Cells contain three main types of cytoskeletal elements - microtubules, microfilaments, and intermediate filaments. Microtubules are the thickest and made of tubulin, providing structural support and enabling cell movement through processes like cell division and intracellular transport. Microfilaments are the thinnest and composed of actin and myosin, allowing for internal cell movements. Intermediate filaments are of intermediate size and contain keratin, reinforcing cell shape and structure.

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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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3. AP Biology Dr.SAS

4. 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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5. 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

11. 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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13. • 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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14. 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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15. • ‘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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16. 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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17. Centrioles
• Cell division
– in animal cells, pair of centrioles
organize microtubules guiding chromosomes in
cell division
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18. 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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19. 1919
• a polarized helical polymer
• filament assembly depends on ATP hydrolysis (requires energy)
Micro Filaments / Actin
Filaments
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20. 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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21. 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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22. Microfilaments -Function
• Shape – Cytoskeleton
• Muscle Contraction
• Amoeboid movement
• Cytoplasmic streaming
• Cytokinesis
• Biconcave shape for RBC is by actin
filaments
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23. 2323
•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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25. 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 25Dr.SAS

26. 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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28. Summary
• Microtubules
– thickest
– cell structure & cell motility
– tubulin
• Microfilaments
– thinnest
– internal movements
within cell
– actin, myosin
• Intermediate filaments
– intermediate
– more permanent fixtures
– keratin 28Dr.SAS