1 cytoskeleton a cytoplasmic system of fibers -> critical to cell motility (movement) macrophage...
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Cytoskeleton
A cytoplasmic system of fibers -> critical to
cell motility (movement)
Macrophage cytoskeleton
Cytoskeleton of a lung cell in mitosis
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The cytosol:
20-30 w% of cytosol are proteins -> ¼ - ½ of total protein is in cytosol
Protein conc. 200-400 mg/ml -> complexes of protein
It is believed that cytosol is highly organized
-> Most soluble proteins are
- bound to filaments
- localized in specific regions
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Filament network in the cell
Filament network (fluorescence)
From the nucleus to the plasma membrane
Microtubules network
Starting from the MT center near nucleus
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Actin monomers assemble into long helical polymers with polarity
1. Microfilaments and Actin Structures
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Actin Filaments grow faster at (+) end than at (-) end
The rate of addition of ATP-G-actin is much faster at the (+) end than at the (-) end (rate of dissociation is similar) -> lower critical concentration (Cc) at (+) end in steady state -> filament grows preferentially at the (+) end
If actin conc. is between Cc- and Cc+ (steady state) -> actin subunits flow through filaments by attaching to (+) end and dissociating from (-) end
Treadmilling phenomen -> involved in movement of cells
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Capping Proteins Block Assembly and Disassembly at Actin Filament Ends
The presence of these 2 proteins at opposite ends prevent actin from dissociating during muscle contraction
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Movement of invaders inside the cell
Most infections are spread by lysed cells.
Some Bacteria (Listeria monocytogenes) or viruses (vaccinia – related to smallbox virus) escape from cell on the end of a polymerizing actin filament.
These organisms or viruses move through the cytosol at rates of 11μm/min.
Actin generates the force necessary for movement
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Filaments attached to Membranes
Microvilli on an epithelial cell showing polarity of actin filaments
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Myosins - Cellular Motor Proteins
Tail:
-> Locatized to cellular membranes
-> vesicle attached (cargo)
Form thick filaments in muscles
S1 motor domain
Head -> Motor domain (S1) -> ATP depentend
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Myosin heads walk along actin filaments -> towards (+) end
Sliding-filament assay:
Myosin tail absorbed onto glass surface -> a solution of actin filaments allowed to flow through
In presence of ATP myosin heads walk towards (+) end of actin filaments -> sliding of filaments
-> Movement of labeled actin filaments
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Myosins – Motor proteins responsible for cell movement
These are the most important 3 myosins (out of ca. 40 we have in humans)Loss of more specialized ones -> causes deafness/blindness
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Myosin motion along actin
Length of the neck domain -> determines rate of movement
Step size
-> Moves in 72 nm steps
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Skeletal muscle contraction is regulated by Calcium and actin binding proteins
Tropomyosin (TM)
Troponin (TN)
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Cell Locomotion
Coordination of motions generated by different parts of the cellMovement of fish epidermal cell Cell locomotion mechanism:
Includes actin polymerization and branching-generated movement at the edge, assembly of adhesion structures, and contractions mediated by myosin II
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Myosin V Carries Many Cargoes
Myosin V: -> carries secretory vesicles, organelles,...
-> Used to prepare nucleus for mitosis
-> used to segregate organelles
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2. Intermediate Filaments
Keratin and lamin IF
IF differ in stability, size and structure from other cytoskeleton fibers:
- IF are extremely stable (hair, nails, wool)
-10 nm diameter
- α-helical rods -> assemble into ropelike filaments
- assemble from different IF proteins
- assembly through several intermediate structures
Intermediate structures in the assembly of IF
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Cross-links between Microtubules and Intermediate Filaments in Fibrioblast cells
Microtubules (red), Intermediate Filaments (blue), connection between fibers (green)
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3. Microtubules
Kinesin-powered movement of a vesicle along a microtubule
Microtubules are involved in cell movement:
- Beating of cilia and flagella
- transport of vesicles in the cytoplasm Microtubules organizing center (MTOC)
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Microtubules organization
2 type of MT in cells:
- Stable and long-lived (found in non-replication cells) -> in cilia, flagella, neurons
- unstable and short-lived (found in mitosis) -> spindle-shaped apparatus that partitions chromosomes equally to daughter cells
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Microtubules assemble from Microtubule Organizing Centers (MTOCs)
MTOCs in non-mitotic cells -> centrosomes
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Microtubules Assembly/Disassembly
Colchicine and Taxol:
Drugs that interfere with Microtubules Assembly/Disassambly
Colchicine: 2500 years ago Egyptians treated heart problems
Nowadys: treatment of gout, skin and joint diseases
Taxol: (stabilizes Microtubules)
Anticancer agents -> treatment of ovarian cancer
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Kinesin and Dynein – two Families of Motor Proteins
Responsible for Transport along Microtubules
Microtubules based vesicle transport
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Microtubule motors in a Cell
Kinesins -> transport to cell periphery (+)Dyneins -> transport to cell center (-)
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Cooperation of Myosin and Kinesin at the cell cortex
Secretory vesicles are handed over from Kinesin to Myosin -> last part of secretory pathway
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Rotory Motors – Cilia and Flagellar
Sperm
Bacterial Flagella (E. coli, Salmonella)
Rotation of Flagellar -> Motion
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Motion of E. coli
The points show the locations of the bacterium at 80 ms intervals.
Changing of direction: Tumbling is caused by an abrupt reversal of the fagellar motor
A second reversal restores smooth swimming -> almost always in a different direction
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Reversal of the direction of Flagellar Rotation is obtained by Proton Transport through Motor
Proton flow drives flagella rotation !!!
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Chemotaxis Signaling Pathway
Direction of bacterial movement depends on chemical substances:
-> Bacteria swim towards high concentrations of glucose - chemoattractans
-> Bacteria swim away from harmful substances, such as phenol - chemorepellants
Receptor in plasma membrane initiate signal pathway ->
Phosphorylation of CheY protein -> P-CheY binds to flagellar motor -> clockwise rotation favored
Attractant binds to receptor -> pathway blocked -> smooth swimming
Repellant binds to receptor -> pathway stimulated -> more frequent clockwise rotation -> tumbling