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The cytoskeleton

Miklós Nyitrai; November 29, 2016

1. What is the cytoskeleton?

2. Polymerisation

3. Filament types, and polymerization

4. Motor proteins

Content

Definition of the cytoskeleton

The dynamic protein framework of eukaryotic cells.

The cytoskeleton

Three main types:1. Intermediate filaments2. Microtubules3. Microfilaments

Movements in biology

� Subcellular, cellular levels

� Requires ATP (energy)

� Cytoskeleton-mediated

� Assembly and disassembly of cytoskeletal

fibers (microfilaments and microtubules)

� Motor proteins use cytoskeletal fibers

(microfilaments and microtubules) as tracks

What roles the cytoskeleton plays?

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Migrating melanocyte expressing GFP-tagged actin (VictorSmall).

Cell motility (crawling) – a movie Cell motility – cytoskeletal elements

Growth of filopodia – cytoskeletal elements The steps of crawling

An example: motility with actinpolymerizationIntracellular 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 building units of microfilaments: actin monomers

Globular (G-) actin MW: 43 kDa, 375 aa, 1 bound ATP or ADPSubdomains (4)

The actin monomer

1

4

3

2

nucleotide

The formation of actin filaments: polymerisation

The polimerization

Three phases:1. Nucleation2. Elongation3. Equilibrium

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Dimer

Trimer

Nucleation

Elongation

Actin monomerActin filament

The polymerisation of actin

Depolymerisation

T. D. Pollard: Cell, 112, 453-465, 2003.

The properties of actin filaments

37 nm

7 nm thick, length in vitro is more than 10 µm, in vivo 1-2 µmDouble helixSemi-flexible polymer chain (persistence length: ~10 µm)"barbed end“ and "pointed end"

(“barbed” =+ rapid polymerization, “pointed” =- slow polymerization)

The equilibrium

+-Actin functions, an example

Microtubules

Subunit: tubulinMW: 50 kD, α- és β-tubulin -> heterodimer1 bound GTP or GDP;

The building units of microtubules:tubulin

α

β

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The microtubules

~25nm thick, tube shape13 protofilaments Right hand, short helixLeft hand, long helixStiff polymer chain (persistence length: a few mm!)Structural polarization:

+ end: rapid polymerization, - end: slow polymerizationGTP-cap

Microtubule functions, an example The intermediate filaments

The polymerisation ofintermediate flaments

protofilamentum

filamentum

Polymerised in cell

lack of dynamic equilibrium

Central rods (α-helix)

hydrofob-hydrofob interactions

-> colied-coil dimer

2 dimer -> tetramer

(antiparallel structure)

Tetramers connected

Longitudinally -> protofilaments

8 protofilaments -> filament

The building units are polymers!

The subunit of filaments:‚coiled-coil’ dimer

Vimentin dimer

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Nuclear lamins A, B, C lamins

(65-75kDa)

Vimentin type Vimentin (54kDa)

Desmin (53kDa)

Peripherin (66kDa)

Keratins Type I (acidic) (40-70kDa)

Type II (neutral/basic) (40-70kDa)

Neuronal IF neurofilament proteins (60-130kDa)

Tissue specific intermediatefilament types

-Intrinsic flexibility-Thermal (entropy) flexibility (persistence length)

A = persistence length

F

Z = end-to-end distance

Lc = contour length

Polymer mechanics

Bending stiffness:F

Longitudinal stiffness:F

Torsion:F

Mechanism:

The direction of force:

Summary