Cytoskeleton Flashcards
1
Q
What are the 3 components of the cytoskeleton?
A
microtubules
microfilaments
intermediate filaments
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)
3
Q
Briefly describe the structure and components of microfilaments
A
structure: solid, thin, flexible, branched, helical
components: actin polymers
4
Q
Briefly describe the structure and components of intermediate filaments
A
structure: tough, rope-like
components: 70+ different proteins
5
Q
What cell types are microtubules in?
A
all eukaryotes
6
Q
What cell types are microfilaments in?
A
all eukaryotes
7
Q
What cell types are intermediate filaments in?
A
animals only
8
Q
Where are microtubules located?
A
cytoplasm
9
Q
Where are microfilaments located?
A
cytoplasm
10
Q
Where are intermediate filaments located?
A
cytoplasm and nucleus
11
Q
Compare the diameter of microtubules, microfilaments, and intermediate filaments
A
microtubules: 25 nm
microfilaments: 8 nm
intermediate filaments: 10-12 nm
12
Q
Which of the 3 cytoskeleton is the thinnest? which is the thickest?
A
thinnest = microfilaments
thickest = microtubules
13
Q
What monomers make up microtubules?
A
alpha tubulin
beta tubulin
14
Q
What monomers make up microfilaments?
A
actin
15
Q
What monomers make up intermediate filaments?
A
varies
16
Q
What is the enzyme activity in microtubules?
A
GTPase
17
Q
What is the enzyme activity in microfilaments?
A
ATPase
18
Q
What is the enzyme activity in intermediate filaments?
A
trick question! there’s no enzyme activity
19
Q
Is there structural polarity in microtubules?
A
yes
20
Q
Is there structural polarity in microfilaments?
A
yes
21
Q
Is there structural polarity in intermediate filaments?
A
no
22
Q
What motor proteins function in microtubules?
A
kinesins and dyneins
23
Q
What motor proteins function in microfilaments?
A
myosin
24
Q
What motor proteins function in intermediate filaments?
A
none!
25
Where does growth occur on microtubules?
+ end
26
Where does growth occur on microfilaments?
+ end
27
Where does growth occur on intermediate filaments?
internal
28
What are 5 functions of the cytoskeleton?
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
29
What are the main functions of microtubules?
structural support
intracellular transport
spatial organization of organelles
cell motility
cell division
30
What are the main functions of microfilaments?
intracellular transport
cell motility
cell contractility
cell division
31
What are the main functions of intermediate filaments?
structural support
mechanical strength
32
What 5 general similarities do microtubules and microfilaments have?
polarity
nucleation
elongation
NTP hydrolysis
+ end caps
33
Briefly describe why microtubules and microfilaments have polarity in common
they both have + and - ends
growth is more common on the + end
34
Briefly describe why microtubules and microfilaments have nucleation in common
Both have a 'lag' phase - a slow process to form the initial aggregate
35
Briefly describe why microtubules and microfilaments have elongation in common
they both see rapid growth after the aggregate forms
36
Briefly describe why microtubules and microfilaments have NTP hydrolysis in common
Both hydrolyze an NTP (ATP for actin, GTP for tubulin) into NDP + Pi to incorporate incoming units at the + end
37
Briefly describe why microtubules and microfilaments have + end caps in common
Both have + caps that indicate elongation (they are bound to NTP and added at the + end)
38
Describe the polymer polarity. Which cytoskeleton elements have this?
unique + and - ends on microtubules and microfilaments
39
What occurs at the + end of microtubules and microfilaments?
addition (growth) of new units (tubulin dimers on microtubules and actin monomers on microfilaments)
40
What is added to the + end of microtubules?
tubulin dimers
41
What is added to the + end of microfilaments?
actin monomers
42
describe nucleation
the formation of an initial aggregate of individual units (tubulin dimers or actin monomers)
43
Is nucleation slow or fast initially? why?
slow because the first few units are not very stable (they lack lots of subunit-subunit interactions)
44
What happens once the initial aggregate has formed?
elongation (rapid growth) is much faster
45
What does polymerization eventually reach over time? when does it reach this?
an equilibrium phase or steady state when the growth of the polymer is equal to the shrinkage of the polymer
46
What does actin bind to?
ATP
47
What does tubulin bind to?
