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CB / U-01 / Cytoskeletal Proteins

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Cytoskeleton: the skeleton of a cell

Cells need a (cyto)skeleton to:

•create shape

•change shape

•allow movement

=

dynamic!

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AP Biology Dr.SAS

Cytoskeleton • Function

– structural support

• maintains shape of cell

• provides anchorage for organelles

– motility

• cell locomotion

• cilia, flagella, etc.

– regulation

• organizes structures &

activities of cell

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Cytoskeleton • Structure

– network of fibers extending throughout

cytoplasm

– 3 main protein fibers

• microtubules

• intermediate filaments

• microfilaments

It’s a matter of size…

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Cells have three

cytoskeletal elements

Microfilaments Intermediate filaments

Microtubules

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Filaments differ in Size, Shape and Flexibility

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Three filamentous networks in eukaryotic cells

MT:

• hollow tubes made of tubulin

• rigid, long, straight

MF/AF:

• helical polymers made of actin

• flexible, organized into 2D networks

and 3D gels

IF: • heterogenous group filamentous

proteins

• rope-like structure used to give cell

mechanical strength

25 nm

10 nm

7 nm

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Microtubules

•heterodimer ( -tubulin and -tubulin) which assembles into 13 polarized

protofilaments

• GTP bound to -subunit is hydrolyzed during assembly (requires energy) Dr.SAS

Microtubules • Function

– structural support & cell movement

• move chromosomes during cell division

– centrioles

• tracks that guide motor proteins carrying organelles

to their destination

– motor proteins: myosin & dynein

• motility

– cilia

– flagella

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• Basic arrangement -Tubulin (helical shape with

13 tubulin molecules/turn

• In addition to the tubulin, there are about 20-25

secondary proteins which is called Microtubule

Associated Proteins (MAPs).

• Few other variety of proteins that move along

them, form cross-bridges or influence their state

of polymerization - Kinesin and Cytoplasmic

dynein

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Microtubule Motor Protein

Kinesin:

• Move from – end to + end

• Transport organelles

• Mitosis and meiosis transport

• Synaptic vesicles along axons

Dynein :

• Move from + end to – end

• Cilia dynein – cilia movement

• Cytoplasmic dynein – organelle transport, mitosis

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• ‘Molecular motor or motor molecule’ in cells which converts the

chemical energy contained in ATP into the mechanical energy of

movement.

• Dynein transports various cellular cargo by "walking"

along cytoskeletal microtubules towards the minus-end of the

microtubule, which is usually oriented towards the cell center.

• Thus, they are called "minus-end directed motors."

• This form of transport is known as retrograde transport.

• In CONTRAST, kinesins are motor proteins that move toward

the microtubules' plus end, are called plus-end directed motors.

Dynein is a motor protein

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Microtubule -Function 1. Mechanical function

2. Cell membrane movement

3. Movement of cilia and flagella

4. Chromosomal movement during cell

division

5. Circulation and transport

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Centrioles

• Cell division

– in animal cells, pair of centrioles

organize microtubules guiding chromosomes in

cell division

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Cilia & flagella

• Extensions of eukaryotic cytoskeleton

• Cilia = numerous & short (hair-like)

• Flagella = 1-2/cell & longer (whip-like)

– move unicellular & small multicellular organisms

by propelling water past them

– cilia sweep mucus & debris from lungs

– flagellum of sperm cells

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• a polarized helical polymer

• filament assembly depends on ATP hydrolysis (requires energy)

Micro Filaments / Actin

Filaments

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Microfilaments (Actin filaments)

• Structure

– thinnest class of fibers

– solid rods of globular protein

subunits, actin

– twisted double chain of actin

subunits

– about 7nm in diameter

• Function

– 3-D network inside cell membrane

– in muscle cells, actin filaments interact with myosin filaments to create muscle contraction

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Microfilaments (actin filaments)

• Dynamic process

– actin filaments constantly form & dissolve making the cytoplasm liquid or stiff during movement

• movement of Amoeba

• cytoplasmic streaming in plant cells

– speeds distribution of materials

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Microfilaments -Function

• Shape – Cytoskeleton

• Muscle Contraction

• Amoeboid movement

• Cytoplasmic streaming

• Cytokinesis

• Biconcave shape for RBC is by actin

filaments

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•not found in all eukaryotes

• heterogenous: tissue specific

-several proteins with different amino acid composition

• NO energy required,

• NO filament polarity

Intermediate filaments

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Higher order organization of IF has functional

consequences

Monomers

Polymers

(protofilament)

Filaments

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Intermediate filaments - Polymers of elementary

unit protein • Structure

– specialized for bearing tension epidermal cells , muscle cells and neurons

– built from keratin proteins

• same protein as hair

– intermediate in size 8-12nm

• Function

– hold “things” in place inside cell

– more permanent fixtures of cytoskeleton

– reinforce cell shape & fix organelle location

• nucleus is held in place by a network of intermediate filaments

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The polymerization steps of

intermediate filaments • two basic units (i.e., monomers) associate in

the same direction to form a dimer.

•Two dimers running in

the reverse direction are line up

to form a tetramer, with the two strands

slightly offset in their opposite directions.

•Tetramers are arranged side by side to form a

protofilament.

•Eight protofilaments are

bundled to form an intermediate filament with

a diameter of approximately 10 nm.

Lack of polarity/Does not require GTP or ATP

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Summary

• Microtubules

– thickest

– cell structure & cell motility

– tubulin

• Microfilaments

– thinnest

– internal movements within cell

– actin, myosin

• Intermediate filaments

– intermediate

– more permanent fixtures

– keratin 28 Dr.SAS