Cytoskeleton Flashcards
1
Q
what is the cell cytoskeleton
A
a network of fibrous proteins that governs the shape and movement of a cell
2
Q
what is the function of the cell cytoskeleton
A
provides structure and organization to cells
resists and transmits stresses
drives shape change and movement
3
Q
two properties of the cell cytoskeleton are
A
self assembly
self organization
4
Q
self assembly
A
release of energy
forms static structures
5
Q
self organization
A
continuously consumes energy
forms dynamic filaments
6
Q
what are the building blocks of the cytoskeleton
A
microtubules
intermediate filaments
actin filaments
7
Q
actin structure
A
helical polymers of the protein actin
organize into a variety of linear bundles and 2-d networks
8
Q
actin function in stress fibers
A
locomotion
provide strength and shape to the membrane
9
Q
actin function in microvilli
A
provide shape and shape to the membrane
10
Q
actin function in striated muscle
A
form a 3D network
11
Q
microtubule structure
A
long hollow cylinders made of the protein tubulin
12
Q
which is more rigid, actin or microtubules
A
microtubules
13
Q
microtubules frequently have one end attatched to a
A
microtubule organizing center called a centrosome
14
Q
microtubule function in cilia
A
form cilia which function as motile rods or sensory devices
15
Q
microtubule function in interphase
A
form tracks to shuttle cargo
16
Q
microtubule function during mitosis
A
form a bipolar mitotic spindle during cell division
17
Q
intermediate filament structure
A
ropelike and elastic fibers
made of intermediate filament proteins
large and heterogeneous family
organize into a loose and flexible network
18
Q
intermediate filament function in neurons
A
extend across the cytoplasm giving cells mechanical strength
19
Q
intermediate filament function in epithelial tissue
A
they span the cytoplasm from one cell-cell junction to another, strengthening the entire epithelium
20
Q
intermediate filaments form the
A
nuclear lamina just beneath the nuclear membrane
21
Q
filaments are highly
A
dynamic
assembly/disassembly
22
Q
how do actin and microtubules play a role in cell division?
A
- the polarized microtubule array rearranges to form the bipolar mitotic spindle which aligns and separates chromosomes
- actin filaments rearrange to stop movement and form a sphere
- actin and myosin form a belt called the contractile ring which constricts to pinch the cell in two
- the daughter cells reorganize the microtubules and actin into smaller versions
23
Q
stages of cytokinesis by the contractile ring
A
- the contractile ring forms around the equator during telophase
- contractile ring constricts to form a cytokinetic furrow in the plasma membrane
- abscission occurs in the cytoplasmic bridge on either side and chromatin decondenses in the nuclei
24
Q
what do animal cells use to separate daughter cells at the end of mitosis
A
a contractile ring of actin filaments and myosin II
25
what does Myosin II do to a band of actin filaments
pulls on it applying tension to the plasma membrane
26
each actin subunit is
a globular monomer
27
each actin monomer has what bound to it
an ATP or ADP bound in the deep cleft at the center of the molecule
28
how do actin subunits assemble
head-to-tail to form a tight right handed helix forming a structure about 8nm wide called filamentous or f-actin
29
why are actin filaments polar
the assymetric actin subun its
30
what are the structurally different ends on actin filaments
the plus end
the minus end
31
microtubule subunits
alpha and beta tubulin
32
both tubulin subunits can bind
GTP
33
which tubulin subunit can hydrolize
beta
34
the Tubulin subunits polymerize to form a
protofilament
35
the microtubule is a hollow tube formed from
13 protofilaments aligned in parallel
36
microtubules are polarized with
a plus end - beta
a minus end - alpha
37
diameter of a microtubule
25nm
38
diameter of actin
8nm
39
where can antimitotic drugs bind to microtubules
diverse sites
plus end
tubulin dimers
interior surface
