

Tau is a MAP that is excessively phosphorylated and causes Alzheimer’s Disease

Excess phosphorylation = cannot bind to neuronal microtubules and therefore cannot stabilize the dynamic instability of microtubules

neuronal microtubules disassemble +
hyper-phosphorylated Tau aggregates = neurons die

80
Q

What does the initial formation of a microtubule require?

A

A very high concentration of tubulin dimers and assistance of other proteins

81
Q

Where are microtubules generated in the cell?

A

Microtubule Organizing Centres (MTOCs)

82
Q

Give an example of an animal MTOC

A

the centrosome

83
Q

Where do microtubules form in plants and fungi?

A

MTOCs embedded in the nuclear envelope

84
Q

What does MTOC stand for?

A

Microtubule Organizing Centre

85
Q

Describe the structure of centrosomes

A

two perpendicular barrel-shaped centrioles are surrounding by a dense mass (looks like a cloud) of insoluble proteins called pericentriolar material

86
Q

In animals, how do microtubules anchor to the centrosome as an MTOC?

A

the minus end of the microtubule anchors in the dense mass of insoluble proteins surrounding the centrioles

87
Q

Where are microtubules nucleated?

A

at their minus ends

88
Q

What allows the centrosome to position itself roughly in the centre of the cell?

A

the position of microtubules

microtubules are nucleated (attached to the centrosome) at their minus end

the + end points outwards into the rest of the cell

89
Q

What is found at the base of the minus end of microtubules?

A

gamma tubulin rings

90
Q

What is the function of gamma tubulin rings in microtubules?

A

they are required for growth and polarity determination in all organisms

they also anchor the minus end of microtubules to insoluble proteins of the pericentriolar material in animal centrosomes

91
Q

How do gamma tubulin rings help anchor the microtubule to centrosomes?

A

they are located at the base of the minus end of microtubules and bind to the insoluble proteins of the pericentriolar material (the dense mass that surrounds the centrioles) of animal centrosomes

92
Q

What binds to the gamma tubulin ring once it is bound to the periocentriolar material of centrosomes? What does this cause?

A

the alpha tubulin of a heterodimer tubulin

this causes the elongation of the microtubule with the plus end facing outwards

93
Q

What are the 2 kinds of microtubule motor proteins?

A

kinesins

dyneins

94
Q

What is the general function of the microtubule motor proteins?

A

they generate the force necessary for moving materials within the cell along microtubules

95
Q

What kind of material do microtubule motor proteins move?

A

organelles
incoming secretory vesicles
outgoing endocytic vesicles
vesicles between the ER and Golgi (COPI and COPII)

96
Q

What direction do kinesins move?

A

toward the + end of the microtubule

97
Q

What direction do dyneins move?

A

towards the minus end of the microtubule

98
Q

What structure does a kinesin have?

A

a tetramer with 2 heavy and 2 light chains

99
Q

Describe the structure of kinesin

A

a tetramer (2 heavy + 2 light chains)

globular HEADS on the heavy chain

heads are connected to the tail by a neck and flexible stalk

100
Q

What is the function of the heads of kinesins?

A

they bind and hydrolyze ATP to create processive movement along the microtubule

101
Q

What is the function of the tails of kinesins?

A

the tail binds the cargo that the kinesin is moving

102
Q

What connects the heads and tail of kinesins?

A

a neck and flexible stalk

103
Q

Describe what it means for the movement of kinesin to be processive

A

one alternating head is attached to the microtubule at all times (like walking - one head at a time)

104
Q

Describe the position of a kinesins 2 heads (rear and leading) at any given time

A

the rear head lags behind the leader head and is bound to ATP and the microtubule

the leader head is ADP-bound and loosely connected to the microtubule

105
Q

What is the first step in a kinesin moving forward one step?

A

the rear head hydrolyzes the ATP it is bound to (now bound to ADP)

this releases phosphate which loosens its attachment to the microtubule

106
Q

What happens after the rear head of kinesin hydrolyzes ATP (Step 2)?

A

the front head replaces the ADP it is bound to with ATP

107
Q

What happens after the leading head of kinesin replaces ADP with ATP (Step 3)?

A

When ATP binds to the front head it causes a conformational change which propels the rear head forwards (becomes the new leading head)

108
Q

What happens after the rear head is propelled forward (Step 4)?

A

the new rear head hydrolyzes ATP to release phosphate and loosen its bind to the microtubule

and

the new leader head will exchange ADP to ATP to take another step forward

109
Q

Briefly describe all the steps for kinesin to move forward one step

A
  1. 1 ADP bound kinesin head binds to a microtubule binding site
  2. this head exchanges ADP for ATP
  3. binding of ATP causes conformational change which propels the 2nd head in front to a new binding site
  4. the new rear head hydrolyzes its ATP to release phosphate and loosen its attachment to the MT
  5. new leader head exchanges ADP with ATP to take another step

repeat repeat

110
Q

Describe the structure of dyneins

A

have both heavy and light chain

2 binding sites for microtubules on globular heads on heavy chains

long stem domains connect heavy and light chains

light chains bind to dynactin

111
Q

What is required for dyneins to bind cargo?

