cell biology cytoskeletal proteins

1. 11
CB / U-01 / Cytoskeletal Proteins
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2. 22
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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6. 6Dr.SAS

7. 77
Cells have three
cytoskeletal elements
Microfilaments Intermediate filaments
Microtubules
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Filaments differ in Size, Shape and Flexibility
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9. 99
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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10. 1010
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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12. 12Dr.SAS

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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27. 27Dr.SAS

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