40
intermediate filament diameter
10nm
41
unlike actin and microtubule filaments, intermediate filaments are exclusively
structural proteins
static structures
42
domains of intermediate filaments
N-terminal domain
C-terminal domain
central or alphahelical rod domain
43
the rod domain has a conserved structure that forms a
alphahelical coiled coil structure
44
what is a coiled coil
a supersecondary structure composed of two alpha helices coiled together like the strands of a rope
45
that is the repeat in coiled coil
7-residue heptad repeat
HxxHCxC
hydrophobic and charged
46
how are coiled coil structures stabilized
hydrophobic weak interactions
the burial of hydrophobic surfaces provides the thermodymanic driving force for oligomerization
the packing in a coiled coil is exceptionally tight with almost complete vanderwaals contact between the side chains of the hydrophobic residues
47
a model of intermediate filament assembly
two monomers form a coiled-coil dimer
two dimers then line up side by side to form an antiparallel tetramer
in the final 10nm roplelike filament, tetramers are packed together in a helical array which has 16 dimers (32 coiled coils) in cross section
48
intermediate filaments act like a
nano-sized flexible rope
easy to bend and stretch but hard to break
impart mechanical stability to animal cells
49
neurofilaments are intermediate filaments important for
structural integrity of neurons and determine axonal diameter
50
what is ALS
amyotrophic lateral sclerosis
an accumulation and abnormal assembly of neurofilaments in moter neuron cell bodies and axon
interferes with normal axon transport
51
cause of ALS
mutation in molecular chaperone (ubiquilin)
52
nucleation phase
For a new actin filament to form, subunits first must nucleate, which is stabilized by subunit–subunit interactions
53
polymerization phase
elongation
54
equilibrium phase
is reached at which there is no net change in polymer length. The % free subunits after polymerization reflects the critical concentration (Cc),
55
key factors that influence filament growth
1) Asymmetry of the actin monomers (head-to-tail assembly;
reverse not allowed)
2) Monomer concentration (above Cc; which is true in the cytosol)
3) Kinetics of monomer binding
4) Nucleotide-bound state of monomers
56
kinetics of monomer binding
Actin Filaments Have Two Distinct Ends That Grow at Different Rates
Growth or shortening of the filament depends on the association/ dissociation rates (kon and koff)
57
The kon/koff are ... at the plus
end than the minus end.
10 x greater
58
ATP-bound state tends to
polymerize
59
ADP bound state tends to
depolymerize
60
hydrolysis reduces the binding affinity of
Hydrolysis reduces the binding affinity of the subunit for the neighboring subunits
and makes it more likely to dissociate from
each end of the filament
61
nucleation
polymerization starts by formation of a trimer aggregate or nucleus
62
polymerization
this is a reversible process in which monomers both associate/dissociate from both ends
63
elongation
the filament grows faster at PLUS end leading to
filament elongation.
64
treadmilling
At some point a steady-state is reached when the free monomer
concentration is starting to go slightly down: the PLUS end will
continue to grow but the MINUS end will shrink.
The length is constant but the filament moves towards (+)
direction
65
treadmilling can generate enough force to
‘push’ cell into one direction (transfers momentum forward due to asymmetric plus end polymerization)
66
the treadmilling cycle depnds on
ATP; the newly added monomers to the PLUS end have ATP-bound.
soon after polymerization ATP is hydrolyzed resulting in a ADP-bound monomers in the MINUS end
67
what is filament treadmilling
where the length of the filament stays roughly constant (steady-state) and the polymerized
monomers within the filament transfer momentum forward due to asymmetric plus end polymerization
68
what types of proteins control the dynamics of G-actin and F-actin?