GTP
48
What happens to new units that join the + end of a polymer?
they bind to either ATP (actin) or GTP (tubulin)
49
What happens to the NTP that has bound the new unit at the + end of a polymer?
it is hydrolyzed
50
T or F: the older units of a polymer are bound to ATP/GTP
false! the NTP they are bound to is hydrolyzed to ADP/GDP
51
Describe the + end caps
when a polymer is extending, a 'cap' of newly added ATP/GTP-bound units are at the plus end
52
What does the presence of a + cap signify?
elongation of the polymer
53
What are microtubules composed of?
heterodimers of alpha and beta tubulin
54
In microtubules, what is alpha tubulin bound to?
GTP
55
T or F: the GTP that alpha tubulin binds to is hydrolyzed
FALSE it is never hydrolyzed
56
In microtubules, what is beta tubulin bound to?
either GDP or GTP
57
On a microtubule, where will alpha tubulin be located? What does this contribute to?
On the bottom or beneath a beta tubulin
contributes to structural polarity
58
On a microtubule, where will beta tubulin be located? What does this contribute to?
on the top or above alpha tubulin
contributes to structural polarity
59
How are alpha-beta tubulin heterodimers oriented in microtubules? What does this form?
head-to-tail in 13 staggered protofilaments to form a hollow tube
60
Describe protofilaments
the way alpha-beta tubulins are organized to form the hollow microtubule
61
How many protofilaments are required to make a microtubule?
13
62
Are microtubules hollow or solid?
hollow
63
In a microtubule, what forms the plus end?
exposed Beta tubulins
64
Where does shrinkage occur on microtubules?
at the plus end
| old dimers can be lost here
65
T or F: only growth occurs at the + end of microtubules
false, both growth and shrinkage occurs here
66
T or F: shrinkage of microtubules occurs at the - end
false! most of the growth and shrinkage occurs at the + end
67
What is added to the microtubule to extend it?
new heterodimers at the plus end
68
Which subunit dictates assembly and disassembly of the microtubule?
beta subunit
69
When will microtubule assembly happen?
when the exposed B subunit at the plus end is bound to GTP
70
What causes disassembly at the + end of microtubules?
the GTP bound to the beta subunit is hydrolyzed and GDP + Pi cause the disassembly at the plus end
71
What prevents disassembly at the plus end of microtubules?
the presence of GTP-containing Beta subunits at the + end
72
What causes the microtubule to shrink?
if GTP hydrolysis is occurring faster than subunit addition, the GTP cap will be lost and the disassembly will outweigh the assembly
73
Describe a catastrophe and what causes it
microtubule disassembly from the plus end caused by GTP hydrolysis occurring more rapidly than the addition of heterodimers (the loss of the GTP cap)
74
Describe a rescue and how it occurs
A rescue is when enough GTP-bound tubulin dimers are added to the shrinking end of a microtubule (GTP cap) and growth can resume
75
Describe dynamic instability
a microtubule's alternation between growth, catastrophe, rescue, catastrophe, etc.
76
What is the function of Microtubule Accessory Proteins (MAPs)?
they stabilize microtubules and promote growth/assembly
77
How do MAPs stabilize microtubules?
one of their domains binds to the microtubule and the other projects outwards
78
What mediates the binding of MAPs to a microtubule?
phosphorylation and dephosphorylation
79
What is an example of a MAP in humans? Describe its relationship to Alzheimer's Disease
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
What does the initial formation of a microtubule require?
A very high concentration of tubulin dimers and assistance of other proteins
81
Where are microtubules generated in the cell?
Microtubule Organizing Centres (MTOCs)
82
Give an example of an animal MTOC
the centrosome
83
Where do microtubules form in plants and fungi?
MTOCs embedded in the nuclear envelope
84
What does MTOC stand for?
Microtubule Organizing Centre
85
Describe the structure of centrosomes
two perpendicular barrel-shaped centrioles are surrounding by a dense mass (looks like a cloud) of insoluble proteins called pericentriolar material
86
In animals, how do microtubules anchor to the centrosome as an MTOC?
the minus end of the microtubule anchors in the dense mass of insoluble proteins surrounding the centrioles
87
Where are microtubules nucleated?
at their minus ends
88
What allows the centrosome to position itself roughly in the centre of the cell?
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
What is found at the base of the minus end of microtubules?
gamma tubulin rings
90
What is the function of gamma tubulin rings in microtubules?
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
How do gamma tubulin rings help anchor the microtubule to centrosomes?