A

the light chain of dyneins binds to a protein called dynactin which can bind to cargo

112
Q

Describe dynactin?

A

a linker protein that binds to the light chains of dyneins and the cargo

113
Q

What direction is anterograde transport?

A

forward

114
Q

What direction is retrograde transport?

A

backward

115
Q

In axoplasmic transport along an axon in nerve cells, where are materials moved in anterograde transport?

A

away from the cell body

116
Q

In axoplasmic transport along an axon in nerve cells, where are materials moved in retrograde transport?

A

toward the cell body

117
Q

What microtubule motor proteins would be involved in axoplasmic anterograde transport?

A

kinesins move cargo from the minus to the plus end (forward = anterograde = from cell body to axon terminus)

118
Q

What microtubule motor proteins would be involved in axoplasmic retrograde transport?

A

dyneins move cargo from plus to minus end (backwards = retrograde = from axon terminus to cell body)

119
Q

What structure do intermediate filaments have?

A

they are tetramers

120
Q

How does the structure of intermediate filaments form?

A

globular terminal domains (N and C termini) for attachment between a long alpha helical region

parallel monomers associate into dimers

dimers are assembled antiparallel and staggered into tetramers to make ONE intermediate filament

121
Q

T or F: all intermediate filaments are the same size and composed of the same number of tetramers

A

false! they can vary

122
Q

What makes intermediate filaments different from MTs and MFs?

A

IFs do not:

have plus or minus ends (no polarity)

bind NTPs

123
Q

Where are new units added to intermediate filaments?

A

in the middle of the filament

124
Q

What is the assembly/disassembly of IFs regulated by?

A

phosphorylation

125
Q

What makes IFs different from MTs and MFs?

A

they do not:

have polarity (no + or - ends) - addition is in the middle of the filament

bind NTP

break under a lot of deforming force

126
Q

Order the cytoskeletal elements in ability to withstand deforming force

A

strongest: IF
middle: MF
weakest: MT

127
Q

What is the major function of intermediate filaments?

A

they provide mechanical support for cells subject to mechanical stress

128
Q

Where would you expect to see many intermediate filaments?

A

in cells that are subjected to a lot of mechanical stress

ex. epithelial cells that line the bladder

129
Q

What is another function of intermediate filaments?

A

in addition to providing strength, they connect other parts of the cytoskeleton together

130
Q

What is an example of protein in an intermediate filament?

A

keratin

131
Q

T or F: microfilaments are less dynamic than microtubules

A

false! MFs are more dynamic

132
Q

What are the main functions of microfilaments?

A

structure and stability for the cell

133
Q

What are 6 examples of microfilament movements?

A

movement of cells over substratum (crawling)

leading edge of axon growing towards synaptic target (axon outgrowth)

organelle movements and vesicle movements

cell division (cytokinesis)

cytoplasmic streaming

muscle contraction

134
Q

What are microfilaments composed of?

A

globular actin subunits (G-actin)

135
Q

What is G-actin?

A

globular actin subunit (monomer) that are incorporated into the filament (F-actin)

136
Q

How does G-actin assemble into actin filaments?

A

G-actin binds ATP and assembles into actin filaments (F-actin)

137
Q

What is F-actin?

A

filamentous actin

138
Q

What are the 2 structures of actin which make up microfilaments?

A

G-actin (globular) = monomer

F-actin (filamentous) = polymer of many G-actins

139
Q

How is an individual G-actin monomer incorporated into an F-actin polymer?

A

by hydrolyzing ATP

140
Q

What are the + and - ends of F-actin based on?

A

the shape of the monomers

141
Q

How are monomers positioned in the filament? (microfilament)

A

positioned so the ATP binding sites are closer to the minus end

142
Q

Does the + or - end of F-actin grow more rapidly?

A

plus

143
Q

Where does disassembly occur in microfilaments?

A

at the minus end of the F-actin

144
Q

Which part of the microfilament is weaker?

A

the part where the actin is bound to ADP is weaker (the minus end)

145
Q

Which end of an F-actin filament has ADP bound?

A

minus

146
Q

Which end of an F-actin filament has ATP bound?

A

plus end

147
Q

What promotes disassembly/depolymerization in F-actin?

A

Hydrolysis of ATP

148
Q

Describe treadmilling in microfilaments

A

when the F-actin polymer is adding units at the same rate the minus end is removing units

it’s like an equilibrium state and the length of the polymer is stable

149
Q

What happens to the length of the F-actin polymer when treadmilling occurs?

A

the length is stable

150
Q

What happens to individual units that have been added to the plus end during treadmilling?

A

they will move toward the minus end

151
Q

roughly how much of a cells total protein is actin? How much of this is assembled into filaments? What happens to the rest?