severing
branching
bundling
crosslinking
capping
sequestering
69
what is a good model system to study locomotion
fish fibroblast
keratocyte
70
Key players in A simple Model for Actin-based Cell Locomotion: The molecular clutch hypothesis
actin filaments
extracellular matrix
integrin
vinculin
71
how is the ECM involved in The molecular clutch hypothesis
provides structural support to the
surrounding cells
72
how is integrin involved in The molecular clutch hypothesis
adhesion proteins that function mechanically, by attaching actin filaments to the ECM
73
how is vinculin involved in the The molecular clutch hypothesis
involved in linking integrin to the actin
74
what are the steps in the molecular clutch hypothesis
At the leading edge
there are actin filaments in
Treadmilling mode
* In order to generate
force actin filaments
need to attach to
Integrin receptors (these
mechanically link the
lamellopodium to the
ECM)
* The Integrin-Actin
linker is Vinculin (which
functions as a ‘clutch’)
* The coordinated action
of these three
molecules (F-actin,
Integrin and Vinculin)
lead to directed
movement
75
what cytoskeletal element has dynamic instability
microtubules
76
what is dynamic instability
Microtubules continually grow from the centrosome added to a cell extract. Suddenly some microtubules stop growing and then shrink back rapidly (dynamic instability).
77
dynamic instability is regulated by
interaction with other proteins
78
like actin filaments, microtubule subunits can
polymerize and depolymerize
79
The rapid interconversion between a growing and shrinking state, at a uniform free subunit concentration, is called
dynamic instability
80
are microtubules stable
no
81
When nucleotide hydrolysis proceeds more rapidly
than subunit addition
there is a switch from slow growth to rapid shrinkage, an event called a “catastrophe” which leads shrinkage
82
what us rescue in dynamic instability
GTP-containing subunits may
still add to the shrinking end, and
if enough add to form a new cap,
then microtubule growth resumes
83
significance of dynamic instability
This event is important for the
fast disassembly of the
mitotic spindle during cell
division
84
during polymerization the tubulin dimers are
in the GTP bound state
85
GTP bound to alphatubulin is
stable
86
the gtp bound to beta tubulin is
hydrolyzed to GDP shortly after assembly
87
GDP-tubulin is more prone to
depolymerization
88
when hydrolysis catches up to the tip of the microtubule, it begins
a rapid depolymerization and shrinkage
89
the switch from growth to shrinking is called
catastrophe
90
the properties of pure tubulin cannot fully explain
microtubule behavior in cells
91
MAPs
microtubule associated proteins
92
what do MAPs regulate
microtubule initiation
elongation
shortening
catastrophes
rescues in the cytoplasm
93
cells can modulate the activity of MAPs to
change microtubule dynamics
94
dynamic instability is more pronounced during
mitosis
95
developmentally programmed gene expression establishes the mix of
MAPs in each cell type
96
in the presence of tau
microtubules grow 3 times
faster, shorten slower, and have catastrophes 50 times less frequently than pure tubulin microtubules.
97
the individual tubulin binding repeats can rearrange with allows tau to
dampen microtubule dynamics without stopping tubulin association and disassociation altogether
98
phosphorylation of the microtubule binding motifs of these MAPs inhibit
microtubule binding and destabilizes microtubules
99
the phosphate group on phosphorylated MAPS are repelled from
the negatively charged surface of the microtubule
100
many tau mutations cause
rare inherited dementias
101
detection of phosphorylated tau in spinal fluid is
used to diagnose Alzheimers
102
an intermediate filament can stretch how much
three times its length
103
how can you stretch filaments
AFM probe
104
what is titan
a giant muscle protein
105
titin is highly
extensible
106
titins extensibility results from
domain unfolding
107
titan forms what pattern when force is applied
a sawtooth pattern
108
titin domains readily
refold after mechanical unfolding
109
folding of titin Ig domains is an important contributor to
the force generated by a contracting muscle
110
titin is a signaling molecule that
regulates muscle development and adaptation to load via a kinase
