13. Cytoskeleton proteins revealed by Commassie staining
Cytoskeleton: filament system
Three types of filaments
and accessory proteins
(assembly of cytoskeleton,
motorproteins that move
organelles
or filaments)
Internal order
Shape and remodel surface
Move organelles
Movement
Cell division
14. Dynamic and adaptable
Actin filaments:
shape of the cell’s surface
and whole cell locomotion
5-9 nm diameter
Microtubules:
positions of membrane-enclosed
organelles,
intracellular transport
25 nm diameter
Intermediate filaments:
mechanical strength and
resistance to shear stress
10 nm diameter
18. Microtubules
• Microtubules
• I. Introduction: Long, hollow cylinders, 25 nm in diameter, made of
tubulin. The basic subunit is a heterodimer of α and β tubulin (9.8);
13 protofilaments in a typical cylinder. See below about GTP
binding, treadmilling, growth and dynamic instability (9.26). There is
a + end, fast growing, w/β tubulin at its end, and a – end, slow
growing, w/α tubulin at its end. The GTP’s are important in
assembly (9.8)
• A. They have MAPs, that influence their use- linking them together,
stabilizing them, or destabilizing them.
• B. They form a network, coming from the microtubule organizing
center, which is usually the centrosome or centriole, w/ the – end
anchored there. (9.10-13, 19)
• C. Also form cilia and flagella, and spindle fibers in mitosis.
23. • F. MICROTUBULE DYNAMICS: 9.25
• Key points- the cap means that subunits
are added easily- loss of GTP = harder to
add subunits, need higher subunit conc. to
add.
• Produces microtubule catastrophes!
27. FIGURE 9.25 The structural cap model
of dynamic instability. According
to the model, the growth or shrinkage of a
microtubule depends
on the state of the tubulin dimers at the
plus end of the microtubule.
Tubulin-GTP dimers are depicted in red.
Tubulin-GDP dimers are depicted
in blue. In a growing microtubule (step 1),
the tip consists of an
open sheet containing tubulin-GTP
subunits. In step 2, the tube has begun
to close, forcing the hydrolysis of the bound
GTP. In step 3, the tube
has closed to its end, leaving only tubulin-
GDP subunits. GDP-tubulin
subunits have a curved conformation
compared to their GTP-bound
counterparts, which makes them less able
to fit into a straight protofilament.
The strain resulting from the presence of
GDP-tubulin subunits
at the plus end of the microtubule is
released as the protofilaments curl
outward from the tubule and undergo
catastrophic shrinkage (step 4).
29. Lateral bonds force GDP-containing
protofilaments into a linear conformation
30. Remodeling
• The fact that MT’s aren’t fixed means that
cells can remodel their shape- plant cells,
our cells in mitosis- round up, as MT’s
used to make spindle fibers
32. The structure of a microtubule and its subunits
GTP!
GTP
hollow and cylindrica and polar
13 parallel protofilaments
heterodimer
33. The structure of an actin monomer and actin filament
monomer
ATP
polar
two parallel protofilaments
that twist around each other
in a right-handed helix
Flexible but cross-linked and
bundled together by accessory
proteins in a living cell
34. The preferential growth of microtubules at the plus end
Plus end: polymerize and
depolymerize faster than
minus end
Microtubules:
Plus end- b subunit
Minus end- a subunit
Actin filaments
Plus end- barbed end
Minus end- pointed end
35. Fig. 6-22 Centrosome
Microtubule
Centrioles
0.25 μm
Longitudinal section
of one centriole
Microtubules Cross section
of the other centriole
37. MT drugs
• Colchicine- prevents MT formation- arrests
cells at metaphase
• Useful in determining role of MT’s in a
process
38. Effect of the drug taxol on microtubule organization
treatment of cancers
39. Actin and tubulin are highly conserved: they have to bind to
many proteins directly and indirectly
Accessory proteins and intermediate filament proteins
are not as conserved
40. A model of intermediate filament construction
Intermediate filaments
are only found in some
metazoans:vertebrates,
nematodes,molluscs
Not required in
every cell type
Ancesters: nuclear lamins
Parallel
Antiparrel
No polarity! “subunit”
8 parallel protofilaments
Easily bent
Hard to break
41. Two types of intermdiate filaments in cells of the nervous system
Neurofilaments:axons
NF-L, NF-M, NF-H proteins coassemble
axon glia
NF-M and NF-H have long C-terminal tails
That bind to neighboring filaments:uniform spacing
Regular spacing
When axons grow, subunits are added at the filament ends
and along the filament length; axon diameter increase 5 fold
In ALS (Lou Gehrig’s Disease), there is an accumulation and abnormal assembly of
Neurofilaments in motor neuron cell bodies and axon--interfere with normal axon transport
42. Summary
1. Three types of cytoskeletal filaments, protofilaments;
2. Subunits, polymerization, treadmilling, dynamic
instability;
3. Intermediate filaments, cell integrity, diseases caused
by mutations in the intermediate filament genes
4. Natural toxins and cytoskeleton
44. Fig. 6-23
5 μm
Direction of swimming
(a) Motion of flagella
Direction of organism’s movement
Power stroke Recovery stroke
(b) Motion of cilia
15 μm
52. Cytoskeletal filaments are all constructed from
smaller protein subunits
Intermediate filaments: smaller
elongated and fibrous subunits
Actin and microtubule filaments:
compact and globular subunits
All form as helical assemblies
of subunits
Noncovalent interactions:
rapid assembly and disassembly
62. The different sides of the
cilium may slide, depending
on the direction of sliding.