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
What binds to the gamma tubulin ring once it is bound to the periocentriolar material of centrosomes? What does this cause?
the alpha tubulin of a heterodimer tubulin
this causes the elongation of the microtubule with the plus end facing outwards
93
What are the 2 kinds of microtubule motor proteins?
kinesins
| dyneins
94
What is the general function of the microtubule motor proteins?
they generate the force necessary for moving materials within the cell along microtubules
95
What kind of material do microtubule motor proteins move?
organelles
incoming secretory vesicles
outgoing endocytic vesicles
vesicles between the ER and Golgi (COPI and COPII)
96
What direction do kinesins move?
toward the + end of the microtubule
97
What direction do dyneins move?
towards the minus end of the microtubule
98
What structure does a kinesin have?
a tetramer with 2 heavy and 2 light chains
99
Describe the structure of kinesin
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
What is the function of the heads of kinesins?
they bind and hydrolyze ATP to create processive movement along the microtubule
101
What is the function of the tails of kinesins?
the tail binds the cargo that the kinesin is moving
102
What connects the heads and tail of kinesins?
a neck and flexible stalk
103
Describe what it means for the movement of kinesin to be processive
one alternating head is attached to the microtubule at all times (like walking - one head at a time)
104
Describe the position of a kinesins 2 heads (rear and leading) at any given time
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
What is the first step in a kinesin moving forward one step?
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
What happens after the rear head of kinesin hydrolyzes ATP (Step 2)?
the front head replaces the ADP it is bound to with ATP
107
What happens after the leading head of kinesin replaces ADP with ATP (Step 3)?
When ATP binds to the front head it causes a conformational change which propels the rear head forwards (becomes the new leading head)
108
What happens after the rear head is propelled forward (Step 4)?
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
Briefly describe all the steps for kinesin to move forward one step
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
Describe the structure of dyneins
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
What is required for dyneins to bind cargo?
the light chain of dyneins binds to a protein called dynactin which can bind to cargo
112
Describe dynactin?
a linker protein that binds to the light chains of dyneins and the cargo
113
What direction is anterograde transport?
forward
114
What direction is retrograde transport?
backward
115
In axoplasmic transport along an axon in nerve cells, where are materials moved in anterograde transport?
away from the cell body
116
In axoplasmic transport along an axon in nerve cells, where are materials moved in retrograde transport?
toward the cell body
117
What microtubule motor proteins would be involved in axoplasmic anterograde transport?
kinesins move cargo from the minus to the plus end (forward = anterograde = from cell body to axon terminus)
118
What microtubule motor proteins would be involved in axoplasmic retrograde transport?
dyneins move cargo from plus to minus end (backwards = retrograde = from axon terminus to cell body)
119
What structure do intermediate filaments have?
they are tetramers
120
How does the structure of intermediate filaments form?
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
T or F: all intermediate filaments are the same size and composed of the same number of tetramers
false! they can vary
122
What makes intermediate filaments different from MTs and MFs?
IFs do not:
have plus or minus ends (no polarity)
bind NTPs
123
Where are new units added to intermediate filaments?
in the middle of the filament
124
What is the assembly/disassembly of IFs regulated by?
phosphorylation
125
What makes IFs different from MTs and MFs?
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
Order the cytoskeletal elements in ability to withstand deforming force
strongest: IF
middle: MF
weakest: MT
127
What is the major function of intermediate filaments?
they provide mechanical support for cells subject to mechanical stress
128
Where would you expect to see many intermediate filaments?
in cells that are subjected to a lot of mechanical stress
ex. epithelial cells that line the bladder
129
What is another function of intermediate filaments?
in addition to providing strength, they connect other parts of the cytoskeleton together
130
What is an example of protein in an intermediate filament?
keratin
131
T or F: microfilaments are less dynamic than microtubules
false! MFs are more dynamic
132
What are the main functions of microfilaments?
structure and stability for the cell
133
What are 6 examples of microfilament movements?
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
What are microfilaments composed of?
globular actin subunits (G-actin)
135
What is G-actin?
globular actin subunit (monomer) that are incorporated into the filament (F-actin)
136
How does G-actin assemble into actin filaments?
G-actin binds ATP and assembles into actin filaments (F-actin)
137
What is F-actin?
filamentous actin
138
What are the 2 structures of actin which make up microfilaments?