111
the signaling function of titin is mediated by a
stretch activated protein kinase
112
the thick filament is composed of
about 300 myosin molecules associated in parallel (quat structure)
113
the thick filament has
bipolar organization
a central bare zone devoid of heads
114
the thin filament is composed of
hundreds of G-(globular) actin subunits
115
in the thin filament the subunits form
a helical quat structure known as F-(filamentous) actin (capped)
116
the major component of myosin is
a long polypeptide known as the myosin heavy chain
117
the myosin heavy chain has
a long alpha helical tail (1000aa) and a globular head
118
the myosin head
also called S1 or proteolytic filament
an ATP dependent molecular motor (ATPase)
119
the essential and regulatory light chains
are two shorter polypeptide chains wrapped around the neck or lever arm of the head
120
the main proteins of thin filaments are
actin, tropomyosin, and troponin
121
globular subunits of actin stick together forming
F-actin filamentous actin
122
actin subunits have sites that can bind
myosin heads
123
tropomyosin molecules are bound to
actin along each chain of the actin double helix
124
each tropomyosin molecule spans
about 8 actin subunits and slightly overlaps with the next tropomyoson molecule
125
how long is tropomyosin
a dimer
42nm
126
at rest tropomyosin molecules
physically cover the sites on actin that could otherwise bind myosin heads
127
troponin is
a heterotrimeric protein complex comprised of three subunits
128
what are the three subunits in troponin
tropomyosin binding subunit, troponin T (TnT)
calcium binding subunit troponin C (TnC)
actomyosin ATPase inhibitory subunit troponin I (TnI)
129
the coordinated actions of the troponin complex are designed to
regulate the functions of actomyosin cross bridges in a calcium-dependent manner
130
mutations in sarcomeric proteins cause
cardiomyopathies
dialated
hypertrophic
restrictive
131
point mutations in the myosin head can lead to
HCM or DCM
132
laminin serves as an
anchor of membrane complexes (Integrin and DGC) to the ECM
133
dystrophin serves as a
mechanical link between membrane complexes and contractile units (Myofibrils)
134
desmin intermediate link protein acts as a
an elastic link between the sarcolemma, myofibrils and nucleus.
It provides maintenance of cellular
integrity and force transmission.
135
basic steps in force generation by myosin
1. Strong attachment of the head
to actin filament (no ATP
bound)
2. ATP comes in and kicks the S1
head off the actin; Hydrolysis
occurs and there is a large
shape change that causes head
to move by about 5nm (size on
actin monomer)
3. The head binds to a new actin
subunit releasing phosphate
(Pi) triggering the “Power Stroke” and actin filament
movement
4. Re-attachment to actin
136
contraction steps: at rest
The Resting State; low intracellular [Ca]
At rest, the head or S1 has ADP and P attached and has a pronounced curvature. S1 cannot bind to actin since the binding sites on actin are physically covered by tropomyosin.
137
when tropomyosin moves out of the way and the head binds to acin it is called
the crossbridge
138
when the crossbridge forms,
ADP and P dissociate from the head and forcefully bends
Power stroke
139
power stroke generates
a force between thick and thin filaments driving the thin filaments toward the center of the sarcomere
140
after the power stroke,
ATP binds immediately causing detachment
141
how does the myosin head return to the pre-stroke state
ATP hydrolysis
142
in the prestroke stat the myosin head contains
ADP and Pi and has a weak affinity for actin
143
what does release of the Pi on the myosin head do
make it bind better to actin
144
like a spring, the lever arm stores
the energy released by atp hydrolysis
145
myosin heads continuously cycle between
actin bound and actin detached states
146
what causes tropomyosin to uncover the actin binding sites
an increase in cytoplasmic calcium
troponin c binds to calcium and the conformational change causes tropomyosin to uncover the sites
147
binding of calcium to troponin c is
reversible
148
how long does calcium bind to troponin c
a few crossbridge cycles before spontaneously unbinding
149
troponin-tropomyosin complexes trigger and resent repetitively with
high calcium levels
150