A

5% is actin

50% of this is assembled into filaments

the other 50% remains as soluble monomers

152
Q

What binds to either F- or G- actin? What does it do?

A

Actin Binding Proteins that modify the properties of the actin

153
Q

What are 6 examples of how Actin Binding Proteins can modify F- or G-actin?

A

nucleate or sequester G-actin monomers

polymerize or depolymerize F-actin filaments

cap filaments to restrict length

cross-link to create branches and bundling to increase strength

sever filaments

attach actin to organelles or cell surface by membrane binding

154
Q

What are 4 examples of the many different proteins that interact with actin (Actin Binding Proteins)?

A

thymosin

Arp2/3 complex

profilin

cofilin

155
Q

How are microfilaments involved in cell movement?

A

the disassembly or reassembly of actin filaments can result in the movement towards site of reassembly

156
Q

What are 3 examples of activities that require cell locomotion?

A

wound healing

development of axons

formation of blood vessels

157
Q

Describe a lamellipodium

A

a broad, flattened area where the cell is reaching towards a stimulus via polymerization

158
Q

What can a lamellipodium also be called?

A

a leading edge

159
Q

Describe the structure of a lamellipodium

A

broad, very flat, fan-shaped with a ruffled edge that contains actin meshwork

160
Q

Describe filopodia

A

the individual projections (ruffled edges of the leading edge) within the lamellipodium that contains a core of long bundled actin filaments

161
Q

What is in the filopodia?

A

a core of long bundled actin filaments

162
Q

What is located near the tail of a cell with a lamellipodium?

A

stress fibres

163
Q

What are stress fibers? what do they do? where are they located?

A

contractile non-muscle actin bundles that contract near the tail of a cell to bring the rest of the cell forwards as the lamellipodium crawls toward a stimulus

164
Q

What is the movement of a cell towards a stimulus called?

A

chemotaxis

165
Q

Briefly describe the steps of cell movement with actin filaments

A
  1. actin polymerization pushes out lamellipodium
  2. new attachments to substratum via integrins
  3. stress fibres contact and cell moves
166
Q

Where in a cell is actin more concentrated?

A

the leading edge

167
Q

Where in a cell is myosin more concentrated?

A

at the tail of the cell

168
Q

What is actin growth at the lamellipodia regulated by?

A

WASP proteins

169
Q

What causes new filaments to form at the lamellipodia?

A

WASP activates Arp 2/3 complex in response to chemotactic signals

170
Q

What does Arp 2/3 stand for?

A

Actin Related Protein

171
Q

What occurs in individuals with Wiskott-Aldrich Syndrome?

A

they are lacking WASP proteins and have a dysfunctional immune system

white blood cells fail to respond to chemotactic signals

172
Q

What activates ARP2/3?

A

WASP

173
Q

What happens when ARP2/3 has been activated?

A

ARP2/3 will bind to the minus end of an actin filament to promote nucleation

174
Q

What happens ARP2/3 binds to the minus end of an actin filament?

A

it can attach to the side of another actin filament to form a branch

175
Q

What is the purpose of ARP2/3 forming branches?

A

it helps individual actin filaments extend faster and extend into a tree-like web

176
Q

What does cofilin do to microfilaments?

A

it is an actin binding protein that encourages the minus ends to dissociate

177
Q

What does profilin do to microfilaments?

A

it is an actin binding protein that encourages the growth at plus ends

178
Q

T or F: actin can form fixed permanent cell projections

A

true

179
Q

What kind of micrograph would be used to visualized microfilaments?

A

TEM

180
Q

What is an example of microfilaments that would require staying the same length?

A

actin filaments in microvilli

181
Q

Describe the structure of microvilli

A

plus ends of actin filaments form microvillii and are bound to the tip of the plasma membrane by minus ends anchored to intermediate filaments

stabilizing proteins run along the length of the microvilli to maintain the length

182
Q

What stabilizes the length of microvilli?

A

stabilizing proteins

183
Q

What type of motor proteins are involved with microfilaments?

A

myosin

184
Q

What are the 2 types of myosin?

A

conventional (type II)

nonconventional

185
Q

Describe the function of conventional (type II) myosins

A

they are used for contraction in actin filaments

186
Q

Describe the function of nonconventional myosins

A

used like kinesins to move things around the cells (like vesicles)

187
Q

What does myosin use to move things?

A

ATP hydrolysis

188
Q

Describe the structure of myosin II

A

a dimer made up of monomers with

1 heavy chain and 2 light chains
2 globular head domains

a coiled coil tail of heavy chain alpha helices (C terminal chains)

189
Q

In myosin, what hydrolyzes ATP?

A

the two globular head domains bind and hydrolyze ATP (N terminus)

190
Q

What can type II myosins assemble into?

A

bipolar filaments (many myosins) with heads in both directions

191
Q

What causes the contractile force of type II myosin?

A

the pulling by bipolar filaments on the actin cytoskeleton

BIOL 2321 - Cell Biology (26 decks)

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