G-actin (globular) = monomer
F-actin (filamentous) = polymer of many G-actins
139
How is an individual G-actin monomer incorporated into an F-actin polymer?
by hydrolyzing ATP
140
What are the + and - ends of F-actin based on?
the shape of the monomers
141
How are monomers positioned in the filament? (microfilament)
positioned so the ATP binding sites are closer to the minus end
142
Does the + or - end of F-actin grow more rapidly?
plus
143
Where does disassembly occur in microfilaments?
at the minus end of the F-actin
144
Which part of the microfilament is weaker?
the part where the actin is bound to ADP is weaker (the minus end)
145
Which end of an F-actin filament has ADP bound?
minus
146
Which end of an F-actin filament has ATP bound?
plus end
147
What promotes disassembly/depolymerization in F-actin?
Hydrolysis of ATP
148
Describe treadmilling in microfilaments
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
What happens to the length of the F-actin polymer when treadmilling occurs?
the length is stable
150
What happens to individual units that have been added to the plus end during treadmilling?
they will move toward the minus end
151
roughly how much of a cells total protein is actin? How much of this is assembled into filaments? What happens to the rest?
5% is actin
50% of this is assembled into filaments
the other 50% remains as soluble monomers
152
What binds to either F- or G- actin? What does it do?
Actin Binding Proteins that modify the properties of the actin
153
What are 6 examples of how Actin Binding Proteins can modify F- or G-actin?
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
What are 4 examples of the many different proteins that interact with actin (Actin Binding Proteins)?
thymosin
Arp2/3 complex
profilin
cofilin
155
How are microfilaments involved in cell movement?
the disassembly or reassembly of actin filaments can result in the movement towards site of reassembly
156
What are 3 examples of activities that require cell locomotion?
wound healing
development of axons
formation of blood vessels
157
Describe a lamellipodium
a broad, flattened area where the cell is reaching towards a stimulus via polymerization
158
What can a lamellipodium also be called?
a leading edge
159
Describe the structure of a lamellipodium
broad, very flat, fan-shaped with a ruffled edge that contains actin meshwork
160
Describe filopodia
the individual projections (ruffled edges of the leading edge) within the lamellipodium that contains a core of long bundled actin filaments
161
What is in the filopodia?
a core of long bundled actin filaments
162
What is located near the tail of a cell with a lamellipodium?
stress fibres
163
What are stress fibers? what do they do? where are they located?
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
What is the movement of a cell towards a stimulus called?
chemotaxis
165
Briefly describe the steps of cell movement with actin filaments
1. actin polymerization pushes out lamellipodium
2. new attachments to substratum via integrins
3. stress fibres contact and cell moves
166
Where in a cell is actin more concentrated?
the leading edge
167
Where in a cell is myosin more concentrated?
at the tail of the cell
168
What is actin growth at the lamellipodia regulated by?
WASP proteins
169
What causes new filaments to form at the lamellipodia?
WASP activates Arp 2/3 complex in response to chemotactic signals
170
What does Arp 2/3 stand for?
Actin Related Protein
171
What occurs in individuals with Wiskott-Aldrich Syndrome?
they are lacking WASP proteins and have a dysfunctional immune system
white blood cells fail to respond to chemotactic signals
172
What activates ARP2/3?
WASP
173
What happens when ARP2/3 has been activated?
ARP2/3 will bind to the minus end of an actin filament to promote nucleation
174
What happens ARP2/3 binds to the minus end of an actin filament?
it can attach to the side of another actin filament to form a branch
175
What is the purpose of ARP2/3 forming branches?
it helps individual actin filaments extend faster and extend into a tree-like web
176
What does cofilin do to microfilaments?
it is an actin binding protein that encourages the minus ends to dissociate
177
What does profilin do to microfilaments?
it is an actin binding protein that encourages the growth at plus ends
178
T or F: actin can form fixed permanent cell projections
true
179
What kind of micrograph would be used to visualized microfilaments?
TEM
180
What is an example of microfilaments that would require staying the same length?
actin filaments in microvilli
181
Describe the structure of microvilli
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
What stabilizes the length of microvilli?
stabilizing proteins
183
What type of motor proteins are involved with microfilaments?
myosin
184
What are the 2 types of myosin?
conventional (type II)
nonconventional
185
Describe the function of conventional (type II) myosins
they are used for contraction in actin filaments
186
Describe the function of nonconventional myosins
used like kinesins to move things around the cells (like vesicles)
187
What does myosin use to move things?
ATP hydrolysis
188
Describe the structure of myosin II
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
In myosin, what hydrolyzes ATP?
the two globular head domains bind and hydrolyze ATP (N terminus)
190
What can type II myosins assemble into?
bipolar filaments (many myosins) with heads in both directions
191
What causes the contractile force of type II myosin?
the pulling by bipolar filaments on the actin cytoskeleton