how much work does a myosin motor generate
5pN
moves about 10nm
151
what is kinesin responsible for
moving vesicles and organelles over microtubules
152
kinesin structure
a tetramer made of two heavy and two light chains
a pair of globular heads: force generating moters
a stalk
a tail that binds cargo
153
kinesin vs myosin head
kineasin head is less than half the size of the myosin head
154
kinesin and myosin lack
similarity in amino acid sequence
155
the kinesin head is folded
in a way similar to the ATP binding core of the myosin head
156
odel for the kinesin-1 “hand over hand” walking mechanism
1. In the ADP bound form the head
binds to MT
2. ATP comes in and kicks off ADP
3. The head rotates and binds to a forward binding site
4. The neck linker is what pulls the head and docks on a new binding site
5. Hand-over-hand over mechanism
157
two kinds of dynein
cytoplasmic
axonemal
158
cytoplasmic dynein
organelle transport and centrosome assembly; movement
of chromosomes and positioning the mitotic spindles for cell division
159
molecular motor vesicle driving movement
kinesin (- to +; away from
the center of the cell) and
dynein (+ to -; towards the
center of the cell)
160
what is the force of myosin II
1-10 pN
160
what is the speed for myosin II
6micro meter/sec
161
step side of myoson II
5nm
size of actin
162
run length of myosin II
0.1 micro meters
163
processivity of myosin II
no
164
stepping of myosin II
single
165
what is the speed of kinesin
1 micro meter per second
166
force of kinesin
7 pN
167
step of kinesin
8nm
size of tubulin dimer
168
run length of kinesin
1 micrometer
169
processivity of kinesin
yes
170
stepping of kinesin
hand over hand
171
dynein speed
axonemal - 7 micro meter per sec
cytoplasmic - 1 micrometer per sec
172
dynein force
7pN
173
dynein step size
8-31 nm
size of multiple tubulin dimers
174
dynein run length
5 micro meter
175
dynein processivity
yes
176
dynein stepping
hand over habd or inchworm
177
what is the typical calcium concentration under resting conditions in a muscle cell
super low
178
what is the typical calcium concentration in muscle contraction
high, activates many pathways
179
in myosin, are the two heads synchronized or cycle independently
independently, makes it move faster
180
what makes myosin heads only walk in one direction
polarized filaments
181
how come the actin filament doesnt slide back when the myosin detaches
many heads acting asynchronously
182
what are the two main roles ATP hydrolysis plays in muscle contraction?
ATP by the hydrolysis motor domain is required for filament sliding in muscle contraction
ATP hydrolysis is required to pump calcium out of the cytosol into the ER to allow the myofibrils to relax
183
actin subunit
actin monomer
184
actin main function
cell dynamics: motility and transport
185
actin shape and flexibility
rod-like
semi flexible
186
actin polarity
yes + and -
187
actin enzymatic activity
yes ATPase
188
actin assembly
weak, electrostatic, vanderwaals, hydrogen bonds
reversible
189
microtubule subunit
tubulin
alpha and beta dimer
190
microtubule main function
cell division: cilia structure
191
microtubule shape and flexibility
hollow cylinders
rigid
192
microtubule polarity
yes + and -
193
microtubule enzymatic activity
yes GTPase
194
microtubule assembly
weak, electrostatic, vanderwaals, hydrogen bonds
reversible
195
intermediate filament subunit
helical rod
196
intermediate filament main function
cell structure mechanical support
197
intermediate filament shape and flexibility
rope like
elastic
198
intermediate filament polarity
no
199
intermediate filament enzymatic activity
no
200
intermediate filament assembly
strong coiled coil hydrophobic
irreversible
201
actin dynamics
dymanic elongation/shrinking, they elongate steadily in the presence of monomers; steady assembly/disassembly needed for locomotion or treadmilling
202
microtubule dynamics
complex dynamics; dynamic instability; switch between two states, stabe growing or rapidly shrinking; dynamics needed for spindle formation and collapse during cell division
203
intermediate filament dynamics
no
form stable filaments, resists sheer stress
204
actin molecular motor
myosins (II and IV)
205
microtubule molecular motors
kinesin (toward plus end)
dynein (toward minus end)
206
intermediate filament molecular motor
none