These sliding more
These sliding more
63.
64. Intermediate filaments
• 10 nm in diameter
• Only in animals! (??plant/fungal nucleus??)
• Variety of types- 60 genes!
• Seem to be involved in providing strength
to cells.
• Able to interact with both MT's and
microfilaments (actin filaments).
65. Intermediate filaments impart mechanical stability
to animal cells
Keratin filaments in epithelial cells
“desmosomes”
The most diverse family
20 in human epithelial cells
10 more in hair and nails
Diagnosis of epithelial
cancers (carcinomas)
66.
67. Octamers of Tetramers make
up the structure. No polarity!
Subunits are filamentous, rather than
globular.
68.
69. • Keratin- epithelial cells, hair, nails
• Neurofilaments- in, well, nerves
• Lamins- lines the nucleus
70. When they are mutant
• Smaller nerve fibers- a natural mutant
quail!
• Fragile skin
• Sometimes muscle weakness
• Sometimes nothing!
76. Microfilaments (Actin)
• Where we’re going:
• Basic structure, polarity, treadmilling
• Muscle contraction
• Amoeboid movement
77. Minus end
Domains 1-4
ATP binding cleft
Subunits= G actin-bound
w/ATP; F-actin=
microfilaments
Looks like
a double
helix!
78. S1 is a myosin
fragment that binds to
actin- the points point
to the minus end
79. Treadmilling of actin filaments
actin sub units can flow through the filaments by attaching preferentially to the
(+) end and dissociating preferentially from the (-) end of the filament.
This treadmilling phenomenon occur in some moving cells.The oldest subunits
In treadmilling filament lie at the (-) end.
80. Treadmilling-it’s easier to
add to the + than – end at
any concentration, and at
some concentrations it’s
adding at the + end at the
rate it’s coming off the –
end= treadmilling.
81. The treadmilling of an actin filament
Structural difference between the two ends
D form polymer leans towards disassembly
82. Muscle Contraction
• Three types of muscle fibers:
• Skeletal, striated, voluntary
• Heart- more like skeletal, but not
multinucleated. Its structure allows the
propagation of an action potential (the
heart beats by itself, w/o outside signals)
• Involuntary, smooth muscle- gut, uterus,
etc.
83. Multinucleated
cell, arises from
fusion; great big
thing- 100mm X
100 um!
2.5 uM length
Muscle contractility
For the Cell Biology Video The Cytoskeleton in a Neuron Growth Cone, go to Animation and Video Files
For the Cell Biology Video Cytoskeletal Protein Dynamics, go to Animation and Video Files.
Figure 6.20 The cytoskeleton
Figure 6.21 Motor proteins and the cytoskeleton
For the Cell Biology Video Actin Network in Crawling Cells, go to Animation and Video Files.
For the Cell Biology Video Actin Visualization in Dendrites, go to Animation and Video Files.
Table 6-1
Table 6-1a
Table 6-1b
Table 6-1c
For the Cell Biology Video Transport Along Microtubules, go to Animation and Video Files.
For the Cell Biology Video Movement of Organelles in Vivo, go to Animation and Video Files.
For the Cell Biology Video Movement of Organelles in Vitro, go to Animation and Video Files.
Figure 6.22 Centrosome containing a pair of centrioles
Figure 6.23a A comparison of the beating of flagella and cilia—motion of flagella
Figure 6.24 Ultrastructure of a eukaryotic flagellum or motile cilium
For the Cell Biology Video Motion of Isolated Flagellum, go to Animation and Video Files.
For the Cell Biology Video Flagellum Movement in Swimming Sperm, go to Animation and Video Files.
For the Cell Biology Video Interphase Microtubule Dynamics, go to Animation and Video Files.
For the Cell Biology Video Microtubule Sliding in Flagellum Movement, go to Animation and Video Files.
For the Cell Biology Video Microtubule Dynamics, go to Animation and Video Files.
For the Cell Biology Video E-cadherin Expression, go to Animation and Video Files.