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The cytoskeleton
(October 09, 2012, Miklós Nyitrai)
1. What is the cytoskeleton?
2. Polymerisation
3. Filament types, and polymerization
4. Motor proteins
Definition of the cytoskeleton
The cytoskeleton
Three main types:
1. Intermediate filaments
2. Microtubules
3. Microfilaments
The dynamic protein framework of eukaryotic cells.
Movements in biology
Subcellular,
Subcellular, cellular levels
Requires ATP (energy)
CytoskeletonCytoskeleton-mediated
„
„
What roles the cytoskeleton plays?
plays?
Assembly and disassembly of cytoskeletal fibers
(microfilaments and microtubules)
Motor proteins use cytoskeletal fibers
(microfilaments and microtubules) as tracks
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Cell motility (crawling)
crawling) – a movie
Cell motility – cytoskeletal elements
Migrating melanocyte expressing GFP-tagged actin (Victor Small).
Growth of filopodia – cytoskeletal elements
The steps of crawling
An example: motility with actin polymerization
Intracellular pathogens
Cytoskeletal filament systems and their
properties
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Microfilaments (actin)
Actin was discovered and named by a
Hungarian scientist, Straub F. Brúnó
The actin monomer
The building units of microfilaments:
actin monomers
Globular (G-) actin
MW: 43 kDa, 375 aa,
1 bound ATP or ADP
Subdomains (4)
nucleotide
4
2
3
1
The polimerization
The formation of actin filaments:
polymerisation
Three phases:
1. Nucleation
2. Elongation
3. Equilibrium
3
The polymerisation of actin
Actin monomer
The properties of actin filaments
Actin filament
37 nm
Depolymerisation
7 nm thick, length in vitro is more than 10 µm, in vivo 1-2 µm
Double helix
Semi-flexible polymer chain (persistence length: ~10 µm)
"barbed end“ and "pointed end"
(“barbed” =+ rapid polymerization, “pointed” =- slow polymerization)
Dimer
Elongation
Trimer
Nucleation
T. D. Pollard: Cell, 112, 453-465, 2003.
The equilibrium
Actin functions,
an example
-
+
Microtubules
The building units of microtubules: tubulin
Subunit: tubulin
MW: 50 kD, α- és β-tubulin -> heterodimer
1 bound GTP or GDP;
α
β
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The microtubules
~25nm thick, tube shape
13 protofilaments
Right hand, short helix
Left hand, long helix
Stiff polymer chain (persistence length: a few mm!)
Structural polarization:
+ end: rapid polymerization, - end: slow polymerization
GTP-cap
Microtubule functions, an example
The intermediate filaments
The polymerisation of intermediate flaments
The subunit of filaments: „coiled-coil” dimer
Polymerised in cell
lack of dynamic equilibrium
Central rods (α-helix)
hydrofob-hydrofob interactions
-> colied-coil dimer
2 dimer -> tetramer
(antiparallel structure)
protofilamentum
Tetramers connected longitudinally
-> protofilaments
8 protofilaments -> filament
filamentum
The building units are polymers!
Vimentin dimer
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Tissue specific intermediate filament types
Nuclear lamins
A, B, C lamins
(65-75kDa)
Polymer mechanics
The direction of force:
Bending stiffness:
Longitudinal stiffness:
Vimentin type
Torsion:
F
F
F
Vimentin (54kDa)
Desmin (53kDa)
Peripherin (66kDa)
Keratins
Mechanism:
-Intrinsic flexibility
-Thermal (entropy) flexibility (persistence length)
Type I (acidic) (40-70kDa)
Type II (neutral/basic) (40-70kDa)
A = persistence length
Neuronal IF
neurofilament proteins (60-130kDa)
Lc = contour length
F
Z = end-to-end
distance
Summary
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